EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2022


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This could, therefore, be considered an example of


tendon ‘regeneration’ at least on a limited functional level. Of course, this is not the same as suggesting that the bridging tissue consists of regenerated normal tendon, but it did demonstrate a similarity to tendon ultrasonographically. ‘Regenerative medicine’ has had the ‘holy grail’ of tissue regeneration as its goal for some time, which was the impetus for the first use of mesenchymal stem cells for the treatment of a tendon injury in 2003 (Smith et al. 2003). However, there has been little evidence that true regeneration has been achieved for any of the ever-expanding list of regenerative medicine products and devices that have become commercially available, even though such claims are often made. To a certain extent, this is because ‘regeneration’ is not well defined. It is important to differentiate normal repair from reformation of ‘normal’ tendon, although tendon does change with ageing. In this author’s opinion, true regeneration should be defined, for tendon at least, by the restoration of three key elements – mechanics, structure and composition – to their pre-injury state, which does give a high bar to reach. Defining these characteristics for musculoskeletal tissues is also not always simple, and a spectrum ranging from nonfunctional repair tissue to regenerated tendon probably exists. The normal healing response in tendon is via a process of scarring with the production of fibrous tissue. This tissue is collagen-rich, like tendon, but with a different ratio between collagen types I and III and is more disorganised in structure, which results in different mechanical properties. It also has higher levels of noncollagenous proteins resulting in a higher tissue content of glycosaminoglycans. However, one of the most striking differences exists at an ultrastructural level where the collagen fibril populations are universally small in scar tissue


while they are bimodal (mixed population of small and large fibrils) in normal mature (adult) superficial digital flexor tendon (but not all tendons). This latter characteristic was, therefore, used in an experimental study in horses to assess the ability of mesenchymal stem cells (MSCs) to induce tendon regeneration, but MSC-treated tendons still showed the same unimodal distribution of small collagen fibres similar to controls (Caniglia et al. 2012), indicating a failure of regeneration. However, that does not mean that they cannot still exert a beneficial effect and they have been shown to improve tissue composition and organisation as well as function (Smith et al. 2013). We now believe this effect to be due to modification of the inflammatory process rather than true regeneration which still results in a clinical benefit (reduced reinjury rate; (Godwin et al. 2012) but not by the recreation of normal tendon tissue. In the case described in this edition, not surprisingly, it was not possible to retrieve any of this ‘regenerated’ tissue for a more detailed analysis to determine if true tendon regeneration had occurred. The only possible evaluation of the nature of this tissue, other than through the observation of limb function and soundness, was with ultrasound. This showed an echogenic material similar in echogenicity to tendon with at least some evidence of a degree of longitudinally arranged fibres on longitudinal views (figure 6 in the case report). We have seen this occur previously in extrathecal locations, including in a case when both digital flexor tendon had been lacerated and the gap healed with one scar involving both tendons. Over time, however, two separate tendons, capable of moving independently, were restored (M. Schramme and R. Smith,


unpublished observations), showing the remarkable propensity for the restoration of tissue function in some situations.


Mechanical loading of the gap tissue is believed to be an


important driver for this functional restoration. However, in the horse, such loads can be both helpful and harmful. It is important to protect the neotendon from high damaging loads when it is forming, but a gradual increase in loading also provides an important stimulus for tissue differentiation. A cast, as used in this case report initially, is the only way to unload the gap in the early stages of healing but this does not allow gradual loading. It also unloads the other weight- bearing tendons and ligaments of the distal limb which weakens them, and has recently been shown to be deleterious to the other musculoskeletal tissues of the limb (Stewart et al. 2020). Therefore, it is important to consider alternative methods of controlling digital flexor tendon (or suspensory ligament) loading. Contrary to what is perceived by the horse-owning public, distal limb bandages fail to provide any significant support to the digital flexor tendons in the adult horse (Smith et al. 2002) and a more robust system is needed. Contoured palmar or plantar splints made from casting tape are effective and can be applied to the palmar/plantar aspect of a clinical bandage. They are useful to apply immediately after removal of the cast when a bandage is still required (Smith et al. 2002; Kuemmerle et al. 2018; Fig 2). However, they are difficult to maintain, can cyclically fail, and are likely to be less effective at controlling loading than specifically designed orthotic devices. The first


Fig 2: A contoured palmar splint used to provide significant fetlock support by being taped to the palmar aspect of a bandage. The splint can be fashioned on the opposite limb or else made by laying two rolls of four inch casting tape onto a precontoured plastic guttering. The angle should be slightly straighter than the normal fetlock angle in the horse to be effective at providing fetlock support.


© 2021 The Authors. Equine Veterinary Education published by John Wiley & Sons Ltd on behalf of EVJ Ltd.


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