Introduction Osteoarthritis (OA) is one of the main reasons for early retirement of sport horses and despite new technologies, radiographic examination is still the most commonly used method of diagnosis. Radiographic changes due to OA in horses are mainly found in the distal limb and tarsal joints (Baccarin et al., 2012). The objectives of the present study were to identify the most frequent radiological findings in these two regions and evaluate the correlation between the affected joints and regions. Materials and Methods Radiographic exams including distal limb and tarsal regions in 113 horses from four different practices were classified retrospectively. Radiographic classification was performed by 3 experienced clinicians, including: distal interphalangeal (DIP), proximal interphalangeal (PIP), metacarpophalangeal (MCP), metatarsophalangeal (MTP), tarsometatarsal (TMT), distal intertarsal (DIT), proximal intertarsal (PIT) and tibiotarsal (TT) joints. At least 2 radiographic projections were performed per region. Radiographic findings were scored using a scale adapted from Kellgren & Lawrence (1957) and de Grauw et al. (2006). The severity of OA findings was classified regarding osteophytes, subchondral bone sclerosis and joint space narrowing as: 0 - normal; 1- doubtful; 2 - minimal; 3 - moderate and 4 - severe. Statistical analysis was performed with Statistical Analysis System software (SAS, version 9.4). Results The population studied included Purebred Lusitano (PBL), Crossbred Lusitano (CL) and Purebred Arabian (PBA) horses with a mean age of 7,1-years-old (range 3-25 years-old). The most frequent radiographic findings were scored as doubtful (score 1), meaning that their clinical relevance is questionable. There were significant differences regarding the DIP joint with CL horses presenting higher mean scores in the four limbs (p < 0,001) when compared to the other two breeds. The CL horses also presented more lesions in TMT (p < 0,05) and DIT joints (p < 0,01). Significantly higher mean score in MCP joints was present in PBL horses (p < 0,05). PBA horses presented significantly lower mean scores in PIP joints (p < 0,05). Age had a significant influence in forelimb PIP (p < 0,01) and fetlock (MCP and MTP) joints (p < 0,01), with older horses presenting higher scores in these regions. The most affected joints (score ≥ 1), were TMT joints (87% in left hind (LH) and 91% in right hind (RH), along with DIT joints (63% and 73% in LH and RH limbs, respectively). Considering forelimbs, the most affected joint (score ≥ 1), was the DIP (71% and 68% in the left (LF) and right forelimb (RF) limbs, respectively). Moderate to severe OA (scores 3 and 4) were mainly found in 3 joints: 10% of TMT and 5% of DIT and 4% of the MCP joints. When analyzing the radiographic findings for the same joint, in contralateral limbs, a strong, positive and significant correlation (r ≥ 0,60, p < 0,001) was systematically found for all joints evaluated, except for the fetlock in the hindlimbs (that showed a moderate, though significant, correlation of r = 0,44, p < 0,001). When analyzing radiographic findings for the same region, a strong, positive and significant correlation (r = 0,56, p < 0,001) was found between the distal limb region of the right diagonal: RF and LH limbs. Also, a moderate, positive and significant correlation (r = 0,36, p < 0,001) was found between the distal limb of the left diagonal: LF and RH limbs. Conclusion In this study Crossbred Lusitano horses presented higher mean scores of OA when compared with Purebred Lusitano and Purebred Arabian horses, in particular in the DIP joint. The most affected joints in the hindlimbs were the small tarsal joints, which is consistent with studies in other breeds (Van Hoogmoed et al., 2003, Baccarin et al., 2012, Contino et al., 2012). The DIP joints presented high scores in forelimbs in a large percentage of horses. Radiological findings in both small tarsal joints in hindlimbs, and DIP joints in forelimbs are frequently encountered in routine examinations and clinical relevance needs to be investigated in every case. To our knowledge, the strong and significant correlation between the existence of radiographic OA changes in the same joint of contralateral limbs, both in the distal limb and tarsal regions, is perceived clinically but was never reported in these breeds. This finding validates the importance of comparing contralateral joints in OA cases and could be explained by genetics and conformational characteristics. Finally, the correlation between radiographic changes in the distal region between diagonal limbs was surprising and could be due to the type of training and exercise in this particular population, mostly dressage horses.

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