Sacral agenesis (SA), also known as caudal regression syndrome, is a rare congenital malformation of the sacrum, the pathology of which still remains uncertain.1–3 Patients with SA always present with sacroiliac joint deformity, and some of them also have combined progressive lumbosacral scoliosis such as hemivertebra. Therefore, surgical decision-making for patients with SA and lumbosacral scoliosis is challenging. The floating pelvis on the defect side may lead to spinopelvic instability with malalignment in the coronal plane and sometimes sagittal imbalance associated with the forward-bending trunk.2 A surgical approach should yield an aligned spine over the pelvis. Thus, how to reconstruct the stability of the lumbosacropelvic junctional region with spinopelvic fixation is the most important issue.
Spinopelvic fixation has been extensively performed in adult spinal deformities. Previous research has led to consensus regarding indications for spinopelvic fixation,4 including long fusion constructs or 3-column osteotomies, high-grade spondylolisthesis, lumbar deformity, and pelvic obliquity. Therefore, spinopelvic fixation is necessary to achieving solid fusion across the lumbosacral junction in such complex congenital deformities. In addition, pelvic fixation may also improve the patient’s walking function as well as the biomechanical stability of the implants.
S1 pedicle screws, iliac screws, and S2-alar-iliac (S2AI) screws are the most commonly used distal anchors in spinopelvic fixation. The advantages and disadvantages of each sacropelvic fixation have already been compared in many studies. S1 pedicle screws in the long spinal fusion had failure rates as high as 44%.5 Iliac screws had greater pullout strength and higher fusion rates at the lumbosacral region, but had more common complications of instrumentation prominence and pain.6 S2AI screws, a relatively new technique, offer many advantages to prior techniques in that they provide stable pelvic fixation that is in line with S1 screws and are less prominent, without the additional dissection required for traditional iliac screws.7 Despite many studies on the outcome of S2AI screws in adult spinal deformity, spinopelvic fixation with S2AI screws in patients with SA has to our knowledge not been previously reported. In this study, we aimed to evaluate the outcome of different distal fixation anchors in lumbosacral spinal deformities associated with SA and to determine the optimal pattern of spinopelvic fixation.
Methods
Cohort
This was a single-institution, retrospective study of SA patients with lumbosacral scoliosis who underwent surgical intervention from 2002 to 2017. Inclusion criteria were 1) diagnosis of SA associated with lumbosacral scoliotic deformities, 2) posterior instrumentation with distal anchors at the sacrum or pelvis, and 3) a minimum 2-year follow-up. Exclusion criteria were 1) coexisting deformities at the cervical or thoracic spine that needed to be corrected, 2) abnormalities of the hip joints and lower extremities, 3) severe neurological symptoms, 4) nonambulatory status, and 5) a history of prior spine surgery.
The patients were divided into three groups according to the different distal anchors: group 1 with S1 screws, group 2 with iliac screws, and group 3 with S2AI screws. The indications for surgery were progressive coronal imbalance and lumbosacral deformity. The decision of choosing different distal anchors was made on the basis of skeletal maturity and severity of the deformity and discussed by the spine surgeon team in our center. For young patients with moderate deformities, we chose S1 screws as distal anchors with short fusion levels to reduce the invasiveness of the surgery. For patients with severe deformities and/or combined coronal imbalance, pelvic fixation (with iliac or S2AI screws) with relative long fusion levels was necessary to achieve deformity correction. For pelvic fixation, we initially used iliac screws. Then we gradually transitioned to the low-profile S2AI screws after this use was proposed by Chang et al. in 2009.8 But under some conditions, such as complete unilateral SA, we had to choose an iliac screw due to the absence of a trajectory for S2AI screws. This study was approved by the institutional review board of our hospital. Informed consent was obtained from each subject.
Radiographic Measurements
Demographic data were obtained from our clinical database. Long-cassette standing posteroanterior and lateral radiographs of the entire spine were obtained preoperatively, immediately postoperatively, and at the last follow-up. The main curve (measured by the Cobb method), coronal balance (the distance between the C7 plumb line and the central sacral vertical line [C7PL–CSVL]), and pelvic obliquity angle (POA; measured between the iliac crest line and the horizontal line) were measured on patient radiographs to analyze the coronal alignment. All radiographical parameters were measured by Surgimap Spine Software (Nemaris Inc.). The pre- and postoperative radiographs were measured independently by two blinded authors, and the mean values of the measurements were used for further analysis. The implant-related complications were also reviewed.
