Femoroacetabular impingement predisposes to traumatic posterior hip dislocation

Abstract

Background: Traumatic posterior hip dislocation in adults is generally understood to be the result of a high-energy trauma. Aside from reduced femoral antetorsion, morphologic risk factors for dislocation are unknown. We previously noticed that some hips with traumatic posterior dislocations had evidence of morphologic features of femoroacetabular impingement (FAI), therefore, we sought to evaluate that possibility more formally.

Questions/purposes: We asked whether hips with a traumatic posterior hip dislocation present with (1) a cam-type deformity and/or (2) a retroverted acetabulum.

Methods: We retrospectively compared the morphologic features of 53 consecutive hips (53 patients) after traumatic posterior hip dislocation with 85 normal hips (44 patients) based on AP pelvic and crosstable axial radiographs. We measured the axial and the lateral alpha angle for detection of a cam deformity and the crossover sign, ischial spine sign, posterior wall sign, retroversion index, and ratio of anterior to posterior acetabular coverage to describe the acetabular orientation.

Results: Hips with traumatic posterior traumatic dislocation were more likely to have cam deformities than were normal hips, in that the hips with dislocation had increased axial and lateral alpha angles. Hips with posterior dislocation also were more likely to be retroverted; dislocated hips had a higher prevalence of a positive crossover sign, ischial spine sign, and posterior wall sign, and they had a higher retroversion index and increased ratio of anterior to posterior acetabular coverage.

Conclusions: Hips with posterior traumatic dislocation typically present with morphologic features of anterior FAI, including a cam-type deformity and retroverted acetabulum. An explanation for these findings could be that the early interaction between the aspherical femoral head and the prominent acetabular rim acts as a fulcrum, perhaps making these hips more susceptible to traumatic dislocation.

Fig. 1

A flowchart illustrates the approach and surgical treatment of the hips with posterior traumatic dislocation. ORIF = open reduction and internal fixation.

Fig. 2A–D

Radiographic parameters for assessment of cam deformity and acetabular retroversion are shown. (A) The AP alpha angle (α1) is formed by the neck axis (n) and a line through the center of the head (C) and the point (D) where the femoral neck exceeds the circle outlying the femoral head (dashed line). (B) The axial alpha angle (α2) is formed in a similar way on the crosstable axial radiograph. (C) The crossover sign is positive when the anterior wall of the acetabulum (solid line) crosses the posterior wall (dashed line). The retroversion index is the ratio of distance (a) to the length of the acetabular cup opening (b). The ischial spine sign is positive when the ischial spine (IS) protrudes into the true pelvis. The posterior wall sign is positive when the posterior wall (dashed line) is projected medially to the femoral head center (C). (D) Anterior and posterior coverage is defined as the percentage of area of the femoral head covered by the anterior (gray area) or posterior (dashed area) wall, respectively. The ratio of anterior to posterior coverage is a measure for the acetabular orientation.

Fig. 3A–B

Mechanisms in posterior traumatic hip dislocation are illustrated. (A) With increasing flexion and internal rotation, the aspherical femoral head-neck junction (asterisk) comes into contact with the prominent acetabular rim (arrows), which then acts as a fulcrum (circle). (B) The increasing force of the long femoral lever arm pushes the femoral head posteriorly. Hips with a fulcrum need less impact and may dislocate without a fracture, even more so when the posterior border is less prominent.

Fig. 4A–C

(A) An AP radiograph shows the hips of a 19-year-old man who sustained a posterior hip dislocation after a car injury. Hips with posterior traumatic dislocation typically present with morphologic features of anterior FAI, including a cam-type deformity and retroverted acetabulum, as indicated by (B) a positive crossover sign (arrow) and increased lateral alpha angle (α1) and (C) increased axial alpha angle (α2).