Distraction Osteogenesis for Treatment of Premature Physeal Closure and Shortening of the Third and Fourth Metatarsals of a Dog

Introduction

Metatarsal bone length is achieved through endochondral ossification at the distal physeal plate. During skeletal development, endochondral ossification provides for bone growth as chondrocytes in the physeal plate progress through phenotypic zones of the plate to allow for subsequent bone production and remodeling.¹ At maturity, the physeal plates close, and growth contributing to bone length ceases. The distal physeal plates of the metatarsal bones are reported to close between 5.5 and 9 months of age in dogs.² Injury or insult to the physeal plate prior to skeletal maturity can result in shortening or deformity of the bone. Numerous reports in the medical literature related to humans have demonstrated metatarsalgia and painful toe deformity as a result of shortening or deformity of the primary weight-bearing metatarsals (III and IV).³⁴ These symptoms in humans have been alleviated by Ilizarov distraction techniques to treat disorders such as clubfoot, and more rare conditions, such as brachymetatarsia.^5–12 To the authors’ knowledge, lameness resulting from metatarsal or metacarpal shortening in dogs has not been reported. However, a biomechanical alteration in the weight-bearing components of the paws of dogs is a relatively common cause of lameness. Therefore, substantial shortening of metatarsal III and IV could be a primary cause of lameness in dogs.

Circular external skeletal fixators (CESF) offer an excellent means of treatment for shortened and deformed long bones.¹³¹⁴ Through the use of CESF motors, shortened bones can be lengthened through the process of distraction osteogenesis.¹³¹⁴ Distraction osteogenesis employs the principles of controlled micromotion to stimulate the formation of bony micro-columns that form along the lines of tension created by the CESF.¹³¹⁵ Indications for use of CESF in dogs include comminuted fractures, leg lengthening procedures, and correction of angular limb deformities.^16–22 To the authors’ knowledge, there have been no reports of using distraction osteogenesis for lengthening metatarsal bones in dogs. The purpose of this report is to describe the use of CESF to lengthen the third and fourth metatarsals in a dog with fracture-induced premature physeal closure with shortening of the bones causing lameness.

Case Report

A 4-month-old, 16-kg, intact male Siberian husky was presented for evaluation of a left hind-limb lameness of 3 weeks’ duration. Ten days prior to presentation, the referring veterinarian diagnosed previous fractures of the left third and fourth metatarsals based on radiographic examination. However, the owners reported no known history of trauma. Physical examination revealed disproportionate growth of the left third and fourth metatarsals with resultant relative shortening and hyperextension of digits III and IV with respect to digits II and V [Figure 1]. A weight-bearing lameness of the left hind limb was present. No other abnormalities were noted on complete physical examination.

Radiographs of both hind limbs, from tarsus to phalanges, were obtained [Figure 2]. The radiographic appearance of the right hind paw was considered to be within normal limits. Radiographs of the left hind paw showed healing fractures of the distal aspects of metatarsals III and IV. Bony callous bridged the fracture of metatarsal IV; however, a radiographic fracture line was still evident. Bone resorption was noted in metatarsal III, but the fracture appeared radiographically healed. The distal physis of metatarsals III and IV were closed, resulting in 1-cm decreased length in comparison to the right hind limb. The radiographic diagnosis was healing fractures of metatarsals III and IV, with premature closure of the distal physeal plates causing shortening of both metatarsals. Resultant shortening and hyperextension of digits III and IV were also evident.

Based on physical and radiographic examination, the hind-limb lameness in this dog was felt to be attributable to the shortened metatarsal bones. Distraction osteogenesis accomplished through the use of a CESF was recommended to the owners as the most appropriate treatment option for this patient. However, the owners were made fully aware of the novelty of this technique for treatment of shortened metatarsals, as well as the surgeons’ concerns regarding complete correction of the hyperextended digits and the hind-limb lameness. After thorough discussion of all treatment options and associated prognoses, the owners opted for distraction osteogenesis using a CESF.

The day following presentation, thorough planning of the surgical procedure was undertaken. After consulting a surgeon^a with extensive experience using CESF, the fixator was constructed based on the gross and radiographic anatomy of the patient and the intended surgical manipulations.

