= = = = = = = = = = ANNALS Of ANATOMY = = = = = = = = = =
Influence of asymmetric occlusal relationships and decreased maxillary width on the growth of the facial skeleton in the guinea pig Kauko P. Isotupa*, David S. Carlson** and Kauko K. Makinen*
* Department of Biologic and Materials Sciences, School of Dentistry, and ** Center for Human Growth and Development, The University of Michigan, Ann Arbor, Michigan 48109-1078, U.S.A.
Summary. The purpose of this study was to evaluate the effect of muscle function and occlusal form on mandibular growth in guinea pigs. We hypothesized that restriction of maxillary width and asymmetric function of the mandible would lead to mandibular asymmetry. The hard palate of 12 animals was exposed at the age often days, and cyanocrylate was used to close the midpalatal suture in order to restrict transverse maxillary growth. The right incisors and right molars were ground obliquely at the time of surgery and every two weeks thereafter until the animals were sacrificed 4, 8 and 12 weeks later (four animals in each time group). Six age-matched animals served as controls. Dorsoversal xrays were taken at sacrifice, followed by the removal of small biopsies from six different sites of the maxillomandibular skeleton for biochemical studies, which will be reported separately. Relative to controls, the treated animals exhibited a narrow maxilla and asymmetry in the height of the ramus and in the length of the mandible from the mental foramen to the angular process. The maximum width between the angular processes of the mandible was extremely narrow, as was the condylar neck especially on the right side. It was concluded that abnormal masticatory muscle function caused by occlusal deviation led to a narrowing and a slight asymmetry of the mandible in the growing guinea pig.
complex and dentition is also often associated with an excessively narrow palate. According to some investigators, malocclusion characterized by transverse palatal restriction and asymmetry of the facial skeleton results primarily from asymmetric function of the muscles of mastication among children with parafunctional habits such as thumb-sucking and mouth-breathing (Linder-Aronson 1970). In animals with continuously erupting teeth like guinea pigs and rats, it has been noted that intermittent, chronic activity of the jaw elevator muscles, particularly the medial pterygoid muscles, is necessary to wear the occlusal surfaces and keep the teeth from supererupting. It has also been demonstrated that sutural growth can be restricted by using cyanoacrylate across the suture in young rats, guinea pigs and rabbits (Persson et al. 1979; Babler and Persing 1982).
Key words: Bone growth - Facial skeleton - Guinea pig
The purpose of this study was to evaluate the effect of transverse restriction of maxillary growth and occlusal asymmetry on the growth of the maxillomandibular complex. Manipulation of the tooth surfaces in the guinea pigs was done in order to produce asymmetric occlusal interferences that would encourage alteration of muscle activity because of the need for increased tooth wear and deviation of the mandible. Restriction of maxillary-palatal growth was done in order to emphasize the effective discrepancy between the upper and lower dentition and thus to enhance asymmetric function.
Introduction
Materials and methods
Asymmetric growth of the mandible may occur as an idiopathic syndrome or following minor trauma to the temporomandibular joint. Asymmetry of the maxillomandibular
Eighteen male guinea pigs were divided into experimental (n = 12) and control groups (n = 6). At ten days of age all animals were anaesthetized (5 mg/kg Rompum plus 30 mglkg Ketamine 1M,
~I
Ann. Anat. (1992) 174: 447-451 Gustav Fischer Verlag Jena
followed by 15 - 30 mg/kg Pentobarbital sodium IP) and positioned in a specially designed cephalostat in order to obtain standardized dorsoventral radiocephalograms. Control animals were returned to their cages immediately after x-raying . In experimental animals, the mouth was propped open and a 1 cm incision was made longitudinally in the midline of the palate. The midpalatal suture was cleared of periosteum using a periosteal elevator. Cyanoacrylate glue was injected in the soft sutural tissue and onto the bone surfaces across the midpalatal suture. The glue was allowed to dry for two minutes, after which time the incision was closed by pressing the flaps against the palatal bones. No sutures were used in closing the incision. With the mouth remaining propped open, a dental handpiece and a carborundum grinding wheel were used to grind away the buccal aspect of the lower molars and the palatal aspect of the upper molars on the right side; the contacts between the upper and lower molars were preserved. The incisors were ground at an angle slanting up toward the left (Fig. lC). The dental grinding was repeated under anaesthesia every two weeks and a dorsoventral cephalogram was taken every four weeks . The guinea pigs were fed normal solid food pellets, hay and water ad libitum. Animals were weighed weekly. Four experimental and two control animals were killed at 4, 8 and 12 weeks postoperatively via an overdose of Pentobarbital sodium. The mandible was detached and contact lateral radiograms were taken from the bisected halves of the mandible. The remaining soft tissues were carefully removed, after which samples of bone and cartilage were taken for subsequent biochemical studies (Isotupa et al. 1992).
