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Segmental Bone Transport for the Treatment of Bone Deficits

Arnold S. Lesser, VMD, Diplomate ACVS

New bone can be formed by stretching an osteotomy 1 mm a day. If done properly, the callus formed will not bridge until distraction is discontinued, and a period of neutralization or compression then will allow the callus to transform into cortical bone. Two cases of infected, tibial nonunions are presented in which this technique was used to fill the bone defect created after sequestrectomy. Bone gaps of 5 cm and 2 cm were filled in a rottweiler and a cat, respectively.

Introduction

The technique of distraction osteogenesis as described by Ilizarov has been used for lengthening bone, correcting angular or rotational deformities, and filling bone defects. Although these techniques are relatively new to western medical practice, some experience in the treatment of angular limb deformities has been documented in the veterinary literature. Less common is the use of this technique for free bone transport to fill in bony gaps. Two cases are described in which segmental bone transport was used to fill bone defects created by removal of infected sequestra.

Case Reports

Case No. 1

A 1.5-year-old rottweiler was referred for the treatment of osteomyelitis and nonunion of the right tibia [Figures 1A, 1B]. Nine weeks previously, the dog fell over backward and sustained a long, spiral, closed fracture of the tibia that was stabilized by open reduction and internal fixation with two intramedullary (IM) pins and seven full cerclage wires. Three weeks later the fixation broke down and a large, infected, mid-diaphyseal sequestrum developed along with multiple draining fistulas. Six weeks later the dog was referred for further evaluation and treatment by the author. Surgery was performed, and the sequestrum, which measure 7 cm, and the IM pins and cerclage wires were removed. A sample of the granulation tissue surrounding the sequestrum was collected for bacterial culture and sensitivity. Escherichi coli and Staphylococcus intermedius were cultured, and appropriate antibiotic therapy was initiated. This case was basically an infected, vascular nonunion with an avascular component (the sequestrum), as evidenced by the proliferative callus present at either end of the fracture fragments and partially surrounding the sequestrum {Figures 1A, 1B]. The exuberant callus provided a ready source of bone graft which was harvested with a rongeur and packed into the gap left by the sequestrectomy. The graft extended along the entire bed but did not fill the gap completely.

An osteotomy was performed 2.5 cm proximal to the distal end of the proximal fragment. The interosseous membrane attachments to the fibula were separated so there would be no impediment to moving the tibial segment distally. A bilateral, external fixator (type 2) was placed on the tibia to stabilize the proximal and distal fragments {Figures 2A, 2B]. A separate 3.2-mm pin was placed through the free cortical segment and attached to two medium Kirschner fixation clamps that also were placed on two 4.8-mm threaded bars. (insert 2 pages figures) These two threaded bars were incorporated into the bilateral fixator. The two clamps were stabilized on the threaded bars with two nuts each [Figure 3]. This allowed the free segment to be distally 1 mm a day by loosening the distal nut and tightening the proximal nut a set amount (corresponding to 1 mm of advancement) daily. The distraction of the bone segment was delayed for a five-day lag period. The owners chose to hospitalize the dog for the lengthening period because of the presence of a second rottweiler at home.

At three weeks the osteotomy was distracted 1.5 cm, and the dog was weight bearing 70% of the time. At six weeks the distraction was 2.5 cm, and the transosseus pins were bending due to consolidation of the cancellous graft [Figure 4]. The consolidation partially explains the reduction in movement of the fragment to less than the calculated 1 mm-a-day over the six-week period, especially during the second three-week period. The graft consolidated restricting movement of the fragment, and even though it was somewhat compressed, it healed half the thickness of the tibial shaft. At nine weeks, calcification of the distracted callus had completely bridged the gap created by the advancement of the bone segment, and the dog was full weight bearing on the leg [Figure 5]. The external fixator was reduced to a unilateral design to dynamize the fracture with the aim of stimulating the grafted section to remodel to full thickness. By 12 weeks postsurgery, the callus was completely healed, and by 15 weeks the grafted area was remodeled to full cortical thickness [Figure 6].

Case No. 2

A nine-month-old, spayed femail, domestic shorthair cat was presented for an infected nonunion of the right tibia. The cat had sustained bilateral, comminuted tibial fractures that had been treated with open reduction. An IM pin and full cerclage wires had been used on the left tibia, and a unilateral external fixator had been used on the right. The left tibia healed, but the right tibia became infected; at presentation, 1 cm of the proximal fragment was protruding from an open wound on the medial aspect of the leg [Figure 7]. Insert one page of figures.

The dead bone and fracture site were exposed medially. In order to leave viable, bleeding edges, 1.5 cm of necrotic bone was removed from the proximal fragment and 0.5 cm from the distal fragment. This left a 2-cm gap between the tibial fragments. Two 0.062 Kirschner wires were inserted through the distal tibial segment in the transverse plane, at 90 degrees to each other. These two pins were attached via four small Kirschner clamps to a ring fabricated from 32-mm 316L stainless steel rod. The Kirschner wires were placed under hand tension before the clamps were tightened. There were two additional medium clamps on the ring. Two transosseus pins were placed in the proximal fragment as positioned for a typical bilateral external fixator. These were connected to the two medium clamps on the ring with two 3/16-inch threaded connecting bars and tightened. This combination of bilateral and ring fixators provided stability to the tibia. The advantage of the ring was to allow the use of small Kirschner wires crossed in the short distal fragment. If these thin wires are placed under tension, they can become stable in bending but still allow slight cyclic motion in compression, which some surgeons believe enhances healing.

An osteotomy was made 1.5 cm from the distal end of the proximal fragment, creating a free segment of bone [Figure 8]. An additional 0.045 Kirschner wire was placed through the free segment of bone and attached to two additional medium clamps that were placed on the threaded bars just below the level of the gap.