OBJECTIVETo solve the problem of screw penetrating the joint surface easily by determining the angle of inclination and the mean longth screw plated on the posterior column.

METHODSTen specimens of adult male cadavers, aged 20 to 74 years old, averaged 54.5 years old, were collected. After removal of the bilateral femurs from the hip joints, and sawing through the sacral and pubic symphysis in the median sagittal plane, 20 semi pelvic specimens were used for this study when the osseous abnormalities were excluded. The specimens were air dried naturelly after the soft tissue attaching to the pelvis had been eliminated. The margin of superior acetabular and inferior acetabular were determined, and the serial cross-sections of the acetabular posterior column were made. The width of posterior column,the width of acetabulum,the width ratio of acetabulum to posterior column, the angle of inclination and the mean length of screw on all entry points were measured. Defined the level parallel to 1/2 section of superior acetabulum was cross-section B; 1/2 section of acetabulum was C; 1/2 section of inferior acettabulum was D. At the different levels, defined the entry point on the outer edge of posterior column of the acetabulum or the trailing edge of acetabulum was B0, C0 or D0; lateral 1/2 of posterior column of the acetabulum was B1, C1 or D1; 1/2 of posterior column of the acetabulum was B2, C2 or D2; medial 1/2 of posterior column of the acetabulum was B3, C3 or D3; the inner edge of posterior column of the acetabulum was B4,C4 or D4.

RESULTSOn cross-section B, the angle of inclination and the mean length of screw at B0 was 41 degrees and 44.0 mm; at B1 was 66 degrees and 42.2 mm; at B2 was 91 degrees and 59.5 mm; at B3 was 107 degrees and 64.0 mm; the maximum angle and the mean length at point B4 was 123 degrees and 65.5 mm; the minimum angle and the mean length at point B4 was 109 degrees and 59.0 mm. On cross-section C,the angle and the mean length at point CO was 39 degrees and 39.0 mm; at C1 was 57 degrees and 36.0 mm; at C2 was 74 degrees and 36.0 mm;at C3 was 90 degrees and 36.0 mm; at C4 was 106 degrees and 76.0 mm. On cross-section D,the angle and the mean length at DO was 42 degrees and 35.5 mm; at D1 was 61 degrees and 33.0 mm; at D2 was 81 degrees and 32.0 mm; at D3 was 100 degrees and 31.0 mm; at D4 was 120 degrees and 74.0 mm.

CONCLUSIONWhen using the fixation technique of acetabular posterior column plate, the angles of screw-posterior column are 40 degrees to 60 degrees, 60 degrees to 75 degrees, 75 degrees to 90 degrees and 90 degrees to the angle of parallel to the square area respectively on the region of outer 1/4,outer-middle 1/4,inner-middle 1/4 and inner 1/4 of the acetabulum region, and the screw length is 30 mm.