Jaw protruder muscles and condylar cartilage growth in the rat Robert d. Hinton, PhD Dallas, Texas
Many studies have explored the role of the protrusive musculature in promoting growth at the condylar cartilage and the overall lengthening of the lower jaw, with emphasis on the lateral pterygoid muscle (LPM). The largely anteroposterior orientation of the superficial part of the masseter muscle (SM) in the rat suggests that it may also function as a protruder of the lower jaw. Accordingly, it is possible that the action of the SM may play a part in the regulation of growth of the condylar cartilage and the lower jaw. To examine this hypothesis, bilateral resection of the superficial portion of the masseter muscle was performed in male Sprague-Dawley rats at 26 days of age. At 5 days after surgery, [3H]-thymidine incorporation in the condylar cartilage was increased (F = 6.93, p -< 0.01) in the SM myectomy group relative to the surgical control and unoperated control groups. However, by 20 days after surgery no differences were present. At this sacrifice interval, lower jaw dimensions relating to areas of muscle attachment, as well as ramus height, were significantly reduced in the SM myectomy group, but overall jaw length (mental foramen to condyle) was unaffected. In contrast, myotomy of the LPM resulted in a significant decrease in mitotic activity of the cartilage 4 days after surgery. This decrease was present, but not more pronounced, in animals subjected to both SM myectomy and LPM myotomy. Hence, myotomy or myectomy of these two muscles, each with a protrusive orientation, produces opposite effects on proliferative activity at the condylar cartilage. (AMJ ORTHODDENTOFACORTHOP1991 ;100:436-42.)
O v e r the past two decades, several investigators have contended that the growth rate of the condylar cartilage can be altered by changes in the postural position of the lower jaw. In particular, an increase in the mitotic activity of the prechondroblastic zone of the cartilage, total cartilage thickness, and overall mandibular length has been documented in conjunction with an anterior and inferior displacement of the condyle subsequent to forced protrusion of the lower jaw) 5 Since tonic activity of the lateral pterygoid muscle (LPM), the principal protruder of the lower jaw, was markedly increased after placement of an appliance that prompted protrusion on closure, 3 the activity of the LPM was initially thought to be critical to the generation of the cellular response) .6 However, subsequent investigations7"8 have shown that increased mitotic activity at the condylar cartilage can also be elicited by the application of Class II intermaxillary elastic forces, a circumstance in which lateral pterygoid activity is minimal. 7 Accordingly, it has been suggested that the
This work was supported by NIH grant DE06982 from the National Institute for Dental Research and by Biomedical Research Support Grant funds from Baylor College of Dentistry. 811125570
436
functional result of LPM activity rather than its activity per se may be the critical factor, that is the transient removal of articular forces from the condylar cartilage ("unloading") engendered by the protrusive posture. 9'2 Viewed in this context, the LPM is an important, but not crucial, factor in the regulation of condylar growth. Its actions, as well as those of Class II appliances, are then instrumental only to the extent that they repetitively unload the condyle. A potentially informative and testable corollary may be derived from these concepts. It has been postulated '3"14that any muscle that protrudes the lower jaw may conceivably aid in regulation of the growth rate of the condylar cartilage. In most animals, including monkeys and human beings, the lateral pterygoid is the only muscle oriented in such a way as to effect significant protrusion. However, the superficial component of the masseter muscle in the rat (Fig. l) has a large, fleshy belly that is largely anteroposteriorly oriented relative to the lower jaw./5 Studies of muscle orientation, '~:6 electromyography,t7 and histochemistry~8 suggest that the superficial masseter (SM) differs in its functional role from that of the deep masseter. Although the SM contributes to jaw elevation during mastication, its primary line of action is also a
Volume 100
Number 5
Fig. 1. Superficial masseter (SM) muscle in the rat (adapted from Hiiemae, 1971). Note that its course is much more oblique than in primates. protrusive one; this muscle is thought t 0 b e active in the control o f jaw-positioning movements ~s and during the final stages of incision. ~7"~9Because of this, it may be hypothesized that if anterior translation were crucial to the regulation of condylar growth, alteration o f the contractile activity in the SM should affect the mitotic rate o f the cartilage. Although several workers have previously reported on the influence of myectomy o f the masseter muscle or denervation of its nerve on form o f the lower j a w and cranium in rats, 2°-27 the specific influence o f the SM has not been studied. Moreover, no study has analyzed the effect of myectomy on condylar cartilage growth in particular. The primary purpose of this study was to investigate the effect o f surgical resection of the superficial component o f the rat masseter muscle on growth o f the lower j a w and the Condylar cartilage. A subsequent group of experiments was then conducted secondary to the SM myectomy studies to investigate the interaction of SM resection with cutting (myotomy) of the LPM.
