Splayleg: a spontaneous limb defect in rabbits. Genetics, gross anatomy, and microscopy.
A limb defect in rabbits known as splayleg arose in a closed breeding colony of Dutch rabbits. Since the causes of this postural deformity are not fully understood, an effort was made to clarify the mode of inheritance and the etiology of the disease. Pedigree analysis and genetic mating tests showed that, in contrast to published reports, the disease is not determined by a single recessive gene with complete expressivity. The underlying genetic system is composed of one recessive gene with reduced expressivity or of more genes with the probable involvement of environmental factors. Femurs of abnormal limbs displayed endotorsion of the shaft and anteversion of the neck as the main features. Endotorsion of the femur was associated with exotorsion of the tibia. Dislocation of the hip was not observed. There was no abnormnal laxity of the joint capsule or the ligamentaum teres. Only minor differences in the acetabulum between control and splayleg rabbits were observed. Enzyme histochemistry and electron microscopy of the gluteus medius, adductor longus- and magnus, sartorius, gracilis, and semitendinosus muscles and the femoral, peroneal, and tibial nerves did not reveal any qualitative differences between splayleg and control rabbits. A study on muscle fiber diameter showed that fibers of splayleg animals were in general smaller, particularly in the semitendinosus. Atypical mitochondria of muscles were observed in all rabbits, but more frequently in splayleg animals. A hypothesis is presented in which imbalanced development of the neural, muscular and skeletal systems of the limb is the primary defect. This results in torsion of the femur. It depends on the degree of femoral torsion and the capacity of the young rabbit for compensation by exorotation of the limb in the hip whether a persistent splayleg posture will develop.