Strain Gauge Analysis of Occlusal Forces on Implant Prostheses at Various Occlusal Heights Young-Eun Cho, DDS, MS1/Eun-Jin Park, DDS, PhD2/Jai-Young Koak, DDS, PhD3/ Seong-Kyun Kim, DDS, PhD3/Seong-Joo Heo, DDS, PhD3/Ji-Man Park, DDS, PhD4 Purpose: The purpose of this study was to evaluate and compare the strain development at different occlusal heights of an implant prosthesis and adjacent teeth through the use of strain gauges. Materials and Methods: A test model was constructed using autopolymerizing polyurethane resin, artificial composite resin teeth, and an implant gold crown in the mandibular first molar area. The resin block containing the implant and the gold crown was sectioned, and two expansion screws were attached perpendicular to the bottom of the resin block on the buccal and lingual sides. The expansion screws were turned to create a gap. Four groups were created based on the occlusal height of the implant gold crown. Three strain gauges were attached to the buccal surfaces of the mandibular right second premolar, implant gold crown, and second molar. Beef jerky, carrot, and bread were used as test foods. A universal testing machine was used to apply compressive forces of 300 N (beef jerky), 250 N (carrot), and 50 N (bread), and the occlusal force was measured in each group. Results: With 300 N, occlusal forces were concentrated on the adjacent teeth when the occlusal height of the implant prosthesis decreased. With 250 and 50 N, when the occlusal height of the implant prosthesis increased, the occlusal force applied to the implant prosthesis increased, but alterations in the implant crown height had little effect on the adjacent teeth. Conclusion: Different amounts of strain in the implant prosthesis and adjacent teeth were recorded depending on the occlusal height of the prosthesis. With 250 or 50 N of force, an increased prosthesis height affected the implant itself. With 300 N of force, decreased occlusal height of the prosthesis resulted in increased force on the adjacent teeth. Int J Oral Maxillofac Implants 2014;29:1034–1041. doi: 10.11607/jomi.3040 Key words: bite force, dental implants, dental stress analysis, pressure transducers, vertical dimension
D
entists and researchers have debated the identification and application of concepts of occlusion to endosseous implants for many years.1–4 People have different mastication habits, chewing force, and chewing efficiency. Bruxism and clenching habits negatively affect the prognosis of endosseous implants because dental implants lack sufficient resistance to lateral
1Graduate
Student, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. 2 Associate Professor, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. 3 Professor, Department of Prosthodontics and Dental Research Institute, Seoul National University, Seoul, Korea. 4 Assistant Professor, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. Correspondence to: Dr Ji-Man Park, Department of Prosthodontics, School of Medicine, Ewha Womans University, 911-1 Mok-5-dong, Yangcheon-gu, Seoul, 158-710, South Korea. Fax: +82-2-2650-5764. Email: [email protected] This paper was presented at the Academy of Osseointegration’s 27th Annual Meeting, March 1–3, 2012, Phoenix, Arizona.
©2014 by Quintessence Publishing Co Inc.
forces.5–8 Appropriate concepts of occlusion according to individual mastication characteristics may improve the long-term prognosis of implant restorations. Few randomized controlled trials have compared different occlusal schemes, and little research has focused on the occlusion of implant prostheses. Unlike natural teeth, osseointegrated implants are ankylosed to the surrounding bone without the periodontal ligament, which has mechanoreceptors and serves a shock-absorbing function.9–12 Because of the ankylotic character of implants, many clinicians prefer to hypoocclude dental implants to protect implant-supported restorations.2–4,13,14 The concept of light contact during heavy biting and no occlusal contact during light biting was recommended by Lundgren and Laurell3 and Kim et al.15 However, some authors have reported that hypo-occlusion may cause occlusal disharmony, with adjacent teeth receiving most of the occlusal force, and they suggested equal occlusion for implant crowns and any remaining natural teeth.16,17 There are several objective methods to measure masticatory functions, including bite force, electromyography, sound transmission, pressure-sensitive sheets, and strain gauges.18 Pressure-sensitive sheets
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Cho et al
a
b
c
Fig 1 Arrangement of teeth and simulation of the PDL in the models. (a) The occlusal surface of the dentiform model was indented with silicone impression material. (b) The PDL was simulated by coating silicone impression material around the roots of the acrylic resin teeth. (c) The resin teeth were then relocated in the putty index.
(eg, T-scan, Tekscan, and Prescale, FUJIFILM) measure occlusal force directly. However, they cannot measure occlusal forces during mastication.19 Strain gauges have been widely used to measure strain development around experimental objects. According to Sakaguchi et al,20 strain gauges bonded to external tooth surfaces can be used to study stress in tooth-related areas. Several studies have employed comparative finite element analyses and strain gauge measurement for the purpose of validation. These tools enable the assessment of bite force corresponding to a single natural or prosthetic tooth, unlike sheet-type pressure sensors.19,21 The goal of this study was to compare strain development in an implant prosthesis and adjacent teeth by using strain gauges with the prosthesis set at different occlusal heights.
MATERIALS AND METHODS Fabrication of Working Model
A test model was constructed with autopolymerizing polyurethane resin (POLYUROCK, Metalor Technologies) and artificial resin teeth (Nissin Dental Products Inc). Maxillary and mandibular teeth were arranged in maximum intercuspation, and group function was established in the dentiform model using an articulator (Hanau), with the mandibular right first molar missing. The arrangement of artificial teeth was confirmed using articulating paper (Accufilm, Parkell) and shim stock (Hanel). The occlusal surface of the dentiform model was indented with silicone impression material (Flexitime-putty type, Heraeus). Then the resin teeth were relocated in the putty index. Periodontal ligament (PDL) tissue was simulated with silicone impression material (Examix Fine, GC) around the roots of the resin teeth, which were embedded in a resin replica of the alveolar bone (Fig 1).22 The thickness of the PDL substitute was between 0.25 and 0.3 mm after the preliminary application of the PDL silicone with
the known PDL thickness, and the color of the material was harmonized by a well-trained operator.23,24 In the mandibular right first molar area, a single 4.0 × 10-mm implant (MK III, Nobel Biocare) was embedded and a screw-retained gold crown was created. All procedures, including cast fabrication, wax-up, casting, and finishing, were carried out in accordance with conventional protocols. The implant gold crown was placed into Class I occlusion by three-point contact with the maxillary teeth. To control the occlusal height of the implant gold crown, orthodontic expansion screws (Dentaurum) were used. The resin block containing the implantcrown assembly was sectioned into a 12 × 20 × 12mm (width × height × depth) segment, and two expansion screws were attached perpendicular to the bottom of the resin block on its buccal and lingual sides. These screw assemblies were fixed parallel to the axes of the occluding teeth in the opposing arch using a surveyor. In this setup, a turn of the expansion screw by 90 degrees moves the resin block 0.4 mm upward. Therefore, the expansion screws were turned by 5 degrees to create a 23-µm gap, which is the thickness of articulating paper (Fig 2). Four groups were assigned according to the occlusal height of the implant crown. The occlusal height of the crown was controlled by turning the screw, and the change in height was confirmed with articulating paper under 100 N of load applied with a universal testing machine (Model 3345, Instron Corp). • Group 1: The occlusal height of the implant gold crown was 46 µm lower than the adjacent teeth. • Group 2: The occlusal height of the implant gold crown was 23 µm lower than the adjacent teeth. • Group 3: The occlusal height of the implant gold crown was the same as that of the other teeth. This group served as the control. • Group 4: The occlusal height of the implant gold crown was 23 µm higher than the adjacent teeth. The International Journal of Oral & Maxillofacial Implants 1035
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Cho et al
a
b
c
Fig 2 Attachment of expansion screw and verification of change in occlusal height. (a) Two expansion screws were fixed parallel to the axis of occluding teeth in the opposite jaw at the buccal and lingual side. (b) The expansion screw was turned 5 degrees to create a 23-µm gap. (c) The height change was confirmed with Accufilm and shim stock.
