Abstract
Background
Two-level osteotomy has emerged as an effective technique for addressing severe kyphosis secondary to ankylosing spondylitis (AS). Despite its efficacy, there remains a lack of consensus regarding the criteria for determining the necessity of two-level osteotomy. This study aimed to investigate precise and direct preoperative predictors for selection of two-level osteotomy in patients with severe AS kyphosis.
Methods
A retrospective cohort of 101 AS patients was analyzed, including 33 patients who underwent two-level modified pedicle subtraction osteotomy (PSO) and 68 patients who underwent one-level modified PSO. Radiographic parameters, including pelvic tilt, pelvic incidence (PI), sacral slope (SS), lumbar lordosis (LL), thoracic kyphosis (TK), global kyphosis (GK), sagittal vertical axis (SVA), chin-brow vertical angle (CBVA), and osteotomized vertebral angle, were measured. Clinical outcomes were assessed using Oswestry Disability Index and Scoliosis Research Society-22 questionnaire. Comparative analyses of radiographic and clinical outcomes were conducted across different patient groups. Preoperative predictors for selecting two-level osteotomy were identified through receiver-operating characteristic curve analysis and logistic regression analysis.
Results
Patients undergoing two-level osteotomy exhibited significantly higher preoperative parameters of SS, LL, TK, GK, SVA, and CBVA compared to those receiving one-level osteotomy (P < 0.05). Notably, CBVA, GK, and SVA were identified as the most influential parameters influencing the selection of two-level osteotomy, with optimal threshold values of 63.9°, 91.1°, and 25.4 cm, respectively. Logistic regression analysis revealed CBVA and GK as independent predictors for selecting two-level osteotomy (P < 0.05). Patients undergoing lumbar region two-level osteotomy demonstrated larger SVA (P < 0.05) and pelvic incidence (PI; P = 0.267), whereas those with thoracic and lumbar osteotomy exhibited increased TK (P = 0.465). All patients achieved favorable clinical outcomes at final follow-up (P < 0.05).
Conclusions
Preoperative CBVA, GK, and SVA are key parameters influencing the selection of two-level modified PSO for AS kyphosis. Specifically, preoperative CBVA > 63.9° and GK > 91.1° serve as independent predictors, with SVA > 25.4 cm acting as an auxiliary criterion. The choice of osteotomy sites is predominantly influenced by preoperative SVA, PI, and TK measurements.
Level of evidence
Level IV, therapeutic study.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12891-025-09075-z.
Keywords: Ankylosing spondylitis, Two-level osteotomy, Preoperative predictor, Severe kyphosis, Decision making
Introduction
Ankylosing spondylitis (AS) is a chronic, progressive inflammatory disorder predominantly affecting the axial skeleton, often culminating in severe thoracolumbar kyphosis in advanced stages. This deformity markedly impairs patients’ ability to maintain an upright posture, lie flat, and sustain a horizontal gaze, thereby substantially compromising daily functioning and quality of life [1, 2]. Surgical correction via three-column osteotomies—such as pedicle subtraction osteotomy (PSO) and its modified variants [3, 4], have emerged as effective approaches for correcting kyphotic deformities and restoring sagittal balance.
In cases of severe AS-related kyphosis requiring substantial angular correction—particularly when the osteotomy vertebral angle (OVA) exceeds 60°—two-level osteotomy is traditionally employed to achieve adequate deformity correction and optimal sagittal alignment [5–8]. However, accurately predicting the requisite OVA remains a complex challenge, with no universally accepted standards or methods. Existing strategies—including trigonometric calculations [9], manual radiographic measurements [10, 11], and digital simulation software [12, 13], —have yielded inconsistent and disparate results [14]. Relying solely on predicted OVA for surgical planning can be problematic, potentially leading to intraoperative errors and suboptimal outcomes. Consequently, there is an urgent need to identify simple, reliable preoperative parameters that can guide surgical decision-making regarding the selection of osteotomy levels.
This study aims to identify preoperative parameters that influence the decision to perform two-level osteotomy in patients with severe AS kyphosis. Furthermore, it involved a comparative analysis of preoperative parameters among patients undergoing two-level osteotomy at different sites, with the goal of elucidating critical factors influencing the choice of osteotomy sites based on individual patient characteristics.
