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BACKGROUND Digital flexor tendon sheath (DFTS) pathology is an important cause of lameness in horses. The outcome after surgical treatment is variable and depends on the exact diagnosis. OBJECTIVES To (1) describe the prevalence of lesions associated with lameness caused by non-septic DFTS tenosynovitis in a large population of German sports and pleasure horses; (2) determine the sensitivity and specificity of diagnostic imaging techniques for identifying lesions within the DFTS with tenoscopic-diagnosis being the gold-standard; (3) explore associations between tenoscopically-diagnosed lesions and signalment, purpose, and limb affected; and (4) describe the outcome following DFTS tenoscopy with non-septic DFTS tenosynovitis in this population. STUDY DESIGN Retrospective case series METHODS: Medical records of horses admitted for tenoscopic surgery of non-septic DFTS tenosynovitis between 2011 and 2020 were reviewed. Follow-up information was obtained via telephone contact. Sensitivity and specificity of ultrasonography and contrast tenography were determined using tenoscopic diagnosis as gold standard and univariable analysis was used to explore associations between signalment, case history, and tenoscopic diagnosis. RESULTS Medical records from 131 horses were retrieved, of which 8 horses had bilateral disease and 6 horses were presented for tenoscopy on two separate occasions (3 for tenoscopy in the same limb, 3 in a different limb), thus, making a total of 145 limbs. Lesions were most commonly diagnosed in the deep (DDFT, n=55 limbs) and superficial (SDFT, n=55 limbs) digital flexor tendons. Manica flexoria (MF) lesions were detected in 44 limbs and palmar/plantar annular ligament (PAL) constriction in 99 limbs. In 36 limbs, only one structure within the DFTS was injured, whereas in 109 limbs a combination of lesions was noted, the most common being the combination of a SDFT lesion with PAL constriction. All affected limbs were examined with diagnostic ultrasonography; contrast tenography was performed in 86 limbs. For diagnosis of MF and DDFT tears, tenography was more sensitive (89% (CI 65.4-95.2%); 72% (CI 46.4-89.3%) respectively) than specific (64% (CI 52.5-77.6%); 53% (CI 42.2-73.3%) respectively) whereas ultrasonography was more specific (92% (CI 84.5-96.3%); 92% (CI 83.6-96.0%) with lower sensitivity (64% (CI 47.7-77.2%); 54% (CI 39.5-67.9%). For SDFT lesions, ultrasonography was highly specific (94% (CI 86.9-97.9%) with lower sensitivity (66% (CI 51.3-77.4%). Follow-up information following first surgery was obtained for 118 horses (132 limbs): 18 (15.3%) of 118 horses remained chronically lame, 40 (33.8%) performed at a reduced level and 60 (50.8%) performed at the same or higher level following rehabilitation after tenoscopy. Horses with DDFT lesions had the poorest outcomes with only 36.6% returning to the same or higher level of exercise. MAIN LIMITATIONS Retrospective analysis of clinical records and subjective outcome assessment based on owner follow-up with potential recall bias. Findings on diagnostic imaging are impacted by many factors including equipment quality and operator expertise and experience. CONCLUSION Diagnostic imaging techniques were complimentary and contrast tenography was sensitive and ultrasonography was specific for the diagnosis of MF and DDFT lesions. Following tenoscopic surgery for non-septic tenosynovitis of the DFTS, approximately half the cases were able to return to pre-injury level of exercise. This article is protected by copyright. All rights reserved.