a
b
Load cell
Strain gauge Circuit board Expansion screw
Fig 4 Compensation work with a load cell. The load cell was located between the maxilla and mandible over the teeth with the strain gauges, and compressive force was applied with a universal testing machine. The strain of the strain gauge and pressure of the load cell were read at the same time and the compensation work was done repeatedly for each tooth.
Measurement Procedure
Three strain gauges (KFG-2-350-C1-11, KYOWA Electronic Instruments) were attached to the buccal surface of the mandibular second premolar, implant crown in the mandibular first molar area, and mandibular second molar at the midpoint of the clinical crown, which was also directly under the mesiobuccal cusp. A dynamic signal–conditioning strain amplifier (CTA-1000, Curiotech Inc) was connected, and an analysis program (DA-1700B, Cas Korea) was used to measure, record, and process the strain values (Fig 3).
Fig 3 Strain gauges and amplifiers. (a) Three strain gauges were attached to the buccal surface of the mandibular right second premolar, the implant gold crown in the mandibular right first molar area, and the mandibular right second molar. Printed circuit boards (green rectangular plates) lengthened the wire and protected the strain gauges’ wires from disconnection. (b) A dynamic signal-conditioning strain amplifier and analysis program was used to measure, record, and process the strain values.
Prior to making the measurements, the model was calibrated to ensure it was free of residual stress, with strain gauges showing zero. Test food was inserted between the arches, over the teeth with strain gauges, and compressive force was applied with a universal testing machine. Compensation work was done with a load cell, which functioned as an electronic scale. Before masticatory action was initiated with the test food between the maxillary and mandibular teeth, a load cell was placed directly on the strain gauge, and the strain values of the load cell and the strain gauge were measured simultaneously while a masticatory (compressive) force of 100 N was applied with the universal testing machine (Fig 4). The electronic resistance value was measured, recorded, and then processed with an analysis program (DA-1700B); the value was read 3 seconds after the beginning of each masticatory cycle. In this way, the load transferred to the teeth or the implant was calibrated by calculating both values. Beef jerky, carrot, and bread were used as test foods. Each material was cut into 12 pieces with dimensions of 2 × 4.5 × 0.5 cm, so that the food specimens could be indented to match the occlusal surfaces of three teeth simultaneously (Fig 5). The testing machine was used to apply compressive forces of 300 N to the beef jerky, 250 N to the carrot, and 50 N to the bread, and the occlusal force was measured in each group.25 The experiment was carried out 12 times under conditions of different occlusal heights and food types, and 20 strokes of mastication were executed for each condition.
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Test food
Strain gauge
Circuit board Expansion screw
Fig 5 Insertion of test food.
Table 1 Mean Strain Values and SDs for Beef Jerky (n = 20) Mean strain (SD) (N) Tooth Second molar (a)
Group 1
Group 2
Group 3
172.51 (85.99)
126.99 (40.42)
105.44 (45.26)
Implant (b)
33.31 (17.54)
34.97 (30.74)
Second premolar (c)
81.66 (18.01)
139.54 (55.74)
89.77 (93) 89.41 (36.78)
Group 4
F (P value)*
97.59 (55.46) 100.27 (98.52) 94.53 (50.87)
4.608 ∙ (.000)
0.057§ (.982)
F (P value)
11.417† (.000) b < a, c‡
†comparison
§comparison
*Statistically significant if P < .05; of tooth effects; ‡result of multiple comparisons by Scheffé; of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Statistical Analysis
RESULTS Beef Jerky
Table 1 and Fig 6 show the average strain values and standard deviations (SDs) for the four different groups according to the occlusal height of the implant prosthesis when biting beef jerky. The mean strain values for the implant prostheses were 33.31 N, 34.97 N, 89.77 N, and 100.27 N for group 1, group 2, group 3, and group 4, respectively. There was a significant interaction between tooth position and occlusal height of the prosthesis. The strain on the second molar was
Molar
Implant
Premolar
300 250 Strain (N)
Two-way analysis of variance (ANOVA) (SPSS for Windows, v. 12.0, SPSS Inc) was performed to analyze the factors of occlusal height and tooth position by themselves and the interaction between these factors. Oneway ANOVA was used to evaluate the difference in load among teeth for each occlusal height. The Scheffé method was used as a post hoc test. Correction was done through the Scheffé multiple-comparisons test, and the strain values on the implant crown and adjacent teeth were compared. An α of .05 was considered to indicate statistical significance.
200 150 100 50 0
–46 µm
–23 µm
0 µm
23 µm
Fig 6 Mean strain values for beef jerky. The strain on the second molar was significantly higher when the implant prosthesis was at the –46-µm level than at the 0- or 23-µm levels. On the second premolar, statistically significant differences were found between the strain measured at –23 µm and that measured at the other levels. Two-way ANOVA: occlusal height, P = .982; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .033; implant, P = .061; second premolar, P = .017.
significantly higher when the implant prosthesis was at the –46-µm level than at the zero or 23-µm level. On the second premolar, statistically significant differences were found between the strain at the –23-µm level and the strain values measured at the other levels. When the occlusal height of the implant gold crown The International Journal of Oral & Maxillofacial Implants 1037
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Cho et al
Table 2 Mean Strain Values and SDs for Carrot (n = 20) Mean (SD) (N) Tooth
Group 1
Group 2
Group 3
Group 4
F (P value)*
Second molar (a)
177.08 (32.66)
156.55 (23.83)
165.17 (23.83)
138.32 (43.13)
Implant (b)
119.96 (38.4)
126.87 (25.18)
167.06 (50.58)
209.14 (63.99)
114.358† (.000) c < a, b‡
45.01 (17.6)
62.61 (6.35)
53.31 (9.78)
40.07 (9.09)
Second premolar (c)
1.520§
F (P value) †comparison
7.224 ∙ (.000)
(.215)
‡result
§comparison
*Statistically significant if P < .05; of tooth effects; of multiple comparisons by Scheffé; of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Molar
300
Implant
Premolar
Strain (N)
250 200 150 100
Bread
50 0
(group 4), the occlusal force applied to the implant prosthesis increased. However, alterations in the occlusal height of the implant prosthesis had little effect on the adjacent teeth, and the mean strain value graph (Fig 7) showed a stairway-like trend from the stack paper cutter–shaped masticatory pattern.
