The use of a transition rod may prevent proximal junctional kyphosis in the thoracic spine after scoliosis surgery: a finite element analysis

Comparison Between Resident and Attending Surgeons as Assistants on Adolescent Idiopathic Scoliosis Surgery: No Differences in Outcomes, Complications Rate, or Pedicle Screw Placement Accuracy

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

The aim of the present study was to determine if the level of training of the first assistant (resident or attending surgeon) has an influence on the radiographic outcome of AIS surgery and on the accuracy rate of the pedicle screws placement.

SUMMARY OF BACKGROUND DATA

Adolescent idiopathic scoliosis (AIS) surgery is a challenging procedure that requires a dedicated team of skilled professionals. Therefore, understanding the learning curve is of outstanding importance to guarantee the best outcomes and the highest safety to the patients.

METHODS

A retrospective analysis of patients who underwent surgery for AIS with a minimum follow-up of 2 years was conducted. All patients were operated by an experienced spine surgeon, assisted by and attending surgeon (group A) or a senior resident (group B). Radiographic outcomes were assessed. Through postoperative CT scan, accuracy of pedicle screw placement was measured (using Gertzbein-Robbins classification). Groups were then statistically compared.

RESULTS

A total of 120 patients were included (mean age 15.3±3.39 y, major curve Cobb 60.7±11.9 degrees). No difference was found between groups in terms of preoperative characteristics and postoperative radiographic (correction rate, thoracic kyphosis, screw density) outcomes or complications. Operative time and estimated blood loss were significantly higher in Group B (P=0.045 and P=0.024, respectively). Of the 2746 pedicle screws inserted (1319 group A and 1427 group B), 2452 had a perfect intrapedicular trajectory (absolute accuracy of 89.29%) and 2697 had a breach <2 mm (relative accuracy of 97.56%). No difference was found among groups in terms of absolute or relative accuracy (P=0.06 and P=0.23, respectively).

CONCLUSIONS

AIS cases assisted by senior residents have longer operative time and higher blood loss, but this does not negatively affect the overall radiographic outcome and does not place the patient at increased risk of complications.

LEVEL OF EVIDENCE

III.

Can We Rely on Prophylactic Two-Level Vertebral Cement Augmentation in Long-Segment Adult Spinal Deformity Surgery to Reduce the Incidence of Proximal Junctional Complications?

Background and Objectives: Proximal junctional kyphosis (PJK) and failure (PJF), the most prevalent complications following long-segment thoracolumbar fusions for adult spinal deformity (ASD), remain lacking in defined preventive measures. We studied whether one of the previously reported strategies with successful results-a prophylactic augmentation of the uppermost instrumented vertebra (UIV) and supra-adjacent vertebra to the UIV (UIV + 1) with polymethylmethacrylate (PMMA)-could also serve as a preventive measure of PJK/PJF in minimally invasive surgery (MIS). Materials and Methods: The study included 29 ASD patients who underwent a combination of minimally invasive lateral lumbar interbody fusion (MIS-LLIF) at L1-2 through L4-5, all-pedicle-screw instrumentation from the lower thoracic spine to the sacrum, S2-alar-iliac fixation, and two-level balloon-assisted PMMA vertebroplasty at the UIV and UIV + 1. Results: With a minimum 3-year follow-up, non-PJK/PJF group accounted for fifteen patients (52%), PJK for eight patients (28%), and PJF requiring surgical revision for six patients (21%). We had a total of seven patients with proximal junctional fracture, even though no patients showed implant/bone interface failure with screw pullout, probably through the effect of PMMA. In contrast to the PJK cohort, six PJF patients all had varying degrees of neurologic deficits from modified Frankel grade C to D3, which recovered to grades D3 and to grade D2 in three patients each, after a revision operation of proximal extension of instrumented fusion with or without neural decompression. None of the possible demographic and radiologic risk factors showed statistical differences between the non-PJK/PJF, PJK, and PJF groups. Conclusions: Compared with the traditional open surgical approach used in the previous studies with a positive result for the prophylactic two-level cement augmentation, the MIS procedures with substantial benefits to patients in terms of less access-related morbidity and less blood loss also provide a greater segmental stability, which, however, may have a negative effect on the development of PJK/PJF.

Time-dependent biomechanical evaluation for corrective planning of scoliosis using finite element analysis - A comprehensive approach

Scoliosis is a medical condition marked by an abnormal lateral curvature of the spine, typically forming a sideways "S" or "C" shape. Mechanically, it manifests as a three-dimensional deformation of the spine, potentially leading to diverse clinical issues such as pain, diminished lung capacity, and postural abnormalities. This research specifically concentrates on the Adolescent Idiopathic Scoliosis (AIS) population, as existing literature indicates a tendency for this type of scoliosis to deteriorate over time. The principal aim of this investigation is to pinpoint the biomechanical factors contributing to the progression of scoliosis by employing Finite Element Analysis (FEA) on computed tomography (CT) data collected from adolescent patients. By accurately modeling the spinal curvature and related deformities, the stresses and strains experienced by vertebral and intervertebral structures under diverse loading conditions can be simulated and quantified. The transient simulation incorporated damping and inertial terms, along with the static stiffness matrix, to enhance comprehension of the response. The findings of this study indicate a significant reduction in the Cobb angle, halving from its initial value, decreasing from 35° to 17°. In degenerative scoliosis, failure was predicted at 109 cycles, with the Polypropylene brace deforming by 10.34 mm, while the Nitinol brace exhibited significantly less deformation at 7.734 mm. This analysis contributes to a better understanding of the biomechanical mechanisms involved in scoliosis development and can assist in the formulation of more effective treatment strategies. The FEA simulation emerges as a valuable supplementary tool for exploring various hypothetical scenarios by applying diverse loads at different locations to enhance comprehension of the effectiveness of proposed interventions.

Proximal junctional kyphosis in Lenke 5C adolescent idiopathic scoliosis after selective posterior thoracolumbar/lumbar fusion: risk factors and predictive index

BACKGROUND

Although several studies have reported that selective posterior thoracolumbar/lumbar (TL/L) fusion can yield satisfactory results in Lenke 5C adolescent idiopathic scoliosis (AIS), the proximal junctional kyphosis (PJK) is still a common complication that occurs after surgery. The purpose of this study is to analyse the risk factors for postoperative proximal junctional kyphosis in Lenke 5C patients who underwent selective posterior TL/L fusion and explore whether PJK can be predicted.

METHODS

A total of 83 AIS patients with Lenke 5C curves who met the inclusion criteria were analysed. All patients were divided into two groups based on the occurrence of postoperative PJK. Univariate and multivariate analyses were used to identify independent factors significantly associated with PJK, and an PJK index was proposed and verified.

RESULTS

PJK was observed in 27 of 83 (32.5%) patients in the study. Preoperative thoracic kyphosis (TK) and the immediate postoperative proximal junctional angle (PJA) were the primary factors identified by the binary logistic regression analysis. The PJK index was defined as 1.1× preoperative TK + 2.3× immediate postoperative PJA. The receiver operating characteristics curve indicated that the occurrence rate of PJK was 85% and non-occurrence rate was 82% when the PJK index was greater than 42.

CONCLUSION

Large preoperative TK and a large immediate postoperative PJA play important roles in the development of PJK in Lenke 5C patients treated with selective posterior thoracolumbar/lumbar fusion. The PJK index can be used to predict the occurrence of PJK with high accuracy. To prevent the occurrence of PJK, we should pay attention to the TLK, and preserving more posterior proximal intervertebral elements at the upper instrumented vertebral level would be an important part of corrective surgery; however, moderate correction of the lumbar curve is recommended.

Proximal Junctional Kyphosis and Failure: Strategies for Prevention

Proximal junctional kyphosis (PJK) and proximal junctional failure/fractures (PJF) are common complications following long-segment posterior instrumented fusions for adult spinal deformity. As progression to PJF involves clinical consequences for patients and requires costly revisions that may undermine the utility of surgery and are ultimately unsustainable for health care systems, preventative strategies to minimize the occurrence of PJF are of tremendous importance. In this article, the authors present a detailed outline of PJK and PJF with a focus on surgical strategies aimed at preventing their occurrence..

Biomechanical evaluation of different semi-rigid junctional fixation techniques using finite element analysis

BACKGROUND

Proximal junctional failure is a common complication attributed to the rigidity of long pedicle screw fixation constructs used for surgical correction of adult spinal deformity. Semi-rigid junctional fixation achieves a gradual transition in range of motion at the ends of spinal instrumentation, which could lead to reduced junctional stresses, and ultimately reduce the incidence of proximal junctional failure. This study investigates the biomechanical effect of different semi-rigid junctional fixation techniques in a T8-L3 finite element spine segment model.

METHODS

First, degeneration of the intervertebral disc was successfully implemented by altering the height. Second, transverse process hooks, one- and two-level clamped tapes, and one- and two-level knotted tapes instrumented proximally to three-level pedicle screw fixation were validated against ex vivo range of motion data of a previous study. Finally, the posterior ligament complex forces and nucleus pulposus stresses were quantified.

