Impact of Femoral Varus on Complications Associated with Corrective Surgery for Medial Patellar Luxation

Introduction
The objective of this study was to evaluate the association of femoral varus with postoperative complications following correction of medial patellar luxation.

Materials and methods
Medical records of dogs that had corrective surgery for non-traumatic medial patellar luxation without concomitant cranial cruciate ligament rupture were reviewed. Signalment, history, grade of luxation, operative technique and clinical outcomes were retrieved. Measurements of femoral varus including femoral varus angle (FVA), inclination angle (ICA), anatomical lateral distal femoral angle (aLDFA), and mechanical lateral distal femoral angle (mLDFA) were taken from caudocranial radiographs of the pelvic limbs by three examiners.

Results
Fifty five stifles (47 dogs) met the inclusion criteria. There were 44 stifles from dogs <20kg, and 11 from dogs ≥20kg. Grade I MPL was noted in 2/55 stifles (3.6%), grade II in 26/55 stifles (50.9%), grade III in 12/55 stifles (36.4%) and grade IV in 4/55 stifles (7.3%). Overall mean FVA was 17° ± 9.8°. Mean FVA for Grade I-II MPL was 16.2° ± 7.2°, grade III was 16.7° ± 9.1°, and grade IV was 27.0° ± 22.9°. No significant correlation was found between the 4 angles measured and grade of patellar luxation or patient age. There was a strong positive correlation between these angles and increasing patient weight; FVA, ICA, aLDFA and mLDFA were all significantly greater for dogs >20kg. Overall frequency of complications was 22% with no cases requiring corrective surgery for patellar reluxation postoperatively. There was no significant association between complications and FVA or patient weight.

Discussion
Previous recommendations have advised distal femoral osteotomy for dogs with excessive femoral varus based on FVAs greater than 12° to help reduce the incidence of patellar reluxation following routine surgery. However, in this study, routine surgery was successful in the surgical correction of MPL in a population of dogs with a mean FVA of 17° ± 9.8°. Given the lack of reluxation requiring revision surgery and lack of association of FVA with complications in this population, the definition of excessive femoral varus and recommendations for distal femoral osteotomy may need to be re-evaluated. The angles at which the degree of femoral varus is likely to increase the incidence of patellar reluxation may be higher than previously thought.