Variation in frontal plane joint angles in horses V. E. UNT*, J. EVANS, S. R. REED, T. PFAU and R. WELLER Department of Veterinary Clinical Sciences, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire, AL9 7TA, UK. Keywords: horse; conformation; biomechanics; angular limb deformity; motion analysis Summary Reasons for performing study: Conformation in horses is often considered an indicator of athletic ability, performance and resistance to orthopaedic disease. Evaluation is performed in the standing horse and repeatability influenced by stance. Ground reaction forces increase in the moving horse as speed increases. Objectives: To determine the effect of locomotion on equine carpus, tarsus, metacarpophalangeal (MCP) and metatarsophalangeal (MTP) joints in the frontal plane. Hypothesis: Valgus/varus angulation will change when moving. Materials and methods: Kinematic data were collected standing, walking and trotting for 2 groups of horses. The change in angle for carpus, tarsus, MCP and MTP joints was calculated standing and midstance for each stride. Comparison of joint angles between left and right limbs, standing, walking and trotting were made. Inter- and intrahorse variations were investigated. Results: Significant differences were observed between groups of horses and left and right forelimbs. Between walk and trot, the MCP joint changed from valgus to varus, and the tarsus and MTP joints increased in valgus deformity. Between standing and walk the carpus increased in valgus deformity. Interhorse variation was significantly different, intrahorse variation was not. Variation in measurements between gaits was minimal. Conclusions: This study validates the measurement of joint angles from the front in the walking and trotting horse using kinematic data, interhorse variation in joint angle measurements exceeding intrahorse variation. The increases in joint angles between standing and walking, and walking and trotting warrant further investigation. Introduction It is widely accepted that a substantial proportion of wastage within the equestrian population can be attributed to lameness (Jeffcott et al. 1982; Rossdale et al. 1985; Dyson et al. 2008); and this has wide implications both economically and ethically. Joint disease, particularly with regard to the metacarpophalangeal and carpal joints (Bailey et al. 1999), has been implicated as one of the greatest causes of lameness in racehorses (Wilsher et al. 2006). The industry has anecdotally made associations between conformation and musculoskeletal injury, although few studies have investigated this (Anderson et al. 2004; Gnagey et al. 2006; van Weeren and Crevier-Denoix 2006; Weller et al. 2006a). It is therefore important to understand any factor that may predispose and/or result in joint injury and wastage, including conformation. Conformation in the horse is often used as an indicator of an individual’s athletic ability, future performance and their resistance to orthopaedic disease (Holmström 2001). The term conformation refers to the structural arrangement of body segments relative to each other. This includes symmetry, segment lengths and shapes, joint angles and the deviations of these segments. Conformation may also refer to the aesthetic appearance of an animal according to the observer’s perception (Blood and Studdert 2000). Certain conformational characteristics may be considered beneficial or detrimental, depending on the discipline and/or breed (Marks 2000). The term angular limb deformity (ALD) is synonymous with a deviation in the limb, when viewing the horse in the frontal plane, or from the front (Mitten and Bertone 1994). Several deviations within the limbs in the frontal plane are observed, including valgus and varus deviations of the carpus, metacarpophalangeal (MCP) joint, tarsus and metatarsophalangeal (MTP) joint. A valgus deformity refers to a deviation of the limb laterally about a reference point, or the joint; and a varus deformity pertains to a medial deviation of the limb. Valgus and varus ALDs may occur in isolation or in combination with compensatory deformities, for example, MCP joint varus could be offset by a carpal valgus (Auer and Stick 2006). Studies have previously investigated the changes in conformational characteristics associated with growth (Anderson and McIlwraith 2004; Santschi et al. 2006) and with increasing risk or incidence of disease. In man, knee malalignment is a risk factor for the progression of knee osteoarthritis (Cerejo et al. 2002), and increases in knee valgus of 1° have been associated with the risk of lateral compartment cartilage defects in subjects with radiographic changes of osteoarthritis (Janakiramanan et al. 2008). In a National Hunt Thoroughbred population, carpal valgus was associated with superficial digital flexor tendonitis and tarsal valgus with pelvic fractures (Weller et al. 2006b). However, in a different Thoroughbred population, it was demonstrated that a degree of carpal valgus was protective against carpal effusions and carpal fracture injuries (Anderson et al. 2004). The differing results are at least partly due to the different techniques used to measure conformation in these studies; it has been shown that measuring conformation reliably is a technical challenge (Weller et al. 2006a). *Corresponding author email: vunt@rvc.ac.uk [Paper received for publication 10.01.10; Accepted 28.05.10] © 2010 EVJ Ltd 444 EQUINE VETERINARY JOURNAL Equine vet. J. (2010) 42 (Suppl. 38) 444-450 doi: 10.1111/j.2042-3306.2010.00192.x