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An experimental surgical reduction of the mandible of a pig
An experimental surgical reduction of the mandible of a pig John D. Atherton, Ph.D., Barry 8. J. Lovius, Lawrence D. Finch, MB., Ch.B., BSc.
Liverpool,
F.D.S., and
England
A
strong opinion exists among oral surgeons and orthodontists that a patient with a prognathic mandible should not be operated upon for reduction of the prognathism until mandibular growth is complete. The logic behind this opinion is that the intrinsic growth pattern of the patient will tend to restore the face to its genetically predetermined shape, should the mandible be surgically reduced at a time when there is active growth. Clinicians also rightly believe that it is impossible to predict the growth of a particular patient. Few cases of relapse have been reported in the literature. Goldstein1 reported two cases in which he attributed relapse to early operation; one patient was 17 years of age and the other was 15. Relapse was attributed to continued mandibular growth of the patient. The successful treatment of young patients is reported by Biederman2 who described the treatment by ostectomy of three paients aged 16, 13, and 12 years. More recently, Knowles3 has reported six cases of osteotomy in patients between 9 and 10 years of age. The facial growth pattern and occlusal changes were reviewed after 3 to 5 years. No sudden relapses occurred during this period; nor were there any significant relapses which necessitated second operations. Knowles attributed this success to two factors: (1) Only patients with deep overbite were selected. (2) The operation was carried out in such a way that the position of the condyle head was not changed so that, in turn, the future growth direction would not be altered. In addition, during the operation the masseter and internal pterygoid muscles were stripped off the surface of the ramus, so that after the operation the muscles became reattached to the surface of the mandible in its new position. In this way the direction of muscle pull was not altered. Knowles4 considers that the operations which lead to the repositioning of the From the University
158
Departments of Liverpool,
of Orthodontics Liverpool L69
and 3BX,
Oral Surgery, England.
School
of
Dental
Surgery,
Volume Number
65 2
Experimental
Fig. 1. The mandible of a pig showing the of bone. The position of the metal implant a black spot near the posterior border.
surgical
line at
reduction
of pig
of cuts made to remove the time of the experiment
mmdible
159
a 1.5 cm. block is shown by
masseter and internal pterygoid along with the mandible are more likely to be followed by relapse than operations which leave these muscles in their original position. Many operations currently in use for correction of mandibular prognathism lead to a displacement of the muscles of mastication along with the distal movement of the mandible. Only two types of operation avoid this: (1) Operations which reduce the mandible by removal of a block of bone from each side of the body of the mandible. This type of operation leaves the ramus and the muscle attached to it in situ. (2) The type of operation described by Knowles in which the muscles are peeled off the ramus prior to its section. Method
We decided to test the possibility of relapse following surgical reduction of the mandible by an operation on an animal. A young pig was considered to be the animal of choice because of its large size and rapid growth. Atherton and Wynne5 found that the facial growth pattern in Class III patients was not unlike that in Class II or Class I patients. It was therefore considered a valid experiment if a normal young animal was operated on rather than a Class III type. One side only of the mandible was reduced, so that the other side might serve as a control. The body of the mandible was the site of choice because of the technical difficulty of operating in the ramus region. Knowles’ concept of not altering the direction of pull of the masseter and internal pterygoid muscles was therefore followed. Operation. Following induction of general anesthesia, a long incision was made along the crest of the ridge of the left alveolus. Buccal and lingual flaps were reflected, and the body of the mandible in the premolar area was completely exposed. Two angled cuts, 1.5 cm. apart, were made with a Lindemann bur. They were extended through the entire width of the mandible except for the lingual plate (Fig. I). Holes were made on the main segments to take two stainless steel wires for fixation, and then the cuts were completed through the lingual plate by means of an osteotome. The 1.5 cm. section was removed, and the
160
Atherton,
Fig. 2. Radiographs Experimental side.
Lovius, and Pinch
of the mandible of the experimental The position of the metal implants
Am. J. Orthod. February 1974
pig. Above, Control is indicated by black
side. spots.