FINDINGS

Simulated range of motions demonstrated the fidelity of the general model and modelling of semi-rigid junctional fixation techniques. All semi-rigid junctional fixation techniques reduced the posterior ligament complex forces at the junctional zone compared to pedicle screw fixation. Transverse process hooks and knotted tapes reduced nucleus pulposus stresses, whereas clamped tapes increased nucleus pulposus stresses at the junctional zone.

INTERPRETATION

The relationship between the range of motion transition and the reductions in posterior ligament complex and nucleus pulposus stresses was complex and dependent on the fixation techniques. Clinical trials are required to compare the effectiveness of semi-rigid junctional fixation techniques in terms of reducing proximal junctional failure incidence rates.

Biomechanical investigation of long spinal fusion models using three-dimensional finite element analysis

BACKGROUND

This study represents the first finite element (FE) analysis of long-instrumented spinal fusion from the thoracic vertebrae to the pelvis in the context of adult spinal deformity (ASD) with osteoporosis. We aimed to evaluate the von Mises stress in long spinal instrumentation for models that differ in terms of spinal balance, fusion length, and implant type.

METHODS

In this three-dimensional FE analysis, FE models were developed based on computed tomography images from a patient with osteoporosis. The von Mises stress was compared for three different sagittal vertical axes (SVAs) (0, 50, and 100 mm), two different fusion lengths (from the pelvis to the second [T2-S2AI] or 10th thoracic vertebra [T10-S2AI]), and two different types of implants (pedicle screw or transverse hook) in the upper instrumented vertebra (UIV). We created 12 models based on combinations of these conditions.

RESULTS

The overall von Mises stress was 3.1 times higher on the vertebrae and 3.9 times higher on implants for the 50-mm SVA models than that for the 0-mm SVA models. Similarly, the values were 5.0 times higher on the vertebrae and 6.9 times higher on implants for the 100-mm SVA models than that for the 0-mm SVA models. Higher SVA was associated with greater stress below the fourth lumbar vertebrae and implants. In the T2-S2AI models, the peaks of vertebral stress were observed at the UIV, at the apex of kyphosis, and below the lower lumbar spine. In the T10-S2AI models, the peaks of stress were observed at the UIV and below the lower lumbar region. The von Mises stress in the UIV was also higher for the screw models than for the hook models.

CONCLUSION

Higher SVA is associated with greater von Mises stress on the vertebrae and implants. The stress on the UIV is greater for the T10-S2AI models than for the T2-S2AI models. Using transverse hooks instead of screws at the UIV may reduce stress in patients with osteoporosis.

Complications after Posterior Lumbar Fusion for Degenerative Disc Disease: Sarcopenia and Osteopenia as Independent Risk Factors for Infection and Proximal Junctional Disease

Proximal Junctional Disease (PJD) and Surgical Site Infection (SSI) are among the most common complications following spine surgery. Their risk factors are not fully understood. Among them, sarcopenia and osteopenia have recently been attracting interest. The aim of this study is to evaluate their influence on mechanical or infective complications after lumbar spine fusion. Patients who underwent open posterior lumbar fusion were analyzed. Through preoperative MRI, central sarcopenia and osteopenia were measured with the Psoas Lumbar Vertebral Index (PLVI) and the M-Score, respectively. Patients were stratified by low vs. high PLVI and M-Score and then by postoperative complications. Multivariate analysis for independent risk factors was performed. A total of 392 patients (mean age 62.6 years, mean follow up 42.4 months) were included. Multivariate linear regression identified comorbidity Index (p = 0.006), and dural tear (p = 0.016) as independent risk factors for SSI, and age (p = 0.014) and diabetes (p = 0.43) for PJD. Low M-score and PLVI were not correlated to a higher complications rate. Age, comorbidity index, diabetes, dural tear and length of stay are independent risk factors for infection and/or proximal junctional disease in patients who undergo lumbar arthrodesis for degenerative disc disease, while central sarcopenia and osteopenia (as measured by PLVI and M-score) are not.

Vertebral Bone Quality Score Independently Predicts Proximal Junctional Kyphosis and/or Failure After Adult Spinal Deformity Surgery

BACKGROUND

Proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) can be catastrophic complications associated with adult spinal deformity (ASD) surgery. These complications are markedly influenced by osteoporosis, leading to additional vertebral fracture and pedicle screw loosening. The MRI-based vertebral bone quality score (VBQ) is a newly developed tool that can be used to assess bone quality.

OBJECTIVE

To investigate the utility of the VBQ score in predicting PJK and/or PJF (PJF/PJK) after ASD correction.

METHODS

We conducted a retrospective chart review to identify patients age ≥50 years who had received ASD surgery of 5 or more thoracolumbar levels. Demographic, spinopelvic parameters, and procedure-related variables were collected. Each patient's VBQ score was calculated using preoperative T1-weighted MRI. Univariate analysis and multivariate logistic regression were performed to determine potential risk factors of PJK/PJF. Receiver operating characteristic analysis and area-under-the-curve values were generated for prediction of PJK/PJF.

RESULTS

A total of 116 patients were included (mean age, 64.1 ± 6.8 years). Among them, 34 patients (29.3%) developed PJK/PJF. Mean VBQ scores were 3.13 ± 0.46 for patients with PJK/PJF and 2.46 ± 0.49 for patients without, which was significantly different between the 2 groups ( P < .001). On multivariate analysis, VBQ score was the only significant predictor of PJK/PJF (odds ratio = 1.745, 95% CI = 1.558-1.953, P < .001), with a predictive accuracy of 94.3%.

CONCLUSION

In patients undergoing ASD correction, higher VBQ was independently associated with PJK/PJF occurrence. Measurement of VBQ score on preoperative MRI may be a useful adjunct to ASD surgery planning.

Failure in Adult Spinal Deformity Surgery: A Comprehensive Review of Current Rates, Mechanisms, and Prevention Strategies

STUDY DESIGN

Literature review.

OBJECTIVE

The aim of this review is to summarize recent literature on adult spinal deformity (ASD) treatment failure as well as prevention strategies for these failure modes.

SUMMARY OF BACKGROUND DATA

There is substantial evidence that ASD surgery can provide significant clinical benefits to patients. The volume of ASD surgery is increasing, and significantly more complex procedures are being performed, especially in the aging population with multiple comorbidities. Although there is potential for significant improvements in pain and disability with ASD surgery, these procedures continue to be associated with major complications and even outright failure.

METHODS

A systematic search of the PubMed database was performed for articles relevant to failure after ASD surgery. Institutional review board approval was not needed.

RESULTS

Failure and the potential need for revision surgery generally fall into 1 of 4 well-defined phenotypes: clinical failure, radiographic failure, the need for reoperation, and lack of cost-effectiveness. Revision surgery rates remain relatively high, challenging the overall cost-effectiveness of these procedures.

CONCLUSION

By consolidating the key evidence regarding failure, further research and innovation may be stimulated with the goal of significantly improving the safety and cost-effectiveness of ASD surgery.

The application of finite element analysis to determine the optimal UIV of growing-rod treatment in early-onset scoliosis

Objectives: To analyze the stress distribution in the proximal vertebral body and soft tissue of dual growing-rod (GR) with different upper instrumented vertebra (UIV) to determine the optimal UIV.

Methods: A ten-year-old male EOS case treated with GR was selected. Based on spiral computed tomography (CT) scanning performed in 0.6 mm thick slices, a finite element model (FEM) of the preoperative state (M0, the original spine state) of the patient was created. Subsequently, four models with different UIV fixations were numerically analyzed by FEM, including M1 (UIV = T1, i.e., the upper-end vertebrae (UEV) of the upper thoracic curve), M2 (UIV = T2), M3 (UIV = T3) and M4 (UIV = T4, i.e., the lower end vertebrae (LEV) of the upper thoracic curve). Displacement and maximum stress in the proximal vertebral body and soft tissue were measured and compared among the five models.

Results: The spine model was fixed with the sacrum, and the gravity conditions were imposed on each vertebral body according to the research of Clin and Pearsall. The results are as follows:M4 model has the largest overall displacement, while M1 has the least displacement among the four models. Except M2, the maximum normalized stress of UIV increases with the downward movement of UIV. M1 has the lowerest annulus fibrosus stress and highest joint capsule stress, which is characterized by the vertebrae backward leaning, while M4 is the opposite. The supraspinous ligament stress of M3 and M4 is significantly higher than that of M1 and M2. This suggests that UIV downshift increases the tendency of the proximal vertebral bodies to bend forward, thereby increasing the tension of the posterior ligaments (PL).

Conclusion: The UIV of the GR is recommended to be close to the UEV of the upper thoracic curve, which can reduce the stress of the proximal PL, thereby reducing the occurrence of proximal junctional kyphosis (PJK).

A biomechanical study of ligament tethers strengthening for the prevention of proximal junctional kyphosis after posterior long-segment spinal fusion

BACKGROUND

Proximal junctional kyphosis is a known complication of posterior long-segment thoracolumbar fusion. Here, the biomechanical effectiveness of ligament tethers strengthening and vertebral body augmentation, in proximal junctional kyphosis prevention was explored using the finite element analysis.

