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.