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Summary Background and objectives Apart from the primary effect of bite-blocks on tooth position, they may also influence the mandibular condylar growth. Our aim was to investigate their influence on the condylar morphology, with variable masticatory forces. Material and methods Fifty-two 4-week old male rats were divided into hard and soft diet groups in order to create individuals with different masticatory muscle capacity. Two weeks later, they were equally divided into bite-block and control groups. After a total of 6 weeks experimental time, the animals were sacrificed. The mandibles were scanned with high-resolution micro-CT and 3D analysis was performed on the condylar neck and head of the condyle. The volume and the length of the condylar process were measured. Statistical analysis was done with a one-way analysis of variance. Results The use of bite-blocks decreased the length of the condylar process (P = 0.001) as well as the volume of the condylar neck (P = 0.001) and head (P = 0.006). The soft diet decreased the volume of the condylar neck (P < 0.001) and head (P < 0.001) two to three times more than the bite-blocks but did not affect the condylar process length. The interaction between the two variables was not statistically significant. Conclusions Both the bite-block appliance and weak masticatory muscle function reduced the volume at all regions of the condylar process, although the functional factor had a substantially greater effect. However, only the bite-block appliance affected the condylar process length. In the presence of both factors, an additive effect was found but no interaction detected. Introduction Posterior bite-blocks are functional appliances, which aim primarily at obtaining vertical dentoalveolar corrections. The effect of the appliance by lowering the mandible is interference with dental eruption, which leads to either decreased dental eruption or intrusion. The influence of the appliance on the vertical position of the posterior teeth results in an autorotation of the mandible, with changes in the skeletal relationship as seen by the decrease of the intermaxillary angle, and a possible influence in the skeletal growth pattern of the individual. Some associated changes have also been described, such as decrease of the gonial angle, the ANB angle and lower face height (1–4). However, to our knowledge, no clinical studies exist to elucidate if these changes influence the condylar growth and if the use of the posterior bite-block appliance may cause certain changes on the condylar morphology. An effort has been done to elucidate the skeletal adaptation of the mandible following posterior bite-block therapy on animal studies, using lateral cephalograms (5–7). In these 2D studies, a smaller lateral condylar process area and a decrease in the condylar inclination have been demonstrated (7), with an overall reduced mandibular ramus and condylar height (5). The posterior bite-block effect is caused by stretching the elevator muscles of the mandible. However, the functional capacity of these muscles varies a lot between individuals (8–10). Thus, it would be important to know if the bite-block effect depends on the functional capacity of these muscles. Previously, it has been shown that changes in the masticatory function may cause alterations in the condylar morphology. It was found that in rats fed a soft diet, the transversal width of the condylar head (11–13), the condylar head length (11) and the condylar height (13) were smaller than in animals fed normal diet. Our hypothesis was that the use of posterior bite-blocks may influence the 3D morphology of different regions of the condylar process, and these may respond differently depending on the functional capacity of the masticatory muscles. Thus, the aim of this study was to investigate, with a three-dimensional approach, the effects of bite-blocks on the condylar morphology in growing rats, in the context of variable functional capacities of masticatory muscles. Material and methods Animals Fifty-two young Sprague-Dawley rats were obtained at the age of 3 weeks and quarantined for 1 week before the start of