SECTION EDITOR
Long-term prognosis of e x t e n s i v e polyunit c a n t i l e v e r e d fixed partial dentures Lars Laurell, OdontDr, DDS, a Dan Lundgren, OdontDr, DDS, a Hanne Falk, LDS, a and Anders Hugoson, OdontDr, DDS b
The Institute for Postgraduate Dental Education, JSnk6ping, Sweden The aim of this r e t r o s p e c t i v e study w a s to elucidate the l o n g - t e r m p r o g n o s e s of e x t e n s i v e fixed partial d e n t u r e s including u n i l a t e r a l or b i l a t e r a l polyunit cantilev e r s in p a t i e n t s w i t h h e a l t h y but reduced periodontal support. F o l l o w i n g periodontal t h e r a p y 36 cross-arch fixed partial d e n t u r e s w i t h two or m o r e c a n t i l e v e r units u n i l a t e r a l l y or b i l a t e r a l l y w e r e fitted in 34 patients. In the prosthodontic design, s p e c i a l attention w a s g i v e n to the retention to long p a r a l l e l preparations, to the d i m e n s i o n s of the f r a m e w o r k , and to the occlusal design. After c o m p l e t i o n of therapy, the p a t i e n t s w e r e enrolled in a r e g u l a r m a i n t e n a n c e care program and f o l l o w e d up for a period of 5 to 12 years. D u r i n g this f o l l o w - u p period one abutm e n t tooth w a s fractured in one patient. One fixed partial denture w i t h e x t r e m e l y reduced periodontal support w a s lost as a result of c o m p l e t e periodontal breakd o w n from occlusal trauma. For 33 fixed partial dentures, n e i t h e r periodontal nor technical c o m p l i c a t i o n s occurred. (J PROSTHET DENT 1991;66:545-52.)
T h e incorporation of posterior cantilevers in fixed partial dentures (FPDs) is sometimes desirable for functional and esthetic reasons. However, posterior cantilevers consisting of two or more units in FPDs are generally considered a threat to the construction, the abutment teeth, and the supporting tissues because of supposed large forces affecting the cantilever segments. Clinical long-term follow-up studies on the prognosis of extensive cantilever prosthodontics that would confirm or reject such postulations are few thus far 16 and the results are, to some extent, contradictory. Thus, Nyman and Lindhe 1 presented a total of less than 8 % technical failure (loss of retention 3.3 %, fracture of the prosthesis 2.1%, and fracture of abutment teeth 2,4 9 ) after 5 to 8 years in 159 periodontally treated patients supplied with extensive cantilever FPDs. During this observation period no further loss of periodontal attachment occurred, probably because of the controlled maintenance care program in which the patients were enrolled. Isikowitz2 presented long-term follow-up results of 87 FPDs with distal extension bases and reported an increasing failure rate over time. Thus, after 5 years, 98 % of the prostheses were still in function, after 10 years 82 9 , after 15 years 69%, and after 20 years 499. It was reported that failures were significantly more frequent when the FPDs occluded with a complete denture. Of 130 fixture-supported cantilever prostheses, Adell et
al.8 reported an incidence of exchanged FPDs because of various technical and biological complications in 20 9 in the maxillae and 9 % in the mandibles. Fracture of FPDs accounted for 5%. These studies all concerned patients treated by specialists. In an epidemiologic study on extensive FPDs made by general practitioners, Randow et al. 4 found technical failures in an average of 33 % of 83 patients with two or more cantilever units. Technical failures increased with an increasing number of cantilever units and years in service. Periodontal problems were rare, occurring in only 7% of the patients. High or very high rates of failures, mainly due to loss of retention of distal retainer crowns in cantilever FPDs, were also reported by Dahl et al. 5 (40%) and Karlsson s (369). In our department, a number of extensive FPDs including two or three unilateral or bilateral cantilever units have been made throughout the years as part of the overall treatment of patients with advanced stages of periodontal breakdown. In view of the inconsistencies in reported prognosis for extensive fixed cantilever prostheses, we considered it important to: (1) present the results of a retrospective long-term follow-up study of patients periodontally treated and prosthetically restored with extensive posterior unilateral or bilateral polyunit cantilever FPDs and, (2) discuss factors that might promote success or failure. MATERIAL
aAssociate Professor, Department of Periodontology. bProfessor, Head, Department of Periodontology. 10/1/30547
THE JOURNAL OF PROSTHETIC DENTISTRY
AND
METHODS
The group of subjects studied included all of the subjects who, during the years of 1973 to 1981, were referred to the department of periodontology at The Institute for Post-
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LAURELL ET AL
108 a:
6
W m
4 Z
2 0 5
6
7
8
9
10
11
12
YEARSIN SERVICE
Fig. 1. Number of fixed partial dentures distributed according to years in service.
Table I. Number of fixed partial dentures distributed according to number of cantilever units Cantilever units
Number of fixed partial dentures
Examinations
2 or 3 unilaterally
2 or ~ on one side and 1 on the other
2 or 3 bilaterally
12
11
12
graduate Dental Education, JSnkSping, Sweden, for the treatment of periodontal diseases of varying severety and in whom extensive fixed partial dentures including two or more unilaterally or bilaterally posterior cantilever units were part of the overall treatment. The choice of cantilevered fixed partial dentures as the treatment modality was based on (1) lack of teeth that could be used as abutments and (2) the refusal of the patient to accept a fixed partial denture to replace teeth that were lost or had to be removed. Before the definitive prosthodontic therapy, sometimes preceded by transitional prostheses, all subjects were given adequate periodontal therapy according to the principles outlined by Lindhe and Nyman. 7 Required endodontic treatment was performed according to standardized methods. Root-filled single-rooted teeth were always supplied with cores and posts that were cast in type III dental gold alloys. Special requirements were set up for the design of the cantilever prostheses: 1. The jaw relationships permitted the establishment of anterior as well as posterior occlusal contacts with axial force direction along the entire arch of teeth. In addition, the occlusal design was characterized by wide freedom in the retrusive-protrusive range and anterior guided lateral movements with no working or nonworking side contacts on the cantilever segments. 2. Optimal retention to long preparations with almost parallel tooth surfaces, at least at the distal abutments, was another prerequisite. To attain this goal, preprosthetic
546
gingival and osteoplastic crown-lengthening surgery was occasionally performed to achieve at least 3 mm of completely parallel cervical embracement to the abutment teeth. The cantilever segments were made 5 to 6 mm in height and 4 to 5 mm in width at the connection to the distal retainer crown. These dimensions were also aimed at along the entire prosthesis, in particular immediately mesial to the distal retainer. After completion of the periodontal and prosthetic therapy, the patients were clinically and radiographically examined to obtain baseline data for the future follow-up controls. The patients were then enrolled in a regular maintenance care program with clinical and, when needed, radiographic reexaminations and required treatment. The final examination, including full mouth radiographs, was performed in 1987, 5 to 12 years after completion of initial therapy.
For the present study the following variables were assessed for each individual: F u n c t i o n a l s t a t u s . (1) Years in service, (2) extension of fixed partial denture, (3) number of abutment teeth, (4) number of root filled distal abutment teeth, (5) number of pontic units, (6) ratio between abutments and pontic units, (7) number of occluding pairs of "teeth," (8) loss of retention of abutment teeth, (9) fracture of materials, and (10) fracture of abutment teeth. P e r i o d o n t a l s t a t u s . (1) Oral hygiene, expressed as percentage of abutment tooth surfaces with plaque, (2) probing depths of 4 mm or more measured at the mesial, distal, buccal, and lingual surfaces of the abutment teeth with a periodontal calibrated probe; (3) bone scores, that is, the height of the alveolar bone around the abutment teeth measured on radiographs according to BjSrn et al. s and expressed as percentage of the root height, (4) periodontal support index, PSI:A, according to Ante, 9 constituting the ratio between the total remaining periodontal ligament of abutments and the total maximal periodontal ligament of replaced teeth, (5) periodontal support index, PSI:L, according to Laurell and Lundgren, 1° representing the ratio between the total remaining periodontal ligament area (mm 2) of abutments and the total maximal periodontal ligament area (mm 2) of the entire extension of the prosthesis, as if all teeth were used as abutments, each with maximal alveolar bone height. This index expresses how much periodontal support is left. The calculations of PSI:A and PSI:L were performed on radiographs by use of the mean values of periodontal ligament areas for different teeth presented by JepsenJ 1 Data
analyses
Data were presented as mean values and standard deviations. For some variables, frequency distributions were assessed. Student's t-test for paired data was used for cal-
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LONG-TERM PROGNOSIS FOR CANTILEVERED FPD~
N
N
~= 10.8 +1.0
N
~= 4 . 6 t l . 3
16-
16'
16.