Materials and methods
Patients
The consecutive AS patients who underwent one- and two-level osteotomy between January 2012 and January 2022 were retrospectively reviewed. The inclusion criteria were as follows [15, 16]: (1) patients who underwent one- or two-level modified PSO [4]; (2) with global kyphosis (GK) larger than 70° [17]; (3) with a minimum follow-up of 2 years. Patients with a history of previous spine surgery, pathologic fracture, and other comorbidities impacting surgical considerations, such as nephrotic syndrome, cerebral infarction, and decompensated liver cirrhosis, were excluded. Finally, 101 patients (91 men, 10 women) met the inclusion criteria and were categorized into two groups: 68 in one-level osteotomy group and 33 in two-level osteotomy group. Among the patients who underwent two-level osteotomy, 21 cases involved thoracic and lumbar osteotomy, and 12 cases involved double lumbar osteotomy.
Data collection
Standing lateral radiographs of the whole spine were obtained for measurements. The radiographic parameters assessed included the following [6, 18]: Pelvic tilt (PT): the angle between vertical plumb line and the line connecting the center of S1 upper endplate to the center of femoral head axis; Pelvic incidence (PI): the angle between a line perpendicular to S1 upper endplate and the line connecting the center of S1 upper endplate to the center of femoral head axis; Sacral slope (SS): the angle between S1 upper endplate and horizontal plane; Lumbar lordosis (LL): the Cobb angle between the upper endplate of L1 and the upper endplate of S1; Thoracic kyphosis (TK): the Cobb angle between the upper endplate of T4 and the lower endplate of T12; Global kyphosis (GK): the angle between the superior endplate of the most tilted upper vertebra and the inferior endplate of the most tilted lower vertebra; Sagittal vertical axis (SVA): the horizontal distance between C7 plumb line and the posterior-superior corner of S1; Chin-brow vertical angle (CBVA): the angle between the line from chin to brow and the plumb line; Osteotomized vertebral angle (OVA): for one-level osteotomy, defined as the change in the angle between the inferior endplate of osteotomized vertebra and the superior endplate of adjacent cranial vertebra pre- and postoperatively; for two-level osteotomy, calculated as the sum of the two individual correction angles (Fig. 1). Clinical outcomes were evaluated using Oswestry Disability Index (ODI) and Scoliosis Research Society-22 (SRS-22) questionnaire.
Fig. 1.
Schematic illustration of OVA measurement
Surgical decision-making
In this study, one-level osteotomy was performed for patients requiring OVA within the range of 30° to 60°; for OVA exceeding 60°, two-level osteotomy was considered [6, 18]. Preoperative assessment of OVA was conducted using either Surgimap simulation or the tracing paper cutting method [14]. Key parameters, including CBVA, PT, and SVA, were carefully evaluated. Given the characteristic posterior pelvic tilt and rotation observed in AS patients, the surgical protocol involved first adjusting PT to an estimated normal range [10, 14], then using the osteotomy vertebra as a rotational pivot to optimize spinal alignment, aiming for postoperative SVA less than 7.4 cm [10, 14, 19]. CBVA was maintained within 10° to 20° to ensure functional and aesthetic outcomes [20]. The site of osteotomy for one-level osteotomy was predominantly located at the apex or within one to two levels proximal or distal to the apical vertebra. In cases requiring two-level osteotomy, the initial osteotomy was typically performed at the apex of kyphosis, with the second osteotomy conducted two to three levels distal to the first. The primary objective of the surgical intervention was to restore the patient’s ability to stand upright, lie flat, and gaze straightforwardly postoperatively.
Surgical technique
A modified pedicle subtraction osteotomy (PSO) was performed as previously described [4, 19]. Under general anesthesia, pedicle screws were inserted at the planned fusion levels. The osteotomy resection included the upper portion of the targeted vertebra, the adjacent cranial intervertebral disc, and the inferior articular processes of the superior vertebra. Notably, the resection extended along the upper part of the pedicles to the anterior vertebral body, removing approximately one-third to half of the vertebral body. Following osteotomy, bilateral temporary rods were placed spanning at least two vertebrae above and below the osteotomy site. During correction, the rods were bilaterally compressed at the osteotomy ends to slightly shorten the spinal cord, facilitating subsequent lengthening. The patient’s shoulders were elevated by the circulating nurse, and postural pads were gradually removed to correct kyphosis. Using the rods as a hinge at the osteotomy gap, surgeons bent the rods while maintaining shoulder elevation to realign the spine. Once satisfactory correction was achieved, a cage was implanted at the osteotomy site to restore spinal alignment.