The following day, the patient was premedicated, anesthetized, and prepared for aseptic surgery of the left metatarsal area. A 5-cm incision was made between the third and fourth metatarsals on the dorsal aspect of the paw. The fascia overlying the third and fourth metatarsals was incised along the same line as the skin incision and was reflected to allow each bone to be isolated. Fluoroscopic guidance^b was used to allow precise placement of the CESF transfixation wires and the osteotomies. An initial Kirschner wire^c (0.045-inch) was placed percutaneously from lateral to medial to incorporate the proximal diaphyses of both metatarsals III and IV. A two-ring (proximal full ring and distal 0.75 ring) CESF^d with medial and lateral linear motors was initially attached to the wire using corresponding bolts, nuts, and washers. Two Kirschner wires (0.045-inch) were then placed percutaneously at divergent angles in each of the distal diaphyses of metatarsals III and IV. A final Kirschner wire (0.0625-inch) was placed percutaneously from lateral to medial and slightly divergent to the initially placed wire, incorporating the fourth and central tarsal bones. The wires were tensioned with a tensioner,^d and the CESF was secured using corresponding bolts, nuts, and washers. The periosteum over the diaphyses of the third and fourth metatarsals was incised in an axial direction and elevated over the entire circumference of the bone at the site of subsequent osteotomy in order to preserve continuity of the periosteum. Transverse osteotomies were made in metatarsals III and IV using a pneumatic sagittal saw.^e The osteotomies were staggered in a proximal-distal direction in order to minimize the chance of synostosis between the metatarsals. Routine closure of the surgical site was performed. Postoperative radiographs were obtained [Figure 3], and a soft-padded bandage was placed over the limb and CESF.

A 2-day latency period was allowed prior to initiation of distraction. Two days after surgery, distraction of the CESF was initiated by manually turning both motors to provide 0.5 mm of linear distraction twice daily (i.e., 1.0 mm per day). Four days after surgery, the amount of distraction was increased to 0.75 mm twice daily (i.e., 1.5 mm per day). Seven days following surgery, radiographs of the left hind paw were obtained [Figure 4]. Radiographic examination revealed linear distraction of 9 mm in each metatarsal, with early regenerate bone formation in the osteotomy gap. The 9-mm gain in length approximated the original discrepancy in metatarsal length between left and right hind limbs. However, based on the dog’s age and radiographic examination, continued growth of metatarsals II and V in the left hind limb was expected. Therefore, continued distraction at a rate of 1.5 mm per day for 8 days was recommended. Eight days after surgery, skin sutures were removed, and the dog was discharged to the owners with instructions to continue the linear distraction at a rate of 1.5 mm per day (i.e., 0.75 mm twice daily) for 8 days. The owners were instructed to encourage controlled walking on a leash at a pace to allow use of the affected hind limb. General incision and CESF care was also discussed.

The owners of the dog successfully completed the remainder of the distraction protocol and reported no complications associated with the surgery. The dog was returned 3 weeks after surgery for follow-up examination. On presentation, the dog was weight bearing on the left hind limb with no evidence of lameness. Radiographs of the left hind limb were obtained and revealed appropriate metatarsal length and alignment with substantial regenerate bone formation [Figure 5]. The CESF was locked in place, and the dog was exercise restricted for a period of 2 more weeks.

The dog returned again for follow-up examination after 2 weeks. The dog had returned to full weight bearing, and no lameness could be appreciated. Radiographs were obtained, and evidence of metatarsal healing was deemed sufficient enough for removal of the CESF [Figure 6]. The CESF was removed under sedation. Examination of the left hind paw after CESF removal revealed appropriate relative length of digits III and IV with respect to digits II and V [Figure 7]. The hyperextension of digits III and IV was substantially improved.

The dog was discharged, and the owners were instructed to allow a progressive return to full function over the ensuing 4 weeks. During telephone conversations 6 and 12 months after surgery, the owners reported that the dog was ambulating normally with no evidence of lameness or morbidity associated with the surgery.

