Coxofemoral instability in equids caused by Ligamentum capitis ossis femoris disruption : The key role of clinical findings and ultrasonography

The ligament of the head of the femur (Ligamentum capitis ossis femoris, LCOF) is reinforced by the accessory ligament (Ligamentum accessorium, LA), a strong band detached from the symphyseal tendon, passing through the acetabular notch dorsal to the transverse acetabular ligament which spans this notch. The LA ends caudal to the LCOF in the notch on the femoral head. Interestingly, the LA is not found in other domestic animals than equids (Nickel et al., 1992). The horse in the case report by Burdette et al. (2025) showed a complete disruption of the ligamentous structures attached to the femoral head including the LA during post-mortem examination. As in previous reports (Sauer et al., 2025), the combined LCOF and LA have been collectively referred to as ‘LCOF’. This ligamentous complex, hereafter addressed as LCOF-LA, is relatively short, which complicates preservation of potentially intact fibres during post-mortem preparation of an equine coxofemoral joint (CFJ) with incomplete disruption of these ligaments. In the case report by Burdette et al. (2025), the ‘sidewinder gait’ was observed approximately 3 weeks after onset of an acute lameness after rolling, raising the general question of mechanisms leading to LCOF-LA disruption. Purely traumatic ruptures causing complete coxofemoral luxations have been associated with falls, catching of the limb and anaesthetic recovery (Brenner & Whitcomb, 2009). Exaggerated attempts to rise after rolling may be associated with forced adduction or abduction of the limb combined with inward or outward rotation generating substantial tensile and shear forces on the LCOF-LA, amplified by the long equine limb lever arm. Compared with humans, the short femoral neck in adult horses usually resists these forces rendering intra-articular ligaments and the acetabular rim more susceptible to injury (Brenner & Whitcomb, 2009). Nevertheless, complete (type I) or partial (type II) ruptures of the LCOF have been categorised in humans after major and minor trauma, respectively (Gray & Villar, 1997). Depending on the magnitude and direction of the traumatic forces and other factors (a) incomplete dislocation (subluxation) of the femur from the acetabulum like in the current case or (b) complete dislocation (luxation) may occur which should be considered as distinct entities (Brenner & Whitcomb, 2009). Acute or chronic disruption of the LCOF may cause CFJ ‘microinstability’ in humans (Cerezal et al., 2012) and does not necessarily lead to complete luxation, although it has to be considered as a potential precursor stage: The CFJ is a ‘ball and socket’ joint, but immediate exarticulation is prevented even after disruption of the LCOF-LA as the acetabulum is deepened by an incomplete ring of fibrocartilage, the acetabular lip (Labrum acetabulare) which is attached to its bony margin and completed medially by the transverse acetabular ligament. It increases the height of the acetabular rim so that it extends over the femoral head beyond its maximal diameter (Nickel et al., 1992). In humans, partial and complete disruption of the Ligamentum teres, the functional equivalent of the LCOF-LA, has been reported to cause hip pain and impaired joint function, although the role of the ligament as a CFJ stabiliser is discussed controversially (Bardakos & Villar, 2009). Besides the mechanical role of the ligament, the ligament contains neurovascular elements in humans indicating its involvement in the perception of pain, potentially caused by impingement of torn ligamentous fibres between articular surfaces. Disrupted neural structures may also contribute to altered proprioception (Perumal et al., 2019), although this clinical finding has not been observed in the case reported by Burdette et al. (2025) and historically, neurovascular structures have not been described in the equine LCOF-LA (Oncken, 1922). Persistent instability can lead to joint effusion, labral tears, osteochondral damage, and eventually, osteoarthritis (Chahla et al., 2016). This pathway of osteoarthritis has also been assumed in the current case with cartilage erosions on the femoral head surrounding the LCOF-LA insertion which is consistent with previous reports of full-thickness cartilage loss in chronic cases including the opposing acetabular joint surface (Sauer et al., 2025). Primary degeneration of the LCOF-LA in the context of long-standing overuse and CFJ osteoarthritis or other orthopaedic disease represents another important mechanism commonly found in humans (type III after Gray & Villar, 1997) potentially leading to subsequent secondary traumatic disruption of the ligament (Bardakos & Villar, 2009). In the current case, histopathology indicated degenerative changes consistent with secondary rupture potentially triggered by low-impact trauma in the context of rolling. Notably, bilateral ligament disruptions have been identified incidentally at necropsy in horses with unilateral clinical signs of LCOF-LA pathology and CFJ osteoarthritis (Aleman et al., 2020; Sauer et al., 2025). This highlights the potential relevance of a primary degenerative pathway in equids, making comparative imaging as well as post mortem examination of both CFJs particularly advisable. Dysplasia of the CFJ may predispose to LCOF pathology but compared to other species remains rare in horses with isolated reports, for example, in a Friesian and a Mangalarga Marchador foal (Hermans et al., 2016; Kneipp et al., 2025). Finally, avulsion fractures of the femoral attachment of the LCOF-LA represent another uncommon cause of CFJ instability (O'Brien et al., 2012). It is noteworthy that avulsion included only the LCOF but not the LA in one case (Meister et al., 1991). Beyond the characteristic posture caused by shifting the centre of mass to the unaffected side while standing and the indicative gait pattern, further clinical findings provide valuable information for the diagnosis of LCOF disruption. Elevation of the contralateral, unaffected hindlimb may be impossible or achievable only after several attempts with discomfort when the horse is required to bear weight on the affected limb (Aleman