For two-level osteotomies, the initial steps mirrored those of one-level osteotomy. After closing the first osteotomy site, a unilateral long temporary rod was placed from the first to the second osteotomy level. The resection and correction at the second osteotomy level were then performed similarly. Following satisfactory correction, the temporary rods were replaced with pre-contoured rods. The osteotomy gap was filled with autogenous bone graft and a cage, and further rod compression was applied. Throughout the procedure, somatosensory and motor-evoked potentials were continuously monitored.
Statistical analysis
All numerical data were presented as mean ± standard deviation. Statistical analysis was conducted using SPSS software (version 22.0, SPSS, Inc., Chicago, IL). Two independent samples t-tests were performed to compare quantitative data between two groups, while qualitative data were assessed using the χ2 test or Fisher’s exact test as appropriate. Paired samples t-tests were performed to evaluate radiographic parameters and clinical outcomes before and after surgery. Receiver-operating characteristic (ROC) curve analyses and logistic regression analysis were performed to identify preoperative predictors for the selection of two-level osteotomy. The Maximum Youden index was employed to determine the optimal threshold values. A P value < 0.05 was considered statistically significant.
Results
Demographic and surgical data
No significant differences were observed in baseline characteristics, including average age, sex, apex location, presence of Andersson lesions, or follow-up duration, between the two groups (P > 0.05). Two-level osteotomy group exhibited longer operative times, higher estimated blood loss, and longer fusion levels compared to one-level osteotomy group (P < 0.05). One-level osteotomy was predominantly performed at L2, L1, and L3. Two-level osteotomy comprised 21 cases of thoracic and lumbar osteotomy and 12 cases of double lumbar osteotomy (Table 1).
Table 1.
Comparison of demographic and surgical data between one- and two-level osteotomy groups
| Variable | One-level osteotomy | Two-level osteotomy | P value |
|---|---|---|---|
| Case (n) | 68 | 33 | - |
| Age (year) | 39.8 ± 8.2 | 39.5 ± 10.1 | 0.171 |
| Sex(male/female) | 60/8 | 31/2 | 0.586 |
| Andersson lesion | 13(19.1%) | 4(12.1%) | 0.378 |
| Operative time (min) | 371.2 ± 90.8 | 550.0 ± 155.2 | 0.008* |
| Apex | |||
| T8 | 1 | 1 | 0.998 |
| T9 | 2 | 1 | |
| T10 | 4 | 2 | |
| T11 | 13 | 8 | |
| T12 | 32 | 13 | |
| L1 | 12 | 6 | |
| L2 | 4 | 2 | |
| Osteotomy sites (one-/two-level) | |||
| T12/T10 and L1 | 3 | 1 | - |
| L1/T11 and L3 | 14 | 1 | |
| L2/T12 and L2 | 40 | 2 | |
| L3/T12 and L3 | 11 | 17 | |
| L1 and L3 | - | 4 | |
| L1 and L4 | - | 4 | |
| L1 and L5 | - | 1 | |
| L2 and L5 | - | 3 | |
| Estimated blood loss (ml) | 856.0 ± 294.1 | 1323.1 ± 541.3 | 0.001* |
| Number of fusion levels (n) | 6.5 ± 0.9 | 7.9 ± 1.5 | 0.049* |
| Follow-up (months) | 28.7 ± 4.7 | 26.9 ± 3.5 | 0.690 |
*Indicates a statistically significant difference between one- and two-level osteotomy groups (P < 0.05)
Radiographic and clinical outcomes
Within two-level osteotomy group, the preoperative measurements of SS, LL, TK, GK, SVA and CBVA were significantly larger than those in the one-level osteotomy group (P < 0.05). Furthermore, larger corrections in TK, GK, SVA and CBVA were achieved in two-level osteotomy group (P < 0.05) (Table 2). All patients in both groups experienced favorable clinical outcomes at the final follow-up (P < 0.05). The improvement of ODI total scores was relatively larger in two-level osteotomy group (P = 0.182) (Supplemental Table 1). Additionally, the rate of complications in two-level osteotomy group was higher than that in one-level osteotomy group (P < 0.05) (Supplemental Table 2).