12.
12
12.
8.
8.
4.
4. 8
9 10 11 12 Bridge units
2
3 4 5 6 Abutments
7
4
5 6 7 8 Pontic units
9
F i g . 2. N u m b e r of fixed partial denture units, a b u t m e n t teeth, and pontic units.
Table IL Periodontal conditions at baseline and final examinations around a b u t m e n t teeth of 35 fixed partial dentures t h a t remained throughout follow-up period
Baseline Final
Plaque score (%)
Sites with probing depths ~ 4 m m (n)
7.6 _+ 16.3 10.9 _+ 24.4 (t = 1.49)
1.9 -- 3.9 2.6 _+ 3.6 (t = 1.81)
culation of significance of differences between obtained data. P-Values of less than 0.05 (p < 0.05) were considered statistically significant. RESULTS
Functional
status
Altogether, 36 F P D s placed in 34 patients were studied. Thirty-two prostheses were conventional gold and acrylic resin F P D s and four were made of metal ceramics. During the observation period one F P D was lost because of complete and rapid periodontal breakdown after 6 years in service and was excluded from further d a t a analyses. At the final examination, the remaining 35 F P D s had served for an average of 8.4 +- 2.3 years, ranging between 5 and 12 years (Fig. 1). The distribution of the F P D units, a b u t m e n t teeth, and pontic units are presented in Fig. 2. The mean number of F P D units amounted to 10.8 -+ 1, range 8 to 12, the most common was 10 and 12 (17 and 14 F P D constructions, respectively). The number of a b u t m e n t teeth averaged 4.6 + 2.2, ranging from two to seven. The corresponding figure for pontic units was 6.2 +- 1.3, range 4 to 10. The mean ratio between a b u t m e n t s and pontic units was 4.6:6.2 varying between 2:10 to 7:5. Thus there were usually fewer abutments than pontic units in each patient. The distribution of prosthodontic constructions according to number of cantilever units is presented in Table I. Twelve patients had two or three cantilever units unilaterally, 11 patients two or three cantilever units on one side and one on the other, whereas the remaining 13 patients had two or three cantilever units bilaterally (two patients had cantilever prostheses in both jaws). The length of each cantilever unit measured approximately 8 mm, corre-
THE JOURNAL OF PROSTHETIC DENTISTRY
Bone score (%)
PSI:A (%)
PSI:L (%)
64 _+ 11 64 _+ 11
57 _+ 27 57 _+ 27
29 ± 9 29 x 9
sponding to the width of a premolar. Root-filled distal a b u t m e n t teeth adjoining cantilever segments usually unilaterally, were found in 18 patients. The mean number of occluding pairs of " t e e t h " was 9.6 _+ 2.2, indicating t h a t in most patients all existing cantilever pontics occluded with their antagonists. Eight patients had a complete denture occluding with the cantilever prosthesis.
Technical
failures
In one patient (3 % ), technical complications occurred in the form of a fractured vital a b u t m e n t tooth in a two abutm e n t - s u p p o r t e d 12-unit fixed b u t facultatively removable prosthesis (Fig. 3). In 1976 this p a t i e n t received periodontal t r e a t m e n t and a cantilevered fixed cross arch prosthetic restoration in both jaws. In the maxillae, two canines were left as abutments supplied with inner crowns to which a 12-unit F P D was attached with the aid of screws (SjSding & Co., Solna, Sweden). In 1983 the vital right canine abutm e n t fractured. The prosthesis was unscrewed, the tooth root-filled, and supplied with a cast post and a new inner crown to fit the original outer crown, after which the original prosthesis was inserted. Two years later (1985) this post lost retention and a new post was made with cervical embracement to improve retention. The original 12-unit F P D with three bilateral cantilever units was still in function after 11 years. No loss of retention or other technical complications had occurred in any of the other 32 patients (34 F P D constructions) t h a t completed the study.
Periodontal
status
Periodontal conditions in terms of plaque score, number of sites with probing depths of >--4mm, bone score, and pe-
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L A U R E L L E T AL
Fig. 3. Panoramic radiographs from 1978 and 1986 showing same maxillary and mandibular cantilever fixed partial dentures of one patient with technical complications in form of fractured right maxillary abutment canine. Series of intraoral radiographs demonstrates condition of maxillary abutment canines throughout 10-year period. Right abutment had undergone endodontic treatment and was supplied with post in 1983 and 1986.
15
[]
10 m
[]
Ba
.m Lt.
5 '~ [] |•[]
.3
[]
[]
.3
0 0
s
15
26
Baseline
Fig. 4. Number of sites with probing depths of >__4mm in 18 patients who exhibited one or more such sites at final examination plotted against number of such sites at baseline examination. (Two subjects who had no such sites at baseline examination had two and two subjects had four at final examination.)
548
riodontal support indices at the baseline and final examinations for all of the 35 FPDs evaluated throughout the study are presented in (Table II). After initial therapy no further detectable detoriation had occurred during the observation time in any of these patients. Oral hygiene. At the baseline examination the mean plaque score was 7.6% _+ 16.3%. At the final examination most patients exhibited excellent oral hygiene as indicated by a mean plaque score of 10.9 _+ 24.4%. Indeed 20 patients were free from detectable plaque. The mean plaque score for the subgroup with plaque (n = 15) was 20 _+ 24% (range 2 to 100). Probing depths. Sites with probing depths of ~ 4 mm averaged 1.9 + 3.9 at the baseline examination. At the final examination the mean number of such sites was 2.6 + 3.6 (p > 0.05). For the 18 subjects in whom these sites were found the mean number was 5.0 -+ 3.4 (range I to 13). All of these sites were in the 4 to 5 mm range. In Fig. 4 the number of sites with probing depths of >__4mm at the final examination are plotted against the number of such sites
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NUMBER 4
L O N G - T E R M P R O G N O S I S FOR C A N T I L E V E R E D FPD~
N
N
N ~=S7+-27%
16
16
12
12
• 50
50-59 60-69 Bone-scoreR
70"
11 > 20
25-49
50-74 PSI:A%
[]
Baseline
•
Final
~=29*-9%
1: 75-99
100-
• 20
20-29
30-39 PSI:L%
40-49
50-
Fig. 5. Bone scores (%), PSI:A (%), and PSI:L (%) at baseline and final examinations.
at the baseline examination, showing that for most of these 18 subjects a slight increase in the number of such sites had occurred during the observation period. Bone s c o r e . At the baseline and final examinations the mean bone score amounted to 64% _+ 11%, range 45% to 85 %. Thus during the observation time no further radiographically detectable loss of periodontal support had occurred. A bone score value of 64% means that almost two thirds of the original bone height of the abutments was left. A frequency distribution analysis revealed that most of the patients had bone score values of 60% or more (Fig. 5). P S I : A . The mean PSI:A score (relationship between remaining pericemental area of abutments and maximal pericemental area of replaced teeth) was 57% -+ 27%, range 19 % to 133 %. While most of the patients scored 25 % to 74% PSI:A (Fig. 5) only three patients met the demands of Ante of equal pericemental area of abutments and replaced teeth. P S I : L . The mean PSI:L score was 29 % _+ 9 %, range 10 to 51%. Thus on average less than one third of the original periodontal support remained related to the extension of the FPD. All patients but one had PSI:L scores below 50% (Fig. 5).
