Acetabular retroversion

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Acetabular retroversion denotes an abnormal posterior angulation of the superolateral acetabular rim, resulting in excessive coverage of the femoral head and metaphysis along the anterior border .

Epidemiology

Acetabular retroversion is a common abnormality affecting 5 to 20% of the general population. It occurs in 16 to 25% of dysplastic hips and affects 31 to 49% of patients with Legg-Calvé-Perthes disease, and 36 to 76% of those diagnosed with slipped femoral epiphysis .

Pathology

The condition is most often caused by an abnormal prominence of the anterosuperior part of the acetabular rim, rather than a shallow posterosuperior rim, entailing a partial overcoverage of the femoral head. Acetabular retroversion is a form of pincer morphology and predisposes to femoroacetabular impingement. It is also a risk factor for early-onset osteoarthritis of the hip, as the decreased area and poor orientation of the posterior acetabular wall result in the formation of stress zones with increased wear and inhomogeneous load distribution throughout the articular surface .

Major forms of acetabular retroversion:

developmental (see: acetabular dysplasia)
acquired (see: os acetabuli)

Radiographic features

Proper assessment of the acetabular configuration necessitates perfectly centered AP radiographs of the pelvis, which should be the first diagnostic imaging test to assess acetabular retroversion.

Signs of acetabular retroversion:

crossover sign (anterosuperior rim seemingly extends laterally to the posterosuperior rim)
ischial spine sign (ischial spine projects medially to the pelvic brim)
posterior wall sign (deficiency of the posterosuperior wall)

Quantitative measures of acetabular orientation and coverage (on radiographs):

lateral center-edge angle (Wiberg angle): formed by a vertical line and a line between the center of the femoral head and the lateral edge of the acetabulum. An angle <20° indicates dysplasia, whilst >30–40° signals acetabular overcoverage.
acetabular index (Tönnis angle): formed by a horizontal line and a line between the most medial and inferior point of the acetabular sclerotic zone to the lateral margin of the acetabular dome. An angle >13° indicates hip dysplasia, while values close to or less than 0° is caused by acetabular overcoverage.
alpha angle (Notzli angle): formed by the axis of the femoral neck and a line between the center of the femoral head to the point where it loses its sphericity. Ideally, it is measured on the Dunn view. An angle >55° indicates a cam morphology.
anterior center-edge angle (Lequesne angle): It can only be measured on the false-profile view of the hip, where it is formed by a vertical line and a line connecting the center of the femoral head to the most anterior point of the acetabular rim. An angle >20° indicates anterior overcoverage.

The acetabular version can be measured precisely on CT/MRI in the axial plane, where it is formed by a line perpendicular to the horizontal axis of the pelvis, and a line connecting the most anterior and posterior points of the acetabular margin. Normally it ranges between 12 to 20° .

Treatment and prognosis

The main goal of treatment is to prevent the degenerative sequelae by early correction of the biomechanical integrity of the joint. It is primarily surgical: periacetabular osteotomy or acetabular rim reconstruction (osteochondroplasty) are the most commonly performed surgical interventions .