Table 2.
Difference of radiographic parameters between one- and two-level osteotomy groups
| Parameters | Groups | Preoperative | Postoperative | Final Follow-up | Correction | Loss of Correction |
|---|---|---|---|---|---|---|
| PT | One-level | 37.4 ± 12.0 | 26.4 ± 11.5 | 31.7 ± 10.6 | 11.1 ± 10.6† | 5.3 ± 5.9 |
| Two-level | 36.0 ± 13.4 | 26.0 ± 9.7 | 34.3 ± 12.0 | 11.2 ± 11.6† | 8.2 ± 5.6 | |
| PI | One-level | 46.0 ± 12.4 | 45.5 ± 12.1 | 44.6 ± 10.8 | 0.4 ± 3.4 | 0.9 ± 4.1 |
| Two-level | 49.9 ± 14.0 | 49.8 ± 12.0 | 50.0 ± 9.8 | 0.1 ± 4.6 | 0.2 ± 3.4 | |
| SS | One-level | 8.5 ± 11.9* | 19.2 ± 10.8 | 13.0 ± 13.2 | 10.7 ± 9.7† | 6.2 ± 7.7 |
| Two-level | 14.4 ± 13.1* | 23.8 ± 11.6 | 15.8 ± 7.5 | 9.3 ± 12.1† | 8.0 ± 5.5 | |
| LL | One-level | 7.4 ± 21.7* | −35.8 ± 16.0 | −33.4 ± 18.9 | 43.3 ± 17.5† | 2.5 ± 8.4 |
| Two-level | 18.9 ± 21.9* | −30.1 ± 22.9 | −25.5 ± 19.6 | 49.0 ± 21.4† | 4.6 ± 5.3 | |
| TK | One-level | 54.3 ± 16.2* | 50.0 ± 15.8 | 52.9 ± 10.7 | 4.3 ± 11.4*, † | 2.9 ± 5.5 |
| Two-level | 65.0 ± 17.2* | 44.3 ± 19.7 | 45.6 ± 21.4 | 20.6 ± 20.1*, † | 1.3 ± 3.8 | |
| GK | One-level | 90.2 ± 11.7* | 35.9 ± 14.2 | 42.2 ± 16.0* | 54.2 ± 13.8*, † | 6.3 ± 7.8 |
| Two-level | 110.2 ± 20.6* | 32.0 ± 18.6 | 31.7 ± 15.5* | 78.2 ± 24.3*, † | 0.3 ± 6.4 | |
| SVA | One-level | 23.9 ± 9.3* | 9.2 ± 5.9* | 7.6 ± 5.1 | 14.7 ± 7.0*, † | 1.5 ± 4.1 |
| Two-level | 31.4 ± 7.2* | 11.9 ± 6.1* | 10.8 ± 7.7 | 19.5 ± 6.6*, † | 1.1 ± 4.1 | |
| OVA | One-level | - | 47.4 ± 13.0* | 45.3 ± 13.3* | - | 2.1 ± 2.9 |
| Two-level | - | 72.4 ± 15.1* | 72.0 ± 14.6* | - | 0.4 ± 5.9 | |
| CBVA | One-level | 41.7 ± 26.2* | 8.6 ± 11.4 | 6.9 ± 9.4 | 33.1 ± 22.4*, † | 1.7 ± 10.8 |
| Two-level | 83.1 ± 33.3* | 10.8 ± 12.2 | 6.9 ± 10.1 | 72.4 ± 30.1*, † | 3.9 ± 5.4 |
PT pelvic tilt, PI pelvic incidence, SS sacral slope, LL lumbar lordosis, TK thoracic kyphosis, GK global kyphosis, SVA sagittal vertical axis, OVA osteotomized vertebra angle, CBVA chin-brow vertical angle
* Indicates a statistically significant difference between one- and two-level osteotomy groups (P < 0.05)
† Indicates a statistically significant difference between preoperative and postoperative values (P < 0.05)
Key parameters for selection of two-level osteotomy
The relationship between preoperative parameters (SS, LL, TK, GK, SVA and CBVA) and the selection of two-level osteotomy was examined using ROC curve analysis. The area under the curves (AUC) for CBVA, GK, SVA, LL, and TK were found to be statistically significant (P < 0.05), indicating a significant influence of these key parameters on the selection of two-level osteotomy (Table 3). Notably, CBVA, GK, and SVA were identified as the top three candidates for two-level osteotomy selection, exhibiting an AUC exceeding 0.7. The optimal threshold values for selecting two-level osteotomy were determined to be 63.9° for CBVA, 91.1° for GK, and 25.4 cm for SVA (Figs. 2 and 3).
