Patrick Loftin, DVM
Colin Mitchell, BVM&S, MS, DACVS
An angular limb deformity (ALD) is a side-to-side deviation of the limb. Where the limb of a horse should be straight when viewed from the front, horses with ALD appear to have limbs that are angled away from the presumed straight line of the limb. The deviation is described by the joint closest to which the deviation occurs: the fetlock (ankle), carpus (knee) or tarsus (hock), followed by the direction which the lower limb is deviated: varus (pointing underneath the body) or valgus (pointing away from the body of the horse). This condition occurs in foals and can be congenital, such that the foal is born with the condition, or acquired after birth as the foal grows. At birth the small cubodial bones located in the carpus and tarsus should be fully developed. If bony formation, or ossification, is not complete, a normal activity can cause crushing of these bones, leading to abnormal angulation. ALD can also be seen in foals whose ligaments, which support the joints of the limbs, are overly lax at birth. Futhermore, as a foal grows, factors such as unbalanced nutrition, excessive exercise or trauma may induce an ALD. Uneven growth of one side of a bone compared to the other will then cause growth in an abnormal direction towards the side of slower growth.
Diagnosis of an ALD relies mainly on a thorough physical exam and radiographs of the affected limb. A small amount of angular deviation can be normal, and as the foal matures these will self correct. Upon manipulation of the limb, if a deformity is easily corrected it is likely to be due to incomplete ossification of the cuboidal bones of the carpus or tarsus, or laxity of the ligaments of the limb. Radiographs are necessary to definitively diagnose incomplete ossification of these bones, although this is usually only seen in premature foals. Radiographs also help localize the origin of the abnormal angulation. Uncorrectable limb deformities have either developed through disproportionate growth from improper nutrition or trauma, or permanent damage to the cuboidal bones.
Methods of treatment revolve around returning the limb to a straight line. Conservative treatment can be utilized through stall rest, controlled exercise, limb splints, casts and hoof trimming. These therapies are used in mild cases or those that are diagnosed early. As each bone of the equine limb has a specific periods of time in which rapid growth occurs, correction of an ALD must occur during this maximal growth period. As such, severe angular limb deformities or those diagnosed later in the foal's life may require more aggressive, surgical treatment. One such surgery is periosteal elevation or stripping. Periosteum is the outer lining of the long bones, and transecting this layer allows the bone to increase its growth rate. This procedure allows the shorter side of the bone to catch up to the longer, convex side. Transphyseal bridging is a second surgical procedure used on the convex aspect of the limb in which screws, wires or plates are placed across the growth plate in order to slow the growth of that region. By slowing the rate of growth on the longer side of the bone, the shorter aspect is again allowed to catch up. The implants used for bridging the physis must be removed to prevent overcorrection of the ALD. Both of these techniques are used to bring the limb back into proper alignment. In the most severe cases, both procedures may be performed in the same limb.
Prognosis for correcting ALDs is generally good, as long as diagnosis and treatment are completed during the maximal growth period for the bones in question. By 3 months of age, the metacarpus, or cannon bone, has already progressed through this maximum growth period, making it imperative to treat fetlock abnormalities early. It is less urgent, time-wise, to treat deformities around the tarsus or carpus, whose surrounding bones remain in their rapid growth phase for 4 to 6 months respectively. Growth of the long bones of the limbs will continue slowly after these initial periods, and correction may still be attempted. However, complete correction may not occur. In conclusion, early diagnosis is important in all cases. This allows the deformity to be fully characterized and proper treatment, whether conservative or surgical, to be instituted in a timely manner.