M. Schramme, S. Hunter, N. Campbell, A. Blikslager, R. Smith Veterinary and Comparative Orthopaedics and Traumatology 2010 4 231-239 Objective: Tendon injuries are common in all athletic activities in both humans and horses. Research of treatment modalities for this disease has typically been performed on a model of collagenase-induced tendonitis. This model has several disadvantages. Our hypothesis was that a reproducible core lesion could be created surgically in superficial digital flexor tendons (SDFT), which could then be evaluated consistently using ultrasonography. Materials and methods: Four horses free of forelimb lameness were used in this study. Each horse underwent general anaesthesia and a synovial resector was used to create a core lesion in the SDFT of each forelimb. Sonographic examination was conducted weekly using 2 cm intervals between a section 7 and 25 cm distal to the accessory carpal bone. At two, four, eight, and 12 weeks after injury, a horse was euthanatized. Histopathological evaluation of the SDFT was performed at the same levels as the sonographic examination. Results: Only mild clinical signs of tendonitis were observed. Ultrasonographic core lesions were 10–16 cm long and had a mean maximum cross-sectional area (CSA) of 18.25 ± 5.91% occurring at 17–23 cm distal to the accessory carpal bone, and a mean volume of 1.86 ± 0.26 cm3. Mean duration taken to achieve maximum lesion CSA and lesion volume was 35 ± 7 days. Histologically, the lesions were characterised by mild inflammation followed by fibroplasia. Conclusion: The reported surgical technique resulted in core lesions that were consistent in size and location, were readily evaluated with ultrasonography, and showed similarities with the ultrasonographic and histological progression of naturally occurring tendonitis lesions.