METHODS

Based on a validated model of T1-L5 with the pedicle screw system instrumented T8-L5, strengthening models with different strategies were created to assess the range of motion in proximal vertebrae, vertebrae stress, pedicle screw stress, and pressure on intervertebral discs during extension, flexion, lateral bending, and axial rotation motions. Strengthening strategies included two- and three-level posterior ligament tethers (TE-T7-T9 and TE-T6-T9), and tethers with T7 &T8 vertebral body augmentation (TECE-T7-T9 and TECE-T6-T9).

FINDINGS

Compared to the spinal fusion model, the ligament tethers strengthening significantly reduced the flexion-extension range of motion difference among the proximal vertebrae. During the flexion-extension motion, the T8 vertebra stresses in the TE-T7-T9, TE-T6-T9, TECE-T7-T9, and TECE-T6-T9 models were distinctively reduced, the values decreased by 26.8%, 28.3%, 28.8%, and 9.6%, respectively, during flexion, and by 21.9%, 35.2%, 23%, and 18.6%, respectively, during extension. In the strengthening models, the maximum stresses on the T7/T8 intervertebral disc in the TE-T6-T9 model were reduced by 13.8% during flexion and by 14.7% during extension.

INTERPRETATION

Based on our results, the ligament strengthening configuration of the three-level posterior tethers produced a more gradual transition in range of motion, vertebrae stresses, and intervertebral discs stress between the fused and non-fused segments, especially during flexion-extension, which may significantly decrease the proximal junctional kyphosis biomechanical risk.

Bone Fusion Morphology after Circumferential Minimally Invasive Spine Surgery Using Lateral Lumbar Interbody Fusion and Percutaneous Pedicle Screws without Bone Grafting in the Thoracic Spine: A Retrospective Study

Background and Objectives: This study aimed to investigate the process and morphology of thoracic and lumbosacral bone fusion in patients with adult spinal deformity (ASD) who underwent circumferential minimally invasive spine surgery (CMIS) by lateral lumbar interbody fusion (LLIF) and percutaneous pedicle screws (PPSs) without bone grafting in the thoracic spine and who have risk factors for bone fusion failure in the thoracic spine. Materials and Methods: This retrospective study included 61 patients with spinal deformities (46 women and 15 men) who underwent CMIS with LLIF and PPSs at our hospital after 2016 and completed a 3-year postoperative follow-up. The rate and morphology of bone fusion and rod fracture rate in the thoracic and lumbosacral vertebrae were evaluated. Patients were divided into the thoracic spine spontaneous bone fusion group and the bone fusion failure group. The data of various spinopelvic parameters and the incidence of complications were compared. The vertebral body conditions in the thoracic spine were classified as less degenerative (type N), osteophyte (type O), and diffuse idiopathic skeletal hyperostosis (DISH) (type D). Results: After three postoperative years, the bone fusion rates were 54%, 95%, and 89% for the thoracic, lumbar, and lumbosacral spine, respectively. Screw loosening in the thoracic vertebrae was significantly higher in the bone fusion failure group, while no significant differences were observed in the spinopelvic parameters, Oswestry Disability Index (ODI), and rate of proximal junctional kyphosis and rod fractures. Type N vertebral body condition and screw loosening were identified as risk factors for spontaneous bone fusion failure in the thoracic spine. Conclusion: This study indicated that spontaneous bone fusion is likely to be obtained without screw loosening, and even if bone fusion is not obtained, there is no effect on clinical results with the mid-term (3-year) results of CMIS without bone grafting in the thoracic spine.

Conditions for Achieving Postoperative Pelvic Incidence-Lumbar Lordosis < 10° in Circumferential Minimally Invasive Surgery for Adult Spinal Deformity

This retrospective study aimed to evaluate the clinical outcomes of circumferential minimally invasive surgery (CMIS) using lateral lumbar interbody fusion (LLIF) and percutaneous pedicle screw (PPS) in adult spinal deformity (ASD) patients, and to clarify the conditions for achieving postoperative pelvic incidence-lumbar lordosis (PI-LL) < 10°. Demographics and other parameters of ASD patients who underwent CMIS and who were divided into groups G (achieved postoperative PI-LL < 10°) and P (PI-LL ≥ 10°) were compared. Of the 145 included ASD patients who underwent CMIS, the average fused level, bleeding volume, operative time, and number of intervertebral discs that underwent LLIF were 10.3 ± 0.5 segments, 723 ± 375 mL, 366 ± 70 min, and 4.0 segments, respectively. The rod material was titanium alloy in all the cases. The PI-LL significantly improved from 37.3 ± 17.9° to 1.2 ± 12.2° postoperatively. Pre- and postoperative PI, postoperative LL, preoperative PI-LL, PI-LL after LLIF, and postoperative PI-LL were significantly larger in group P. PI-LL after LLIF was identified as a significant risk factor of postoperative PI-LL < 10° by logistic regression, and the cut-off value on receiver operating characteristic curve analysis was 20°. Sufficient correction was achieved by CMIS. If PI-LL after LLIF was ≤20°, it was corrected to the ideal alignment by the PPS procedure.

Proximal junctional kyphosis in pediatric spinal deformity surgery: a systematic review and critical analysis

PURPOSE

Proximal junctional kyphosis (PJK) is a commonly encountered clinical and radiographic phenomenon after pediatric and adolescent spinal deformity surgery that may lead to post-operative deformity, pain, and dissatisfaction. Understanding the risk factors of PJK can be useful for pre-operative informed consent as well as to identify any potential preventative strategies.

METHODS

We performed a systematic review and critical analysis following the PRISMA statement in July 2019 by searching the PubMed, Scopus, and Embase databases, including all prior published studies. We included articles with data on PJK in patients with operative pediatric and adolescent scoliosis and those that detailed risk factors and/or preventative strategies for PJK. Levels of evidence were determined based on consensus. Findings were summarized and grades of recommendation were assigned by consensus. This study was registered in the PROSPERO database; 202,457.

RESULTS

Six hundred and thirty five studies were identified. Thirty-seven studies met criteria for inclusion into the analysis. No studies including neuromuscular scoliosis met inclusion criteria. No findings had Grade A evidence. There were 4 findings found to contribute to PJK with Grade B evidence in EOS: higher number of distractions, disruption of posterior elements, greater sagittal plane correction. There was no difference in incidence noted between etiology of the curvature. Five findings with Grade B evidence were found to contribute to PJK in AIS populations: higher pre-operative thoracic kyphosis, higher pre-operative lumbar lordosis, longer fusion constructs, greater sagittal plane correction, and posterior versus anterior fusion constructs.

CONCLUSION

Greater sagittal plane correction has Grade B evidence as a risk factor for PJK in both EOS and AIS populations. In EOS patients, an increased number of distractions and posterior element disruption are Grade B risk factors. In AIS patients, longer fusion constructs, higher pre-operative thoracic kyphosis and lumbar lordosis, and posterior (as opposed to anterior) constructs also contributed to PJK with Grade B evidence. These findings can guide informed consent and surgical management, and provide the foundation for future studies.

Computational modelling of the scoliotic spine: A literature review

Scoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed.

Novel Distributed Loading Technique Using Multimaterial, Long-Segment Spinal Constructs to Prevent Proximal Junctional Pathology in Adult Spinal Deformity Correction-Operative Technique and Radiographic Findings

BACKGROUND

Proximal junctional kyphosis (PJK) and proximal junction failure are common and costly complications after long-segment adult spinal deformity (ASD) correction. Although much research has focused on the concept of "softening the landing" to prevent proximal junction pathologies, long-segment constructs largely deviate from the force-deformation curve of the physiologic spine. Our novel distributed loading technique for ASD correction is described using multimaterial, long-segment constructs to create a biomechanically sound, yet physiologic, decremental stiffness toward the rostral end.

METHODS

Operative steps detail the custom-designed constructs of dual-headed pedicle screws and varied rod diameters and materials (cobalt chromium or titanium) for an initial 20 patients (mean 66.6 ± 4.8 years). Standing scoliosis films were obtained preoperatively and at regular intervals postoperatively to assess for PJK.

RESULTS

No patient had evidence of PJK or proximal junction failure at latest radiographic follow-up (mean 17.9 months, range 13-25 months). Radiographic findings for sagittal vertical axis averaged 11.2 ± 5.6 cm preoperatively and 3.6 ± 2.3 cm postoperatively. Compared with preoperative parameters, postoperative reductions in pelvic incidence-lumbar lordosis mismatch averaged 28.7 ± 12.9 degrees, and sagittal vertical axis averaged 7.6 ± 5.2 cm while PJA was essentially unchanged.

CONCLUSIONS

Preliminary results suggest that the distributed loading technique is promising for prevention of PJK with stiffness gradients that mimic the force-deformation curve of the physiologic posterior tension band. Our technique may optimize the degree of stress at the proximal junction without overwhelming the anterior column bony while remodeling and mature arthrodesis takes place.

