Case Report Lipoma of the long digital extensor tendon sheath in a horse C. Owens † , A. Young ‡ , P. Loukopoulos † and B. Hilbert* † † Veterinary Clinical Centre and Veterinary Diagnostic Laboratory, School of Animal and Veterinary Sciences, Charles Sturt University, Wagga Wagga, New South Wales, Australia; and ‡ University Veterinary Teaching Hospital, Faculty of Veterinary Science, University of Sydney, Camden, New South Wales, Australia. *Corresponding author email: bhilbert@csu.edu.au Keywords: horse; lipoma; tendon sheath; weanling Summary This paper describes the occurrence of a lipoma associated with the long digital extensor tendon sheath at the level of the tarsus in a 5-month-old female Thoroughbred. Radiographic and ultrasonographic images were interpreted as being an atypically located lipoma. Its position and character have some similarities to lipoma arborescens, which is reported to occur in and around human joints and tendon sheaths. Surgical removal is considered to be the treatment of choice. Introduction Pedunculated mesenteric lipomas occur in the abdomen of older horses, and are usually only detected at post mortem examinations or when they cause a strangulating intestinal obstruction (Edwards and Proudman 1994; Freeman and Schaeffer 2001; Garcia-Seco et al. 2005). In 2002, Hammer et al. described a lipoma in a 2-year-old Quarter Horse that had arisen from the sheath of the common digital extensor tendon at the level of the carpus. The tumour was thought to be associated with a traumatic injury that the gelding sustained one year earlier. Histologically, the mass consisted of vascularised adipose tissue and collagen bundles but was otherwise unremarkable. While there have been sporadic reports of lipomas occurring externally in horses, they have not been typically associated with the synovial lining of tendon sheaths or joints (Blackwell 1972; Bristol and Fubini 1984; Baker and Kreeger 1987; Anderson and King 1988; Lepage et al. 1993; Hamir et al. 1994; Carson Dunkerley et al. 1997). This report describes the putative, spontaneous occurrence of an intrathecal lipoma of the long digital extensor tendon sheath in a Thoroughbred filly. Case History A 5-month-old female Thoroughbred was presented for assessment of a firm, soft tissue mass on the dorsal aspect of the right tarsus (Fig 1). The filly was not lame and there did not appear to be any restriction of joint motion. The swelling had been present for approximately 4 months and had slowly grown in size to 7 9 10 cm. There was a granulating wound over the dorsal and medial aspect of the swelling which was related to a previous biopsy attempt in the field. The swelling had previously been injected with corticosteroids but remained unchanged. Fluid could not be aspirated from the swelling and there was no associated effusion of the tarsocrural joint. Imaging Radiographic examination of the right tarsus revealed a large, approximately 9 cm long mass dorsal to the tarsocrural joint and trochlear ridges of the talus (Fig 2). The mass was of mixed fat and soft tissue opacity. The lesser opacity of the fat enabled visualisation of a thin dorsal soft tissue capsule and multiple, irregular but subtle soft tissue opacities coursing throughout the centre of the mass. A small (2 cm long) soft tissue defect was seen at the distal dorsomedial margin of the mass, consistent with the location of the granulating external wound from the previous biopsy. There was no radiographic evidence of tarsocrural effusion and the mass did not communicate with the joint on a positive contrast arthrogram study. Linear soft tissue opacities consistent with at least one of the extensor tendons could be seen passing distally into the central region of the mass. The adjacent osseous structures were unremarkable. The mass was ultrasonographically heterogeneous and had a thin, well-defined hyperechoic capsular margin dorsally (Fig 3). The deeper margins of the mass were less distinct. The adjacent long and lateral digital extensor tendons were unremarkable despite their close proximity to the mass. There was at least one small pocket of echogenic fluid within the mass. Differential diagnoses at this stage included an atypically located lipoma, granuloma or organising haematoma. The former was thought to be most likely given the mixed fat/soft tissue opacity on the radiographic study. Surgical findings With the horse anaesthetised and positioned in dorsal recumbency and with the affected limb suspended in extension, a 15 cm curvilinear skin incision was made over the dorsal aspect of the swelling. An encapsulated mass (Fig 4) was exposed using a combination of sharp and blunt dissection. The tumour was intimately associated with the synovial lining of the sheath of the long digital extensor tendon and this made it difficult to define the anatomic limits of the tendon sheath accurately. An incision into the tumour revealed a white fat-like tissue mass that seemed to have its origins within the tendon sheath (Figs 5 and 6). The tumour was isolated using blunt dissection and careful efforts were made to ensure as much abnormal tissue as possible was removed. A defect that was created in the tendon sheath was repaired using USP 2/0 poly-p-diaxanone (MonoPlus) 1 placed in a horizontal mattress pattern. The subcutaneous tissues were apposed and dead space eliminated by placing © 2015 EVJ Ltd 1 EQUINE VETERINARY EDUCATION Equine vet. Educ. (2015)  () - doi: 10.1111/eve.12528