Study Design. An experimental laboratory-based biomechanical study. Objective. To investigate the correlation between cage size and subsidence and to quantify the resistance to subsidence that a larger cage can provide. Summary of Background Data. The assumption that a bigger interbody cage confers less subsidence has not been proven. There was no previous study that has shown the superiority of lateral cages to bullet cages in terms of subsidence and none that has quantified the correlation between cage size and subsidence. Methods. A cage was compressed between two standardized polyurethane foam blocks at a constant speed. Four sizes of bullet cages used for transforaminal lumbar interbody fusion (TLIF) and six sizes of lateral cages used for lateral lumbar interbody fusion (LLIF) were tested. The force required for a 5 mm subsidence, axial area of cages, and stiffness were analyzed. Results. A larger cage required a significantly higher force for a 5 mm subsidence. Longer bullet cages required marginal force increments of only 6.2% to 14.6% compared to the smallest bullet cage. Lateral cages, however, required substantially higher increments of force, ranging from 136.4% to 235.7%. The average force of lateral cages was three times that of bullet cages (6426.5 vs. 2115.9 N), and the average stiffness of the LLIF constructs was 3.6 times that of the TLIF constructs (635.5 vs. 2284.2 N/mm). There was a strong correlation between the axial area of cages and the force for a 5 mm subsidence. Every 1 mm2 increment of axial area corresponded to approximately 8 N increment of force. Conclusion. Cage size correlated strongly with the force required for a 5 mm subsidence. The LLIF constructs required higher force and were stiffer than the TLIF constructs. Among bullet cages, longer cages only required marginal increments of force. Lateral cages, however, required substantially higher force. Level of Evidence: N/A

Study Design. Cross-sectional radioanatomical study. Objective. The aim of this study was to analyze the prevalence, size, and location of the oblique corridor (OC), and the morphology of the psoas muscle at the L4-L5 disc level. Summary of Background Data. Lateral lumbar interbody fusion via the OC has the advantage of avoiding injury to the psoas muscle and lumbar plexus. However, the varying anatomy of major vascular structures and the iliopsoas may preclude a safe oblique access to the L4-L5 level. Methods. Five hundred axial magnetic resonance images of the L4-L5 disc level were shortlisted. OCs were categorized into four grades: Grade 0 = no corridor, Grade 1 = small corridor (≤1 cm), Grade 2 = moderate corridor (1–2 cm) and Grade 3 = large corridor (>2 cm). OC location was labeled as antero-oblique, oblique, or oblique-lateral. Psoas morphology was categorized based on a modified Moro's classification, where the anterior section was further subdivided into types AI-AIV. Oblique approach was considered nonviable either when there was no corridor due to vascular obstruction (Grade 0) or when the psoas was high-rising (Types AII-AIV). Results. 10.5% of the selected 449 patients had no measurable OC (grade 0) at the L4-L5 level. There were 35% and 37.2% patients with a grade 1and 2 OC, respectively. The location of the OC was anterior oblique, oblique, and oblique lateral in 3.7%, 89.6%, and 6.7%, respectively. According to the modified Moro's classification, 19.4% had a high-rising psoas. Predominantly, psoas was either in line with the disc (Type I; 30.7%) or low-rising (Type AI; 47.4%). Conclusion. Twenty-five percent of the patients did not have an accessible OC either due to obstruction by vascular structures or due to a high-rising psoas. Hence, proper evaluation of the relevant anatomy preoperatively is recommended for early adopters of this technique, as varying anatomy precludes universal suitability of oblique lateral interbody fusion for the L4-L5 level. Level of Evidence: 3

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Study Design: Retrospective cohort study. Objective: We intend to evaluate the accuracy and safety of cervical pedicle screw (CPS) insertion under O-arm-based 3-dimensional (3D) navigation guidance. Methods: This is a retrospective study of patients who underwent CPS insertion under intraoperative O-arm-based 3D navigation during the years 2009 to 2018. The radiological accuracy of CPS placement was evaluated using their intraoperative scans. Results: A total of 297 CPSs were inserted under navigation. According to Gertzbein classification, 229 screws (77.1%) were placed without any pedicle breach (grade 0). Of the screws that did breach the pedicle, 51 screws (17.2%) had a minor breach of less than 2 mm (grade 1), 13 screws (4.4%) had a breach of between 2 and 4 mm (grade 2), and 4 screws (1.3%) had a complete breach of 4 mm or more (grade 3). Six screws were revised intraoperatively. There was no incidence of neurovascular injury in this series of patients. 59 of the 68 breaches (86.8%) were found to perforate laterally, and the remaining 9 (13.2%) medially. It was noted that the C5 cervical level had the highest breach rate of 33.3%. Conclusions: O-arm-based 3D navigation can improve the accuracy and safety of CPS insertion. The overall breach rate in this study was 22.9%. Despite these breaches, there was no incidence of neurovascular injury or need for revision surgery for screw malposition.

Study Design: Retrospective case series.Objectives: This study aims to determine the prevalence and risk factors for orthostatic hypotension (OH) in patients undergoing cervical spine surgery.Methods: Data was collected from records of 190 consecutive patients who underwent cervical spine procedures at our center over 24 months. Statistical comparison was made between patients who developed postoperative OH and those who did not by analyzing characteristics such as age, gender, premorbid medical comorbidities, functional status, mechanism of spinal cord injury, preoperative neurological function, surgical approach, estimated blood loss, and length of stay.Results: Twenty-two of 190 patients (11.6%) developed OH postoperatively. No significant differences in age, gender, medical comorbidities, or premorbid functional status were observed. Based on univariate comparisons, traumatic mechanism of injury (P ¼ .002), poor ASIA (American Spinal Injury Association) grades (A, B, or C) (P < .001), and posterior surgical approach (P ¼ .045) were found to significantly influence occurrence of OH. Among the significant variables, after adjusting for mechanism of injury and surgical approach, only ASIA grade was found to be an independent predictor. Having an ASIA grade of A, B, or C increased the likelihood of developing OH by approximately 5.978 times (P ¼ .003). Conclusion:Our study highlights that OH is not an uncommon manifestation following cervical spine surgery. Patients with poorer ASIA grades A, B, or C were more likely to have OH when compared with those with ASIA grades D or E (43.5% vs 7.2%). Hence, we suggest that postural blood pressure should be routinely monitored in this group of patients so that early intervention can be initiated.