Tendon transfer mechanics and donor muscle properties : implications in surgical correction of upper limb muscle imbalance

Sammanfattning: Tendon transfer surgery is used to improve the hand function of patients with nerve injuries, spinal cord lesions, cerebral palsy (CP), stroke, or muscle injuries. The tendon of a muscle, usually with function opposite that of the lost muscle function, is transferred to the tendon of the deficient muscle. The aim is to balance the wrist and fingers to achieve better hand function. The position, function, and length at which the donor muscle is sutured is essential for the outcome for the procedure. In these studies the significance of the transferred muscle’s morphology, length and apillarization was investigated using both animal and human models. Immunohistochemical, biochemical, and laser diffraction techniques were used to examine muscle structure. In animal studies (rabbit), the effects of immobilization and of tendon transfers at different muscle lengths were analyzed. Immobilization of highly stretched muscles resulted in fibrosis and aberrant regeneration. A greater pull on the tendon while suturing a tendon transfer resulted in larger sarcomere lengths as measured in vivo. On examination of the number of sarcomeres per muscle fiber and the sarcomere lengths after 3 weeks of immobilization and healing time, we found a cut-off point up to which the sarcomerogenesis was optimal. Transfer at too long sarcomere lengths inhibited adaptation of the muscle to its new length, probably resulting in diminished function. In human studies we defined the sarcomere lengths of a normal human flexor carpi ulnaris muscle through the range of motion, and then again after a routinely performed tendon transfer to the finger extensor. A calculated model illustrated that after a transfer the largest force was predicted to occur with the wrist in extension. Morphological studies of spastic biceps brachii muscle showed, compared with control muscle, smaller fiber areas and higher variability in fiber size. Similar changes were also found in the more spastic wrist flexors comparing with wrist extensors in children with CP. In flexors, more type 2B fibers were found. These observations could all be due to the decreased use in the spastic limb, but might also represent a specific effect of the spasticity. In children and adults with spasticity very small fibers containing developmental myosin were present in all specimens, while none were found in controls. These fibers probably represent newly formed fibers originating from activated satellite cells. Impaired supraspinal control of active motion as well as of spinal reflexes, both typical of upper motor syndrome, could result in minor eccentric injuries of the muscle, causing activation of satellite cells. Spastic biceps muscles had fewer capillaries per cross-sectional area compared to age-matched controls, and also a smaller number of capillaries around each fiber. Nevertheless, the number of capillaries related to the specific fiber area was normal, and hence the spastic fibers are sufficiently supplied with capillaries. This study shows that the length of the muscle during tendon transfer is crucial for optimization of force output. Laser diffraction can be used for accurate measurement of sarcomere length during tendon transfer surgery. Wrist flexor muscles have more morphological alterations typical of spasticity compared to extensors.