Statistical Analysis
IBM SPSS 22.0 software (IBM Corp.) was used for statistical analysis. Demographic data and radiographic parameters were compared using one-way ANOVA, and p < 0.05 was considered statistically significant.
Results
In total, 24 SA patients (14 males and 10 females) were included in this study: 8 patients were stratified into group 1 (S1 screws), 9 into group 2 (iliac screws), and 7 into group 3 (S2AI screws). The mean age of all patients at the time of surgery was 10.2 years (range 3–25 years): 7.0 years in group 1, 11.4 years in group 2, and 12.5 years in group 3. The mean follow-up time was 3.6 years. The mean numbers of fusion segments were 3.5, 6.0, and 7.0, respectively, in groups 1, 2, and 3 (p < 0.05). The preoperative radiographic parameters were not different among three groups (Table 1). According to the Renshaw classification,9 18 cases were type I (partial or total unilateral SA) and 6 cases were type II (partial SA with bilaterally symmetrical defect). Among the study patients, 12 patients had SA combined with lumbar hemivertebra, 5 with failure of lumbar vertebral segmentation, 6 with tethered spinal cord, 2 with diastematomyelia, 2 with syringomyelia, and 4 with intraspinal lipoma. None of these patients showed neurological symptoms related to these intraspinal disorders, and after consultation with neurosurgeons, none of the patients underwent additional neurosurgery to deal with these conditions.
Comparison of demographic data and preoperative parameters in the patient groups
| No. of Patients | Age (yrs) | Follow-Up (yrs) | No. of Fused Segments | Main Curve (°) | Coronal Balance (mm) | Pelvic Obliquity (°) | |
|---|---|---|---|---|---|---|---|
| Group 1 | 8 | 7.0 ± 3.5 | 3.6 ± 1.5 | 3.5 ± 1.6 | 42.6 ± 9.2 | 20.4 ± 14.3 | 3.6 ± 2.4 |
| Group 2 | 9 | 11.4 ± 5.8 | 3.8 ± 2.2 | 6.0 ± 2.4 | 47.5 ± 16.2 | 20.1 ± 15.0 | 3.7 ± 1.9 |
| Group 3 | 7 | 12.5 ± 7.8 | 3.2 ± 1.6 | 7.0 ± 1.4 | 52.3 ± 15.5 | 17.8 ± 9.5 | 3.3 ± 1.8 |
| p value | — | 0.170 | 0.813 | 0.008 | 0.448 | 0.932 | 0.928 |
Values are presented as the mean ± SD unless otherwise indicated.
Deformity Correction
As shown in Table 2, in all three groups the major curve showed significant improvement after surgery (all p < 0.05): from 42.6° to 25.6° in group 1, from 47.5° to 21.1° in group 2, and from 52.3° to 27.7° in group 3. At the final follow-up, no significant loss of correction was found in each group. However, coronal balance showed a tendency to deteriorate during follow-up in patients with S1 screws (from 18.8 mm immediately postoperatively to 27.0 mm at the final follow-up). Pelvic obliquity showed significant correction in patients with both iliac (group 2) and S2AI (group 3) screws (from 3.7° to 2.3° in group 2 and from 3.3° to 1.6° in group 3), but remained unchanged after surgery with S1 screws (group 1) (p > 0.05).
Comparison of radiographic outcomes in the patient groups
| Preop | Postop | Follow-Up | F Value | p Value | |
|---|---|---|---|---|---|
| Main curve correction (°) | |||||
| Group 1 | 42.6 ± 9.2 | 25.6 ± 19.0 | 23.1 ± 16.9 | 3.694 | 0.042 |
| Group 2 | 47.5 ± 16.2 | 21.1 ± 10.8 | 24.4 ± 19.4 | 7.355 | 0.003 |
| Group 3 | 52.3 ± 15.5 | 27.7 ± 10.1 | 22.0 ± 9.9 | 10.609 | 0.001 |
| Coronal balance correction (mm) | |||||
| Group 1 | 20.4 ± 14.3 | 18.8 ± 9.2 | 27.0 ± 9.7 | 1.185 | 0.325 |
| Group 2 | 20.1 ± 15.0 | 19.4 ± 10.1 | 18.6 ± 11.3 | 0.038 | 0.963 |
| Group 3 | 17.8 ± 9.5 | 11.5 ± 6.0 | 12.3 ± 6.6 | 1.266 | 0.310 |
| POA correction (°) | |||||
| Group 1 | 3.6 ± 2.4 | 4.1 ± 2.5 | 4.2 ± 3.7 | 0.100 | 0.905 |
| Group 2 | 3.7 ± 1.9 | 2.3 ± 1.2 | 1.9 ± 0.8 | 4.374 | 0.024 |
| Group 3 | 3.3 ± 1.8 | 1.6 ± 0.6 | 1.5 ± 0.4 | 4.912 | 0.023 |
Values are presented as the mean ± SD unless otherwise indicated.