Biomechanics of adjacent segment after three-level lumbar fusion, hybrid single-level semi-rigid fixation with two-level lumbar fusion

Multi-level spinal fusion has been reported in some cases to lead to adjacent segment disease (ASD) and proximal junctional kyphosis (PJK). The purpose of this study was to demonstrate a polyether-ether-ketone (PEEK) rod fixation system implanted adjacent to a two-level lumbar fusion would have a lower risk of PJK than three-level lumbar fusion, which was investigated by comparing the biomechanical effects on the adjacent level after surgical procedures. Four finite element (FE) models of the lumbar-sacral spine (intact model (INT), L4-S1 fusion model (L4-S1 FUS), L3-S1 fusion model (L3-S1 FUS), and single-level PEEK rod semi-rigid fixation adjacent to L4-S1 fusion model (FUSPRF)) were established. Displacement-controlled finite element (FE) analysis was used during the simulation. Compared with the two-level fusion model (L4-S1 FUS), both three-level implanted models (L3-S1 FUS and FUSPRF) showed an increase intersegmental rotation angle, and maximum von-Mises stress on the disc annulus. The results also showed that the intersegmental rotation, stress on the disc annulus and maximum stress on the rod were lower in the FUSPRF model than the L3-S1 FUS model. Though the maximum screw stress was higher in the FUSPRF model than the L3-S1 FUS model under all moments except for torsion, the maximum screw stress in the two models were far below the yield strength of titanium alloy. As the parameters above have been indicated as risk factors for PJK, it can be concluded that hybrid single-level PEEK rod semi-rigid fixation and two-level lumbar fusion have a lower risk of PJK than three-level lumbar fusion.

A biomechanical study of proximal junctional kyphosis after posterior long segment fusion with vertebral body augmentation

Background Proximal junction kyphosis is a common clinical complication of posterior long-segment spinal fusion and vertebral body augmentation method is one of the effective approaches to prevent it. The purpose of this study was to explore the biomechanical effect of proximal junction kyphosis after posterior long-segment thoracolumbar fusion with different vertebral augmentation schemes using finite element analysis. Methods 3D nonlinear finite element models of T1-L5 spine posterior long-segment T8-L5 thoracolumbar fusion combined with T7, T8 and T7&T8 vertebral bone cement augmentation were constructed from human spine CT data and clinical surgical operation scheme to analyze the von Mises stress in the vertebrae, intervertebral discs pressure and pedicle screws system loads under the flexion, extension, lateral bending and axial rotation motion. Findings Compared with thoracolumbar posterior long-segment fusion model, T7 maximum stress in T7, T8 and T7&T8 vertebrae augmentation models were reduced by 8.64%, 7.17%, 8.51%;0.79%, -3.88%,1.67%;4.02%, 5.30%, 4.27% and 3.18%, 3.06%, -6.38% under the flexion, extension, lateral bending and axial rotation motion. T7/T8 intervertebral disc pressure in T7, T8, T7&T8 vertebral augmentation models were 36.71Mpa,29.78Mpa,36.47Mpa;22.25Mpa,18.35Mpa,22.06Mpa;84.27Mpa,68.17Mpa, 83.89Mpa and 52.23Mpa, 38.78Mpa,52.10Mpa under the same condition. The maximum stress 178.2Mpa of pedicle screws is mainly distributed at the root of screw. Interpretation Thoracolumbar posterior long-segment fusion with proximal double-segment vertebral augmentation should be recommended to prevent proximal junction kyphosis than single-segment augmentation. Simulation results can provide theoretical foundations and assist surgeons in selecting the appropriate operation scheme.

State of the art: proximal junctional kyphosis-diagnosis, management and prevention

Proximal junctional kyphosis (PJK) is a common problem that may occur following the surgical treatment of adult patients with spinal deformity. It is defined as the proximal junctional sagittal angle from the UIV and UVI + 2 of at least 10° AND at least 10° greater than the preop measurement. The reported incidence of radiographic PJK in the literature varies between 17 and 46%. A smaller subset of these patients may need revision surgery and are defined as proximal junctional failure (PJF), which can be associated with vertebral fracture, vertebral subluxation, failure of instrumentation, and neurological deficits. Several risk factors for development of PJK have been proposed. However, large-scale prospective studies are needed to better identify strategies to reduce the incidence of PJK.

Biomechanical Evaluation of a Dynamic Stabilization System for the Prevention of Proximal Junctional Failure in Adult Deformity Surgery

STUDY DESIGN

Biomechanical spine model. Comparison of stress in the implant and the adjacent cranial segment was done with conventional rigid versus dynamic stabilization system (DS) fixation.

OBJECTIVE

The aim of this study was to study stress at the proximal end of spinal fixation with a novel DS.

SUMMARY OF BACKGROUND DATA

High stress at the implant bone junction may cause proximal junctional failure (PJF) in adult deformity surgery.

METHODS

Five life-size spine models were instrumented with pedicle screws and a 5.5-mm Titanium rod from T8-S1. The same models were subsequently instrumented with a similar rod and DS between T8-9 pedicle screws. The spine model was loaded with 25 Nm static load cranial to the proximal fixation in six directions. Strains were measured from the proximal screws. Disc pressure was measured from the proximal instrumented segment (T8-9) and cranial adjacent segment (T7-8).

RESULTS

Rigid fixation produced highest strain at T8, followed by T10 then T9. In contrast, DS fixation produced highest strain at T10, followed by T9 then T8. Strain at T8 was significantly less with DS fixation than rigid fixation (P = 0.019). The T10 screw strain was not significantly higher with DS stabilization compared to rigid fixation (P = 0.091). Rigid fixation allowed no load-sharing or pressure rise at T8-9 but an abrupt rise at T7-8. DS system permitted load-sharing and pressure rise in T8-9; the difference compared to rigid fixation was significant in flexion loading (P = 0.04) and similar trend but not significant in extension (P = 0.09). DS system produced a rise in the adjacent segment disc pressure (T7-8), which was smaller than rigid fixation but not significant.

CONCLUSION

Long spinal fixation using rigid rods produces maximum stress at the proximal end screw and increases adjacent disc pressure, possibly leading to PJF. Dynamic stabilization at the cranial end segment may prevent PJF by reducing these factors.Level of Evidence: N/A.

Rod contour and overcorrection are risk factors of proximal junctional kyphosis after adult spinal deformity correction surgery

PURPOSE

To examine the risk factors of proximal junctional kyphosis (PJK) after surgery for adult spinal deformity (ASD) focusing on rod contour.

METHODS

Sixty-three patients with ASD who underwent surgery using lateral lumbar interbody fusion and percutaneous pedicle screws were analyzed. Fixation range was from the lower thoracic spine to the pelvis in all cases. Patients were divided into two groups. The PJK group consisted of 16 patients with PJK. The non-PJK group had 47 patients without PJK. We examined various spinopelvic parameters and parameters related to rod contour.

RESULTS

Among the various spinal and pelvic parameters, those in the PJK group were significantly larger in terms of preoperative SVA and were significantly smaller in terms of postoperative "PI-LL." For parameters related to rod contour, the rod kyphotic curve at the thoracic spine in the PJK group was significantly less than that in the non-PJK group. The inclination of the pedicle screw at the upper instrumented vertebra (UIV) was significantly more cranial in the PJK group than in the non-PJK group. The kyphotic curve of the rod at the UIV was more parallel in the PJK group than in the non-PJK group. On logistic regression analysis, insufficient kyphotic curve at the thoracic spine along with UIV and overcorrection of the lumbar spine were identified as significant risk factors.

CONCLUSIONS

Insufficient kyphotic curve of the rod in the thoracic spine along with UIV and overcorrection of the lumbar spine were noted as significant risk factors of PJK.

Semitendinosus Tendon Augmentation for Prevention of Proximal Junctional Failure

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

The aim of this study was to compare the incidence of proximal junctional kyphosis (PJK), proximal junctional failure (PJF), and clinical outcomes of patients who did and did not receive posterior ligament complex (PLC) augmentation using a semitendinosus allograft when undergoing long-segment posterior spinal fusion for adult spinal deformity.

SUMMARY OF BACKGROUND DATA

Clinical research on the augmentation of the PLC to prevent PJK and PJF has been limited to small case series without a comparable control group.

METHODS

From 2014 to 2019, a consecutive series of patients with adult spinal deformity who underwent posterior long-segment spinal fusion with semitendinosus allograft to augment the PLC (allograft) or without PLC augmentation (control) were identified. Preoperative and postoperative spinopelvic parameters were measured. PJK, PJF, and Oswestry Disability Index (ODI) scores were recorded and compared between the two groups. Univariate and multivariate analysis was performed. P ≤ 0.05 was considered significant.

RESULTS

Forty-nine patients in the allograft group and 34 patients in the control group were identified. There were no significant differences in demographic variables or operative characteristics between the allograft and control group. Preoperative and postoperative spinopelvic parameters were also similar between the two groups. PJK was present in 33% of patients in the allograft group and 32% of patients in the control group (P = 0.31). PJF did not occur in the allograft group, whereas six patients (18%) in the control group developed PJF (P = 0.01). Postoperative absolute ODI was significantly better in the allograft group (P = 0.007).

CONCLUSION

The utilization of semitendinosus allograft tendon to augment the PLC at the upper instrumented vertebrae in patients undergoing long-segment posterior spinal fusion for adult deformity resulted in a significant decrease in PJF incidence and improved functional outcomes when compared to a cohort with similar risk of developing PJK and PJFLevel of Evidence: 3.