Below,
segments were secured with two wire loops, The tissues were replaced, and the wound was closed. At the end of the operation metal implants or markers consisting of 2 mm. lengths of stainless steel wire 0.5 mm. in diameter were inserted into the posterior border of the mandible with a purpose built plunger (Figs. 1 and 2). The head was radiographed at a distance of 36 inches. Intravenous tetracyline was given postoperatively. Recovery was uneventful, except for the loss of one wire ligature along with a small sequestrum. The pig was kept alive for 4 months. The head was macerated to give a clean bone surface and then radiographed. The two flat on the surface of halves of the mandible were separated and radiographed the film (Fig. 2). A sibling pig was kept as a control. At the end of the experiment the control pig was slightly larger than the experimental pig. Results
Immediately following the operation, the pig was severely retrognathic with a deviation of the center of the mandible to the experimental side. With 4 months’ continued growth, the appearance of retrognathia diminished slightly. The situation is shown diagrammatically in Fig. 3, compiled from tracings of lateral radiographs of the head. In Fig. 3, views 1 and 2 show the experimental
vozume Number
65 2
Experimental
surgical
reduction
of pig mandible
3. Diagram made from radiographs of the experimental and control pigs. mental pig at the termination of the experiment. 2, Experimental pig immediately ing operation. 3, Control pig at the end of the experiment. 4, Control pig at the of the experiment.
Fig.
161
Experifollowbeginning 1,
pig and 3 and 4 show the control pig. The experimental pig was severely retrognathic immediately following the operation but recovered to an edge-to-edge incisor relationship. The normal relationship is for the lower incisors to be in advance of the upper incisors, and this is shown in the control pig. The length of the left (operation) side of the mandible of the experimental animal was measured from the incisal tip to the posterior surface of the ramus and found to be 20.2 cm. The right side of the mandible, similarly measured, was 18.5 cm. The difference between these two measurements was 1.7 cm., which corresponds almost exactly with the 1.5 cm. segment of bone removed from the body at the time of operation, if allowance is made for the width of the cuts. The relative shortness of the surgically treated side of the mandible is well shown in Figs. 2 and 4. The radiograph of the two sides of the mandible (Fig. 2) shows the location of the steel bone markers placed immediately after the operation. Each marker lies in a similar position in relation to the posterior border. The anterior direction of growth of the pig mandible can be very well seen from the orientation of the bone trabeculae of the ramus. This growth has carried the markers from near the posterior borders to positions beneath the crypts of the second molars. The amount of bone deposited on the posterior surface of the mandible was similar on each side. The maxilla and cranial base (Fig. 5) were slightly distorted by the experiment. The maxilla inclined toward the operation side. In the region of the temporal bones there was an asymmetry, with the glenoid fossa being more anteriorly placed than on the unoperated side, This displacement anteriorly involved the pterygoid plates, the bony capsule of the internal ear, and the articular fossa of the temporal bone. Discussion
It was expected that the operation growth might produce a compensatory
during the phase of active mandibular acceleration of bone deposition on the
162
Atherton,
Lovius,
Fii. 4. The mandible occlusal surface. The than the right side.
Am. J. Orthod. 1974 February
and Finch
of the experimental pig left side of the mandible
and of
that the
of the control experimental
pig viewed from the pig is 1.7 cm. shorter
experimental side, leading to the relapse that is feared will occur in man following early surgical procedures for reduction of mandibular prognathism. However, it would appear that in a period when growth was most prolific, no compensatory growth took place during the 4 months following removal of 1.7 cm. of bone (a 1.5 cm. block and the width of two cuts) from one side of the body of the mandible of a young pig. The amount of bone deposited on the posterior surfaces of the rami was similar on both sides, so that the operation side remained 1.7 cm. shorter. Although dimensionally there was no compensatory postoperative acceleration of growth in the bone of the mandible, the clinical appearance of retrognathia was less severe at the end of the experiment than in the immediate postoperative period. Two factors appeared to compensate for the bone deficiency: (1) The maxilla had deviated slightly toward the operation side and the whole area involving the condylar fossa on the operation side was displaced anteriorly, so that there was some compensation in the skull as a whole for the reduction in the mandible. (2) The 1.7 cm. of bone removed from the mandible represented a much higher proportion of the over-all size of the mandible at the time of operation than at the end of the experiment. The experiment can be said to support the clinical treatment of young prognathic patients by early operation. The growth mechanism of the mandible of the pig and man is, as far as is known, similar and therefore it can be inter-
Volume Number
65 2
Experimental
surgical
reduction
of pig mandible
163
Fig. 5. The skulls of the experimental pig (left] and control pig (right] viewed from the inferior surface. There is some distortion in the shape of the skull of the experimental pig, most noticeable in this photograph in the region of the maxilla, the medial and lateral pterygoid plates of the sphenoid bone, and the inferior surface of the temporal bone.
