Angular limb deformities are usually diagnosed in foals in the carpal-, third metacarpal/metatarsal- and tarsal region. There are two major types of deformities:
valgus deformity, which is most frequently diagnosed in the carpus and to some extend in the tarsus. Definition: a lateral deviation of the limb distal to the location of the deformity.
varus deformity, which is most frequently diagnosed in the distal third metacarpal/metatarsal bones. Occasionally it is seen in the carpus. Definition: a medial deviation of the limb distal to the location of the deformity.
In most cases the deformity is recognized bilaterally, however, one side may be more affected than the other.
Foal with a valgus deformity in the right carpal region and a varus deformity in the left carpal region.
First the owner is questioned whether the foal was born prematurely or after a normal length of gestation period. A prematurely born foal is rarely a surgical candidate because the deformity is usually not caused by an aberrant growth.
Twin foals are always skeletally premature and should be treated as such, even if they were born after a prolonged gestation period.
The picture shows twin foals born after a gestation period of 341 days.
The nature of the angular limb deformity can visually not be assessed. Therefore some manipulations help in narrowing the possibilities in. The foal is manually restrained in standing or sternal position. In a foal with a valgus deformity (left figure in the carpal region) one hand is placed at medial aspect of the carpus to apply a force in lateral direction, while the other hand counteracts this force at the lateral aspect of the metacarpophalangeal region. If the deformity can be corrected while the forces are applied, the animal suffers from a laxity of the periarticular structures or incomplete ossification of the cuboidal carpal bones because a deformity in bone cannot be manually corrected.
Note: this manipulation is only helpful in very young foals.
The foal is then observed as it stands squarely and when it walks. It is important to stand in front of the foal perpendicular to the frontal plane of the carpus. Because young foals frequently rotate their entire front limb outward to brace the elbow to the chest wall, the observer has to position him/herself perpendicularly relative to the frontal plane of the carpal region. If the hoof points directly towards the observer, no immediate management action is needed because the entire limb is rotated outwardly. A mild deformity in the carpal region can be neglected at this point of time. If the hoofs point straight forward in the direction as the foal stands but the carpus is rotated outwardly, the problem should be treated within the 6 weeks of life because the growth plate of the distal third metacarpal/metatarsal bone closes at approximately four months of age.
For the observation of the patient while walking, the deformity can best be assessed when the foal walks towards the observer (for deformities in the front limbs) and away from observer (for evaluations in the hind limbs).
To verify the nature and degree of the deformity dorsopalmar/dorsoplantar radiographs are necessary.
It is advisable to take a long narrow cassette to include as much as possible of the bones proximal and distal to the location of the deformity. The direction of the x-ray beam should be perpendicular to the frontal plane through the location of the deformity.
The picture shows dorsopalmar radiographic view of the carpus and lateromedial radiographic view of the tarsus of the smaller twin foal shown above. The cuboidal carpal and tarsal bones have not started to ossify.
Note: radiographically, the deformity is difficult to accurately depict in the distal tibia because the third metatarsal bone, the tarsal bones and the tibia are not located in the same frontal plane.
Dorsopalmar radiographic views of the carpus of the smaller (left) and the larger twin foal (right) shown above. The cuboidal carpal bones show different stages of ossification. Ossification is clearly further advanced in the larger foal.
Severe bilateral carpal valgus deformity in a young foal showing disproportional growth at the distal radius including physeal ectasia (arrows).
Radiographs to depict deformities of the distal third metacarpal bone are best taken with the limb flexed at the carpus and the phalanges extended in the same plane as the third metacarpal bone.
Example of a foal suffering from a bilateral distal McIII varus deformity. The dorsopalmar radiographic view shows the location of the deformity in the epiphysis of the left McIII and a medial osseous proliferation mainly in the metaphyseal region of the bone.
Note: the growth plate of the distal McIII/MtIII closes at 3-4 months of age. It is therefore imperative that the diagnosis of angular limb deformities is made at a very early age.
Compensatory deformity in the proximal phalanx
Severe McIII/MtIII deformities may lead to compensatory deformities in the proximal phalanx. This can happen already at an early age. Example: this foal was admitted at two weeks of age with a severe distal MtIII varus deformity in the right limb.
Note: the metatarsophalangeal joint space, the level of the proximal physis of the proximal phalanx, the proximal interphalangeal joint space and the level of the proximal physis of the middle phalanx are all parallel to each other while the distal MtIII physis is oriented at a different angle.
The 2-week dorsoplantar radiograph shows a correction of the deviation angle from 14 degrees to 8 degrees. It can be noted that the angle between the distal MtIII physis and the metatarsophalangeal joint space is still the same while the angle between the level of the proximal physis of the proximal phalanx and the level of the proximal interphalangeal joint space has changed demonstrating a compensatory deformity in the proximal phalanx. Therefore, it is imperative that during the diagnosis of a distal McIII/MtIII deformity special attention is also given to the proximal phalanx as a whole. If diagnosed early, these deformities can be corrected.