–46 µm
–23 µm
0 µm
23 µm
Fig 7 Mean strain values for carrot. The strain on the implant was significantly larger when it was 23 µm higher than the adjacent teeth compared to the strain measured at the –46- or –23-µm level. On the second premolar, the strain at the –23-µm level was significantly larger than that seen at the 23-µm level. In comparison with the 0-µm level, no implants or teeth showed a difference in strain related to the height of the implant. Twoway ANOVA: occlusal height, P = .215; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .100; implant, P = .001; second premolar, P = .002.
decreased (groups 1 and 2), occlusal forces were concentrated on the adjacent teeth.
Carrot
Table 2 and Fig 7 show the average absolute strain values and SDs according to the occlusal height of the prosthesis when biting carrot. The mean strain values for the implant prosthesis were 119.96 N, 126.86 N, 167.06 N, and 209.14 N for group 1, group 2, group 3, and group 4, respectively. There was a significant interaction between tooth and occlusal height of the prosthesis. The strain on the implant was significantly higher when it was 23 µm higher than the adjacent teeth, compared to the strain measured at the –46- or –23-µm levels. On the second premolar, the strain at the –23-µm level was significantly greater than that at the 23-µm level. In comparison with the 0-µm level, no implants or teeth showed a significant difference in strain related to the height of the implant. When the occlusal height of the implant gold crown increased
Table 3 and Fig 8 show the average absolute strain values and SDs for the four different groups according to the occlusal height of the implant prosthesis when biting bread. The mean strain values for the implant prosthesis were 3.76 N (group 1), 3.84 N (group 2), 5.75 N (group 3), and 8.50 N (group 4). There was a significant interaction between tooth and occlusal height of the implant prosthesis. With regard to tooth position, the occlusal load on the second premolar was significantly lower than the loads on the implant prosthesis and the second molar. With respect to prosthesis height, the occlusal load in group 4 was higher than those of the other groups. The strain on the implant with high occlusion was significantly larger than the strain seen with low- or even-occlusion configurations. On the second premolar, higher strain was measured when the implant prosthesis height was the same as the adjacent teeth compared to the other groups. When the occlusal height of the implant crown increased, the occlusal force transmitted to the implant increased. The graph of mean strain values for bread (Fig 8) showed a similar pattern to that obtained for carrot.
DISCUSSION A strain gauge is used to measure the strain on an object. In the field of dentistry, a strain gauge is used to analyze occlusal force.26–28 Fontijn-Tekamp et al27 used a strain gauge transducer to evaluate bite forces of mandibular implant-retained overdentures, and Soares et al29 evaluated the biomechanical behavior
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Cho et al
Table 3
Mean Strain Values and SDs for Bread (n = 20) Mean (SD) (N) Group 1
Group 2
Group 3
Group 4
F (P value)*
Second molar (a)
Tooth
6.34 (1.36)
5.59 (1.09)
5.71 (2.73)
4.83 (0.81)
Implant (b)
3.76 (1.70)
3.84 (2.77)
5.75 (2.27)
8.50 (2.17)
49.311† (.000) c < a, b‡
Second premolar (c)
1.82 (0.36)
2.00 (0.29)
2.49 (0.33)
1.98 (0.59)
F (P value)*
6.903 ∙ (.000)
3.575§ (.017)1, 2, 3 < 4‡
*Statistically significant if P < .05; †comparison of tooth effects; ‡result of multiple comparisons by Scheffé; §comparison of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Molar
30
Implant
Premolar
25 Strain (N)
of premolar teeth by strain measurement and strain distribution. When a strain gauge is attached to a tooth or prosthesis, occlusal stress can be evaluated directly without such devices as pressure-measuring sheets. When a strain gauge is attached to each tooth, it is possible to assess the bite force of each natural or prosthetic tooth independently while food is bitten or chewed.20,26,28–30 In this study, the strain gauge was attached to the buccal surfaces of the mandibular right second premolar, an implant crown in the first molar area, and the second molar. When compressive force was applied by a universal testing machine, the load applied to the occlusal surface and the amount of displacement were measured. The compressive force reproduced the masticatory force transmitted by the masticatory muscles, with the measured strain value indicating the occlusal force. Because the surface characteristics of the gold crown were different from those of the adjacent resin teeth, calibration was conducted with a load cell. The load cell was placed directly on the strain gauge, and the strain values of the load cell and the strain gauge were measured simultaneously. In this way, because both values were calculated, the load transferred to each tooth or the implant was calibrated regardless of the distance of the strain gauge from the occlusal surface. The aim of this study was to evaluate the changes in relative strain development in an implant and adjacent teeth when the implant prosthesis was in uniform occlusion, hypo-occlusion, or hyper-occlusion. To do this, all measurements were conducted in an identical test model using an adjustable-height apparatus. As a result, all other conditions were controlled, except for the implant prosthesis height and food type. After calibration, the change in relative strain development could be evaluated. Previous studies have found differences in occlusal forces according to race, sex, age, and skeletal type. Yoshinari et al31 and Strub et al32 reported that the average occlusal force exerted during mastication and swallowing was 40 N, whereas the maximum occlusal
20 15 10 5 0
–46 µm
–23 µm
0 µm
23 µm
Fig 8 Mean strain values for bread. The strain on the implant with high occlusion was significantly larger than on those with low or even occlusion. On the second premolar, larger strain was measured when the implant prosthesis height was the same as the adjacent teeth compared to the other groups. Two-way ANOVA: occlusal height, P = .017; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .037; implant, P = .000; second premolar, P = .018.