MATERIALS AND METHODS Experimental set 1 Three experiments were initially performed to evaluate the effect of superficial masseter muscle rcsection on both lower jaw growth and condylar cartilage growth. In each, identical surgerical procedures were performed, but with different sacrifice schedules. In each experimental set, 26-dayold male Sprague-Dawley rats were randomly divided into three groups. Before surgery, all animals were anesthetized with a mixture of ketamine (100 mg/ml) and Rompun (100 mg/ml) at a dose of 1 ml/kg body weight. In one group, the superficial portion of the masseter muscle was removed bilaterally. Care was taken to resect only that part of the muscle in which fibers ran most superficially and at an acute angle to the long axis of the lower jaw (Fig. 1), leaving intact the more vertically oriented fibers of the deeper portions of the masseter muscle. A second group, in which an identical sur-
Protruder muscles and jaw growth
437
g,~i~,,;~i~!~-~i~ -~''~''~.~,,.~ " ~, . ;~ ',~ ~ Fig. 2. Photomicrograph of a typical condylar cartilage removed before biochemical an~ilysis. Note that the tissue has been largely severed from the ramus at the cartilage-bone interface and contains little bone. As a result, [SH]-thymidine incorporation should reflect primarily mitotic activity taking place in the prechondroblastic zone of the cartilage.
gical approach was made to expose the superficial portion of the masseter but no resection ;,vas performed, served fis a surgical control group. A third group in which no surgery was performed served as a Control group. After surgery, all animals were fed standard rat pellets arid water ad libituin. All animals in one experimemal set were killed by ~ileth.al intravenous injection of sodium pentoba~'bital (Nembutal, 50 mg/mi) at 5 days afte r surgei'y, in another set at 10 days after surgery, and in a third set at 20 days afier surgery. One hour before they were killed, all animals were intravenously injected (femoral vein) with 1 p-ci/g body weight of [~H]thymidine (New England Nuclear, specific activity = 6.7 Ci/mmole). Immediately after death, one mandibular condyle was removed and the articular disk was carefully detached so hs not to damage or strip any part of the prechondroblastic zone of ihe cartilage. The condylar cartilage was severed from the ramus at the cartilage-b0ne interface (Fig. 2), dissected free of excess tissue, and frozen at - 2 0 ° C for later biochemical assay. In the animals killed at 20 days after surgery, the 10wer jaw On the opposite side was removed and defteshed to facilitate measurement of the lower jaw dimensions. Biochemical analysis. Condyles reserved for biochemical analysis to cletermine [3H]-thymidine incorporation were riomogenized in ice-cold 10% trichloroacetic acid (TCA) containing 1 mM thymidine. The homogenate s were placed on ice for 30 minutes, then centrifuged at 3000 g and 4 ° C for 15 mintites. The precipitates were washed twice with ice-cold 10% TCA without thymidine, then hydrolyzed for 20 minutes in 2 ml of 10% TCA at 90 ° C. After hydrolysis, the tubes were kept on ice for 15 minutes, then centrifuged at 3000 g for 10 minutes at 4 ° C. An aliquot (500 ~1) of the supematant was added to 10 ml of ACS (Aqueous Counting Scintillant, Amersham, Arlington Heights, I11.) and assayed for radioactivity in a Beckman LS 7500 scintillation counter. A further aliquot was used to estimate DNA content according to the
438
Hinton
Am. J. Orthod. Dentofac. Orthop. November 1991
cS
Fig. 3. Lower jaw of a rat showing dimensions measured in this study. MF-C, Mental foramen to posterior border of condyle; MF-N, mental foramen to notch on posterior border of ramus; MF-A, mental foramen to tip of angular process; MH1, height from superior surface of condyle to inferior border of mandible; MH2, height from mandibular foramen to inferior border of mandible.
