The kyphotic deformity caused by AS can impair the ability to look forward and lie flat, and in severe cases, it could endanger cardiopulmonary and digestive function. [21, 22] Osteotomy surgery can not only correct the sagittal imbalance of the spine but also enhance cardiopulmonary and digestive function. [5, 23, 24] SPO, PSO, Ponte, and VCR are the four corrective osteotomies that have been reported so far for the treatment of AS-related kyphosis, with SPO and PSO being the most widely used. SPO is a posterior chevron-shaped osteotomy that obtains 10° correction with a single level. [7, 25] Ponte osteotomy and SPO are often mistakenly used in scientific articles. The notable differences between the two osteotomies are Schwab’s Osteotomy Classification and the extent of resection. Ponte osteotomy requires adequate resection of the laminae to correct thoracolumbar kyphosis by substantially shortening the posterior column.[19] PSO is a closing wedge osteotomy that does not lengthen the anterior column and accomplishes a correction of approximately 30° to 40°. [8, 12, 26] Neither of the two single-level osteotomies is sufficient to correct severe thoracolumbar kyphotic deformities. One-stage two-level PSO is regarded as an effective treatment for severe AS-related kyphosis since it can correct an angle of 100°. However, this method is technically challenging, necessitating a longer operation time and more blood loss, which raises the difficulty and risk of surgery. [27, 28] As a result, we recommend staged osteotomy surgeries for the treatment of severe AS-related rigid kyphosis. Zhong et al,[29] reported a mean blood loss was 2560 ± 1109ml in 10 patients with severe kyphotic AS who were treated with one-stage two-level PSO. Zhang et al,[26] also reported that the mean blood loss was 3311 ± 523ml in 9 patients with severe kyphotic AS who received a one-stage operation. In our study, the total blood loss in staged osteotomies was 1998 ± 613ml, which was less than that of the one-stage two-level PSO. Due to the older age of the patients enrolled in our study, less perioperative blood loss was safer for the patients. Furthermore, patients with severe kyphosis typically cannot tolerate one-stage two-level osteotomies due to poor physical condition and chronic nutritional deficiencies. Compared to PSO, the Ponte osteotomy we performed in the lateral position is less aggressive, results in less blood loss, and carries a lower surgical risk. Therefore, we believe that staged surgery is a relatively safe method of treating severe kyphosis in AS patients, lowering the complexity and risk of surgeries.
22 patients underwent a single-level Ponte osteotomy in the lateral position in the first stage, which is a Grade 2 osteotomy according to Schwab’s Osteotomy Classification. We performed the Ponte osteotomy from pedicle to pedicle (Fig. 3), with an average correction of 29.1 °. Correction is achieved mainly by shortening the posterior column and lengthening the anterior column, which could lead to damage to major vessels, especially the abdominal aorta. [30] However, no severe vascular injury was recorded in our study. This might be because no abdominal aorta calcification was detected on preoperative CT angiography in all cases, which allowed the elastic aorta to adapt well to the elongation of the spine during the correction procedure. In our experience, the osteotomy width of the Ponte osteotomy is safe within 25mm.
It is difficult to place AS patients with severe kyphosis in a prone position, which often calls for a specially prepared reverse “V” shaped folding bed. In some cases, patients cannot even undergo corrective surgery in the prone position because of the attachment of the chest and abdomen. (Fig. 4) Blindness was a frequent consequence during prone spine surgery that could be brought on by local extrusion and ischemic optic neuropathy. [31] Qian et al,[32] reported that brachial plexus palsy may occur during prone spinal osteotomy due to local compression and excessive abduction of the shoulder. In this study, we performed the first stage osteotomy in the lateral position for the first time, which not only resolved the problem of the intraoperative position setting but also prevented the complications of postoperative blindness and brachial plexus palsy. The complication rate of the first-stage lateral surgery was 4.3%, which was lower than the average complication rate (13.4%) for osteotomy reported by Qian et al. [33]
In this study, GK, TK, LL, SVA, and CBVA were used to evaluate the sagittal balance of the spine. Our results showed significant improvements in all parameters after surgery and at the last follow-up. Qian et al,[33] explored the feasibility of using a single-stage skipping two-level PSO in 10 patients with AS-related severe kyphosis (Cobb > 100 °). Their results showed that GK, LL, and SVA corrected from 113.4 °, 41.9° and 25.2cm preoperatively to 71.6 °, -44.1°and 5.8cm postoperatively, respectively. Zhong et al,[29] showed that the kyphosis angle, CBVA and SVA improved from 92.0°, 37.6° and 24.1cm preoperatively to 30.0°, − 0.6°and 7.5cm postoperatively. In our study, the kyphosis angle was corrected from 115.0° to 46.5° postoperatively with a mean correction of 68.5 °, which was similar to the previous studies. Although the Ponte osteotomy we performed in the lateral surgery was a Grade 2 osteotomy, it achieved a mean correction of 29.1 °. The correction angle was almost equal to the PSO procedure. The reason might be that we expanded the laminae resection range from pedicle to pedicle, thus providing an adequate osteotomy gap. The wider the osteotomy gap, the greater the correction angle. The mean SVA improved from 21.2 cm to 5.1cm postoperatively with an average correction of 16.1 cm, which was superior to previous studies. As we know, the lower the osteotomy level, the longer the corrected lever arm, therefore more SVA can be restored and better sagittal balance can be achieved. In the first stage of surgery, L3/4 was selected as the osteotomy level in 16 cases (69.6%), which might explain the superiority of our results.
In this study, 23 patients underwent a total of 46 operations in two stages. The complication rate of the first stage, second stage, and the total were 4.3%, 13%, and 8.7%, respectively. Cerebrospinal fluid leakage was the most common complication due to the adhesive lesions between the dura and the ligamentum flavum, the facet, or lamina. [34] In addition, postoperative blindness and brachial plexus paralysis were avoided due to the lateral position.
Limitations
This study also has limitations. The sample size was relatively small. Besides, retrospective studies have several inherent limitations, such as selection bias and data availability. Sagittal alignment parameters of the spine were evaluated in this study (GK, TK, LL, SVA); however, the spinopelvic alignment parameters(e.g., such as pelvic incidence, pelvic tilt, and sacral slope) were not fully assessed due to the low quality of X-ray imaging.