Carpal sheath hernia in a cob: Unusual case, unorthodox fix

Abstract

The case report presented by Neild et al. (2025) describes a rare and unusually complex case of carpal sheath synovial herniation in a cob-cross mare following tenoscopic treatment of a radial physeal exostosis and intra-thecal deep digital flexor tendon (DDFT) tear - an injury more commonly seen in racehorses due to carpal hyperextension (Southwood et al., 1998). The use of a synthetic polypropylene mesh to support the compromised carpal sheath wall represents a novel adaptation of herniorrhaphy techniques to equine synovial pathology and adds a valuable perspective to the growing body of surgical solutions for complex synovial outpouchings. Beyond its technical innovation, the case report also invites thoughtful discussion on several broader clinical themes, including the underlying aetiopathogenesis in an atypical breed, the localisation and assessment of pain in such cases, the interpretation of intrathecal tissue recovery and the contextualisation of this case within the existing literature.

Synovial herniation of the carpal sheath is rarely seen in cob-type horses, whose upright limb conformation limits carpal hyperextension. While there is no published evidence specifically linking this condition to Thoroughbreds or other high-motion sport horses, it is thought to be more likely in these types due to the greater frequency of carpal overextension in such populations (Nixon et al., 2004; Southwood et al., 1998). In this case, however, the mare was diagnosed with a radial physeal exostosis where a concurrent deep dorsolaterally located DDFT tear would suggest significant bony pathology given that it matched the location of the exostosis. However, it is also possible that the tear could have been created or exacerbated by dynamic carpal over-extension; although, notably, the report does not document this. An injury to the accessory ligament of the DDFT (ALDDFT) injury has also been associated with carpal over-extension and could have been responsible for chronic effusion, requiring repeat sheath injections, but was not reported by the authors.

The terminology used to describe fluid-filled swellings associated with synovial structures, particularly the digital flexor tendon sheath (DFTS) and tarsal sheath, has long been inconsistent in the veterinary literature (Crawford et al., 2011; Hawkins et al., 2021; Laverty, 2009; Minshall & Wright, 2012). Terms such as synovial hernia, synoviocoele, synovial ganglion cyst and adventitious bursa have been applied somewhat interchangeably, though each has specific pathological and anatomical implications. In this case report, Neild et al. (2025) have elected to use the term ‘synovial hernia’ which traditionally refers to a protrusion of the synovial membrane through a defect in the joint capsule or tendon sheath, often still maintaining continuity with the synovial cavity. This implies that the stratum synoviale has remained intact but has extended abnormally through a fascial or capsular defect. However, given that histopathology was not performed in this case, there is no definitive evidence as to whether the herniated sac retained its synovial lining or was instead the result of rupture and subsequent extra-synovial fluid accumulation. As shown in Crawford et al. (2011), similar fluid-filled structures in the DFTS often lack a synovial lining when examined histologically and instead demonstrate myxomatous inflammation consistent with a synovial ganglion cyst. Minshall and Wright (2012) initially proposed the term ‘synoviocoele’ as the most neutral and anatomically descriptive term, simply denoting a cavity of synovial origin without implying a specific pathogenesis or histological profile. Thus, in the absence of histological confirmation in this case, ‘synoviocoele’ would have been the more conservative and technically accurate descriptor. It acknowledges the synovial origin of the lesion while avoiding assumptions about the integrity or composition of the synovial membrane. In future publications regarding these structures, it would be helpful for authors to clarify the rationale behind terminology choice, particularly when the underlying lesion's nature - whether herniation, rupture or cystic degeneration remains speculative in the absence of a biopsy (Table 1).

The authors attribute the formation of the synovial hernia primarily to post-operative effusion and increased intrasynovial pressure, which may have forced synovial fluid and potentially the synovial membrane through a fascial defect at the site of tenoscopic portal creation. They propose that repeated intra-thecal corticosteroid injections may have further compromised local tissue integrity by suppressing fibroblast activity, contributing to fascial weakening (Stankler & Ewen, 1972). Additionally, they acknowledge that multiple instrument passages during the initial tenoscopy could have enlarged the fascial rent, particularly between the ulnaris lateralis and lateral digital extensor muscles, an area that naturally experiences dynamic loading.

While these are plausible contributors, the authors do not explore several other likely mechanisms. The cumulative biomechanical strain from exercise during the post-operative period may have exacerbated tension at the weakened portal site. Moreover, the potential for incomplete fascial healing, especially in a site of high motion and pressure fluctuation, could have predisposed the area to delayed herniation. The role of local ischaemia or microvascular disruption during surgery, particularly if compounded by corticosteroid use, was also not discussed but could have further impaired normal healing processes.

Taken together, it is likely that the hernia developed as a multifactorial sequela of focal surgical trauma, prolonged synovial effusion, pharmacological tissue suppression and mechanical stress over time. This case serves as a valuable reminder of the importance of portal site management, controlled rehabilitation and consideration of long-term tissue resilience following tenoscopy, particularly in horses with conformational or workload factors that may place additional strain on surgical sites.