the experiment, as was previously described (6). At week 4 of age they were split into two equal groups receiving either hard or soft diet. The hard diet consisted of ‘normal’ hard pellets (R34; Lactamin, Stockholm, Sweden) and the soft diet was made from the same pellets, ground to powder and mixed with water to a porridge texture. At week 6 of age they were again equally separated into two groups, a control group and an experimental group. The experimental group was fitted under anaesthesia with a permanently bonded resin bite-block on the three upper molars on both sides. This bite-block opened the bite by 1 mm at the third molar and 2 mm at the first molar (6). The rats wore this appliance for 4 weeks and at the age of week 10 they were sacrificed, the head dissected and preserved in 99% ethanol. Throughout the experimental period their weight was monitored weekly. Micro-CT The dissected mandibles were scanned by Quantum GX micro-CT imaging system, Perkin Elmer®, Waltham, Massachusetts, USA. The following settings were employed: 90 kV, 88 µA, FOV 36 mm, scan mode ‘High resolution’, scan time 14 min, 3D filter off, nominal resolution 9 microns. The scans were reconstructed using the integrated scanner software by choosing the condyle as the region of interest. This scan was then exported in DICOM format. Condylar process length, cross-sectional surface, and volume The DICOM files were analyzed in Osirix® software, Geneva, Switzerland. The scans were reoriented according to the base of the condylar process. This was defined by the tangent passing through the two deepest points of the two posterior mandibular notches: the point in the coronoid-condylar notch was marked as point A and the other in the condylar-angular notch as point B. The most remote point of the condyle head from the AB line was defined as the point S (Figure 1). The measurements performed included one-, two-, and three-dimensional data. Figure 1. View largeDownload slide Three-dimensional reconstructions of the mandible and the condylar process region, obtained by microcomputed tomography. Landmarks and items used for analysis are shown here. A: deepest point in the coronoid-condylar notch, B: deepest point in the condylar-angular notch, S: most remote point of the condyle head from the AB line. (a) Slices at the location of the 25th, 50th, and 75th percentiles. CS: cross-sectional surface. (b) Reconstructed volumes of the three regions of interest of the condylar process. VH: head volume, VN: neck volume, VB: base volume. Figure 1. View largeDownload slide Three-dimensional reconstructions of the mandible and the condylar process region, obtained by microcomputed tomography. Landmarks and items used for analysis are shown here. A: deepest point in the coronoid-condylar notch, B: deepest point in the condylar-angular notch, S: most remote point of the condyle head from the AB line. (a) Slices at the location of the 25th, 50th, and 75th percentiles. CS: cross-sectional surface. (b) Reconstructed volumes of the three regions of interest of the condylar process. VH: head volume, VN: neck volume, VB: base volume. The length of the condylar process was calculated as the orthogonal distance between S and the AB line. This one-dimensional distance was measured using the three-dimensional coordinates, thus taking into account the three dimensions unlike the two-dimension projection on a radiograph. The cross-sectional surfaces of the condylar process were traced and measured on every slice perpendicular to condylar length. We defined the 1st percentile as the closest slice from the S point, and the 100th percentile as the farthest slice, that is to say the condylar base. The 25th (CS25), 50th (CS50), and 75th (CS75) percentiles were used to evaluate three representative sections of the condylar process (Figure 1a). The volume of the condylar process was assessed in three regions (Figure 1b): 1. the condylar head, which constitutes the mandibular part of the temporomandibular joint, 2. the condylar neck, and 3. the condylar base, which links the condylar process to the mandibular ramus. The condylar process was divided into these three equal regions using the condylar process length. The