TRANSVERSESTRENGTHSOF DENTURE BASE RESINS
CLINICAL
IMPLICATIONS
The results indicate that the transverse strength of Triad denture base resin is significantly lower than the heat polymerized brands Lucitone 199 and Accelar 20. Clinically, a resin material exhibiting a lower transverse strength may be more prone to fracture during function as a denture base than would a resin with a higher transverse strength. This potential for fracture may increase with use because of the water sorpti0n for some types of materials. CONCLUSION 1. Lucitone long-cured resin significantly decreases in transverse strength from dry to 30 days in water storage. 2. Accelar 20 resin shows minimal decrease in transverse strength from dry to 30 days in water storage. 3. The transverse strength of Triad resin was significantly lower than the other resins during all the time periods measured but was unaffected by water storage. 4. Changes in transverse strengths for all materials from 30 to 90 days of water storage were small and not statistically significant.
REFERENCES 1. Sweeney WA, Paffenbarger GC, Caul HJ, Sweeney WT. American Dental Association specification No. 12 for denture base resin: second revision. J Am Dent Assoc 1953;46:54-66. 2. Sweeney WT, Paffenbarger GC, Beall JR. Acrylic resins for dentures. J Am Dent Assoc 1942;29:7-33. 3. Sweeney WT, Caul H J, Gneug W. A transverse testing machine for denture resins. J Am Dent Assoc 1954;49:174-6. 4. Stafford GD, Handley RW. Transverse bend testing of denture base polymers. J Dent 1975;3:251-5. 5. Craig RG, ed. Restorative dental materials. 8th ed. St Louis: CV Mosby, 1989:512-3. 6. Gilbert AS, Pethrick RA, Phillips DW. Acoustic relaxation and infrared spectroscopic measurements of the plasticization of polymethyl methacrylate by water. J Appl Polymer Sci 1977;21:319-30. 7. Stafford GD, Smith DC. Some studies of the properties of denture base polymers. Br Dent J 1968;125:337-42. 8. Stafford GD, Bates JF, Huggett R, Handley RW. A review of the properties of some denture base polymers, J Dent 1980;8:292-306. 9. Braden M. Water absorption characteristics of dental microfine composite filling materials, II. Experimental materials. Biomater 1984;5:3735.
Reprint requests to: DR. DONNAL. DIXON COLLEGE OF DENTISTRY UNIVERSITYOF IOWA IOWACITY, IA 52242
An a n a l y s i s of the r e l a t i o n s h i p b e t w e e n m a n d i b u l a r a l v e o l a r bone loss and a low F r a n k f o r t - m a n d i b u l a r plane a n g l e J o h n W. U n g e r , D D S , a C h a r l e s W. E l l i n g e r , DDS, MS, b a n d J o h n C. G u n s o l l e y , D D S , M S c
Medical College of Virginia, School of Dentistry, Richmond, Va., and University of Kentucky, College of Dentistry, Lexington, Ky. A group of c o m p l e t e denture p a t i e n t s w a s studied to d e t e r m i n e the effect of a l o w F r a n k f o r t - m a n d i b u l a r plane angle on the loss o f r e s i d u a l ridge h e i g h t in the mandible. M e a s u r e m e n t s w e r e m a d e from tracings of c e p h a l o m e t r i c films. The Iow-FMA group did not e x p e r i e n c e s t a t i s t i c a l l y g r e a t e r a m o u n t s of a l v e o l a r ridge loss w h e n c o m p a r e d w i t h the group w i t h an FMA larger than 20 d e g r e e s . The loss of r e s i d u a l ridge h e i g h t for both groups f o l l o w e d a l i n e a r r e l a t i o n s h i p from y e a r 5 to y e a r 20. There w a s s o m e indication that the r e s i d u a l ridge h e i g h t w a s s m a l l e r initially in the Iow-FMA group. The r e l a t i v e effect of the loss of r e s i d u a l ridge height, c o m b i n e d w i t h s m a l l e r r e s i d u a l ridge height originally, indicates that the l o w - F M A p a t i e n t s are m o r e l i k e l y to h a v e a n a t o m i c deficiencies and p r o b l e m s w i t h c o m p l e t e d e n t u r e s a s s o c i a t e d w i t h this l a c k o f m a n d i b u l a r r e s i d u a l ridge height. (J PROSTHET DENT 1991;66:513-6.)
