Commentary on
"Intrasynovial Flexor Tendon Repair.
An Experimental Study Comparing Low and High Levels of in Vivo Force During Rehabilitation in Canines"
By Martin I. Boyer, MD et al.

Commentary by Andrew Weiland, MD*, and Jess Ting, MD*,
Hospital for Special Surgery, New York, NY

Much attention has focused in recent years on the effects of postoperative rehabilitation methods on outcomes of flexor tendon repair. A large body of data has demonstrated improved outcomes after early passive mobilization, resulting in a fundamental change in the postoperative management of these injuries. Indeed, the vast majority of hand surgeons today employ some sort of early passive-mobilization regimen. Extrapolating from these data, some now advocate "high-force" rehabilitation protocols, in the belief that increased mechanical stress at the repair site will result in improved outcomes. This hypothesis, however, has never been definitively tested.

Lieber et al.¹ previously reported on a canine model of passive mobilization utilizing synergistic wrist and digital motion that enables tendon excursion and tendon force to be analyzed as independent variables. Using this model to examine the first independent variable-tendon excursion—Silva et al.² recently showed that increasing tendon excursion beyond 1.7 mm did not significantly improve range of motion or tensile strength. In the present study, Boyer et al. turn their attention to the second independent variable in this model, i.e., tendon force, and demonstrate that ultimate force, gap formation, and tendon rupture rates do not significantly differ between high-force and low-force passive-mobilization rehabilitation groups.

These findings suggest that the stiffness and strength of the repair site do not increase proportionally with increases in the applied forces employed in the rehabilitation protocol in the first six weeks after flexor tendon repair, at least in the setting of contemporary four and eight-strand suture repairs. The authors postulate that this may be because the strength of these multistrand repairs "shields" the tendon-tendon interface from the increased extrinsic forces. The authors also found that increasing the number of core strands from four to eight did not increase softening or necrosis of the tendon, but was the only factor which significantly increased ultimate strength and resistance to gap formation, a finding consistent with the literature.

This study is important because it sheds light on the complex interactions that take place among healing-tendon, suture, and the extrinsic forces acting on them. It challenges the prevailing concept that increased extrinsic tendon forces and excursion, such as those seen with more vigorous rehabilitation protocols, promote tendon-healing. While it may not be possible to directly extrapolate the results of this study to our clinical practice, it certainly points to the need for prospective, controlled trials before we advocate the deployment of high-force, high-excursion rehabilitation protocols.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

References

1. Lieber RL, Silva MJ, Amiel D, Gelberman RH. Wrist and digital motion produce unique flexor tendon force and excursion in the canine forelimb. J Biomech. 1999;32:175-81.
2. Silva MJ, Brodt MD, Boyer MI, Morris TS, Dinopoulos H, Amiel D, Gelberman RH. Effects of increased in vivo excursion on digital range of motion and tendon strength following flexor tendon repair. J Orthop Res. 1999;17:777-83.