Abstract
Introduction
The cortical bone trajectory (CBT) is a novel lumbar pedicle screw trajectory. There is a dearth of literature on CBT screw insertion techniques. The aim of this study was to conduct a detailed morphometric measurement of the lumbosacral spine for CBT pedicle screw starting point, trajectory, length and diameter, using the inferior facet of the cephalad level as a bony landmark.
Methods
The 3-dimensional reconstruction computed tomography (CT) scans of 86 adults who underwent examination of the lumbosacral spine were studied. The junction of the inferior and medial border of the smallest coronal section of the lumbar pedicle was traced in lumbar segments while the junction of the medial border of the sacral 1 (S1) pedicle and the inferior border of the L5 inferior facet was traced in S1 segments. The starting point was defined to be the projective point on the vertebral plate by this junction. The distances from the starting point to the inferior, lateral and medial border of the inferior facet of the cephalad level, and the inferior border of the transverse process were measured. The trajectory of pedicle screws for the lumbar and S1 segments was determined in the axial and sagittal planes. The angle formed between this trajectory and a line from the vertebral midline was defined as the transverse angle (TA). The angle formed between this trajectory and a line from superior margin of the vertebral body was defined as the cephalad angle 1 (CA1). Cephalad angle 2 (CA2) was defined as the angle formed between the same trajectory on the sagittal plane and a line from the posterior margin of the pars interarticularis. The maximum pedicle screw diameter, as well as the maximum pedicle screw length of the lumbosacral segments was measured on axial and sagittal CT scans, respectively.
Results
The distances from the inferior border of inferior facet to the starting point from L1 to S1 were 8.9, 6.3, 4.1, 2.9, 1.4 and 0 mm, respectively. The distances from the medial border of the inferior facet to the starting point from L1 to S1 were between 3 to 4mm. The percentages of the distance from the starting point to the medial border of the inferior facet from L1 to S1 were 80.5, 70.1, 61.6, 48.6, 40.8 and 29.3%, respectively. TA from L1 to S1 were 9.0, 9.6, 11.3, 13.5, 15.5, and 8.2 degrees, respectively. CA1/CA2 from L1 to S1 were 26.7/38.7, 26.0/38.7, 26.9/38.0, 24.4/37.2, 22.9/35.1 and 18.4/47.8 degrees, respectively. The maximum diameters of the trajectory from L1 to S1 were 4.8, 5.1, 6.1, 6.8, 7.8, and 6.1mm, respectively. Twenty-five mm was a safe border as the maximum length of the pedicle screw from L1 to S1.
Conclusions
The inferior facet of the cephalad level is an attractive bony landmark for establishing a starting point of CBT for minimally-invasive spine surgery.