Table 3.
ROC curve analysis of preoperative parameters for two-level osteotomy selection
| Preoperative parameters | AUC | P value | 95% Confidence interval | |
|---|---|---|---|---|
| Lower | Upper | |||
| CBVA | 0.844 | < 0.001 | 0.761 | 0.927 |
| GK | 0.828 | < 0.001 | 0.746 | 0.910 |
| SVA | 0.735 | < 0.001 | 0.632 | 0.837 |
| LL | 0.686 | 0.007 | 0.558 | 0.814 |
| TK | 0.653 | 0.025 | 0.521 | 0.785 |
ROC receiver-operating characteristic, AUC area under the ROC curve, CBVA chin-brow vertical angle, GK global kyphosis, SVA sagittal vertical axis, LL lumbar lordosis, TK thoracic kyphosis. Significance set as P < 0.05
Fig. 2.
A 41-year-old man with severe thoracolumbar kyphosis secondary to AS underwent two-level modified PSO at L1 and L4. A, D Preoperative assessments showed a severe thoracolumbar kyphosis, with CBVA of 107.8° (> 63.9°), GK of 126.7° (> 91.1°), and SVA of 33.2 cm (> 25.4 cm), indicating the need for two-level osteotomy based on the current findings. B, E Two-level osteotomy was performed at L1 and L4, resulting in a total OVA of 83.5°(OVA1(red lines) + OVA2(yellow lines)); the kyphosis was successfully corrected with CBVA of 14.8°, GK of 41.4° and SVA of 13.5 cm postoperatively. C, F At 2-year follow-up, the correction was well-maintained with CBVA of 12.5°, GK of 38.4° and SVA of 3.4 cm; and the clinical outcome was considered satisfactory with ODI score of 5.0 and SRS-2 score of 4.20. There was a substantial improvement in patient’s appearance from preoperative (D) to postoperative (E) and at the 2-year follow-up (F)
Fig. 3.
Receiver-operating characteristic curve analyses of preoperative parameters for two-level modified PSO selection. A The optimal threshold value of preoperative CBVA was 63.9° with a sensitivity of 80.6%, a false-positive rate (1-Specificity) of 22.7% and an area under the curve (AUC) of 0.844. B The optimal threshold value of preoperative GK was 91.1° with a sensitivity of 87.9%, a false-positive rate of 33.8% and an AUC of 0.828. C The optimal threshold value of preoperative SVA was 25.4 cm with a sensitivity of 83.3%, a false-positive rate of 41.2% and an AUC of 0.735
Independent predictors for selection of two-level osteotomy
Logistic regression analysis was conducted to identify independent predictors influencing the selection of two-level osteotomy. Among the top three candidates (CBVA, GK, SVA), only CBVA and GK were included in the model (P < 0.05). The results indicated that CBVA and GK were the independent predictors in the selection of two-level osteotomy, and should be considered first in decision-making (Table 4).
Table 4.
Logistic regression analysis of preoperative parameters for two-level osteotomy selection
| Variables | B | SE | Wald | P value | Exp (B) | 95% Confidence interval | |
|---|---|---|---|---|---|---|---|
| Lower | Upper | ||||||
| Preoperative CBVA | 0.043 | 0.014 | 9.327 | 0.002 | 1.044 | 1.016 | 1.074 |
| Preoperative GK | 0.062 | 0.024 | 6.863 | 0.009 | 1.064 | 1.016 | 1.114 |
| Constant | −9.587 | 2.344 | 16.730 | < 0.001 | 0.000 | - | - |
Using the method of forward LR analysis; CBVA, chin-brow vertical angle; GK, global kyphosis. Significance set as P < 0.05
Differences in parameters for selecting thoracic and lumbar osteotomy versus double lumbar osteotomy
Based on osteotomy sites, patients undergoing two-level osteotomy were classified into thoracic and lumbar osteotomy group and double lumbar osteotomy group. Patients in double lumbar osteotomy group showed significantly larger preoperative SVA (P < 0.05) and relatively larger preoperative PI (P = 0.267); and these patients achieved larger corrections in LL and SVA postoperatively. Conversely, patients in thoracic and lumbar osteotomy group had relatively larger preoperative TK (P = 0.465) and achieved larger TK correction postoperatively (P < 0.05). Both groups demonstrated similar OVA postoperatively. There were no significant differences in the correction of PT, PI, SS, GK, and CBVA between these two groups (P > 0.05) (Table 5).
