Finite element assessment of a disc-replacement implant for treating scoliotic deformity.

BACKGROUND: Bracing and spinal fusion surgery have long been the primary methods for idiopathic scoliosis correction; however, there exist multiple limitations with both techniques. Growth modulation techniques have recently been attempted, but are typically performed across multiple vertebral elements. The aim of this study was to quantify the corrective abilities of a dual-angled, wedge shaped, rigid disc implant designed to correct spinal deformity. METHODS: The 3D spinal geometry of four patients was reconstructed using calibrated radiographs, from which personal finite element models were created. Coronal and sagittal Cobb angles and axial stress distribution were calculated pre- and post- simulation of device implantation at the apical vertebral element. FINDINGS: Insertion of a rigid wedged implant resulted in up to 90.1% coronal correction with kyphotic normalization, and reduced axial stress differential within adjacent vertebrae by up to 83.3%. This correction in axial stress differential was seen to propagate to subjacent vertebrae in both rostral and caudal directions. Insertion of two implants yielded greater correction with respect to all three measures. INTERPRETATION: Local Cobb angle correction, increased kyphotic angle, and a decrease in axial stress differential with adjacent and subjacent vertebral levels demonstrate a potential for deformity correction from within the disc space. The decrease in axial stress differential demonstrates a capacity for growth modulation and reversal of the Heuter-Volkmann principle. Based on qualitative views of spinal shape following device implantation, the wedged implant proved more efficacious in correcting single thoracic curves than double major curves.

Defining criteria for optimal lumbar curve correction following the selective thoracic fusion surgery in Lenke 1 adolescent idiopathic scoliosis: developing a decision tree.

OBJECTIVE: The aim of this study was to identify the range of optimal versus suboptimal rates of spontaneous lumbar Cobb correction (SLCC%) and the factors predicting such outcomes in a cohort of Lenke 1 adolescent idiopathic scoliosis (AIS) after posterior spinal fusion surgery. METHODS: Seventy-one consecutive Lenke1 B and C AIS patients with a fusion level to L1 and higher with two-year follow-up were included. Thoracic kyphosis (T1-T4 and T4-T12 TK), lumbar lordosis (L1-S1 LL), thoracic and lumbar Cobb angles, thoracic and lumbar apical vertebral rotations and translations (AVR and AVT), pelvic incidence, sacral slope, and sagittal and frontal balances were measured at preoperative, early postoperative, and two-year follow-up. The SLCC% was calculated between preoperative and two-year follow-up. A clustering analysis determined the subgroups of patients with significantly higher and lower (optimal versus suboptimal) rate of SLCC% in the cohort at two-year follow-up. The cutoff values of the preoperative and early postoperative radiographic parameters that significantly predicted the optimal and suboptimal SLCC% were determined using a decision tree. RESULTS: The averages of the optimal versus suboptimal range of SLCC% in the cohort were 72% [55%, 105%] versus 39% [- 7%, 42%]. Preoperative and early postoperative spinal parameters predicted the optimal versus suboptimal SLCC% with an accuracy of 82%, 95%CI [0.73-0.94]. Preoperative AVTLumbar < 10 mm was a predictor of optimal SLCC%. In patients with a preoperative AVTLumbar > 10 mm, early postoperative T4-T12 TK < 24° (but not less than 17°) accompanied by - 5° < AVRThoracic < 5° were the main predictors of optimal SLCC% in our cohort. CONCLUSION: Quantitative clustering of the SLCC% into optimal and suboptimal groups allowed identifying the cutoff values of preoperative (AVTLumbar) and early postoperative (T4-T12 TK and AVRThoracic) spinal parameters that can predict the optimal range of SLCC% at two-year postoperative in our cohort of Lenke 1 AIS. LEVEL OF EVIDENCE: IV.

Costs and Length of Stay for the Acute Care of Patients with Motor-Complete Spinal Cord Injury Following Cervical Trauma: The Impact of Early Transfer to Specialized Acute SCI Center.

