The use of pedicle screws in the lumbar region is a well-established technique that has been shown to provide immediate stability and rigid fixation that facilitates correction of a deformity in both sagittal and coronal planes [
1-
5]. However, to ensure optimal placement for achieving the requisite stability, the screw must be meticulously placed and insertion obtained in good quality bone. Various techniques have been developed to ensure optimal placement of pedicle screws in the pedicles. In the straight-ahead technique as described by Roy-Camille [
6], screw insertion begins at the intersection of a horizontal line bisecting the transverse process and a longitudinal line bisecting the facet joint. The screw is then inserted straight ahead, parallel to the vertebral endplates. The Magerl [
7] technique uses the same horizontal landmark for screw insertion as the Roy-Camille technique, but for the longitudinal line, the landmark is just lateral to the angle of the superior facet. The screw is then angled laterally to medially while kept parallel to the vertebral endplates. The up-and-in method of screw placement uses the same longitudinal reference line as described by Magerl et al., but with a horizontal reference line that crosses the lower third of the transverse process. The screws are then placed in a caudad-to-cephalad direction toward, but not into, the vertebral endplate. The screws are also angled slightly medially as in the Magerl technique [
8]. Beyond the conventional techniques using intraoperative landmarks, recent advancements in a developed navigation technique have begun to help surgeons insert pedicle screws more accurately [
9-
14]. However, despite experience with conventional techniques and advancements in the field of intraoperative navigation, intraoperative lateral pedicular wall perforation is not uncommon. This has been attributed to the morphology of the pedicle and also the fact that the lateral wall is the weakest among all walls making up the pedicle [
15]. Castro [
16] reported that 14 (11%) of 131 screws penetrated the lateral wall of the pedicle in 30 patients after lumbar spinal fusion, as assessed using computed tomography. Silbermann [
9] compared the accuracy rates of pedicle screw placement between the free-hand and O-arm-based navigation techniques and found that 34 (22.4%) of 152 screws showed medial encroachment and 14 (9.2%) screws showed lateral encroachment with free-hand placement in comparison to 2 (1.1%) and 7 (3.7%) of 187 screws, respectively, with O-arm-based navigation. Thus, lateral perforation seemed more likely to occur with O-arm-based navigation than with free-hand placement. Galalis [
17] also published a systematic review comparing free-hand placement, fluoroscopy guidance, and navigation techniques. Twenty-six prospective clinical studies were included in the analysis, and these studies included 1,105 patients in whom 6,617 screws were inserted. When evaluating the position of perforation, in the studies using the free-hand technique, a range from 12 to 67% was found for lateral perforation. When fluoroscopy was used, the pedicles were perforated laterally with an incidence of 16 to 79%. In patients in whom computed tomography (CT) navigation was used, the proportion of screws that perforated the lateral wall was significantly increased, ranging from 29 to 80%, compared to the percentage of screws that perforated the medial wall, which ranged from 8 to 29%. All of these studies suggest that meticulous attention should be paid to the lateral placement of pedicle screws, especially when using navigation and assistance techniques, as with these the incidence of lateral perforation is significantly higher. In addition to biomechanical changes, lateral wall perforation may also result in vascular injuries, especially when the aorta and other retroperitoneal structures are located close to the screw trajectory and the vertebral bodies [
18,
19]. Anatomic studies have shown that, even in severe scoliosis, the aorta persistently follows and adheres to the abnormal curves of the spine [
20]. Many studies on the accuracy of the screw trajectory and its actual placement in the pedicle have revealed that a large percentage of screws penetrate the pedicle lateral cortex, placing major vascular structures at risk for injury [
21]. Many case series and reports have reported similar findings, and Minor [
22] reported a case of a patient who underwent surgical correction of a spinal deformity and had to receive endovascular treatment following iatrogenic injury. Postoperative CT scans of the case revealed a laterally misplaced pedicle screw, which was impinging on the descending aortic wall. The patient was brought to the operating room, where a thoracic stent graft was deployed under fluoroscopic guidance as the malpositioned screw was manually retracted. Although vascular injuries associated with spinal surgery have delayed presentation, occurring only after chronic irritation of the pulsating aortic wall against a metallic implant, immediate intervention is still indicated to prevent potentially serious, future complications.
It is thought that lateral pedicle wall perforation negatively impacts the purchase of the screw in the pedicle and may consequently reduce its pull-out strength. However, there is still a lack of quantitative evidence demonstrating this negative correlation. Thus, the present biomechanical study was undertaken to explore and quantify the impact of perforation of the lateral wall during pedicle screw insertion on its pull out strength.