polated that if a portion of bone is removed from the premolar region of the mandible of a growing patient (the operation of ostectomy), the mandible will stay smaller than it would have been if the operation had not been carried out. It seems probable that compensatory changes that would reduce the effect of the operation would take place in the human skull. Also, the same argument applies to the human patient that has been applied to the pig, namely, that the amount of bone removed in the young animal is a greater proportion of the mandible at the time of operation than that removed in the fully grown mature animal. For both these reasons, it would seem a practical precaution to set the mandible back a litle more than appears necessary to allow for some compensatory change. This experiment does not accurately simulate the operation of osteotomy of the ramus. There may be a fundamental difference between the operation of ostectomy in which a piece of bone is removed and osteotomy in which the mandible is repositioned. However, the clinical experience of Knowles3 seems to indicate that, if the masseter and internal pterygoid muscles are detached from the ramus and allowed to reposition themselves after the operation, then there is little subsequent relapse. Summary
In this experiment 1.7 cm. of bone was removed from one side of the mandible of a young pig (the operation of ostectomy) and the pig was allowed to grow for 4 months. At the end of the experiment the mandible was 1.7 cm. shorter on the operation side. The amount of bone deposited (5.5 cm.) on the posterior
164
Athertox,
Lovius,
and Pinch
Am.
J. Orthod.
Februarg1974
surface of the ramus was the same on each side, as measured by metal implants. The face was more retrognathic immediately following operation than at the end of the experiment because compensatory changes had occurred in the cranial base, so that the condylar fossa area was more anteriorly placed on the operation side and also the amount of bone removed represented a greater proportion of the mandible at the time of operation than at the end of the experiment. The experiment can be said to support the clinical treatment of young prognathic patients by early operation. REFERENCES
1. Goldstein, A.: Appraisal of results of surgical correction of Class III malocclusions, Angle Orthod. 17: 59-91, 1964. 2. Biederman, W.: The orthodontist’s role in resecting the prognathic mandible, AM. J. ORTHOD. 53: 356-375, 1970. 3. Knowles, C. C.: Long term results of mandibular osteotomy: An interim report on the treatment of young subjects, Trans. Br. Sot. Study Orthod. 56: 12-26, 1969. 4. Knowles, C. C.: Personal communication, 1972. 5. Atherton, J. D., and Wynne, T. H. M.: A long term assessment of the facial pattern in children who had received orthodontic treatment, Dent. Pratt. Dent. Rec. 14: 317-322, 1964. Pembroke P. 0. Box
Place 147
Liverpool,
F.D.S., and
England
A
strong opinion exists among oral surgeons and orthodontists that a patient with a prognathic mandible should not be operated upon for reduction of the prognathism until mandibular growth is complete. The logic behind this opinion is that the intrinsic growth pattern of the patient will tend to restore the face to its genetically predetermined shape, should the mandible be surgically reduced at a time when there is active growth. Clinicians also rightly believe that it is impossible to predict the growth of a particular patient. Few cases of relapse have been reported in the literature. Goldstein1 reported two cases in which he attributed relapse to early operation; one patient was 17 years of age and the other was 15. Relapse was attributed to continued mandibular growth of the patient. The successful treatment of young patients is reported by Biederman2 who described the treatment by ostectomy of three paients aged 16, 13, and 12 years. More recently, Knowles3 has reported six cases of osteotomy in patients between 9 and 10 years of age. The facial growth pattern and occlusal changes were reviewed after 3 to 5 years. No sudden relapses occurred during this period; nor were there any significant relapses which necessitated second operations. Knowles attributed this success to two factors: (1) Only patients with deep overbite were selected. (2) The operation was carried out in such a way that the position of the condyle head was not changed so that, in turn, the future growth direction would not be altered. In addition, during the operation the masseter and internal pterygoid muscles were stripped off the surface of the ramus, so that after the operation the muscles became reattached to the surface of the mandible in its new position. In this way the direction of muscle pull was not altered. Knowles4 considers that the operations which lead to the repositioning of the From the University
158
Departments of Liverpool,
of Orthodontics Liverpool L69
and 3BX,
Oral Surgery, England.
