Comparison of axial tibial-metatarsal alignment in dogs of varying size with and without medial patellar luxation

Objectives: To compare tibial torsion angle (TTA) and proximal tibial metatarsal angle (PTMTA) between dogs weighing < 10 kg and dogs weighing ≥ 10 kg with and without medial patellar luxation (MPL) to assess whether transverse plane tibial-metatarsal malalignment is size dependent. Study design: Retrospective computed tomographic analysis. Sample population: One hundred and one limbs from 55 client-owned dogs. Methods: Computed tomographic (CT) scans of pelvic limbs that met inclusion criteria were categorized by dog size (dogs weighing less or over 10 kg) and presence and grade of medial patellar luxation (normal, Grade 2, Grade 3 and Grade 4). A single investigator measured both TTA and PTMTA in each limb. Both TTA and PTMTA were compared between dogs of varying degrees of MPL and between dogs weighing < 10 kg and dogs weighing ≥ 10 kg. Results: A clear association between increasing TTA and higher grades of MPL was not discerned in dogs weighing < 10 kg. However, dogs weighing < 10 kg did exhibit a trend of worsening tibial-metatarsal malalignment as patellar luxation grade increased with grade 4 MPLs having a significantly larger PTMTA than all other groups. Among dogs weighing ≥ 10 kg, those with grade 3 MPLs possessed a significantly larger TTA compared to all other groups, but dogs with grade 4 MPLs were not different from normal dogs. However, dogs weighing ≥ 10 kg did exhibit a trend of increasing PTMTA values associated with worsening grades of patellar luxation with grade 2 and 4 MPL dogs showing more tibial-metatarsal malalignment than normal dogs. There were differences between dogs weighing < 10 kg and dogs weighing ≥ 10 kg with respect to TTA measurements with dogs < 10 kg showing more tibial torsion in MPL Grades 2 and 4. However, no differences were detected in PTMTA between sizes with any grade of MPL. Conclusion: Tibial-metatarsal malalignment measurable through an increased PTMTA may be a more predictably associated transverse plane deformity with MPL than tibial torsion alone as measured by the TTA. Both dogs weighing less or over 10 kg appear to exhibit tibial-metatarsal malalignment with increasing severity in cases of higher grades of MPL.