Analysis of relative motion splint in the treatment of zone VI extensor tendon injuries

Purpose: Early protected motion after extensor tendon repair is desirable. The low-profile relative motion splint, described previously by Merritt et al, holds the affected digit in 15 degrees of extension relative to the uninjured digits to allow less-cumbersome early protected motion versus dynamic splinting. Although early clinical results have been favorable, formal biomechanic testing of this approach is lacking. We used an in vitro model to assess the effect of the low-profile relative motion splint on the biomechanics of zone VI extensor tendons by measuring tendon elongation with and without the splint. Tendon elongation also was measured after transection and repair of extensor tendons in zone VI with and without splint protection. Methods: Ten fresh-frozen cadeveric upper extremities were prepared and mounted on a testing apparatus with the wrist in 25 degrees of extension. Alternating applications of extension and flexion loads to the tendons induced a full range of motion for 25 cycles. Differential variable reluctance transducers were applied to zone VI of the index, middle, and ring extensor tendons. Measurements of intact tendon microelongation (or strain) were obtained with and without the relative motion splint. The middle finger extensor tendon then was transected (in zone VI) and was repaired immediately. Measurements were repeated with and without splint protection. Elongation ratios were calculated and analyzed statistically. Results: For the intact tendon of the middle finger splinting reduced the elongation by 1% in extension, by 2% in flexion, and by 3% in neutral position. After the transection and repair of this same tendon, the splint reduced the elongation by 5% in extension, by 7% in flexion, and by 6% in neutral position. Cycling without splint protection caused permanent stretching at the repair site. Reapplication of the splint decreased elongation at the repair site by 2% in extension, by 3% in flexion, and by 3% in neutral position. Conclusions: The relative motion splint reduces the effective strain on intact and repaired zone VI middle finger extensor tendons and supports its clinical use.