Toward Patient Specific Models of Pediatric IVDs: A Parametric Study of IVD Mechanical Properties

Patient specific finite element (FE) modeling of the pediatric spine is an important challenge which offers to revolutionize the treatment of pediatric spinal pathologies, for example adolescent idiopathic scoliosis (AIS). In particular, modeling of the intervertebral disc (IVD) is a unique challenge due to its structural and mechanical complexity. This is compounded by limited ability to non-invasively interrogate key mechanical parameters of a patient's IVD. In this work, we seek to better understand the link between mechanical properties and mechanical behavior of patient specific FE models of the pediatric lumbar spine. A parametric study of IVD parameter was conducted, coupled with insights from current knowledge of the pediatric IVD. In particular, the combined effects of parameters was investigated. Recommendations are made toward areas of importance in patient specific FE modeling of the pediatric IVD. In particular, collagen fiber bundles of the IVD are found to dominate IVD mechanical behavior and are thus recommended as an area of primary focus for patient specific FE models. In addition, areas requiring further experimental research are identified. This work provides a valuable building block toward the development of patient specific models of the pediatric spine.

Risk and Predictive Factors for Proximal Junctional Kyphosis in Patients Treated by Lenke Type 5 Adolescent Idiopathic Scoliosis Correction

OBJECTIVE

Proximal junctional kyphosis (PJK) is a common sagittal complication of adolescent idiopathic scoliosis (AIS) after corrective surgery, leading to new deformities, pain, and, even, revision surgery. In the present study, we investigated the risk and predictive factors for PJK in patients who had undergone Lenke type 5 AIS correction to identify the parameters relevant to intraoperative guidance.

METHODS

A total of 35 patients with Lenke type 5 AIS who had undergone corrective surgery at our hospital from January 2008 to February 2016 were divided into the PJK (n = 15) and non-PJK (n = 20) groups. Correlation and receiver operating characteristic curve analyses were performed to screen the parameters for significance and calculate the thresholds. A survival analysis was performed to examine the differences between the 2 groups.

RESULTS

Independent t tests revealed significant differences between the 2 groups in the preoperative pelvic incidence, preoperative pelvic tilt, postoperative proximal junctional angle (PJA), and postoperative thoracic kyphosis (TK). The postoperative PJA, postoperative TK, and other parameters correlated significantly with changes in the PJA at the final follow-up. The receiver operating characteristic curves revealed that the postoperative PJA and postoperative TK effectively predicted for the occurrence of PJK, with a threshold of 9.45° and 25.25°, respectively. The estimated survival times were 14.7 months for a PJA >9.45° and TK >25.25°, 19.2 months for a PJA >9.45°, and 33.9 months for TK >25.25°.

CONCLUSIONS

The results of the present study have shown that the postoperative PJA and postoperative TK can be used to effectively predict for the occurrence of PJK in patients with Lenke type 5 AIS after corrective surgery, with a threshold of 9.45° and 25.25°, respectively.

Using Satellite Rod Technique in Patients with Severe Kyphoscoliosis Undergoing Three-Column Osteotomy: A Minimum of 2 Years' Follow-up

OBJECTIVE

To introduce the satellite rod technique utilized in severe spinal deformity after three-column osteotomy (3CO) and to evaluate the radiographic and clinical outcomes at 2-year follow-up, further discussing its utilization in this particular cohort.

METHODS

A total of 32 (19 females and 13 males) with an average age of 32.9 ± 18.3 years from December 2012 to March 2016 were retrospectively reviewed. Radiographic measurements were performed on standing full-spine anteroposterior and lateral radiographs preoperatively, postoperatively, and at last follow-up. The coronal parameters including Cobb angle and distance between C₇ plumb line and center sacral vertical line (C7PL-CSVL), as well as the sagittal parameters including global kyphosis (GK) and sagittal vertical axis (SVA) were measured at three time points. The Scoliosis Research Society-22 questionnaire (SRS-22) was fulfilled preoperatively and at each follow-up. Paired t test would be used to determine whether there was a significant difference between time points.

RESULTS

A total of 32 patients were enrolled in this study with mean age of 32.9 ± 18.3 (range, 12 to 66) years old. Twenty patients underwent pedicle subluxation osteotomy (PSO) and 12 patients underwent vertebral column resection (VCR). The pathogenesis of this cohort included neuromuscular scoliosis (11 cases), congenital kyphoscoliosis (seven with hemivertebrae and five with segmentation failure), degenerative spinal deformity (five cases), and thoracolumbar tuberculosis with angular kyphosis (four cases). The post-operative Cobb angle decreased significantly from 49.1° ± 28.0° to 19.0° ± 16.7° with a correction rate of 65.2% ± 21.8%. At final follow-up, the average Cobb angle was 19.4° ± 16.9° and no obvious loss of correction was found. The preoperative, postoperative, and last follow-up C7PL-CSVL were 23.9 ± 14.5 mm, 15.7 ± 11.1 mm, and 12.1 ± 7.4 mm, respectively. Significant postoperative improvement was attained while there was no change observed at last follow-up. Postoperative GK significantly improved from 73.8° ± 28.1° to 23.2° ± 11.7° with the correction rate of 66.0% ± 17.9%. SVA decreased significantly from 42.9 ± 33.9 mm to 24.1 ± 21.1 mm. The average GK and SVA at final follow-up were 22.7° ± 10.1° and 23.5 ± 21.1 mm, respectively and no obvious loss of correction was observed of them during follow-up. In addition, no change or loss of motor or somatosensory evoked potential occurred during surgery. During the follow-up, two malposition screws and one rod breakage were found.

CONCLUSION

The satellite rod used in patients with severe kyphoscoliosis undergoing 3CO could yield favorable radiological and clinical outcomes. With the utilization of this technique, the coronal and sagittal balance could be well-maintained during follow-up.

Surgical Design Optimization of Proximal Junctional Kyphosis

Purpose

The objective of this study was to construct a procedural planning tool to optimize the proximal junction angle (PJA) to prevent postoperative proximal junctional kyphosis (PJK) for each scoliosis patient.

Methods

Twelve patients (9 patients without PJK and 3 patients with PJK) who have been followed up for at least 2 years after surgery were included. After calculating the loading force on the cephalad intervertebral disc of upper instrumented vertebra of each patient, the finite-element method (FEM) was performed to calculate the stress of each element. The stress information was summarized into the difference value before and after operation in different regions of interest. A two-layer fully connected neural network method was applied to model the relationship between the stress information and the risk of PJK. Leave-one-out cross-validation and sensitivity analysis were implemented to assess the accuracy and stability of the trained model. The optimal PJA was predicted based on the learned model by optimization algorithm.

Results

The mean prediction accuracy was 83.3% for all these cases, and the area under the curve (AUC) of prediction was 0.889. And the output variance of this model was less than 5% when the important factor values were perturbed in a range of 5%.

Conclusion

Our approach integrated biomechanics and machine learning to support the surgical decision. For a new individual, the risk of PJK and optimal PJA can be simultaneously predicted based on the learned model.

Biomechanics of prophylactic tethering for proximal junctional kyphosis: effects of cyclic loading on tether strength and failure properties

STUDY DESIGN

Biomechanical evaluation of woven polyester tethers.

OBJECTIVES

To quantify changes in tether elongation, stiffness, and failure characteristics after cyclic loading. Ligamentous augmentation is gaining interest as a technique to prevent proximal junctional kyphosis (PJK) in adult spinal fusions. There are a lack of data regarding the effects of cyclic loading on polyester tether mechanical properties. Tether stretch may lead to loss of stabilization and increased risk of tether failure. Biomechanical data are needed to determine the effects of cyclic loading on tether integrity.

METHODS

Testing was done in two materials: (1) a synthetic cortical bone composite to determine baseline mechanical properties, and (2) nine cadaveric L1 spinous processes. 5 mm woven polyester tethers were looped through 2.5 mm holes drilled in each material. First, five tethers were tested directly to failure in the synthetic bone to establish baseline failure properties. Next, tethers were tested at one of the three cyclic load ranges [5%, 25%, and 50% (n = 5 each) of baseline failure] for 1000 cycles and then loaded to failure. Cadaveric tests were done at the 25% range and compared to synthetic bone tests at the same range. Cadaveric failure tests were classified as either tether failure or spinous process bone failure.

RESULTS

Greater cyclic loading range had a significant effect on tether loop elongation, increased stiffness, and decreased ultimate tensile force. Among the cadaveric failure tests, 56% resulted in tether failure and the remaining 44% resulted in bone failure.

CONCLUSIONS

Polyester tethers stretch significantly when loaded to physiological ranges. Anticipation of tether stretch may be an important consideration for a tethering strategy to prevent PJK. Improved understanding of tether material properties can provide guidance for the evaluation of clinical outcomes associated with techniques to reduce the risk of PJK caused by ligamentous laxity.

LEVEL OF EVIDENCE

Biomechanical study.

Evaluation of Spinous Process Tethering at the Proximal End of Rigid Constructs: In Vitro Range of Motion and Intradiscal Pressure at Instrumented and Adjacent Levels

BACKGROUND

Adult spinal deformity surgery requires use of long thoracolumbar instrumentation, which is associated with risk of postoperative proximal junctional kyphosis (PJK). Tethering has been used in spinal surgery but not around the spinous process (SP) in the context of preventing PJK.