force on posterior teeth varied from 200 to 540 N. Ferrario et al7 studied single-tooth occlusal forces in healthy young adults with a strain gauge. They reported occlusal forces of 206 to 291 N for premolars, 234 to 306 N for first molars, and 221 to 294 N for second molars, with the subjects asked to clench maximally without food. Kim and Lee25 investigated the maximum fracture load of various foods with a universal testing machine. They reported that the mean fracture load of dried anchovy was 41 N, that of dried squid was 169 N, that of boiled crab was 331 N, and that of boiled chicken with bone was 382 N. On the basis of these data and pilot experiments, loads of 50 N for bread, 250 N for carrot, and 300 N for beef jerky were selected for this study. In the case of biting beef jerky, occlusal forces were evenly distributed to the adjacent teeth when the occlusal height of the implant prosthesis was increased. In the control group (group 3) and the hypo-occlusion groups (groups 1 and 2), the occlusal force applied to the implant prosthesis decreased but the force applied The International Journal of Oral & Maxillofacial Implants 1039
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Cho et al
to the adjacent teeth increased when the occlusal height of the implant prosthesis decreased. In other words, when the implant was in hypo-occlusion, occlusal forces were concentrated on the adjacent teeth. When pressure was applied on the beef jerky, the teeth flattened out the food bolus. A large share of the occlusal force seemed to be absorbed by the beef jerky, and the rest seemed to be transmitted to the teeth. Consequently, less than 300 N was recorded on the strain gauges. It should be considered that in actual conditions, the beef jerky would be torn and crushed by lateral excursion. However, in this study, only hinge movement was allowed by the one-directional vertical pressure of the universal testing machine. Carrot, in contrast to beef jerky, is more elastic and requires squashing. It demands elastic movement until it is smashed. Therefore, the actual occlusal force applied to the teeth would seem to be higher than the 300 N of force applied in the case of beef jerky. When the occlusal height of the implant prosthesis decreased, the occlusal force transmitted to the implant prosthesis decreased, but there was very little effect on the adjacent teeth. When the occlusal height of the implant prosthesis increased (group 4), the occlusal forces on it increased. The occlusal load was evenly distributed to the tooth surfaces, and greater occlusal force was transmitted to the posterior teeth via a leverage effect. Because carrot has elastic characteristics, a posterior tooth will withstand a large amount of pressure when biting this food, leading to a steplike distribution of strain. On the other hand, beef jerky can be flattened without breakage; thus, the strain distribution was different from other foods. Bread, which is a soft food, was easily torn when the experiments were repeated. Occlusal force was evenly distributed to the adjacent teeth until the bread was ground. Mastication of carrot and mastication of bread showed similar patterns of force distribution. Therefore, the occlusal height of an implant prosthesis would not be a critical issue in people who usually eat less leathery and relatively soft food, such as carrot and bread. However, for people who frequently eat tough food such as beef jerky, the height of any implant restorations should be adjusted carefully to avoid undesirable results. Although the occlusion of implant prostheses has been a highly controversial topic, few studies have examined the strain development on adjacent teeth and on the implant itself at various occlusal heights. In the case of chewing carrot and bread, through which occlusal forces are evenly transmitted to teeth, a decrease in the implant restoration’s occlusal height did not lead to harmful effects on adjacent teeth, which
seems to corroborate earlier theories.3,4 On the other hand, tough jerky, which is mangled during mastication, may give rise to hazardous loading when the occlusal height of the prosthesis is low. Because there was little difference between the 0-µm and the 23-µm prosthesis levels on the resultant strain with carrot or bread, equal occlusal height between adjacent teeth and implant restoration is recommended. There are several limitations to this study. First, the experimental model could not simulate proprioceptors, which are sensory receptors that respond to mechanical stimuli. Regulation of occlusal forces is not a simple somatic mechanism but a complex process involving the brain. The feedback mechanisms relevant to muscle spindles were excluded in this experimental model. In addition, the horizontal displacement of the premolars and molars was restrained by the hinge movement of the articulator, and the shear load to crush the food was not applied. Considering these limitations, further in vivo studies that would reproduce the lateral movement and feedback mechanism are warranted to confirm the results of this study.
CONCLUSIONS In this study, the following conclusions were reached. 1. For carrot and bread, foods that evenly distributed occlusal forces, increasing the height of the implant crown had a significant effect on the implant itself. 2. For tough food, such as beef jerky, decreased occlusal height resulted in concentration of the occlusal forces on the adjacent teeth, and the hypo-occlusion may have caused occlusal disharmony. 3. For patients with unhealthy periodontal tissue around adjacent teeth, the longevity of these teeth may be shortened when the occlusal height of the crown is lower than that of the adjacent teeth. Moreover, in patients with nocturnal bruxism or clenching, failure of osseointegration may occur if the implant prosthesis is taller than the adjacent teeth.
ACKNOWLEDGMENTS The authors would like to acknowledge Im-Hee Shin for providing statistical analysis of the manuscript. This work was supported by Ewha Womans University Research Grant no. 1-2011-1684001-2. The authors reported no conflicts of interest related to this study.
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REFERENCES 1. Carlsson GE. Dental occlusion: Modern concepts and their application in implant prosthodontics. Odontology 2009;97:8–17. 2. Rilo B, da Silva JL, Mora MJ, Santana U. Guidelines for occlusion strategy in implant-bone prostheses. A review. Int Dent J 2008;58:139–145. 3. Lundgren D, Laurell L. Biomechanical aspects of fixed bridgework supported by natural teeth and endosseous implants. Periodontol 2000 1994;4:23–40. 4. Davis SJ. Occlusal considerations in implantology: Good occlusal practice in implantology. Dent Update 2010;37:610620. 5. Lobbezoo F, Brouwers JE, Cune MS, Naeije M. Dental implants in patients with bruxing habits. J Oral Rehabil 2006;33:152–159. 6. Conrad HJ, Schulte JK, Vallee MC. Fractures related to occlusal overload with single posterior implants: A clinical report. J Prosthet Dent 2008;99:251–256. 7. Ferrario VF, Sforza C, Serrao G, Dellavia C, Tartaglia GM. Single tooth bite forces in healthy young adults. J Oral Rehabil 2004;31:18–22. 8. Misch CE. The effect of bruxism on treatment planning for dental implants. Dent Today 2002;21:76–81. 9. Rudd KD, O’Leary TJ, Stumpf AJ Jr. Horizontal tooth mobility in carefully screened subjects. Periodontics 1964;2:65–68. 10. Sekine H, Komiyama Y, Hotta H, Yoshida K. Mobility characteristics and tactile sensitivity of osseointegrated fixture-supporting systems. In: Van Steenberghe D (ed). Tissue Integration in Oral Maxillofacial Reconstruction. Amsterdam: Elsevier, 1986:306–332. 11. Shulte W. Implants and periosteum. Int Dent J 1995;45:16–26. 12. Misch CE. An implant is not a tooth: A comparison of periodontal indexes. In: Misch CE (ed). Dental Implant Prosthetics, ed 1. St Louis: Mosby, 2005:18-–31. 13. Jacobs R, van Steenberghe D. Comparison between implant-supported prostheses and teeth regarding passive threshold level. Int J Oral Maxillofac Implants 1993;8:549–554. 14. Hammerle CH, Wagner D, Bragger U. Threshold of tactile sensitivity perceived with dental endosseous implants and natural teeth. Clin Oral implants Res 1995;6:83–90. 15. Kim Y, Oh TJ, Misch CE, Wang HL. Occlusal considerations in implant therapy: Clinical guidelines with biomechanical rationale. Clin Oral Implants Res 2005;16:26–35. 16. Millstein P. Communication: A case against underocclusion. J Mass Dent Soc 2008;57:34. 17. Matsutani Y, Tanaka M, Kawazoe T. Effect of occlusal contact height in masticatory function and muscle activity in posterior unilateral osseointegrated implant bridge. J Jpn Soc Oral Implantol 2003;16:18–31.