Periodontal complications During the observation period, one patient with a 10unit fixed partial denture with bilateral two-unit cantilevers supported on three abutments with a mean baseline bone score of 67% and a PSI:L of 20% exhibited rapid and complete periodontal breakdown during the sixth year after treatment. The series of radiographs in Fig. 6 illustrates the rapid course of the destruction. In 1977 the combined periodontal and prosthetic therapy was terminated. In 1982, at which time his periodontal condition was still excellent, his wife divorced him. He had to sell their house and a small cottage in Spain. Mental depression, drinking, self-neglect, and excessive bruxism with increasing abutment tooth mobility developed for some years. In a short time the abutments were exfoliated, mainly as a result of traumatic breakdown of the periodontal tissues. Periodontal treatment, endodontic therapy, stress-breaking acrylic resin bases attached
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 6. Radiographs of three abutment teeth supporting 10-unit maxillary cantilever fixed partial denture of one patient who exhibited complete and rapid periodontal breakdown during a 1-year period (1982 to 1983).
to the pontic units, and occlusal splints as well as psychosocial therapy failed to arrest the process. One subject with a mandibular 10-unit bilateral posterior cantilever FPD occluding with a complete denture and supported on three abutment teeth, with an averaged bone score of 50% and a PSI:L score of 21%, with time exhibited increasing abutment tooth mobility without detect-
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LAURELL ET AL
Fig. 7. Tooth-supported mandibular cantilever fixed partial denture retained to postprosthodontic treatment with installed titanium implants to prevent increasing tooth mobility of abutments and prosthesis.
able further loss of periodontal support. After 6 years (final examination), widened periodontal space could be detected on the radiographs and the patient was supplied with mandibular bilateral posterior titanium implants connected to the cantilever segments to stabilize the fixed prosthesis and prevent further increase of abutment tooth mobility (Fig. 7). DISCUSSION From the results of this retrospective study it can be postulated that cross-arch FPDs with unilateral or bilateral posterior two-unit cantilever segments to replace lost teeth and restore function and esthetics when distal end abutments are not available constitute a prosthodontic treatment modality with a predictably good prognosis. In this respect our results concur with those of Nyman and Ericsson 12 but disagree with those of Randow et ah, a Dahl et al., 5 and Karlsson. 6 However the good prognosis presupposes that certain critical periodontal and prosthetic prerequisites in the treatment, planning, and performance are fulfilled. Periodontally these prerequisites include adequate initial periodontal therapy followed by a well-designed maintenance care program based on individual needs and aimed at maintaining periodontal health. 7 These prerequisites were fulfilled in the patients studied as demonstrated by the periodontal status at the final examination 5 to 12 years after initial therapy. Although there was a slight but insignificant increase in the mean number of deepened periodontal pockets from 1.9 to 2.6 (which from a clinical point of view could be neglected) there was no further, radiographically detectable, loss of periodontal support in any of the patients, excluding the one patient with complete and rapid periodontal deterioration. In all probability this was caused by psychosocially elicited excessive occlusal trauma leading to mechanical tissue destruction. This is in agreement with the results of previous studiesfl 4,12,18The mean bone score of the abutment teeth was maintained throughout the entire observation period (64 % _+ 11% ), although
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Fig. 8. Anterior abutment teeth with cast posts before and after gingivoplastic and osteoplastic crown-lengthening surgery performed to secure retention of fixed partial denture with bilateral polyunit cantilevers.
on the average only 30% of the original periodontal ligament area remained as indicated by the PSI.L. This markedly reduced periodontal support was obviously sufficient to withstand long-term operating functional forces. It might be argued that markedly reduced periodontal support, as in these patients, will influence the sensory input from periodontal pressoreceptors on masticatory muscle activity, resulting in reduced chewing and closing forces. 14 However, recent studies have revealed that in dentitions with cross-arch bilateral posterior two-unit cantilever FPDs, maximal closing and mean chewing forces can be of considerable magnitudes (mean values 308 N and 112 N, respectively).16 The present findings thus support previous statements 1,12,17 that teeth with severely reduced periodontal support and with increased tooth mobility can serve as reliable abutment teeth for extensive FPDs provided periodontal health has been (re-)established and can be maintained in the remaining dentition. Prosthodontically, the prerequisites include occlusion, retention, and dimension. Occlusion. Anterior occlusal contacts with axial force direction should be established in the retruded and the intercanine position. This implies that the palatal aspect of the incisor and canine crown is given a functional instead of anatomic morphology with horizontal shelves and possibilities of gliding contacts in the retrusive-protrusive range, is Experimental studies in our clinic have shown that lack of anterior occlusal contacts in cross-arch bilateral cantilever prostheses results in increased chewing and closing forces over the cantilever segments and a considerable increase of bending moment in the distal abutment. This implies a clearly increased risk of abutment tooth
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VOLUME 66
NUMBER 4
LONG-TERM PROGNOSIS FOR CANTILEVERED F P D s
fracture or loss of retention of distal retainers. Therefore, if anterior occlusal contacts cannot be established, for example, in patients with postnormal jaw relationships, polyunit posterior cantilevers should be avoided. The occlusal design of the present FPDs meant that the occlusal contacts were evenly distributed over the tooth arch, including the cantilever units. With this occlusal design, closing and chewing forces decrease in the distal direction along the cantilever segments. 16,19However, during the patients' regular recall visits it became obvious that the occlusal wear pattern was not uniform along the tooth arch. Occlusal wear was thus less pronounced in the cantilever segments as compared with the anterior abutment-supported area. This difference in occlusal wear was probably the result of the slight elastic apical deflection of the cantilever segments when loaded, 19 which partly spared them from wear. Over time this resulted in a clinically detectable increase in hardness of occlusal contacts over the cantilever segments, which eventually might convert into small supracontacts. Such alterations in occlusal contact pattern will result in increased cantilever load. Experimental studies16, 2o have revealed that small (80 #m) supracontacts on the second cantilever unit will dramatically increase the local load over this cantilever unit, resulting in a reversed (distally increasing) pattern of force distribution along the cantilever segment and increased bending moments and stress in the FPD construction and supporting components. The regular recall program should therefore include careful occlusal diagnosis by use of microthin occlusal ribbons and, when needed, selective occlusal grinding to maintain diminutive point-shaped occlusal contacts on the distal cantilever unit in particular. Lateral movements were anterior guided with intentional exclusion of working or nonworking side contacts on the cantilever segments. This arrangement aimed at minimizing lateral force components and lateral bending and torsion moments around the distal retainer crown and its abutment components. Eight of the patients had complete maxillary dentures occluding with cantilever prostheses. Dentitions with complete dentures are generally believed to generate comparatively small closing and chewing forces. Recent studies on occlusal forces in dentitions with complete maxillary dentures occluding with mandibular implant-supported FPDs 21 or tooth-supported FPDs (unpublished data) with bilateral two-unit cantilever segments have, however, revealed that not only can the closing and chewing forces be expected to be of the same magnitudes as in dentitions where the cantilevered prostheses occlude with natural teeth, the forces also increase in the distal direction along the cantilever segment. These factors may at least partly explain the increasing abutment tooth mobility in one of the patients with a complete maxillary denture occluding with a bilateral cantilever prosthesis and also the reported high prevalence of technical failures in such dentitions. 2, 4 Retention. Loss of retention of anchor crowns is reported
THE JOURNAL OF PROSTHETIC DENTISTRY
