Morphometric assessment of the femoral trochlea, patella and patellar ligament in small breed dogs with and without grade II medial patellar luxation.

BACKGROUND: Medial patellar luxation is a common orthopedic condition in small breed dogs, though its etiology remains poorly understood. The aim of this study was to assess the morphometry of the femoral trochlea, patella and patellar ligament in small breed dogs with and without grade 2 medial patellar luxation. This retrospective study evaluated small breed dogs weighing ≤ 15 kg including 80 stifles with Grade 2 MPL and 80 controls. The femoral trochlear length to patellar length ratio (TL: PL), trochlear curvature to condylar curvature ratio (TC: CC) and patellar ligament length to patellar length ratio (PLL: PL) were assessed. In addition, these parameters were determined comparing dogs with and without a chondrodysplastic phenotype. T-tests and ANOVA were used for statistical analysis.

RESULTS: The TL: PL ratio was increased in dogs with grade II MPL compared to controls with 1.7 ± 0.3 and 1.6 ± 0.2. (p<.001), respectively. This indicates an increased length of the trochlea. No differences were found in the TC: CC and PLL: PL ratios between both groups. Within the grade II MPL group, there was an increased PLL: PL ratio (2.0 ± 0.2; 1.9 ± 0.2; p=.002) in dogs with chondrodysplastic phenotype in comparison with non-chondrodysplastic phenotype. A decreased femoral trochlear length, or a decreased curvature of the femoral trochlea does not seem to be related to grade II patella luxation in small breed dogs. In dogs with a chondrodysplastic phenotype the relatively longer patellar ligament could play a role in the development of grade II MPL.

CONCLUSION: Trochlear length and curvature did not seem to be related to the development of grade II MPL. In dogs with a chondrodysplastic phenotype, grade II MPL could be related to a longer patellar ligament.