volume was constructed using all the cross-sectional surfaces perpendicular to the condylar length. The 1–33 percentiles were used for the volume of the condylar head (VH), the 34–66 percentiles for the volume of the condylar neck (VN), and the 67–100 percentiles for the volume of the condylar base (VB). Statistical analysis The data were analyzed with SPSS Statistics, IBM Corp., Armonk, New York, USA. Each dependent variable was analyzed with a linear regression model including the following independent variables: 1. presence or absence of bite-block, 2. functional condition of the masticatory muscles according to the soft or hard diet and the possible interaction between these two factors. The level of significance was set at P <0.05. Method-error Random and systematic errors (Table 1) of the condylar process length, cross-sectional surface and volume were assessed on measurements, which were repeated with a 1-week interval on 30 rats. Random error was calculated with Dahlberg’s formula: Table 1. Calculation of method error: systematic error was assessed using a paired t-test, random error using Dahlberg’s formula, and the coefficient of reliability was calculated according to Houston (14). Variable Random error Systematic error (P value) Coefficient of reliability (%) Condylar process length (mm) 0.002 0.87 99.99 Cross-sectional surface (mm2) 0.005 0.69 99.99 Volume (mm3) 0.238 0.89 94.67 Variable Random error Systematic error (P value) Coefficient of reliability (%) Condylar process length (mm) 0.002 0.87 99.99 Cross-sectional surface (mm2) 0.005 0.69 99.99 Volume (mm3) 0.238 0.89 94.67 View Large Table 1. Calculation of method error: systematic error was assessed using a paired t-test, random error using Dahlberg’s formula, and the coefficient of reliability was calculated according to Houston (14). Variable Random error Systematic error (P value) Coefficient of reliability (%) Condylar process length (mm) 0.002 0.87 99.99 Cross-sectional surface (mm2) 0.005 0.69 99.99 Volume (mm3) 0.238 0.89 94.67 Variable Random error Systematic error (P value) Coefficient of reliability (%) Condylar process length (mm) 0.002 0.87 99.99 Cross-sectional surface (mm2) 0.005 0.69 99.99 Volume (mm3) 0.238 0.89 94.67 View Large Se=∑d22n where d stands for the difference between the repeated measures and n for the number of rats undergoing the replicate measurements. The coefficient of reliability was calculated as described by Houston (14): 1 – Se2/St2, where St is the total variance of performed measurements. The systematic error was assessed with a paired t-test. No systematic error was detected for any duplicated measurement. Coefficients of reliability were above 99.9% except for the volume measurements (94.67%), results related to low random errors: 0.002 mm, 0.005 mm2, and 0.238 mm3 for length, cross-sectional surface, and volume, respectively. Results The effects of a posterior bite-block appliance on the condyle morphology were investigated, in the context of different masticatory muscle capacity (Table 2). By using multiple regression analysis model, we examined the effects of each factor, as well as the possible interaction between these two factors. Table 2. Mean and standard deviations (SD) of the 7 measured variables (condylar process length, cross-sectional surfaces, and volumes) for each group. CH, control hard; CS, control soft; BH, bite-block hard; BS, bite-block soft. Variables CH CS BH BS Mean (SD) Mean (SD) Mean (SD) Mean (SD) Length (mm) 6.06 (0.22) 6.06 (0.19) 5.77 (0.32) 5.90 (0.21) Cross-sectional surfaces 25th percentile (mm2) 3.01 (0.22) 2.04 (0.40) 2.56 (0.43) 1.76 (0.39) 50th percentile (mm2) 2.54 (0.20) 1.64 (0.20) 2.28 (0.33) 1.59 (0.29) 75th percentile (mm2) 2.91 (0.15) 1.99 (0.15) 2.81 (0.59) 1.96 (0.32) Volumes Head (mm3) 5.00 (0.37) 3.99 (0.57) 4.46 (0.74) 3.58 (0.63) Neck (mm3) 5.47 (0.45) 3.52 (0.49) 4.67 (0.59) 3.28 (0.59) Base (mm3) 6.31 (0.38) 4.42 (0.41) 5.69 (1.16) 4.37 (1.13) Variables CH CS BH BS Mean (SD) Mean (SD) Mean (SD) Mean (SD) Length (mm) 6.06 (0.22) 6.06 (0.19) 5.77 (0.32) 5.90 (0.21) Cross-sectional surfaces 25th percentile (mm2) 3.01 (0.22) 2.04 (0.40) 2.56 (0.43) 1.76 (0.39) 50th percentile (mm2) 2.54 (0.20) 1.64 (0.20) 2.28 (0.33) 1.59 (0.29) 75th percentile (mm2) 2.91 (0.15) 1.99 (0.15) 2.81 (0.59) 1.96 (0.32) Volumes Head (mm3) 5.00 (0.37) 3.99 (0.57) 4.46 (0.74) 3.58 (0.63) Neck (mm3) 5.47 (0.45) 3.52 (0.49) 4.67 (0.59) 3.28 (0.59) Base (mm3) 6.31 (0.38) 4.42 (0.41) 5.69 (1.16) 4.37 (1.13) View Large Table 2. Mean and standard deviations (SD) of the 7 measured variables (condylar process length, cross-sectional surfaces, and volumes) for each group. CH, control hard; CS, control soft; BH, bite-block hard; BS, bite-block soft. Variables CH CS BH BS Mean (SD) Mean (SD) Mean (SD) Mean (SD) Length (mm) 6.06 (0.22) 6.06 (0.19) 5.77 (0.32) 5.90 (0.21) Cross-sectional surfaces 25th percentile (mm2) 3.01 (0.22) 2.04 (0.40) 2.56 (0.43) 1.76 (0.39) 50th percentile (mm2) 2.54 (0.20) 1.64 (0.20) 2.28 (0.33) 1.59 (0.29) 75th percentile (mm2) 2.91 (0.15) 1.99 (0.15) 2.81 (0.59) 1.96 (0.32) Volumes Head (mm3) 5.00 (0.37) 3.99 (0.57) 4.46 (0.74) 3.58 (0.63) Neck (mm3) 5.47 (0.45) 3.52 (0.49) 4.67 (0.59) 3.28 (0.59) Base (mm3) 6.31 (0.38) 4.42 (0.41) 5.69 (1.16) 4.37 (1.13) Variables CH CS BH BS Mean (SD) Mean (SD) Mean (SD) Mean (SD) Length (mm) 6.06 (0.22) 6.06 (0.19) 5.77 (0.32) 5.90 (0.21) Cross-sectional surfaces 25th percentile (mm2) 3.01 (0.22) 2.04 (0.40) 2.56 (0.43) 1.76 (0.39) 50th percentile (mm2) 2.54 (0.20) 1.64 (0.20) 2.28 (0.33) 1.59 (0.29) 75th percentile (mm2) 2.91 (0.15) 1.99 (0.15) 2.81 (0.59) 1.96 (0.32) Volumes Head (mm3) 5.00 (0.37) 3.99 (0.57) 4.46 (0.74) 3.58 (0.63) Neck (mm3) 5.47 (0.45) 3.52 (0.49) 4.67 (0.59) 3.28 (0.59) Base (mm3) 6.31 (0.38) 4.42 (0.41) 5.69 (1.16) 4.37 (1.13) View Large Length of the condylar process The use of bite-block appliance showed a coefficient of −0.29 in the condylar process length (P = 0.003), indicating a shorter length for the animals wearing the appliance (Table 3). The soft diet did not affect the length of the condylar process and there was no interaction between the two independent variables. The model explained 22% of the variation in the condylar process length (P = 0.008). Table 3. Condylar process length. R-squared = 0.22; Model P value = 0.008. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: condylar process length. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value Constant 6.06 0.07 <0.001 Function −0.00 0.09 0.982 Appliance −0.29 0.09 0.003 Interaction 0.13 0.13 0.342 Coefficient Standard error P value Constant 6.06 0.07 <0.001 Function −0.00 0.09 0.982 Appliance −0.29 0.09 0.003 Interaction 0.13 0.13 0.342 View Large Table 3. Condylar process length. R-squared = 0.22; Model P value = 0.008. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: condylar process length. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value Constant 6.06 0.07 <0.001 Function −0.00 0.09 0.982 Appliance −0.29 0.09 0.003 Interaction 0.13 0.13 0.342 Coefficient Standard error P value Constant 6.06 0.07 <0.001 Function −0.00 0.09 0.982 Appliance −0.29 0.09 0.003 Interaction 0.13 0.13 0.342 View Large Cross-sectional surfaces In the section CS25, in the region of the condylar head, the use of bite-block appliance showed a coefficient of −0.44 (P = 0.004), indicating a smaller cross-sectional area in the animals wearing the appliance (Table 4). Feeding the animals soft diet showed a coefficient of −0.97 (P < 0.001), indicating smaller cross-sectional area in the animals with the reduced masticatory function. Sixty-five percent of the variation in the cross-sectional measurements was explained by the model (P < 0.001). Table 4. Cross-sectional surfaces. In 25th percentile, R-squared = 0.65; model P value <0.001. In 50th percentile, R-squared = 0.73; model P value <0.001. In 75th percentile, R-squared = 0.64; model P value <0.001. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: cross-sectional surface. