RECOGNITION AND TREATMENT OF ANGULAR AND FLEXURAL LIMB DEFORMITIES Susan M. Stover, DVM, PhD, DiplACVS Angular and flexural limb deformities are among a spectrum of disorders that can result from the abnormal distribution of biomechanical forces on rapidly growing bones and soft connective tissues of young animals. An angular limb deformity is a deviation of a limb in the frontal plane (seen when viewed from the front or back of the animal). In most domestic animals a straight line dropped perpendicular to the ground from the shoulder joint should bisect the long bones of the limb. A varus deformity is a deviation of the distal portion of the limb from this line toward the midline of the animal, and a valgus deformity is a deviation of the distal portion of the limb away from the midline of the animal. The most common condition in foals is a valgus deformity of the carpus (or distal limb), and that condition is also referred to as “medial deviation of the carpus”. Abnormal conformation(s) of the limb can arise from several etiologies/sources. Although the clinical appearance is similar, it is important to determine the anatomic source of the abnormal conformation, because the treatment and prognosis differ for some of these conditions. In general, the angular limb deformity can be classified as being due to one or more of the following conditions: 1) laxity of periarticular ligaments, 2) physeal/metaphyseal growth imbalance, 3) epiphyseal growth imbalance, 4) delayed endochondral ossification/hypoplasia of a cuboidal bone/osteochondrosis of a cuboidal bone, or 5) overt trauma. Most of these conditions arise from an abnormal distribution of forces from the medial to lateral aspects of the limb on the growing regions of a bone. Therefore, it is prudent to review the regions of growth of long and cuboidal bones (of the carpus in this example). Growth of the distal ½ of the radius, the carpus, and the proximal ½ of the metacarpus is centered in the regions of the distal physis of the radius, the distal epiphysis of the radius, and the cuboidal bones of the carpus. Although the basilar layers of the articular cartilage of the metacarpus may contribute some growth, a physis of the proximal aspect of the metacarpus is not present after birth – and therefore, this region does not usually contribute to the development of angular limb deformities. The diagnosis of these conditions is largely based on the clinical appearance of the angular limb deformity, the time of onset of the deformity, and the radiographic appearance (geometric configuration and morphologic appearance of the bony structures). Careful assessment of the clinical appearance of some angular limb deformities can localize the anatomic location serving as the major source of a deformity. For example, a wedge shaped metaphyseal region with a deformity centered at the same location would indicate uneven growth of the physis. The detection of joint effusion with an angular limb deformity centered at that joint is good evidence of cuboidal bone hypoplasia (providing the effusion is in a joint that can be detected clinically – for example, middle carpal joint effusion can be detected clinically, but distal intertarsal joint effusion is not detectable clinically because of the tight periarticular ligam ents surrounding this joint). Based on the time of onset, angular limb deformities can also be classified as either 1) present at birth (congenital) or 2) acquired within the first weeks or months of life. Angular limb deformities present at birth are usually due to periarticular ligament laxity or hypoplastic (incomplete ossification) cuboidal bones. Periarticular ligament laxity is due to weak collateral ligaments (periarticular supporting structures, e.g., tendons crossing the joints) simply because the animal has never used them outside of the uterus, or from malpositioning of the affected joint in the uterus. Hypoplastic cuboidal bones (in this example, carpal bones) are present in animals usually born prematurely. The growth of cuboidal bones occurs by interstitial growth of the cartilaginous template, which subsequently undergoes endochondral ossification. This process is occurring rapidly around the time of birth. The cuboidal bones of animals that are born prematurely are still largely cartilaginous (versus having a greater bony component), and may not be strong enough to sustain the forces associated with weight bearing. Under these conditions, growth and/or subsequent ossification of the cartilaginous template is delayed. The cuboidal bones either do not develop normal morphologic shapes or are actually damaged by excessive compressive forces. The remaining types of deformities are acquired through asymmetrical growth across the medial to lateral aspects of the limb. Animals may be born with normal limb conformation, but over a period of weeks or months gradually acquire a deformity. These deformities are centered at regions of rapid growth, the distal physis/metaphysis of the radius and the distal epiphysis of the radius. In this instance, excessive compression is associated with a retardation of growth in comparison to the side of the limb under relative tension. Uneven growth across the region of rapid growth in the bone results in an angular limb deformity. For example, excessive compression on the lateral aspect of the distal physis of the radius results in a retardation of growth laterally in relation to the rate of growth on the medial aspect of the physis. Over a period of time this results in a valgus deformity because the medial side of the physis grows faster than the lateral