Chapter 4 Biomechanics of Core and Peripheral Tendon Repairs

Outline

Surgical sutures with core and epitendinous stitches are the mainstay of tendon repair. Adequate mechanical strength is essential for a repair to resist gapping and failure. A repair using conventional two-strand core sutures is weak; repairs using four- or six-strand core sutures are stronger. To ensure surgical repair strength, a sufficient number of strands (four or more) passing through the repair site, an optimal core suture purchase (0.7 to 1.2 cm), appropriate suture calibers (4-0 or 3-0), and optimal size (2 mm) of locking or grasping anchors are essential. Maintaining a certain baseline tension on the core suture during surgery greatly benefits gap resistance. It should be realized that repair strength decreases considerably as the finger flexes, and traumatized tendon ends tend to soften during the first 2 weeks postsurgery, thus reducing the holding power of the suture. The innately weak and slow biological healing creates a “no gain” lag period in tendon strength during the first 3 to 4 weeks after surgery. We should ensure that the strength of surgical repairs is greater than the forces generated during unresisted active finger flexion (1 to 35 N for a normal adult) plus a certain safety margin.

The gap resistance, ultimate strength, and stiffness of repaired tendons are the primary parameters that define the mechanical properties of a surgical repair. To evaluate a surgical repair, the tendon is usually tested under a single cycle load-to-failure or under cyclic loading; both generate load-displacement curves. The cycle numbers at which gapping occurs and the repair completely fails indicate performance of the repair.

Surgical repairs of the tendon are divided into end-to-end repairs (for acutely lacerated tendons), repairs of tendon–bone junctions, and tendon repairs by other methods (such as interweave repairs for tendon grafting or transfer). This chapter will be devoted to end-to-end repairs for primary tendon surgery. These repairs have received the most attention, and their success is most dependent on repair mechanics. Strong surgical repair is the mechanical basis of early tendon mobilization and a prerequisite for biological healing.

The end-to-end repair commonly consists of two parts: (1) core sutures that provide essential strength and (2) peripheral sutures (also called epitendinous sutures) that serve to “tidy up” the tendon approximation. Strength of the repairs relates to the tendons’ resistance to gapping at the repair site and to complete failure during tendon movement, which consequently relates closely to adhesions and ruptures of the tendons. The essential goal of a surgical repair is to provide the tendon with strength sufficient to withstand early active tendon motion postsurgery but offering minimal resistance to tendon gliding. A surgical repair should also be simple (or at least uncomplicated) for most surgeons to learn and to practice. Therefore, understanding the biomechanics of tendon repairs is imperative not only to design and planning of an optimal repair technique but also to its appropriate application to clinical cases and to guide postoperative rehabilitation. Surgeons or therapists should also be aware of the mechanical properties of surgical repairs when establishing postoperative therapies.

Essential Mechanical Requirements of a Surgical Repair

Forces generated during normal hand action range from 1 to 35 N, except tip pinch, according to in vivo measurements.¹ Schuind et al¹ measured tendon forces up to 6 N during passive mobilization of the wrist, up to 9 N during passive mobilization of the fingers, and up to 35 N during active unrestricted finger motion. Therefore, a surgically repaired tendon should be able to withstand at least 40 N, with power sufficient to resist gap formation. The repair should be able to withstand cyclic loads under both linear and curvilinear load conditions. In vitro laboratory tests have shown that a conventional two-strand core repair plus running peripheral sutures yields a maximal strength of 20 to 30 N²; this is lower than forces generated during normal hand actions and explains why some such repairs are disrupted during postoperative motion exercise. Studies have shown that forces necessary to cause failure of four-strand repairs are around or beyond 40 N³^–⁵; six-strand repairs fail with loads over 50 to 60 N.^6,⁷

Clinically, patients are encouraged to start moving the tendons in the first few days after surgery. Edema of traumatized tissues (tendons, subcutaneous tissue, sheath, and pulleys), bulkiness of the tendons, and the normal healing responses of tissues increase the resistance of the tendons. In the first 1 to 2 weeks, the tendon ends tend to soften, which decreases their power to hold the sutures. Therefore, a certain safety margin should be maintained by increasing the baseline surgical repair strength. Taking these points into consideration, one expects a baseline ultimate strength of 40 to 50 N and the capacity to resist gapping of 20 to 30 N for a surgically repaired tendon.

