Surgical Lengthening of the Mandible of a Bactrian Camel by Distraction Osteogenesis
A female Bactrian camel (Camelus bactrianus) suffered trauma to her lower jaw shortly after birth due to the overenthusiastic attempts by her mother to get her to stand. She experienced extensive damage to the lateral aspect of the rostral portion of the left mandible. Bone fragments were evident although the jaw was not completely fractured. The wounds were debrided and sutured, and she was separated from her mother for hand-rearing. Her subsequent development was normal except her mandible failed to grow as fast as her maxilla, presumably due to trauma to the rostral growth center. As a result, by the time she had reached 2 years of age her mandible was approximately 4 cm shorter than the maxilla and her tongue protruded from the left side of her mouth. By 3 years of age, brachygnathism had increased to 5 cm and the camel was showing increasing problems in food prehension. She was referred to Ontario Veterinary College for corrective surgery at the age of 3 years 3 months.
The camel, which weighed 477 kg, was anesthetized and her head was placed in dorsal recumbency. Following standard preparation for aseptic surgery, the ventral surface of the ramus of each mandible was exposed at its narrowest point, approximately 4 cm rostral to the first premolar. A custom-made distraction apparatus consisted of five semicircular perforated aluminum alloy plates, held approximately 2 cm apart, on two 10-cm long threaded expansion bars. Three of the plates were positioned caudal to the proposed osteotomy site, two rostrally. The plates were secured to the mandible through attachment by locking nuts to four threaded Steinmann pins inserted perpendicularly into the ventrolateral aspect of each ramus: two caudal to the osteotomy site and two rostral. One additional pin was inserted centrally into the mandibular symphysis and affixed to the most rostral plate. The pins were threaded into, but not through, the distal cortex. Once the device was secured to the mandible and stabilized, a cortical osteotomy of each mandible was performed at the exposed sites. Two holes were drilled from the ventral aspect through the ramus of each mandible to weaken the bone, and transection of the mandible was performed using an osteotome with care being taken to minimize trauma to the gingiva, periosteum, and the medullary blood supply. The surgical wound was closed and, once the device was considered stable, the camel was allowed to recover.
Following surgery, ceftiofur (Excenel, Upjohn Animal Health, Orangeville, Ontario L9W 3T3, Canada) 500 mg IV was given for 3 weeks, and butorphanol (Torbugesic, Ayerst, Guelph, Ontario N1K 1E4) 240 mg IM BID was given for 3 days. From the sixth postoperative day, the rostral segment of the mandible was advanced relative to the caudal section 1 mm each day by rotating the threaded expansion bar one full turn on each side. A light bandage and a waterproof vinyl bag applied over the device, provided protection, and insulation. At first the camel showed some discomfort during the distractions but was soon highly cooperative. Localized infection was observed around several of the rostral pins after 30 days. Staphylococcus aureus and Enterococcus faecalis were cultured from the exudate. Topical treatment was combined with a second course of ceftiofur for 10 days. Daily 1-mm distractions were continued for 40 days. Expansion of the device was evident by the separation of the semicircular plates along the expansion bar. After about 35 days of expansion, a progressive bend was seen in the expansion bars and it was apparent the apparatus was expanding more than the mandible. There was also some deviation of the mandible to the right side. The camel again showed some discomfort during the daily distraction procedure. Although the distraction device had been expanded 4 cm, the mandible had not lengthened to the same degree. Further expansion was not attempted due to limitations in the length of the expansion bars, and the rapid healing of one mandible. Throughout the post-surgical period the camel continued to eat, although she would only consume a cubed ration and refused hay.
Following completion of the expansion phase, the device was maintained but no further distractions were performed for an additional 38 days. Eighty-five days postoperatively, radiography revealed adequate ossification of the healing callus and the device was removed in two stages. One of the two sets of caudal pins and one of the rostral sets were removed initially, without anesthesia. The remainder of the device was removed under anesthesia 27 days later, 112 days after the original surgery. The jaw had been lengthened approximately 1 cm.
Distraction osteogenesis is the process of new bone formation between bone segments that are gradually separated by incremental traction.2 The process begins when a distraction force is applied to the healing callus and continues as long as the distraction is continued. The distraction force applied to bone creates tension in the surrounding soft tissues, initiating a sequence of changes termed distraction histogenesis.2 Although the principles of bone remodeling and lengthening have been familiar to surgeons for centuries, the science behind distraction osteogenesis was not known, and complications including non-unions and delayed healing prevented widespread acceptance of the procedure until the late 20th century.
The Russian surgeon Gavriil Ilizarov refined the technique in the 1950s with the use of circular fixators which provide greater stability than the earlier linear versions. He is widely credited as the pioneer of this procedure. The biologic principles he espoused: minimal disturbance of bone, a delay before distraction, consistent rate (amount of expansion per day) and rhythm (number of distractions per day) of distraction, and the importance of the site and the technique of osteotomy have been well accepted. Originally used for lengthening and correcting deformities of the long bones, the technique has gained great popularity over the past 10 years, particularly for the correction of craniofacial bone deformities, including lengthening of the mandible.1,2
Typically, the bone is separated by means of a low-energy osteotomy of the cortex preserving local blood supply to both the periosteum and medullary canal. Following a 5-day period of latency to allow the formation of a stable blood clot and granulation tissue, the ends of the bone are uniformly separated at a constant rate. Mechanical tension promotes growth of the healing connective tissue faster than occurs with normal fracture healing through increased blood supply at the fracture site, fibroblast hyperplasia and hypertrophy, alignment of the fibroblasts along the axis of the direction of tension, and subsequent osteoblast proliferation and ossification. Optimum osteogenesis of long bones occurs at a distraction rate of 1 mm/day.1-3 Distraction is continued for a period of approximately 6 weeks, or less if the desired correction has been achieved. After distraction ceases, the device is left in place for an additional period to allow the fibrous callus to ossify completely.
In animals, besides its widespread use in laboratory animals for experimental purposes, the technique has been used to correct long bone deformities in horses and dogs, and for jaw lengthening in horses.3 Our experience has shown that the technique is applicable to zoo animals provided they tolerate the external fixator and the daily manipulation of the device during the distraction period. In this case we were aided by the camel’s exceptional disposition. Unfortunately, the degree of lengthening achieved was not as much as desired. This is likely due to more rapid ossification of the osteotomy site than expected. Although there was some improvement in her ability to eat, she still eats slowly, and her appearance was not changed significantly. In retrospect, it may have been necessary to increase the rate and/or rhythm of the distractions.
1. Aronson J. Principles of distraction osteogenesis. In: McCarthy JG, ed. Distraction of the Craniofacial Skeleton. New York, NY: Springer; 1999:51–66.
2. Samchukov ML, Cherkashin AM, Cope JB. Distraction osteogenesis: origins and evolution. In: McNamara JA, Trotman C. eds. Distraction Osteogenesis and Tissue Engineering: Including Proceedings from the 24th Annual Moyers Symposium; March 1–2, 1997; Ann Arbor, MI: University of Michigan. 1998.
3. Welch RD, Lewis DD. Distraction osteogenesis. Vet Clin North Am Small Anim Pract. 1999;29:1187–1205.