The Retroacetabular Angle Determines the Safe Angle for Screw Placement in Posterior Acetabular Fracture Fixation

Introduction. A method for the determination of safe angles for screws placed in the posterior acetabular wall based on preoperative computed tomography (CT) is described. It defines a retroacetabular angle and determines its variation in the population. Methods. The retroacetabular angle is the angle between the retroacetabular surface and the tangent to the posterior acetabular articular surface. Screws placed through the marginal posterior wall at an angle equal to the retroacetabular angle are extraarticular. Medial screws can be placed at larger angles whose difference from the retroacetabular angle is defined as the allowance angles. CT scans of all patients with acetabular fractures treated in our institute between September 2002 to July 2007 were used to measure the retroacetabular angle and tangent. Results. Two hundred thirty one patients were included. The average (range) age was 42 (15–74) years. The average (range) retroacetabular angle was 39 (30–47) degrees. The average (range) retroacetabular tangent was 36 (30–45)？mm. Conclusions. Placing the screws at an average (range) angle of 39 (33–47) degrees of anterior inclination with the retroacetabular surface makes them extraarticular. Angles for medial screws are larger. Safe angles can be calculated preoperatively with a computer program. 1. Introduction Fractures involving the posterior acetabular wall and column are the most common acetabular fractures [1–3]. The midportion of the posterior acetabulum above the ischial spine was considered by Matta et al. to be the danger zone of the acetabulum [4], a fact which was emphasized by more recent anatomical studies [5]. Some authors advised avoiding insertion of screws in the danger zone to avoid the negative consequence of intraarticular screw penetration [6]. The latter is a recognized complication of acetabular fracture surgery [7, 8]. Ebraheim et al. verified the boundaries of the danger zone by a cadaver study, and they recommended that a screw placed at an angle of 30–49 degrees of medial angulation to the perpendicular of the posterior wall should be extraarticular [9]. They measured this angle at one centimeter medial to the posterior margin of the acetabulum. Consequently, the safe angle for fixing marginal posterior wall fractures cannot be predicted depending on their study. Bosse advised that placing a screw in the coronal plane perpendicular to the long axis of the patient’s body would be extraarticular [10]. However, placing all posterior wall screws in the plane defined by Bosse [10] might compromise the stability of fracture