Table 5.
Difference of radiographic parameters between thoracic and lumbar osteotomy, and double lumbar osteotomy
| Parameters | Groups (T + L/L + L two-level osteotomy = 21/12) |
Preoperative | Postoperative | Correction |
|---|---|---|---|---|
| PT | T + L osteotomy | 35.7 ± 13.7 | 27.6 ± 10.6 | 8.1 ± 9.1† |
| L + L osteotomy | 36.5 ± 13.5 | 23.3 ± 7.7 | 13.2 ± 15.1† | |
| PI | T + L osteotomy | 48.8 ± 10.7 | 48.9 ± 10.9 | 0.1 ± 3.0 |
| L + L osteotomy | 53.5 ± 17.2 | 51.3 ± 14.1 | 2.2 ± 6.4 | |
| SS | T + L osteotomy | 12.1 ± 9.5 | 21.2 ± 9.7 | 9.0 ± 8.8† |
| L + L osteotomy | 18.3 ± 17.4 | 28.1 ± 13.7 | 9.8 ± 16.7† | |
| LL | T + L osteotomy | 17.4 ± 22.0 | −18.7 ± 16.6* | 36.1 ± 12.6*,† |
| L + L osteotomy | 21.3 ± 22.5 | −49.2 ± 19.0* | 70.5 ± 14.5*,† | |
| TK | T + L osteotomy | 66.7 ± 17.1 | 37.9 ± 16.3* | 28.7 ± 15.8*,† |
| L + L osteotomy | 62.0 ± 17.8 | 55.6 ± 20.8* | 6.5 ± 19.6* | |
| GK | T + L osteotomy | 111.3 ± 21.5 | 34.2 ± 14.9 | 77.1 ± 20.6† |
| L + L osteotomy | 108.2 ± 19.5 | 28.2 ± 23.9 | 80.0 ± 30.6† | |
| SVA | T + L osteotomy | 29.6 ± 7.3* | 12.3 ± 5.7 | 17.3 ± 5.1*,† |
| L + L osteotomy | 34.8 ± 6.0* | 11.2 ± 7.2 | 23.6 ± 6.6*,† | |
| OVA | T + L osteotomy | - | 70.0 ± 16.1 | - |
| L + L osteotomy | - | 76.6 ± 12.4 | - | |
| CBVA | T + L osteotomy | 83.3 ± 36.2 | 12.1 ± 12.9 | 71.3 ± 33.4† |
| L + L osteotomy | 82.6 ± 28.9 | 8.6 ± 11.1 | 73.9 ± 24.7† |
T + L thoracic and lumbar osteotomy, L + L lumbar and lumbar osteotomy, PT pelvic tilt, PI pelvic incidence, SS sacral slope, LL lumbar lordosis, TK thoracic kyphosis, GK global kyphosis, SVA sagittal vertical axis, OVA osteotomized vertebra angle, CBVA chin-brow vertical angle
* Indicates a statistically significant difference between one- and two-level osteotomy groups (P < 0.05)
† Indicates a statistically significant difference between preoperative and postoperative values (P < 0.05)
Discussion
Three-column osteotomies, such as PSO and its modified variants, are frequently employed in surgical correction of AS-related kyphosis [21–23]. However, in patients with advanced disease presenting with severe kyphotic deformity, one-level osteotomy may be insufficient to achieve the desired correction [7, 24]. In such cases, two-level osteotomy is often recommended, as it offers a greater corrective potential and improved spinal alignment at two sites [7, 24]. Currently, the use of two-level osteotomy has become prevalent in clinical practice [5, 6, 25, 26]. Nonetheless, this approach necessitates careful consideration due to increased operative time, greater blood loss, and a higher risk of complications [18, 27]. Traditionally, the decision to perform a two-level osteotomy was primarily guided by the predicted OVA, calculated using specific formulas [9, 11, 12] or software applications [12, 13]. However, the predicted OVA derived from these methods exhibits variability and lacks standardization, rendering it complex and less practical in routine clinical settings [8, 14]. Consequently, there is an urgent need to establish reliable, universally applicable preoperative predictors to facilitate decision-making regarding two-level osteotomy.