School
of
Dental
Surgery,
Volume Number
65 2
Experimental
Fig. 1. The mandible of a pig showing the of bone. The position of the metal implant a black spot near the posterior border.
surgical
line at
reduction
of pig
of cuts made to remove the time of the experiment
mmdible
159
a 1.5 cm. block is shown by
masseter and internal pterygoid along with the mandible are more likely to be followed by relapse than operations which leave these muscles in their original position. Many operations currently in use for correction of mandibular prognathism lead to a displacement of the muscles of mastication along with the distal movement of the mandible. Only two types of operation avoid this: (1) Operations which reduce the mandible by removal of a block of bone from each side of the body of the mandible. This type of operation leaves the ramus and the muscle attached to it in situ. (2) The type of operation described by Knowles in which the muscles are peeled off the ramus prior to its section. Method
We decided to test the possibility of relapse following surgical reduction of the mandible by an operation on an animal. A young pig was considered to be the animal of choice because of its large size and rapid growth. Atherton and Wynne5 found that the facial growth pattern in Class III patients was not unlike that in Class II or Class I patients. It was therefore considered a valid experiment if a normal young animal was operated on rather than a Class III type. One side only of the mandible was reduced, so that the other side might serve as a control. The body of the mandible was the site of choice because of the technical difficulty of operating in the ramus region. Knowles’ concept of not altering the direction of pull of the masseter and internal pterygoid muscles was therefore followed. Operation. Following induction of general anesthesia, a long incision was made along the crest of the ridge of the left alveolus. Buccal and lingual flaps were reflected, and the body of the mandible in the premolar area was completely exposed. Two angled cuts, 1.5 cm. apart, were made with a Lindemann bur. They were extended through the entire width of the mandible except for the lingual plate (Fig. I). Holes were made on the main segments to take two stainless steel wires for fixation, and then the cuts were completed through the lingual plate by means of an osteotome. The 1.5 cm. section was removed, and the
160
Atherton,
Fig. 2. Radiographs Experimental side.
Lovius, and Pinch
of the mandible of the experimental The position of the metal implants
Am. J. Orthod. February 1974
pig. Above, Control is indicated by black
side. spots.
Below,
segments were secured with two wire loops, The tissues were replaced, and the wound was closed. At the end of the operation metal implants or markers consisting of 2 mm. lengths of stainless steel wire 0.5 mm. in diameter were inserted into the posterior border of the mandible with a purpose built plunger (Figs. 1 and 2). The head was radiographed at a distance of 36 inches. Intravenous tetracyline was given postoperatively. Recovery was uneventful, except for the loss of one wire ligature along with a small sequestrum. The pig was kept alive for 4 months. The head was macerated to give a clean bone surface and then radiographed. The two flat on the surface of halves of the mandible were separated and radiographed the film (Fig. 2). A sibling pig was kept as a control. At the end of the experiment the control pig was slightly larger than the experimental pig. Results
Immediately following the operation, the pig was severely retrognathic with a deviation of the center of the mandible to the experimental side. With 4 months’ continued growth, the appearance of retrognathia diminished slightly. The situation is shown diagrammatically in Fig. 3, compiled from tracings of lateral radiographs of the head. In Fig. 3, views 1 and 2 show the experimental
vozume Number
65 2
Experimental
surgical
reduction
of pig mandible
3. Diagram made from radiographs of the experimental and control pigs. mental pig at the termination of the experiment. 2, Experimental pig immediately ing operation. 3, Control pig at the end of the experiment. 4, Control pig at the of the experiment.
Fig.
161
Experifollowbeginning 1,
pig and 3 and 4 show the control pig. The experimental pig was severely retrognathic immediately following the operation but recovered to an edge-to-edge incisor relationship. The normal relationship is for the lower incisors to be in advance of the upper incisors, and this is shown in the control pig. The length of the left (operation) side of the mandible of the experimental animal was measured from the incisal tip to the posterior surface of the ramus and found to be 20.2 cm. The right side of the mandible, similarly measured, was 18.5 cm. The difference between these two measurements was 1.7 cm., which corresponds almost exactly with the 1.5 cm. segment of bone removed from the body at the time of operation, if allowance is made for the width of the cuts. The relative shortness of the surgically treated side of the mandible is well shown in Figs. 2 and 4. The radiograph of the two sides of the mandible (Fig. 2) shows the location of the steel bone markers placed immediately after the operation. Each marker lies in a similar position in relation to the posterior border. The anterior direction of growth of the pig mandible can be very well seen from the orientation of the bone trabeculae of the ramus. This growth has carried the markers from near the posterior borders to positions beneath the crypts of the second molars. The amount of bone deposited on the posterior surface of the mandible was similar on each side. The maxilla and cranial base (Fig. 5) were slightly distorted by the experiment. The maxilla inclined toward the operation side. In the region of the temporal bones there was an asymmetry, with the glenoid fossa being more anteriorly placed than on the unoperated side, This displacement anteriorly involved the pterygoid plates, the bony capsule of the internal ear, and the articular fossa of the temporal bone. Discussion
It was expected that the operation growth might produce a compensatory
during the phase of active mandibular acceleration of bone deposition on the
162
Atherton,
Lovius,
Fii. 4. The mandible occlusal surface. The than the right side.