to dominate among causes of technical failures. 1, 4-6 In this respect root-filled, post-supplied distal abutments seemed to dominateJ In the present study no retention complications occurred even though root-filled abutments adjoining the cantilever segments were present in half the number of the FPDs. Access to long clinical crowns usually enabled preparation with optimal retention. When this was not the case, preprosthetic gingival and osteoplastic crown-lengthening surgery was performed to obtain at least 3 mm of cervical embracement around parallel root surfaces. Especially in patients with root-filled post-supplied abutments this should be carefully considered as illustrated in Fig. 8. Dimensioning. The metal framework of the present FPDs measured 5 to 6 mm in height (axial force direction) and 4 to 5 mm in width along the entire prosthesis. To avoid fatigue and fracture of the FPD, a minimum dimension of 5 mm in height and 4 mm in width at least around the distal retainer crown has been recommended, 22 based on the maximal local forces that the distal cantilever unit might be subjected to under extreme, provoked situations. If such dimensions cannot be achieved, posterior two-unit cantilevers should be avoided. In the present material with an observation time of 5 to 12 years, technical complications occurred in only one patient (3%). The patients are continuously subjected to regular follow-up and the failure rate might increase with time as a result of fatigue of the prostheses and the abutment teeth. 2, 4 In a retrospective study on 83 extensive FPDs with cantilever units made by general practitioners, Randow et al. 4 reported an average technical failure rate of 37 % after 5 to 7 years, the failures increasing with time in function and number of cantilever units. Thus with two bilateral posterior cantilever units, technical failures amounted to 44 %. These findings were based on questionnaires to the dentists but none of the discussed factors critical for the prognosis (occlusion, retention, and dimension) was analyzed. With the well-performed periodontal therapy and maintenance care and with the occlusal and dimensional designs of the prostheses that we have suggested, neither periodontal problems nor fatigue of the FPDs in the present study are to be expected in the future. Complications that might be more difficult to control over extended time are loss of retention and/or fractures of distal--in particular, root-filled--abutment teeth, although the results of this study indicate that such problems can also be prevented for a considerable time. The question still arises as to how long a treatment modality should be followed up for a clear statement on its prognoses. The results of this retrospective investigation do not contradict the condition that posterior cantilevers in fixed prostheses imply an inherent risk for technical complications. The general recommendation that distal end abutment support should always be used when available, still holds. The introduction of well-documented implant techniques to create support for fixed prosthesis, for instance the Br~_nemark technique, 15might open new possibilities of
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LAURELLET AL
achieving distal end abutments when teeth are missing. However, if natural teeth or implants are not available as end abutments, FPDs with two or even more posterior cantilever units might be a realistic alternative to removable partial dentures provided certain prerequisites are fulfilled. In extensive prosthodontics, failures do occur. Failures in cantilever prostheses have mainly been of technical character. An imbalance between functional forces and the ability of the various prosthetic components, including the abutment teeth, to withstand these forces must thus have existed. The nature of and reasons behind failures should always be carefully analyzed to improve this treatment modality and prevent future failures. In this study we have pointed out some important factors that should be considered. CLINICAL
IMPLICATIONS
Where distal end abutments are not available as support for extensive fixed prosthodontics, posterior polyunit cantilevers can be successfully incorporated in the FPD provided: 1. The jaw relationships permit the establishment of occlusal contacts anteriorly as well as posteriorly with an occlusal morphology guiding the occlusal forces in an apical direction. 2. Lateral movements are anterior-guided with no cantilever contacts. 3. Great care is given to the preparation procedure to secure the retention of the abutment crowns. This might call for preprosthodontic crown-lengthening surgery. 4. The dimension of the metal framework at least mesial and distal to the distal retainer crown should measure a minimum of 5 mm in height and 4 mm in width. If any of these prerequisites cannot be fulfilled, posterior polyunit cantilevers should be avoided. SUMMARY Thirty-six extensive fixed partial dentures with polyunit cantilevers unilaterally or bilaterally, made with special considerations on retention, dimension, and occlusal design were studied during a period of 5 to 12 years. During this period one fixed partial denture was lost after 6 years in service as a result of a rapid traumatic periodontal breakdown related to psychosocial circumstances. One fixed partial denture exhibited increasing abutment-tooth mobility and was stabilized by the installation of and connection to bilateral Br~nemark implants. Fracture of one abutment tooth occurred in one patient with a facultatively removable 12-unit prosthesis supported on two canine abutments. This patient could be treated without loss of the prosthesis. The remaining 33 cantilever prostheses exhibited no signs of either biologic or irreversable technical complications. Factors influencing the prognosis of extensive fixed cantilever protheses were reviewed. 552
REFERENCES 1. Nyman S, Lindhe J. A longitudinal study of combined periodontal and prosthetic treatment of patients with advanced periodontal disease. J Periodontol 1979;50:469-14. 2. Isikowitz L. A long-term prognosis for the free-end saddle bridge. J Oral Rehabil 1985;12:247-62. 3. Adell R, Ekholm V, Rockler B, Br~nemark P-I. A 15 year study of osseointegrated implants in the treatment of the edentolous jaw. Int J Oral Surg 1981;10:387-416. 4. Randow K, Glantz PO, Z6ger Bo. Technical failures and some related clinical complications in extensive fixed prosthodontics. Acta Odontol Scand 1986;44:241-55. 5. Dahl B, 0rstavik D, Karlsan K. Multi-unit bridges on reduced periodontol support. The Scandinavian Division of IADR Special Issue 1987; report no 120, p 84. 6. Karlsson S. Failures and length of service in fixed prosthodontics after long-term function. Swed Dent J 1989;13:185-192. 7. Lindhe J, Nyman S. The effect of plaque control and surgical pocket elimination on the establishment and maintenance of periodontal health. A longitudinal study of periodontol therapy in cases of advanced disease. J Clin Periodontol 1975;2:67-79. 8. Bj6rn H, Hailing A, Tyberg H. Radiographic assessment of marginal bone loss. Odontologisk Revy 1969;20:165-79. 9. Ante I. The fundamental principles of abutments [Thesis]. Michigan State Dental Society Bulletin 8, 1926;14-23. 10. Laurell L, Lundgren D. Periodontal ligament areas and occlusal forces in dentitions restored with cross-arch bilateral end abutment bridges. J Clin Periodontol 1985;12:850-60. 11. Jepsen A. Root surface measurements and a method for x-ray determination of root surface area. Acta Odontol Scand 1963;21:35-46. 12. Nyman S, Ericsson I. The capacity of reduced periodontal tissues to support fixed bridgework. J Clin Periodontol 1982;9:409-14. 13. Isidor F, Budtz-J~rgensen E. Periodontal conditions following treatment with cantilever bridges or removable partial dentures in geriatric patients. A 2-year study. Gerodontics 1987;3:117-21. 14. Laurell L, Lundgren D. Periodontal ligament areas and occlusal forces in dentitions restored with cross-arch unilateral posterior two-unit cantilever bridges. J Clin Periodontol 1985;12:850-860. 15. Br~nemark P-I, Hansson B-C, Adell R, Breine U, LundstrSm J, Hallen O, Ohman A. Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period. Scand J Plast Reconstr Surg 1977;ll(Suppl 16). 16. Laurell L, Lundgren D. Influence of occlusion on closing and chewing forces in cross-arch bilateral posterior cantilever fixed partial dentures. J PROSTHET DENT (in press). 17. Nyman S, Lindhe J, Lundgren D. The role of occlusion for the stability of fixed bridges in patients with advanced periodontal disease. J Clin Periodontol 1975;2:53-66. 18. Beyron H. Occlusion. Points of significance in planning restorative procedures. Proceedings in the first international prosthodontic congress. J PROSTHETDENT 1973;30:641-52. 19. Laurell L, Lundgran D. Distribution of occlusal forces along unilateral posterior two-unit cantilever segments in cross-arch fixed partial dentures. J PROSTHETDENT 1988;60:106-12. 20. Laurell L, Lundgren D. Interfering occlusal contacts and distribution of chewing and biting forces in dentitions with fixed cantilever prostheses. J PROSTHET DENT 1987;58:197-203. 21. Falk H' Laurell L' Lundgren D" Occlusal f°rce pattern in dentiti°ns with mandibular implant-supported cantilever prostheses occluding with complete dentures. Int J Oral Maxillofac Implant 1989;4:55-62. 22. Lundgren D, Laurell L. Occlusal force pattern during chewing and biting in dentitions restored with fixed bridges of cross-arch extension. II Unilateral posterior two-unit cantilevers. J Oral Rehabil 1986;13:191203.