In our study, the two-level osteotomy group demonstrated significantly larger preoperative parameters, including SS, LL, TK, GK, SVA, and CBVA. Among these, CBVA, GK, and SVA showed higher AUC values, positioning them as the top three candidate parameters for guiding the selection of two-level osteotomy. This highlights the critical role of preoperative CBVA, GK, and SVA in influencing surgical decision-making. Subsequently, these three parameters underwent logistic regression analysis to identify independent predictors for selecting two-level osteotomy. The analysis revealed that only CBVA and GK remained as independent predictors, whereas SVA was excluded. This exclusion can be attributed to SVA being a distance parameter that is affected by both correction magnitude and osteotomy site [9, 28]. Specifically, osteotomies performed at lower spinal sites tend to permit larger SVA corrections for a given OVA [7, 9]. Therefore, preoperative SVA may not serve as a decisive factor in determining the need for two-level osteotomy. The AUC for SVA in predicting two-level osteotomy was 0.735, ranking third behind CBVA (AUC 0.844) and GK (AUC 0.828). Given that CBVA and GK are primary indicators of horizontal gaze capability and the severity of kyphotic deformity, respectively, they are inherently more sensitive to the extent of correction achievable through osteotomy [29–31]. Severe CBVA and GK deformities often necessitate substantial osteotomy corrections to restore horizontal vision and optimal spinal alignment. Consequently, preoperative CBVA and GK can be considered robust independent predictors for selecting two-level osteotomy, with SVA serving a secondary or adjunctive role in this decision-making process.
Rather than relying on intricate measurements or calculations of the desired OVA [9–11], our study identified clear and practical preoperative predictors for severe AS kyphosis, thereby facilitating clinical decision-making regarding two-level osteotomy. Specifically, the optimal threshold values for preoperative CBVA, GK, and SVA in determining the need for two-level osteotomy were determined to be 63.9°, 91.1°, and 25.4 cm, respectively. Patients meeting the criteria of CBVA > 63.9°, GK > 91.1°, and SVA > 25.4 cm were recommended for two-level osteotomy, which were similar with the results reported in previous studies [6–8, 15]. Notably, CBVA > 63.9° and GK > 91.1° were identified as essential criteria for selecting two-level osteotomy, with SVA > 25.4 cm serving as a supplementary factor. In patients with flexible cervical spines, CBVA was less relevant [7]. However, for individuals with ankylosed cervical spine, preoperative CBVA > 63.9° should be carefully considered to preserve horizontal gaze and avoid overcorrection. Furthermore, CBVA was correlated with postoperative clinical outcomes, underscoring the importance of monitoring this parameter during correction, particularly in patients with ankylosed cervical spines [20]. Postoperatively, maintaining CBVA within 10°–20° is recommended to restore horizontal vision and optimize clinical results [20, 31]. Although modified osteotomy techniques capable of achieving larger OVA via one-level osteotomy have been described [32–34], two-level osteotomy remains strongly recommended for patients with GK > 91.1° to mitigate the risk of significant local spinal cord shortening and excessive internal fixation angulation, which could lead to stress concentration and an increased likelihood of rod breakage [27].
The selection of osteotomy sites is also critical in two-level osteotomy. This study examined preoperative radiographic characteristics that influence the decision between thoracic and lumbar osteotomies or double lumbar osteotomies. Patients undergoing thoracic and lumbar osteotomies generally exhibited larger preoperative TK and achieved substantial postoperative TK correction. These findings suggest that in cases of severe AS kyphosis with predominant thoracic deformity, a combined thoracic and lumbar osteotomy may be appropriate [25]. This surgical strategy involves performing an initial thoracic osteotomy to address substantial thoracic or thoracolumbar kyphosis, followed by a lumbar osteotomy to correct overall sagittal imbalance. Among the cohort, the most common combination was T12 and L3 osteotomies. Conversely, patients who underwent double lumbar osteotomy had larger preoperative SVA and PI, and achieved notable postoperative improvements in LL and SVA. These results indicate that for individuals with severe sagittal imbalance or larger preoperative PI, double lumbar osteotomy may be preferred to comprehensively restore sagittal alignment via dual lower lumbar osteotomies [21, 35]. Additionally, this approach effectively reestablishes lumbar lordosis, aligning postoperative LL values with increased PI and thereby promoting a harmonious spinopelvic balance [21, 25]. The most frequently selected sites for double lumbar osteotomy in this study were L1 and L3 or L4.