et al., 2020). Interestingly, manipulation of the affected limb while being elevated often leads to inconclusive results as in the current case report (Burdette et al., 2025). The CFJ has its greatest range of motion during flexion and extension, and the main function of the LCOF-LA is to limit abduction of this joint. In summary, flexion of the CFJ with external rotation of the limb and extension combined with internal rotation are assumed to impose the greatest tensile loads on these ligaments. Accordingly, provocation tests should not only include passive flexion and extension but also abduction and adduction. If feasible in an adult horse, the limb should be abducted in flexion and with external rotation, followed by adduction in extension with concurrent internal rotation to provoke an abnormal mobility or elicit a painful response. Depending on the duration of the disorder, the degree of gluteal muscle atrophy is variable and typically more pronounced unilateral on the affected side in chronic cases (Sauer et al., 2025). In the case report by Burdette et al. (2025), mild gluteal and epaxial muscle atrophy was symmetrical and likely attributed to the onset of severe lameness only 3 weeks prior to presentation. After clinical examination, transcutaneous ultrasonography using a convex probe at a frequency of 2–3.5 MHz represents a valuable next diagnostic step in patients with CFJ disorders. Muscle atrophy facilitates ultrasonography by reducing penetration depth requirements and improving probe angulation as it allows a more perpendicular probe coupling angle relative to the dorsal and craniodorsal contours of the joint. However, image quality may be markedly reduced in heavy breeds due to increased skin thickness (Sauer et al., 2025). Parts of the dorsolateral and dorsolaterocranial contours of the neck of the femur, of the femoral head and the acetabulum can be visualised by transcutaneous ultrasonography. This comprises the joint space as well as the fibrous cartilage of the Labrum acetabulare merging into the joint capsule. In the current case, mild irregularity of the right acetabulum was observed during basic ultrasonographic examination showing that even minor findings have the potential to be indicative of clinically relevant pathology. However, instability of the coxofemoral joint caused by disruption of the LCOF-LA may not lead to an obvious ultrasonographic widening of the joint space during static weight bearing examination, so that only joint effusion, abnormalities of the joint capsule or signs of secondary osteoarthritis may be detected. In contrast, dynamic ultrasonography as described by Brenner and Whitcomb (2009) with the weight of the horse being shifted away from the affected limb and back in a latero-lateral direction during the examination can reveal joint instability. Abduction and adduction of the elevated limb can also be actively performed by a second person lifting the limb during ultrasonography. Consequently, this procedure should be incorporated into diagnostic protocols as it is the key diagnostic modality to detect CFJ instability in the standing horse. Conclusive evidence of CFJ instability other than luxation cannot be expected from a standing latero-lateral oblique radiographic view (Barrett et al., 2006). In contrast, standing modified ventro-dorsal radiography with the affected limb being abducted leads to an anatomical representation of the CFJ resembling that in dorsal recumbency and to images of acceptable to excellent quality depending on the size of the equid and the degree of muscle atrophy (Sauer et al., 2025). Apart from secondary findings of OA in long-standing cases, an abnormal width of the joint space or position of the femur may theoretically be found. However, reliable interpretation requires exact comparative views of the contralateral joint, which are often difficult to obtain in standing horses with restricted weightbearing on the affected limb. Arthrocentesis and intra-articular anaesthesia of the CFJ are ideally performed under ultrasound guidance, which allows for a more reliable approach to the joint than the blind technique (David et al., 2007). The inevitable instability caused by LCOF-LA disruption leads to mechanical alterations of the gait, so that only limited improvement of lameness can be expected (Sauer et al., 2025). Advanced imaging modalities of potential high value to diagnose LCOF-LA pathology and CFJ osteoarthritis include computed tomography (CT) (Ogden et al., 2025) and contrast arthrography. While the latter has proven to reliably detect intra-synovial ligament pathology of the equine stifle (Bergman et al., 2007), its application to LCOF-LA has not been reported to date (Ogden et al., 2025). Currently, the maximum diameter of CT scanner gantries in clinical settings is 85–90 cm so that their use is limited relative to the size of the pelvis. Arthroscopy of the CFJ is a routine procedure in humans to visualise and potentially debride the LCOF (Chahla et al., 2016). In normal horses, this ligament and the notch of the acetabulum are visible after distraction of the limb. However, longer instruments are needed in adult horses weighing more than 300 kg. In a series of nine horses, tearing of the LCOF was identified in two cases, with improvement reported in one case with partial disruption (Nixon, 1994). The potential relevance of arthroscopic LCOF-LA debridement in equids should be further investigated. The case report by Burdette et al. (2025) provides additional evidence that primary or secondary disruption of the LCOF-LA, including osteoarthritis, should be considered as a highly relevant source of significant CFJ pain causing a non-neurological ‘sidewinder gait’. In addition to careful clinical examination, bilateral in-depth ultrasonography including dynamic evaluation offers substantial diagnostic value in cases of LCOF-LA disruption. Contrast-enhanced CT arthrography and diagnostic arthroscopy should be considered in smaller equids or in cases that remain inconclusive. Many thanks to Dr. Fay Sauer, PhD for her constructive comments. Open Access funding enabled and organized by Projekt DEAL. There are no funders to report for this submission. No conflicts of interest have been declared. Not applicable.