PMC6881767_01
Composite radiograph of the dog's pelvic limbs depicting limb length disparity. These radiographs were obtained 6 weeks following initial fracture stabilization. The left femur was 18% shorter than the right femur. The left tibia was 7% shorter than the right tibia. Left pelvic limb muscle atrophy, proximal tibial varus and internal torsion were also evident.
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Radiographs obtained at the time of the initial evaluation revealed remodeling of the healed left diaphyseal femoral, tibial, and fibular fractures (Figure 1).
Lateral radiograph of the dog's left pelvic limb obtained 6 weeks following initial fracture stabilization. Premature closure of the caudal portion of the tibial plateau has resulted in a tibial plateau angle of 50 .
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TPA was 50  (Figure 2) as compared to the contralateral tibia, which had a TPA of 27 . There was moderate effusion of the left stifle and generalized osteopenia of the left pelvic limb with thinning of the cortices and moderate muscle atrophy.
Sagittal deformity correction plan and projected results. (a) The proximal tibial axis (orange line) was established by defining the tibial plateau (red line) and transferring the mechanical caudal proximal tibial angle as measured on the contralateral normal tibia (63 ; tibial plateau angle (TPA) of 27 ) and positioning that angle to extend proximally through the tibial eminences. The distal tibial mechanical axis (Kelly green line) was estimated based on the position this line extended proximally from the talus on the normal, contralateral limb. The intersection of these two axes defined the neutral center of rotation of angulation (CORA). The transverse bisecting line (purple dashed line), which is a line passing through the neutral CORA and located equidistant (78 ) from the proximal and distal tibial axes, was drawn. An opening CORA, located cranially on the transverse bisecting line at the apex of the tibial tuberosity (large black dot), was selected for the location of the hinge axis. (b) A radial osteotomy was planned in the region of the proximal tibial metaphyseal-diaphyseal junction. Distraction would result in rotation of the proximal tibial segment about the hinge axis (magenta arrows) and separation of both the cranial and caudal tibial cortices along the osteotomy margin resulting in lengthening of the limb segment as the TPA was sequentially reduced to 7 . Locating the hinge axis on the transverse bisecting line was done to reduce the TPA and perform lengthening without inducing a secondary translational deformity. During distraction the position of the tibial eminences, which were used as the proximal landmark to define the mechanical tibial axis, were translated slightly cranial which also translated the final mechanical tibial axis (forest green line) cranial. (c) Regenerate bone (orange stippled area) would form in the distraction gap establishing a confluent column of tibial bone.
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Surgery was planned to place a hinged circular external fixator (IMEX Veterinary, Inc., Longview, TX) and perform distraction osteogenesis to sequentially reduce the TPA while simultaneously lengthening the crus (Figure 3).
Photograph of the left pelvic limb obtained immediately following surgery. The stretch ring and associated fixation wires stabilized the plateau segment. The hinges, attached directly to the proximal ring, were placed to position the hinge axis through the apex of the tibial tuberosity. One of the two angular motors which were used to perform the distraction is visible caudolaterally. The second, caudomedial motor cannot be seen in this photograph. Half-pins were placed cranial and medial on the distal ring to improve construct stability while minimizing postoperative morbidity.
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A pair of angular motors were attached caudolaterally and caudomedially, articulating the two rings (Figure 4).
A sequence of lateral postoperative radiographs of the dog's left pelvic limb which were obtained immediately following placement of the fixator (a), 19 days following surgery, at the termination of distraction (b), 6 weeks following surgery (c), and 12 months following surgery at the time of the final long-term follow-up evaluation (d).
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Immediate post-operative radiographs confirmed appropriate location of the tibial and fibular osteotomies as well as the hinge axis (Figure 5(a)). Distraction was terminated 19 days post-operatively, with 6 mm and 17 mm of distraction along the cranial and caudal tibial cortex, respectively, and continued regenerate bone formation in both distraction gaps (Figure 5(b)). At 6 weeks following surgery, the regenerate bone had undergone extensive consolidation and remodeling, but there was still a narrow, well-marginated 1-2 mm fibrous interzone present in the tibia (Figure 5(c)). TPA of 3  (Figure 5(d)).