Patterns of deformity of the developing hip.

The hip joint presents a coaptive relationship between the head of the femur and the acetabulum. Developmental changes in either component affect the responsive growth of its companion. Normal pressure of the head of the femur in the acetabulum maintains a balance in the proliferation of triradiate and acetabular growth cartilage. Secondary deformities occur as a result of failure of head-acetabular relationships as in congenital dislocation of the hip and primary epiphyseodesis of the triradiate cartilage (TRC). Anatomically the proximal femur adapts to the growth of the primary epiphyses and associated biomechanical forces. The femoral head and greater trochanter enlarge by appositional growth, and their subjacent growth plates, nourished by epiphyseal vascularity that penetrates the bony end plate, contribute to length and width of the neck, respectively. During infancy and early childhood the longitudinal growth plate (LGP) of the neck is horizontal. The shape of the proximal femur depends primarily upon lateral contributions of the trochanteric growth plate (TGP) and the femoral neck isthmus (FNI) balanced against the longitudinal and slightly medial contributions of the LGP. The resultant longitudinal vector of axial and medial (LGP) and lateral (TGP and FNI) directions of growth maintains biomechanical forces about the hip while at the same time insuring the trajectory of growth in line with the axis of the femur. An understanding of the interrelationships of the growth zones of the proximal femur and their alterations as a result of genetic, environmental, and developmental influences permits early diagnosis of growth deformities and the opportunity to plan early corrective procedures.