The aim of this study was to determine the incidence of pedicle screw misplacement during MITLIF. Cannulated pedicle screws are inserted percutaneously under fluoroscopic guidance during MITLIF. Theoretically, cannulated screws are more safely and accurately inserted than conventional screws [4
]. However, although wide variations exist, partly due to the lack of a standardized evaluation method, pedicle wall penetration after percutaneous pedicle screw insertion is currently reported to be around 10% [4
]. In the present series of 488 percutaneous pedicle screws, frank penetration was found for 54 (11.1%) screws, and 8 (1.6%) screws penetrated by more than 2 mm. Two screws (0.4%) showing medial penetration required repositioning. These results for the percutaneous insertion of pedicle screws during MITLIF concur with the published literature in terms of misplacement and neurological complication rates [4
We also sought to identify the preoperative and intraoperative factors that affect pedicle screw positioning. A meta-analysis on pedicle screw placement accuracy concluded that thoracic screws were more frequently inserted inaccurately than lumbar screws [5
]. Belmont et al. [18
] found that coronal plane deformity is a significant risk factor of pedicle screw penetrations. Facet osteoarthritis may distort the contour of the superior articular process, making this structure difficult to use as an anatomical landmark [3
]. The insertion of S1 screws is frequently interrupted by the iliac crest [4
Based on our experiences of difficult cases, we hypothesized that several demographic and surgical factors could be an important predictor of an inaccurate placement of pedicle screw. Statistical analysis revealed that obesity, higher pedicle convergent angle, EBL more than 400 cc, and greater multifidus cross-sectional area affect the accurate placement of pedicle screw.
Although it is unclear if obesity adversely affects the rate of intraoperative complication associated with surgical procedures, obese patients present more technical challenges during spinal surgery [19
]. Positioning of obese patients without abdominal compression was very difficult. Intraoperative fluoroscopic or radiographic identification of anatomical landmarks are frequently blurred. Moreover, surgical procedures were performed in deeper place with poor visualization and illumination. Obesity also brings more bleeding [21
Minimally invasive techniques have a virtue of treating obese patients [22
]. Rosen et al. [22
] reported that obesity per se is not a risk factor of complications during MITLIF. However, accurate insertion of pedicle screw is affected by obesity in our series. In addition, the quantity of the multifidus muscle was also found to be a significant risk factor of pedicle screw misplacement. These results infer that fluoroscopic images are frequently blurred in bulky patient and procedures in deep space are more difficult to perform. Therefore, we conclude that surgeons should pay more attention to inserting pedicle screws in bulky patients.
EBL more than 400 cc was also an associated factor for pedicle screw misplacement in the presenting study. Excessive EBL implies hemodynamic problem, longer operation time, and blurred surgical fields. While a direct correlation of EBL to technical complication has not been defined, excessive bleeding during spinal surgery may influence the surgeon’s surgical performance [24
Another relevant factor for screw misplacement in our series was pedicle convergent angle. Except for L2 and L3 pedicles in which the sample size was too small, more discrepancy between the pedicle convergent angle and the screw trajectory angle was found to the lower levels, i.e., more convergent angle should be given to the lower levels. We assumed that it was due to the surgeon’s tendency. However, the impingement effect of iliac crest could explain the high misplacement rate of S1 screw insertion.
We thought about other demographic and anatomical variables. However, these variables were not found to be associated with the pedicle screw misplacement in the present study.
This study had several limitations. The retrospective design involving possible data associated with the use of medical records, miscoding, and a lack of clinical information may cause uncertainty in the results. Another limitation was that the diameter and length of inserted screws were mostly 6.5 and 45 mm, (98.5 and 82.4%, respectively). Last, this study does not include other radiologic outcome including fusion rates or screw failure.