Friction of the gliding surface. Implications for tendon surgery and rehabilitation

Abstract

Finger flexor tendon rehabilitation has come a long way, but further advances are possible. Ideally, a healing tendon should move, but under the minimum load necessary to achieve motion. It is possible to design suture repairs that minimize the friction between tendon and sheath while simultaneously maintaining adequate strength to provide a wide margin of safety during therapy. A looped, four-strand modified Kessler repair is a good example of this type of high-strength, low-friction repair. At the same time, rehabilitation methods can also be optimized. A new modified synergistic motion protocol is described in which wrist flexion and finger extension is alternated with wrist and metacarpophalangeal joint extension and finger interphalangeal joint flexion. Based on evidence from basic science studies, the authors hypothesize that this new protocol will deliver more effective proximal tension on the tendon repair than either passive flexion/active extension or synergistic protocols, and may be useful in patients who are not ready for, or are not reliable with, active motion or place and hold protocols. The scientific basis for these new methods is reviewed, and the concept of the "safe zone" for tendon loading, in which tendon motion occurs without gapping of the repair site, is developed.

FIGURE 1

Tendon friction testing device. In this example, the proximal phalanx and its attached A2 pulley are mounted in the center, and the FDP tendon is passing through the pulley at a proximal angle of 30 degrees and a distal angle of 20 degrees. Force transducers are attached to the proximal and distal ends of the tendon. Proximal transducer F2 is attached to a mechanical actuator, which can pull the tendon proximally or release it to move distally, under the influence of a distal load. Distal transducer F1 is attached between the tendon and the load. When the tendon is moved toward the actuator, the force recorded at F1 is equal to the load, and the force at F2 is equal to the load plus the friction force.

FIGURE 2

Gapped tendon caught on the pulley edge.

FIGURE 3

The safe zone for a typical four-strand repair with a 40-N (9-lb) breaking strength is actually within a much smaller range, perhaps between 5 and 10 N (1-2 lb).

FIGURE 4

Examples of repairs resulting high and low friction between adjacent surfaces.

FIGURE 5

Modified synergistic early motion protocol. A: extension phase; B: flexion phase.

FIGURE 6

An expanded safe zone.