Tendon Injury Repair

Tendons are a fibrous connective tissue which attach muscle to bone. Their key mechanical properties include:

  • high tension strength (high ultimate failure load), and
  • high stiffness (when a load is placed on a tendon it will resist stretching; tendons are only slightly elastic, usually only stretching up to 6 – 10% when loaded).

These properties allow tendon to efficiently transfer load from the muscle, attached to one end of the tendon, to the bone attached at the other end of the tendon, resulting in the bone moving when the muscle contracts.

Tendon injuries may be acute as a result of a trauma, or may be a chronic injury as a result of a progressive deterioration of the tissue. Tendons usually fail by tearing away from the bone (common for rotator cuff and bicep tendon injuries), or rupture within the tendon itself (frequent in Achilles tendon injury).

Tendons may heal through a conservative treatment, or may require surgery. The surgical approach involves repairing the torn tissue back to its original position (or as close as possible), with the tendon-bone or tendon-tendon attachment requiring months to be complete.

Some patients are at risk for the surgical repair to fail, and the tear or rupture to re-occur. For patients with rotator cuff tears these patients include those with (a) large or massive tears, (b) chronic tears, (c) re-tears, (d) older patients, and (e) those who smoke.

X-Repair is specifically designed to have mechanical properties similar to tendon such as the ones present in human rotator cuff. That means it has high ultimate failure load and high stiffness, at levels similar to tendon, and high suture retention strength, in order to hold it in place and allow X-Repair to function.

Pre-clinical studies show that at the time of surgery the mechanical properties of the surgical repair site are significantly improved with X-Repair, and the outcome at 12 weeks after surgery is substantially improved with reduced gap formation between the tendon and bone, and increased mechanical properties of the repair site[6, 18].

When surgery is performed to repair tendon and X-Repair is used to reinforce that repair, the functional properties (strength and stiffness) of X-Repair allow X-Repair to assume some of the load that would usually be applied from the muscle to the tendon and through the surgically repaired site. It therefore is designed to protect the tendon during its repair process after surgery, and at the same time allows some load to be seen across the repair site, optimizing the repair response. X-Repair is made of a polymer which slowly degrades in the body (poly-L-lactic acid) such that it will not fully dissolve until after the tendon repair process is complete.

Structure and Function of Tendon
The function of a tendon is to transmit force created by the muscle to the bone and to generate movement; thus, the tendons are part of a bone-tendon-muscle-tendon-bone complex. The tendon is composed primarily of type I collagen, the major fibrillar collagen in the body, organized into fibrils. The fibrils are assembled into parallel arrays of fibers, which are then organized into fiber bundles and whole tendons [23]. The parallel bundles of collagen fibers are aligned in the direction of the load applied [24]. In contrast, ligaments function by attaching bone to bone, and are present around all joints, functioning to stabilize the movement in the joint. However, the structure of ligaments is similar to tendons.

The collagen I molecule consists of two alpha I (I) chains and one alpha 2(I) chains, and these assemble into micro-fibrils in a quarter-staggered array, by electrostatic forces that is then stabilized by the formation of intermolecular crosslinks, providing the tensile strength to the microfibril. Further maturation of these crosslinks infer additional mechanical properties to the fibrils and fibers. Other collagens (for example collagens III, V), proteoglycans (including biglycan and decorin) participate in matrix organization and stabilization. The cells of the tendon, tenocytes, are responsible for the synthesis of the tendon, its maintenance and repair [2].

Tendon attaches to bone at the insertion site, which is characterized by 4 distinct zones: tendon, unmineralized fibrocartilage, mineralized fibrocartilage, and bone. This transition area effectively attaches the tendon to the bone with high strength, and provides efficient transfer of forces from the tendon to the bone [3].

Mechanical properties
Tendon functions primarily to transfer tensile loads, from muscle to bone, and the collagen fibers are aligned in the direction of the load. The tensile properties of tendon are usually determined using uniaxial tensile tests, to provide the load-displacement curve, or if the cross-sectional area is considered and displacement is converted to strain, the stress-strain curve. The load-displacement curve can be used to assess whole tendons, whereas the stress-strain curve allows easy comparison between tissues [4, 24].

