Chapter 14 Partial Tendon Lacerations

Outline

Many patients with partial tendon laceration come to the clinic several weeks after the injury due to persistence of symptoms, by which time the cutaneous wound may be healed and closed. Evidence of triggering or entrapment is adequate indication for exploration of the wound. Partial tendon lacerations can also be seen in a large wound together with complete lacerations of other tendons, which are easy to diagnose.

The degree of laceration directly impacts tensile strength of the injured tendon. However, with even larger degrees of laceration, the tendon retains enough tensile strength to withstand most active motion. The volar location of the laceration may lower a clinician’s threshold for surgical repair. A lesser degree laceration (<40% to 50%) may not need surgical repair. A tendon laceration >50% to 60% usually needs surgical repair.

Repair of partial tendon lacerations decreases the chance of tendon triggering and improves tendon excursion. The partially lacerated tendon can be treated with a circumferential suture with or without core suture technique. Core sutures are used in large degree (>60% to 75%) partial laceration. The lacerated parts can be trimmed to reduce triggering in partial tendon lacerations up to 75%. Complications of untreated lacerations include triggering, entrapment, and rupture of the tendons.

While little controversy exists today on necessity of complete tendon laceration repair, the topic of treating partial tendon laceration remains a subject of debate in hand surgery. Opinions are divided on whether management of partial tendon laceration is best achieved through surgical or nonsurgical approach.¹ This controversy, in part, stems from fewer in vivo studies of partial as opposed to complete tendon laceration in the literature many with conflicting results. In part, this could also be due to fewer cases of partial tendon laceration properly diagnosed in clinical practice. Thus, majority of studies have used in vitro cadaveric or animal models. Additionally, a majority of these studies focus on complete laceration of flexor tendons with much smaller focus on partial tendon laceration.

The controversy centered on the repair of partial tendon laceration was initiated by the work of Wray and Weeks ² in a chicken model where nonrepaired partially lacerated flexor tendons were deemed better than those repaired. This result was similarly confirmed by Reynolds et al³ in a similar model. Just a few years later, the concept of not repairing partially lacerated tendons was challenged by Kleinert and others.⁴ They advocated that partially lacerated tendons should be treated similarly to complete lacerations, emphasizing that this type of injury required the same diligence in surgical repair and postoperative care. In their series, they reported significant rise in complications following nonrepair of partial flexor tendon lacerations. Similarly, around the same time, Janecki⁵ reported on two cases of entrapment following nonoperative treatment of partially lacerated flexor tendons. The sentiment to repair this type of tendon injury was further echoed by Schlenker and coworkers,⁶ who reported a significant increase in triggering, entrapment, and rupture among other complications when these lacerations were not repaired. They recommended the repair of all lacerations greater than 25%. They favored modified Kessler core suture technique along with a running epitendinous suture for lacerations greater than 50% while for those between 25% and 50%, a running braided polyester suture was suggested. For laceration less than 25%, excision of the distally based flap was recommended to avoid triggering or entrapment in the pulley. However, since these studies, a series of in vitro biomechanical studies along with in vivo animal studies has resulted a better understanding of the properties of partially lacerated tendons and a shift in paradigm of treatment of partial tendon injury.

Diagnosis and Physical Examination

Clinically most patients have normal active motion, and cutaneous wounds are either rather small or healed. Identifying partial tendon laceration is a challenge. Partial tendon laceration classically presents as painful and weakened tendon excursion as opposed to absence of active finger flexion or any excursion as would be expected with complete laceration. However, without direct visualization through an open wound or an imaging modality, determining the degree of laceration based on clinical exam is not possible. In majority of cases, an opening in the skin at the site of trauma can serve as a window for initial examiner to characterize the degree of tendon laceration as a percentage of intact tendon width. This measurement is commonly done using a caliper. Interestingly, it has been showed that neither caliper measurement nor estimation with naked-eye provides consistent and reliable means of assessing the degree of laceration when managing partial tendon injuries.⁷ Yet, these are tools used by most in estimating degree of tendon injury.

Many patients with partial tendon laceration may present several weeks after the injury due to scheduling or persistence of symptoms, by which time the wound may be healed and closed. Evidence of triggering or entrapment is adequate indication for exploration of the wound. If a moderate index of suspicion exists for exploration, an imaging study such as ultrasound or magnetic resonance imaging is beneficial. Partial tendon laceration can also occur with other tendons lacerated completely in a larger wound, which is easy to diagnose.

