At its most basic, biomechanics is the study of the effects of bending, twisting, pulling, pushing and rubbing (shear) forces on living tissue. These effects provide, as limits, a mechanical description of biological tissue; as they relate to loading experienced in vivo, they describe the mechanical milieu in which living tissues operate. To the extent that the latter affect the former, one can speak of a “Wolff’s Law of Soft Tissue”, to describe the effect of function on form.
Within the realm of hand surgery, no topic exceeds tendon injury and repair in the wealth of biomechanical data available, the thought that has gone into the analysis of that data, and the knowledge that has been gained as a result. This review will summarize the influence of biomechanical thought and research on the management of flexor tendon injury.
Conceptually, the loads applied to tendons physiologically become the lower limit for the material properties of the tissue, if it is to function normally. Thus it is relevant to know the tensile strength of normal tendon, of various tendon repairs, and the loads that might be applied to healing tendons either during daily activity or with rehabilitation. Tendon repairs commonly fail by breaking at some point during the healing period. In vitro studies have shown that thicker core sutures, repairs with more strands crossing the laceration, and repairs with locking loops are stronger, and such repairs have been adopted clinically. A running peripheral suture does not increase the ultimate breaking strength much, but does increase the load needed to cause the repair to gap, especially when the running suture is locked. This may be useful as well, for several mechanical reasons discussed below, and on the basis of these mechanical studies, peripheral finishing sutures have been incorporated into tendon repairs, although the details of such sutures remain subject to discussion.
Tendon repairs have also been studied in vivo, in animal models. It has been known since the 1940s, when Mason and Allen wrote their classic study, that repairs tend to weaken for the first few weeks, especially in immobilized tendons. More recently, it has become clear that this effect can be moderated considerably if tendons are moved postoperatively, and so early motion regimens have become incorporated into all tendon rehabilitation protocols. Whether loading of the tendon is also important remains controversial. Loading clearly stimulates isolated tendon cells and, in some cases, tendon tissue in vitro, but the results of loading programs in vivo, either in animal models or in clinical studies, have been unimpressive when compared to similar protocols which assure motion, but with minimal loading.
Some unanticipated findings have been noted in the studies of partial tendon injuries, which again have influenced clinical practice. For partial lacerations that affect less than 90% of the tendon cross-section, a repair results in a weaker tendon postoperatively than no repair. Even acutely, for partial lacerations of 75% or less, repairs add little to the mechanical strength of the tendon, since the breaking strength of the residual tendon far exceeds that of the suture.
More recently, biomechanical thought on tensile testing has evolved further, to match repair strength with the requirements of in vivo loading in normal or repaired tendons. As a result, thresholds are now known for the loads experienced by normal human and animal tendons in vivo, as well as those experienced by repaired animal tendons in vivo.
Biomechanical thought has also been critical to the understanding of tendon kinematics, and the role of the pulley system in controlling the relationship between tendon excursion and joint motion. Studies of the tendon pulleys have shown that the A2 and A4 pulleys are the critical ones, and that if these pulleys are lost the tendon needs more excursion to achieve full flexion, excursion that might not be available due to the physiological constraints on muscle contraction. More recent studies have shown that the essential function of these pulleys can be preserved, even if only half of the length of the pulley is intact. Tensile testing of pulley reconstructions has shown that a double or triple loop around bone is the strongest configuration, with weaving into the remaining pulley rim the weakest.
One does not often think about compression forces on tendon, but tendons experience these forces where they contact a pulley. These forces are responsible for the load that the tendon applies to the pulley, and are dependent on both tendon load and joint angle. This biomechanical understanding has led to insights regarding the specialized gliding surface of tendons under pulleys, the study of which has opened up the field of tendon tribology, the study of tendon friction, and tendon gliding.
It is clear that a tendon repair that does not move is at greater risk of adhesion formation, and takes longer to build up its mechanical strength, than a tendon that moves. Thus it becomes important to know how it is that tendons move, how far they should move, and how it is possible to know that they have moved.
Biomechanical studies have given us much information on how tendons move normally. By understanding the pennation of muscles, the contractibility of actin and myosin, and muscle anatomy, it is possible to calculate the amount of tendon motion that muscle contraction can provide. By understanding the radian concept, it is possible to know how much joint motion can be induced by that tendon excursion, and how much additional tendon excursion can be created by joint motion. Finally, and most recently, it has been possible to calculate the coefficient of friction of normal tendon, which gives an idea of how much tension is needed to induce tendon gliding in a normal tendon or, of more interest, an injured or repaired one.
It is now known, for example, that repaired tendons have to overcome several loads which are significantly increased from normal, which add to the tension which much be applied to a tendon repair. The friction of the tendon within its sheath is increased, by factors that include the roughness of the injured surface, the exposed loops of suture from the repair, and the bulk of the repair. It has been shown, for example, that some of the increased strength of newer repairs is in effect “wasted” in overcoming the increased gliding resistance associated with such repairs. Other factors, such as posttraumatic and postoperative oedema and joint stiffness also increase the load needed to induce tendon gliding after injury. All this may explain why passive motion programmes sometimes fail: it may not be possible to generate sufficient tendon loading with passive manoeuvers to overcome these sources of gliding resistance after injury and repair.
Biomechanical thought has also led to strategies to overcome these difficulties. Trimming of partial tendon lacerations can improve gliding without sacrificing strength. Resection of a slip of superficialis may aid gliding of the profundus beneath A2. Modifications of active and passive rehabilitation programmes have been described, to generate forces similar to those resisting tendon motion after tendon repair, while trying to stay within the safe harbour of loading which avoids repair rupture or gapping.
It is clear that biomechanical insights permeate every aspect of tendon surgery. Repairs, rehabilitation, and even our surgical exposures are based on biomechanical principles. If it is true that we have seen farther by standing on the shoulders of giants, it is also true that without properly balancing loads and forces, we would have toppled from our perch long before we could take our first glance.