Posterior acetabular arc angle of the femoral head assesses instability of posterior fracture-dislocation of the hip

Abstract

Purpose Unstable posterior fracture-dislocation of the hip is determined by the wall defect or acetabular fracture index. The unstable hip is a result of inadequate posterior acetabular coverage of the femoral head from the posterior acetabular wall fracture. In order to measure total posterior acetabular coverage of the femoral head and avoid using the contralateral acetabulum as a calculation reference, the posterior acetabular arc angle of the femoral head was measured to assess stability of posterior fracture-dislocation of the hip. Methods Using coronal computed tomography (CT) scan of the normal contralateral acetabulum at the level of the widest acetabular diameter and thinnest medial wall of 60 acetabular fractures, posterior acetabular arc angles of the femoral head in intact, 20 % and 50 % defects of posterior acetabular walls were measured. The angles were measured from the acetabular centre to the thinnest medial wall and to the top, inner cortex of 80 % and 50 % posterior acetabular walls. Results Average intact, 80 % and 50 % posterior acetabular walls were 33.82±4.30, 26.88±3.33 and 16.91±2.15 mm which corresponded to 92.25 ± 11.34, 77.42 ± 10.04 and 50.63±6.58°of posterior acetabular arc angles of the femoral head. The intraclass correlation coefficient (ICC) of the measurements including correlation of conversion of posterior acetabular wall depths to posterior acetabular arc angles of the femoral head were more than 0.82 and 0.89. ConclusionsThe measurement technique of posterior acetabular arc angle of the femoral head has strong reliability. Therefore, stable or unstable posterior fracture-dislocation of the hip can be determined in terms of more than 77 degrees or less than 50 degrees of posterior acetabular arc angles of the femoral head instead of less than 20 % or more than 50 % posterior acetabular wall defect.

Posterior acetabular arc angle of unstable posterior hip fracture–dislocation

“…The study showed 54.48°coronal PAAA of the unstable posterior hip fracture-dislocation and 57.43°of 50 % of the posterior wall of the normal contralateral posterior acetabulum, with no statistically significant difference (Table 1). Moreover, coronal PAAA and 50 % of the posterior acetabular wall corresponded to the study by Harnroongroj et al [6]. Measuring vertical PAAA of the posterior acetabular wall fracture in the vertical plane using 3D CT reconstruction of the lateral pelvis may be another beneficial method for more detailed evaluation of the unstable posterior hip fracture-dislocation.…”

“…Moreover, patients 17 and 18 shown in Table 1 displayed 75°and 76°coronal PAAA, respectively, with a 22.42-and 24.60-mm coronal posterior wall, respectively. Both cases had clinical unstable posterior hip fracture-dislocations, which did not show on the images [4,6]. On the other hand, both cases had outstanding 119°and 112°of vertical PAAAs of the wall fracture compared with 120°and 118°of intact vertical PAAA of normal contralateral acetabulum (Table 2).…”

“…Then, intact posterior acetabular wall depth (AB) was measured from the top of the wall (A) perpendicularly to the PQ line (B). Fifty percent of the posterior wall (BD) was calculated and marked as D. The centre of the acetabulum (O) was identified with a circle template, using anterior, posterior and medial walls as references [6]. Then, the OX line was drawn from the acetabular centre to the thinnest part of the wall (X).…”

“…Moreover, neither measurement allows evaluation of whether the posterior acetabular wall involves the superior acetabular dome. A previous study as shown that the coronal posterior acetabular arc angle (PAAA) of the femoral head is another method by which to determine a unstable posterior hip dislocation [6]. Measurement included arc coverage of the posterior half of the medial wall and posterior acetabular wall.…”

Acetabular roof arc angles and anatomic biomechanical superior acetabular weight bearing area

“…Acetabular fracture involving superior weight bearing area shows significantly poor results 12. Moreover, stability of the hip depends on adequate acetabular coverage of the femoral head 34. Roof arc angle is a method to evaluate adequate acetabular coverage and stability of the femoral head, Matta and Merritt study based on clinical findings and suggested that the fracture crosses acetabular weight bearing dome when <45° medial, anterior and posterior roof arc angles 56.…”