Translaminar Screw Fixation in the Upper Thoracic Spine: Computed Tomography-Based Quantitative Laminar Analysis and Feasibility Study of Translaminar Virtual Screw Placement

Abstract

Translaminar screws (TLS) offer an alternative to pedicle screw fixation in the upper thoracic spine. Although some studies have described the anatomy of the laminae at the upper thoracic spine, computed tomography (CT) imaging is the modality of choice for presurgical planning. The characteristics of upper thoracic lamina in healthy Han adults have not been ascertained for the safe placement of TLSs. To determine the measurements (the diameter, maximal screw length, and optimal screw trajectory) required for safe TLSs placement in the T1–3 by CT scan evaluation, forty patients (20 men, 20 women), age (48.2 ± 13.9) years (range 19–78 years), were involved in the study from December 2008 to June 2012. Patients received a standardized axial bone-window CT imaging at T1–3, and had no evidence of spinal trauma, bone defect, or deformity. For each lamina, the height, thickness (outer cortical and inner cancellous), maximal screw length, and the crossing angle of the lamina were measured using Syngo 3D software and statistically analyzed. There were no significant differences between the left and right sides for each of the measurements (P > 0.05), but significant differences were determined between males and females for some measurements. The thickness of the laminar outer cortical diameter for the axial reconstruction was higher than that for the sagittal reconstruction (P < 0.01). The T1 lamina was estimated to accommodate, on average, a 4.2-mm longer screw than the T2 lamina, and a 5.7-mm longer screw than the T3 lamina. Most laminae could accept a 4.0-mm screw with a 1.0-mm clearance. Thus, it is more reasonable and accurate to select screw sizes according to the measurement of laminar thickness in the parasagittal reconstruction of the CT scan. T₁ crossing laminar screws were successfully placed, without impingement to the spinal canal. Based on the CT evaluation, there are no anatomical limitations to the placement of TLSs at T1–3 in Chinese patients. Gender differences were noted in the superior thoracic vertebrae for dimensions of the laminae, which must be considered when placing TLSs at these levels. The TLSs add to the surgeon’s armamentarium for instrumented fixation of the upper thoracic vertebrae.