Reprint requests to: DR. LARS LAURELL THE INSTITUTEFOR POSTGRADUATEDENTALEDUCATION BOX 1030 S-551 11 0:6NKSPING SWEDEN
OCTOBER 1991
VOLUME 66
NUMBER 4
Long-term prognosis of e x t e n s i v e polyunit c a n t i l e v e r e d fixed partial dentures Lars Laurell, OdontDr, DDS, a Dan Lundgren, OdontDr, DDS, a Hanne Falk, LDS, a and Anders Hugoson, OdontDr, DDS b
The Institute for Postgraduate Dental Education, JSnk6ping, Sweden The aim of this r e t r o s p e c t i v e study w a s to elucidate the l o n g - t e r m p r o g n o s e s of e x t e n s i v e fixed partial d e n t u r e s including u n i l a t e r a l or b i l a t e r a l polyunit cantilev e r s in p a t i e n t s w i t h h e a l t h y but reduced periodontal support. F o l l o w i n g periodontal t h e r a p y 36 cross-arch fixed partial d e n t u r e s w i t h two or m o r e c a n t i l e v e r units u n i l a t e r a l l y or b i l a t e r a l l y w e r e fitted in 34 patients. In the prosthodontic design, s p e c i a l attention w a s g i v e n to the retention to long p a r a l l e l preparations, to the d i m e n s i o n s of the f r a m e w o r k , and to the occlusal design. After c o m p l e t i o n of therapy, the p a t i e n t s w e r e enrolled in a r e g u l a r m a i n t e n a n c e care program and f o l l o w e d up for a period of 5 to 12 years. D u r i n g this f o l l o w - u p period one abutm e n t tooth w a s fractured in one patient. One fixed partial denture w i t h e x t r e m e l y reduced periodontal support w a s lost as a result of c o m p l e t e periodontal breakd o w n from occlusal trauma. For 33 fixed partial dentures, n e i t h e r periodontal nor technical c o m p l i c a t i o n s occurred. (J PROSTHET DENT 1991;66:545-52.)
T h e incorporation of posterior cantilevers in fixed partial dentures (FPDs) is sometimes desirable for functional and esthetic reasons. However, posterior cantilevers consisting of two or more units in FPDs are generally considered a threat to the construction, the abutment teeth, and the supporting tissues because of supposed large forces affecting the cantilever segments. Clinical long-term follow-up studies on the prognosis of extensive cantilever prosthodontics that would confirm or reject such postulations are few thus far 16 and the results are, to some extent, contradictory. Thus, Nyman and Lindhe 1 presented a total of less than 8 % technical failure (loss of retention 3.3 %, fracture of the prosthesis 2.1%, and fracture of abutment teeth 2,4 9 ) after 5 to 8 years in 159 periodontally treated patients supplied with extensive cantilever FPDs. During this observation period no further loss of periodontal attachment occurred, probably because of the controlled maintenance care program in which the patients were enrolled. Isikowitz2 presented long-term follow-up results of 87 FPDs with distal extension bases and reported an increasing failure rate over time. Thus, after 5 years, 98 % of the prostheses were still in function, after 10 years 82 9 , after 15 years 69%, and after 20 years 499. It was reported that failures were significantly more frequent when the FPDs occluded with a complete denture. Of 130 fixture-supported cantilever prostheses, Adell et
al.8 reported an incidence of exchanged FPDs because of various technical and biological complications in 20 9 in the maxillae and 9 % in the mandibles. Fracture of FPDs accounted for 5%. These studies all concerned patients treated by specialists. In an epidemiologic study on extensive FPDs made by general practitioners, Randow et al. 4 found technical failures in an average of 33 % of 83 patients with two or more cantilever units. Technical failures increased with an increasing number of cantilever units and years in service. Periodontal problems were rare, occurring in only 7% of the patients. High or very high rates of failures, mainly due to loss of retention of distal retainer crowns in cantilever FPDs, were also reported by Dahl et al. 5 (40%) and Karlsson s (369). In our department, a number of extensive FPDs including two or three unilateral or bilateral cantilever units have been made throughout the years as part of the overall treatment of patients with advanced stages of periodontal breakdown. In view of the inconsistencies in reported prognosis for extensive fixed cantilever prostheses, we considered it important to: (1) present the results of a retrospective long-term follow-up study of patients periodontally treated and prosthetically restored with extensive posterior unilateral or bilateral polyunit cantilever FPDs and, (2) discuss factors that might promote success or failure. MATERIAL
aAssociate Professor, Department of Periodontology. bProfessor, Head, Department of Periodontology. 10/1/30547
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AND
METHODS
The group of subjects studied included all of the subjects who, during the years of 1973 to 1981, were referred to the department of periodontology at The Institute for Post-
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LAURELL ET AL
108 a:
6
W m
4 Z
2 0 5
6
7
8
9
10
11
12
YEARSIN SERVICE
Fig. 1. Number of fixed partial dentures distributed according to years in service.
Table I. Number of fixed partial dentures distributed according to number of cantilever units Cantilever units
Number of fixed partial dentures
Examinations
2 or 3 unilaterally
2 or ~ on one side and 1 on the other
2 or 3 bilaterally
12
11
12
graduate Dental Education, JSnkSping, Sweden, for the treatment of periodontal diseases of varying severety and in whom extensive fixed partial dentures including two or more unilaterally or bilaterally posterior cantilever units were part of the overall treatment. The choice of cantilevered fixed partial dentures as the treatment modality was based on (1) lack of teeth that could be used as abutments and (2) the refusal of the patient to accept a fixed partial denture to replace teeth that were lost or had to be removed. Before the definitive prosthodontic therapy, sometimes preceded by transitional prostheses, all subjects were given adequate periodontal therapy according to the principles outlined by Lindhe and Nyman. 7 Required endodontic treatment was performed according to standardized methods. Root-filled single-rooted teeth were always supplied with cores and posts that were cast in type III dental gold alloys. Special requirements were set up for the design of the cantilever prostheses: 1. The jaw relationships permitted the establishment of anterior as well as posterior occlusal contacts with axial force direction along the entire arch of teeth. In addition, the occlusal design was characterized by wide freedom in the retrusive-protrusive range and anterior guided lateral movements with no working or nonworking side contacts on the cantilever segments. 2. Optimal retention to long preparations with almost parallel tooth surfaces, at least at the distal abutments, was another prerequisite. To attain this goal, preprosthetic
546
gingival and osteoplastic crown-lengthening surgery was occasionally performed to achieve at least 3 mm of completely parallel cervical embracement to the abutment teeth. The cantilever segments were made 5 to 6 mm in height and 4 to 5 mm in width at the connection to the distal retainer crown. These dimensions were also aimed at along the entire prosthesis, in particular immediately mesial to the distal retainer. After completion of the periodontal and prosthetic therapy, the patients were clinically and radiographically examined to obtain baseline data for the future follow-up controls. The patients were then enrolled in a regular maintenance care program with clinical and, when needed, radiographic reexaminations and required treatment. The final examination, including full mouth radiographs, was performed in 1987, 5 to 12 years after completion of initial therapy.
For the present study the following variables were assessed for each individual: F u n c t i o n a l s t a t u s . (1) Years in service, (2) extension of fixed partial denture, (3) number of abutment teeth, (4) number of root filled distal abutment teeth, (5) number of pontic units, (6) ratio between abutments and pontic units, (7) number of occluding pairs of "teeth," (8) loss of retention of abutment teeth, (9) fracture of materials, and (10) fracture of abutment teeth. P e r i o d o n t a l s t a t u s . (1) Oral hygiene, expressed as percentage of abutment tooth surfaces with plaque, (2) probing depths of 4 mm or more measured at the mesial, distal, buccal, and lingual surfaces of the abutment teeth with a periodontal calibrated probe; (3) bone scores, that is, the height of the alveolar bone around the abutment teeth measured on radiographs according to BjSrn et al. s and expressed as percentage of the root height, (4) periodontal support index, PSI:A, according to Ante, 9 constituting the ratio between the total remaining periodontal ligament of abutments and the total maximal periodontal ligament of replaced teeth, (5) periodontal support index, PSI:L, according to Laurell and Lundgren, 1° representing the ratio between the total remaining periodontal ligament area (mm 2) of abutments and the total maximal periodontal ligament area (mm 2) of the entire extension of the prosthesis, as if all teeth were used as abutments, each with maximal alveolar bone height. This index expresses how much periodontal support is left. The calculations of PSI:A and PSI:L were performed on radiographs by use of the mean values of periodontal ligament areas for different teeth presented by JepsenJ 1 Data
analyses
Data were presented as mean values and standard deviations. For some variables, frequency distributions were assessed. Student's t-test for paired data was used for cal-
OCTOBER 1991
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LONG-TERM PROGNOSIS FOR CANTILEVERED FPD~
N
N
~= 10.8 +1.0
N
~= 4 . 6 t l . 3
16-
16'
16.
