Pedicle subtraction osteotomy (PSO) represents an efficient technique to correct severe and rigid spinal deformities. One of the challenges, in addition to stabilize adequately the spinal segment, is to reach the desired angular correction that was defined preoperatively. The objective of this work is to describe and evaluate a simple, reproducible and relevant method based on trigonometrical concepts, guiding the surgeon to achieve the targeted correction. Comparative retrospective CT-scan based consecutive series of PSO. The concept of the trigonometric method is based on the geometrical relation between the angular correction obtained at the site of the PS0 (angle α) and the height of bony resection achieved at the posterior wall of the osteotomized vertebra (h). The proposed formula is: h = 2.L.tan(α/2) with L corresponding to the antero-posterior length of the vertebral body. Therefore, according to the corrected angle planned, the height of bony resection can be established helping the surgeon intra-operatively. Adult patients included in the study were operated for a grade 3 or 4PSO between January 2012 and October 2020. For these patients, we compared the correction obtained on the post-operative CT-scan versus the correction predicted by the formula. A total of 55 patients were included with 34 women and 21 men. Mean age was 65.2 ± 9.9 years. 75% of PSO were performed on L4. Degenerative changes were the most frequent cause of the deformity (45%). The mean difference (defined as the delta Δm) between the correction effectively achieved surgically and the correction angle predicted by the trigonometric formula was 1.4 ± 1.5° 0-5.6. Mean angular correction achieved at the level of the PSO was 22.8°±6.5, with a minimum and maximum correction of 7.5° and 36.6°, respectively.When thebone resection was limited strictly to the pedicle height, mean correction was found to be 24°, while resection 2/3 mm less than pedicle height resulted into a mean correction of 16°, and for resection 2/3 mm more than pedicle height, a mean correction of 27°can be expected. Finally, the correction could reach 32° if the bone resection was 5 mm superior to pedicle height. The mathematical formula seems to be enough accurate and reliable to guide the spine surgeons and to be utilized intra-operatively and during preoperative planning in order to define the amount of vertebral bony resection required at the PSO level according to the desired angular correction.
Rivollier et al. (Tue,) studied this question.