Historical Reviews

The main body of information regarding the biomechanics of tendon repair was accrued over the past few decades; the principal contributing research groups are summarized in Box 4-1. The investigatory topics included (1) development of new repair methods; (2) comparisons of existing repairs; (3) factors affecting strength; and (4) new means to test or to record the mechanical properties of the repairs. Human and porcine tendons are the most common materials used in in vitro tests of repair strength,²^–⁵ though tendons from sheep, rabbits, monkeys, and cows are also reported.^7,⁸ Canine, rabbit, and chicken are used in in vivo investigations.⁹^–¹²

Box 4-1 Groups Contributing to Major Information of Tendon Repair Biomechanics (Listed in Alphabetic Order of the Leading Investigators)

Amadio-Mayo Group: In vitro model: human, canine tendons; in vitro model: canine tendons

Gliding friction of the tendon against sheath and pulley (Uchiyama et al), gapping and resistance of grasping and locking repairs (Tanaka et al), partial tendon repairs (Zobitz et al), effects of different repairs, knot sizes and locations (Momose et al), and relation of gap sizes and gliding resistance (Zhao et al).

Gelberman–St Louis Group: In vitro and in vivo models: canine tendons

Significance of gap formation on adhesion and repair failure (Gelberman et al), effects of forces of rehabilitation on the healing strength (Boyer et al), zone 1 flexor tendon repair including tendon–bone junction (Silva, Dinopoulos, et al), and development of eight-strand repair (Winster et al).

Manske–St Louis Group: In vitro model: human, canine tendons; in vivo model: canine, chicken tendons

Development of tendon splint repair (Aoki et al), knot locations (Pruitt et al), cyclic loading (Pruitt et al), differences between grasping and locking loops (Hotokezaka and Manske), effects of area of locking loops, suture sizes, and development of the modified Pennington locks (Hatanaka and Manske), and tendon strength during early mobilization (Kubuta, et al).

Mass-Chicago Group: In vitro model and in vivo models: human tendons

In situ tendon repair strength in curvilinear test model using cadaveric hands (Komanduri et al), comparison of repair strength of different techniques (Angeles et al), repair strength during cyclic load (Choueka et al), and partial tendon repairs (Manning et al)

Tang-Nantong Group: In vitro model: human, porcine tendons; in vivo model: chicken tendons

Development of four- and six-strand repairs (Wang et al), effects of sizes and configurations of locks (Xie et al), core suture purchase (Cao et al) and oblique or partial tendon cuts (Tan et al), effects of tension direction and tendon curvature (Tang et al), strength of the healing tendon (Wu et al), and effects of a major pulley on strength (Cao and Tang).

Trumble-Seattle Group: In vitro model: human tendons

In situ test of repair strength in cadaveric hands and cyclic loading (Thurman et al), zone 1 tendon repairs (McCallister et al), and repair with Fiberwire (Miller et al and Hwang et al).