The stress-strain curve or load-displacement testing of tendons generate a characteristic curve, showing an early toe region where collagen fibrils become fully aligned, a steep linear portion where the tendon takes the load applied, with stretching of the tendon being restricted to as little as 10% strain, and the tendon exhibits elastic properties, returning to its original length when the load is removed. The slope of the stress-strain curve provides the modulus of the tissue, and tendon tensile modulus is very high. Extremely high loads will result in failure, where the tendon will tear, either within the tendon mid-substance, or at the tendon-bone interface.

Tendon Injury and Repair
Tendon injuries can be acute or chronic. The acute injuries are usually a result of trauma (including many sports injuries), whereas chronic injuries are a result of accumulation of small repetitive failures within the tissue that fail to heal.

Rotator cuff tendon acute injury frequently occurs in athletes of overhead sports, whereas chronic degeneration and tearing tends to occur in older individuals. Indeed the major patient population with rotator cuff tears is between 45 and 65 years of age [20]. Untreated partial tears, whether they were initially acute or chronic, can progress to large or massive rotator cuff tears. Of course, the function of the rotator cuff tendon is compromised, dramatically effecting function of the shoulder joint.

Achilles tendon tears may also be a result of acute or chronic injury, and frequently occurs in active individuals where the tendon was subjected to unusually high loads, and may be a consequence of a combination of chronic degeneration combined with exposure to high loads.

Tendons repair and heal through a well-described process common to most connective tissues. It involves inflammation providing oxygen, nutrients, and clot formation. Macrophages invade and digest the clot, release growth factors, fibroblasts are recruited, and a vascularized granulation tissue is formed. New collagen synthesis occurs, the collagen is deposited and organized as new fibers primarily along the axis of the tendon. Covalent crosslinks are formed and mature into stable crosslinks over a period of several weeks. This comprehensive and dynamic repair system, which occurs over weeks and months and eventually restores function to the tendon.

Tendon to bone healing (for example, as frequently required in rotator cuff repair) does not result in the restoration of a normal tendon-bone insertion site, rather there is a scar formation and the zone of calcified cartilage does not form [15]. There is frequently the re-separation of the tendon from the bone repair site, resulting in gap formation, which can fill in with scar tissue. However the resultant repair, with our without the gap formation, is usually inferior to the original tissue mechanical properties [5, 8]. Gap formation after surgical repair can result in retraction of the muscles, fatty infiltration into the muscles, muscle atrophy, and loss of muscle strength. Chronic retraction can result in irreversible damage to the muscle and muscle-tendon unit of the rotator cuff.

Tendon and Tendon Repair Mechanical Properties
Traditionally normal tendons are tested in tension to determine their stiffness, modulus, and ultimate failure properties [12, 17, 19, 21]. To determine structural properties for a particular geometry, force and displacement are analyzed directly. To determine material properties that allow comparison between tissues with different dimensions, force is normalized by the tissue’s initial cross-sectional area (tensile stress) and displacement is normalized by the initial tissue length (tensile strain). Plotting the two parameters against each other (force vs. displacement or stress vs. strain) up to failure results in a nonlinear stress-strain response composed of three regions: the toe region where the modulus or slope continually increases; the linear region where the modulus becomes nearly constant; the failure region. Stiffness (modulus) in the linear region and strength (maximum stress) are common indices of tendon function. For material properties, modulus and maximum stress for tendon can range from 500 to 1000 MPa and 60 to 100 MPa, respectively, depending on tissue organization, anatomical location, and species[1, 7, 16, 22, 26]. Corresponding strains to maximum stress can range from 13% to 22% when measured between grips and even lower (~5% to 8%) if strains are measured optically in the tendon midsubstance[7, 22, 25]. For structural properties, human rotator cuff has stiffness in the range of 228 – 843 N/mm, and ultimate load being 424 – 2757 N/mm [10, 11, 14]. Normal and repair tissues do not routinely function at high strain levels, and therefore test protocols more relevant to normal function, address a tendon’s subfailure properties. Indeed, repair tendon has been shown to reach a stiffness of only 10% of normal tissue[9] and maximum force only 25 – 37% that of normal tissue [9, 13].

Studies with X-Repair and Rotator Cuff
Preclinical studies with X-Repair augmentation of rotator cuff repair showed that the device enhanced the mechanical properties of the surgical repair site at the time of surgery [18]. At 12 weeks after surgery there were significantly enhanced mechanical properties of the repair site, and substantially reduced gap formation[6]. These results indicate that augmentation of tendon repair with X-Repair has the potential to improve the outcome of surgical repair of tendons.


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