The correlations between the anchor density and the curve correction of adolescent idiopathic scoliosis surgery

Pedicle screw-based instrumentation has gained popularity in surgery of adolescent idiopathic scoliosis (AIS) patients in the past twenty years [1‐3]. From a mechanical standpoint, it provides three columns fixation of the vertebrae and shows a better curve correction for AIS patients. However, increased anchor density may lead to higher implant costs, risks of implant malposition, and blood loss [4‐6].

AIS curves are complex three-dimensional deformities that require adequate correction in all three dimensions, including coronal, sagittal and axial planes. Controversies still exist in determining the relation between anchor density and curve correction in all three dimensions [7‐17]. In a recent consensus, most experienced panelists agreed that less than 1.60 of the anchor density was optimal in treating AIS with smaller major curve (40°-70°) [18]. The lower safe limit of anchor density to maintain long-term adequate correction remains to be determined.

The main purpose of the present study is to determine the relations between anchor density and curve correction in coronal, sagittal and axial planes of adolescent idiopathic scoliosis surgery.

A total of 127 patients (17 males and 110 females) aged 14.4 years old at the time of surgery were enrolled in the present study. The numbers of Lenke type 1, 2, 3, 4, 5 and 6 curve patients were 59, 19, 12, 6, 22 and 9, respectively. The mean anchor density of all patients was 1.60 (ranging from 1.14 to 2.0). The demographic data and radiographic parameters for all patients and Lenke 1–6 subgroup patients are shown in Table 1.

Pedicle screws instrumentation and posterior fusion have become the treatment of choice in AIS surgery [1‐3]. Although, with the advancement of pedicle screw insertion techniques and electroneurophysiological monitoring systems, neurologic complications, increased intraoperative blood loss and implant costs remained a concern for spinal surgeons [4‐6]. Correcting scoliotic deformity in the coronal plane is one of the earliest established surgical goals for AIS patients. The correlation of anchor density and AIS coronal curve correction has been widely studied and reported over the past decades. However, studies have shown contradictory results regarding the correlation of anchor density and coronal curve correction in AIS surgery [7‐17].

Several studies have shown positive correlations between AIS coronal curve correction and anchor density. Clements et al. [7] reviewed 250 major thoracic and 42 major lumbar curves within all 6 Lenke types and observed weak but significant correlations with mixed types of implants. Yang et al. [9] proposed a similar weak correlation within 58 Lenke 1A and 1B patients. Chen et al. [11] further demonstrated a mild correlation (r = 0.43, P <  0.05) between thoracolumbar/lumbar curve correction and anchor density in 39 Lenke 5 AIS patients. A large series study that consisted of 584 Lenke 1, 245 Lenke 2 and 123 Lenke 5 AIS patients was conducted by Larson et al. [13] The results showed that the high anchor density (AD ≥1.54) group has a significantly better coronal curve correction than the low anchor density (AD < 1.54) group in Lenke 1 and Lenke 2 patients, and the significant difference continued to exist within postoperative 2 years. However, Lenke 5 patients did not demonstrate a significant difference in coronal curve correction between high- and low-density groups in the same study. Two studies in 2016 showed support for a positive correlation between coronal curve correction and anchor density. Ketenci et al. [15] studied 76 matched Lenke 1 patients and equally divided these patients into the consecutive pedicle screw group (mean anchor density 2) and the interval pedicle screw group (mean anchor density 1.14). A significantly better coronal and rotational correction of thoracic curve was observed in the consecutive pedicle screw group. Nevertheless, Mac-Thiong et al. [17] reviewed the coronal main curve and main thoracic curve correction of 137 AIS patients. The results showed that the curve correction of anchor density < 1.4 was significantly inferior to the curve correction of anchor density ≥ 1.8, whereas the curve correction of anchor density between 1.4 to 1.8 showed comparable results to an anchor density ≥ 1.8.

In contrast, several studies reported no correlations between coronal curve correction and anchor density. Quan et al. [8] reviewed 49 Lenke 1 patients and found no correlation between anchor density and coronal curve correction. Bharucha et al. [10] divided 91 Lenke 1 patients into 34 high-density and 57 low-density patients based on mean anchor density (1.3), and no differences of curve correction, thoracic kyphosis or apical thoracic rotation were observed. Gebhard et al. [12] also found no correlation between main thoracic curve correction and anchor density within 119 AIS patients. Rushton et al. [14] demonstrated no correlation between correction of coronal curve, thoracic kyphosis or lumbar lordosis in 106 AIS patients (78 Lenke 1). Kempaninem et al. [16] compared 26 high-density (mean anchor density 1.68) AIS patients with 26 low-density (mean anchor density 1.28) AIS patients and found no differences in major MT curve correction, coronal balance, sagittal balance or apical vertebral translation.

Our study results were more compatible with studies reporting no correlation between anchor density and coronal curve correction [8, 10, 12, 14, 16]. The coronal curve correction parameters in our study were not correlated with anchor density in all patients subgroups, with the account of different curve types, curve magnitudes and curve flexibilities. (Tables 2 & 3) Similarly, there were no differences between low-density, middle-density and high-density groups for coronal curves correction. (Table 4).