18. Rottner K, Richter EJ. Effect of occlusal morphology on the accuracy of bite force measurements using thin film transducers. Int J Prostodont 2004;17:518–523. 19. Bachus KN, DeMarco AL, Judd KT, Horwitz DS, Brodke DS. Measuring contact area, force, and pressure for bioengineering applications: Using Fuji Film and Tekscan systems. Med Eng Phys 2006;28:483–488. 20. Sakaguchi RL, Brust EW, Cross M, DeLong R, Douglas WH. Independent movement of cusps during occlusal loading. Dent Mater 1991;7:186–190. 21. Hoshino K, Miura H, Morikawa O, Kato H, Okada D, Shinki T. Influence of occlusal height for an implant prosthesis on the periodontal tissues of the antagonist. J Med Dent Sci 2004;51:187–196. 22. Sohn BS, Heo SJ, Chang IT, Koak JY, Kim SK. A strain gauge analysis of implant-supported cantilevered fixed prosthesis under distal static load. J Korean Acad Prosthodont 2007;4:717–723. 23. Sarafidou K, Stiesch M, Dittmer MP, Borchers L, Kohorst P. Load-bearing capacity of implant-supported, tooth-supported, and combined zirconia-fixed dental prostheses. Implant Dent 2011;20:311–317. 24. Stiesch-Scholz M, Schulz K, Borchers L. In vitro fracture resistance of four-unit fiber-reinforced composite fixed partial dentures. Dent Mater 2006;22:374–381. 25. Kim JH, Lee JB. A comparative study on the correlation between Korean foods and fractures of PFG and all ceramic crowns for posterior application. J Korean Acad Prosthodont 2009;47:156–163. 26. Kim HK, Chang IT, Heo SJ, Koak JY, Yim SH. In vitro comparison of bite force and bending moment development with various occlusal surface materials used on implants. J Korean Dent Assoc 2004;42:232–238. 27. Fontijn-Tekamp FA, Slagter AP, van’t Hof MA, Geertman ME, Kalk W. Bite forces with mandibular implant-retained overdentures. J Dent Res 1998;77:1832–1839. 28. Glantz PO, Rangert B, Svensson A, et al. On clinical loading of osseointegrated implants. A methodological and clinical study. Clin Oral Implants Res 1993;4:99–105. 29. Soares PV, Santos-Filho PC, Gomide HA, Araujo CA, Martins LR, Soares CJ. Influence of restorative technique on the biomechanical behavior of endodontically treated maxillary premolars. Part II: Strain measurement and stress distribution. J Prosthet Dent 2008;99:114–122. 30. Toparli M, Gökay N, Aksoy T. An investigation of the stress values on a tooth restored by amalgam. J Oral Rehabil 1999;26:259–263. 31. Yoshinari M, Derand T. Fracture strength of all-ceramic crowns. Int J Prosthodont 1994;7:329–338. 32. Strub JR, Beschnidt SM. Fracture strength of 5 different all-ceramic crown systems. Int J Prosthodont 1998;11:602–609.
The International Journal of Oral & Maxillofacial Implants 1041 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
D
entists and researchers have debated the identification and application of concepts of occlusion to endosseous implants for many years.1–4 People have different mastication habits, chewing force, and chewing efficiency. Bruxism and clenching habits negatively affect the prognosis of endosseous implants because dental implants lack sufficient resistance to lateral
1Graduate
Student, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. 2 Associate Professor, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. 3 Professor, Department of Prosthodontics and Dental Research Institute, Seoul National University, Seoul, Korea. 4 Assistant Professor, Department of Prosthodontics, Ewha Womans University, Seoul, Korea. Correspondence to: Dr Ji-Man Park, Department of Prosthodontics, School of Medicine, Ewha Womans University, 911-1 Mok-5-dong, Yangcheon-gu, Seoul, 158-710, South Korea. Fax: +82-2-2650-5764. Email: [email protected] This paper was presented at the Academy of Osseointegration’s 27th Annual Meeting, March 1–3, 2012, Phoenix, Arizona.
©2014 by Quintessence Publishing Co Inc.
forces.5–8 Appropriate concepts of occlusion according to individual mastication characteristics may improve the long-term prognosis of implant restorations. Few randomized controlled trials have compared different occlusal schemes, and little research has focused on the occlusion of implant prostheses. Unlike natural teeth, osseointegrated implants are ankylosed to the surrounding bone without the periodontal ligament, which has mechanoreceptors and serves a shock-absorbing function.9–12 Because of the ankylotic character of implants, many clinicians prefer to hypoocclude dental implants to protect implant-supported restorations.2–4,13,14 The concept of light contact during heavy biting and no occlusal contact during light biting was recommended by Lundgren and Laurell3 and Kim et al.15 However, some authors have reported that hypo-occlusion may cause occlusal disharmony, with adjacent teeth receiving most of the occlusal force, and they suggested equal occlusion for implant crowns and any remaining natural teeth.16,17 There are several objective methods to measure masticatory functions, including bite force, electromyography, sound transmission, pressure-sensitive sheets, and strain gauges.18 Pressure-sensitive sheets
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Cho et al
a
b
c
Fig 1 Arrangement of teeth and simulation of the PDL in the models. (a) The occlusal surface of the dentiform model was indented with silicone impression material. (b) The PDL was simulated by coating silicone impression material around the roots of the acrylic resin teeth. (c) The resin teeth were then relocated in the putty index.
(eg, T-scan, Tekscan, and Prescale, FUJIFILM) measure occlusal force directly. However, they cannot measure occlusal forces during mastication.19 Strain gauges have been widely used to measure strain development around experimental objects. According to Sakaguchi et al,20 strain gauges bonded to external tooth surfaces can be used to study stress in tooth-related areas. Several studies have employed comparative finite element analyses and strain gauge measurement for the purpose of validation. These tools enable the assessment of bite force corresponding to a single natural or prosthetic tooth, unlike sheet-type pressure sensors.19,21 The goal of this study was to compare strain development in an implant prosthesis and adjacent teeth by using strain gauges with the prosthesis set at different occlusal heights.