METHODS

Researchers applied a nondestructive hybrid loading protocol to 7 T8-L2 cadaveric specimens in flexion-extension, lateral bending, and axial rotation (AR). A rigid construct (pedicle screws and rods) and 1- and 2-level SP constructs were tested, as was a hand-tie technique. SP tethering (SPT) constructs use clamps on both sides of the SP; SPT helix constructs use 1 clamp and wrap around the SP.

RESULTS

All tether constructs showed greater motion at the instrumented level and less motion at adjacent levels compared to rigid constructs. In AR, 1- and 2-level SPT constructs restricted first instrumented level motion to a greater extent when compared with other tether constructs (P ≤ .05). Passing the band through the T10 SP did not produce significant biomechanical differences compared to passing it through the T9-T10 interspinous ligament (P > .05). Hand-tied constructs demonstrated more motion compared to tensioned constructs (P > .05). Intradiscal pressure results corroborated motion data.

CONCLUSIONS

SPT at the proximal end of a rigid construct produced more favorable biomechanical outcomes at instrumented and adjacent levels than were seen with a completely rigid construct. Clinical research is needed to determine whether these methods reduce the risk of PJK among patients.

LEVEL OF EVIDENCE

3.

CLINICAL RELEVANCE

This work sheds light on the biomechanical stability of proximal tethering constructs in an effort to enhance the surgeon's ability to reduce rates of proximal junctional kyphosis and failure in thoracolumbar spinal fusion surgery.

Protection of L1 nerve roots by pre-relieve tension in parallel endplate osteotomy for severe rigid thoracolumbar spine deformity

BACKGROUND

This is a retrospective study of the use of parallel endplate osteotomy (PEO) for correction of severe rigid thoracolumbar spine deformity.

METHODS

From July 2016 to January 2019, 12 patients with severe rigid thoracolumbar spine deformity underwent PEO on T12 or L1 vertebrae were studied.

RESULTS

Following PEO at T12 or L1, the mean kyphosis and scoliosis correction rates reached 77.0 ± 8.9% and 75.5 ± 8.0%, respectively and the intraoperative estimated blood loss was 1950 ± 1050 mL, and the mean operative time was 6.98 ± 4.02 h. The SF-36 scores of physical function, role-physical, bodily pain, general health, vitality, social function, role-emotional and mental health changed from 63 ± 28, 50 ± 25, 50 ± 30, 34 ± 19, 53 ± 28, 45 ± 30, 30 ± 36 and 54 ± 18 at baseline to 83 ± 18, 69 ± 19, 72 ± 12, 66 ± 21, 75 ± 15, 72 ± 22, 66 ± 34 and 76 ± 12 at 1 year postoperatively, 83 ± 8, 68 ± 32, 83 ± 17, 73 ± 17, 82 ± 18, 76 ± 26, 70 ± 37 and 88 ± 12 at 18 months postoperatively, 86 ± 6, 83 ± 33, 90 ± 16, 81 ± 16, 89 ± 14, 88 ± 25, 83 ± 17 and 94 ± 10 at 24 months postoperatively, respectively (P < 0.01). Three patients had symptoms of L1 nerve root injury, as reflected by lower limb weakness and inner thigh numbness on knee extension and hip flexion, which was further confirmed by electromyography.

CONCLUSIONS

PEO is easier to operate, and the spinal cord and nerve root are under direct vision and can effectively and safely correct severe rigid thoracolumbar spine deformity with satisfactory clinical results. However, it is important to identify, separate and protect L1 nerve roots during surgery in cases where patients have symptoms of back pain, muscle weakness and leg numbness on the convex side after surgery.

Proximal junctional kyphosis following T10-pelvis fusion presenting with neurologic compromise: case presentations and review of the literature

Purpose: Proximal Junctional Kyphosis (PJK) is a well-documented phenomenon following spinal instrumented fusion. Myelopathy associated with proximal junctional failure (PJF) is poorly described in the literature. Adjacent segment disease, fracture above the upper instrumented vertebrae and subluxation may all cause cord compression, ambulatory dysfunction, and/or lower extremity weakness in the postoperative period.Materials and methods: We review the literature on PJK and PJF, and discusses the postoperative management of three patients who experienced myelopathy associated with PJF following T9/10 to pelvis fusion at a single institution.Results and conclusions: PJF with myelopathy must be diagnosed and surgically corrected early on so as to minimize permanent neurologic injury. Patients requiring significant sagittal deformity correction are at greater risk for PJF, and may benefit from constructs terminating in the upper thoracic spine.

Population-based design and 3D finite element analysis of transforaminal thoracic interbody fusion cages

OBJECTIVE

To compare the biomechanical characteristics of two transforaminal thoracic interbody fusion cages based on the Chinese population thoracic anatomy.

METHOD

Computed tomography scans of the thoracic spine of 150 patients from our institution were collected and analysed. Two cages were designed based on the anatomical parameters of these patients. Further, we used 3D finite element analysis models to compare the stability of two cages by using Mimics 17.0 and ANSYS 15.0 software.

RESULT

Two kinds of thoracic cages (box and kidney-shaped) were designed. Under the displacement working condition, the two new fusion cages could achieve immediate postoperative stability, but the kidney-shaped cage was better than the box-shaped cage. Under the stress working condition, no highly focused stress area was found in either cages, but the kidney-shaped cage experienced less stress than the box-shaped cage.

CONCLUSION

The kidney-shaped cage is more stable and experiences lesser stress than the box-shaped cage after thoracic intervertebral fusion, and it is more suitable for Chinese transforaminal thoracic interbody fusion.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

This article is about thoracic fusion cage design and finite element analysis (FEA) analysis based on the thoracic anatomy parameters. For there is currently no suitable thoracic fusion cage for transforaminal thoracic interbody fusion, the results in this article may have the potential of transferring the two designed cages into clinical use.

The effect of posterior tethers on the biomechanics of proximal junctional kyphosis: The whole human finite element model analysis

Little is known about the effects of posterior tethers on the development of proximal junctional kyphosis (PJK). We evaluated the ability of posterior tethers to the proximal motion segment stiffness in long instrumented spinal instrumentation and fusion using a whole body human FE model. A series of finite element (FE) analysis of long segmental spinal fusion (SF) from the upper thoracic vertebra (T1) or lower thoracic vertebra (T9) to the sacrum with pedicle screws and rods were performed using an entire human body FE model (includes 234,910 elements), and compressive stresses (CS) on the anterior column, and tensile stresses (TS) on the posterior ligamentous complex (PLC) in the upper-instrumented vertebra (UIV) and the vertebra adjacent to the UIV (UIV + 1) were evaluated with posterior tethers or without posterior tethers. The models were tested at three T1 tilts (0, 20, 40 deg.), with 20% muscle contraction. Deformable material models were assigned to all body parts. Muscle-tendon complexes were modeled by truss elements with a Hill-type muscle material model. The CS of anterior column decreased with increasing T1 slope with tethers in both models, while the CS remained relatively large in T9 model compared with T1 model (T1 UIV; 0.96 to 1.56 MPa, T9 UIV; 4.79 to 5.61 MPa). The TS of the supraspinous ligament was markedly reduced in both T1 and T9 models with posterior tethers (11-35%). High vertebral CS on UIV and UIV + 1 were seen in the T9 UIV model, and the TS on the PLC were increased in both UIV models. Posterior tethers may decrease PJK development after SF with a proximal thoracic UIV, while both posterior tethers and vertebral augmentation may be necessary to reduce PJK development with a lower thoracic UIV.

Revision surgery in proximal junctional kyphosis

INTRODUCTION

Proximal junctional kyphosis (PJK) is a relatively common complication following spinal deformity surgery that may require reoperation. Although isolating the incidence is highly variable, in part due to the inconsistency in how PJK is defined, previous studies have reported the incidence to be as high as 39% with revision surgery performed in up to 47% of those with PJK. Despite the discordance in reported incidence, PJK remains a constant challenge that can result in undesirable outcomes following adult spine deformity surgery.

METHODS

A comprehensive literature review using Medline and PubMed was performed. Keywords included "proximal junctional kyphosis," "postoperative complications," "spine deformity surgery," "instrumentation failure," and "proximal junctional failure" used separately or in conjunction.

RESULTS

While the characterization of PJK is variable, a postoperative proximal junction sagittal Cobb angle at least 10°, 15°, or 20° greater than the measurement preoperatively, it is a consistent radiographic phenomenon that is well defined in the literature. While particular studies in the current literature may ascertain certain variables as significantly associated with the development of proximal junctional kyphosis where other studies do not, it is imperative to note that they are not all one in the same. Different patient populations, outcome variables assessed, statistical methodology, surgeon/surgical characteristics, etc. often make these analyses not completely comparable nor generalizable.

CONCLUSIONS

The goal of adult spine deformity surgery is to optimize patient outcomes and mitigate postoperative complications whenever possible. Due to the multifactorial nature of this complication, further research is required to enhance our understanding and eradicate the pathology. Patient optimization is the principal guideline in not only PJK prevention, but overall postoperative complication prevention. These slides can be retrieved under Electronic Supplementary Material.