Am. J. Orthod. 1974 February
and Finch
of the experimental pig left side of the mandible
and of
that the
of the control experimental
pig viewed from the pig is 1.7 cm. shorter
experimental side, leading to the relapse that is feared will occur in man following early surgical procedures for reduction of mandibular prognathism. However, it would appear that in a period when growth was most prolific, no compensatory growth took place during the 4 months following removal of 1.7 cm. of bone (a 1.5 cm. block and the width of two cuts) from one side of the body of the mandible of a young pig. The amount of bone deposited on the posterior surfaces of the rami was similar on both sides, so that the operation side remained 1.7 cm. shorter. Although dimensionally there was no compensatory postoperative acceleration of growth in the bone of the mandible, the clinical appearance of retrognathia was less severe at the end of the experiment than in the immediate postoperative period. Two factors appeared to compensate for the bone deficiency: (1) The maxilla had deviated slightly toward the operation side and the whole area involving the condylar fossa on the operation side was displaced anteriorly, so that there was some compensation in the skull as a whole for the reduction in the mandible. (2) The 1.7 cm. of bone removed from the mandible represented a much higher proportion of the over-all size of the mandible at the time of operation than at the end of the experiment. The experiment can be said to support the clinical treatment of young prognathic patients by early operation. The growth mechanism of the mandible of the pig and man is, as far as is known, similar and therefore it can be inter-
Volume Number
65 2
Experimental
surgical
reduction
of pig mandible
163
Fig. 5. The skulls of the experimental pig (left] and control pig (right] viewed from the inferior surface. There is some distortion in the shape of the skull of the experimental pig, most noticeable in this photograph in the region of the maxilla, the medial and lateral pterygoid plates of the sphenoid bone, and the inferior surface of the temporal bone.
polated that if a portion of bone is removed from the premolar region of the mandible of a growing patient (the operation of ostectomy), the mandible will stay smaller than it would have been if the operation had not been carried out. It seems probable that compensatory changes that would reduce the effect of the operation would take place in the human skull. Also, the same argument applies to the human patient that has been applied to the pig, namely, that the amount of bone removed in the young animal is a greater proportion of the mandible at the time of operation than that removed in the fully grown mature animal. For both these reasons, it would seem a practical precaution to set the mandible back a litle more than appears necessary to allow for some compensatory change. This experiment does not accurately simulate the operation of osteotomy of the ramus. There may be a fundamental difference between the operation of ostectomy in which a piece of bone is removed and osteotomy in which the mandible is repositioned. However, the clinical experience of Knowles3 seems to indicate that, if the masseter and internal pterygoid muscles are detached from the ramus and allowed to reposition themselves after the operation, then there is little subsequent relapse. Summary
In this experiment 1.7 cm. of bone was removed from one side of the mandible of a young pig (the operation of ostectomy) and the pig was allowed to grow for 4 months. At the end of the experiment the mandible was 1.7 cm. shorter on the operation side. The amount of bone deposited (5.5 cm.) on the posterior
164
Athertox,
Lovius,
and Pinch
Am.
J. Orthod.
Februarg1974
surface of the ramus was the same on each side, as measured by metal implants. The face was more retrognathic immediately following operation than at the end of the experiment because compensatory changes had occurred in the cranial base, so that the condylar fossa area was more anteriorly placed on the operation side and also the amount of bone removed represented a greater proportion of the mandible at the time of operation than at the end of the experiment. The experiment can be said to support the clinical treatment of young prognathic patients by early operation. REFERENCES
1. Goldstein, A.: Appraisal of results of surgical correction of Class III malocclusions, Angle Orthod. 17: 59-91, 1964. 2. Biederman, W.: The orthodontist’s role in resecting the prognathic mandible, AM. J. ORTHOD. 53: 356-375, 1970. 3. Knowles, C. C.: Long term results of mandibular osteotomy: An interim report on the treatment of young subjects, Trans. Br. Sot. Study Orthod. 56: 12-26, 1969. 4. Knowles, C. C.: Personal communication, 1972. 5. Atherton, J. D., and Wynne, T. H. M.: A long term assessment of the facial pattern in children who had received orthodontic treatment, Dent. Pratt. Dent. Rec. 14: 317-322, 1964. Pembroke P. 0. Box
Place 147