Complications
There were no major neurological or vascular complications related to the screws inserted in either group. In group 1, there were 3 cases of unilateral S1 pedicle screw misplacement. Loosening of the S1 screw was not observed during follow-up in group 1. In group 2, 3 patients encountered rod breakage, and 1 patient had a surgical site infection. In group 3, no implant failure was observed during follow-up.
Discussion
SA is a rare congenital spinal malformation and is complex for surgeons to deal with because of many comorbidities.10–12 To the best of our knowledge, this is the first article that describes the surgical outcome after spinopelvic fixation in patients with SA combined with lumbosacral scoliosis.
A limited number of cases with surgical management of SA with different types of distal fixation have been reported previously in the literature. Zhang et al.13 presented a case of SA with spinopelvic dissociation, but no lumbosacral scoliosis. The patient was treated with ilium-sacrum-ilium internal fixation and fusion in order to achieve sustained stability of the sacroiliac joint. Yazici et al.14 reported 3 cases with a total absence of sacrum and lower lumbar vertebrae. All of these patients underwent posterior lumbopelvic fixation with the aid of an anterior tibial cortical structural graft for laminopelvic bridging and obtained satisfactory correction and solid fusion. Furthermore, Ferland et al.15 described a novel type of spinopelvic fixation with bilateral vascularized rib grafts spanning the spine to the pelvis to provide long-term stability. This procedure was applied in 6 patients with SA, but failed in 4 patients, who then underwent revision surgeries. Unlike prior case reports, our study, for the first time, compared a large case series of SA patients with different types of distal fixation anchors. Our results showed that lumbosacral scoliosis in SA patients may be corrected effectively with spinopelvic fixation. However, S1 screws may have worse coronal balance maintenance and iliac screws may have more implant-related complications.
For patients using S1 screws as distal anchors, the aggravation of coronal malalignment postoperatively was more than our expectation. At last follow-up, the coronal balance distance (CBD) increased to 27.0 mm, while the preoperative and immediately postoperative CBDs were only 20.4 and 18.8 mm, respectively. It is informative that SA patients treated with S1 screws as distal anchors showed worsening trunk shift during follow-up. Previous studies have suggested that S1 screws, when used alone in long constructs, are prone to pullout failure or allow for motion, and thus increase the rates of pseudarthrosis, especially at the L5–S1 level.16 This means that placing anchors in the sacrum alone is frequently insufficient to maintain the coronal balance. In addition, for SA patients specifically, sometimes we could not find the traditional anatomical landmarks of the sacrum during surgery due to its dysplasia. In this condition, the entrance point for S1 screws may differ from the traditional one, and the trajectory of S1 screws was not standard. This altered trajectory may correlate with decreased biomechanical stability, followed by coronal imbalance (Fig. 1).
A 5-year-old boy was diagnosed with congenital lumbosacral deformities associated with SA (A–C). He underwent surgery with a distal anchor at S1. The postoperative image of a CT transverse section showed that the left S1 trajectory was not standard (D). After surgery, coronal imbalance occurred and the trunk shifted to the left side (E and F).
Regarding the improvement of pelvic obliquity, it was affected by another weakness of S1 screws. The POA was 3.6° before surgery, 4.1° after surgery, and 4.2° at follow-up, findings indicating that no improvement occurred with S1 screws. In comparison, both iliac and S2AI screws were associated with significant correction. Previous studies had reached a consensus that pelvic obliquity is an indication for pelvic fixation, and our results corresponded to this conclusion.4 Lee et al.17 made a comparison between S2AI and traditional iliac screws for pediatric neuromuscular deformity. These authors found no significant difference of correction of pelvic obliquity between groups (from 24° to 11° vs from 20° to 9°), and the same was true with our research (from 3.7° to 1.9° in group 2 and from 3.3° to 1.5° in group 3). Therefore, compared to pelvic fixation alone, S1 screws may not be optimal distal anchors in SA.