Discussion

Distraction osteogenesis is an effective method for regaining proper alignment of deformed limbs, restoring functional length of bones, and replacing bone defects.¹³¹⁴ In the case described in this report, a traumatic event created fractures involving the distal physeal plates of the metatarsals. The end result was premature closure of the physeal plates, shortening of the metatarsals, hyperextension of the digits, and disproportionate length of the third and fourth phalanges. Hyperextension of the digits was attributed to abnormal tendon lengths secondary to the abnormal development of the associated metatarsals. As a result, the distal phalanges remained in an extended position. Progressive correction of the shortened metatarsals should allow for the associated tendons to achieve a more anatomically functional length. This was evident in this case, as the digital hyperextension was greatly improved 6 weeks after surgery. Based on the dog’s lameness on presentation, it is likely that further skeletal growth during maturation would cause the affected metatarsals to become increasingly disproportionate, exacerbating the dysfunction. Therefore, the timing and method in regaining proper length and function in the left hind limb of this patient were considered integral in obtaining the successful outcome seen in this case.

Although other methods for correction of length discrepancies in bone can be utilized, all of the techniques require acute correction of the deformity after the osteotomy. These techniques have disadvantages, including the inability to adjust bone length and alignment, lack of axial micromotion when using rigid fixation, and inappropriate rates of tendon, nerve, and vessel stretching. Circular external ring fixators, although technically more difficult to apply, allow for adjustment, biological lengthening rates, angular correction, rotational correction, and gradual soft-tissue stretching.¹³¹⁶²³

Distraction osteogenesis occurs through three distinct phases. The first stage involves the osteotomy or corticotomy. A corticotomy involves cutting two-thirds of the diameter of a bone and then twisting the bone to fracture it. This method was first described by Ilizarov as an attempt to preserve medullary blood supply.¹³¹⁴ Presently, it is more common to perform a complete osteotomy and preserve the associated periosteum as much as possible. It is now known that medullary blood supply is quickly reestablished after a complete osteotomy, making this technique a more practical alternative.¹³¹⁶ Complete osteotomies were used in the present case, and careful consideration was given to the placement of the osteotomy sites. The osteotomies were staggered to reduce the chance of synostosis during the healing period. Although metatarsal synostosis has not been reported in dogs, it is possible that gait dysfunction and pain could result if a synostosis was to occur between metatarsals III and IV. It has been reported in humans that synostosis of the metatarsals resulted in flattening of the transverse arch of the foot, creating a painful gait.²⁴

The second phase of distraction osteogenesis is described as the lag phase. During this time, the osteotomy site is flooded with blood, growth factors, and bone precursor cells, and revascularization occurs. In a mature animal, it has been proposed that 4 to 7 days is the optimal lag period.¹³¹⁴²⁵ Based on the age of the dog in this case, a lag period of 2 days was used. A shortened lag time is often instituted in immature animals because of the rapid healing characteristics of young animals. Problems, including synostosis and loss of distraction capabilities, have been reported in young animals and children as exuberant callus formation occurs at the osteotomy site.²⁴ As a result, no lag phase or a short, 1- to 2-day lag phase has been recommended for young animals.¹³

As the lag phase is complete, fibrovascular tissue, growth factors, and osteogenic precursor cells are firmly attached to the opposing bone ends. As the ends of these bones are distracted, regenerate bone forms into micro-columns along tension lines created by the distraction device. There is some debate regarding the ideal rate for distraction. When possible, micro-amounts of distraction approximately 2 mm per day or 0.001 mm per second would occur throughout the daily distraction period. However, this can only be accomplished through the use of costly, computer-driven, motorized devices. More commonly in veterinary patients, manually controlled motors on the CESF are incrementally turned anywhere from one to four times per day.¹³ In the present case, distraction was started at 1 mm per day and was increased to 1.5 mm per day. The distraction was divided into two distraction intervals, approximately 12 hours apart. This distraction rhythm has been shown to be appropriate for lengthening procedures when continuous distraction cannot be employed.¹³ After the distraction period is complete, the fixator is locked to provide stability for the interpositional tissue to mature and remodel into functional bone. The dog in this case was distracted for a total period of 15 days. The CESF was maintained an additional 2 weeks before removal. The final outcome of treatment for this dog was proportionate metatarsal length with excellent limb function.

Conclusion

Distraction osteogenesis using CESF has been employed in human and animal patients for corrective osteotomies, comminuted fractures, and shortened bones. In this case, an obvious shortening deformity in metatarsals III and IV caused hind-limb dysfunction. Through the use of a CESF construct and gradual distraction, the metatarsals were lengthened to a clinically functional status. The end result was a completely healed osteotomy site, as well as a return to normal function. It is the authors’ hopes that this case represents yet another way that the CESFs are applicable to veterinary orthopedics.