12.
12
12.
8.
8.
4.
4. 8
9 10 11 12 Bridge units
2
3 4 5 6 Abutments
7
4
5 6 7 8 Pontic units
9
F i g . 2. N u m b e r of fixed partial denture units, a b u t m e n t teeth, and pontic units.
Table IL Periodontal conditions at baseline and final examinations around a b u t m e n t teeth of 35 fixed partial dentures t h a t remained throughout follow-up period
Baseline Final
Plaque score (%)
Sites with probing depths ~ 4 m m (n)
7.6 _+ 16.3 10.9 _+ 24.4 (t = 1.49)
1.9 -- 3.9 2.6 _+ 3.6 (t = 1.81)
culation of significance of differences between obtained data. P-Values of less than 0.05 (p < 0.05) were considered statistically significant. RESULTS
Functional
status
Altogether, 36 F P D s placed in 34 patients were studied. Thirty-two prostheses were conventional gold and acrylic resin F P D s and four were made of metal ceramics. During the observation period one F P D was lost because of complete and rapid periodontal breakdown after 6 years in service and was excluded from further d a t a analyses. At the final examination, the remaining 35 F P D s had served for an average of 8.4 +- 2.3 years, ranging between 5 and 12 years (Fig. 1). The distribution of the F P D units, a b u t m e n t teeth, and pontic units are presented in Fig. 2. The mean number of F P D units amounted to 10.8 -+ 1, range 8 to 12, the most common was 10 and 12 (17 and 14 F P D constructions, respectively). The number of a b u t m e n t teeth averaged 4.6 + 2.2, ranging from two to seven. The corresponding figure for pontic units was 6.2 +- 1.3, range 4 to 10. The mean ratio between a b u t m e n t s and pontic units was 4.6:6.2 varying between 2:10 to 7:5. Thus there were usually fewer abutments than pontic units in each patient. The distribution of prosthodontic constructions according to number of cantilever units is presented in Table I. Twelve patients had two or three cantilever units unilaterally, 11 patients two or three cantilever units on one side and one on the other, whereas the remaining 13 patients had two or three cantilever units bilaterally (two patients had cantilever prostheses in both jaws). The length of each cantilever unit measured approximately 8 mm, corre-
THE JOURNAL OF PROSTHETIC DENTISTRY
Bone score (%)
PSI:A (%)
PSI:L (%)
64 _+ 11 64 _+ 11
57 _+ 27 57 _+ 27
29 ± 9 29 x 9
sponding to the width of a premolar. Root-filled distal a b u t m e n t teeth adjoining cantilever segments usually unilaterally, were found in 18 patients. The mean number of occluding pairs of " t e e t h " was 9.6 _+ 2.2, indicating t h a t in most patients all existing cantilever pontics occluded with their antagonists. Eight patients had a complete denture occluding with the cantilever prosthesis.
Technical
failures
In one patient (3 % ), technical complications occurred in the form of a fractured vital a b u t m e n t tooth in a two abutm e n t - s u p p o r t e d 12-unit fixed b u t facultatively removable prosthesis (Fig. 3). In 1976 this p a t i e n t received periodontal t r e a t m e n t and a cantilevered fixed cross arch prosthetic restoration in both jaws. In the maxillae, two canines were left as abutments supplied with inner crowns to which a 12-unit F P D was attached with the aid of screws (SjSding & Co., Solna, Sweden). In 1983 the vital right canine abutm e n t fractured. The prosthesis was unscrewed, the tooth root-filled, and supplied with a cast post and a new inner crown to fit the original outer crown, after which the original prosthesis was inserted. Two years later (1985) this post lost retention and a new post was made with cervical embracement to improve retention. The original 12-unit F P D with three bilateral cantilever units was still in function after 11 years. No loss of retention or other technical complications had occurred in any of the other 32 patients (34 F P D constructions) t h a t completed the study.
Periodontal
status
Periodontal conditions in terms of plaque score, number of sites with probing depths of >--4mm, bone score, and pe-
547
L A U R E L L E T AL
Fig. 3. Panoramic radiographs from 1978 and 1986 showing same maxillary and mandibular cantilever fixed partial dentures of one patient with technical complications in form of fractured right maxillary abutment canine. Series of intraoral radiographs demonstrates condition of maxillary abutment canines throughout 10-year period. Right abutment had undergone endodontic treatment and was supplied with post in 1983 and 1986.
15
[]
10 m
[]
Ba
.m Lt.
5 '~ [] |•[]
.3
[]
[]
.3
0 0
s
15
26
Baseline
Fig. 4. Number of sites with probing depths of >__4mm in 18 patients who exhibited one or more such sites at final examination plotted against number of such sites at baseline examination. (Two subjects who had no such sites at baseline examination had two and two subjects had four at final examination.)
548
riodontal support indices at the baseline and final examinations for all of the 35 FPDs evaluated throughout the study are presented in (Table II). After initial therapy no further detectable detoriation had occurred during the observation time in any of these patients. Oral hygiene. At the baseline examination the mean plaque score was 7.6% _+ 16.3%. At the final examination most patients exhibited excellent oral hygiene as indicated by a mean plaque score of 10.9 _+ 24.4%. Indeed 20 patients were free from detectable plaque. The mean plaque score for the subgroup with plaque (n = 15) was 20 _+ 24% (range 2 to 100). Probing depths. Sites with probing depths of ~ 4 mm averaged 1.9 + 3.9 at the baseline examination. At the final examination the mean number of such sites was 2.6 + 3.6 (p > 0.05). For the 18 subjects in whom these sites were found the mean number was 5.0 -+ 3.4 (range I to 13). All of these sites were in the 4 to 5 mm range. In Fig. 4 the number of sites with probing depths of >__4mm at the final examination are plotted against the number of such sites
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L O N G - T E R M P R O G N O S I S FOR C A N T I L E V E R E D FPD~
N
N
N ~=S7+-27%
16
16
12
12
• 50
50-59 60-69 Bone-scoreR
70"
11 > 20
25-49
50-74 PSI:A%
[]
Baseline
•
Final
~=29*-9%
1: 75-99
100-
• 20
20-29
30-39 PSI:L%
40-49
50-
Fig. 5. Bone scores (%), PSI:A (%), and PSI:L (%) at baseline and final examinations.
at the baseline examination, showing that for most of these 18 subjects a slight increase in the number of such sites had occurred during the observation period. Bone s c o r e . At the baseline and final examinations the mean bone score amounted to 64% _+ 11%, range 45% to 85 %. Thus during the observation time no further radiographically detectable loss of periodontal support had occurred. A bone score value of 64% means that almost two thirds of the original bone height of the abutments was left. A frequency distribution analysis revealed that most of the patients had bone score values of 60% or more (Fig. 5). P S I : A . The mean PSI:A score (relationship between remaining pericemental area of abutments and maximal pericemental area of replaced teeth) was 57% -+ 27%, range 19 % to 133 %. While most of the patients scored 25 % to 74% PSI:A (Fig. 5) only three patients met the demands of Ante of equal pericemental area of abutments and replaced teeth. P S I : L . The mean PSI:L score was 29 % _+ 9 %, range 10 to 51%. Thus on average less than one third of the original periodontal support remained related to the extension of the FPD. All patients but one had PSI:L scores below 50% (Fig. 5).
