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EQUINE VETERINARY EDUCATION


Equine vet. Educ. (2022) 34 (2) 74-76 doi: 10.1111/eve.13465


Clinical Commentary


‘Bridging the gap’: A spontaneous demonstration of regenerative medicine?


R. K. W. Smith Department of Clinical Sciences and Services, The Royal Veterinary College, Hatfield, UK *Corresponding author email: rksmith@rvc.ac.uk


Keywords: horse; orthotic device; resection; superficial digital flexor tendon


A case report published in this issue of Equine Veterinary Education (Lenoir et al. 2022) documents the radical surgical resection of a portion of the superficial digital flexor tendon in an area that usually shows poor natural healing. This case report is interesting because the gap became filled with tissue with similarities to tendon on ultrasound and enabled the horse to regain soundness with the aid of an orthotic device. The resection of the tendon was deemed necessary


because of suspected infection although this was not possible to confirm with positive culture or histology. While infection is rare in tendon, it does carry a poor prognosis (Kidd et al. 2002) which, therefore, justifies radical debridement. However, this left a large gap between the two ends of the superficial digital flexor tendon. Gap healing of equine tendons is seen following tendon lacerations with the formation of bridging fibrous ‘callus’ (Jordana et al. 2011). However, this can be a protracted process and may ultimately become exaggerated with restrictive adhesion formation. Reflection of the tendon ends can also occur which can delay healing further, and so, there have been


many attempts to assist healing, especially for the digital flexor tendons because of their importance in normal weight- bearing limb function. These approaches have included the use of a variety of biologic and synthetic implants to bridge the gap. The choice of a suitable material has progressed from inextensible carbon fibre (Vaughan et al. 1985), through artificial nonabsorbable materials (Gibson et al. 2002; Barrett et al. 2014; Fig 1), to long-lasting absorbable materials (Eliashar et al. 2001; Jenson et al. 2005), and even autologous tendon tissue (Valdes-Vazquez et al. 1996). Their role has been postulated to act as a scaffold to provide a conduit along which cells and healing vasculature can grow as well as maintain the tendon ends in alignment. This is in contrast to the use of scaffold in smaller animals where they can act as a robust mechanical ‘bridge’ because of the smaller forces involved. This is not possible to achieve in horses and, indeed, it has been our observation in the past, especially in hindlimbs supported with a distal limb cast post-operatively


which has immobilised the fetlock but allowed hock and stifle flexion, that the implants have become detached from the transected tendon ends. This observation, the poor availability of appropriately sized, long-lasting but absorbable, implants, and the failure to show significant benefit over debridement alone in the few limited studies published, has meant that implants are now rarely used. As a result, clinicians have relied on simple debridement and natural repair while the region is protected from loading using external coaptation, as described in this case report.


Fig 1: A nonabsorbable (terylene) implant has been used to bridge the large gap between tendon ends following blunt laceration in both digital flexor tendons.


What is, however, surprising in this case report is that this


gap healing has occurred in a region which is particularly problematical for healing. Plantar to the fetlock, where the tendon is subjected to compression as well as tension, requires a more complex extracellular matrix. Moreover, its intrathecal location means that the tendon is surrounded by synovial fluid and lacks a paratenon. We have recently established that synovial fluid has toxic effects on the cells within tendon which helps explain why many tendon and ligament lesions that communicate with the synovial cavity of a tendon sheath, bursa or joint often fail to heal spontaneously (Garvican et al. 2017). The paratenon is considered to be a major player in extrinsic (coming from surrounding tissues) healing, which is believed to dominate extrathecal tendon repair, at least in experimental animal models (Kajikawa et al. 2007). The paratenon is continuous with the endotenon, or interfascicular matrix, which is believed to be the source of endogenous tendon stem cells and neovasculature (Cauvin 2000; Marr et al. 2017). However, the lack of a paratenon in intrathecal locations, means that, in the absence of adhesion formation, repair relies more heavily on intrinsic (from within the tendon) healing which is less effective and slower. Adhesions will enhance gap healing by providing a route for cells and vasculature to access the lesion, and by isolating the injury from the synovial environment, but they may also result in restriction of tendon movement, causing pain and lameness. Hence, it is impressive that the surgically created gap in the superficial digital flexor tendon within the digital flexor tendon sheath in this case became bridged with tissue without restrictive adhesion formation.


© 2021 The Authors. Equine Veterinary Education published by John Wiley & Sons Ltd on behalf of EVJ Ltd. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.


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