Supported by United States Public Health Service Research Grant No. R01 009401 from the Department of Health, Education, and Welfare. aAssociate Professor and Chairman, Department of Removable Prosthodontics, Medical College of Virginia, School of Dentistry. bprofessor, Department of Oral Health Practice, University of Kentucky, College of Dentistry. CDirector, Officeof Applied Research, Medical Collegeof Virginia, School of Dentistry. 10/1/25051 THE JOURNAL OF PROSTHETIC DENTISTRY
A l v e o l a r bone loss can have a profound effect on the success of complete denture treatment and is encountered most often in the mandible. A troublesome aspect of this problem is the wide variation frequently observed in the extent and amount of bone loss. The ability to predict which patients are likely to suffer great amounts of bone loss would be of importance to both patient and dentist. Reference has been made in the literature to a connection between residual ridge loss and a low 513
UNGER, ELLINGER, AND GUNSOLLEY
to the cranium, the mandibular occlusal line parallel to the mandibular line, and small anterior facial height. These observations also would aptly describe a patient with a low FMA. A definitive link between low FMA and residual ridge loss in edentulous patients does not seem to exist in the literature, although there is ample evidence to suggest that a relationship may well be identified. The present study was undertaken to discover whether such an association exists.
..p'----
M E T H O D S A N D MATERIAL Mandibular
Fig. 1. Frankfort-mandibular plane angle is formed by intersection of Frankfort horizontal plane (porion [Po] to orbitale [Or]) and the mandibular plane (gonion [Go] to menton [MeJ). Mandibular ridge height was measured from highest point on residual ridge in midline to menton.
Frankfort-mandibular plane angle (FMA). 1 This study determined whether greater than normal residual ridge loss should be expected in patients with low Frankfort-mandibular plane angles.
LITERATURE REVIEW The importance of the FMA to prosthodontics was presented clearly and carefully to the profession by DiPietro and Moergeli. 1 They indicated that the low-FMA patient has increased biting force and in turn transmits more force to the residual ridge. Presumably this could result in a greater amount of residual ridge loss for these patients. They further described the low-FMA patient as tending to have flat, broad palatal vaults, shallow buccal vestibules, and muscle attachments near the crest of the residual ridge. They also advised that even minimal resorption can complicate treatment in these patients because they have smaller residual ridges initially. Curtis et al. 2 reported that residual ridge loss may be more evident in patients with a low FMA. They believe that this could be caused by the more vertically directed forces of mandibular closure seen in these patients. In addition, they quoted an earlier study by Tallgren 3 indicating that edentulous patients with a low FMA have considerably greater amounts of alveolar bone loss. Tallgren 3, 4 found greater amounts Of residual ridge loss in patients who were described as having marked mandibular basal bend. This is defined further as a large basal angle and a small gonial angle. This type of patient is illustrated in the later article 3 by a diagram that would seem to be a suitable depiction of a patient with a low FMA. In the earlier article, Tallgren 4 describes the patient with marked basal bend as one with a small inclination of the mandible
514