Periodontal complications During the observation period, one patient with a 10unit fixed partial denture with bilateral two-unit cantilevers supported on three abutments with a mean baseline bone score of 67% and a PSI:L of 20% exhibited rapid and complete periodontal breakdown during the sixth year after treatment. The series of radiographs in Fig. 6 illustrates the rapid course of the destruction. In 1977 the combined periodontal and prosthetic therapy was terminated. In 1982, at which time his periodontal condition was still excellent, his wife divorced him. He had to sell their house and a small cottage in Spain. Mental depression, drinking, self-neglect, and excessive bruxism with increasing abutment tooth mobility developed for some years. In a short time the abutments were exfoliated, mainly as a result of traumatic breakdown of the periodontal tissues. Periodontal treatment, endodontic therapy, stress-breaking acrylic resin bases attached
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 6. Radiographs of three abutment teeth supporting 10-unit maxillary cantilever fixed partial denture of one patient who exhibited complete and rapid periodontal breakdown during a 1-year period (1982 to 1983).
to the pontic units, and occlusal splints as well as psychosocial therapy failed to arrest the process. One subject with a mandibular 10-unit bilateral posterior cantilever FPD occluding with a complete denture and supported on three abutment teeth, with an averaged bone score of 50% and a PSI:L score of 21%, with time exhibited increasing abutment tooth mobility without detect-
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LAURELL ET AL
Fig. 7. Tooth-supported mandibular cantilever fixed partial denture retained to postprosthodontic treatment with installed titanium implants to prevent increasing tooth mobility of abutments and prosthesis.
able further loss of periodontal support. After 6 years (final examination), widened periodontal space could be detected on the radiographs and the patient was supplied with mandibular bilateral posterior titanium implants connected to the cantilever segments to stabilize the fixed prosthesis and prevent further increase of abutment tooth mobility (Fig. 7). DISCUSSION From the results of this retrospective study it can be postulated that cross-arch FPDs with unilateral or bilateral posterior two-unit cantilever segments to replace lost teeth and restore function and esthetics when distal end abutments are not available constitute a prosthodontic treatment modality with a predictably good prognosis. In this respect our results concur with those of Nyman and Ericsson 12 but disagree with those of Randow et ah, a Dahl et al., 5 and Karlsson. 6 However the good prognosis presupposes that certain critical periodontal and prosthetic prerequisites in the treatment, planning, and performance are fulfilled. Periodontally these prerequisites include adequate initial periodontal therapy followed by a well-designed maintenance care program based on individual needs and aimed at maintaining periodontal health. 7 These prerequisites were fulfilled in the patients studied as demonstrated by the periodontal status at the final examination 5 to 12 years after initial therapy. Although there was a slight but insignificant increase in the mean number of deepened periodontal pockets from 1.9 to 2.6 (which from a clinical point of view could be neglected) there was no further, radiographically detectable, loss of periodontal support in any of the patients, excluding the one patient with complete and rapid periodontal deterioration. In all probability this was caused by psychosocially elicited excessive occlusal trauma leading to mechanical tissue destruction. This is in agreement with the results of previous studiesfl 4,12,18The mean bone score of the abutment teeth was maintained throughout the entire observation period (64 % _+ 11% ), although
550
Fig. 8. Anterior abutment teeth with cast posts before and after gingivoplastic and osteoplastic crown-lengthening surgery performed to secure retention of fixed partial denture with bilateral polyunit cantilevers.
on the average only 30% of the original periodontal ligament area remained as indicated by the PSI.L. This markedly reduced periodontal support was obviously sufficient to withstand long-term operating functional forces. It might be argued that markedly reduced periodontal support, as in these patients, will influence the sensory input from periodontal pressoreceptors on masticatory muscle activity, resulting in reduced chewing and closing forces. 14 However, recent studies have revealed that in dentitions with cross-arch bilateral posterior two-unit cantilever FPDs, maximal closing and mean chewing forces can be of considerable magnitudes (mean values 308 N and 112 N, respectively).16 The present findings thus support previous statements 1,12,17 that teeth with severely reduced periodontal support and with increased tooth mobility can serve as reliable abutment teeth for extensive FPDs provided periodontal health has been (re-)established and can be maintained in the remaining dentition. Prosthodontically, the prerequisites include occlusion, retention, and dimension. Occlusion. Anterior occlusal contacts with axial force direction should be established in the retruded and the intercanine position. This implies that the palatal aspect of the incisor and canine crown is given a functional instead of anatomic morphology with horizontal shelves and possibilities of gliding contacts in the retrusive-protrusive range, is Experimental studies in our clinic have shown that lack of anterior occlusal contacts in cross-arch bilateral cantilever prostheses results in increased chewing and closing forces over the cantilever segments and a considerable increase of bending moment in the distal abutment. This implies a clearly increased risk of abutment tooth
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VOLUME 66
NUMBER 4
LONG-TERM PROGNOSIS FOR CANTILEVERED F P D s
fracture or loss of retention of distal retainers. Therefore, if anterior occlusal contacts cannot be established, for example, in patients with postnormal jaw relationships, polyunit posterior cantilevers should be avoided. The occlusal design of the present FPDs meant that the occlusal contacts were evenly distributed over the tooth arch, including the cantilever units. With this occlusal design, closing and chewing forces decrease in the distal direction along the cantilever segments. 16,19However, during the patients' regular recall visits it became obvious that the occlusal wear pattern was not uniform along the tooth arch. Occlusal wear was thus less pronounced in the cantilever segments as compared with the anterior abutment-supported area. This difference in occlusal wear was probably the result of the slight elastic apical deflection of the cantilever segments when loaded, 19 which partly spared them from wear. Over time this resulted in a clinically detectable increase in hardness of occlusal contacts over the cantilever segments, which eventually might convert into small supracontacts. Such alterations in occlusal contact pattern will result in increased cantilever load. Experimental studies16, 2o have revealed that small (80 #m) supracontacts on the second cantilever unit will dramatically increase the local load over this cantilever unit, resulting in a reversed (distally increasing) pattern of force distribution along the cantilever segment and increased bending moments and stress in the FPD construction and supporting components. The regular recall program should therefore include careful occlusal diagnosis by use of microthin occlusal ribbons and, when needed, selective occlusal grinding to maintain diminutive point-shaped occlusal contacts on the distal cantilever unit in particular. Lateral movements were anterior guided with intentional exclusion of working or nonworking side contacts on the cantilever segments. This arrangement aimed at minimizing lateral force components and lateral bending and torsion moments around the distal retainer crown and its abutment components. Eight of the patients had complete maxillary dentures occluding with cantilever prostheses. Dentitions with complete dentures are generally believed to generate comparatively small closing and chewing forces. Recent studies on occlusal forces in dentitions with complete maxillary dentures occluding with mandibular implant-supported FPDs 21 or tooth-supported FPDs (unpublished data) with bilateral two-unit cantilever segments have, however, revealed that not only can the closing and chewing forces be expected to be of the same magnitudes as in dentitions where the cantilevered prostheses occlude with natural teeth, the forces also increase in the distal direction along the cantilever segment. These factors may at least partly explain the increasing abutment tooth mobility in one of the patients with a complete maxillary denture occluding with a bilateral cantilever prosthesis and also the reported high prevalence of technical failures in such dentitions. 2, 4 Retention. Loss of retention of anchor crowns is reported
THE JOURNAL OF PROSTHETIC DENTISTRY
