Impacts of resection level and baseline deformity features on upper lumbar pedicle subtraction osteotomy closure ratio in ankylosing spondylitis-related thoracolumbar kyphosis

Abstract

Background: Pedicle subtraction osteotomy (PSO) is an unique technique widely applied in the correction of thoracolumbar kyphosis arise from ankylosing spondylitis (AS). During the osteotomy closure procedure, both the upper part of the osteotomized vertebra and the lower part of that contributed to the spinal realignment jointly. However, the relative proportion of truncal and pelvic sagittal correction which occurs with the closure of PSO has not been detailed described in AS. This study aims to investigate whether the upper lumbar osteotomy level and baseline deformity characteristics impact the distribution of global correction.

Methods: 73 AS patients who underwent single-level lumbar PSO were reviewed. Radiographic parameters included sagittal vertical axis (SVA), spino-sacral angle (SSA), global kyphosis (GK), spinal tilt (ST) and T1 pelvic angle (TPA), thoracic kyphosis (TK), lumbar lordosis (LL), pedicle subtraction angle (PSA), osteotomized vertebra angle (OVA), pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). The truncal and pelvic closures were calculated by the change of superior or inferior endplate angle of the osteotomized vertebra. Besides, in accordance with Diebo’s project, all patients were divided into three groups based on the ratio of osteotomy closure (ROC). The impact of preoperative radiographic parameters and upper lumbar PSO level on the truncal versus pelvic closures were also investigated.

Results: No significant difference between ROC and osteotomy level was observed (P=0.155>0.05). Furthermore, there were significant differences in SVA (P<0.001), PI (P<0.001), PT (P<0.001) and ST (P<0.001) among three groups. Preoperative SVA (r=0.581, P<0.001) was positively correlated with ROC while the PI (r=-0.510, P<0.001), PT (r=-0.547, P<0.001) and ST (r=-0.517, P<0.001) were negatively correlated with ROC. Significant improvement of all the spinopelvic parameters except TK and PI were noted postoperatively. The correction of SVA (r=0.692, P<0.001) and ST (r=0.629, P<0.001) was positively correlated with ROC, whereas the variation of PT (r=-0.570, P<0.001) and SS (r=-0.461, P<0.001) was negatively correlated with ROC.

Conclusion: The osteotomy level is not the driver in the ratio of osteotomy closure but the baseline deformity features. Specifically, AS patients with preoperative larger PT realized a proportionally greater pelvic closure (improvement of pelvic retroversion) and less correction of SVA. Additionally, it’s important to get more pelvic closure as much as possible when closing the osteotomy gap for AS patients with higher PI.

Background

Ankylosing spondylitis (AS) is an immune-mediated chronic rheumatic inflammatory disease that commonly affecting the axial skeleton[1]. It might cause progressive heterotopic ossification of the spinal ligaments and eventually lead to inflexible thoracolumbar kyphosis (TLK), restricting patient’s daily living activities[1, 2]. Therefore, surgical intervention is recommended for cases with severe kyphosis to improve patient’s functional status[3]. So far, it has been well documented that pedicle subtraction osteotomy (PSO) can effectively correct thoracolumbar kyphosis caused by AS through realignment of the spino-pelvic complex, irrespective of the rigidity of spinal curvature[3–6].

Having a better knowledge of PSO realizing the restoration of sagittal imbalance is warranted eagerly. Although the correlation between the correction amount and the osteotomized angle is apparent, the relationship between the magnitude of correction and the osteotomy level remains debated. Several scholars reported that the lower the osteotomized vertebra (OV) performed, the greater correction of translating C7 SVA posteriorly[7–9]. On the contrary, Lafage et al.[10] discovered that the osteotomy level didn’t affect the absolute sagittal correction, while more caudal osteotomy level correlated with higher correction of pelvic retroversion. Actually, during the osteotomy closure procedure, both the upper part of the OV and the lower part of that contributed to the spinal realignment collectively, making the patient’s pelvis and shoulder be in the same horizontal line rather than full truncal or pelvic closure.

