Translaminar facet screw placement: an anatomic study

Anatomic measurements for screw path length, caudal and lateral angles, and superior and inferior lamina border thicknesses from L1 to L5 were measured for translaminar facet screw fixation in 30 dried lumbar spines. All measured values were fairly constant from L1 to L5. The mean values of the length of the screw path and lateral angle gradually increased from L1-2 to L5-S1 levels (41 to 54 mm, 39 degrees to 60 degrees, respectively). The caudal angle of screw placement relative to transverse plane gradually decreased from L1-2 to L5-S1 levels (60 degrees to 38 degrees). The superior border of the lamina was relatively thinner, with the mean thickness ranging from a minimum of 1.3 mm at L1 to a maximum of 2.0 mm at L5. The thickness of the inferior border of the lamina increased from L1 to L5 (from 6.7 mm to 7.8 mm). This study confirmed that a translaminar facet screw, 40 mm to 50 mm long at L1 to L5 levels, 60 mm long at L5-S1 level, and 4.5 mm in diameter, should be inserted through the lumbar facet joint at an angle of 40 degrees to 50 degrees laterally at L1 to L5 levels, 50 degrees to 55 degrees laterally at L5-S1 level, 45 degrees to 60 degrees caudally at L1 to L5 levels, and 35 degrees to 40 degrees at L5-S1 level caudally.