Discussion
This case series describes meniscal involvement in 3 foals with septic arthritis of the LFTJs. To the authors’ knowledge, this form of meniscal involvement has not been reported heretofore. Conventional classification systems of septic arthritis and osteomyelitis in horses do not make provision for involvement of IA soft-tissue structures, such as the menisci (Richardson DW and Stewart S, no date). This highlights the need for an additional classification subtype, to extend the S-type infection from involving not only the synovial membrane and fluid, but also to other soft tissue structures within the joint.
The progression of the pathophysiology in these cases is unclear. Given that the meniscal lesions were most marked at the site of attachment with the capsular synovium, it was theorized that extension of purulent inflammation occurred from the joint via the vascular network of the capsular synovium supplying the meniscus at this location (Arnoczky and Warren, 1982). Purulent osteomyelitis was found at the junction between articular cartilage and subchondral bone (Firth and Goedegebuure, 1988). The presence of septic canals below erosive articular cartilage lesions, in the absence of any lesions in the underlying epiphysis, suggests that the inflammation spread from the joint to the subchondral bone through septic cartilage canals (Wormstrand et al. , 2018). Therefore, the authors believe that, due partial/complete failure of passive transfer, the original IA septic focus established within the synovial membrane and fluid, and subsequently extended to the meniscus, articular cartilage and subchondral bone, leading to their disruption.
From the current series, it appears that a septic meniscus can act as a persistent nidus of infection perpetuating septic arthritis, and also leading to septic osteomyelitis and subchondral bone cyst formation. In these cases, once infection became established within the menisci, arthroscopic lavage and debridement of IA fibrin alone was inadequate to address the sepsis within the meniscus, which did not resolve until the purulent focus within the meniscus itself was debrided. For this reason, in cases where foals with septic arthritis of the LFTJ relapse following arthroscopic debridement and lavage, the authors advocate paying close attention to the appearance and position of the meniscus using US, and CT where available, to examine for evidence of displacement or disruption. Meniscal disruption should also be considered in cases where synovial parameters resolve post-operatively following arthroscopic debridement and lavage for LFTJ septic arthritis in foals, however lameness persists for no apparent reason.
It has been reported that sepsis of the LFTJ usually occurs as an extension of infection from the FPJ, as the two communicate in 1-17% of cases (Reeves, Trotter and Kainer, 1991; Vacek, Ford and Honnas, 1992; Hennessy et al. , 2012). In this case series, only one of the three foals was presented with additional joints affected in the stifle (Foal 2), with the other two foals having only LFTJ involvement. Furthermore, in adults, meniscal tears occur three times more often in the medial meniscus than the LM (Walmsley, Phillips and Townsend, 2003; Cohen et al. , 2009), which was not the case in this series.
In adults, advanced imaging of the stifle is challenging due to size limitations, and thus arthroscopy has traditionally been relied on for the diagnosis of pathology of menisci and meniscal ligaments(McIlwraith CW, Nixon AJ, Wright IM, no date). US does not always accurately predict meniscal tears in adults (Cohen et al. , 2009). The small size of the patients in this series allowed for CT to be performed in 2 of the foals, providing valuable additional information regarding IA soft tissue involvement in these cases.
In adult horses, only limited portions of the meniscus are visible with arthroscopy, requiring examination of the cranial and caudal pouches of the femorotibial joints from separate approaches (McIlwraith CW, Nixon AJ, Wright IM, no date). Grade 3 tears, which extend beneath the femoral condyle, are typically inaccessible (Walmsley, Phillips and Townsend, 2003; McIlwraith CW, Nixon AJ, Wright IM, no date). Furthermore, according to McIlwraith et al. , accessibility to the LFTJ can be difficult due to the IA location of the long digital extensor and popliteal tendons (McIlwraith CW, Nixon AJ, Wright IM, no date). In the current series, the authors found the foal LFTJs relatively easy to maneuver within, most likely because of laxity due to their young age, which facilitated better access to a greater extent of the tears for debridement.
Regenerative therapies, including IA platelet rich plasma (PRP) and interleukin-1 receptor antagonist protein (IRAP) have been proposed as adjunctive therapies for meniscal injuries (Fowlie JG, Richardson DW, Ortved KF, no date). A clinical study in adult horses has shown that IA administration of BMSCs may promote meniscal regrowth and decrease the subsequent development of osteoarthritis (Ferris et al. , 2014). Similarly, caprine models have demonstrated significant regeneration of medial meniscal tissue, with implanted cells shown to be incorporated within newly formed tissue, following IA delivery of adults BMSCs (Murphy et al. , 2003). The use of BMSCs for treatment of similar injuries in foals has not been previously reported until now.
Prognosis for grade III meniscal tears in adults horses is poor, with only 6% of horses returning to previous athletic function (Walmsley, Phillips and Townsend, 2003). In adults, grade III tears preclude complete removal of torn tissue (McIlwraith CW, Nixon AJ, Wright IM, no date). The results of the current series suggest that the short-medium term prognosis in foals with meniscal disruption associated with septic arthritis, is also guarded to poor for return to soundness, with only 1 Foal lame-free out of 3 after 1 year. The prognosis in foals may be more favorable than adults, due to their greater healing potential, low weight and easier access to the tears by arthroscopy due to their small size, allowing more complete debridement. It has been hypothesized that dystrophic mineralization of the meniscus, visible on radiographs and US, may lower the prognosis for return to athleticism (Fowlie JG, Richardson DW, Ortved KF, no date). Early signs of mineralization within the soft tissue lateral to the LFTJ, and within the LM, in Foals 2 and 3, seen at 6 months, may indicate a poorer long-term prognosis for soundness.