Werner H, Winkels P, Grevel V, Oechtering G, Böttcher P. Hinnerk Werner, DVM, University of Leipzig, Deparment of Small Animal Medicine, An den Tierkliniken 23, Leipzig, 04103, Germany, Phone: +49 163 568 8263, Fax: +49 341 973 8799, E-mail: h.werner@kleintierklinik.uni-leipzig.de. VCOT 2009 22 6 Objectives: To determine the sensitivity and specificity of arthroscopic estimation of positive and negative radio-ulnar incongruence (RUI) in the canine elbow joint. Methods: Experimental radial shortening and lengthening by 1 and 2 mm increments were performed in nine right elbow joints, extending an established surgical in vitro model of RUI. Arthroscopic estimation of each artificially produced radio-ulnar joint conformation (RUJC) was done using a graduated hook probe. A total of 72 RUJC were blindly evaluated in a random manner by an independent investigator and estimated in 1 mm increments (-2, -1, 0, +1, +2). Results: The sensitivity for identification of an incongruent joint was 0.98 (95% CI: 0.90 to 0.99). The specificity for identification of a congruent joint was 0.89 (95% CI: 0.65 to 0.98). Analysing the data only in respect to a congruent joint versus one with a shortened radius (positive RUI) resulted in a sensitivity of 0.96 (95% CI: 0.80 to 0.99) and a specificity of 1.00 (95% CI: 0.92 to 1.00). Clinical significance: Accurate estimation of RUI in dogs affected by elbow dysplasia might improve functional outcome and prevent osteoarthritis when corrective or modifying osteotomies are being considered as part of the treatment plan. Arthroscopy has been shown to be highly accurate and precise in detecting both positive and negative RUI in vitro . However, its diagnostic strength under clinical conditions still has to be proven.