More recently, Diebo et al.[11] described the relative proportion of truncal (SVA) and pelvic correction (PT, pelvic tilt) which occurs after three-column osteotomy closure in adult spinal deformity (ASD) patients, thus the conception of ratio of osteotomy closure (ROC) was introduced. In their series, the ROC was negatively correlated with the osteotomy level (r=-0.259, P = 0.006). However, to our knowledge, the distribution of truncal and pelvic correction was still unknown in AS patients following PSO. Unlike ASD patients, the adult AS patients presented rigid thoracolumbar kyphosis with hardly thoracic compensated adaptation. Moreover, AS-related kyphosis usually underwent PSO at upper lumbar spine (L1-L3) to correct the kyphotic deformity and to obtain a harmonious lumbar lordosis profile, which might be quite different from ASD[12, 13]. Hence, the purpose of this study was to investigate whether the upper lumbar spine PSO level and baseline deformity natures impact the relative distribution of truncal and pelvic sagittal correction.

Methods

Results

A total of 73 patients (7 females and 66 males) with an average age of 34.6 years (range, 21–59 years) were included (Table 1). There were 38 patients with a mean age of 32.9 years in the TC group, 22 patients with an average age of 34.5 years in the EC group and 13 patients with a mean age of 39.8 years in the PC group. There was no statistical significance in the age among three groups (P = 0.111). The mean follow-up duration was 3.13 years, with no significant difference in the three groups (P = 0.139), so did the gender distribution (P = 0.728). For fused levels, no patients underwent spinopelvic fixation in this study. Each patient was treated with a single-level PSO in the upper lumbar spine (L1-L3). The distribution of osteotomy level in the three groups is listed in Table 1, and there was no statistical significance (P = 0.257).

The average ROC value was 57.89%/42.11%, which means 57.89% of the osteotomy closure applied in the truncal restoration (SVA) and 42.11% of that was used in pelvic retroversion (PT). ROC was not significantly different among osteotomy levels (P = 0.155, Table 2). However, there was a increasing tendency of truncal closure and decreasing tendency of pelvic closure when osteotomy level changed from cranial to caudal.

The analysis of preoperative radiographic parameters among closure groups revealed that there were significant differences in SVA (P < 0.001), PI (P < 0.001), PT (P < 0.001) and ST (P < 0.001) across three groups (Table 3). The patients in the TC group had a significantly greater pre-op SVA than patients in EC, which in turn had larger pre-op SVA than patients in the PC group. As for pre-op PT and ST, the opposite results could be observed. Besides, the patients in the PC group had a significantly higher PI than patients in the TC group (P < 0.001). Remarkable improvement was noticed in sagittal parameters (GK, SSA, SVA, LL, PT and SS) after PSO in all three groups (P < 0.05). The TLK was corrected from an average of pre-op GK of 69.9° to 23.1° in TC group, from 65.1° to 23.5° in EC group and from 75.7° to 36.3°, respectively. No significant loss of correction was observed at the final follow-up. The Pearson correlation analysis revealed that pre-op SVA (r = 0.581, P < 0.001) was positively correlated with ROC while the pre-op PI (r=-0.510, P < 0.001), PT (r=-0.547, P < 0.001) and ST (r=-0.517, P < 0.001) were negatively correlated with ROC (Table 4). Furthermore, the improvement of SVA (r = 0.692, P < 0.001) and ST (r = 0.629, P < 0.001) was positively correlated with ROC, and the variation of PT (r=-0.570, P < 0.001) and SS (r=-0.461, P < 0.001) in perioperation was negatively correlated with ROC.

Discussion

As the course of the disease progress, the rigid TLKD of AS patients may deteriorate, causing the center of gravity shifts anteriorly accordingly, which triggers backward pelvic rotation associated with the maximal hips’ extension to compensate for the sagittal malalignment[4, 12, 15]. Currently, PSO has been widely recognized as the effective and preferable technique to reconstruct sagittal alignment, placing the pelvis in a neutral position and restoring the ability to see ahead[4, 8–10, 16, 17]. In operation, the closure of osteotomy gap is obtained by collective rotation of the cranial and caudal vertebral column at the hinge[17]. Therefore, suitable adjustment of ROC should be taken into consideration based on the individual preoperative deformity features or osteotomy level.