Spinal surgeons were more aware of the correction of thoracic kyphosis and lumbar lordosis since the introduction of three-dimensional correction concept of AIS surgery [1, 23]. Furthermore, hypokyphosis has been associated with pulmonary function compromise and risk of future proximal junctional kyphosis in AIS patients [24, 25]. Maintaining adequate lumbar lordosis is also essential to balanced sagittal profiles in long-segments instrumented spinal surgery [26]. Larson et al. [13] found high anchor density was associated with postoperative hypokyphosis in Lenke 1,2 AIS patients, while several other studies reported no correlations between anchor density and thoracic kyphosis/lumbar lordosis correction [14, 15, 17].

In our study, mild positive correlation existed between anchor density and thoracic kyphosis correction in all patients, Lenke 1 patients, and Lenke 1–3 patients. (Table 2) When divided into low-density, middle-density, and high-density groups, the differences in thoracic kyphosis correction was accompanied by fused level differences. (Table 4) Post-hoc Scheffé tests revealed that with about two addition fusion levels, the thoracic kyphosis correction was about 9° less in low-density group, as compared to high-density group. (Table 5) However, the thoracic kyphosis corrections were about 10° less with no significant differences in fusion level in low-density group when comparing to other two groups in Lenke 1 patients.

Several factors influence thoracic kyphosis corrections included preoperative thoracic kyphosis, main thoracic curve flexibility, anchor density of concave side, rod materials and facetectomy levels [27‐29]. Although statistical radiographic differences of thoracic kyphosis correction were observed between low-density and other two groups, clinical significances of thoracic kyphosis correction were not reached since the mean differences were only 5°-10° between the three groups.

Studies have shown no significant correlations between anchor density and lumbar lordosis in instrumented thoracolumbar/lumbar curves [11, 13], which is compatible to our study in both Lenke 5 and patients with major TL/L curves ± other minor structural curves (Lenke 5–6). (Table 2) For patients with instrumentations in both structural thoracic and structural thoracolumbar/lumbar curves (Lenke 3,4,6), anchor density was not correlated with thoracic kyphosis (r = − 0.12, p = 0.55) and lumbar lordosis (r = − 0.22, p = 0.27) based on our study statistics.

The correction of the axial rotation of the deformed vertebrae is one of the key components of AIS surgery. Few studies have examined the relationship between anchor density and the rotational correction of axial plane. Bharucha et al. [10] reported no differences of trunk rotation correction between anchor density above or below 1.3 (mean anchor density of all 91 patients). Whereas Ketenci et al. [15] found consecutive pedicle screws construct (anchor density 2.0) has significant better apical vertebral rotation correction over interval pedicle screw construct (anchor density 1.14). In our study, there was no correlation between anchor density and apical vertebral rotation in all patients and all subgroups of different curve types, curve magnitudes and curve flexibilities. (Tables 2 & 3) Nevertheless, no differences were observed between low-density, middle-density, and high-density groups in terms of apical vertebral rotation correction (p = 0.60) in all patients. (Table 4).

To investigate the effect of anchor density in different curve magnitudes and flexibilities, Pearson correlation coefficients between anchor density and curve correction were calculated in small curves (40°-60°), large curves (> 60°), stiff curves (flexibility ≤40%) and flexible curves (flexibility > 40%) (Table 3). From the literatures [30, 31], the flexibility assessed by supine bending views were averaged more than 40%, which was set as the cutoff value between stiff curves and flexible curves in our study. In large and stiff curves, no correlations between anchor density and all curve correction parameters were observed. While thoracic kyphosis was positively correlated with anchor density in small curves (r = 0.38, p <  0.001) and flexible curves (r = 0.34, p = 0.01). Therefore, for small and flexible curves, increase the anchor density may result in more thoracic kyphosis correction. But the difference may not be obvious when the anchor density exceeds more than 1.7. However, for large and stiff curves, the anchor density was not correlated with all curve correction parameters, other correction techniques or longer fusion length were typically utilized to achieve adequate correction.

There were several limitations of this study, including the retrospective nature and lack of patient-reported outcome evaluations. The patients’ standing postures were not standardized in erect whole spine lateral view, which resulted in interpretation and reporting bias. Sagittal radiographic parameters were easily influenced by the patients’ positioning and motion [32, 33]. In addition, evaluating the axial vertebral rotation with the Nash-Moe method may not reflect the true axial rotation, since the classification only divides the rotations into four categories [22]. Further prospective studies and long-term surgical outcome comparisons can provide stronger evidence to clarify the true relationships between three-dimensional curve correction and anchor density.

In our study, the anchor density was not related to coronal or axial curve corrections. Mild positive correlations with anchor density were found in thoracic kyphosis correction, especially in patients with smaller and flexible curves. Low anchor density with longer fusion level achieves similar curve corrections with middle or high anchor density in adolescent idiopathic scoliosis surgery. Therefore, spinal surgeons should consider the influences of anchor density on correcting deformities when planning the distributions of implants preoperatively.