Physiology of Tendon Healing

Through significant contribution early in the 20th century by Mason, Allen, and Shearon ^8,⁹ an understanding of various stages of tendon healing was developed. Following injury, the tendons enter an exudative phase of tendon healing (0 to 14 days) followed by a reparative phase (15 to 35 days). An understanding of the phases in tendon healing is important in choosing appropriate time point for biomechanical and biological studies. The healing tendon is strongest at day 1 with this strength dropping by day 5. The lost strength is gradually restored starting around day 15 and continues until the end of the reparative phase. The exact length of time for the tendon to reach maximum strength appears to extend beyond the 5th week. The above authors also demonstrated that the weakest sight on an undamaged tendon corresponded to the musculotendinous junction and the tendon–bone insertion. This group was among the first to suggest that early motion resulted in increase in strength and function of partially lacerated tendons. In addition to tensile strength, the concepts of tendon gliding and resistance against motion were emphasized in the context of tendon repair. Many have shown the deleterious effects of immobilization on tendons and the subsequent change in substance constituents and weakening of their insertion sites.¹⁰^–¹²

The degree of laceration has an effect on tendon healing. In an in vivo canine model of partial flexor tendon injury, Cooney and others ¹³ noted that the greater lacerations had less evidence of healing. While tendon with 30% laceration had histological evidence of amorphous connective tissue in the tendon gap with nascent collagen fibers bridging across the tendon edges, the 60% and 90% lacerated tendons showed little evidence of healing with no collagen present in the gap at 35 days.

Biomechanical Properties of Partial Tendon Injury

The majority of experimental data on partial tendon injuries is the result of years of investigation using in vitro cadaveric and animal tendons in addition to a smaller number of in vivo studies in animal models. The popular animal models for their proximity to human tendon anatomy are the canine, chicken, sheep, and pig models. Majority of effort in understanding partial tendon injury has been associated with flexor tendons specifically in zone 2.

Gap formation in the context of partial tendon injury is an important point of clinical interest because its increase is associated with higher gliding resistance, triggering, and delay in healing.^14,¹⁵ Gapping beyond 2 mm is harmful to tendon healing.¹⁴

Tensile Strength and Cross-Sectional Area

Feared sequelae of improperly treated partial tendon laceration include triggering, loss of entrapment, adhesion formation, or late rupture. It is intuitive that partial lacerated tendons would be weaker than intact tendons. Additionally, some had suggested formation of a scar mass at the laceration sites if a partial injury were to go nonrepaired, thus adversely affecting tendon function.⁴ It is our presumption that many such tendons if they were to be repaired would like be subjected to more rigorous immobilization postrepair than if they were left alone. In the following sections, the evidence that has shaped our current approach to partial tendon injury is reviewed. Of note, the great focus in study partial tendon lacerated has involved mainly flexor tendons and attention specifically to zone 2 injuries in part due to the historical poor outcome of injury in this area. Many times, the clinicians inadvertently extrapolate this information to treatment of flexor tendon in other zones and to extensor tendon injuries.

In the recent decades, several studies have pointed toward early active finger motion to decrease adhesion formation and improve tendon gliding.¹⁶^–¹⁸ While rupture of partially lacerated tendons has been discussed as a complication of early motion and no repair, many have shown this to be a more minimal concern than previously thought. In a series of 34 partial tendon lacerations ranging from 25% to 95% laceration of the cross-sectional area of the tendon, no rupture occurred following non surgical management even in the 11 cases with more than 75% laceration. All but one case had good to excellent function.¹⁹ Several studies have characterized the strength of partially lacerated tendon. While the tensile strength of the tendon decreases as the cross-sectional area of laceration increase, these studies support that a partially lacerated tendon in most cases retains sufficient tensile strength to withstand motion and loading. In chickens, Reynolds and others³ showed the mean tensile strength of unsutured tendons to be higher than the sutured repairs at both 2 and 4 weeks of unrestricted motion after surgery. Dobyns and others²⁰ demonstrated that 30% of lacerated tendons retain 80% of their original strength, while 75% of lacerations maintain 40% and 90% of lacerations have 25% of the strength. Hariharan and others⁷ looked at the tensile strength of 50% and 75% volarly lacerated flexor tendons in cadavers and compared the failure loads to in vivo forces measured in human flexor tendons during unresisted active finger movement. Failure loads for 50% lacerated tendon were almost twice as much as the failure load for 75% lacerated ones. However, the failure values for both degrees of laceration far exceeded the in vivo values required for unresisted active finger movement.²¹

McCarthy and others ⁷ noted a tendency for tendons to fail at the site of partial laceration when the cross-sectional area of laceration was equal or greater than 60% in an in vitro canine model. At this degree of laceration, they noted a 22.8% decrease in stiffness, 41.5% decrease in failure loads, 15.6% decrease in percent elongation, and 56.2% decrease in energy absorbed compared with intact tendons. The structural properties change adversely with increasing degree of laceration.⁷

As inflicted lacerations through tendons are most commonly not transverse, the effect of the direction of laceration and its impact on tensile strength becomes important. Tan and others ²² have shown that obliquity of tendon laceration affects the strength of partially lacerated tendons. Using pig tendons lacerated to 90% of their diameter, they found that lacerations with 45° and 60° had significantly less ultimate tensile strength compared with transverse, 15° or 30° oblique laceration.²²