A total of 11 linear and angular measurements were taken from the dry skulls or from the cephalograms using a Helios digital caliper (0.1 mm accuracy) (Table 1; Fig. lA, B). Mean values were calculated for comparison of intragroup craniometric differences between right and left sides and intergroup differences between experimental and control animals . No more extensive statistical analysis was possible due to the number of animals in the study.
A
e
L -_ _ _ _ _ _ _ _
5 ---
Table 1. Craniometric measurements taken from dry skulls, dorsoventral cephalometric x-rays, and lateral contact radiographs of the mandible and lateral x-rays of the entire skull.
Dry Skull 1. palatal width between the right and left first molars 2. palatal length from the foramen incisivum to the posterior margin of the palatal bone Dorsoventral Cephalograms
3. maximum length of the skull 4. maximum width of the mandible Lateral Radiograms of the Mandible 5. maximum height of the mandible measured perpendicularly from the mandibular line to the highest point of the condyle , right and left 6. distance from the foramen mentale to the most distal end of the angular process 7. width of the condylar neck just below the condylar cartilage 8. height of the angular process Lateral Radiograms of the Mandible and of the Skull
9. angle between the mandibular baseline and the mesial tangent to the first molar 10. angle between the anterior mandibular base line and the occlusal plane 11. angle between the anterior mandibular line and the anterior cranial line
Fig. 1. Panels A and B: Representation of the dimensional measurements performed in the skulls and the mandibles . Panel C: diagram indicating the procedure used to modify the dentition of the animals in the experimental group. The right molars and the upper and lower incisors were ground obliquely using a dental handpiece to encourage the mandible to deviate to the left upon closure. The arrow shows the direction of occlusion as a result of grinding.
448
Results
from the treated groups was one of the most strongly affected guinea pigs , and died after one week) .
Clinical Observations The guinea pigs in the experimental group lagged slightly behind the controls .in weight gain, though this was variable. Two animals in the experimental group seemed to be affected more than the others early in the experiment. The first died after one week. The weight gain of the other was dramatically less than that of other experimental animals at the 4-week interval. Another experimental animal died during anaesthesia at the 8-week interval. The skulls of these three guinea pigs were found to be abnormal. Their teeth were overerupted and the bite was opened. In addition, the posterior part of the lower jaw was found to be very narrow and the angular processes were resorbed totally on both sides.
Cephalometric Analysis (Table 2) The dorsoventral cephalograms did not reveal any major asymmetry in the shape of the skulls in the experimental or in the control animals. The lenght of the skull was normal in the experimental group. However, the width of the posterior mandible was more narrow in the experimental group than in the control animals. The ratio between the length of the skull and the width of the mandible was substantially higher in experimental animals than in controls. This difference was most obvious at the 4-week interval, but was also notable at the 8- and 12-week intervals . Measurements from the lateral head films indicated that the angle between the anterior line of the mandible and the anterior cranial line of the skull was smaller in the experimental group. The orientation of the occlusal plane in the experimental animals was also different: it was essentially parallel to the mandibular line while the corresponding angle in control animals averaged 2.5 0 in the 4-week group and 4.7 0 in the 12-week group. The angular measurements from the lateral mandibular radiograms showed that the lower molars leaned mesially in the experimental groups at the 4- and 8-week intervals, but were normally upright again by the 12-week interval. The length of the mandible from the mental foramen to condylion was similar to control animals . However, the height of the mandible was less in experimental animals as was the distance from the mental foramen to the tip of the angular process. The condylar neck was more narrow in the experimental group than in the controls. Comparing measurements from the right and the left sides of the mandible in experimental animals , it was found that (1) the condylar neck was narrower on the right side than on the left side, (2) the height of the mandible on the right side had a tendency to be lower than on the left, and (3) the length from the mental foramen to the tip of the angular process was lower on the left side than the right. Fig. 2 shows the ventral and lateral views of skulls from the control and the treated group (the animal
Table 2. Cephalometric data of treated and control animals determined 4, 8 and 12 weeks after the beginning of the test. Time (weeks)
Control animals
Treated animals
Mean
S.D.
