Quantitative anatomic basis of antegrade lag screw placement in posterior column of acetabulum

Lag screw fixation has been recommended for treatment of acetabular and pelvic fracture for several years. The aim of the present study was to determine the projection of the axis of the posterior column on the inner table of the iliac wing in the supra-acetabular region.

Thirty adult dried bony hemipelves specimens and other five intact adult dried pelvic specimens were included in this study. The projection point of the axis of the posterior column of the acetabulum was determined on the inner table of the iliac wing of the hemipelves specimens. The perpendicular distance from the optimal entry point to the linea terminalis of pelvis was measured and recorded as the lateral distance. The same measurement along the linea terminalis from the optimal entry point to the junction between the anterior border of iliosacral articulation and the linea terminalis of pelvis was made and recorded as the posterior distance. The depth of the anchor path and the corresponding average retroversion angulation and extraversion angulation were also measured. According to the results acquired from this study, a series of 6.5 mm lag screws were inserted into the posterior column of each side of the other five intact specimens, respectively, to evaluate the position of the screws. The data were expressed as mean ± SD and analyzed by using the descriptive methods with SPSS 10.0.

The average length of lag screw was 104.8 ± 4.2 mm. The average lateral distance was 16.8 ± 2.1 mm. The average posterior distance was 23.5 ± 3.4 mm. The corresponding average retroversion angulation and extraversion angulation were 57°36′ ± 4°28′ and 119°18′ ± 2°32′, respectively. The insertion of the single 6.5 mm lag screw of adequate length was possible in the posterior column along its anchor path and no accidental extraosseous or intraarticular screw placement had occurred.

The present study describes a safe anchor path of antegrade lag screw fixation in the posterior column. Insertion of the lag screws of adequate length is possible in the posterior column along its functional axis.