Is the postoperative pedicle screw position after dorsal instrumentation with or without intraoperative cone beam CT imaging worse in patients with obesity than in normal-weight patients?

The purpose of this retrospective study was to evaluate whether the use of intraoperative imaging by cone beam CT (CBCT) leads to an improved postoperative pedicle screw position compared to conventional fluoroscopy independent of body weight in patients with obesity and normal-weight patients. The hypotheses were that (1) the use of intraoperative CBCT imaging in dorsal instrumentation in patients with obesity and normal-weight patients may reduce the rate of misplaced screws on postoperative CT and that (2) patients with obesity show worse postoperative pedicle screw positions.

The results of this retrospective study suggest that (1) intraoperative CBCT imaging in comparison with conventional fluoroscopy leads to an improved postoperative screw position among both patients with obesity and normal-weight patients. However, (2) the patients with obesity showed significantly worse pedicle screw positions postoperatively after dorsal instrumentation with intraoperative CBCT imaging than normal-weight patients.

Moreover, in the analysis of subgroups, the patients with obesity, especially those with BMI > 35 kg/m2, who received pedicle screws with intraoperative conventional fluoroscopy had postoperatively higher rates of pedicle screw perforations compared to those patients who received intraoperative CBCT imaging. However, the differences between the subgroups of the patients with obesity were not statistically significant. Furthermore, the results suggest that the normal-weight patients who received pedicle screws with intraoperative CBCT imaging showed significantly better pedicle screw positions postoperatively than those patients who received only conventional fluoroscopy intraoperatively. However, even despite the intraoperative CBCT scans, perforation rates of 16.4% and 8.7% were still observed in the SG1 and SG2 groups. Simultaneously, it was also shown that the perforations were mainly lateral perforations (Table 1). This is also consistent with our in-house intraoperative procedure, in which lateral screw deviations are not targeted but tolerated with sufficient clinical stability, as well as medial screw deviations without neurological limitations.

The placement of pedicle screws during dorsal instrumentation is challenging, especially because intraoperative visualization of the pedicle screw position using conventional fluoroscopy is considered to be only of limited reliability [10]. Corresponding malpositions of pedicle screws in all directions (medial, lateral, cranial and caudal) have been described, but medial and lateral deviations can be clinically relevant. While medial pedicle screw deviations may be associated with injuries of important nervous structures of the spinal cord [3], lateral pedicle screw dislocations are considered to be negative for the biomechanical stability of the dorsal instrumentation. Due to these described factors, a pedicle screw position centrally in the pedicle is desired by the surgeon but is not easy to achieve.

Malposition rates (1.7–31% and 15–72%, respectively) in postoperative CT with the free-hand technique without intraoperative imaging or only with the use of conventional fluoroscopy can be significant [10, 18]. Accordingly, there is an increasing desire for improved intraoperative imaging. Therefore, CT has been established as the standard method in the assessment of pedicle screw position [5,6,7,8], but the intraoperative availability is low. Therefore, other techniques, such as CBCT, have been developed to close this intraoperative gap. Additionally, good (type A + B according to Gertzbein) pedicle screw positions have been observed in 88.8–94.1% patients after dorsal instrumentation with intraoperative imaging by a CBCT scan [15, 19]. Moreover, it has been shown that intraoperative imaging with CBCT is not inferior to intraoperative CT [20, 21]. However, a disadvantage of a CBCT scan compared to conventional fluoroscopy is the higher radiation exposure [22], which is certainly not negligible.

Studies that specifically examine the intraoperative value of CBCT for dorsal instrumentation in patients with obesity (BMI > 30 kg/m2) are not available in the current literature. Independent of intraoperative imaging, there are few studies examining the pedicle screw position during dorsal instrumentation in patients with obesity. Winder et al. observed a nonsignificantly higher rate of pedicle screw perforation in patients with obesity after dorsal instrumentation and intraoperative conventional fluoroscopy [14]. Additionally, Park et al. investigated the pedicle screw position after dorsal instrumentation with conventional fluoroscopy in patients with obesity with a BMI > 30 kg/m2 who received 89 pedicle screws; the authors did not observe a significant difference in postoperative pedicle screw positions compared to normal-weight patients [13].

However, a significantly higher inaccuracy in pedicle screw positioning could be observed in patients with obesity using intraoperative CBCT imaging in contrast to normal-weight patients. Simultaneously, this finding could not be monitored in pedicle screw positioning in normal-weight and patients and with obesity when using conventional fluoroscopy. Nevertheless, the results suggest that regardless of body weight, the accuracy of the pedicle screw positions was reduced when using only conventional fluoroscopy in comparison with an additional usage of intraoperative CBCT imaging. However, since precise pedicle screw placement goes along with an improved stability of the dorsal instrumentation, an optimal pedicle screw positioning appears relevant in obese patients, who particularly seem to benefit from intraoperative CBCT imaging.

But, the results of this study must be interpreted in light of several limitations. (1) This is a retrospective study with a small sample size. Accordingly, it is difficult to draw a definitive conclusion regarding the influence of BMI on postoperative pedicle screw position. Furthermore, due to the small subgroup sizes among the patients with obesity, no significant difference can be observed between these groups. (2) The distribution of pedicle screws showed that more pedicle screws were inserted in the cervical spine among the normal-weight patients than among patients with obesity. Considering that the screw–pedicle ratio in the cervical spine is less favorable than in the thoracic and lumbar spine, this may have led to an increased screw perforation postoperatively in the cohort of normal-weight patients. (3) Despite intraoperative CBCT imaging, postoperative screw perforations were also observed. These were also registered intraoperatively. However, as these mainly showed lateral deviation and did not lead to neurological or vascular complaints, they were tolerated. (4) Only the radiologic results of intraoperative 3D imaging were assessed but not the functional outcomes. All screws that would have caused neurologic or vascular complications would have been revised. Since no clinically relevant complications were observed, no screw was revised based on postoperative CT imaging. (5) Two different types of CBCT were used in this study: Cios Spin (Siemens, Erlangen, Germany) and ARCADIS Orbic 3D (Siemens, Erlangen, Germany). The quality of imaging of the Cios Spin was superior to that of the ARCADIS Orbic 3D. Even if not specified, a certain influence cannot be excluded with absolute certainty.

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