side of the physis. Although we are using the carpus of the foal as an example, deformities can center on any portion of the limb that is undergoing rapid growth (e.g., the distal physis of the metacarpus, the tarsus). Overt trauma, either through fracture or ligamentous disruption, can occur at any time during the pre- or post-natal life of the animal. Trauma is usually associated with the immediate development of a deformity. Two types of things are evaluated from radiographs of a limb affected with a deformity. Firstly, the geometric configuration is assessed. From the craniocaudal (dorsopalmar) radiograph, lines are drawn that bisect the longitudinal axis of the long bones. Normally, the line bisecting the radius and the line bisecting the third metacarpal bone should be parallel and superimposed. If an angular deformity is present, the lines will not be parallel and they will intersect one another. The severity of the deformity can be evaluated by measuring the angle of the intersection. This information is useful for assessing the initial severity of the condition, but is most useful for comparison with the angle at later times to assess progress following treatment of the condition. The location of the intersection is an indicator of the anatomic region that is the source of the abnormal growth. For example, the intersection will occur in the region of the physis/metaphysis with abnormalities of growth in this region; and in the region of the carpus with either periarticular ligamentous laxity or cuboidal bone hypoplasia. Abnormal growth may be occurring in more than one anatomic location, in which case, the intersection may occur in an intermediate location. Secondly, radiographs are useful for assessing the morphology of the bones of the limb. A wedge-shaped appearance to the metaphysis would be associated with uneven growth across the medial to lateral aspects of the metaphysis, etc. Most importantly, the morphology of the ossification center of the cuboidal bones can be assessed. Abnormal shape and uneven radiodensity of the ossification center is indicative of hypoplasia of the affected cuboidal bone(s). Conservative treatment of these conditions is based on correcting the uneven distribution of forces on the rapidly growing regions of affected bones. The success of treatment is dependent on continued growth of these regions. Since growth occurs most rapidly in the neonate and decreases exponentially until, in the case of the physis, closure occurs; treatment is most successful when instituted as early as possible in the life of the animal. For example, closure of the distal physis of the radius occurs a t approximately 2-3 years of age; however, rapid growth of this region occurs only during the first 8 months of life. Therefore, treatment should be instituted prior to 4 months of age in order to have continued growth and be most effective. It is important to correct uneven loading of the limbs because once an angular limb deformity is present, the abnormal conformation accentuates the uneven distribution of forces and may promote worsening of the condition. Alleviation of the uneven distribution of forces can usually be achieved by corrective trimming of the feet and/or the use of orthotics. Decreasing the magnitude of the forces can usually be achieved by controlling the exercise of the animal in a manner that allows enough exercise to stimulate strengthening of periarticular supporting structures, but limits excessive exercise that would result in fatigue of periarticular supporting structures or direct damage to developing bones. The early detection of angular limb deformities and management by foot care and controlled exercise is associated with a good prognosis for return to normal limb conformation and soundness. In the young foal when the deformity is not improving with corrective hoof trimming and exercise management, splints and/or casts can be used to obtain more immediate straightening of the limb and ensure physiologic loading of the limbs. Splints and casts require daily nursing care and frequent re-applications to prevent the development of skin sores in these fast growing foals; and to minimize weakening of tendons and ligaments that are also immobilized during treatment of the angular deformity. The one condition associated with a guarded to poor prognosis for eventual soundness is hypoplasia (incomplete ossification) of the cuboidal bones. With this disorder, immediate correction of the angulation is warranted to relieve excessive compression on the affected cuboidal bones. Early relief of the uneven distribution of forces can result in return of normal interstitial growth of the cartilaginous template of the cuboidal bone and subsequent endochondral ossification to form a normally sized and shaped bone. This is often done by application of a cast that maintains the limb in a normal conformation. If the cuboidal bone does not develop to it‟s normal conformation, secondary degenerative joint disease will usually result. In older animals that have not responded to conservative therapy, surgical techniques can be used to temporarily retard or enhance growth of selective regions of the physis. For example, metallic implants can be placed to bridge the medial or lateral aspect of the physis to retard growth of that aspect of the physis while they are in place (transphyseal bridging). Alternately, the growth of an aspect of the physis can be enhanced by transecting the periosteum in the region of the metaphysis on that side of the metaphysis (periosteal stripping). In both of these procedures, the prognosis for