to dominate among causes of technical failures. 1, 4-6 In this respect root-filled, post-supplied distal abutments seemed to dominateJ In the present study no retention complications occurred even though root-filled abutments adjoining the cantilever segments were present in half the number of the FPDs. Access to long clinical crowns usually enabled preparation with optimal retention. When this was not the case, preprosthetic gingival and osteoplastic crown-lengthening surgery was performed to obtain at least 3 mm of cervical embracement around parallel root surfaces. Especially in patients with root-filled post-supplied abutments this should be carefully considered as illustrated in Fig. 8. Dimensioning. The metal framework of the present FPDs measured 5 to 6 mm in height (axial force direction) and 4 to 5 mm in width along the entire prosthesis. To avoid fatigue and fracture of the FPD, a minimum dimension of 5 mm in height and 4 mm in width at least around the distal retainer crown has been recommended, 22 based on the maximal local forces that the distal cantilever unit might be subjected to under extreme, provoked situations. If such dimensions cannot be achieved, posterior two-unit cantilevers should be avoided. In the present material with an observation time of 5 to 12 years, technical complications occurred in only one patient (3%). The patients are continuously subjected to regular follow-up and the failure rate might increase with time as a result of fatigue of the prostheses and the abutment teeth. 2, 4 In a retrospective study on 83 extensive FPDs with cantilever units made by general practitioners, Randow et al. 4 reported an average technical failure rate of 37 % after 5 to 7 years, the failures increasing with time in function and number of cantilever units. Thus with two bilateral posterior cantilever units, technical failures amounted to 44 %. These findings were based on questionnaires to the dentists but none of the discussed factors critical for the prognosis (occlusion, retention, and dimension) was analyzed. With the well-performed periodontal therapy and maintenance care and with the occlusal and dimensional designs of the prostheses that we have suggested, neither periodontal problems nor fatigue of the FPDs in the present study are to be expected in the future. Complications that might be more difficult to control over extended time are loss of retention and/or fractures of distal--in particular, root-filled--abutment teeth, although the results of this study indicate that such problems can also be prevented for a considerable time. The question still arises as to how long a treatment modality should be followed up for a clear statement on its prognoses. The results of this retrospective investigation do not contradict the condition that posterior cantilevers in fixed prostheses imply an inherent risk for technical complications. The general recommendation that distal end abutment support should always be used when available, still holds. The introduction of well-documented implant techniques to create support for fixed prosthesis, for instance the Br~_nemark technique, 15might open new possibilities of
551
LAURELLET AL
achieving distal end abutments when teeth are missing. However, if natural teeth or implants are not available as end abutments, FPDs with two or even more posterior cantilever units might be a realistic alternative to removable partial dentures provided certain prerequisites are fulfilled. In extensive prosthodontics, failures do occur. Failures in cantilever prostheses have mainly been of technical character. An imbalance between functional forces and the ability of the various prosthetic components, including the abutment teeth, to withstand these forces must thus have existed. The nature of and reasons behind failures should always be carefully analyzed to improve this treatment modality and prevent future failures. In this study we have pointed out some important factors that should be considered. CLINICAL
IMPLICATIONS
Where distal end abutments are not available as support for extensive fixed prosthodontics, posterior polyunit cantilevers can be successfully incorporated in the FPD provided: 1. The jaw relationships permit the establishment of occlusal contacts anteriorly as well as posteriorly with an occlusal morphology guiding the occlusal forces in an apical direction. 2. Lateral movements are anterior-guided with no cantilever contacts. 3. Great care is given to the preparation procedure to secure the retention of the abutment crowns. This might call for preprosthodontic crown-lengthening surgery. 4. The dimension of the metal framework at least mesial and distal to the distal retainer crown should measure a minimum of 5 mm in height and 4 mm in width. If any of these prerequisites cannot be fulfilled, posterior polyunit cantilevers should be avoided. SUMMARY Thirty-six extensive fixed partial dentures with polyunit cantilevers unilaterally or bilaterally, made with special considerations on retention, dimension, and occlusal design were studied during a period of 5 to 12 years. During this period one fixed partial denture was lost after 6 years in service as a result of a rapid traumatic periodontal breakdown related to psychosocial circumstances. One fixed partial denture exhibited increasing abutment-tooth mobility and was stabilized by the installation of and connection to bilateral Br~nemark implants. Fracture of one abutment tooth occurred in one patient with a facultatively removable 12-unit prosthesis supported on two canine abutments. This patient could be treated without loss of the prosthesis. The remaining 33 cantilever prostheses exhibited no signs of either biologic or irreversable technical complications. Factors influencing the prognosis of extensive fixed cantilever protheses were reviewed. 552
REFERENCES 1. Nyman S, Lindhe J. A longitudinal study of combined periodontal and prosthetic treatment of patients with advanced periodontal disease. J Periodontol 1979;50:469-14. 2. Isikowitz L. A long-term prognosis for the free-end saddle bridge. J Oral Rehabil 1985;12:247-62. 3. Adell R, Ekholm V, Rockler B, Br~nemark P-I. A 15 year study of osseointegrated implants in the treatment of the edentolous jaw. Int J Oral Surg 1981;10:387-416. 4. Randow K, Glantz PO, Z6ger Bo. Technical failures and some related clinical complications in extensive fixed prosthodontics. Acta Odontol Scand 1986;44:241-55. 5. Dahl B, 0rstavik D, Karlsan K. Multi-unit bridges on reduced periodontol support. The Scandinavian Division of IADR Special Issue 1987; report no 120, p 84. 6. Karlsson S. Failures and length of service in fixed prosthodontics after long-term function. Swed Dent J 1989;13:185-192. 7. Lindhe J, Nyman S. The effect of plaque control and surgical pocket elimination on the establishment and maintenance of periodontal health. A longitudinal study of periodontol therapy in cases of advanced disease. J Clin Periodontol 1975;2:67-79. 8. Bj6rn H, Hailing A, Tyberg H. Radiographic assessment of marginal bone loss. Odontologisk Revy 1969;20:165-79. 9. Ante I. The fundamental principles of abutments [Thesis]. Michigan State Dental Society Bulletin 8, 1926;14-23. 10. Laurell L, Lundgren D. Periodontal ligament areas and occlusal forces in dentitions restored with cross-arch bilateral end abutment bridges. J Clin Periodontol 1985;12:850-60. 11. Jepsen A. Root surface measurements and a method for x-ray determination of root surface area. Acta Odontol Scand 1963;21:35-46. 12. Nyman S, Ericsson I. The capacity of reduced periodontal tissues to support fixed bridgework. J Clin Periodontol 1982;9:409-14. 13. Isidor F, Budtz-J~rgensen E. Periodontal conditions following treatment with cantilever bridges or removable partial dentures in geriatric patients. A 2-year study. Gerodontics 1987;3:117-21. 14. Laurell L, Lundgren D. Periodontal ligament areas and occlusal forces in dentitions restored with cross-arch unilateral posterior two-unit cantilever bridges. J Clin Periodontol 1985;12:850-860. 15. Br~nemark P-I, Hansson B-C, Adell R, Breine U, LundstrSm J, Hallen O, Ohman A. Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period. Scand J Plast Reconstr Surg 1977;ll(Suppl 16). 16. Laurell L, Lundgren D. Influence of occlusion on closing and chewing forces in cross-arch bilateral posterior cantilever fixed partial dentures. J PROSTHET DENT (in press). 17. Nyman S, Lindhe J, Lundgren D. The role of occlusion for the stability of fixed bridges in patients with advanced periodontal disease. J Clin Periodontol 1975;2:53-66. 18. Beyron H. Occlusion. Points of significance in planning restorative procedures. Proceedings in the first international prosthodontic congress. J PROSTHETDENT 1973;30:641-52. 19. Laurell L, Lundgran D. Distribution of occlusal forces along unilateral posterior two-unit cantilever segments in cross-arch fixed partial dentures. J PROSTHETDENT 1988;60:106-12. 20. Laurell L, Lundgren D. Interfering occlusal contacts and distribution of chewing and biting forces in dentitions with fixed cantilever prostheses. J PROSTHET DENT 1987;58:197-203. 21. Falk H' Laurell L' Lundgren D" Occlusal f°rce pattern in dentiti°ns with mandibular implant-supported cantilever prostheses occluding with complete dentures. Int J Oral Maxillofac Implant 1989;4:55-62. 22. Lundgren D, Laurell L. Occlusal force pattern during chewing and biting in dentitions restored with fixed bridges of cross-arch extension. II Unilateral posterior two-unit cantilevers. J Oral Rehabil 1986;13:191203.
Reprint requests to: DR. LARS LAURELL THE INSTITUTEFOR POSTGRADUATEDENTALEDUCATION BOX 1030 S-551 11 0:6NKSPING SWEDEN
OCTOBER 1991
VOLUME 66
NUMBER 4
