Radiographic and clinical assessment of a freehand lateral mass screw fixation technique: Is it always safe in subaxial cervical spine?

Background context Many spine surgeons use the freehand technique for lateral mass (LM) screw fixation. However, issues about its safety still exist. Purpose To examine the safety of the freehand technique after LM screw insertion. Study design Retrospective case series. Patient sample A total of 26 patients (21 men and five women) who underwent LM screw fixation via the freehand technique were included. Outcome measures Postoperative computed tomography images and medical records were analyzed. Methods During surgery, the lateral trajectory of screws was set using the adjacent spinous process (SP) after the cranial trajectory was set by palpating the joint surface. Computed tomography analyses were performed for the measurement of screw angles, and axial images were evaluated to determine the SP length that would be long enough to not involve the transverse foramen. The medical records were evaluated for the investigation of surgery-related complications. Results Each axial and sagittal angle of the screws showed a significant difference. A total of 18 screws (13.5%) involved the transverse foramen, and the mean axial angle of the screws was significantly smaller than the group not involving the foramen (p=.0078). A total of eight screws (6.0%) violated the facet, and the mean sagittal angle was significantly smaller than the group not violating the facet. The average difference in angles between the screw and the actual joint surface was largest at C6 (p=.0472). The mean maximum length of the SP, long enough to not involve the transverse foramen was significantly short at C3 and C6 (p=.0015). The actual SP length in one patient was longer than the maximum SP length determined through analysis in the case of C6. If the SP of C7 was used in C6, the SP length would not be safe in six patients. Conclusions Excellent outcomes were observed with the use of the freehand technique for the insertion of LM screw at the subaxial C spine. However, this technique using the SP may pose a relative risk at C6.