In a study at the University of Kentucky, College of Dentistry, complete maxillary and mandibular dentures were constructed for 64 patients. The patients were divided into two groups of equal size. One group received dentures made with a complex technique; for the other group, a standard technique was used. Patients included in this study had to have been edentulous for at least 1 year. The age range for the participants was 30 to 65 years. Cephalometric films were made at the beginning of treatment and at various yearly intervals thereafter. 57 After 5 years cephalometric films were available for 34 patients from the original group of 64. This meant that films were present at both initial placement of the dentures and at the 5-year interval for this group. Twenty years after insertion, cephalometric films were obtainable for 24 of the group of 34 patients; 20-year data were unavailable for 10 patients. The FMA was determined on the films made at initial placement of the dentures. This was done by measuring the angle formed by the intersection of the Frankfort horizontal plane and the mandibular plane. The height of the mandibular alveolar bone was determined by measuring from the highest point on the surface of the mandible in the midline to menton, the most inferior point on the mandibular symphysis (Fig. 1). This linear measurement Was done at initial placement, 5 years after insertion, and also at the 20-year postinsertion period. The change in alveolar bone was calculated by subtracting the height at the 5- and 20year postinsertion periods from that present at the initial placement. Measurements were made on tracings of the available films. All the tracings were done by the same person to avoid introducing errors related to the process of tracing. RESULTS The FMA as determined from the films made at initial placement, for all 34 patients, ranged from 14 to 41 degrees. Those patients with an FMA of 20 degrees or less were considered to have a low FMA. 1 This group included 10 patients. At the 5-year postinsertion period, the entire group of 34 patients had lost a total of 36 mm of bone. The range of loss was up to 3 mm. The mean loss per patient was 1.06 mm with a mean loss per year of 0.21 ram. The low-FMA group of 10 patients lost a total of 13 mm of bone
OCTOBER 1991
VOLUME 66
NUMBER 4
RESIDUAL RIDGE LOSS AND LOW FMA
T a b l e I. FMA (degrees)
IP* (mm)
5 yrt (mm)
Difference (IP-5 yr)
20 yr~ (ram)
Difference (IP-20 yr)
22 28 25 14 17 15 23 26 24 21 31 31 19 27 41 18 30 17 25 29 21 40 2O 25 20 30 29 17 31 25 24 37 16 28
29 26 24 19 29 13 27 32 19 26 31 29 35 29 25 15 29 11 26 31 18 33 26 26 33 23 27 19 20 23 29 37 17 27
27 26 21 16 28 12 26 31 18 24 31 29 35 29 23 13 28 10 26 30 17 32 24 25 32 22 25 19 19 23 27 37 15 27
2 0 3 3 1 1 1 1 1 2 0 0 0 0 2 2 1 1 0 1 1 1 2 1 1 1 2 0 1 0 2 0 2 0
----------29 27 35 28 21 11 25 9 26 29 16 30 22 25 31 21 22 17 18 22 25 37 14 26
----------2 2 0 1 4 4 4 2 0 2 2 3 4 1 2 2 5 2 2 1 4 0 3 1
*Residual ridge height at initial placement of complete dentures. tResidual ridge height 5 years after complete denture placement. :~Residual ridge height 20 years after complete denture placement.
i n c l u d i n g a m e a n loss p e r p a t i e n t of 1.30 m m a n d a 0.26 m m loss p e r y e a r ( T a b l e I). R e c o r d s f r o m 24 p a t i e n t s of t h e o r i g i n a l g r o u p of 34 were a v a i l a b l e for s t u d y a t t h e 2 0 - y e a r p o s t i n s e r t i o n period. T h e e n t i r e g r o u p h a d a t o t a l b o n e loss of 53 m m . T h i s r e s u l t e d i n a m e a n loss p e r p a t i e n t of 2.21 m m w i t h a m e a n loss p e r y e a r of 0.11 m m . T h e r a n g e of b o n e loss a t t h i s s t a g e was u p to 5 m m . T h e l o w - F M A g r o u p c o n s i s t e d of s e v e n p a t i e n t s , f r o m t h e o r i g i n a l n u m b e r of 10, w h o h a d a t o t a l of 17 m m of b o n e loss, p r o d u c i n g a m e a n loss of 2.42 m m p e r p a t i e n t a n d a m e a n loss p e r y e a r of 0.12 m m ( T a b l e I). S t a t i s t i c a l a n a l y s i s of t h e d a t a was m a d e f r o m two aspects. T h e first a n a l y s i s c o n c e r n e d t h e h y p o t h e s i s t h a t p a t i e n t s i n t h e l o w - F M A g r o u p w o u l d h a v e g r e a t e r losses in a l v e o l a r ridge h e i g h t t h a n t h o s e p a t i e n t s w i t h a n F M A g r e a t e r t h a n 20 degrees. T h r e e s e p a r a t e a n a l y s e s of v a r i -