MATERIALS AND METHODS Fabrication of Working Model
A test model was constructed with autopolymerizing polyurethane resin (POLYUROCK, Metalor Technologies) and artificial resin teeth (Nissin Dental Products Inc). Maxillary and mandibular teeth were arranged in maximum intercuspation, and group function was established in the dentiform model using an articulator (Hanau), with the mandibular right first molar missing. The arrangement of artificial teeth was confirmed using articulating paper (Accufilm, Parkell) and shim stock (Hanel). The occlusal surface of the dentiform model was indented with silicone impression material (Flexitime-putty type, Heraeus). Then the resin teeth were relocated in the putty index. Periodontal ligament (PDL) tissue was simulated with silicone impression material (Examix Fine, GC) around the roots of the resin teeth, which were embedded in a resin replica of the alveolar bone (Fig 1).22 The thickness of the PDL substitute was between 0.25 and 0.3 mm after the preliminary application of the PDL silicone with
the known PDL thickness, and the color of the material was harmonized by a well-trained operator.23,24 In the mandibular right first molar area, a single 4.0 × 10-mm implant (MK III, Nobel Biocare) was embedded and a screw-retained gold crown was created. All procedures, including cast fabrication, wax-up, casting, and finishing, were carried out in accordance with conventional protocols. The implant gold crown was placed into Class I occlusion by three-point contact with the maxillary teeth. To control the occlusal height of the implant gold crown, orthodontic expansion screws (Dentaurum) were used. The resin block containing the implantcrown assembly was sectioned into a 12 × 20 × 12mm (width × height × depth) segment, and two expansion screws were attached perpendicular to the bottom of the resin block on its buccal and lingual sides. These screw assemblies were fixed parallel to the axes of the occluding teeth in the opposing arch using a surveyor. In this setup, a turn of the expansion screw by 90 degrees moves the resin block 0.4 mm upward. Therefore, the expansion screws were turned by 5 degrees to create a 23-µm gap, which is the thickness of articulating paper (Fig 2). Four groups were assigned according to the occlusal height of the implant crown. The occlusal height of the crown was controlled by turning the screw, and the change in height was confirmed with articulating paper under 100 N of load applied with a universal testing machine (Model 3345, Instron Corp). • Group 1: The occlusal height of the implant gold crown was 46 µm lower than the adjacent teeth. • Group 2: The occlusal height of the implant gold crown was 23 µm lower than the adjacent teeth. • Group 3: The occlusal height of the implant gold crown was the same as that of the other teeth. This group served as the control. • Group 4: The occlusal height of the implant gold crown was 23 µm higher than the adjacent teeth. The International Journal of Oral & Maxillofacial Implants 1035
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Cho et al
a
b
c
Fig 2 Attachment of expansion screw and verification of change in occlusal height. (a) Two expansion screws were fixed parallel to the axis of occluding teeth in the opposite jaw at the buccal and lingual side. (b) The expansion screw was turned 5 degrees to create a 23-µm gap. (c) The height change was confirmed with Accufilm and shim stock.
a
b
Load cell
Strain gauge Circuit board Expansion screw
Fig 4 Compensation work with a load cell. The load cell was located between the maxilla and mandible over the teeth with the strain gauges, and compressive force was applied with a universal testing machine. The strain of the strain gauge and pressure of the load cell were read at the same time and the compensation work was done repeatedly for each tooth.
Measurement Procedure
Three strain gauges (KFG-2-350-C1-11, KYOWA Electronic Instruments) were attached to the buccal surface of the mandibular second premolar, implant crown in the mandibular first molar area, and mandibular second molar at the midpoint of the clinical crown, which was also directly under the mesiobuccal cusp. A dynamic signal–conditioning strain amplifier (CTA-1000, Curiotech Inc) was connected, and an analysis program (DA-1700B, Cas Korea) was used to measure, record, and process the strain values (Fig 3).
Fig 3 Strain gauges and amplifiers. (a) Three strain gauges were attached to the buccal surface of the mandibular right second premolar, the implant gold crown in the mandibular right first molar area, and the mandibular right second molar. Printed circuit boards (green rectangular plates) lengthened the wire and protected the strain gauges’ wires from disconnection. (b) A dynamic signal-conditioning strain amplifier and analysis program was used to measure, record, and process the strain values.
Prior to making the measurements, the model was calibrated to ensure it was free of residual stress, with strain gauges showing zero. Test food was inserted between the arches, over the teeth with strain gauges, and compressive force was applied with a universal testing machine. Compensation work was done with a load cell, which functioned as an electronic scale. Before masticatory action was initiated with the test food between the maxillary and mandibular teeth, a load cell was placed directly on the strain gauge, and the strain values of the load cell and the strain gauge were measured simultaneously while a masticatory (compressive) force of 100 N was applied with the universal testing machine (Fig 4). The electronic resistance value was measured, recorded, and then processed with an analysis program (DA-1700B); the value was read 3 seconds after the beginning of each masticatory cycle. In this way, the load transferred to the teeth or the implant was calibrated by calculating both values. Beef jerky, carrot, and bread were used as test foods. Each material was cut into 12 pieces with dimensions of 2 × 4.5 × 0.5 cm, so that the food specimens could be indented to match the occlusal surfaces of three teeth simultaneously (Fig 5). The testing machine was used to apply compressive forces of 300 N to the beef jerky, 250 N to the carrot, and 50 N to the bread, and the occlusal force was measured in each group.25 The experiment was carried out 12 times under conditions of different occlusal heights and food types, and 20 strokes of mastication were executed for each condition.
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Cho et al
Test food
Strain gauge
Circuit board Expansion screw
Fig 5 Insertion of test food.
Table 1 Mean Strain Values and SDs for Beef Jerky (n = 20) Mean strain (SD) (N) Tooth Second molar (a)
Group 1
Group 2
Group 3
172.51 (85.99)
126.99 (40.42)
105.44 (45.26)
Implant (b)
33.31 (17.54)
34.97 (30.74)
Second premolar (c)
81.66 (18.01)
139.54 (55.74)
89.77 (93) 89.41 (36.78)
Group 4
F (P value)*
97.59 (55.46) 100.27 (98.52) 94.53 (50.87)
4.608 ∙ (.000)
0.057§ (.982)
F (P value)
11.417† (.000) b < a, c‡
†comparison
§comparison
*Statistically significant if P < .05; of tooth effects; ‡result of multiple comparisons by Scheffé; of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Statistical Analysis
RESULTS Beef Jerky
Table 1 and Fig 6 show the average strain values and standard deviations (SDs) for the four different groups according to the occlusal height of the implant prosthesis when biting beef jerky. The mean strain values for the implant prostheses were 33.31 N, 34.97 N, 89.77 N, and 100.27 N for group 1, group 2, group 3, and group 4, respectively. There was a significant interaction between tooth position and occlusal height of the prosthesis. The strain on the second molar was
Molar
Implant
Premolar
300 250 Strain (N)
Two-way analysis of variance (ANOVA) (SPSS for Windows, v. 12.0, SPSS Inc) was performed to analyze the factors of occlusal height and tooth position by themselves and the interaction between these factors. Oneway ANOVA was used to evaluate the difference in load among teeth for each occlusal height. The Scheffé method was used as a post hoc test. Correction was done through the Scheffé multiple-comparisons test, and the strain values on the implant crown and adjacent teeth were compared. An α of .05 was considered to indicate statistical significance.