correction of the deformity is related to the amount of growth potential remaining in the affected region of the limb. The surgical procedure itself does not correct the deformity. The deformity is corrected through continued growth in an uneven manner dictated by the surgical procedure. In severe cases, wedge osteotomy can be considered. A flexural limb deformity is a deviation of a limb in the sagittal plane (seen when viewed from the side of the animal) usually associated with flexor tendons that prohibit normal range of joint motion or allow excessive joint motion. These deformities are usually manifested in hyperextension or hyperflexion of the metacarpophalangeal or distal interphalangeal joint(s). Congenital flexural limb deformities can be related to weak digital flexor tendons, recognized by lowering of the fetlock toward the ground and rising of the toe off of the ground during stance. The prognosis is often good with conservative therapy consisting of controlled exercise and corrective shoeing. Attaching a shoe with an extended heel to affected hooves decreases the magnitude of the joint moments (forces that flex and extend the joints) by decreasing the length of the moment arm of the forces associated with weight bearing around the metacarpophalangeal, phalangeal, and digital joints. Under these conditions, the weak tendons have sufficient strength to sustain normal joint conformation. With controlled exercise, the tendons gradually strengthen, the heel on the shoes can be shortened until the shoe can ultimately be removed. Contractural flexural limb deformities are recognized by limited hyperextension (dorsiflexion) of the metacarpophalangeal and/or distal interphalangeal joints(s). These deformities are usually associated with „contraction of the superficial and/or deep digital flexor tendons‟ and can be congenital or acquired. In either situation, it is important to determine if the deformity is primary or secondary to another cause. Any apparent nutritional imbalances should be resolved. A deformity secondary, for example to a contralateral limb lameness, is unlikely to improve until resolution of the primary problem. Conservative treatment for contractural deformities includes corrective trimming and shoeing, and splinting and casting – with or without analgesic medication. The goal is to stretch out the „contracted tendon‟. For example, a shoe with an extended toe will increase the extensor moment on the distal interphalangeal joint and enhance joint extension during weight bearing. Analgesic medication may be needed to alleviate pain associated with stretching of the deep digital flexor tendon during weight bearing. If splints are necessary to help position the joint, it may be advantageous to place a splint on only the dorsal surface of the limb so that the limb is not prevented from further extension and tendon stretching. Recently, intravenous oxytetracycline (3 g in 250-500 ml of physiologic saline solution) has been advocated for congenital contractural deformities. In theory, oxytetracycline chelates calcium making it less available for muscle, resulting in muscle relaxation. Surgical treatment is indicated for cases unresponsive to conservative treatment. The goal of surgical treatment is to obtain normal joint extension by effectively lengthening the deep and/or superficial digital flexor muscle-tendon unit. Each muscle-tendon unit has an accessory ligament (distal and proximal check ligaments, respectively) that protects the muscle from overstretching while preventing fetlock overextension as part of the stay apparatus of the limb. These functions are a consequence of the stiff components in the bone-check ligament-tendon-bone structure that imparts considerable passive resistance to fetlock (and digital) hyperextension. Transection of the check ligament disrupts this series of structures and their passive contribution to limitation of extension. As the joints extend, a greater proportion of tendon load is transferred directly to the associated muscle belly, which is less stiff than tendon and stretches more. Flexural deformities of the distal interphalangeal joint are associated with the distal check ligament and deep digital flexor tendon because this is the only tendon unit that crosses the flexor surface of the distal interphalangeal joint. Surgical transection of the distal check ligament is indicated in cases that do not respond to conservative treatment. Conservative treatment (corrective hoof trimming/shoeing analgesics) is always indicated with surgical treatment. In cases unresponsive to check ligament transection, the deep digital flexor tendon itself may be transected, but the prognosis is markedly worse in these cases. Flexural deformities of the metacarpophalangeal joint may be associated with the proximal check ligament and superficial digital flexor tendon, distal check ligament and deep digital flexor tendon, and/or suspensory apparatus because all of these structures cross the flexor surface of the metacarpophalangeal joint. Surgical transection of the proximal check ligament is first indicated in cases that do not respond to conservative treatment. Conservative treatment (corrective hoof trimming/shoeing analgesics) is always indicated with surgical treatment. In cases unresponsive to proximal check ligament transection, the proximal check ligament, superficial digital flexor tendon, deep digital flexor tendon, and/or the suspensory ligament may be transected, but the prognosis is markedly worse in these cases. In long standing cases, the metacarpophalangeal joint capsule may also be contracted.