Consistent with Wang’s classification [36], this study included 96 cases of Type II (thoracolumbar) kyphosis and 5 cases of Type III (thoracic) kyphosis. Variations in spinopelvic alignment necessitate tailored surgical strategies. For patients with milder GK (< 91°, Types II and III A), when SVA is not excessively elevated and spinopelvic harmony is maintained, one-level modified PSO within L1–L2 region is generally sufficient for deformity correction. In cases with large SVA and spinopelvic mismatch, osteotomies at L3 or below are typically preferred to achieve greater SVA correction and restore lumbar lordosis, thereby optimizing spinopelvic alignment. For patients with severe GK (> 91°, Types II and III B), two-level modified PSO are frequently performed, with the first near the apex of kyphosis and the second at least two vertebral levels below (e.g., T12 and L2 or L1 and L3/L4) [25, 36]. This dual-approach maximizes kyphosis correction
Limitations
Firstly, as a retrospective study, there is an inherent risk of selection bias. Although patient allocation to different groups was not entirely arbitrary, the study nonetheless offers valuable insights into the decision-making process regarding the selection of two-level osteotomy in patients with severe AS kyphosis. Secondly, due to the limited sample size, this study was not able to assess the influence of cervical range of motion on the choice of two-level osteotomy. Thirdly, beyond preoperative parameters such as CBVA, GK, and SVA, other factors—including overall health status and economic considerations—may have affected patient suitability for two-level osteotomy. To validate these findings, a prospective, multicenter study with a larger sample size is warranted.
Conclusions
For severe AS kyphosis, preoperative parameters of CBVA, GK and SVA play crucial roles in the decision-making for selecting two-level osteotomy. Specifically, preoperative CBVA > 63.9° and GK > 91.1° were identified as independent predictors for the recommendation of two-level modified PSO, with preoperative SVA > 25.4 cm serving as supplementary criteria. Furthermore, the selection of sites for two-level osteotomy was predominantly influenced by distinct preoperative parameters, including SVA, PI and TK.
Supplementary Information
Acknowledgements
Not applicable.
Abbreviations
- AS
Ankylosing spondylitis
- PT
Pelvic tilt
- PI
Pelvic incidence
- SS
Sacral slope
- LL
Lumbar lordosis
- TK
Thoracic kyphosis
- GK
Global kyphosis
- SVA
Sagittal vertical axis
- OVA
Osteotomized vertebral angle
- CBVA
Chin-brow vertical angle
- PSO
Pedicle subtraction osteotomy
- ROC
Receiver-operating characteristic curve analyses
- AUC
The area under the curves
- SRS-22
Scoliosis Research Society-22 questionnaire
- ODI
Oswestry Disability Index
Authors’ contributions
JZ L and TL W collected the radiographic and clinical data, ZL Y and YZ W measured parameters. JZ L and TL W analyzed the clinical data and measurements. JZ L wrote the manuscript, CG D and HR T revised the manuscript. HR T and JZ L conceived the idea and designed the study. All authors reviewed the manuscript.
Funding
This study was funded by National Natural Science Foundation of China (81970761), Shenzhen Medical Research Fund (D2402016), Shenzhen Major Project of Fundamental Research (JCYJ20200109114233670) and Sanming Project of Medicine in Shenzhen (SZSM201911011).
Data availability
No datasets were generated or analysed during the current study.
Declarations
Ethics approval and consent to participate
This retrospective study involving human data was in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. This study was approved by the Institutional Review Board of Shenzhen University General Hospital (KYLL-20230309 A). Informed consent was obtained from all patients included in the study.
Consent for publication
Written informed consent for publication of Fig. 2 was obtained from the patient.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Chunguang Duan, Email: duanchunguangdor@163.com.
Huiren Tao, 1910244002@email.szu.edu.cn, Email: huiren_tao@163.com.
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Associated Data
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Supplementary Materials
Data Availability Statement
No datasets were generated or analysed during the current study.