diphenylamine method. :~ Calf thymus DNA (Sigma) served as the standard, and DNA content was read at 600 nm. The results were expressed as disintegrations per minute per microgram of DNA in the acid-insoluble precipitate. Morphometric analysis. On the defteshed lower jaw of each animal killed 20 days after surgery, the following dimensions were measured with Helios dial calipers to the nearest 0.1 mm (Fig. 3): 1. Mental foramen to posterior border of condyle (MF-C) 2. Mental foramen to greatest concavity of notch on posterior ramal border (MF-N) 3. Mental foramen to tip of angular process (MF-A) 4. Mandibular height 1, measured from the inferior surface of the mandible to the superior surface of the condyle (MHI) 5. Mandibular height 2, measured from the inferior surface of the mandible to the mandibular foramen (MH2) Statistical analysis. Morphometric and biochemical data were evaluated at each time interval per experiment for the presence of intergroup differences with a one-way analysis of variance. Comparisons of data between experiments or time intervals (i.e., with a two-way analysis of variance with time as a factor) were not performed because of complications introduced by differing activities of isotope batches over time. Differences between specific groups were assessed with Scheffe's post hoc test.
Experimental set 2 Because of the unexpected findings of the SM resection experiments, additional investigations were performed to compare the effects of resection of the SM with that of the LPM and to determine whether an interactive (i.e., synergistic) effect could be detected. Because of its deep placement and small size, resection of the LPM is necessarily a traumatic and difficult procedure. To minimize the influence of this trauma on feeding behavior, the belly of the LPM was severed near its midline (i.e., a myotomy), but no removal was at-
tempted. This technique has been successfully employed in a previous study-'~and results in an almost complete or complete vertical cut of the fibers that persists 4 days after myotomy. Male Sprague-Dawley rats, 26 days of age, were randomly assigned to one of four groups. One group undenvent bilateral LPM myotomy in the following manner. The fascia overlying the posterior temporalis muscle was incised, and the muscle fibers were carefully elevated to expose the superior aspects of the infratemporal region. The condylar neck was then grasped with forceps and pulled laterally, thereby straightening the course of the LPM and enabling it to be cut by microscissors thrust downward into the infratemporal fossa. This straightening of the muscle usually resulted in its being cut near its midpoint or closer to its origin, thereby minimizing trauma near the condyle that might result in fibrosis or limitation of movement. In a second group, the SM was resected bilaterally as described for experimental set 1. In a third group, each animal underwent both SM myectomy and LPM myotomy bilaterally. All animals in the final group had the same surgical exposures as in group 3 (i.e., for SM resection and LPM cut), but no muscle cuts were performed. To attempt to alleviate the large losses of body weight that had been observed in a previous study-'9after LPM myotomy, all animals were fed small food pellets that could be obtained from a bowl inside the cage. Because of the possibility that the small pellet might affect the jaw movements invoh'ed in food acquisition, this experiment was repeated (minus the SM myectomy group for which such data had already been acquired) with large rat pellets. All animals were killed 4 days after surgery by a lethal intravenous injection of sodium pentobarbital (Nembutal, 50 mg/ml). One hour before death, each animal was intravenously injected (femoral vein) with 1 ptCi/g body weight of [3H]-thymidine (New England Nuclear, specific activity = 6.7 Ci/mmole). After the animal was killed, the condylar cartilage was removed, the articular disk detached and frozen at - 20° C. Subsequently, the cartilage was severed from the ramus at the cartilage-bone interface and assayed for [3HIthymidine incorporation (disintergrations per minute per microgram of DNA) as described under"Biochemical Analysis".
RESULTS Experimental set 1 The S M myectomy animals tolerated the surgical procedures extremely well, and postoperative recovery was uneventful. At death, the masseter muscle in animals that underwent myectomy was composed only of vertically oriented fibers; no regeneration of the SM was encountered. Although minor retardation in the rate of weight gain was apparent after 5 days, the differences (in body weight at death or in percent weight gained) were not statistically significant. The surgical control animals experienced a rate of weight gain intermediate between the control and myectomy samples. There were no significant differences in weight gain at the 20-day sacrifice interval (Table I).
