Antegrade versus retrograde screw fixation of anterior column acetabular fractures: a biomechanical in vitro study

To compare the mechanical strength of antegrade versus retrograde lag screw fixation of anterior column acetabular fractures.

Standardised anterior column fractures were created in synthetic pelvis models and stabilised by either antegrade (ANTE, n = 4) or retrograde (RETRO, n = 4) anterior column screw fixation. In a validated setup, a cyclic loading protocol was applied with increasing axial force (750 cycles, 250–750 N) followed by load to failure. Construct survival, energy absorbed, construct stiffness, and load to failure were assessed. Descriptive and opto-metric methods were used to describe the mode of failure.

All constructs failed with loads below 1500 N. With regard to energy absorbed until failure, the ANTE group resisted to 3.763 × 10⁵ N*cycles (range 3.760 × 10⁵–3.763 × 10⁵) and the RETRO group to 3.762 × 10⁵ N*cycles (range 3.761 × 10⁵–3.765 × 10⁵; p = 1.0). The load to failure was 1254 N (range 977–1299) in the ANTE group and 1234 N (range 1087–1456) in the RETRO group (p = 1.0). Construct stiffness with 250 N was not different between the two groups (ANTE 192 N/mm vs. RETRO 215 N/mm, p = 0.486). In all samples, the mode of failure was a transiliac fracture with screw breakout due to rotation of the pubic fragment around the axis of the screw with a range of rotational motion [ROM] during cyclic testing of 0.96° in one ANTE sample and 0.82° in one RETRO sample for 750 N, and ROM at failure of 2.53° in one ANTE sample and 2.23° in one RETRO sample. There was some plastic deformation of the screws in all cases but no breakage.

In this in vitro mechanical study, antegrade screw fixation of an anterior column acetabular fracture was not different in construct survival, load to failure, stiffness, and mode of failure when compared to retrograde screw fixation.