Rupture of the cranial cruciate ligament (CCL) is one of the most frequent causes of lameness of the rear limb in the dog. Regardless of the type of treatment, CCL rupture inevitably leads to knee osteoarthritis (OA). The purpose of this study was to evaluate the efficacy of associating surgical treatment of spontaneous rupture of the CCL with a chondroprotector, that is called 'supraadditive' because it is formulated to counteract not only chondrodegeneration, but also the oxidative and inflammatory processes of OA. The open-label controlled study used proton NMR spectroscopy to evaluate the synovial fluid of the stifle of 10 dogs with monolateral rupture of the CCL, selected for the study based on specific inclusive criteria. The dogs were assigned randomly into two groups. Five dogs received the supra-additive chondroprotector for 60 days, starting on the day after surgery. Five dogs only underwent surgical reconstruction of the CCL. The results were analysed with the ANOVA unstructured variance matrix-covariance test. The trend over time of the synovial concentration of four metabolites (lactate, alanine, acetyl groups of N-acetylated sugars on glycoproteins and alpha-anomers of glucose) was found to differ to a statistically significant extent between the two groups, suggesting that the supra-additive chondroprotector produces an intra-articular metabolic rebalance. The results support the adjuvant use of the chondroprotector in the management of CCL rupture, in view of its control of the OA changes that accompany this orthopaedic disabling condition.