As the condition progresses, AS patients may develop a series of postural changes, such as, reduced lumbar lordosis, pelvis back-tilt, hip hyperextension, knee flexion, and cervical flexion deformity, affecting the global trunk balance. Spinal kyphosis deformity is a prominent feature of these patients, usually leading to shift the center of gravity of the trunk anteriorly. It was reported that about 30% of AS patients without standard conservative treatment may develop thoracolumbar or lumbar kyphosis . In order to restore the balance of the trunk, a series of compensatory mechanisms will be initiated . When the anterior sagittal imbalance of the spine excesses the compensatory capacity of the body, AS patients will lose the ability of lying down flat, walking, and horizontal visual axis, and have impaired function of the respiratory and digestion systems resulted from compressed visceral organs in severe cases . To improve the quality of life in such patients, surgical intervention is an indispensable treatment protocol.
Osteotomy is widely used to correct spinal deformity and improve health-related quality of life in AS patients with thoracolumbar or lumbar kyphosis . Among them, pedicle subtraction osteotomy is a common procedure to improve postoperative sagittal parameters. Due to the specialty and complexity of ankylosing spondylitis, however, the relationship between the optimal postoperative sagittal parameters and HRQOL remains obscure. Schwab et al.  reported that SVA, PT and PI \(-\) LL were three most important parameters for surgeons to make an operative proposal in adult spinal deformity, furthermore, proposed that postoperative SVA \(<\) 5 cm, PT \(<\) 25 \(^\circ\), and PI \(-\) LL \(=\pm\) 9 \(^\circ\) were favorable indicators for good clinical prognosis. Kim et al.  also reported that SVA was an important prognostic index for AS patients with rigid kyphosis deformity, undergoing posterior osteotomy procedure. The authors indicated that patients with postoperative SVA less than 70 mm were able to obtain better ODI scores (17.4 \(\pm\) 8.2). Recently, Huang et al.  analyzed the clinical data of AS patients with thoracolumbar kyphosis deformity treated by single-level PSO procedure, and proposed that postoperative PT \(<\) 24 \(^\circ\), SSA \(>\) 108 \(^\circ\) and TPA \(>\) 152 \(^\circ\) indicated better clinical outcomes (ODI \(<\) 20). In addition, Lee et al.  found that cervical sagittal alignment in AS patients was different from that of normal population, and obviously related to HRQOL. In present study, all patients’ ODI scores were less than 20 and the value of SVA decreased from 19.21 \(\pm\) 5.04 cm preoperatively to 5.50 \(\pm\) 3.26 cm postoperatively. In addition, compared to preoperative parameters, postoperative PT, SS, and LL were significantly improved. These data indicated that all patients enrolled in this study achieved good clinical outcomes and deformity correction in treatment with posterior single-level PSO.
Currently, the osteotomy correction of rigid sagittal deformity remains a tricky procedure. The reason is, partly, that it is difficult to accurately quantify the osteotomy angle required to restore optimal sagittal alignment before surgery. In 2006, Ondra  and Yang  proposed a trigonometric method for calculating osteotomy size of PSO to correct fixed sagittal deformity. In this method, the vertex of the selected osteotomy segment was used as the rotation axis, and the included angle formed by the rotation and translation of the center of C7 vertebrae to the plumb line ascending from the posterior corner of S1 was the osteotomy angle. Because of insufficient correction due to ignoring the compensatory role of pelvis and lower limbs, this method has been rarely used in clinical practice. Then, van Royen et al.  designed a computational program for preoperative planning in AS patients and defined the normal sacral endplate angle at 40 \(^\circ\). Although it integrated the role of pelvis compensation, but the compensatory effect of lower limbs did not be considered. Furthermore, defining the postoperative SEA at 40 \(^\circ\) was ambiguous and unreasonable, especially in patients with small PI.
In the past decade, with the deepening understanding the global balance, scholars have proposed different calculation method of the osteotomy angle. But the effectiveness and rationality of various methods need to be further evaluated. From a biomechanical standpoint, the ideal method for maintaining sagittal balance is to shift the center of gravity (CG) of the trunk over the hip axis when the pelvic and lower extremity joints are in the neutral position. In the sagittal plane, the center of both acoustic meati overhang almost coincides with the center of mass of head, and the whole spine is well-balanced if the CAM overhang is less than or equal to 2 cm. Based on abovementioned findings, Aurouer et al.  proposed a CAM-HA method for preoperative planning for correction of sagittal deformity of the spine. The authors normalized the value of PT according to the formula of tPT \(=\) 0.37 \(\times\) PI \(-\) 7, then simulated osteotomies to make CAM overhang less than 2 cm. In 2013, Song et al.  found that the hilus pulmonis was located over HA in normal subjects, then proposed the HP-HA method for calculating the osteotomy angle. In this method, the postoperative individual PT was identified similar to that of CAM-HA method, then the HP was served as the CG of the upper trunk to calculate the angle of osteotomy by taking the HP through the plumb line of HA.