THE JOURNAL OF PROSTHETIC DENTISTRY
a n c e were r u n to t e s t t h i s h y p o t h e s i s . T h e r e s p o n s e v a r i a b l e for t w o of t h e t h r e e was t h e d i f f e r e n c e a t 5 years. I n o n e a n a l y s i s all t h e d a t a were u s e d a n d in t h e s e c o n d a n a l y s i s o n l y t h e s u b j e c t s w i t h v a l i d 2 0 - y e a r d a t a were i n c l u d e d . T h e t h i r d a n a l y s i s u s e d t h e d i f f e r e n c e a t 20 y e a r s as t h e res p o n s e v a r i a b l e . I n t h e a n a l y s i s t h e h e i g h t of t h e r e s i d u a l r i d g e a t i n i t i a l p l a c e m e n t was u s e d as a c o v a r i a t e . N o stat i s t i c a l l y s i g n i f i c a n t r e s u l t s were f o u n d w h e n (1) t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h all a v a i l a b l e d a t a a t t h a t t i m e (p < 0.67), (2) w h e n t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h t h o s e of s u b j e c t s w i t h o n l y v a l i d 2 0 - y e a r d a t a (p < 0.59), or (3) w h e n finally t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h t h e d i f f e r e n c e s f o u n d a t 20 y e a r s (p < 0.93). T h e s e c o n d a n a l y s i s was d o n e to d e t e r m i n e w h e t h e r t h e r e was a l i n e a r r e l a t i o n s h i p a m o n g t h e following variables; d i f f e r e n c e s a f t e r 20 years, d i f f e r e n c e s a f t e r 5 years,
515
UNGER, ELLINGER, AND GUNSOLLEY
initial placement, and the FMA. Pearson correlation coefficients were estimated for all possible combinations of variables. A strong relationship was found between the loss of bone during the first 5 years and the total loss found over 20 years (r = 0.83, p < 0.0001). A moderate relationship was found between the residual ridge height at initial placement and the FMA (r = 0.51, p
CLINICAL
IMPLICATIONS
The results indicate that the transverse strength of Triad denture base resin is significantly lower than the heat polymerized brands Lucitone 199 and Accelar 20. Clinically, a resin material exhibiting a lower transverse strength may be more prone to fracture during function as a denture base than would a resin with a higher transverse strength. This potential for fracture may increase with use because of the water sorpti0n for some types of materials. CONCLUSION 1. Lucitone long-cured resin significantly decreases in transverse strength from dry to 30 days in water storage. 2. Accelar 20 resin shows minimal decrease in transverse strength from dry to 30 days in water storage. 3. The transverse strength of Triad resin was significantly lower than the other resins during all the time periods measured but was unaffected by water storage. 4. Changes in transverse strengths for all materials from 30 to 90 days of water storage were small and not statistically significant.
REFERENCES 1. Sweeney WA, Paffenbarger GC, Caul HJ, Sweeney WT. American Dental Association specification No. 12 for denture base resin: second revision. J Am Dent Assoc 1953;46:54-66. 2. Sweeney WT, Paffenbarger GC, Beall JR. Acrylic resins for dentures. J Am Dent Assoc 1942;29:7-33. 3. Sweeney WT, Caul H J, Gneug W. A transverse testing machine for denture resins. J Am Dent Assoc 1954;49:174-6. 4. Stafford GD, Handley RW. Transverse bend testing of denture base polymers. J Dent 1975;3:251-5. 5. Craig RG, ed. Restorative dental materials. 8th ed. St Louis: CV Mosby, 1989:512-3. 6. Gilbert AS, Pethrick RA, Phillips DW. Acoustic relaxation and infrared spectroscopic measurements of the plasticization of polymethyl methacrylate by water. J Appl Polymer Sci 1977;21:319-30. 7. Stafford GD, Smith DC. Some studies of the properties of denture base polymers. Br Dent J 1968;125:337-42. 8. Stafford GD, Bates JF, Huggett R, Handley RW. A review of the properties of some denture base polymers, J Dent 1980;8:292-306. 9. Braden M. Water absorption characteristics of dental microfine composite filling materials, II. Experimental materials. Biomater 1984;5:3735.
