Introduction

The role of excessive distal femoral varus (DFV) in the pathophysiology of medial patellar luxation (MPL) in large-breed dogs has been recently emphasized in the literature. Although not clearly validated, a cut-off value of 12° has been used to define excessive femoral varus and to select cases that may require corrective distal femoral closing wedge ostectomy (DFO). In most cases, the center of rotation of angulation (CORA) is located at the distal aspect of the femur. Consequently, if, as recommended, the ostectomy is performed at the level of the CORA, the distal fragment may not be of sufficient size to safely accommodate standard bone plates. To circumvent these limitations, the ostectomy may be performed proximal to the CORA, or special plates specifically designed for DFO may be used. In both cases, challenging plate contouring may jeopardize accurate restoration of alignment. In addition, in order to limit the risk of implant failure, apposition of the medial cortices should be achieved when using a lateral bone plate. Alternatively, the use of an interlocking nail (ILN) may provide both technical and mechanical advantages over bone plating. Indeed, the intramedullary location of ILNs inherently facilitates realignment in the frontal plane. Furthermore, the angle-stable properties of recent ILN designs, which has been shown to be effective in the treatment of unreconstructed metaphyseal fractures, would allow the surgeon to perform a simpler opening wedge osteotomy since perfect fragment apposition becomes unnecessary.

Preoperative Planning (Femur Deformity)

Preoperative planning may be performed using dedicated software. The anatomical lateral distal femoral angle (aLDFA) is determined according to established principles developed by Tomlinson et al. This angle ranges between 94° and 98° in large-breed dogs. Distal femoral realignment is recommended in dogs with MLP and an aLDFA greater than 102°. In these cases, the location of the CORA and magnitude of the distal femoral varus are determined by joining the proximal and distal femoral anatomical axes. Using a digital planning software, the distal femoral segment can be realigned using an opening wedge osteotomy to achieve an aDFLA equal to either, the normal opposite angle or, in case of bilateral DFV, an angle specific of the breed as reported in the literature. Because of the natural cranial bowing of the femur, the distal segment is placed in a relative recurvatum to accommodate the straight ILN. Thus, femoral realignment is accomplished via a single osteotomy that creates a wedge opening medially and caudally.

Surgical Technique Summary

The dog is placed in lateral recumbency with the affected femur in an uppermost position. The procedure starts with a lateral parapatellar arthrotomy to evaluate the orientation and depth of the femoral trochlea and the integrity of the cruciate ligaments and menisci. Next, a superficial, straight, proximal to distal score line is performed along the lateral femoral cortex using a reciprocating bone saw. This line will be used to verify rotational alignment. An osteotomy is performed parallel to the joint orientation line at the level of the predetermined CORA. The distal femoral metaphysis is then reamed to facilitate deep seating of the interlocking nail. The reamer is oriented nearly parallel to the femoral trochlea in the frontal plane and toward the caudal aspect of the condyles in the sagittal plane. The latter direction provides a slight recurvatum as well as an option for performing an abrasion or block sulcoplasty if necessary without interfering with the ILN. A standard lateral subtrochanteric approach is then performed to allow insertion of the ILN through the intertrochanteric fossa into the proximal femoral segment. The distal femoral segment is realigned and the nail is pushed into the previously reamed distal meta/epiphysis. Rotational alignment is ascertained by juxtaposing the ends of the score line on the lateral cortex. Finally, the AS-ILN is secured using 1 or 2 locking bolts in the proximal and distal segments.

The use of a bone graft to fill the opening wedge is left at the discretion of the surgeon. Retinacular release incisions and/or imbrications are performed as needed prior to routine closure in layers. Ancillary procedures, including transposition of the tibial tuberosity and/or high tibial osteotomies may be necessary and are dictated by the existence of concomitant pathological processes such as tibial internal rotation and/or cranial cruciate ligament rupture.

Postoperative Management

Standard craniocaudal and mediolateral radiographic views are taken to verify proper femoral realignment as well as implant position. Postoperative bandaging is usually unnecessary. Controlled physical rehabilitation along with restricted activity are routinely suggested to hasten functional recovery and limit the risk of postoperative complications. Regular radiographic follow-ups are recommended until evidence of bone healing.

Clinical Outcome

This technique has been used successfully by our group at Michigan State University in a limited case series of ~20 mid- to large-breed, young dogs affected with MLP secondary to excessive DFV. An abrasion or block sulcoplasty was performed in half of the dogs while tibial tuberosity transpositions and/or TPLOs were necessary in 15% of the cases. The procedure was effective in correcting DFV from a mean preoperative aLDFA of 4.2°±106.6° to a mean postoperative aLDFA of 4.0°±93.0°. Clinical union was observed by 8 weeks postoperatively without major complications. In particular, infection, reluxation and/or implant failure were not documented. Functional recovery was subjectively deemed good to excellent in all cases. These results compare favorably to those reported by Brower et al. who used closing wedge ostectomies and lateral plating to correct DFV in 41 dogs. In that retrospective case series, major complications (11%) included infection, persistent lameness and implant failure and clinical union was documented by the ~10th postoperative week.

Conclusion - Subjective Impression

This case series suggests that DFV correction using a femoral opening wedge osteotomy and fixation via an AS-ILN is a valid alternative to the currently performed technique that relies on a closing wedge ostectomy with lateral plate fixation. Potential benefits of this new approach include 1) simplicity of performing an opening compared to a closing wedge, 2) relatively limited surgical exposure required to lock the ILN compared to plate fixation, 3) reduced risk of implant failure despite, the limited distal femoral bone stock, 4) ability to perform an osteotomy somewhat away from the CORA without inducing a lateral displacement of the distal femoral segment, 5) no need for cortical continuity and/or implant contouring.