Protruder muscles attd jaw growth
Volume 100 Number 5
439
Table I. Body weight gain in experimental groups at 5 and 20 days after surgery*
5 days after surgery Weight at death Weight gain--surgery to death (%) 20 days after surgery Weight at death Weight gain--surgery to death (%)
I
Surgical
Superficial masseter
myectomy
F value
112.6 ± 6.9 (7) 53.9 ± 12.3 (7)
107.7 ± 7.3 (7) 48.6 ± 5.3 (7)
105,6 ± 5.8 (7) 46.0 ± 5.7 (7)
2.01 (NS)
205.4 ± 7.7 (11) 179.3 ± 15.1 (11)
210.6 __+ 8.2 (lO) 183.7 ± l l . O (10)
209.8 __- 14.6 (i0) 173.6 ± 11.4 (10)
0.76 (NS)
I
Control
control
1.31 (NS)
1.58 (NS)
NS = not significant.
*Body weight data were not available for 10 days after surgery.
Table II. Intergroup differences in selected lower jaw dimensions at 20 days after surgery: Mean ___ SD
Dimensions Mental foramen to angular process Mental foramen to notch Mental foramen to condyle Superior surface of condyle to inferior border of mandible ( M H I ) Mandibular foramen to inferior border of mandible (MH2)
Surgical Control 17.71 --(14) 15.00 (17) 18.38 (17) 10.33 ± (15) 6.41 ± (15)
0.59 0.50 0.50 0.39 0.24
control 17.43 -'(14) 14.60 --(15) 18.00 --(15) 10.37 -'(15) 6.42 ± (12)
0.41 0.28 0.34 0.61 0.27
Superficial masseter myectomy 17.17 --- 0.36* (15) 14.33 ± 0.31"* (16) 18.04 --- 0.43 (16) 9.48 ± 0.51"** (15) 5.99 ± 0.29*** (9)
F
value 4.98 12.83 3.65 14.45 8.51
*Different from control and surgical controls at p -----0.05. **Different from controls at p ---
Many studies have explored the role of the protrusive musculature in promoting growth at the condylar cartilage and the overall lengthening of the lower jaw, with emphasis on the lateral pterygoid muscle (LPM). The largely anteroposterior orientation of the superficial part of the masseter muscle (SM) in the rat suggests that it may also function as a protruder of the lower jaw. Accordingly, it is possible that the action of the SM may play a part in the regulation of growth of the condylar cartilage and the lower jaw. To examine this hypothesis, bilateral resection of the superficial portion of the masseter muscle was performed in male Sprague-Dawley rats at 26 days of age. At 5 days after surgery, [3H]-thymidine incorporation in the condylar cartilage was increased (F = 6.93, p -< 0.01) in the SM myectomy group relative to the surgical control and unoperated control groups. However, by 20 days after surgery no differences were present. At this sacrifice interval, lower jaw dimensions relating to areas of muscle attachment, as well as ramus height, were significantly reduced in the SM myectomy group, but overall jaw length (mental foramen to condyle) was unaffected. In contrast, myotomy of the LPM resulted in a significant decrease in mitotic activity of the cartilage 4 days after surgery. This decrease was present, but not more pronounced, in animals subjected to both SM myectomy and LPM myotomy. Hence, myotomy or myectomy of these two muscles, each with a protrusive orientation, produces opposite effects on proliferative activity at the condylar cartilage. (AMJ ORTHODDENTOFACORTHOP1991 ;100:436-42.)