Management of Catastrophic Proximal Junctional Failure Following Spinal Deformity Correction in an Adult with Osteogenesis Imperfecta: Case Report and Technical Note

BACKGROUND

Proximal junctional failure (PJF) is a major and sometimes devastating problem following adult spinal deformity (ASD) correction surgery. Common consensus still lags on guidelines for preventing and managing these complications. Surgical treatment of scoliosis in the presence of osteogenesis imperfecta (OI) in the pediatric population is well described. The complication rates are unusually higher in this special subset of patients owing to poor quality of bone. There is a paucity of literature focusing on surgical techniques, strategies, and problems involved in the management of ASD associated with OI.

CASE DESCRIPTION

We report a 59-year-old female with type 1 OI and adult scoliosis who underwent T10-to-pelvis fusion for ASD according to the principles of adult deformity correction. At a 1-year follow-up, she presented with asymptomatic proximal junctional kyphosis of 45° and 2 weeks later had PJF along with spinal cord injury after a fall. On computed tomography scan, kyphosis was increased to 60° at T9-T10. She underwent decompression and revision deformity correction using quadruple rods, with extension of instrumentation to T2 with soft landing using rib bands. At a 4-year follow-up, she had a good functional outcome after revision surgery.

CONCLUSIONS

This is the first report of successful management of PJF following ASD correction in the presence of OI using this technique. Suboptimal hold of implants due to poor bone quality must be at the focus of any surgical planning for these patients. All possible strategies to prevent PJF must be considered when planning the deformity correction in adults with OI.

Preliminary report on the flexible rod technique for prevention of proximal junctional kyphosis following long-segment fusion to the sacrum in adult spinal deformity

OBJECTIVE

The incidence of proximal junctional kyphosis (PJK) after long-segment fixation in patients with adult spinal deformity (ASD) has been reported to range from 17% to 61.7%. Recent studies have reported using "hybrid" techniques in which semirigid fixation is introduced between the fused and flexible segments at the proximal level to allow a more gradual transition. The authors used these hybrid techniques in a clinical setting and analyzed PJK to evaluate the usefulness of the flexible rod (FR) technique.

METHODS

The authors retrospectively selected 77 patients with lumbar degenerative kyphosis (LDK) who underwent sagittal correction and long-segment fixation and had follow-up for > 1 year. An FR was used in 30 of the 77 patients. PJK development and spinal sagittal changes were analyzed in the FR and non-FR groups, and the predictive factors of PJK between a PJK group and a non-PJK group were compared.

RESULTS

The patient population comprised 77 patients (75 females and 2 males) with a mean (± SD) follow-up of 32.0 ± 12.7 months (36.7 ± 9.8 months in the non-FR group and 16.8 ± 4.7 months in the FR group) and mean (± SD) age of 71.7 ± 5.1 years. Sagittal balance was well maintained at final follow-up (10.5 and 1.5 mm) in the non-FR and FR groups, respectively. Thoracic kyphosis (TK) and lumbar lordosis (LL) were improved in both groups, without significant differences between the two (p > 0.05). PJK occurred in 28 cases (36.4%) in total, 3 (10%) in the FR and 25 (53.2%) in the non-FR group (p < 0.001). Postoperatively, PJK was observed at an average of 8.9 months in the non-FR group and 1 month in the FR group. No significant differences in the incidence of PJK regarding patient factors or radiological parameters were found between the PJK group and non-PJK group (p > 0.05). However, FR (vs non-FR) and interbody fusion except L5-S1 using oblique lumbar interbody fusion (vs non-oblique lumbar interbody fusion), demonstrated a significantly lower PJK prevalence (p < 0.001 and p = 0.044) among the surgical factors.

CONCLUSIONS

PJK was reduced after surgical treatment with the FR in the patients with LDK. Solid long-segment fixation and the use of the FR may become another surgical option for spine surgeons who plan and make decisions regarding spine reconstruction surgery for patients with ASD.

Scheuermann's Kyphosis: Diagnosis, Management, and Selecting Fusion Levels

Scheuermann's kyphosis (SK) is a rigid structural deformity of the thoracic spine defined radiographically as three or more contiguous vertebrae with at least 5° of wedging anteriorly. Prevalence of the disease is thought to be between 0.4% and 10%. The true cause of SK remains unclear; however, various theories include growth irregularities, mechanical factors, genetic factors, and/or poor bone quality as the causes. Patients with mild disease (less than 70°) generally have a favorable prognosis with good clinical outcomes. Most patients with SK are successfully treated nonsurgically with observation, anti-inflammatory medications, and physical therapy. Surgical intervention is indicated in patients with greater than 70° to 75° thoracic curves, greater than 25° to 30° thoracolumbar curves, intractable pain, neurologic deficit, cardiopulmonary compromise, or poor cosmesis. Because of advances in posterior spinal instrumentation, surgery can typically be performed through a posterior-only approach. When surgical treatment is planned, appropriate selection of the upper- and lower-instrumented vertebrae is important to achieve a well-balanced spine, preserve motion segments, and reduce the risk of junctional kyphosis.

Spinal Balance/Alignment - Clinical Relevance and Biomechanics

In the normal spine due to its curvature in various regions, C7 plumb line (C7PL) passes through the sacrum so that the head is centered over the pelvis-ball and socket hip joints and ankle joints. This configuration leads to the least muscular activities to maintain the spinal balance. For any reason like deformity, scoliosis, kyphosis, trauma, and/or surgery this optimal configuration gets disturbed requiring higher muscular activity to maintain the posture and balance. Several parameters like the thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), sacral slope (SS), Hip- and leg position influence the sagittal balance and thus the optimal configuration of spinal alignment. Global sagittal imbalance is energy consuming and often painful compensatory mechanisms are developed, that in turn negatively influence the quality of life. This review looks at the clinical aspects of spinal imbalance, and the biomechanics of spinal balance as dictated by the deformities- ankylosing spondylitis, scoliosis and kyphosis; surgical corrections- pedicle subtraction osteotomies and long segment stabilizations and consequent postural complications like the proximal and distal junctional kyphosis. This review suggests several potential research topics as well.

A biomechanical study on proximal junctional kyphosis following long-segment posterior spinal fusion

Posterior long-segment spinal fusion may lead to proximal junctional kyphosis (PJK). The present study sought to identify the appropriate fusion levels required in order to prevent PJK using finite element analysis. A finite element model was constructed based on the whole-spine computed tomography findings of a healthy adult. Nine commonly used posterior spinal fusion methods were selected. Stress on the annulus fibrosis fibers, the posterior ligamentous complex, and the vertebrae after various spinal fusions in the upright position were compared. This study was divided into two groups: non-fusion and fusion. In the former, the stress between the T10 and the upper thoracic vertebrae was higher. Comparing thoracic and lumbar segments in the fusion group, the peak stress values of the upper instrumented vertebrae (UIV) were mainly observed in T2 and L2 whilst those of the UIV+1 were observed in T10 and L2. After normalization, the peak stress values of the UIV and UIV+1 were located in T2 and L2. Similarly, the peak stress values of the annulus fibrosus at the upper adjacent level were on T10 and L2 after normalization. However, the peak stress values of the interspinal/supraspinal complex forces were concentrated on T11, T12, and L1 after normalization whilst the peak stress value of the pedicle screw was on T2. Controversy remains over the fusion of T10, and this study simulated testing conditions with gravitational loading only. However, further assessment is needed prior to reaching definitive conclusions.

Biomechanics of Prophylactic Tethering for Proximal Junctional Kyphosis: Characterization of Spinous Process Tether Pretensioning and Pull-Out Force

STUDY DESIGN

Biomechanical evaluation of cadaver functional spinal units (FSUs).

OBJECTIVES

Demonstrate the effect of increasing spinous process (SP) tether pretension on FSU flexion range of motion (ROM), intervertebral disc (IVD) pressure, and SP force. Quantify SP tether pull-out forces and relate them to SP forces generated at maximum flexion.

SUMMARY OF BACKGROUND DATA

There has been recent interest in the use of SP tethering for prophylactic treatment of proximal junctional kyphosis (PJK). There is currently no consensus on standard tethering technique and no biomechanical data on the effect of tether pretension.

METHODS

Nine T11-T12 FSUs were tested to 5 Nm of flexion-extension bending. A strain gauge was applied at the base of the T11 SP to measure force. Two custom pressure sensors were inserted into the anterior and posterior thirds of the IVD. Motion kinematics were measured by a motion capture system. An untethered test was done to describe baseline behavior. A 5-mm polyester tether was looped through holes drilled at the base of each SP and pretensioned to five different pretensions ranging from 0 to 88 N. Following ROM testing, specimens were dissected into individual vertebra and then SP pull-out testing was done at each level.

RESULTS

Increasing pretension significantly reduced flexion ROM, reduced IVD pressures, and increased SP force. All pretensions, including the minimum, significantly reduced flexion ROM. SP pull-out forces were significantly greater than SP forces generated at maximum flexion.

CONCLUSIONS

Tether pretension significantly affects segmental FSU biomechanics. Pretension should be considered an integral factor in the overall success of a tethering strategy. Efforts should be made to control and record pretension intraoperatively.

LEVEL OF EVIDENCE

Level V, biomechanical study.

