Bilateral Muscle Architecture Differences in a Rabbit Model of Unilateral Hindlimb Transtibial Amputation: A Pilot Study

Residual muscles after amputation experience mechanical unloading, loss of distal tendon insertion, and reduced excursion, conditions known to cause muscle degeneration. However, the architecture of residual muscles following amputation remains largely unreported. In this pilot, exploratory study, five healthy, skeletally mature, male New Zealand white rabbits underwent unilateral hind-paw ankle disarticulation. The contralateral intact limb served as the within-subject control. Six hindlimb muscles that cross the intact biological ankle were harvested bilaterally at four weeks post-amputation, and muscle mass, length, fiber length, pennation angle, sarcomere length, optimal fiber length (OFL), and physiologic cross-sectional area (PCSA) were measured. All statistical comparisons were based on raw, paired differences in values between sides. Muscle mass and OFL were, on average, lower on the residual side for all muscles; raw differences between sides were significant for mass of LG (p = 0.029) and FDS (p = 0.026) and for OFL of LG (p = 0.026). Based on percent difference between sides, degeneration was most severe in the soleus, which had 63 ± 26% less mass and 51 ± 40% lower optimal fiber length in the residual limb than in the intact contralateral limb. Despite lower mass, average PCSA was generally preserved or increased (though differences were not statistically significant) across all muscles except FDS; this was possibly attributable to disproportionately greater reductions in optimal fiber length than in mass. These results have important potential implications for prosthesis function and, therefore, motivate additional research to identify influential factors and evaluate clinical strategies for preserving residual muscle architecture.