O v e r the past two decades, several investigators have contended that the growth rate of the condylar cartilage can be altered by changes in the postural position of the lower jaw. In particular, an increase in the mitotic activity of the prechondroblastic zone of the cartilage, total cartilage thickness, and overall mandibular length has been documented in conjunction with an anterior and inferior displacement of the condyle subsequent to forced protrusion of the lower jaw) 5 Since tonic activity of the lateral pterygoid muscle (LPM), the principal protruder of the lower jaw, was markedly increased after placement of an appliance that prompted protrusion on closure, 3 the activity of the LPM was initially thought to be critical to the generation of the cellular response) .6 However, subsequent investigations7"8 have shown that increased mitotic activity at the condylar cartilage can also be elicited by the application of Class II intermaxillary elastic forces, a circumstance in which lateral pterygoid activity is minimal. 7 Accordingly, it has been suggested that the
This work was supported by NIH grant DE06982 from the National Institute for Dental Research and by Biomedical Research Support Grant funds from Baylor College of Dentistry. 811125570
436
functional result of LPM activity rather than its activity per se may be the critical factor, that is the transient removal of articular forces from the condylar cartilage ("unloading") engendered by the protrusive posture. 9'2 Viewed in this context, the LPM is an important, but not crucial, factor in the regulation of condylar growth. Its actions, as well as those of Class II appliances, are then instrumental only to the extent that they repetitively unload the condyle. A potentially informative and testable corollary may be derived from these concepts. It has been postulated '3"14that any muscle that protrudes the lower jaw may conceivably aid in regulation of the growth rate of the condylar cartilage. In most animals, including monkeys and human beings, the lateral pterygoid is the only muscle oriented in such a way as to effect significant protrusion. However, the superficial component of the masseter muscle in the rat (Fig. l) has a large, fleshy belly that is largely anteroposteriorly oriented relative to the lower jaw./5 Studies of muscle orientation, '~:6 electromyography,t7 and histochemistry~8 suggest that the superficial masseter (SM) differs in its functional role from that of the deep masseter. Although the SM contributes to jaw elevation during mastication, its primary line of action is also a
Volume 100
Number 5
Fig. 1. Superficial masseter (SM) muscle in the rat (adapted from Hiiemae, 1971). Note that its course is much more oblique than in primates. protrusive one; this muscle is thought t 0 b e active in the control o f jaw-positioning movements ~s and during the final stages of incision. ~7"~9Because of this, it may be hypothesized that if anterior translation were crucial to the regulation of condylar growth, alteration o f the contractile activity in the SM should affect the mitotic rate o f the cartilage. Although several workers have previously reported on the influence of myectomy o f the masseter muscle or denervation of its nerve on form o f the lower j a w and cranium in rats, 2°-27 the specific influence o f the SM has not been studied. Moreover, no study has analyzed the effect of myectomy on condylar cartilage growth in particular. The primary purpose of this study was to investigate the effect o f surgical resection of the superficial component o f the rat masseter muscle on growth o f the lower j a w and the Condylar cartilage. A subsequent group of experiments was then conducted secondary to the SM myectomy studies to investigate the interaction of SM resection with cutting (myotomy) of the LPM.
MATERIALS AND METHODS Experimental set 1 Three experiments were initially performed to evaluate the effect of superficial masseter muscle rcsection on both lower jaw growth and condylar cartilage growth. In each, identical surgerical procedures were performed, but with different sacrifice schedules. In each experimental set, 26-dayold male Sprague-Dawley rats were randomly divided into three groups. Before surgery, all animals were anesthetized with a mixture of ketamine (100 mg/ml) and Rompun (100 mg/ml) at a dose of 1 ml/kg body weight. In one group, the superficial portion of the masseter muscle was removed bilaterally. Care was taken to resect only that part of the muscle in which fibers ran most superficially and at an acute angle to the long axis of the lower jaw (Fig. 1), leaving intact the more vertically oriented fibers of the deeper portions of the masseter muscle. A second group, in which an identical sur-
Protruder muscles and jaw growth
437
g,~i~,,;~i~!~-~i~ -~''~''~.~,,.~ " ~, . ;~ ',~ ~ Fig. 2. Photomicrograph of a typical condylar cartilage removed before biochemical an~ilysis. Note that the tissue has been largely severed from the ramus at the cartilage-bone interface and contains little bone. As a result, [SH]-thymidine incorporation should reflect primarily mitotic activity taking place in the prechondroblastic zone of the cartilage.
