Case reveal

Findings:

• The entirety of the tarsal sheath in the field of view is severely thickened and hyperintense, containing a moderate amount of fluid with an intermediate signal intensity on T2 and STIR images (yellow asterisk). At this level, the lateral digital flexor tendon is moderately thickened with irregular margins and disruption of the normal fiber pattern, as characterized by the presence of a linear hyper intensity coursing within the tendon dorsally. The fibrocartilagenous layer along the groove of the sustentaculum tali is variably severely thinned and there is variable close association of the lateral digital flexor tendon at this level.

• At the level of the groove, the sustentaculum tali is moderately STIR hyperintense with regions of lysis and has a severely irregular plantar margin (blue arrow).
• At the level of the tarsometatarsal joint, the superficial digital flexor tendon is focally moderately thickened with an irregular fiber pattern and contains a thin central linear hyperintensity (yellow outline). The surrounding soft tissues are mildly thickened and moderately STIR hyperintense.

• In the plantarodistal aspect of the tarsus, the subcutaneous tissues are moderately thickened and STIR/T2 hyperintense.

Conclusions:

• Septic tenosynovitis of the right tarsal sheath with associated superficial and lateral digital flexor tendinopathy (+/- adhesions), sustentaculum tali osteomyelitis and cellulitis, compatible with prior trauma.

Radiographs (acquired 1.5wks before MRI):

• Severe, interrupted, spicular periosteal proliferation associated with the plantar aspect of the right sustentaculum tali. Moth eaten lysis is also present in the region of the sustentaculum tali.
• Severe soft tissue thickening surrounds the tarsus and is more severe plantaromedially.
• The cutaneous margins in this region are irregular.
• There is increased mineral opacity resulting in decreased compact/trabecular distinction associated with the dorsal aspect of the third and central tarsal bones.
• Mild osteophyte formation is seen along the dorsal periarticular margins of the metatarsal joint.

Ultrasound:

• Severe enlargement of the superficial digital flexor and deep digital flexor tendons. The plantar margin of the sustentaculum tali is severely irregular.

Follow-up:

• Fungal culture: no growth
• Bacterial culture: moderate growth (Enterococcus)
• The patient was given a guarded prognosis in light of the noted sustentaculum osteomyelitis.  Surgery was offered but not heavily encouraged due to concerns as to how much bone would actually have to be curetted. Humane euthanasia was elected.

A little bit more…

• The tarsal sheath is a synovial structure which envelops the lateral digital flexor tendon (Straticò et al.).
• Measuring 25 to 30 cm in length, it starts 6 to 7 cm above the medial tibial malleolus and ends at the proximal third of the metatarsus (Straticò et al.).
• The sustentaculum tali and the calcaneus protect its dorsal and lateral sides, while the thick, transversely oriented plantar retinaculum covers its plantar and medial sides (Straticò et al.).

• Tarsal sheath synovitis may be idiopathic, seen in both young and older horses (Straticò et al., Caspers et al.).
• Lateral digital flexor tendonitis can occur without tarsal sheaths effusion and may be overlooked by concurrent proximal suspensory desmitis (Davis et al.).
• Septic tarsal tenosynovitis is caused by an injury to the medial side of the hock, often linked to a fracture of the medial edge of the sustentaculum tali, the most prominent feature on the medial aspect of the tarsus (Straticò et al., Wereszka et al., Schneider et al.).
  • Without treatment, it can result in infectious tenovaginitis, destruction of fibrocartilage, and eventually osteitis or osteomyelitis of the calcaneus +/- sequestrum formation (Straticò et al., Vajs et al.).
  • Osteomyelitis of the ST has a guarded survival prognosis and poor outlook for athletic performance, depending on the extent of damage to the lateral digital flexor tendon and the lesion’s chronicity (Straticò et al., Schneider et al.).
  • In cases with chronic lesions, the lateral digital flexor tendon often forms adhesions with the synovial sheath wall and undergoes soft tissue mineralization. These changes typically impair function and lead to ongoing lameness (Straticò et al., Vajs et al.).
  • From 1976 to 2011, multiple reports of tarsal sheath sepsis, with or without ST lesions, were documented in the literature. Following antimicrobial therapy and, in some cases, fragment removal and tarsal sheath lavage, only three horses regained sufficient soundness to return to work (Straticò et al.).
• Standard imaging methods, most radiography and ultrasonography, are necessary to guide a diagnosis and to determine the need for tenovaginoscopy of the tarsal sheath (Straticò et al., Vajs et al.).
• Radiography, specifically the dorsomedial-plantarolateral view of the hock and the flexed skyline projection of the plantar aspect of the sustentaculum tali, are necessary to identify fractures of the senstaculum tali, assess the remodeling of the groove, and rule out fractures or osteitis of the tuber calcanei (Straticò et al.).
• Conventional radiography may be sufficient. Double contrast radiography is reported to be more sensitive but requires general anesthesia and has been largely replaced by ultrasonography, which is currently the preferred method for evaluating soft tissue structures of the crus and hock (Straticò et al.).

• MRI is rarely performed due to its limited available and higher cost but may also be considered to guide a diagnosis. In the current case, MRI was done for teaching purposes.
• Tenovaginoscopy of the tarsal sheath is seldom required solely for diagnostic purposes; however, it can be valuable for assessing potential fibrocartilage damage and detecting fibrillations or adhesions of the lateral digital flexor tendon that may not be visible on ultrasonography. Additionally, it is valuable for releasing adhesions, curetting infected bone and enhancing the prognosis for a return to activity (Straticò et al., Schneider et al., Vajs et al.).

References:

• Straticò P, Varasano V, Suriano R, Sciarrini C, Petrizzi L. Traumatic septic tenosynovitis of the tarsal sheath with fragmentation of the sustentaculum tali: surgical treatment and outcome in 3 horses. Journal of Equine Veterinary Science. 2014 Apr 1;34(4):538-43.
• Caspers MK, Gier CJ, Reesink HL. Equine non‐septic tenosynovitis: A systematic literature review of site‐specific pathological lesions, outcomes and surgical complications. Equine Veterinary Journal. 2024 Sep;56(5):842-57.
• Wereszka MM, White NA, Furr MO. Factors associated with outcome following treatment of horses with septic tenosynovitis: 51 cases (1986–2003). Journal of the American Veterinary Medical Association. 2007 Apr 15;230(8):1195-200.
• Davis W, Caniglia CJ, Lustgarten M, Blackwelder T, Robertson I, Redding WR. Clinical and diagnostic imaging characteristics of lateral digital flexor tendinitis within the tarsal sheath in four horses. Veterinary Radiology & Ultrasound. 2014 Mar;55(2):166-73.
• Vajs T, Nekouei O, Biermann NM. A descriptive study of the clinical presentation, management, and outcome of horses with acute soft tissue trauma of the tarsus and the association with synovial involvement. Animals. 2022 Feb 21;12(4):524.
• Schneider RK, Bramlage LR, Moore RM, MECKLENBURG LM, KOHN CW, Gabel AA. A retrospective study of 192 horses affected with septic arthritis/tenosynovitis. Equine Veterinary Journal. 1992 Nov;24(6):436-42.