A tendon is a dense band of fibrous connective tissue which acts as an intermediary component in the attachment of the muscle to bone.
When operating within a range of normal physiological forces, tendons exhibit high compliance, great tensile strength and low extensibility. When supraphysiologic forces are placed on tendons, their mechanical characteristics change, and apparently irreversible structural changes take place. Whether or not such changes eventually result in a clinical lesion is dependent upon a number of poorly defined factors. The healing of injured tendons presents the veterinary surgeon with a number of different management decisions as opposite objectives seem to be required in the same wound. Successful restoration of injured tendons requires a rapid gain in tensile strength without adherence to other tissues. For a single scare to provide strength in one area yet not restrict motion in another, a complex series of events must occur. This series of events is dependent upon the anatomy and vascular supply of the tendons and the adjacent tissues.
A tendon may receive its blood supply from four sources: the muscle or bone to which the tendon is attached (intrinsic vessels), a mesotendon within a synovial sheath, and the paratendon if no sheath exists (extrinsic vessels). Both intrinsic and extrinsic components can be involved in tendon healing. As tendon injuries are often accompanied by injuries to the surrounding soft tissues and/or bone, their healing does not take place in an isolated environment. The gain in tensile strength and adhesions that develop are part of a single healing process, resulting in the tendon and the surrounding tissues healing according to the “one wound-one scar” principle. There is no question that if the wounded tendon could be managed independently of the adjacent soft tissue wound, the problem of tendon repair would be simplified.
The healing process of tendons can be further divided into healing of sheathed versus non-sheathed tendons. In a non-sheathed tendon, healing depends less on intrinsic blood supply because of the contributions of the wound bed from the paratendon and peritendonous tissues. In sheathed tendons under ideal conditions (i.e., if the primary intrinsic blood supply is not damaged), the potential for primary intrinsic repair exists. Maximization of the intrinsic healing and minimization of extrinsic healing will lead to fewer problems with peritendonous adhesions. Unfortunately, the majority of tendon injuries involves the tendon and tendon sheath and primary intrinsic repair is overshadowed by an extrinsic response by the peritendonous tissues. This response results in adhesion formation in addition to tendon healing and may preclude restoration of normal gliding function.
In an effort to erect an artificial barrier between the healing tendon and the rest of the wound, numerous materials have been placed around the anastomatic site. In all cases, retardation of the healing process has occurred. This is because in the overwhelming majority of tendon injuries, although numerous intrinsic vessels are present, these vessels are not capable of nourishing the tendon without collateral connections to extrinsic vessels. In addition, tendon healing is dependent upon migration of cells from outside the tendon into the defect between tendon ends. Therefore, successful isolation of a tendon anastomosis from the extrinsic tissues invariably results in failed healing. The best approach to minimizing adhesion formation and subsequent restricted gliding function is to use proper surgical technique and postoperative care.
Obviously, the importance of adhesions in tendon surgery depends upon the necessity for restoration of normal gliding function. The return of sufficient tensile strength may be more important than restoration of normal gliding function in a number of instances. For example, in treating injuries involving large weight-bearing tendons, provision of tensile strength adequate to prevent distraction during weight-bearing, rather than prevention of adhesions, should be the primary concern of the surgeon. This is because the formation of adhesions which would restrict the motion of these structure is rare, and a successful clinical outcome depends primarily on the maintenance of close opposition of the sutured tendon ends throughout healing.
The goals of tendon repair are apposition of the severed tendon ends with minimal disruption of blood flow, minimal suture bulk, and maximum strength of the overall repair. As is true with any surgical technique, suture materials and tendon suture patterns have been developed and recommended in an attempt to optimize results. These patterns have evolved in an attempt to maximize both tensile strength and normal gliding function.
Monofilament suture material is recommended for tendon repair because of its ability to glide within tissue and may be less likely to initiate tearing or separating of the tendon. While synthetic, monofilament, non-absorbable suture material has been the preferred suture material in the past, polydioxanone (PDS*) is absorbed slowly and loses its strength slowly. Therefore, enough strength would remain until the tendon begins to acquire intrinsic tensile strength. In addition, PDS* is less likely than non-absorbable suture materials to create a suture sinus in a contaminated environment.
As mentioned previously, several suture patterns have been designed for the surgical repair of severed tendons, including the Bunnell, Bunnell-Mayer, locking loop or modified Kessler, and three lopp pulley techniques. In the immediate postoperative healing period the sutures are relied upon to maintain tendon apposition and resist gap formation. They provide mechanical support and serve as a scaffold for initial cellular migration. The suture pattern should not restrict blood flow within the tendon or enhance scar formation by irritating the surrounding tissues. In light of these criteria, the locking loop and three loop pulley techniques are favored, as they are less restrictive of the intrinsic blood supply and provide greater tensile strength than do Bunnell sutures. The three loop pulley pattern has been shown to provide more tensile strength and resistance to gap formation than the locking loop pattern; however, it may compromise gliding function because of the quantity of suture material on the surface of the tendon. With this in mind, the locking loop pattern would seem best suited for use in situations where maximum gliding function is necessary, while the three loop pulley pattern may be used advantageously in high-load situations where provision of early tensile strength rather than restoration of normal gliding function is of primary concern.
Postoperative management of a surgical repair of a tendon rupture should consist of external support and immobilization for three weeks, followed by an additional three to four week period of restricted activity as the intrinsic tensile strength of the healing tendon increase. There should then be a gradual return to normal activity. Recent evidence indicates that limited passive motion aids in the longitudinal orientation of tendon fibrils in tendon repair while active motion will inhibit early repair of the tendon. When controlled passive motion is utilized, tendons heal more rapidly than in immobilized repairs. The difficulty encountered in veterinary surgery is how to conveniently implement limited passive motion without placing too much stress on the healing tendon too soon in a potentially uncooperative patient. Hopefully, additional advances will be made in the near future to help overcome these difficulties and to optimize the healing of tendon injuries in general.