Introduction
Limb press models have been widely reported for investigation of stifle joint stability and efficacy of surgical interventions ex vivo in both dogs and cats (1–3). Muscle loads in the quadriceps and gastrocnemius are typically simulated by the interposition of cables and turnbuckles between the patella and proximal femur, and the distal femur and calcaneus, respectively. These fix the stifle and hock joint angles, and subsequent application of a compressive force to the limb results in cranial tibial thrust forces developing at the stifle joint. How well these simulated muscle forces reflect in vivo measurements or in silico models is unknown. Peak vertical force for the hindlimb of walking cats is 40%-50% of body weight (4,5).
Methods
A limb press was constructed inspired by reported models ((1,2). Entire hindlimbs including hemipelvis were harvested from four cat cadavers. Soft tissues were removed, preserving the periarticular structures of the hip, stifle and hock joints. A screw eye hook was placed in the proximal femur, and connected to a tunnel in the patella via a calibrated 10 kg load cell, turnbuckle and steel wire. Two screws were placed at the origins of the gastrocnemius heads, and a second calibrated 10 kg load cell connected to a tunnel in the calcaneus in similar fashion. The hemipelvis was connected to the top-plate with two positive profile threaded pins, and the foot placed in a support cup hot-glued to a digital scale. Sequential loads of 0%, 10%, 20%, 30% and 40% of body weight were applied after fixing the stifle and hock angles at 120°, and orientating the femur at 60° to the horizontal. Loads were reduced stepwise to 0% and repeated for three cycles. Load cell outputs were measured using an Arduino Uno and two HX711 ADC units.
Results
Mean body mass was 2.95 kg (SD 0.52 kg). Peak quadriceps and gastrocnemius loads reached 63±14 N and 29±6 N, respectively, corresponding to 99% and 219% of body weight. The quadriceps: gastrocnemius load ratio was 2.2:1 at maximum limb load. Measurement repeatability improved after exclusion of the first loading cycle, with coefficients of variation of 9% and 5% for quadriceps and gastrocnemius loads, respectively. Hysteresis was observed between measured forces on the “up” and “down” load cycles for both simulated muscles.
Conclusion
Reported feline gastrocnemius forces in vivo are 19.7 N (6) and ca. 23.3 N (7), with quadriceps forces of ca. 80 N (6), consistent with the values reported here, but based on limited data. Quadriceps forces were greater than patellar ligament forces reported previously (8), which may raise concerns over earlier models. While this hind limb model is neither physiologically nor biomechanically accurate, and the gastrocnemius force neglects the contribution of the soleus muscle, our results suggest that reasonable construct validity for this feline limb press model may be achieved in practice. Investigators should use multiple loading cycles and be aware of possible hysteresis with different loading protocols.
Literature
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