Mechanical compromise of partially lacerated flexor tendons.

Tendons connect muscle to bone and transmit loads from muscle to move and stabilize joints and absorb impacts. They stretch up to 4%–6% strain during in vivo loading [1–3] and display both elastic and viscoelastic behaviors. Tendons with tears and lacerations exhibit diminished mechanical properties compared to normal. Lacerations have been shown in vitro to decrease ultimate load [4–9] and stiffness [6,7] and increase compliance (defined as increased strain under a given load) [4,10] compared to normal tendons. Clinically, it has been suggested that tendons torn more than 50% should be surgically repaired [11–16]. This “50% rule” was originally developed for hand surgery, but has grown to become the standard surgical recommendation for many other tendons, such as rotator cuff [17]. However, little evidence is present to confirm the practice's accuracy or predictive ability [17], and few studies have investigated the mechanical basis of this guideline, and those studies that have, report conflicting results. For example, Mazzocca et al. demonstrated that strain reached during cyclic loading to 100 N does not increase significantly compared to a normal tendon until a 50% rotator cuff tear is present and therefore provided a mechanical basis for the 50% rule [4]. However, Bey et al. reported that extrapolation of their strain data, taken when a constant load of 31 N was applied to normal and 33% cut human cadaver rotator cuff tendons, did not support the 50% rule because the increase in strain would be less than 50%, with the value still falling below the estimated failure strain in vivo [10]. Tear size criteria for surgical repair of digital flexor tendons is debated in the literature, and few studies have addressed the effect of partial tears on the mechanical properties of flexor tendons. Separate clinical studies have concluded that tendons with tear sizes of up to 30% [8], 60% [18], 80% [9], and 95% [19] can be treated successfully without repair and utilizing early mobilization. Bishop et al. concluded that tendons with lacerations up to 60% of the cross-sectional area should be conservatively treated without surgical repair and with early mobilization. This was determined by comparing ultimate load, energy absorption, and stiffness of repaired to nonrepaired lacerated tendons [7]. Supporting Bishop's claim of only considering surgical repair at cut sizes greater than 60%, McCarthy et al. compared the mechanical properties of lacerated tendons to previously measured in vivo forces [6]. This approach, comparing mechanical properties of a lacerated tendon to forces measured in vivo by Schuind [20], was also completed by Manning et al. and Hariharan et al. to demonstrate that surgical repair should be considered with cuts greater than 90% [21] and 75% [5], respectively. The guideline for the 50% rule is generally supported by clinical trials [11,13–15], and the differing surgical criteria for digital flexor tendons are based primarily on ultimate strength [5–9,21] without consideration of remaining tissue behavior under physiologic loads. A robust mechanical study investigating the elastic and viscoelastic properties of partially torn tendons at physiologic load levels has not yet been reported, although such a study would add insight into the early, post-laceration mechanical behavior for the 50% rule. This study will therefore analyze the effects various laceration sizes have on mechanical properties, both elastic and viscoelastic, in flexor tendons. Based on a damage parameter defined by Lemaitre as the difference between the total area and the effective resisting area divided by the total area [22], we hypothesize that laceration will decrease nominal tendon mechanical properties and percent decrease will be equal in magnitude to percent loss of cross-sectional area.