Mean
S.D.
0.35 0.07 0.35
0.45 0.80 0.97
0.01 0.24 0.29
Palatal Width (mm) 4 8 12
0.75 1.50 2.65
Maximum Height of the Mandible (mm) Right 4 8 12
16.29 16.39 17.05
0.25 1.12 0.06
15 .10 16.33 16.55
0.11 0.65 0.85
Left 4 8 12
16.55 16.39 17.17
0.06 1.12 0. 13
15 .28 16.71 16.65
0.08 0.47 0.75
Mandibular Length (mm) Right 4 8 12
36.40 36.95 39.75
0.14 0.13 0.03
35 .10 36.65 37.60
0.14 0.11 0.17
Left 4 8 12
36.35 37.05 39.60
0.12 0.09 0.00
35.45 37.08 38.15
0.Q2 0.17 0.22
6.22 6.16 6.23
0.43 0.29 0.37
58.77 62 .96 67 .13
1.85 1.07 1.39
Maximum Width of the Mandible (mm) 4 25.15 27.43 1.03 8 29 .17 1.19 27 .51 30.85 12 1.0 29.40
0.49 0.91 1.39
Condylar Neck Width (mm) 4 8 12
6.17 6.65 6.80
0 .22 0.47 0.37
Maximum Length of the Skull (mm) 4 59.65 2.39 63.45 8 1.73 12 67.13 1.76
Skull Length/Mandibular Width (Ratio) 4 8 12
2.17 2.17 2.18
2.34 2.29 2.28
The linear measurements from the skull indicated that fusion of the midpalate suture had hindered efficiently the growth in the lateral direction but was associated with an increase in the length of the palate. There were no other noteworthy growth-related differences found between the experimental and control groups.
449
Fig. 2. Ventral and lateral views of skulls from treated (top) and control (bottom) guinea pigs four weeks after the start of the experiment. Note: (1) the transverse restriction of the posterior mandible; (2) resorption of the angular processes ; (3) the upward deflection of the maxillary complex in the treated animal.
Discussion The method used to produce interferences in the occlusion in this study was very simple. Not adding anything onto the occlusal surfaces, we only cut the comer of the teeth on one side. Grinding the occlusal surfaces slanting to the opposite direction compared to the nonnal medially slanting situation in guinea pigs, our purpose was to create an occlusal asymmetry and thereby to encourage the mandible to deviate and grow to the left. The lesser weight gain of the experimental animals showed that the occlusal interferences were very effective, especially in young animals. The animals did not eat much during two days after each episode of grinding. The animals attemped to get rid ofthe disturbing interference, and started to wear their teeth almost incessantly for about a day. The attrition of the teeth resulted in a lower facial height that was found in the angular measurements from the lateral head mms. The mesial leaning of the lower molars in two experimental groups can also be seen to indicate the same increased activity of the muscles. The purpose of closing the midpalatal suture was to imitate the situation in children with the mouth-breathing habit when the maxilla is often found to grow too narrow to fit well together with the lower jaw. At the same time we
also added the possibility of developing interferences in occlusion. This procedure was effective for the purpose of this study as the ,growth in width of the palatal suture of the guinea pig is small and the upper dental arch, even in experimental animals, was wide enough to fit together with the lower molars. The lack of asymmetry in the dorsoventral radiograms suggests that increased activity in masticatory musculature occurred symmetrically although the occlusal interferences were produced asymmetrically. Lack of enamel on the occlusal surfaces and lack of cusps made it possible for the mandible to stay in the middle of the face in spite of a discrepancy between the width of the upper and lower tooth arches. Grinding only the medial comers of the upper molars, contact was left between the upper and the lower molars. The efficiency of the attrition following our manipulation made it possible to reduce the height of the molars more rapidly on the treated right side than on the untreated left side. It is conceivable that the pressure on the right condyle against fossa mandibularis was increased and that the reduced height of the right ramus and the smaller width of the condylar neck were results of that increased condylar pressure. The most important observation made in this study was the deceased growth in width of the posterior part of the
450
mandible. It has been shown that the m. pterygoideus medialis has an active role during active tooth grinding (Weijs and Dautuma 1981). We hypothesize that contraction of the m. pterygoideus medialis brought about an excursion causing a narrowing of the mandible, which was more apparent in young animals with thin bones than in older animals. In the most severe cases, the narrowing effect of this excessive activity of the m. pterygoideus medialis was so dramatic that there was no space for the mandible to open. In a few cases, resorption of the angular processes was total and the animals were no longer able to eat. Even when the mandible was significantly narrower in the 8- and 12-week groups the mandible had space to open and the animals could eat normally. Thus, the artificially increased activity in m. pterygoideus medialis seemed to have a drastic influence upon the growth of the mandible in the guinea pig, not in the size but in the shape of the mandible. No evidence of corresponding growth alterations in the mandibular width in human children has been obtained, although somewhat similar activities in masticatory musculature caused by the interferences in the occlusion are possible on the basis of sore muscles in the morning after trying to wear the interfering cusps away during the night. Following the guidance of the unworn cusps the mandible may then become guided mediolaterally or forward from its correct
position and, untreated, asymmetrical growth of the whole maxillomandibular complex may result.
