Developmental Coxa Vara
– a rare disorder (1/20,000) in which the proximal femur develops in a varus fashion due to an abnormality of enchondral ossification in the medial aspect of the femoral neck. The cause is unknown. There is no racial or gender predilection, and the disorder may be unilateral or bilateral.
The pathology is that of abnormal fibrous or fibrocartilaginous tissue in the medial part of the femoral neck. It is biomechanically inferior, and under the normal stresses of body weight this characteristically triangular area collapses and the neck “bends” into varus. The upper femoral physis then moves from a horizontal alignment to a more oblique/vertical attitude, subjecting it to more shear stress.
The clinical presentation occurs as a painless lurching limp, and can manifest at age 2-3 years. Note: it is painless. There may be associated shortening of the femur, the degree of which depends on the severity of the varus deformity. Like DDH, the thigh creases may be uneven if it is unilateral. Trendelburg test is positive, and lumbar lordosis may be increased, especially if bilateral. Abduction is limited, and internal rotation is limited by increased anteversion of the neck. The limited internal rotation distinguishes this from DDH.
Coxa vara can also occur with various bone dysplasia – spondyloepiphyseal dysplasia, Morquio’s disease. It can also be the result of AVN from Perthes or sepsis, or from malunion of a femoral neck fracture or from a slipped capital femoral epiphysis.
Radiographically, the neck shaft angle is decreased, which can only be assessed with the hips in neutral or slight internal rotation. The medial femoral neck defect is radiolucent and appears as an inverted Y with the physis, which is tilted in a more vertical direction.
The Hilgenreiner-epiphyseal angle is used to quantify the deformity. This is the angle between Hilgenreiner’s line (through the triradiate cartilage) and the a line along the METAPHYSEAL side of the defect in the femoral neck. The HE angle is used to predict which deformities will get worse:
60o: coxa vara tends to get worse
45-60o: difficult to predict – need close follow-up.
If the HE angle is greater than 60, treatment is an intertrochanteric abduction/derotation osteotomy, with the aim to bring the physis into a more horizontal position and reduce the abnormal shear stresses across it. It would appear that the surgery should be done as soon as the child begins to walk.
Note: differs from congenital coxa vara, which does not have the abnormal fibrous/fibrocartilaginous tissue, and is often associated with PFFD.