Relevance of trigonometric method to achieve the correction angle for pedicular subtraction osteotomy in lumbar spine

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Introduction:

PSO (Pedicle Subtraction Osteotomy) is the only way to effectively correct large rigid spinal deformities. The main advantage is that correction can be achieved in a single operation. In addition to the technique of performing the osteotomy, one of the difficulties is to achieve the pre-operative angular correction objective. Automated methods based on artificial intelligence are being developed, but they are still being evaluated and require substantial technical resources. This study proposes a simple, reproducible, relevant method based on trigonometric concepts and reliable intraoperative anatomical landmarks.

 

Material and method:

This study is an observational, analytical, retrospective study carried out in the spine and spinal cord surgery department of Lyon.  It was carried out on patients with a rigid spinal deformity. This paper focuses on grade 3 and grade 4 osteotomies performed between 2013 and 2021. The idea of this study is to describe the accuracy of the trigonometric method used during the surgeries, inspired by trigonometric concepts used in tibial osteotomies in orthopaedics. To do this, we first measured the angle of correction achieved surgically at the PSO vertebra on the postoperative CT scan. Secondly, we calculated the theoretical angle of correction as a function of the height of the resected vertebral wall using the mathematical formula and the pre-operative CT scan. The difference in these two angles gives the delta angle.

 

Results:

The study included 55 patients of whom 34 were women and 21 men. The average age at the time of surgery was 66 years. 75% of PSO were performed on L4. Degenerative pathology was the main indication (45%). The average angular delta was 1.4°. The minimum delta is 0.01°. The maximum delta is 5.64°. The standard deviation is 1.47. The average of the correction achieved at the level of the vertebra that underwent PSO is 22.8°. The average of the theoretical correction expected with the mathematical formula is 21.9°. The ODI score significantly improved by 6 points between pre- and post-op at 1 year. Similarly, with the VAS which is significantly improved by 5 points.

 

Conclusion:

The mathematical method appears to be sufficiently accurate and reliable to be used in pre-operative planning to define the vertebral bone resection required for the desired local correction. A prospective study is needed to validate this technique.