The morphologic variations of low and high hip dislocation

Abstract

Three different types of congenital hip disease in adults have been distinguished based upon the position of the femoral head relative to the acetabulum and the underlying pathoanatomy of the joint: (1) dysplasia; (2) low dislocation; and (3) high dislocation. To facilitate classification of borderline or ambiguous cases, we studied the morphologic variations of low and high dislocation as observed on the radiographs of 101 hips with low and 74 hips with high dislocation. In low dislocation, 54 hips (53.5%) had extended coverage of the true acetabulum (Type B1) and 47 hips (46.5%) had limited coverage (Type B2). Among the cases with high dislocation, a false acetabulum with an adjacent femoral head occurred in 46 hips (62.2%) (Type C1), and the femoral head was floating within the gluteal muscles in 28 hips (37.8%) (Type C2). The kappa value for interobserver agreement between two raters who made radiographic measurements was 0.963, and for intraobserver agreement between the two evaluations of the same observer it was 0.946 and 0.971, respectively. The two types of low and high dislocation were associated with high intra- and interobserver agreement. Whether these distinctions have clinical utility requires further validation.

Level of evidence: Level III, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Fig. 1A–C

The three main types of congenital hip disease in adults are (A) dysplasia, (B) low dislocation, and (C) high dislocation.

Fig. 2

The radiograph and a line drawing of a hip with Type B1 low dislocation are shown. Three points must be recognized on the radiograph: (A) the superior limit of the true acetabulum; (B) the inferior point of the teardrop; and (C) the most inferior point of the false acetabulum. The overlap (distance AC) between the true and the false acetabulum is more than 50% of the vertical diameter (distance AB) of the true acetabulum.

Fig. 3

The radiograph and a line drawing of a hip with Type B2 low dislocation are presented. The false acetabulum covers almost completely the true acetabulum. The overlap (distance AC) between the true and the false acetabulum is less than 50% of the vertical diameter (distance AB) of the true acetabulum.

Fig. 4A–F

The radiographic appearance and the corresponding 3-D computed tomography scan with and without the femoral head in the two subtypes of low dislocation are presented. (A–C) A case with the extended coverage Type B1 low dislocation (coverage more than 50%) is presented. Plain radiography of the hip (A) and 3-D computed tomography scan with (B) and without the femoral head (C) are shown. (D–F) A case with the limited coverage Type B2 low dislocation (coverage less than 50%) is shown. Plain radiographs of the hip (D) and 3-D computed tomography with (E) and without the femoral head (F) are presented. The arrowhead indicates the position of the true acetabulum and the asterisk the position of the false acetabulum.

Fig. 5A–B

The two subtypes of high dislocation are shown. (A) In Type C1 high dislocation, a false acetabulum is present in the iliac wing with which the femoral head articulates. The false acetabulum may be in contact with the true one or it may lie at a distant site. (B) In Type C2 high dislocation, there is no false acetabulum and the femoral head is free-floating within the gluteal muscles. The displacement of the femoral head may be small or significant. The arrowhead shows the position of the true acetabulum and the asterisk the position of the false acetabulum.

Fig. 6A–F

The radiographic appearance and three-dimensional computed tomographic scan with and without the femoral head in the two subtypes of high dislocation are presented. (A–C) A case with Type C1 high dislocation; the false acetabulum (denoted by an asterisk) is in contact with the true one (denoted by an arrowhead). (D–F) In Type C2 high dislocation, there is no false acetabulum and the femoral head is free-floating within the gluteal muscles.