In the surgical decision-making of one-level PSO for AS, the option of the ideal osteotomy level was a pivotal factor contributing to the results of correction. There were usually two options being suggested: osteotomy at the apical vertebra or osteotomy distal to apex[3, 18]. Generally speaking, the apical vertebra was at the angular pathology where it was easiest to operate in the prone position, and a favorable sagittal thoracolumbar curve could be obtained. However, most osteotomies tend to be performed over more caudal vertebra for the followed reasons: first, it decreased the incidence of injury to the neural elements; second, a more caudal osteotomy level might generate more profound correction in AS patients, due to a larger part of trunk being moved more posteriorly. In AS patients, thoracolumbar and lumbar regions are usually the most impacted regions by kyphosis, and the junction of superior and inferior body and the stress concentration in prone position commonly lied at L2 or L3, so facilitating easier and more efficient correction following osteotomy. Furthermore, for AS patients with severe TLK deformity, the goal of PSO is not only the reconstruction of LL but also the improvement of kyphotic deformity to avoid the postoperative inability of lying flat, which was distinct from ASD patients. As Chen et al[3] observed, the maximal correction might be achieved when performing PSO at L2 or L3 in AS-related thoracolumbar kyphosis. Diao et al[18] also pointed out that patients with lordotic lumbar profiles are suitable for L1/L2 PSO while L2/L3 PSO is appropriate for patients with kyphotic lumbar profiles. Hence, the upper lumbar vertebra (L1-L3) was the most commonly used osteotomy site, and currently this study mainly focused on the impact of the upper lumbar PSO level on ROC.

There has been a general assumption that a greater OVA might achieve more correction amount, whereas the relationship between the osteotomized level and the magnitude of correction has been debated extensively[8–10, 19]. Several studies revealed that a more caudal osteotomy level might generate more profound correction in AS patients, due to a larger part of trunk being moved more posteriorly[7–9]. While Lafage et al[10] confirmed that the level of osteotomy correlated with the variation of PT in a line fashion, which didn’t impact the absolute sagittal correction (SVA). In the present study, the average ROC value was 57.89%/42.11%, which means 57.89% of the osteotomy closure applied in the truncal restoration and 42.11% of that was used in pelvic retroversion. Nevertheless, there was no significant difference between the osteotomy level and ROC (P = 0.155, Table 2). Specifically, a more caudal osteotomy level didn’t decrease the percentage of truncal closure, which was different from the results of Diebo et al for ASD patients[11]. Because the rebalance of the spine couldn’t be simplified as a “lever arm” above or beyond the PSO site but involved the spontaneous pelvic adaptation[20]. However, for AS patients with limited hip function or complete ankylosed hip joint, the correction of TLKD is accomplished by the closure of upper part of the OV absolutely due to the difficulty of postoperative adaptive change of pelvis. Hence, osteotomy distal to apex could be considered to avoid the risk of negative spinal imbalance caused by excessive osteotomy amount. In a general way, the selection of OV is not as straightforward and the option is based on other factors, such as the preoperative sagittal lumbar profile, the location of kyphotic apex, hip joint function and surgeon’s preference[3, 18].

The role of osteotomy level itself is not the driver for ROC based on the present findings. Therefore, surgeons should pay more attention to preoperative characteristic deformity. For example, Van Royen et al[9] adopted the combined use of SVA, CBVA and SS for planning corrective osteotomies in AS. However, this method was not individual since the SS was set as an identical value for each patient. Subsequently, Min et al[21] reported the significance of the whole-body kyphosis angle in surgical planning but ignored the role of pelvic parameters. Similarly, Le Huec et al[22] described a method based on a global evaluation of the whole-body balance to predict the desired correction amount for an appointed patient, whereas Le Huec et al didn’t consider the osteotomy level. The above-mentioned methods either undervalued the response of pelvis adapted to the sagittal imbalance or achieved equivalent pelvic correction regardless of osteotomy level. Song et al[23] in 2013 defined postoperative PT on the basis of an equation proposed by Vialle and calculated the needed OVA for individual patient. It’s worth noting the method raised by Song et al considered both pelvic compensation and spinal alignment[23].