Reprint requests to: DR. DONNAL. DIXON COLLEGE OF DENTISTRY UNIVERSITYOF IOWA IOWACITY, IA 52242
An a n a l y s i s of the r e l a t i o n s h i p b e t w e e n m a n d i b u l a r a l v e o l a r bone loss and a low F r a n k f o r t - m a n d i b u l a r plane a n g l e J o h n W. U n g e r , D D S , a C h a r l e s W. E l l i n g e r , DDS, MS, b a n d J o h n C. G u n s o l l e y , D D S , M S c
Medical College of Virginia, School of Dentistry, Richmond, Va., and University of Kentucky, College of Dentistry, Lexington, Ky. A group of c o m p l e t e denture p a t i e n t s w a s studied to d e t e r m i n e the effect of a l o w F r a n k f o r t - m a n d i b u l a r plane angle on the loss o f r e s i d u a l ridge h e i g h t in the mandible. M e a s u r e m e n t s w e r e m a d e from tracings of c e p h a l o m e t r i c films. The Iow-FMA group did not e x p e r i e n c e s t a t i s t i c a l l y g r e a t e r a m o u n t s of a l v e o l a r ridge loss w h e n c o m p a r e d w i t h the group w i t h an FMA larger than 20 d e g r e e s . The loss of r e s i d u a l ridge h e i g h t for both groups f o l l o w e d a l i n e a r r e l a t i o n s h i p from y e a r 5 to y e a r 20. There w a s s o m e indication that the r e s i d u a l ridge h e i g h t w a s s m a l l e r initially in the Iow-FMA group. The r e l a t i v e effect of the loss of r e s i d u a l ridge height, c o m b i n e d w i t h s m a l l e r r e s i d u a l ridge height originally, indicates that the l o w - F M A p a t i e n t s are m o r e l i k e l y to h a v e a n a t o m i c deficiencies and p r o b l e m s w i t h c o m p l e t e d e n t u r e s a s s o c i a t e d w i t h this l a c k o f m a n d i b u l a r r e s i d u a l ridge height. (J PROSTHET DENT 1991;66:513-6.)
Supported by United States Public Health Service Research Grant No. R01 009401 from the Department of Health, Education, and Welfare. aAssociate Professor and Chairman, Department of Removable Prosthodontics, Medical College of Virginia, School of Dentistry. bprofessor, Department of Oral Health Practice, University of Kentucky, College of Dentistry. CDirector, Officeof Applied Research, Medical Collegeof Virginia, School of Dentistry. 10/1/25051 THE JOURNAL OF PROSTHETIC DENTISTRY
A l v e o l a r bone loss can have a profound effect on the success of complete denture treatment and is encountered most often in the mandible. A troublesome aspect of this problem is the wide variation frequently observed in the extent and amount of bone loss. The ability to predict which patients are likely to suffer great amounts of bone loss would be of importance to both patient and dentist. Reference has been made in the literature to a connection between residual ridge loss and a low 513
UNGER, ELLINGER, AND GUNSOLLEY
to the cranium, the mandibular occlusal line parallel to the mandibular line, and small anterior facial height. These observations also would aptly describe a patient with a low FMA. A definitive link between low FMA and residual ridge loss in edentulous patients does not seem to exist in the literature, although there is ample evidence to suggest that a relationship may well be identified. The present study was undertaken to discover whether such an association exists.
..p'----
M E T H O D S A N D MATERIAL Mandibular
Fig. 1. Frankfort-mandibular plane angle is formed by intersection of Frankfort horizontal plane (porion [Po] to orbitale [Or]) and the mandibular plane (gonion [Go] to menton [MeJ). Mandibular ridge height was measured from highest point on residual ridge in midline to menton.
Frankfort-mandibular plane angle (FMA). 1 This study determined whether greater than normal residual ridge loss should be expected in patients with low Frankfort-mandibular plane angles.
