The fixation of posterior wall acetabular fractures often utilizes interfragmentary screws with varying length. Intricate pelvic anatomy and overhanging greater trochanter make obtaining proper screw trajectory difficult. A large measurement may represent aberrant trajectory and breach of the articular surface. This study aims to identify a preferred maximum screw length that avoids intra-articular penetration.

We hypothesized that a screw measured 40 millimeters or longer has a high likelihood of being intra-articular.

A retrospective review included CT scans of 151 consecutive patients collected at a level-1 trauma center was analyzed by two observers. On axial imaging, a straight line was measured at the largest extraarticular portion of the posterior wall simulating ideal screw placement. Another line was measured tangent to the articular surface simulating longest possible extraarticular screw. Measurements were taken at 2-millimeter increments.

The intra-class correlation coefficient between both observers was excellent (0.75–1.00) for most recorded values. The maximum mean length for straight line measured (m=32.18mm, SD=3.74) which was smaller than the mean length for tangent line (m=38.44, SD=4.29). Exploratory multivariate logistic regression analyses demonstrated increased height, age, and acetabular version were associated with larger measurements greater than 40mm (p<0.05).

This study demonstrates that most acetabular posterior walls cannot accommodate a 40 millimeter lag screw. If the measured drill hole is greater than this length, then careful reconsideration of the screw trajectory is warranted to ensure the screw is not intra-articular. Older and taller patients may be able to accommodate longer screws.

III.