Wolfe–Yale–New York Group: In vitro model: human tendons

Development of the cruciate repair (McLarney et al), cyclic loading test of locked repairs and gap formation (Barrie et al), effect of peripheral suture purchase (Merrell et al), and comparisons of four-strand repairs with Teno Fix (Wolfe et al)

Current Tendon Repair Techniques

Techniques currently available are, first, repair with core and peripheral sutures. Core suture repairs are further divided according to the number of strands passing through the repair site: two-strand (conventional) and multistrand (including four-strand, six-strand, and eight-strand) repairs.²^–⁸ ^,13 ^,14 Alternatively, suture repairs can be categorized by type of tendon–suture junctions: grasping, locking, and mixed grasping-locking repairs. A repair is often referred to using the type of tendon–suture junctions and the number of strands (e.g., a locking four-strand repair). Peripheral sutures include interrupted, simple running, locking running, cross-stitch (Silfverskiöld),¹⁵ locking cross-stitch (Dona),¹⁶ interlocking horizontal mattress (IHM),¹⁶ and horizontal mattress (Halsted) sutures.¹⁷ Second, repair can be completed with devices. The use of devices is not common; methods include tendon splints,¹⁸ barbed repair devices,^19,²⁰ and the Teno Fix device.^21,²²

Surgical repairs using sutures are the mainstay of current clinical practice. In experimental settings, repairs with use of the above-mentioned devices have yielded impressive strength. The Teno Fix system, a stainless steel rod connecting stainless steel soft tissue anchors with screw threads in each end, was tested in vitro and shown to have strength similar to or greater than a four-strand repair.²¹ Clinical use of Teno Fix to repair zone 2 flexor tendons has been reported with good outcomes.²² However, Teno Fix is likely to make the tendon bulky and may even impinge upon the already narrow and swollen pulleys, which is a concern. Implanting a stainless steel rod not only risks increasing the volume of the tendon but also may require another operation to remove. A barbed suture device was reported to produce strength equal to a four-strand core suture.^19,²⁰ There are similar concerns regarding repair bulkiness. The idea of placing a splint inside the tendon to bridge the cut ends was tested experimentally by Aoki et al,¹⁸ but this method has not yet been used clinically.

Types of Tendon–Suture Junctions and Their Strength

In a broader sense, all parts of any sutures embedded within tendon stumps constitute tendon–suture junctions. However, the tendon–suture junction usually refers to the site where the suture forms a “locking” or “grasping” configuration to encompass the tendon substance. These sites are anchor points of the surgical suture—by either grasps or locks on the tendon—that help secure the power of surgical repairs.

Locking is defined as the configuration that tightens around a bundle of tendon fibers when tensile forces are applied to the suture ends (Figure 4-1).²³ In contrast, grasping refers to the configuration that holds the tendon fiber bundles but does not tighten around their entire circumference and tends to pull through the fiber bundles when tensile forces are applied to the suture ends (see Figure 4-1).

Figure 4-1 Locking and grasping tendon-suture junctions have different interactions between the suture and tendon fibers. Locking repairs tighten around tendon fibers under tension, but grasping repairs tend to pull through tendon fibers under tension.

Grasping junctions are either open circle or open loop. Locking junctions include cross-locks (either exposed or embedded), circle-locks (looped or double-circled), and Pennington locks (original or modified) (Figure 4-2). Mechanically, the effectiveness of grasps or locks of a core suture is greatest when (1) suture materials are sufficiently strong—allowing the sutures to grasp or lock the tendon substance tightly without disruption of sutures; (2) the anchor point is sufficiently distant from the tendon laceration—so grasps or locks will not slip; and (3) grasp or lock sizes are sufficient—so the suture anchors to ample tendon substance.

Figure 4-2 Different tendon–suture junctions: different locking junctions and a grasping junction.

In other words, one should not expect a locking or grasping junction to be secure if all three requirements above are not met. In clinical settings, we should ensure that the calibers of the suture are equal to or greater than 4-0, the suture purchase no less than 7 to 10 mm, and the size of the lock or grasp equal to or greater than 2 mm. These requirements are critical to the repair strength and are more important than whether a locking or a grasping junction is used or which lock is incorporated into the repair.