gical approach was made to expose the superficial portion of the masseter but no resection ;,vas performed, served fis a surgical control group. A third group in which no surgery was performed served as a Control group. After surgery, all animals were fed standard rat pellets arid water ad libituin. All animals in one experimemal set were killed by ~ileth.al intravenous injection of sodium pentoba~'bital (Nembutal, 50 mg/mi) at 5 days afte r surgei'y, in another set at 10 days after surgery, and in a third set at 20 days afier surgery. One hour before they were killed, all animals were intravenously injected (femoral vein) with 1 p-ci/g body weight of [~H]thymidine (New England Nuclear, specific activity = 6.7 Ci/mmole). Immediately after death, one mandibular condyle was removed and the articular disk was carefully detached so hs not to damage or strip any part of the prechondroblastic zone of ihe cartilage. The condylar cartilage was severed from the ramus at the cartilage-b0ne interface (Fig. 2), dissected free of excess tissue, and frozen at - 2 0 ° C for later biochemical assay. In the animals killed at 20 days after surgery, the 10wer jaw On the opposite side was removed and defteshed to facilitate measurement of the lower jaw dimensions. Biochemical analysis. Condyles reserved for biochemical analysis to cletermine [3H]-thymidine incorporation were riomogenized in ice-cold 10% trichloroacetic acid (TCA) containing 1 mM thymidine. The homogenate s were placed on ice for 30 minutes, then centrifuged at 3000 g and 4 ° C for 15 mintites. The precipitates were washed twice with ice-cold 10% TCA without thymidine, then hydrolyzed for 20 minutes in 2 ml of 10% TCA at 90 ° C. After hydrolysis, the tubes were kept on ice for 15 minutes, then centrifuged at 3000 g for 10 minutes at 4 ° C. An aliquot (500 ~1) of the supematant was added to 10 ml of ACS (Aqueous Counting Scintillant, Amersham, Arlington Heights, I11.) and assayed for radioactivity in a Beckman LS 7500 scintillation counter. A further aliquot was used to estimate DNA content according to the
438
Hinton
Am. J. Orthod. Dentofac. Orthop. November 1991
cS
Fig. 3. Lower jaw of a rat showing dimensions measured in this study. MF-C, Mental foramen to posterior border of condyle; MF-N, mental foramen to notch on posterior border of ramus; MF-A, mental foramen to tip of angular process; MH1, height from superior surface of condyle to inferior border of mandible; MH2, height from mandibular foramen to inferior border of mandible.
diphenylamine method. :~ Calf thymus DNA (Sigma) served as the standard, and DNA content was read at 600 nm. The results were expressed as disintegrations per minute per microgram of DNA in the acid-insoluble precipitate. Morphometric analysis. On the defteshed lower jaw of each animal killed 20 days after surgery, the following dimensions were measured with Helios dial calipers to the nearest 0.1 mm (Fig. 3): 1. Mental foramen to posterior border of condyle (MF-C) 2. Mental foramen to greatest concavity of notch on posterior ramal border (MF-N) 3. Mental foramen to tip of angular process (MF-A) 4. Mandibular height 1, measured from the inferior surface of the mandible to the superior surface of the condyle (MHI) 5. Mandibular height 2, measured from the inferior surface of the mandible to the mandibular foramen (MH2) Statistical analysis. Morphometric and biochemical data were evaluated at each time interval per experiment for the presence of intergroup differences with a one-way analysis of variance. Comparisons of data between experiments or time intervals (i.e., with a two-way analysis of variance with time as a factor) were not performed because of complications introduced by differing activities of isotope batches over time. Differences between specific groups were assessed with Scheffe's post hoc test.
