The effect of range of motion on remodeling of small intestinal submucosa (SIS) when used as an Achilles tendon repair material in the rabbit

Small intestinal submucosa (SIS) is a biomaterial that has proven successful as a xenogeneic tissue graft in a variety of vascular, urological, dermatological, and orthopaedic applications. The effect of a single variable, the applied range of motion, on SIS graft remodeling was examined in a rabbit model of Achilles tendon replacement in the present study. A 1.5-cm long segment of the Achilles tendon was replaced by a porcine origin, braided SIS material in 36 New Zealand White rabbits. Following 2 weeks of complete immobilization in a long leg cast, rabbits were placed in ankle braces for an additional 4 weeks. These braces were especially designed to allow either no motion or full range of motion or to limit the range of motion to between 60 degrees and 90 degrees hock flexion. Histopathological examination of the remodeled tendons showed that immobilization slowed the wound healing response. The SIS material in rabbits with no motion was associated with an increased presence of fibroblasts, extracellular matrix, mononuclear cells, and blood vessels in the remodeling Achilles tendons compared with native tendons. However, either partial or full motion was associated with greater cellular infiltrate and more organized connective tissue matrix than was no motion. We conclude that early postsurgical motion in the presence of SIS facilitates new tissue deposition along the lines of stress in this rabbit model. Total immobilization slows the inflammatory phase of healing, including extracellular matrix deposition, to the midsubstance of the tendon. Unrestricted motion and weightbearing during the early remodeling phase clearly result in changes consistent with accelerated tissue remodeling compared with totally immobilized tendons following placement of tendon grafts in this animal model.