LITERATURE REVIEW The importance of the FMA to prosthodontics was presented clearly and carefully to the profession by DiPietro and Moergeli. 1 They indicated that the low-FMA patient has increased biting force and in turn transmits more force to the residual ridge. Presumably this could result in a greater amount of residual ridge loss for these patients. They further described the low-FMA patient as tending to have flat, broad palatal vaults, shallow buccal vestibules, and muscle attachments near the crest of the residual ridge. They also advised that even minimal resorption can complicate treatment in these patients because they have smaller residual ridges initially. Curtis et al. 2 reported that residual ridge loss may be more evident in patients with a low FMA. They believe that this could be caused by the more vertically directed forces of mandibular closure seen in these patients. In addition, they quoted an earlier study by Tallgren 3 indicating that edentulous patients with a low FMA have considerably greater amounts of alveolar bone loss. Tallgren 3, 4 found greater amounts Of residual ridge loss in patients who were described as having marked mandibular basal bend. This is defined further as a large basal angle and a small gonial angle. This type of patient is illustrated in the later article 3 by a diagram that would seem to be a suitable depiction of a patient with a low FMA. In the earlier article, Tallgren 4 describes the patient with marked basal bend as one with a small inclination of the mandible
514
In a study at the University of Kentucky, College of Dentistry, complete maxillary and mandibular dentures were constructed for 64 patients. The patients were divided into two groups of equal size. One group received dentures made with a complex technique; for the other group, a standard technique was used. Patients included in this study had to have been edentulous for at least 1 year. The age range for the participants was 30 to 65 years. Cephalometric films were made at the beginning of treatment and at various yearly intervals thereafter. 57 After 5 years cephalometric films were available for 34 patients from the original group of 64. This meant that films were present at both initial placement of the dentures and at the 5-year interval for this group. Twenty years after insertion, cephalometric films were obtainable for 24 of the group of 34 patients; 20-year data were unavailable for 10 patients. The FMA was determined on the films made at initial placement of the dentures. This was done by measuring the angle formed by the intersection of the Frankfort horizontal plane and the mandibular plane. The height of the mandibular alveolar bone was determined by measuring from the highest point on the surface of the mandible in the midline to menton, the most inferior point on the mandibular symphysis (Fig. 1). This linear measurement Was done at initial placement, 5 years after insertion, and also at the 20-year postinsertion period. The change in alveolar bone was calculated by subtracting the height at the 5- and 20year postinsertion periods from that present at the initial placement. Measurements were made on tracings of the available films. All the tracings were done by the same person to avoid introducing errors related to the process of tracing. RESULTS The FMA as determined from the films made at initial placement, for all 34 patients, ranged from 14 to 41 degrees. Those patients with an FMA of 20 degrees or less were considered to have a low FMA. 1 This group included 10 patients. At the 5-year postinsertion period, the entire group of 34 patients had lost a total of 36 mm of bone. The range of loss was up to 3 mm. The mean loss per patient was 1.06 mm with a mean loss per year of 0.21 ram. The low-FMA group of 10 patients lost a total of 13 mm of bone
OCTOBER 1991
VOLUME 66
NUMBER 4
RESIDUAL RIDGE LOSS AND LOW FMA
T a b l e I. FMA (degrees)
IP* (mm)
5 yrt (mm)
Difference (IP-5 yr)
20 yr~ (ram)
Difference (IP-20 yr)
22 28 25 14 17 15 23 26 24 21 31 31 19 27 41 18 30 17 25 29 21 40 2O 25 20 30 29 17 31 25 24 37 16 28
29 26 24 19 29 13 27 32 19 26 31 29 35 29 25 15 29 11 26 31 18 33 26 26 33 23 27 19 20 23 29 37 17 27
27 26 21 16 28 12 26 31 18 24 31 29 35 29 23 13 28 10 26 30 17 32 24 25 32 22 25 19 19 23 27 37 15 27
2 0 3 3 1 1 1 1 1 2 0 0 0 0 2 2 1 1 0 1 1 1 2 1 1 1 2 0 1 0 2 0 2 0
----------29 27 35 28 21 11 25 9 26 29 16 30 22 25 31 21 22 17 18 22 25 37 14 26
----------2 2 0 1 4 4 4 2 0 2 2 3 4 1 2 2 5 2 2 1 4 0 3 1
*Residual ridge height at initial placement of complete dentures. tResidual ridge height 5 years after complete denture placement. :~Residual ridge height 20 years after complete denture placement.