Experimental set 2 Because of the unexpected findings of the SM resection experiments, additional investigations were performed to compare the effects of resection of the SM with that of the LPM and to determine whether an interactive (i.e., synergistic) effect could be detected. Because of its deep placement and small size, resection of the LPM is necessarily a traumatic and difficult procedure. To minimize the influence of this trauma on feeding behavior, the belly of the LPM was severed near its midline (i.e., a myotomy), but no removal was at-
tempted. This technique has been successfully employed in a previous study-'~and results in an almost complete or complete vertical cut of the fibers that persists 4 days after myotomy. Male Sprague-Dawley rats, 26 days of age, were randomly assigned to one of four groups. One group undenvent bilateral LPM myotomy in the following manner. The fascia overlying the posterior temporalis muscle was incised, and the muscle fibers were carefully elevated to expose the superior aspects of the infratemporal region. The condylar neck was then grasped with forceps and pulled laterally, thereby straightening the course of the LPM and enabling it to be cut by microscissors thrust downward into the infratemporal fossa. This straightening of the muscle usually resulted in its being cut near its midpoint or closer to its origin, thereby minimizing trauma near the condyle that might result in fibrosis or limitation of movement. In a second group, the SM was resected bilaterally as described for experimental set 1. In a third group, each animal underwent both SM myectomy and LPM myotomy bilaterally. All animals in the final group had the same surgical exposures as in group 3 (i.e., for SM resection and LPM cut), but no muscle cuts were performed. To attempt to alleviate the large losses of body weight that had been observed in a previous study-'9after LPM myotomy, all animals were fed small food pellets that could be obtained from a bowl inside the cage. Because of the possibility that the small pellet might affect the jaw movements invoh'ed in food acquisition, this experiment was repeated (minus the SM myectomy group for which such data had already been acquired) with large rat pellets. All animals were killed 4 days after surgery by a lethal intravenous injection of sodium pentobarbital (Nembutal, 50 mg/ml). One hour before death, each animal was intravenously injected (femoral vein) with 1 ptCi/g body weight of [3H]-thymidine (New England Nuclear, specific activity = 6.7 Ci/mmole). After the animal was killed, the condylar cartilage was removed, the articular disk detached and frozen at - 20° C. Subsequently, the cartilage was severed from the ramus at the cartilage-bone interface and assayed for [3HIthymidine incorporation (disintergrations per minute per microgram of DNA) as described under"Biochemical Analysis".
RESULTS Experimental set 1 The S M myectomy animals tolerated the surgical procedures extremely well, and postoperative recovery was uneventful. At death, the masseter muscle in animals that underwent myectomy was composed only of vertically oriented fibers; no regeneration of the SM was encountered. Although minor retardation in the rate of weight gain was apparent after 5 days, the differences (in body weight at death or in percent weight gained) were not statistically significant. The surgical control animals experienced a rate of weight gain intermediate between the control and myectomy samples. There were no significant differences in weight gain at the 20-day sacrifice interval (Table I).
Protruder muscles attd jaw growth
Volume 100 Number 5
439
Table I. Body weight gain in experimental groups at 5 and 20 days after surgery*
5 days after surgery Weight at death Weight gain--surgery to death (%) 20 days after surgery Weight at death Weight gain--surgery to death (%)
I
Surgical
Superficial masseter
myectomy
F value
112.6 ± 6.9 (7) 53.9 ± 12.3 (7)
107.7 ± 7.3 (7) 48.6 ± 5.3 (7)
105,6 ± 5.8 (7) 46.0 ± 5.7 (7)
2.01 (NS)
205.4 ± 7.7 (11) 179.3 ± 15.1 (11)
210.6 __+ 8.2 (lO) 183.7 ± l l . O (10)
209.8 __- 14.6 (i0) 173.6 ± 11.4 (10)
0.76 (NS)
I
Control
control
1.31 (NS)
1.58 (NS)
NS = not significant.
*Body weight data were not available for 10 days after surgery.
Table II. Intergroup differences in selected lower jaw dimensions at 20 days after surgery: Mean ___ SD
Dimensions Mental foramen to angular process Mental foramen to notch Mental foramen to condyle Superior surface of condyle to inferior border of mandible ( M H I ) Mandibular foramen to inferior border of mandible (MH2)
Surgical Control 17.71 --(14) 15.00 (17) 18.38 (17) 10.33 ± (15) 6.41 ± (15)
0.59 0.50 0.50 0.39 0.24
control 17.43 -'(14) 14.60 --(15) 18.00 --(15) 10.37 -'(15) 6.42 ± (12)
0.41 0.28 0.34 0.61 0.27
Superficial masseter myectomy 17.17 --- 0.36* (15) 14.33 ± 0.31"* (16) 18.04 --- 0.43 (16) 9.48 ± 0.51"** (15) 5.99 ± 0.29*** (9)
F
value 4.98 12.83 3.65 14.45 8.51
*Different from control and surgical controls at p -----0.05. **Different from controls at p ---
