Optimizing osteotomy in ankylosing spondylitis-induced kyphosis: anatomical insights and novel instrumentation

Ankylosing spondylitis (AS) is a chronic inflammatory disorder marked by spinal ossification that leads to progressive thoracolumbar kyphosis (TLK). We previously introduced vertebral body compression osteotomy (VBCO) as a modified approach for deformity correction. Given the unique anatomical variations and increased intraoperative bleeding risk in AS-induced TLK, it is imperative for clinicians to thoroughly understand these anatomical characteristics and develop specialized osteotomy instruments. This study aims to develop a compressive osteotomy instrument specifically designed for the unique vertebral anatomy of AS patients, with the goal of reducing intraoperative blood loss and enhancing surgical safety. A retrospective study was performed on AS patients treated at our hospital from 2012 to 2022, with a gender- and number-matched cohort of degenerative disc disease (DDD) patients serving as controls. Key anatomical parameters were measured from CT images, including osteotomy length (OL), width (OW), angle (OA), height (OH), and the distance (AL) and angle (AA) from the osteotomy starting point to the aorta. Custom osteotomy instruments were subsequently designed based on AS-specific data and applied clinically. Compared to the DDD group, AS patients showed a significantly longer OL at L5 (3.92 vs. 3.63 cm, p < 0.001) and a wider OW at L5 (1.70 vs. 1.44 cm, p = 0.012). Additionally, the aorta in AS patients was positioned anteromedially from T12 to L4, thereby reducing the risk of vascular injury during lumbar osteotomy. Notably, a two-level VBCO performed with the novel osteotomy knife achieved a 45.2° kyphosis correction with only 500 mL intraoperative blood loss, significantly lower than conventional pedicle subtraction osteotomy averages. Although the vertebral morphology of AS patients is similar to that of DDD patients, osteotomy parameters vary along the spinal levels, with the lumbar spine exhibiting more favorable characteristics for osteotomy. The novel compression osteotomy instrument developed based on these findings could enhance surgical safety by reducing blood loss and optimizing vertebral correction.