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value a. 25th percentile Constant 3.01 0.10 <0.001 Function −0.97 0.14 <0.001 Appliance −0.44 0.14 0.004 Interaction 0.16 0.20 0.437 b. 50th percentile Constant 2.54 0.07 <0.001 Function −0.90 0.10 <0.0001 Appliance −0.26 0.10 0.014 Interaction 0.20 0.14 0.169 c. 75th percentile Constant 2.91 0.10 <0.001 Function −0.92 0.14 <0.001 Appliance −0.10 0.14 0.487 Interaction 0.07 0.19 0.736 Coefficient Standard error P value a. 25th percentile Constant 3.01 0.10 <0.001 Function −0.97 0.14 <0.001 Appliance −0.44 0.14 0.004 Interaction 0.16 0.20 0.437 b. 50th percentile Constant 2.54 0.07 <0.001 Function −0.90 0.10 <0.0001 Appliance −0.26 0.10 0.014 Interaction 0.20 0.14 0.169 c. 75th percentile Constant 2.91 0.10 <0.001 Function −0.92 0.14 <0.001 Appliance −0.10 0.14 0.487 Interaction 0.07 0.19 0.736 View Large Table 4. Cross-sectional surfaces. In 25th percentile, R-squared = 0.65; model P value <0.001. In 50th percentile, R-squared = 0.73; model P value <0.001. In 75th percentile, R-squared = 0.64; model P value <0.001. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: cross-sectional surface. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value a. 25th percentile Constant 3.01 0.10 <0.001 Function −0.97 0.14 <0.001 Appliance −0.44 0.14 0.004 Interaction 0.16 0.20 0.437 b. 50th percentile Constant 2.54 0.07 <0.001 Function −0.90 0.10 <0.0001 Appliance −0.26 0.10 0.014 Interaction 0.20 0.14 0.169 c. 75th percentile Constant 2.91 0.10 <0.001 Function −0.92 0.14 <0.001 Appliance −0.10 0.14 0.487 Interaction 0.07 0.19 0.736 Coefficient Standard error P value a. 25th percentile Constant 3.01 0.10 <0.001 Function −0.97 0.14 <0.001 Appliance −0.44 0.14 0.004 Interaction 0.16 0.20 0.437 b. 50th percentile Constant 2.54 0.07 <0.001 Function −0.90 0.10 <0.0001 Appliance −0.26 0.10 0.014 Interaction 0.20 0.14 0.169 c. 75th percentile Constant 2.91 0.10 <0.001 Function −0.92 0.14 <0.001 Appliance −0.10 0.14 0.487 Interaction 0.07 0.19 0.736 View Large In midsection CS50, the same pattern was observed, as bite-block appliance had a coefficient of −0.26 (P = 0.014), and the soft diet showed a coefficient of −0.90 (P < 0.001). Seventy-three percent of the variation of the cross-sectional surface was explained by the model (P < 0.001). In the section CS75, the use of the appliance did not seem to influence the size of the measured area. However, the soft diet showed a similar coefficient as the above described sections, of −0.92 (P < 0.001). The model explained 64% of the variation of the results in this cross-sectional surface (P < 0.001). No interaction between the use of the posterior bite-block appliance and the consistency of diet was detected at any of above-mentioned models. Volumes In the condylar head region, the use of the bite-block appliance showed a coefficient of −0.54 (P = 0.025), corresponding to a smaller bone volume in the animals wearing the appliance (Table 5). Decrease of functional demand showed a coefficient of −1.01 (P < 0.001), also indicating a smaller volume for the animals with weak masticatory muscles. There was no interaction between the appliance and the reduced function. The model in this region explained 46% of the variations in the volume measurements (P < 0.001). Table 5. Volumes. In condylar head region, R-squared = 0.46; model P value < 0.001. In neck region, R-squared = 0.75; model P value <0.001. In base region, R-squared = 0.51; model P value <0.001. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: volume. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value a. Condylar head region Constant 5.00 0.16 <0.001 Function −1.01 0.23 <0.001 Appliance −0.54 0.23 0.025 Interaction 0.13 0.33 0.686 b. Neck region Constant 5.47 0.15 <0.001 Function −1.95 0.21 <0.001 Appliance −0.80 0.21 <0.001 Interaction 0.55 0.30 0.068 c. Base region Constant 6.31 0.24 <0.001 Function −1.88 0.34 <0.001 Appliance −0.61 0.34 0.075 Interaction 0.56 0.47 0.248 Coefficient Standard error P value a. Condylar head region Constant 5.00 0.16 <0.001 Function −1.01 0.23 <0.001 Appliance −0.54 0.23 0.025 Interaction 0.13 0.33 0.686 b. Neck region Constant 5.47 0.15 <0.001 Function −1.95 0.21 <0.001 Appliance −0.80 0.21 <0.001 Interaction 0.55 0.30 0.068 c. Base region Constant 6.31 0.24 <0.001 Function −1.88 0.34 <0.001 Appliance −0.61 0.34 0.075 Interaction 0.56 0.47 0.248 View Large Table 5. Volumes. In condylar head region, R-squared = 0.46; model P value < 0.001. In neck region, R-squared = 0.75; model P value <0.001. In base region, R-squared = 0.51; model P value <0.001. Regression