Case reveal » Veterinary Diagnostic Imaging » College of Veterinary Medicine

Findings:

Beginning proximal to the calcaneus and extending distally to the level of the tibiotarsal joint, there is pocketing of effusion surrounded by thickened synovial membrane (yellow arrowheads). The effusion is dorsal to the calcaneus and gastrocnemius tendon, plantar the to deep digital flexor tendon and its musculotendinous junction, and extends from the medial and lateral aspects of the limb. The effusion is much more prominent laterally (green arrowheads) and terminates distally at the proximal aspect of the plantar lateral tibiotarsal joint recess.

In the mid aspect of the musculotendinous junction of the deep digital flexor tendon there is a defect in the synovial membrane and overlying muscle fascia (pink arrow) with fluid pooling plantar and medial to the muscle (orange and yellow circles, respectively). In addition, there is mild to moderate fluid interspersed within the plantar lateral aspect of the muscle (green arrows). At the defect there is retracted undulating tissue within the effusion (blue circle). The fluid pooling plantar and medial to muscle continues distally as mild to moderate tarsal sheath effusion and terminates at the mid aspect of the sustentaculum tali.

On the lateral aspect of the calcaneus, there is moderate subcutaneous thickening and edema.
The distal aspect of the gastrocnemius tendon is enlarged with mild fiber abnormalities.
The appearance of the mid aspect of the distal intermediate ridge of the tibia and the corresponding articular surface of the talus is consistent with normal anatomic variation.

Synoviocele, mild to moderate effusion and thickening of synovial membrane with focal synovial defect, tarsal sheath
Focal interspered fluid, deep digital flexor tendon muscle
Moderate subcutaneous thickening and edema

The synoviocele was previously injected with sodium hyaluronan and triamcinolone. The swelling improved markedly and the patient’s lameness resolved. The horse resumed racing but the swelling and lameness returned 2 weeks later, prompting the MRI evaluation.

Normal tarsal sheath anatomy:

The tarsal sheath surrounds the conjoined tendons of the lateral head of the deep digital flexor tendon and caudalis tibialis, as well as the lateral head of the deep digital flexor tendon, including their combined portion distally. The tarsal sheath follows these tendons as they course over the tarsus caudomedially. Distally, the tarsal sheath terminates dorsomedial to the deep digital flexor tendon in the proximal third of the metatarsus. Although there are other tendon sheaths associated with the tarsus, this is usually referred to as the tarsal sheath (Minshall et al.).

The tarsal sheath starts caudodorsal in the distal crural region near the musculotendonous junction of the lateral digital flexor. The proximal extend of the tarsal sheath forms a large pouch between the lateral digital flexor and common calcaneal tendons, which is largest laterally when distended (Minshall et al).
The tarsal sheath is separated from the tarsocrural joint only by their synovial and thin fibrous walls. Further distally, the tarsal sheath is separated from the remaining tarsal joints by thick fibrocartilagenous tissue. At the level of the sustentaculum tali, the tarsal sheath is surrounded by the tarsal flexor retinaculum, in addition to both superficial and deep tarsal fascia. The proximal and distal extents of the tarsal sheath are not enclosed by these tissues and hard therefore more voluminous and forms the proximal and distal recesses of the tarsal sheath, respectively (Minshall et al, Whitcomb et al).

Intersynovial fistula: communication between two synovial structures (e.g. joint or tendon sheath, for example).
Synovial cyst and ganglion cyst/hernia: Variable definition in the literature. Some sources use the terms synonymously (Minshall et al.). Crawford et al. described distinguishing features:
- Synovial cyst/hernia: Protrusion of intact synovial membrane through a tear in the fibrous capsule of a joint or tendon sheath (stratum fibrosum)
- Ganglion cyst: Cystic or myxoid degeneration of connective tissue with pocketing of fluid near a joint capsule or tendon sheath (no synovial lining). They contain mucinous material ﬁlled with gelatinous ﬂuid rich in hyaluronic acid and other mucopolysaccharides.

Synoviocoele (cavity of synovial origin): Disruption of both fibrous and synovial layers of a synovial structure resulting in a contiguous cavity filled with synovial fluid in the adjacent tissue (Minshall et al.).

Tarsal sheath synoviocoeles (Minshall et al.):

Variable age range (2-13 year old horses).
May have a higher incidence in race horse.
Lameness usually gradually worsen over time; however, a few horses may present with acute signs.
Clinical findings: Large unilateral swelling (usually worse laterally) on the caudodistal aspect of the crus between the tibia and tendons of insertion of gastrocnemius and superficial digital tendon.
Ultrasound: Fluid cavity, occasionally contains straw-like material and/or amorphous debris, and surrounded by a smooth lining of variable thickness. The distal aspect extends to the musculotendonous junction of the lateral digital flexor, where a communication between the two synovial structures may be visualized.

The defect may form a one-way valve resulting in a progressive accumulation of fluid within the synoviocele and increased pressure that in turn causes local pressure and irritation in the surrounding tissues.
Lavage and drainage of the synoviocele with debridement may have variable results. If recurrence is noted, resection of the synoviocele may be considered.

Minshall, G. J., and I. M. Wright. “Synoviocoeles associated with the tarsal sheath: description of the lesion and treatment in 15 horses.” Equine Veterinary Journal 44.1 (2012): 71-75.
Whitcomb, Mary Beth. “Ultrasonography of the equine tarsus.” (2006): 13-30.
Crawford, A., O’Donnell, M., Crowe, O., Eliashar, E., & Smith, R. K. (2011). Digital sheath synovial ganglion cysts in horses. Veterinary Surgery, 40(1), 66-72.