i n c l u d i n g a m e a n loss p e r p a t i e n t of 1.30 m m a n d a 0.26 m m loss p e r y e a r ( T a b l e I). R e c o r d s f r o m 24 p a t i e n t s of t h e o r i g i n a l g r o u p of 34 were a v a i l a b l e for s t u d y a t t h e 2 0 - y e a r p o s t i n s e r t i o n period. T h e e n t i r e g r o u p h a d a t o t a l b o n e loss of 53 m m . T h i s r e s u l t e d i n a m e a n loss p e r p a t i e n t of 2.21 m m w i t h a m e a n loss p e r y e a r of 0.11 m m . T h e r a n g e of b o n e loss a t t h i s s t a g e was u p to 5 m m . T h e l o w - F M A g r o u p c o n s i s t e d of s e v e n p a t i e n t s , f r o m t h e o r i g i n a l n u m b e r of 10, w h o h a d a t o t a l of 17 m m of b o n e loss, p r o d u c i n g a m e a n loss of 2.42 m m p e r p a t i e n t a n d a m e a n loss p e r y e a r of 0.12 m m ( T a b l e I). S t a t i s t i c a l a n a l y s i s of t h e d a t a was m a d e f r o m two aspects. T h e first a n a l y s i s c o n c e r n e d t h e h y p o t h e s i s t h a t p a t i e n t s i n t h e l o w - F M A g r o u p w o u l d h a v e g r e a t e r losses in a l v e o l a r ridge h e i g h t t h a n t h o s e p a t i e n t s w i t h a n F M A g r e a t e r t h a n 20 degrees. T h r e e s e p a r a t e a n a l y s e s of v a r i -
THE JOURNAL OF PROSTHETIC DENTISTRY
a n c e were r u n to t e s t t h i s h y p o t h e s i s . T h e r e s p o n s e v a r i a b l e for t w o of t h e t h r e e was t h e d i f f e r e n c e a t 5 years. I n o n e a n a l y s i s all t h e d a t a were u s e d a n d in t h e s e c o n d a n a l y s i s o n l y t h e s u b j e c t s w i t h v a l i d 2 0 - y e a r d a t a were i n c l u d e d . T h e t h i r d a n a l y s i s u s e d t h e d i f f e r e n c e a t 20 y e a r s as t h e res p o n s e v a r i a b l e . I n t h e a n a l y s i s t h e h e i g h t of t h e r e s i d u a l r i d g e a t i n i t i a l p l a c e m e n t was u s e d as a c o v a r i a t e . N o stat i s t i c a l l y s i g n i f i c a n t r e s u l t s were f o u n d w h e n (1) t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h all a v a i l a b l e d a t a a t t h a t t i m e (p < 0.67), (2) w h e n t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h t h o s e of s u b j e c t s w i t h o n l y v a l i d 2 0 - y e a r d a t a (p < 0.59), or (3) w h e n finally t h e 5 - y e a r r e s u l t s were c o m p a r e d w i t h t h e d i f f e r e n c e s f o u n d a t 20 y e a r s (p < 0.93). T h e s e c o n d a n a l y s i s was d o n e to d e t e r m i n e w h e t h e r t h e r e was a l i n e a r r e l a t i o n s h i p a m o n g t h e following variables; d i f f e r e n c e s a f t e r 20 years, d i f f e r e n c e s a f t e r 5 years,
515
UNGER, ELLINGER, AND GUNSOLLEY
initial placement, and the FMA. Pearson correlation coefficients were estimated for all possible combinations of variables. A strong relationship was found between the loss of bone during the first 5 years and the total loss found over 20 years (r = 0.83, p < 0.0001). A moderate relationship was found between the residual ridge height at initial placement and the FMA (r = 0.51, p