analysis with formula: y = const. + x1Function + x2Appliance + x1x2Interaction. Function: 0 = hard diet; 1 = soft diet. Appliance: 0 = control; 1 = bite-block. Dependent variable: volume. Values in bold signify statistical significance (P <0.05). Coefficient Standard error P value a. Condylar head region Constant 5.00 0.16 <0.001 Function −1.01 0.23 <0.001 Appliance −0.54 0.23 0.025 Interaction 0.13 0.33 0.686 b. Neck region Constant 5.47 0.15 <0.001 Function −1.95 0.21 <0.001 Appliance −0.80 0.21 <0.001 Interaction 0.55 0.30 0.068 c. Base region Constant 6.31 0.24 <0.001 Function −1.88 0.34 <0.001 Appliance −0.61 0.34 0.075 Interaction 0.56 0.47 0.248 Coefficient Standard error P value a. Condylar head region Constant 5.00 0.16 <0.001 Function −1.01 0.23 <0.001 Appliance −0.54 0.23 0.025 Interaction 0.13 0.33 0.686 b. Neck region Constant 5.47 0.15 <0.001 Function −1.95 0.21 <0.001 Appliance −0.80 0.21 <0.001 Interaction 0.55 0.30 0.068 c. Base region Constant 6.31 0.24 <0.001 Function −1.88 0.34 <0.001 Appliance −0.61 0.34 0.075 Interaction 0.56 0.47 0.248 View Large In the condylar neck region, the two independent variables showed similar pattern as in the condylar head region. The use of the bite-block appliance had a coefficient of −0.80 (P < 0.001), and reduced function after use of soft diet presented a coefficient of −1.95 (P < 0.001). Interestingly, we observed a tendency of interaction between these two factors, decreasing the negative effect they had each of them separately on the volume of the condylar neck, though this interaction did not reach the statistical significant level (P = 0.068). Seventy-five percent of the variation of the volume of this region was explained by the model (P < 0.001). In the condylar base region, the use of the appliance seemed to decrease the bone volume although this effect did not reach the statistical significant level. The decreased functional demand showed a coefficient of −1.88 (P < 0.001), indicating less bone volume in the animals fed a soft diet. There was no interaction between the two independent factors. The model explained 51% of the variation of the volume measured in this region (P < 0.001). Discussion The present study has shown that the two tested variables, i.e. the use of posterior bite-blocks and the reduced masticatory muscle function, both decreased the cross-sectional surfaces and volumes of the condylar process. In contrast to these findings, the condylar process length was reduced by the use of the posterior bite-block while no effect was measured on the animals with different functional demands. The tested variables had an additive effect on all the performed measurements, and no clear interaction was found between them. Our results, dealing with the use of posterior bite-blocks, are in accordance with the previous work of Mavropoulos et al. (7), who found that bite-blocks reduced the lateral surface area of the condylar process and its inclination, and the findings of McNamara (15), who reported a decrease in vertical growth of the condyle. Furthermore, our results are also in line with the findings of previous studies on the condylar size, after alteration of masticatory function by feeding the animals a soft diet (11,12,16). The present study had the possibility to investigate different regions of the condylar process and examine the importance of the tested factors on each region. Wearing the bite-block appliance led to a 5% shorter condylar process. The appliance changes the position of the condyle, probably altering the mechanical loads on the structure and therefore affecting the morphology and growth of the condyle. We can speculate that the bite-blocks may reduce condylar growth and therefore should be used preferably in patients with an Angle class III tendency. Clinical studies are needed to verify these findings. We measured cross-sectional surfaces of the condylar process, and calculated the bone volume of three defined regions of this process based on the consecutive series of sections. The minor differences observed in the condylar process length were not expected to influence the distribution of the condylar process regions, which is the reason why the