Further analysis of patients by closure group (TC, EC, PC) disclosed that there were statistical significances among three groups concerning preoperative SVA, ST, PT and PI (Table 3). The patients (Figure. 2) in the TC group presented with a significant greater pre-op SVA than patients in the EC group which in turn had a significant greater SVA than patients (Figure. 3) in the PC group. The inverse result could be noticed in regard to PT and ST. This phenomenon revealed that the patients with more pelvic retroversion (larger PT), up to a desired SVA, achieved more pelvic closure to eliminate the compensation of pelvis and lower extremities as much as possible, as illustrated in Table 4. Theoretically, AS patients with a high PI would be more able to compensate for TLKD by regulating pelvic retroversion than one with low PI, as the maximal retroversion produced by a high PI pelvis is superior to a low PI one. However, Roussly et al[5] observed that the mean value of SS was easier to reach 0° in low PI than in high PI. They accredited this observation to the restricted hip extension which limited a greater backward rotation of the pelvis. Qian et al[24] investigated the impact of preoperative sagittal parameters on the postoperative sagittal alignments and revealed that there was more likely trend toward failed sagittal realignments for AS patients with either larger preoperative SVA or PI following one-level lumbar PSO. Similarly, Schwab et al[25] concluded that ASD patients with failed realignments had greater preoperative spinopelvic deformity (large SVA, PI and PT). Apparently, AS patients with larger PI require more correction of LL to generate the harmonious relationship between the pelvis and the spine. The current study demonstrated that patients with larger PI (TC vs. PC, P < 0.001) achieved more pelvic closure (Table 4) and each group of patients obtained satisfactory postoperative sagittal alignments. Therefore, it’s essential to reach a more elevation of lower extremities for AS patients with higher PI intraoperatively.

There are several limitations of the present study. Firstly, the patient might not stand in a standard neutral postures due to the patients’ functional recovery period when taking radiographic imaging postoperatively. Secondly, the present study couldn’t take parameters of lower extremities into consideration on account of incomplete imaging of lower extremities, whereas it is worthy of further intensive study.

Conclusions

This study enhances the comprehension of these involved interactions that the proportion of global correction is not based on the osteotomy level but on the preoperative deformity characteristics in AS patients following one-level lumbar PSO. Accordingly, AS patients presented with a preoperative larger SVA and less pelvic compensation obtained more improvement of truncal position after PSO. Furthermore, for AS patients with high PI, achieving more pelvic closure as much as possible when closing the osteotomy gap is vital to reconstruct suitable sagittal alignment involves harmonization with pelvic profile and spinal shape.

Abbreviations

PSO (pedicle subtraction osteotomy)

AS (ankylosing spondylitis)

SVA (sagittal vertical axis)

SSA (spino-sacral angle)

GK (global kyphosis)

ST (spinal tilt)

TPA (T1 pelvic angle)

TK (thoracic kyphosis)

LL (lumbar lordosis)

PSA (pedicle subtraction angle)

OVA (osteotomized vertebra angle)

PI (pelvic incidence)

PT (pelvic tilt)

SS (sacral slope)

ROC (ratio of closure)

TLK (thoracolumbar kyphosis)

ASD (adult spinal deformity)

SOVA (superior endplate angle of the osteotomized vertebra angle)

IOVA (inferior endplate angle of the osteotomized vertebra angle)

Declarations

Compliance with Ethical Standards:

All methods were performed in accordance with the relevant guidelines and regulations.

Ethics approval and consent to participate

The Drum Tower Hospital’s review board approved this study and all participating subjects signed the written informed consents.

Consent for publication

Not applicable.

Availability of data and material

All data generated or analyzed during this study are included in this article. We confirm that the availability of data and materials refers to the raw data generated and used for this study. Meanwhile, the datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests.

Funding

This work received funding from Jiangsu Provincial Key Medical Center (YXZXA2016009).

Author’s contributions

CX: Data collection, data analysis, drafting paper, preparing figure 1-3, paper modification. QBP: Proposed idea, paper modification. QY: Paper modification. YY: Paper modification. WB: Paper modification. All authors read and approved the final manuscript.

Acknowledgements

Not applicable

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