In the current study, 47.1% of the patients (72/153) presented DPSL followed posterior decompression and instrumented fusion for DLS, we found that low BMD, large correction of both main curve and fractional curve, posterolateral fusion were significantly and independently associated with the occurrence of DPSL, and can be assessed before surgery, these results were not confounded by other variables potentially affect DPSL.
A critical point for the investigation of screw loosening is the assessment of whether a screw is loosened or not, planar radiographs are most commonly used, whereas a significant minority of papers employed clinical CT or low-dose CT [19–21]. Potential health risks associated with increased radiation exposure as well as financial costs limit postoperative CT use to selected patients with persistent symptoms or where there is specific concern regarding the possibility of implant-related complications . Wu et al. proved that CT might not be superior to X-ray in assessment of screw loosening, especially considering its higher cost and radiation exposure, as X-ray had a sensitivity of 24% and a specificity of 98%, while CT scan had a sensitivity of 22% and a specificity of 95% regarding extraction torque as criterion of screw loosening . Sanden et al. reported that a radiolucent zone around a PS is a good indicator of screw loosening . That is the reason why we chose plain radiographs to evaluate pedicle screw loosening in the present study.
Since its first description by Watkins in 1953, the technique of posterolateral fusion (PLF) has evolved into the widely adopted method of lumbar arthrodesis for degenerative disease . Tsirikos et al. proved that posterolateral arthrodesis in situ with autologous iliac crest bone without instrumentation achieved a solid fusion and produced excellent clinical outcomes and high patient satisfaction . Moreover, the use of pedicle screw fixation to support the PLF could achieve fusion rates as high as 96% [25–27]. Addition of interbody fusion via a transforaminal approach (TLIF) has become a popular surgical option for treatment of degenerative lumbar conditions. Although technically more complicated than PLF, it has been suggested that TLIF provides superior immediate stability and protects against early pedicle screw loosening. Kim et al proved that TLIF appears to have a protective effect in reducing rates of early screw loosening by approximately 60% versus PLF . A retrospective study comparing the outcomes between TLIF and PLF found that the PLF construct was more prone to induce instrument breakage and pseudoarthrosis compared with the TLIF technique, indicating the importance of anterior column support in the maintenance of construct stability . In recent years, TLIF has replaced PLF to be the most commonly used lumbar fusion surgery . However, postoperative screw loosening was not completely avoided, especially in patients undergoing long segment pedicle screw fixation . Results from the current study suggest that supplementing the TLIF with PLF potentially reduces the rates of DPSL followed corrective surgery for DLS. Following decortication of transverse processes and posterolateral bone, morselized local autograft was placed in the posterolateral intertransverse space in order to create posterolateral arthrodesis, which could reduce the segmental motion of posterior spinal column, and minimize the risk of screw loosening.
It has been widely accepted that osteoporosis is a potential risk factor for pedicle screw loosening. Fixation failure rates due to screw loosening range from 1–15% and can rise in osteoporotic spines for which the strength of fixation with pedicle screws decreases along with lower bone density [6–8]. In a study of osteoporotic cadavers, pedicle screw pullout strength was found to be highly correlated with BMD . BMD measuring with DEXA scans could not perform differentiation of the cancellous bone from the cortical bone, T-scores can be overestimated due to abdominal vessel wall calcification, degenerative bony spurs, and facet hypertrophy [32, 33]. It has been proved that older patients or patients with degenerative lumbar scoliosis are more likely to have unreliable lumbar T-scores . Measuring HU from computed tomography has been proposed as a useful technique for assessing vertebral bone quality, the reliability and accuracy of HU to determine osteoporosis have been proved in the literature with many reports [35–38]. In the current study, our findings are consistent with Jan et al that the determination of bone density by HU value could predict the risk of screw loosening and inform the decision to use cement augmentation to reduce the incidence of screw loosening . Screw augmentation with bone cement as well as expandable screws demonstrated improved screw pullout stability, better fixation strength and clinical results in comparison with standard pedicle screws in human cadaver material in vitro [40–43].
The typical major curve of DLS always lies in the mid-lumbar spine, it is always compensated by a distal curve at the lumbosacral region as the body attempts to maintain coronal balance, named the fractional curve [44, 45]. Both pedicle screws compression at the convex side and pedicle screws distraction at the concave side could achieve correction of the main curve and fractional curve [46, 47]. No matter which technique is used, the cutting effect of pedicle screw on bone should not be ignored. The more scoliosis correction, the greater the cutting effect of pedicle screw on bone, and the greater the risk of pedicle screw loosening. Based on our previous study, we put forward two suggestions. Firstly, asymmetrical vertebral degeneration in DLS patients is manifested as high HU value within concavity and low HU value within convexity both in major and fractional curve. Concave side of the vertebrae theoretically withstand more cutting forces than convex side based on the HU value measurement, distraction of pedicle screws at concave side should be a priority to correct both main curve and fractional curve . Secondly, inadequate release of the posterior spinal bone elements in PLIF or TLIF technique may hinder the correction of the fractional curve, because the lumbosacral junction tends to be particularly rigid and may already be fused into an abnormal position. Posterior column osteotomy facilitate effective correction of the scoliosis through complete release of dural sac as well as the asymmetrical intervertebral reconstruction by cage is recommended .
There are several potential limitations in this study. First, the number of patients is relatively small, and the study may be under powered to detect the significance of some risk factors. Second, the study was conducted retrospectively by case selection, and was not randomized and controlled, the subjects selected are all Chinese Han individuals, whether the conclusion is applicable to other ethnic groups needs to be further investigated in the future. Even with these issues, three suggestions are provided for surgical planning based on the findings in the current study. Firstly, screw augmentation with bone cement or expandable screws is recommended for patients with low BMD measured from preoperative CT scan. Secondly, posterior column osteotomy plus unilateral cage strutting technique on the concavity is recommended for scoliosis correction, screw distraction instead of compression should be preferred. Thirdly, posterolateral fusion is recommended for DLS patients that underwent posterior long segment fusion.