Optimal tether configurations and preload tensioning to prevent proximal junctional kyphosis: a finite element analysis

OBJECTIVE

Proximal junctional kyphosis (PJK) is, in part, due to altered segmental biomechanics at the junction of rigid instrumented spine and relatively hypermobile non-instrumented adjacent segments. Proper application of posteriorly anchored polyethylene tethers (i.e., optimal configuration and tension) may mitigate adjacent-segment stress and help prevent PJK. The purpose of this study was to investigate the impact of different tether configurations and tensioning (preloading) on junctional range-of-motion (ROM) and other biomechanical indices for PJK in long instrumented spine constructs.

METHODS

Using a validated finite element model of a T7-L5 spine segment, testing was performed on intact spine, a multilevel posterior screw-rod construct (PS construct; T11-L5) without tether, and 15 PS constructs with different tether configurations that varied according to 1) proximal tether fixation of upper instrumented vertebra +1 (UIV+1) and/or UIV+2; 2) distal tether fixation to UIV, to UIV-1, or to rods; and 3) use of a loop (single proximal fixation) or weave (UIV and/or UIV+1 fixation in addition to UIV+1 and/or UIV+2 proximal attachment) of the tether. Segmental ROM, intradiscal pressure (IDP), inter- and supraspinous ligament (ISL/SSL) forces, and screw loads were assessed under variable tether preload.

RESULTS

PS construct junctional ROM increased abruptly from 10% (T11-12) to 99% (T10-11) of baseline. After tethers were grouped by most cranial proximal fixation (UIV+1 vs UIV+2) and use of loop versus weave, UIV+2 Loop and/or Weave most effectively dampened junctional ROM and adjacent-segment stress. Different distal fixation and use of loop versus weave had minimal effect. The mean segmental ROM at T11-12, T10-11, and T9-10, respectively, was 6%, 40%, and 99% for UIV+1 Loop; 6%, 44%, and 99% for UIV+1 Weave; 5%, 23%, and 26% for UIV+2 Loop; and 5%, 24%, and 31% for UIV+2 Weave.Tethers shared loads with posterior ligaments; consequently, increasing tether preload tension reduced ISL/SSL forces, but screw loads increased. Further attenuation of junctional ROM and IDP reversed above approximately 100 N tether preload, suggesting diminished benefit for biomechanical PJK prophylaxis at higher preload tensioning.

CONCLUSIONS

In this study, finite element analysis demonstrated UIV+2 Loop and/or Weave tether configurations most effectively mitigated adjacent-segment stress in long instrumented spine constructs. Tether preload dampened ligament forces at the expense of screw loads, and an inflection point (approximately 100 N) was demonstrated above which junctional ROM and IDP worsened (i.e., avoid over-tightening tethers). Results suggest tether configuration and tension influence PJK biomechanics and further clinical research is warranted.

Biomechanical assessment of proximal junctional semi-rigid fixation in long-segment thoracolumbar constructs

OBJECTIVE

Proximal junctional kyphosis (PJK) and failure (PJF) are potentially catastrophic complications that result from abrupt changes in stress across rigid instrumented and mobile non-fused segments of the spine (transition zone) after adult spinal deformity surgery. Recently, data have indicated that extension (widening) of the transitional zone via use of proximal junctional (PJ) semi-rigid fixation can mitigate this complication. To assess the biomechanical effectiveness of 3 semi-rigid fixation constructs (compared to pedicle screw fixation alone), the authors performed cadaveric studies that measured the extent of PJ motion and intradiscal pressure changes (ΔIDP).

METHODS

To measure flexibility and ΔIDP at the PJ segments, moments in flexion, extension, lateral bending (LB), and torsion were conducted in 13 fresh-frozen human cadaveric specimens. Five testing cycles were conducted, including intact (INT), T10–L2 pedicle screw-rod fixation alone (PSF), supplemental hybrid T9 Mersilene tape insertion (MT), hybrid T9 sublaminar band insertion (SLB1), and hybrid T8/T9 sublaminar band insertion (SLB2).

RESULTS

Compared to PSF, SLB1 significantly reduced flexibility at the level rostral to the upper-instrumented vertebral level (UIV+1) under moments in 3 directions (flexion, LB, and torsion, p ≤ 0.01). SLB2 significantly reduced motion in all directions at UIV+1 (flexion, extension, LB, torsion, p < 0.05) and at UIV+2 (LB, torsion, p ≤ 0.03). MT only reduced flexibility in extension at UIV+1 (p = 0.02). All 3 constructs revealed significant reductions in ΔIDP at UIV+1 in flexion (MT, SLB1, SLB2, p ≤ 0.02) and torsion (MT, SLB1, SLB2, p ≤ 0.05), while SLB1 and SLB2 significantly reduced ΔIDP in extension (SLB1, SLB2, p ≤ 0.02) and SLB2 reduced ΔIDP in LB (p = 0.05). At UIV+2, SLB2 similarly significantly reduced ΔIDP in extension, LB, and torsion (p ≤ 0.05).

CONCLUSIONS

Compared to MT, the SLB1 and SLB2 constructs significantly reduced flexibility and ΔIDP in various directions through the application of robust anteroposterior force vectors at UIV+1 and UIV+2. These findings indicate that semi-rigid sublaminar banding can most effectively expand the transition zone and mitigate stresses at the PJ levels of long-segment thoracolumbar constructs.

Sagittal Alignment After Surgical Treatment of Adolescent Idiopathic Scoliosis-Application of the Roussouly Classification

STUDY DESIGN

Retrospective cohort study.

OBJECTIVES

To investigate spinopelvic alignment and spine shape in patients surgically treated for adolescent idiopathic scoliosis (AIS) and to assess the distribution and clinical applicability of the Roussouly classification.

SUMMARY OF BACKGROUND DATA

How spinopelvic alignment is affected in AIS patients is not well established. Roussouly et al. proposed a classification based on the sagittal spinal profile and spinopelvic alignment that may have clinical utility in these patients.

METHODS

A consecutive cohort of 134 surgically treated AIS patients were included. Whole-spine standing lateral radiographs were analyzed preoperatively, one-week postoperatively and at two-year follow-up. Patients were categorized using the modified Roussouly classification and analyzed for sagittal alignment.

RESULTS

Postoperatively, global thoracic kyphosis (TK) decreased by 2.6° and lumbar lordosis (LL) decreased by 6.2°(p ≤ .012) while Pelvic tilt (PT) increased 1.4° (p = .024). At two-year follow-up, TK and LL had returned to preoperative values (p ≥ .346) while PT had decreased from preoperative 9.7 ± 7.6° to 7.0 ± 7.5° (p > .001). Proximal junctional angle increased from 8.4 ± 5.0° preoperatively to 12.8 ± 8.9 (p < .001). Preoperatively, Roussouly curve types were distributed equally apart from a lower rate of type 1 (12%). At final follow-up, 30% were categorised as type 3 with pelvic anteversion which is considerably higher than the normal adolescent population. Only three patients were type 1 at the final follow-up. Overall, we found a high rate of proximal junctional kyphosis (16%), PI-LL mismatch (60%) and pelvic anteversion (38%). In preoperative type 1 patients, the rate was 50%, 82% and 64%, respectively.

CONCLUSION

We found that immediate postoperative changes in lordosis and kyphosis were reversed at final follow-up and found evidence of proximal junctional kyphosation and pelvic anteversion as the main compensatory mechanisms. Poor sagittal alignment was frequent in type 1 curves, and surgical treatment may need to be individualized according to the sagittal profile.

LEVEL OF EVIDENCE

III.

Prevention of Proximal Junctional Kyphosis & Failure Using Sublaminar Bands in a Hybrid Construct in Pediatric Kyphosis Deformity

Purpose

The purpose of this study was to evaluate short term proximal junctional kyphosis and failure (PJK/PJF) rates and clinical outcomes in patients who underwent posterior spinal fusion (PSF) and deformity correction using sublaminar bands in a hybrid construct.

Methods

This is a retrospective review of pediatric spinal deformity cases performed by a single surgeon from January 2008 to December 2012. One hundred thirty-six pediatric deformity patient charts were reviewed for inclusion into the study. All 136 patients reviewed had been operatively managed for a spinal deformity and were younger than 18 years of age. Our study's inclusion criteria comprised patients with a kyphosis greater than 60°, the use of sublaminar polyester bands in a hybrid surgical construct, and a minimum of 2 years of follow-up. Of the 136 spinal deformity cases reviewed, 17 cases met our inclusion criteria. No cases were excluded because of technical difficulty of band placement or for not using a hybrid construct with sublaminar bands (as it was the primary surgeon's principal construct of choice). PJK was defined as proximal junction sagittal Cobb angle (PJA) of at least 10° greater than the preoperative measurement. This was assessed by comparison of preoperative and postoperative radiographs. Our hypothesis was that PSF performed with our technique would have a lower rate of PJK than previously reported with other methods of fixation.

Results

The range of the preoperative kyphosis was 62°-111°, and postoperative kyphosis was 12°-55°. There was one case of PJK (5.8%) and no cases of PJF.

Conclusion

This study on short term outcomes of PSF using sublaminar bands in a hybrid technique for treatment of kyphosis demonstrated a lower rate of PJK than has been reported in prior studies. This technique protects the fixation within the upper instrumented vertebrae of long segment PSFs.