Gold RM, Gregor TP, Huck JL, McKelvie PJ, Smith GK. Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104. JAVMA 2009 June 15 234 12 1549-54 Objective-To determine effects of hip joint osteoarthritis on radiographic measures of hip joint laxity and congruence. Design-Longitudinal study. Animals-40 Labrador Retrievers. Procedures-Dogs were assigned to 2 groups based on radiographic evidence of osteoarthritis. Dogs in the osteoarthritis group were free of osteoarthritis at initial radiographic evaluation (t(1)) and developed osteoarthritis by a subsequent radiographic evaluation (t(2)). Dogs in the nonosteoarthritis group had no radiographic osteoarthritis at either evaluation. Hip joint laxity was quantified by use of the distraction index (DI) from a distraction radiographic view and use of the Norberg angle (NA) from a ventrodorsal hip-extended radiographic view. The compression index (CI) from a compression radiographic view was used as a measure of joint congruence (concentricity). Results-Hip joint laxity (NA or DI) did not change over time in the nonosteoarthritis group. Mean hip joint laxity (NA and DI) for the osteoarthritis group was greater at t(1) than for the nonosteoarthritis group. With the onset of osteoarthritis, mean NA decreased significantly and mean CI increased significantly, but mean DI remained unchanged. Conclusions and Clinical Relevance-No radiographic evidence for compensatory hip joint tightening associated with osteoarthritis was detected. Hip-extended radiography revealed that hip joints got looser with osteoarthritis and NA decreased. Hip joint laxity (DI) on distraction radiographs was unchanged by the onset of osteoarthritis and remained constant in the osteoarthritis and nonosteoarthritis groups at both evaluations. However, the CI increased with osteoarthritis, as reflected in nonzero indices (incongruence). The CI may be a valid marker for early hip joint osteoarthritis.