Perforations and angulations of 324 cervical medial cortical pedicle screws: a possible guide to avoid lateral perforations with use of pedicle screws in lower cervical spine

Abstract

Background ContextMore than half of the perforations reported with usage of cervical pedicle screws (CPS) are lateral perforations, endangering the vertebral artery. The medial cortical pedicle screw (MCPS) technique with partial drilling of the medial cortex shifts the trajectory of pedicle screws medially, decreasing the lateral perforations. PurposeTo evaluate the decrease in lateral perforations of CPS with use of MCPS technique, in relation to medial angulation. Study Design/SettingRetrospective analysis and technical report of the MCPS technique and its safety. Patient SampleA total of 58 patients operated on between December 2011 and May 2015 with insertion of pedicle screws from C3 to C7 were included in the study. Outcome MeasuresAxial reconstructed computed tomography (CT) scan images of the inserted screws were evaluated for placement, perforations, and transverse plane angulations using the Surgimap software (Surgimap Spine 1.1.2.271 Intl. 2009 Nemaris LLC). The angulations of screws were analyzed by the type and level of placement through unpaired t test and analysis of variance test. MethodsA total of 58 patients operated on between December 2011 and May 2015 with insertion of pedicle screws from C3 to C7 were included in the study. There were 49 males and 9 females. Thirty-seven patients had cervical trauma, 17 had cervical spondylotic myelopathy, two had tumors, and two had ankylosing spondylitis. The average age was 49 years (range 18 to 80 years). The screws were inserted using the MCPS technique. All patients underwent postoperative CT scans with GE Optima CT540 16 slice CT scanner (GE Healthcare Chalfont St. Giles, Buckinghamshire, UK). Axial reconstructed images along the axis of the inserted screws were evaluated for placement and perforations. Further, all the screws were evaluated for transverse plane angulations using the Surgimap software. The angulations of screw were analyzed by the type and level of placement through unpaired t test and analysis of variance test. No funds were received by any of the authors for the purpose of the present study. ResultsA total of 324 screws were assessed with postoperative CT scans. Two hundred fifty-six were found to be placed within the pedicle and 68 (20.98%) screws were found to have perforations. Forty screws (12.34%) had grade I medial perforations, 14 screws (4.32%) had grade I lateral perforations, 10 screws (3.08%) had grade II medial perforations, and 4 screws (1.23%) had grade IIlateral perforations. The average angulation of the nonperforated screws (n=256) was 28.6° (43°–17°), that of laterally perforated screws was 20.33° (13°–24°), and that of the medially perforated screws was 34.94° (45°–20°). On statistical analysis with each series, the 99% CI range for the in-screw angles was 27.91° to 29.34°; for the laterally perforated screw series, it was 18.42° to 22.23°; and that for the medially perforated screw series was 32.97° to 36.9°. ConclusionsThe MCPS technique represents a shift in the concept of placement of CPS from the cancellous core to the medial cortex, avoiding screw deflection laterally by the thick proximal medial cortex. The present study shows that the lateral perforations can be consistently avoided, with a medial angulation of more than 27.91°, which is the primary concern with the use of pedicle screws in lower cervical spine. Further, the MCPS technique reduces the lateral perforations at a lesser insertion angle, which is technically desirable.