Minshall and Wright (2012) proposed a ‘one-way valve’ hypothesis for the pain caused by synoviocoeles, where pressure-related pain is central to their significance. In the discussion of the case report, the authors suggest that the clinical significance was likely a consequence of increased intrasynovial pressure secondary to post-operative effusion, with synovial fluid and possibly synovial membrane being forced through a fascial defect at the tenoscopic portal site. While these are reasonable explanations, they are not explicitly linked to the clinical signs of pain observed in this case. The authors stop short of discussing how these structural changes might have resulted in significant lameness - up to Grade 4/5 (AAEP scale) at walk, which is unusual for many synovial outpouchings unless they are large, tense, or associated with deeper pathology. The presence of visibly thinned, compromised skin overlying the hernia was well described in the report but not directly discussed as a pain source. In clinical practice, excessive tension on thinning dermis, particularly in a mobile region like the proximolateral antebrachium, is highly likely to cause nociceptive discomfort and may have been a significant contributor to the horse's lameness (Muir, 2010). Furthermore, the anatomical location of the hernia - situated between the ulnaris lateralis (UL) and lateral digital extensor fasciae places it within a region of considerable muscular activity. The anatomical location of the hernia between the ulnaris lateralis and lateral digital extensor fasciae places it in an area of substantial muscular activity. This is reminiscent of the biomechanical interplay proposed in accessory carpal bone fracture pathogenesis, where asynchronous contraction of the ulnaris lateralis and flexor carpi ulnaris occurs due to their differing embryological origins from dorsal versus ventral limb musculature (Diogo & Abdala, 2010; Radue, 1981). Despite UL's developmental classification as an extensor, its functional role as a flexor, combined with flexor carpi ulnaris's distinct insertion on the accessory carpal bone, creates opposing mechanical forces on the bone.

Repetitive mechanical irritation, friction, or stretching of tissues during movement may have further sensitised the area. Although the authors do mention persistent effusion and synovitis, they do not explore whether this inflammatory environment might have sustained or amplified the horse's pain. Furthermore, given the proximity of superficial nerves in this area, nerve stretch or entrapment cannot be ruled out, especially in a large, space-occupying lesion (Hawkins et al., 2021). In these authors' opinion, the pain observed in this case is most likely to have arisen from a combination of dermal tension, mechanical irritation, persistent low-grade synovitis and possibly neuropathic mechanisms. A more comprehensive assessment of pain pathways in similar cases, including diagnostic blocks and dynamic palpation, would enhance our understanding of when such hernias are clinically significant and warrant surgical correction.

While the pathology is self-evident, synovial herniation is not always associated with pain and lameness, which creates a diagnostic challenge to the clinician. A lack of pain on palpation along with the ability to deflate the hernia on non-weight bearing examination has been proposed previously to be useful clinical indicators of significance (Hawkins et al., 2021), but the authors provide no mention of any pain on digital palpation or compression and would argue against the hernia as the cause of the lameness. The authors acknowledge that diagnostic anaesthesia of the carpal sheath was not performed during the recurrent lameness episodes. This omission leaves uncertainty regarding the actual source of pain and whether the hernia was causative or coincidental.

In this report, the authors note diffuse sheath inflammation at repeat tenoscopy of the carpal sheath, but without correlating that finding directly with lameness. Furthermore, the authors state that the tendon injury demonstrated good healing at the repeat tenoscopy, therefore unlikely to be contributing to the ongoing lameness. Tendon pain can exist in the absence of surface defects and during healing. Unfortunately, however, intra-synovial diagnostic analgesia is not always able to differentiate these sites of pain.

The pressure-related pain is central to the previously proposed surgical indication of tenoscopic decompression (Hawkins et al., 2021; Minshall & Wright, 2012). In this case, it would appear that the communicating defect was large and therefore could not be associated with a ‘one-way valve’ phenomenon. Thus, an alternative approach was necessary and their choice to use a synthetic mesh was an important innovation. The authors describe clear surgical rationale: the fascial defect was too large for simple closure, and resection of the hernial sac would have created a large defect in the synovial membrane with attendant risks of synovial sepsis (Hawthorn et al., 2016). The mesh provided tensile support and allowed for imbrication of synovium without excess tension - a sound approach, especially given the persistent effusion that may have otherwise predisposed to recurrence. However, the post-operative superficial necrosis, likely due to horizontal mattress sutures, suggests the need for alternative skin-closure techniques in future applications. Importantly, the mesh-induced fibrosis likely played a role in preventing re-herniation, as seen in other species (Fitzgerald & Kumar, 2014), but long-term consequences of mesh use in equine synovial structures remain unknown and warrant further study. It should be noted, however, that these authors have observed a large synovial herniation resolving in time although the mechanism why this can occur in some cases but not others is unknown (Figure 1).

This case report offers a creative and successful solution for treating a carpal sheath synovial hernia in a cob-cross horse following tenoscopy - an infrequent but frustrating complication. The authors are commended for introducing mesh herniorrhaphy into equine orthopaedic practice in a synovial context. That said, clarification of the origin of the pain would help limit this approach to those herniations which can be linked definitively with pain and help differentiate the need for a conservative approach or other surgical approaches, such as tenoscopic decompression (Hawkins et al., 2021; Minshall & Wright, 2012). Future directions could include better defining the origin and antalgic effects of synovial herniation, the indications for mesh repair and exploring long-term effects of mesh-induced fibrosis in the synovial environment.

A. Hawkins: Conceptualization; data curation; formal analysis; investigation; project administration; writing – original draft. R. Smith: Writing – review and editing.

There are no funders to report for this commentary.

No conflicts of interest have been declared.

Not applicable to this commentary.