References Babler WJ, Persing JA (1982) Experimental alteration of cranial suture growth : Effect of the neurocranium, basicranium and midface. In: Dixon A, Sarnat B (eds) Factors and mechanisms influencing bone growth. Progress in clinical and biological research 101: 333-345, Alan R Liss , New York Isotupa K, Makinen KK, Carlson D (1992) Proteinase, phosphatase and glucuronidase activities in the growing mandible and temporo-mandibular joint of the guinea pig . Anat Anz 174: 441-446 Linder-Aronson S (1970) Adenoids - their effect on mode of breathing and nasal airflow and their relationship to characteristics of the facial skeleton and the dentition. Acta Otolaryngol Suppl. 265 Persson KM, Roy WA, Persing JA, Rodehearer GT, Winn HR (1979) Craniofacial growth following experimental craniosynostosis and cranioectomy in rabbits . J Neurosurg 50: 187-197 Weijs WA, Dautuma R (1981) Functional anatomy of the masticatory apparatus in the rabbit (Orystolagus cuniculus L). Meth J Zoo131 : 99-147 Accepted October 30, 1991
451
Influence of asymmetric occlusal relationships and decreased maxillary width on the growth of the facial skeleton in the guinea pig Kauko P. Isotupa*, David S. Carlson** and Kauko K. Makinen*
* Department of Biologic and Materials Sciences, School of Dentistry, and ** Center for Human Growth and Development, The University of Michigan, Ann Arbor, Michigan 48109-1078, U.S.A.
Summary. The purpose of this study was to evaluate the effect of muscle function and occlusal form on mandibular growth in guinea pigs. We hypothesized that restriction of maxillary width and asymmetric function of the mandible would lead to mandibular asymmetry. The hard palate of 12 animals was exposed at the age often days, and cyanocrylate was used to close the midpalatal suture in order to restrict transverse maxillary growth. The right incisors and right molars were ground obliquely at the time of surgery and every two weeks thereafter until the animals were sacrificed 4, 8 and 12 weeks later (four animals in each time group). Six age-matched animals served as controls. Dorsoversal xrays were taken at sacrifice, followed by the removal of small biopsies from six different sites of the maxillomandibular skeleton for biochemical studies, which will be reported separately. Relative to controls, the treated animals exhibited a narrow maxilla and asymmetry in the height of the ramus and in the length of the mandible from the mental foramen to the angular process. The maximum width between the angular processes of the mandible was extremely narrow, as was the condylar neck especially on the right side. It was concluded that abnormal masticatory muscle function caused by occlusal deviation led to a narrowing and a slight asymmetry of the mandible in the growing guinea pig.
complex and dentition is also often associated with an excessively narrow palate. According to some investigators, malocclusion characterized by transverse palatal restriction and asymmetry of the facial skeleton results primarily from asymmetric function of the muscles of mastication among children with parafunctional habits such as thumb-sucking and mouth-breathing (Linder-Aronson 1970). In animals with continuously erupting teeth like guinea pigs and rats, it has been noted that intermittent, chronic activity of the jaw elevator muscles, particularly the medial pterygoid muscles, is necessary to wear the occlusal surfaces and keep the teeth from supererupting. It has also been demonstrated that sutural growth can be restricted by using cyanoacrylate across the suture in young rats, guinea pigs and rabbits (Persson et al. 1979; Babler and Persing 1982).
