MAXIM MOREAU, MSc 1 , ÉRIC TRONCY, DV, PhD 1 , SYLVAIN BICHOT, MSc 1 , and BERTRAND LUSSIER, DMV, MSc, Diplomate ACVS 1 1 From the Companion Animal Research Group & GREPAQ and Department of Veterinary Biomedicine, Faculty of Veterinary Medicine, Université de Montréal, St. Hyacinthe, Canada Corresponding Author: Maxim Moreau, GREPAQ, Faculty of Veterinary Medicine, Unive Vet Surg January 2010 39 1 43-47 ABSTRACT Objective—Force platform gait analysis is a recognized clinical evaluation tool that captures and documents the in vivo pathomechanics of osteoarthritis (OA). In a clinical trial designed to evaluate the impact of 2 specific diets, an increase in body weight (BW) was observed in lame client-owned dogs. Covariance analysis was used to evaluate the interference of BW changes toward the evolution of peak vertical force (PVF) values. These secondary findings are reported in this study. Study Design—Prospective study. Animals—Lame dogs (n=26). Methods—Dogs with radiographic evidence of OA and low PVF values were fed with 2 specific diets for 30 and 60 days. PVF and BW were recorded at baseline, day 30 (D30), and D90. Results—Mean (±SD) PVF values (%BW) did not differ significantly over time (D0: 63.9±17.2; D30: 65.5±17.4; and D90: 66.5±20.1). In contrast, BW (kg) was significantly higher at D90 (41.3±7.9) when compared with D30 (39.9±8.4) and D0 (40.0±8.7). Upon covariance analyses, BW changes interfere significantly with PVF values already normalized in %BW (P=.013). Values of PVF adjusted using BW as a covariate were then 63.4±17.1 (D0), 65.0±17.3 (D30), and 67.6±20.5 (D90), whereas D90 was significantly higher than D0. Conclusion—These findings highlighted the interference of changes in BW toward locomotor function of OA dogs when using PVF values normalized in %BW. Exacerbation of lameness when a gain in BW occurred was also sustained, raising a possible bias in clinical study outcomes. Clinical Relevance—A BW increase in dogs with OA could exacerbate a preexisting lameness and induce a bias in clinical trials.