Avulsion of the proximal digital annular ligament in five hindlimbs

Abstract

Summary Avulsion of the proximal attachment of the proximal digital annular ligament (PDAL) was identified in five horses based on characteristic radiographic findings and supported by ultrasonographic examination in four horses. In two cases, PDAL avulsion was associated with acute onset lameness based on physical examination (both animals) and diagnostic analgesia (one animal), and was the only lesion identified. Both horses became sound after a period of rest. Radiographs repeated 11 years later in one animal showed minimal change in the appearance of the lesion. PDAL avulsion was associated with a substantial tear of the deep digital flexor tendon in another horse and in the remaining two cases PDAL avulsion was not associated with lameness. A dissection study found that the proximodistal extent of the proximal PDAL attachment, expressed as a proportion of the length of the proximal phalanx, was significantly (P = 0.011) greater in hindlimbs (11.48 ± 1.01%) compared with forelimbs (8.55 ± 1.06%). This finding may explain why hindlimbs appear at greater risk of PDAL avulsion. In conclusion, while PDAL avulsion can be a cause of acute lameness which, in uncomplicated cases, responds to rest, it can also be an incidental radiographic finding.

The palmar/plantar aspect of the proximal phalanx – a site of a variety of bone and soft tissue pathologies

“…In the current case series, (O'Brien et al . ) there was one case (Case 4) where the proximal attachment of the PDA was not enlarged – and in the other cases, there were no soft tissue abnormalities detected associated with the PDAL; thus, substantiating the conclusion that as long as there is no soft tissue pathology associated with the fragmentation, then persistent lameness should not be a feature. Given the nonarticular nature of the fragmentation and the lack of soft tissue damage associated with the pathology, this is perhaps not unsurprising.…”

“…The report in this current issue (O'Brien et al . ) describes a further type of avulsion fracture associated with the proximal P1 and the attachment of the proximal digital annular ligament that may or may not be associated with lameness and that resolves with rest. The proximal digital annular ligament is a tight fibrous band extending from the lateral palmar/plantar process of the proximal phalanx across the palmar/plantar aspect of the digital tendon sheath to the medial plantar/palmar process (Carnicer et al .…”

“…As described by O'Brien et al . (), fragments were often round and smoothly marginated as distinct separate osseous bodies leaving a defect in the parent bone. Histological examination of the fragments in this case series was not performed to determine the presence of osteochondritic characteristics that may underline a different aetiology to the traumatic/avulsion one.…”

“…In the report by O'Brien et al . (), it is surprising that failure occurs at the bone to bone interface as opposed to failure at the tendon ligamentous interface as the PDAL proximal attachment is a thin unsubstantial piece of tissue that would not appear to provide much structural support to the plantar MCT/MCP region. As the authors state in their discussion, this overloading would either result in bony attachment avulsion or ligamentous damage resulting in tearing of the attachment of the ligament.…”

“…In only one of two cases reported by O'Brien et al . () in which the PDAL avulsion was the sole source of lameness was the pathology blocked. It would be expected that an abaxial sesamoid nerve block would be sufficient to abolish pathology at this site; however, it took a lateralised low four‐point nerve block to abolish lameness.…”