In terms of complications, there were 3 cases of rod breakage and 1 case of surgical site infection during follow-up, and all of them were in group 2, the iliac screw group (Fig. 2). Three patients who encountered rod breakage did not present with sagittal deformities, and the decreased biomechanical stability across the lumbopelvic junction may be the main reason for hardware failures. Remarkably, rod breakage after pelvic fixation with iliac screws in SA patients has been reported in other literature. In their case series, Balioğlu et al.11 reviewed 38 patients with SA, of whom spinal instrumentation had been used in 10 patients to manage severe scoliosis and/or kyphosis. These authors reported a case of a 14-year-old boy with T3–iliac instrumentation, and there was a rod breakage at the level of the osteotomy 2 years later. In contrast, no implant-related complications were observed in the S2AI screw group. Elder et al.18 compared clinical outcomes between S2AI and iliac screws in adult spinal deformities. They found that the S2AI screws had lower rates of reoperation, surgical site infection, wound dehiscence, and symptomatic screw prominence than the iliac screws. Hoernschemeyer et al.19 conducted biomechanical testing of S2AI screws versus traditional iliac screw fixation and demonstrated a trend toward greater stiffness in the S2AI screw cohort. Combining the experience from adult spinal deformities and our current results, we recommend the utilization of S2AI screws as distal anchors for SA patients with lumbosacral scoliosis (Fig. 3). But in some patients with large defects of the sacrum, we could not find a trajectory for the sacral screw and had to choose an iliac screw as a distal anchor during surgery instead.
A 3-year-old boy was diagnosed with SA combined with failure of L4–5 vertebral segmentation (A–C). He underwent spinopelvic fixation with iliac screws but encountered bilateral rod breakage and coronal decompensation 5 years later (D and E). Therefore, a revision surgery was performed and coronal balance was achieved again (F and G).
A 13-year-old girl was diagnosed with lumbar hemivertebra and SA (A and B). The 3D reconstruction CT image shows the distal absence of S3–5 vertebral segments (C). Posterior spinopelvic fixation and fusion were performed with bilateral S2AI screws (D and E). The coronal and sagittal balance was maintained well at the 2-year follow-up (F and G).
Our study had several limitations. First, it was a retrospective study with the inherent risk of data inaccuracy. Second, the sample size was relatively small due to the rarity of this disease. Third, our study focused on the correction of coronal deformities and did not analyze the sagittal profile. We tried to measure the sagittal parameters initially; however, we could barely find the accurate sacrum plate on the sagittal images owing to the complex deformity. Overall, the sagittal profile of our patients was acceptable and not the principal question. In addition, we mainly analyzed the radiographic outcomes, and clinical outcomes were not compared because many of our patients were too young to complete Scoliosis Research Society (SRS)–22 scores. Finally, SA itself is a heterogeneous entity due to the different morphological deficiencies possible for the sacrum. Thus, the heterogeneity in this cohort is an inherent part of this disease and hard to avoid.
Conclusions
Spinopelvic fixation is a safe and effective procedure that can achieve coronal correction in patients with lumbosacral scoliosis associated with SA. Compared with S1 and iliac screws, S2AI screws as distal fixation anchors can achieve a more satisfactory correction with fewer implant-related complications.
Acknowledgments
This work was supported by the Natural Science Foundation of Jiangsu Province (grant no. BK20180122) and the Special Funds for Health Science and Technology Development of Nanjing City (grant no. YKK18092).
Disclosures
The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
Author Contributions
Conception and design: Zhu, Bao, Liu, Sun, Wang, Qiu. Acquisition of data: Zhang, Shu, Qiu. Analysis and interpretation of data: Zhu, Zhang, Bao, Shu. Drafting the article: Zhang. Critically revising the article: Zhu, Liu, Sun, Wang, Qiu. Reviewed submitted version of manuscript: Zhu, Zhang, Bao, Liu, Sun, Wang, Qiu. Approved the final version of the manuscript on behalf of all authors: Zhu. Statistical analysis: Zhang, Bao. Administrative/technical/material support: Bao, Qiu. Study supervision: Wang, Qiu.
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