Osseous Structures Contributing To Hip Stabilization

The adult hip is a multiaxial ball-and-socket synovial joint composed of two bony structures: the femur and the acetabulum. This bony architecture provides the hip with inherent stability. Three biomechanical and anatomic geometries of the femur and acetabulum are significant to joint stability and preservation of the labrum and articular cartilage: appropriate femoral head-neck offset, acetabular anteversion, and acetabular coverage of the femoral head. Proper function of the hip joint necessitates that the amount of offset from the femoral head to the femoral neck be enough to allow a full range of motion without impinging upon the acetabular labrum. A lack of offset from the femoral head to the femoral neck has been described as a cause for femoroacetabular impingement [1]. Flexion at the hip may cause the osseous femoral head-neck junction to come into contact with the acetabular labrum, resulting in impingement [1-3]. A large femoral head can compensate for a flat head-neck junction by simulating offset and adding stability to the joint [4].

Large variations exist in the rotational axis that characterizes the relationship between the acetabular and femoral osseous structures. The range of acetabular anteversion to femoral anteversion affects the rotation of the extremity and changes from the time of birth and through mature skeletal development. The transfer of dynamic and static load to the ligamentous and osseous structures is dependent on this relationship. Abnormal distribution of force or pressure in an incongruent joint precipitates chronic or acute injury. Normal adult acetabular positioning intersects the sagittal plane at 40° and the transverse plane at 60°, opening anteriorly and laterally [5]. The acetabulum is positioned approximately 45° caudally and 15° anteriorly [6,7]. Normal anteversion of the acetabulum is essential to maintaining a normal relationship with the femoral head and is critical in avoidance of impingement [8]. Normal range of acetabular anteversion as defined by Tonnis and Heinecke [9] is 15° to 20°, decreased anteversion is 10° to 14°, and increased anteversion is 21° to 25°. An increase in external rotation is commonly found with decreased acetabular anteversion.

In addition to recognizing acetabular anteversion, it is also important to appreciate the degree of femoral head coverage provided by the acetabulum. This can be measured radiographically as the central edge angle of Wiberg, which is defined as the angle between the horizontal line through the center of the femoral head and a line tangent to the superior and inferior acetabular rims. The normal center edge angle is 30° and a decrease in this angle (dysplasia) has been associated with rapid onset of osteoarthritis [10-13]. Center edge angles of less than 20° correlate with an abnormal orientation of the acetabulum, providing less than satisfactory head coverage and load transfer.

Anteversion of the femur is also important in maintaining proper static and dynamic mechanics in the hip. Anteversion of the femur diminishes with age. A healthy 1 year old has an average anteversion of 31°. This anteversion decreases to 24° at 8 years and to 15° by 15 years [14]. The McKibbin instability index is based on the sum of the angles of the femoral and acetabular anteversion. This ratio will affect range of motion. The sum of the angles of femoral and acetabular anteversion predicts instability for summed angles of 60° or more and predicts low instability for angles of less than 20°. The authors found that, of 290 hips tested, 38% had a low and 6% had a high index.

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