Key words: Bone growth - Facial skeleton - Guinea pig
The purpose of this study was to evaluate the effect of transverse restriction of maxillary growth and occlusal asymmetry on the growth of the maxillomandibular complex. Manipulation of the tooth surfaces in the guinea pigs was done in order to produce asymmetric occlusal interferences that would encourage alteration of muscle activity because of the need for increased tooth wear and deviation of the mandible. Restriction of maxillary-palatal growth was done in order to emphasize the effective discrepancy between the upper and lower dentition and thus to enhance asymmetric function.
Introduction
Materials and methods
Asymmetric growth of the mandible may occur as an idiopathic syndrome or following minor trauma to the temporomandibular joint. Asymmetry of the maxillomandibular
Eighteen male guinea pigs were divided into experimental (n = 12) and control groups (n = 6). At ten days of age all animals were anaesthetized (5 mg/kg Rompum plus 30 mglkg Ketamine 1M,
~I
Ann. Anat. (1992) 174: 447-451 Gustav Fischer Verlag Jena
followed by 15 - 30 mg/kg Pentobarbital sodium IP) and positioned in a specially designed cephalostat in order to obtain standardized dorsoventral radiocephalograms. Control animals were returned to their cages immediately after x-raying . In experimental animals, the mouth was propped open and a 1 cm incision was made longitudinally in the midline of the palate. The midpalatal suture was cleared of periosteum using a periosteal elevator. Cyanoacrylate glue was injected in the soft sutural tissue and onto the bone surfaces across the midpalatal suture. The glue was allowed to dry for two minutes, after which time the incision was closed by pressing the flaps against the palatal bones. No sutures were used in closing the incision. With the mouth remaining propped open, a dental handpiece and a carborundum grinding wheel were used to grind away the buccal aspect of the lower molars and the palatal aspect of the upper molars on the right side; the contacts between the upper and lower molars were preserved. The incisors were ground at an angle slanting up toward the left (Fig. lC). The dental grinding was repeated under anaesthesia every two weeks and a dorsoventral cephalogram was taken every four weeks . The guinea pigs were fed normal solid food pellets, hay and water ad libitum. Animals were weighed weekly. Four experimental and two control animals were killed at 4, 8 and 12 weeks postoperatively via an overdose of Pentobarbital sodium. The mandible was detached and contact lateral radiograms were taken from the bisected halves of the mandible. The remaining soft tissues were carefully removed, after which samples of bone and cartilage were taken for subsequent biochemical studies (Isotupa et al. 1992).
A total of 11 linear and angular measurements were taken from the dry skulls or from the cephalograms using a Helios digital caliper (0.1 mm accuracy) (Table 1; Fig. lA, B). Mean values were calculated for comparison of intragroup craniometric differences between right and left sides and intergroup differences between experimental and control animals . No more extensive statistical analysis was possible due to the number of animals in the study.
A
e
L -_ _ _ _ _ _ _ _
5 ---
Table 1. Craniometric measurements taken from dry skulls, dorsoventral cephalometric x-rays, and lateral contact radiographs of the mandible and lateral x-rays of the entire skull.
Dry Skull 1. palatal width between the right and left first molars 2. palatal length from the foramen incisivum to the posterior margin of the palatal bone Dorsoventral Cephalograms
3. maximum length of the skull 4. maximum width of the mandible Lateral Radiograms of the Mandible 5. maximum height of the mandible measured perpendicularly from the mandibular line to the highest point of the condyle , right and left 6. distance from the foramen mentale to the most distal end of the angular process 7. width of the condylar neck just below the condylar cartilage 8. height of the angular process Lateral Radiograms of the Mandible and of the Skull
9. angle between the mandibular baseline and the mesial tangent to the first molar 10. angle between the anterior mandibular base line and the occlusal plane 11. angle between the anterior mandibular line and the anterior cranial line
Fig. 1. Panels A and B: Representation of the dimensional measurements performed in the skulls and the mandibles . Panel C: diagram indicating the procedure used to modify the dentition of the animals in the experimental group. The right molars and the upper and lower incisors were ground obliquely using a dental handpiece to encourage the mandible to deviate to the left upon closure. The arrow shows the direction of occlusion as a result of grinding.