patterns of change in surfaces and volumes are similar between the groups. The bite-blocks affected the cross-sectional surfaces and volumes of the condylar process. The most reduced region was the head and neck of the condylar process. However, the influence of the bite-block on the bone volume was much less than the one induced by the decreased functional capacity of the masticatory muscles, after feeding the animals soft diet. Indeed, the soft diet decreased the surface and volume 2–3 times more than the bite-blocks. A possible explanation on the effect of the posterior bite-blocks in the reduction of bone volume of the condylar process is likely the reduced muscle activity of the masticatory muscles induced after the insertion of the appliance, as it has been shown in clinical and animal experimental studies (17–20). However, the decrease of this activity seems to be significantly lower than the one induced by changing the consistency of the diet. Not all regions of the condylar process have been influenced equally by the appliance and the reduced muscular activity. It is possible that the observed bone volume differences affected mainly the condylar neck, i.e. the region of the condylar process that under normal conditions (hard diet) is the most exposed one to bending forces during functional loading. Thus, an adaptation of this region may have occurred for the animals with higher functional demands fed a hard diet by increasing bone apposition and bone volume. The other possible physiologic procedure to reinforce a bone structure by increasing the bone density have not been detected by previous study in this region (21). Another explanation of the differences observed in the region of the condylar head is the effect of the appliance and of the reduced function on the growth of the cartilage. Previous studies have shown a substantial reduction in the thickness of the condylar cartilage and the subchondral bone volume in animals with reduced functional demand in contrast to animals fed a hard diet (5,21,22). The base of the condylar process that was used as a reference point for comparison of the condylar processes relies on the coronoid-condylar and the condylar-angular notches. It cannot be excluded that experimental conditions may have induced morphological changes, which may have influenced the position of these notches. However, our intention was to evaluate the form of the entire condylar process, as it can be defined by these accurate landmarks, to separate the condylar process from the mandibular ramus. Therefore, we considered separating the process in three regions and calculated separately their volume, as well as compared the 25th, 50th, and 75th percentiles. We have shown that bite-block appliances in rats reduce the cross-sectional surface and volume, as well as the total length of the condylar process. The soft diet reduces the cross-sectional surface and volume of the condylar process by a factor 2–3 times larger than the bite-blocks, though it does not affect the condylar length. The tested variables had an additive effect on all the above measurements, but no clear interaction was found between them. Conflict of interest None to declare. References 1. Kuster , R. and Ingervall , B . ( 1992 ) The effect of treatment of skeletal open bite with two types of bite-blocks . 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Google Scholar PubMed 21. Bresin , A. , Kiliaridis , S. and Strid , K.G . ( 1999 ) Effect of masticatory function on the internal bone structure in the mandible of the growing rat . European Journal of Oral Sciences , 107 , 35 – 44 . Google Scholar CrossRef Search ADS PubMed 22. Chen , J. , Sorensen , K.P. , Gupta , T. , Kilts , T. , Young , M. and Wadhwa , S . ( 2009 ) Altered functional loading causes differential effects in the subchondral bone and condylar cartilage in the temporomandibular joint from young mice . Osteoarthritis and Cartilage , 17 , 354 – 361 . Google Scholar CrossRef Search ADS PubMed © The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

The European Journal of Orthodontics – Oxford University Press

**Published: ** Oct 11, 2017

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