Effect of Amputation on Muscle Structure Properties in a Rabbit Model

After amputation, muscles in the residual limb are detached from their insertion points and no longer span the missing joints. Our objective was to quantify the effect of amputation-induced disuse on residual muscle structure, an indirect indicator of muscle force-generating capacity. One hind paw was surgically removed at the ankle joint of ten rabbits. At two weeks (n=5) and 4 weeks (n=5) post-amputation and for select muscles (gastrocnemius, soleus, tibialis cranialis, extensor digitorum, and flexor digitorum superficialis), we measured and computed several muscle structure properties. Additionally, we qualitatively assessed the muscle fiber appearance of histological samples at each timepoint. At 2 weeks post-amputation, most muscle parameters were comparable between the residual and contralateral intact limbs. At 4 weeks post-amputation, there was a non-significant but general trend of degeneration in all the residual muscles. Degeneration, by structure and histological appearance, was most pronounced in the flexor digitorum longus and soleus muscles. The observed muscle structure changes following amputation were similar to those reported for other disuse conditions such as tenotomy and joint immobilization. Our findings are clinically relevant because muscle degeneration could impair motor function of both the residual limb and emergent muscle-attached limb prostheses.

Though residual muscles support prosthetic and residual limb function, their architectural integrity following amputation is unknown. Following amputation, it is expected that residual muscles retract and essentially become immobilized in a shortened position. Residual muscles that cross an intact residual joint will still experience forces and length changes, though they are likely altered from those of the intact limb. Such changes in geometry and use are similar to those of tenotomy⁴ and limb immobilization⁵, both of which lead to atrophy, reduced normalized fiber length (i.e., fewer sarcomeres in series), and reduced fiber cross-sectional area.

The objective of the thesis is to quantify muscle architecture in select residual muscles (gastrocnemius, soleus, tibialis cranialis, extensor digitorum, and flexor digitorum superficialis) following unilateral hind-paw amputation. For evaluation of the effect of amputation, independent of muscle retraction (shortening) after tendon release, the muscles were held approximately at their pre-amputation in situ lengths by suturing their tendons to the distal end of the tibia. We hypothesized that muscle architecture property values of the select muscles would be lower, or more degenerated, in the residual limb than in the intact contralateral limb.