448
Results
from the treated groups was one of the most strongly affected guinea pigs , and died after one week) .
Clinical Observations The guinea pigs in the experimental group lagged slightly behind the controls .in weight gain, though this was variable. Two animals in the experimental group seemed to be affected more than the others early in the experiment. The first died after one week. The weight gain of the other was dramatically less than that of other experimental animals at the 4-week interval. Another experimental animal died during anaesthesia at the 8-week interval. The skulls of these three guinea pigs were found to be abnormal. Their teeth were overerupted and the bite was opened. In addition, the posterior part of the lower jaw was found to be very narrow and the angular processes were resorbed totally on both sides.
Cephalometric Analysis (Table 2) The dorsoventral cephalograms did not reveal any major asymmetry in the shape of the skulls in the experimental or in the control animals. The lenght of the skull was normal in the experimental group. However, the width of the posterior mandible was more narrow in the experimental group than in the control animals. The ratio between the length of the skull and the width of the mandible was substantially higher in experimental animals than in controls. This difference was most obvious at the 4-week interval, but was also notable at the 8- and 12-week intervals . Measurements from the lateral head films indicated that the angle between the anterior line of the mandible and the anterior cranial line of the skull was smaller in the experimental group. The orientation of the occlusal plane in the experimental animals was also different: it was essentially parallel to the mandibular line while the corresponding angle in control animals averaged 2.5 0 in the 4-week group and 4.7 0 in the 12-week group. The angular measurements from the lateral mandibular radiograms showed that the lower molars leaned mesially in the experimental groups at the 4- and 8-week intervals, but were normally upright again by the 12-week interval. The length of the mandible from the mental foramen to condylion was similar to control animals . However, the height of the mandible was less in experimental animals as was the distance from the mental foramen to the tip of the angular process. The condylar neck was more narrow in the experimental group than in the controls. Comparing measurements from the right and the left sides of the mandible in experimental animals , it was found that (1) the condylar neck was narrower on the right side than on the left side, (2) the height of the mandible on the right side had a tendency to be lower than on the left, and (3) the length from the mental foramen to the tip of the angular process was lower on the left side than the right. Fig. 2 shows the ventral and lateral views of skulls from the control and the treated group (the animal
Table 2. Cephalometric data of treated and control animals determined 4, 8 and 12 weeks after the beginning of the test. Time (weeks)
Control animals
Treated animals
Mean
S.D.
Mean
S.D.
0.35 0.07 0.35
0.45 0.80 0.97
0.01 0.24 0.29
Palatal Width (mm) 4 8 12
0.75 1.50 2.65
Maximum Height of the Mandible (mm) Right 4 8 12
16.29 16.39 17.05
0.25 1.12 0.06
15 .10 16.33 16.55
0.11 0.65 0.85
Left 4 8 12
16.55 16.39 17.17
0.06 1.12 0. 13
15 .28 16.71 16.65
0.08 0.47 0.75
Mandibular Length (mm) Right 4 8 12
36.40 36.95 39.75
0.14 0.13 0.03
35 .10 36.65 37.60
0.14 0.11 0.17
Left 4 8 12
36.35 37.05 39.60
0.12 0.09 0.00
35.45 37.08 38.15
0.Q2 0.17 0.22
6.22 6.16 6.23
0.43 0.29 0.37
58.77 62 .96 67 .13
1.85 1.07 1.39
Maximum Width of the Mandible (mm) 4 25.15 27.43 1.03 8 29 .17 1.19 27 .51 30.85 12 1.0 29.40
0.49 0.91 1.39
Condylar Neck Width (mm) 4 8 12
6.17 6.65 6.80
0 .22 0.47 0.37
Maximum Length of the Skull (mm) 4 59.65 2.39 63.45 8 1.73 12 67.13 1.76
Skull Length/Mandibular Width (Ratio) 4 8 12
2.17 2.17 2.18
2.34 2.29 2.28
The linear measurements from the skull indicated that fusion of the midpalate suture had hindered efficiently the growth in the lateral direction but was associated with an increase in the length of the palate. There were no other noteworthy growth-related differences found between the experimental and control groups.
449
Fig. 2. Ventral and lateral views of skulls from treated (top) and control (bottom) guinea pigs four weeks after the start of the experiment. Note: (1) the transverse restriction of the posterior mandible; (2) resorption of the angular processes ; (3) the upward deflection of the maxillary complex in the treated animal.