Both of two methods, though, take the compensatory effects of pelvis and lower extremity into account, but there still exist some drawbacks. As we know, ankylosing spondylitis generally starts from the sacroiliac joint and gradually erodes the spine cephalad to cranio-cervical junction region. In different stages of disease progression, the thoracic and cervical region can retain a certain amount of mobility. Ankylosing spondylitis with thoracolumbar or lumbar kyphosis leads to lean forward, often accompanied by a compensatory decrease in thoracic kyphosis and a compensatory increase in cervical lordosis or C2-7 SVA, which may be spontaneously corrected after osteotomy surgery. In addition, the position of CAM is subject to variation with that of head, and the high quality of full spine radiographs covering the landmark of CAM are sometimes difficult to obtain, all of which contribute to the inaccuracy of osteotomy angle calculation by CAM-HA method. Although the center of gravity of the trunk is replaced with HP, which eliminates the influence of body position, in HP-HA method, but the anatomical variation of the hilus pulmonis and the unclear image due to overlapped soft tissue structures decrease its location accuracy in X-ray. Second, the position of HP is usually located at the level of T4 vertebrae, so it is not suitable for patients with relatively high osteotomy site. In this study, the osteotomy angles predicted by CAM-HA method (56.61 \(\pm\) 8.58 \(^\circ\)) and HP-HA method (60.07 \(\pm\) 13.58 \(^\circ\)) were significantly greater than the actual angle (46.57 \(\pm\) 2.32 \(^\circ\)) at final follow up, which were usually not achieved only by a single-level PSO procedure.
In 2011, Le Huce et al.  proposed a full balance integrated (FBI) technique for osteotomy planification of thoracolumbar imbalance. The following factors were included in this method: C7TA, FOA, and PTCA, which took both effects of pelvis and lower extremity into account. In terms of PTCA, the authors suggested that if the PT was less than 25 \(^\circ\) or more than 25 \(^\circ\), 5 or 10 \(^\circ\) of PT compensation should be added, respectively, by experience. However, Lamartina et al.  thought that FBI method just roughly estimated the amount of pelvic tilt excess and lack of consideration of thoracic hypo-lordosis, therewith, proposed a new method to avoid potential drawbacks of FBI method, while keeping its simplicity. This method is based on the measurement of a single angle, the spino-femoral angle (SFA). Beyond that, the hip extension reserve (10 \(^\circ\)) and increase in thoracic kyphosis after surgery are taken into consideration. Recently, Akbar et al.  offered a process in corrective osteotomy surgery with respect to the calculation of the osteotomy angle needed via using Surgimap software. The desired postoperative PT can be reached by rotating the image, next bring C7 in line with the posterosuperior corner of S1, then the resection angle needed can be measured by Surgimap Spine software. Our results showed that no significant difference was found between the osteotomy angle of SFA method (51.24 \(\pm\) 12.14 \(^\circ\)) and FBI method (48.08 \(\pm\) 12.49 \(^\circ\)) and the actual angle at final follow up, but the angle of Surgimap method (53.80 \(\pm\) 9.79 \(^\circ\)) was slightly larger than the actual osteotomy angle.
In fact, the PSO can result in approximately 30 degrees of correction with maximum bony resection when performed at the apex of a sharp deformity . Therefore, for AS patients without need for performing two-level PSO procedure, the SPO at adjacent levels may be served as a supplement to obtain the desired outcome of osteotomy. Second, both pre-bending of titanium rod and enhanced fixation by internal fixation system have an influence on postoperative sagittal alignment. Therefore, combining experimental results with our experiences, SFA, FBI, and Surigmap method are all suitable for calculating the osteotomy angle preoperatively in AS patients with thoracolumbar or lumbar kyphosis, however, considering the simplicity and rationality of all three methods, SFA may be superior to the others.
This study was associated with several limitations. First, AS patients enrolled in this study were characterized by thoracolumbar or lumbar kyphosis. The results of this study are not applicable to AS patients with cervical deformity or flexion contracture deformity of the hip. Second, this study was retrospective in nature. Finally, the sample size was small, and multicenter, large sample, and the long-term clinical observations are needed to confirm this conclusion.