200 150 100 50 0
–46 µm
–23 µm
0 µm
23 µm
Fig 6 Mean strain values for beef jerky. The strain on the second molar was significantly higher when the implant prosthesis was at the –46-µm level than at the 0- or 23-µm levels. On the second premolar, statistically significant differences were found between the strain measured at –23 µm and that measured at the other levels. Two-way ANOVA: occlusal height, P = .982; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .033; implant, P = .061; second premolar, P = .017.
significantly higher when the implant prosthesis was at the –46-µm level than at the zero or 23-µm level. On the second premolar, statistically significant differences were found between the strain at the –23-µm level and the strain values measured at the other levels. When the occlusal height of the implant gold crown The International Journal of Oral & Maxillofacial Implants 1037
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Cho et al
Table 2 Mean Strain Values and SDs for Carrot (n = 20) Mean (SD) (N) Tooth
Group 1
Group 2
Group 3
Group 4
F (P value)*
Second molar (a)
177.08 (32.66)
156.55 (23.83)
165.17 (23.83)
138.32 (43.13)
Implant (b)
119.96 (38.4)
126.87 (25.18)
167.06 (50.58)
209.14 (63.99)
114.358† (.000) c < a, b‡
45.01 (17.6)
62.61 (6.35)
53.31 (9.78)
40.07 (9.09)
Second premolar (c)
1.520§
F (P value) †comparison
7.224 ∙ (.000)
(.215)
‡result
§comparison
*Statistically significant if P < .05; of tooth effects; of multiple comparisons by Scheffé; of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Molar
300
Implant
Premolar
Strain (N)
250 200 150 100
Bread
50 0
(group 4), the occlusal force applied to the implant prosthesis increased. However, alterations in the occlusal height of the implant prosthesis had little effect on the adjacent teeth, and the mean strain value graph (Fig 7) showed a stairway-like trend from the stack paper cutter–shaped masticatory pattern.
–46 µm
–23 µm
0 µm
23 µm
Fig 7 Mean strain values for carrot. The strain on the implant was significantly larger when it was 23 µm higher than the adjacent teeth compared to the strain measured at the –46- or –23-µm level. On the second premolar, the strain at the –23-µm level was significantly larger than that seen at the 23-µm level. In comparison with the 0-µm level, no implants or teeth showed a difference in strain related to the height of the implant. Twoway ANOVA: occlusal height, P = .215; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .100; implant, P = .001; second premolar, P = .002.
decreased (groups 1 and 2), occlusal forces were concentrated on the adjacent teeth.
Carrot
Table 2 and Fig 7 show the average absolute strain values and SDs according to the occlusal height of the prosthesis when biting carrot. The mean strain values for the implant prosthesis were 119.96 N, 126.86 N, 167.06 N, and 209.14 N for group 1, group 2, group 3, and group 4, respectively. There was a significant interaction between tooth and occlusal height of the prosthesis. The strain on the implant was significantly higher when it was 23 µm higher than the adjacent teeth, compared to the strain measured at the –46- or –23-µm levels. On the second premolar, the strain at the –23-µm level was significantly greater than that at the 23-µm level. In comparison with the 0-µm level, no implants or teeth showed a significant difference in strain related to the height of the implant. When the occlusal height of the implant gold crown increased
Table 3 and Fig 8 show the average absolute strain values and SDs for the four different groups according to the occlusal height of the implant prosthesis when biting bread. The mean strain values for the implant prosthesis were 3.76 N (group 1), 3.84 N (group 2), 5.75 N (group 3), and 8.50 N (group 4). There was a significant interaction between tooth and occlusal height of the implant prosthesis. With regard to tooth position, the occlusal load on the second premolar was significantly lower than the loads on the implant prosthesis and the second molar. With respect to prosthesis height, the occlusal load in group 4 was higher than those of the other groups. The strain on the implant with high occlusion was significantly larger than the strain seen with low- or even-occlusion configurations. On the second premolar, higher strain was measured when the implant prosthesis height was the same as the adjacent teeth compared to the other groups. When the occlusal height of the implant crown increased, the occlusal force transmitted to the implant increased. The graph of mean strain values for bread (Fig 8) showed a similar pattern to that obtained for carrot.
DISCUSSION A strain gauge is used to measure the strain on an object. In the field of dentistry, a strain gauge is used to analyze occlusal force.26–28 Fontijn-Tekamp et al27 used a strain gauge transducer to evaluate bite forces of mandibular implant-retained overdentures, and Soares et al29 evaluated the biomechanical behavior
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Cho et al
Table 3
Mean Strain Values and SDs for Bread (n = 20) Mean (SD) (N) Group 1
Group 2
Group 3
Group 4
F (P value)*
Second molar (a)
Tooth
6.34 (1.36)
5.59 (1.09)
5.71 (2.73)
4.83 (0.81)
Implant (b)
3.76 (1.70)
3.84 (2.77)
5.75 (2.27)
8.50 (2.17)
49.311† (.000) c < a, b‡
Second premolar (c)
1.82 (0.36)
2.00 (0.29)
2.49 (0.33)
1.98 (0.59)
F (P value)*
6.903 ∙ (.000)
3.575§ (.017)1, 2, 3 < 4‡
*Statistically significant if P < .05; †comparison of tooth effects; ‡result of multiple comparisons by Scheffé; §comparison of group effects; ∙ comparison of interaction (group × tooth) effects. Group 1 = hypo-occlusion with occlusal height of the implant gold crown 46 µm lower than adjacent teeth; group 2 = hypo-occlusion with occlusal height of the implant gold crown 23 µm lower than adjacent teeth; group 3 = equiocclusion with occlusal height of the implant gold crown the same as that of the other teeth; group 4 = hyper-occlusion with occlusal height of the implant gold crown 23 µm higher than adjacent teeth.