When all the above conditions are met, grasping and locking junctions provide reliable anchors for the suture in the tendon, although a locking repair is generally somewhat stronger than a grasping repair. In practice, when the primary requirements of the repairs are met, the differences in the strength between locking and grasping repairs are actually narrow or insignificant. Depending on the locks or grasps used, the strength of repairs changes (usually less than 10 N).²³^–²⁹ The increase in strength brought about by incorporation of locking anchors is much smaller than increases in strength through other means—such as increasing the strength of suture materials or the number of suture strands passing through the repair site.²^–⁷ From this perspective, surgeons should pay ample attention to factors that exert the greatest influence on strengths and ensure that these requirements are met primarily. It is not appropriate to rely on incorporating locks, hoping that this measure alone increases strength, while neglecting other more influential factors.

Whether various locks have different mechanical strengths is under question. We tested the strength of the different locks and found they have essentially the same holding power.²⁷ A cross-lock and a circle-lock are identical in holding power; an exposed cross-lock and an embedded cross-lock are the same as well.²⁷ However, our in vitro tests showed that the Kessler-type repairs with the original Pennington locks are weaker than repairs with cross- or circle-locks.²⁸

Pennington locks are frequently incorporated in Kessler-type repairs. Most surgeons believe that Pennington locks help secure the repair. However, this consideration actually lacks sufficient experimental support. According to the definitions of Pennington ³⁰ in 1979, in the locking configuration of the Kessler repair, “the transverse suture component passes superficial to the longitudinal component so that the sutures lock a bundle of tendon fibers when tensile forces are applied to the suture.” Pennington did not present its strength data when describing this repair.³⁰ In a letter communication directed to Pennington,³¹ Silfverskiöld commented regarding this locking repair: “As to Dr. Pennington’s claim that the locking loop design is crucial to the tensile strength of the suture, I agree that in theory this should be true, but in actual practice this may not be the case. … Another practical consideration is that even if one attempts to place the transverse part superficial to the longitudinal part, to create the locking design, it is often difficult to be certain of success.” Practically, we agree that it is difficult for surgeons to ascertain the relation of transverse and longitudinal sutures in the repaired digital tendon clinically.

Hatanaka and Manske ^9,²⁴ modified the Pennington locking repair by placing some of the longitudinal strands over the dorsal tendon surface, making locked tendon–suture junctions confirmatively. The modified repair is stronger than the grasping Kessler repair when the suture calibers are 2-0 and 3-0 but not 4-0.²⁴ It is noteworthy that in an in vivo setting,⁹ the strength gained by this modified locking repair was noted only at days 3 and 21, not at days 7 and 14, even with the 2-0 suture. In other words, modified Pennington locks do not improve repair strength during the second week (days 7 to 14), when the tendon stumps usually soften and most ruptures occur. We should be cautious in relying upon Pennington locks to increase strength.

Recently we examined the effectiveness of the original Pennington lock in a four-strand repair; we detected no differences in strength between repairs with the Pennington locks and those with random assignment of grasping and locking. When a two-strand Kessler repair with original Pennington locks was compared with the repair with grasping anchors using 4-0 suture, our tests indicated a modest difference (12%) in gap resistance but no difference in ultimate strength.²⁹ When comparing two grasping anchors (open circle and open loop), we found no difference in their gap resistance and ultimate strength. Taken together, the evidence above leads us to believe that differences between the grasping and the original Pennington’s locking tendon–suture junctions may not be substantial in a clinical setting, and incorporating Pennington locks in multistrand repairs does not appear to affect strength.

Strengths of Core Sutures

Strengths of core sutures have been tested extensively both in vitro and in vivo and in single cycle load-to-failure test 2–14,23–28,32–34 or cyclic test setup.³⁵^–³⁷ Using a 4-0 nylon suture, a two-strand repair has a strength of about 20 to 25 N, a four-strand repair has a strength of about 35 to 45 N, and a six-strand repair has a strength of about 50 to 70 N. Thus we see the strength is roughly proportionate to the number of strands (Table 4-1).^{2–8,13,14,32–35} Strength is greatly increased when suture caliber increases³⁶^–³⁸ or stronger suture material is used (see Table 4-1).^38,³⁹