Flexural deformity pathogenesis

Author: Vic Cox

Flexural deformities are the most common congenital defect in foals.  In a retrospective study of 608 cases of congenital defects in foals at the University of Kentucky, more than half were flexural deformities.  When involving the coffin (DIP) joint, the condition is often referred to as club foot.   In a nutshell, the most likely cause is failure to get enough “exercise” in utero.  Developing bone (and to a lesser extent adult bone) is very plastic.  The shape is determined by the pushes and pulls acting on bones (Wolf’s law).    These forces are akin to the hands of an artist molding clay.  Every mother knows from personal experience how often a fetus moves around and even “kicks” occasionally.

In the early 1950’s the embryologist Viktor Hamburger studied these movements using the convenient and inexpensive model system of developing chicks which can be observed thru a window in the egg shell.  After thousands of observations he concluded that there was no pattern to the movements; they were totally random.  This observation led him to hypothesize that these movements provided a wide variety of stress on bone.  In contrast, if the forces on a bone were lopsided, the shape of the bone would be skewed in relation to the forces acting on it.  Hamburger tested his hypothesis by applying the neuromuscular blocking agent curarie to chick “embryos”.  This curtailed movement and when the chicks hatched they had widespread musculoskeletal deformities including joint ankylosis (fusion).

Contracted tendons are common in long legged species such as horses and cattle but uncommon with short legged species such as pigs or dogs.  The incidence of tendon contracture is more common in oversized calves of double muscled breeds which presumably is due to insufficient intrauterine space to move.   The very long legs of foals may be a predisposing factor as well.

Other factors that may reduce fetal movement are neurological deficits or delayed neurological development.  Copper deficiency leads to swayback lambs.  In the early 70’s three calves were born on a Missouri dairy farm with torticollis (wry neck) and were linked to earlier spraying with a chlorinated hydrocarbon insecticide, a classic neurologic toxin.  Fetal positioning occurs late in gestation when a fetus turns around if needed so that the forelimbs and head are ready to exit the uterus first.  Failure to do so results in breech birth and may be due to the same factors involved in contracted tendons.

In short, intrauterine “exercise” is needed to stretch out developing tendons.   A principle to be learned here is to consider possibilities beyond the obvious.  That is, the obvious tendon defect may be due to a hidden neurologic deficit.

Recent work in Minnesota indicates that mare hypothyroidism may be a factor also.