Molar
30
Implant
Premolar
25 Strain (N)
of premolar teeth by strain measurement and strain distribution. When a strain gauge is attached to a tooth or prosthesis, occlusal stress can be evaluated directly without such devices as pressure-measuring sheets. When a strain gauge is attached to each tooth, it is possible to assess the bite force of each natural or prosthetic tooth independently while food is bitten or chewed.20,26,28–30 In this study, the strain gauge was attached to the buccal surfaces of the mandibular right second premolar, an implant crown in the first molar area, and the second molar. When compressive force was applied by a universal testing machine, the load applied to the occlusal surface and the amount of displacement were measured. The compressive force reproduced the masticatory force transmitted by the masticatory muscles, with the measured strain value indicating the occlusal force. Because the surface characteristics of the gold crown were different from those of the adjacent resin teeth, calibration was conducted with a load cell. The load cell was placed directly on the strain gauge, and the strain values of the load cell and the strain gauge were measured simultaneously. In this way, because both values were calculated, the load transferred to each tooth or the implant was calibrated regardless of the distance of the strain gauge from the occlusal surface. The aim of this study was to evaluate the changes in relative strain development in an implant and adjacent teeth when the implant prosthesis was in uniform occlusion, hypo-occlusion, or hyper-occlusion. To do this, all measurements were conducted in an identical test model using an adjustable-height apparatus. As a result, all other conditions were controlled, except for the implant prosthesis height and food type. After calibration, the change in relative strain development could be evaluated. Previous studies have found differences in occlusal forces according to race, sex, age, and skeletal type. Yoshinari et al31 and Strub et al32 reported that the average occlusal force exerted during mastication and swallowing was 40 N, whereas the maximum occlusal
20 15 10 5 0
–46 µm
–23 µm
0 µm
23 µm
Fig 8 Mean strain values for bread. The strain on the implant with high occlusion was significantly larger than on those with low or even occlusion. On the second premolar, larger strain was measured when the implant prosthesis height was the same as the adjacent teeth compared to the other groups. Two-way ANOVA: occlusal height, P = .017; tooth position, P = .000; interaction, P = .000. One-way ANOVA: second molar, P = .037; implant, P = .000; second premolar, P = .018.
force on posterior teeth varied from 200 to 540 N. Ferrario et al7 studied single-tooth occlusal forces in healthy young adults with a strain gauge. They reported occlusal forces of 206 to 291 N for premolars, 234 to 306 N for first molars, and 221 to 294 N for second molars, with the subjects asked to clench maximally without food. Kim and Lee25 investigated the maximum fracture load of various foods with a universal testing machine. They reported that the mean fracture load of dried anchovy was 41 N, that of dried squid was 169 N, that of boiled crab was 331 N, and that of boiled chicken with bone was 382 N. On the basis of these data and pilot experiments, loads of 50 N for bread, 250 N for carrot, and 300 N for beef jerky were selected for this study. In the case of biting beef jerky, occlusal forces were evenly distributed to the adjacent teeth when the occlusal height of the implant prosthesis was increased. In the control group (group 3) and the hypo-occlusion groups (groups 1 and 2), the occlusal force applied to the implant prosthesis decreased but the force applied The International Journal of Oral & Maxillofacial Implants 1039
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Cho et al
to the adjacent teeth increased when the occlusal height of the implant prosthesis decreased. In other words, when the implant was in hypo-occlusion, occlusal forces were concentrated on the adjacent teeth. When pressure was applied on the beef jerky, the teeth flattened out the food bolus. A large share of the occlusal force seemed to be absorbed by the beef jerky, and the rest seemed to be transmitted to the teeth. Consequently, less than 300 N was recorded on the strain gauges. It should be considered that in actual conditions, the beef jerky would be torn and crushed by lateral excursion. However, in this study, only hinge movement was allowed by the one-directional vertical pressure of the universal testing machine. Carrot, in contrast to beef jerky, is more elastic and requires squashing. It demands elastic movement until it is smashed. Therefore, the actual occlusal force applied to the teeth would seem to be higher than the 300 N of force applied in the case of beef jerky. When the occlusal height of the implant prosthesis decreased, the occlusal force transmitted to the implant prosthesis decreased, but there was very little effect on the adjacent teeth. When the occlusal height of the implant prosthesis increased (group 4), the occlusal forces on it increased. The occlusal load was evenly distributed to the tooth surfaces, and greater occlusal force was transmitted to the posterior teeth via a leverage effect. Because carrot has elastic characteristics, a posterior tooth will withstand a large amount of pressure when biting this food, leading to a steplike distribution of strain. On the other hand, beef jerky can be flattened without breakage; thus, the strain distribution was different from other foods. Bread, which is a soft food, was easily torn when the experiments were repeated. Occlusal force was evenly distributed to the adjacent teeth until the bread was ground. Mastication of carrot and mastication of bread showed similar patterns of force distribution. Therefore, the occlusal height of an implant prosthesis would not be a critical issue in people who usually eat less leathery and relatively soft food, such as carrot and bread. However, for people who frequently eat tough food such as beef jerky, the height of any implant restorations should be adjusted carefully to avoid undesirable results. Although the occlusion of implant prostheses has been a highly controversial topic, few studies have examined the strain development on adjacent teeth and on the implant itself at various occlusal heights. In the case of chewing carrot and bread, through which occlusal forces are evenly transmitted to teeth, a decrease in the implant restoration’s occlusal height did not lead to harmful effects on adjacent teeth, which
seems to corroborate earlier theories.3,4 On the other hand, tough jerky, which is mangled during mastication, may give rise to hazardous loading when the occlusal height of the prosthesis is low. Because there was little difference between the 0-µm and the 23-µm prosthesis levels on the resultant strain with carrot or bread, equal occlusal height between adjacent teeth and implant restoration is recommended. There are several limitations to this study. First, the experimental model could not simulate proprioceptors, which are sensory receptors that respond to mechanical stimuli. Regulation of occlusal forces is not a simple somatic mechanism but a complex process involving the brain. The feedback mechanisms relevant to muscle spindles were excluded in this experimental model. In addition, the horizontal displacement of the premolars and molars was restrained by the hinge movement of the articulator, and the shear load to crush the food was not applied. Considering these limitations, further in vivo studies that would reproduce the lateral movement and feedback mechanism are warranted to confirm the results of this study.
CONCLUSIONS In this study, the following conclusions were reached. 1. For carrot and bread, foods that evenly distributed occlusal forces, increasing the height of the implant crown had a significant effect on the implant itself. 2. For tough food, such as beef jerky, decreased occlusal height resulted in concentration of the occlusal forces on the adjacent teeth, and the hypo-occlusion may have caused occlusal disharmony. 3. For patients with unhealthy periodontal tissue around adjacent teeth, the longevity of these teeth may be shortened when the occlusal height of the crown is lower than that of the adjacent teeth. Moreover, in patients with nocturnal bruxism or clenching, failure of osseointegration may occur if the implant prosthesis is taller than the adjacent teeth.
ACKNOWLEDGMENTS The authors would like to acknowledge Im-Hee Shin for providing statistical analysis of the manuscript. This work was supported by Ewha Womans University Research Grant no. 1-2011-1684001-2. The authors reported no conflicts of interest related to this study.
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Cho et al
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