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

Figure 1

Diagrammatic illustration showing the retroacetabular surface (𝑆) and the retroacetabular tangent (𝑇) with the retroacetabular angle in between (𝜃). The latter is geometrically equivalent to angle 𝛿 which is a safe angle for placing screw 1 at the lateral margin of the posterior wall of the acetabulum. Screw 1 is parallel to the retroacetabular tangent and would not penetrate the joint. Screws 2 and 3 that are placed at distances 𝑎 and 𝑏, respectively, from the margin of the posterior wall (medialization distance) can be inserted safely at bigger angles than the retroacetabular angle (𝛿^′ and 𝛿^′′).