The significance of rudimentary ulnas and fibulas during development of angular limb deformities in foals

Abstract

Carpal valgus deformities remain a common challenge in foals, yet their aetiology is often multifactorial and rarely attributable to a single anatomical anomaly. Recent case reports by Müller et al. (2025) suggest that rudimentary ulnas may play a causative role in these deformities, but such claims warrant closer scrutiny. Since the early 1980s, when Auer reported partial ossification of rudimentary ulnas in about 20% of foals with a valgus deformity, these structures have been documented radiographically without consistent evidence of causation. In most breeds, they represent incidental findings rather than pathogenic agents. Only in specific small breeds, such as Shetland Ponies and Miniature Horses, has the presence of complete ulnas or fibulas been clearly linked to severe developmental changes requiring early surgical correction. Outside such breed-specific forms of atavisms, the argument for causality is weak. A closer look at the three cases reported by Müller et al. (2025) illustrates this point. The first case reflected inadequate neonatal management by the initial veterinarian, with abnormal (most likely incomplete at the time of birth) ossification of the cuboidal carpal bones, which were more likely to be relevant than any ulnar remnant. The second case showed an acceptable surgical response, yet retained ossification gaps that do not completely support the claim of primary causality. The third case barely demonstrated a rudimentary ulna radiographically, making the benefit of surgery questionable. In all three cases, the presence of an ossified ulna could have been incidental rather than determinative. The real challenge remains proper diagnosis and timing of intervention. Radiographic assessment must be performed orthogonally to the carpal frontal plane, and clinicians should remain aware of methodological differences between bisecting line and parallel line measurements. In terms of treatment, transphyseal screw placement is effective at the distal metacarpus and metatarsus, but far less reliable at the distal radius, where prolonged growth increases the risks of overcorrection, physeal injury and screw failure. These technical and biological realities deserve more emphasis than speculative associations with rudimentary ulnas. In light of the available evidence, the conclusions by Müller et al. (2025) might overstate the role of rudimentary ulnas in carpal valgus deformities. They could be regarded, as in most foals, as evolutionary curiosities rather than pathological drivers. The key determinants of outcome remain accurate radiographic assessment, careful case selection and timely intervention. Misattributing causality potentially risks misguiding treatment strategies and obscuring the real, multifactorial nature of angular limb deformities in foals.