Discussion The method used to produce interferences in the occlusion in this study was very simple. Not adding anything onto the occlusal surfaces, we only cut the comer of the teeth on one side. Grinding the occlusal surfaces slanting to the opposite direction compared to the nonnal medially slanting situation in guinea pigs, our purpose was to create an occlusal asymmetry and thereby to encourage the mandible to deviate and grow to the left. The lesser weight gain of the experimental animals showed that the occlusal interferences were very effective, especially in young animals. The animals did not eat much during two days after each episode of grinding. The animals attemped to get rid ofthe disturbing interference, and started to wear their teeth almost incessantly for about a day. The attrition of the teeth resulted in a lower facial height that was found in the angular measurements from the lateral head mms. The mesial leaning of the lower molars in two experimental groups can also be seen to indicate the same increased activity of the muscles. The purpose of closing the midpalatal suture was to imitate the situation in children with the mouth-breathing habit when the maxilla is often found to grow too narrow to fit well together with the lower jaw. At the same time we
also added the possibility of developing interferences in occlusion. This procedure was effective for the purpose of this study as the ,growth in width of the palatal suture of the guinea pig is small and the upper dental arch, even in experimental animals, was wide enough to fit together with the lower molars. The lack of asymmetry in the dorsoventral radiograms suggests that increased activity in masticatory musculature occurred symmetrically although the occlusal interferences were produced asymmetrically. Lack of enamel on the occlusal surfaces and lack of cusps made it possible for the mandible to stay in the middle of the face in spite of a discrepancy between the width of the upper and lower tooth arches. Grinding only the medial comers of the upper molars, contact was left between the upper and the lower molars. The efficiency of the attrition following our manipulation made it possible to reduce the height of the molars more rapidly on the treated right side than on the untreated left side. It is conceivable that the pressure on the right condyle against fossa mandibularis was increased and that the reduced height of the right ramus and the smaller width of the condylar neck were results of that increased condylar pressure. The most important observation made in this study was the deceased growth in width of the posterior part of the
450
mandible. It has been shown that the m. pterygoideus medialis has an active role during active tooth grinding (Weijs and Dautuma 1981). We hypothesize that contraction of the m. pterygoideus medialis brought about an excursion causing a narrowing of the mandible, which was more apparent in young animals with thin bones than in older animals. In the most severe cases, the narrowing effect of this excessive activity of the m. pterygoideus medialis was so dramatic that there was no space for the mandible to open. In a few cases, resorption of the angular processes was total and the animals were no longer able to eat. Even when the mandible was significantly narrower in the 8- and 12-week groups the mandible had space to open and the animals could eat normally. Thus, the artificially increased activity in m. pterygoideus medialis seemed to have a drastic influence upon the growth of the mandible in the guinea pig, not in the size but in the shape of the mandible. No evidence of corresponding growth alterations in the mandibular width in human children has been obtained, although somewhat similar activities in masticatory musculature caused by the interferences in the occlusion are possible on the basis of sore muscles in the morning after trying to wear the interfering cusps away during the night. Following the guidance of the unworn cusps the mandible may then become guided mediolaterally or forward from its correct
position and, untreated, asymmetrical growth of the whole maxillomandibular complex may result.
References Babler WJ, Persing JA (1982) Experimental alteration of cranial suture growth : Effect of the neurocranium, basicranium and midface. In: Dixon A, Sarnat B (eds) Factors and mechanisms influencing bone growth. Progress in clinical and biological research 101: 333-345, Alan R Liss , New York Isotupa K, Makinen KK, Carlson D (1992) Proteinase, phosphatase and glucuronidase activities in the growing mandible and temporo-mandibular joint of the guinea pig . Anat Anz 174: 441-446 Linder-Aronson S (1970) Adenoids - their effect on mode of breathing and nasal airflow and their relationship to characteristics of the facial skeleton and the dentition. Acta Otolaryngol Suppl. 265 Persson KM, Roy WA, Persing JA, Rodehearer GT, Winn HR (1979) Craniofacial growth following experimental craniosynostosis and cranioectomy in rabbits . J Neurosurg 50: 187-197 Weijs WA, Dautuma R (1981) Functional anatomy of the masticatory apparatus in the rabbit (Orystolagus cuniculus L). Meth J Zoo131 : 99-147 Accepted October 30, 1991
451
