Proximal femoral fixation method and axial load affect simulated muscle forces in an ex vivo feline limb press

Abstract Objective To quantify how increasing axial loads combined with different femoral fixation methods impact simulated quadriceps and gastrocnemius muscle forces. Study design Experimental, non‐randomized, ex vivo study. Methods A custom limb press permitting axial loads of 10%–40% bodyweight with three femoral fixation models was tested with 24 limbs from 12 cats (4 per method). Fixation models were: one–complete hip mobility, two–rigid femoral fixation, three–flexion‐extension hip mobility. Femoral angulation to horizontal of 60° ±5° and stifle/hock angulations of 120° ±5° were maintained using turnbuckles. Primary outcomes were bodyweight normalized quadriceps and gastrocnemius forces, and their ratio. Secondary outcomes were radiographic limb angulation and relative foot position for models 2 and 3. Results Normalized quadriceps forces increased more with axial load in models 1 and 3 than model 2 ( p = .04), whereas normalized gastrocnemius force increased more with model 2 than models 1 and 3 ( p = .009). Force ratios were unaffected by axial load ( p = .4), but model 2 ratios were lower than models 1 and 3 ( p = .007). Femoral angulation did not differ with load ( p = .1) or model ( p = .9), but both stifle and hock flexed with increasing load ( p < .001) but remained largely within target. Relative foot position was mean 3.9 mm more caudal in model 2 than model 3. Conclusion Simulated muscle forces were proportionate to axial load, while rigid femoral fixation negatively affected both forces and ratios. Clinical significance Retaining hip flexion‐extension yields normalized quadriceps forces closer to in vivo values and could improve current ex vivo models.