OBJECTIVE: Acute spinal cord injury (SCI) centers aim to optimize outcome following SCI. However, there is no timeframe to transfer patients from regional to SCI centers in order to promote cost-efficiency of acute care. Our objective was to compare costs and length of stay (LOS) following early and late transfer to the SCI center. DESIGN: A retrospective cohort study involving 116 individuals was conducted. Group 1 (n = 87) was managed in an SCI center promptly after the trauma, whereas group 2 (n = 29) was transferred to the SCI center only after surgery. Direct comparison and multivariate linear regression analyses were used to assess the relationship between costs, LOS, and timing to transfer to the SCI center. RESULTS: Length of stay was significantly longer for group 2 (median, 93.0 days) as compared with group 1 (median, 40.0 days; P < 10), and average costs were also higher (median, Canadian $17,920.0 vs. $10,521.6; P = 0.004) for group 2, despite similar characteristics. Late transfer to the SCI center was the main predictive factor of longer LOS and increased costs. CONCLUSIONS: Early admission to the SCI center was associated with shorter LOS and lower costs for patients sustaining tetraplegia. Early referral to an SCI center before surgery could lower the financial burden for the health care system. TO CLAIM CME CREDITS: Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME CME OBJECTIVES: Upon completion of this article, the reader should be able to: (1) Determine the optimal timing for transfer of individuals with cervical traumatic spinal cord injury (SCI) in order to decrease acute care resource utilization; (2) Determine benefits of a complete perioperative management in a specialized SCI center; and (3) Identify factors that may influence resource utilization for acute care following motor-complete tetraplegia. LEVEL: Advanced ACCREDITATION: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.The Association of Academic Physiatrists designates this activity for a maximum of 1.5 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.

Assessment of Regional Bone Density in Fractured Vertebrae Using Quantitative Computed Tomography.

STUDY DESIGN: Cohort study. PURPOSE: The aim of this study is to propose and evaluate a new technique to assess bone mineral density of fractured vertebrae using quantitative computed tomography (QCT). OVERVIEW OF LITERATURE: There is no available technique to estimate bone mineral density (BMD) at the fractured vertebra because of the alterations in bony structures at the fracture site. METHODS: Forty patients with isolated fracture from T10 to L2 were analyzed from the vertebrae above and below the fracture level. Apparent density (AD) was measured based on the relationship between QCT images attenuation coefficients and the density of calibration objects. AD of 8 independent regions of interest (ROI) within the vertebral body and 2 ROI within the pedicles of vertebrae above and below the fractured vertebra were measured. At the level of the fractured vertebra, AD was measured at the pedicles, which are typically intact. AD of the fractured vertebral body was linearly interpolated, based on the assumption that AD at the fractured vertebra is equivalent to the average AD measured in vertebrae adjacent to the fracture. Estimated and measured AD of the pedicles at the fractured level were compared to verify our assumption of linear interpolation from adjacent vertebrae. RESULTS: The difference between the measured and the interpolated density of the pedicles at the fractured vertebra was 0.006 and 0.003 g/cm3 for right and left pedicle respectively. The highest mean AD located at the pedicles and the lowest mean AD was found at the anterior ROI of the vertebral body. Significant negative correlation exist between age and AD of ROI in the vertebral body. CONCLUSIONS: This study suggests that the proposed technique is adequate to estimate the AD of a fractured vertebra from the density of adjacent vertebrae.

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement.

BACKGROUND: Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation. METHOD: Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated. FINDING: The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors. INTERPRETATION: Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Non-neurological outcomes after complete traumatic spinal cord injury: the impact of surgical timing.

It remains unclear whether the benefits of early surgical timing are significant in neurologically complete spinal cord injury (SCI). We wanted to compare the effects of early and late surgical timing on non-neurological outcomes in persons with traumatic complete SCI. All cases of traumatic complete SCI referred to a single institution between 2000 and 2011 were retrospectively reviewed. The occurrence of pneumonia, urinary tract infection (UTI), pressure ulcer (PU), and all other postoperative complications were recorded. Cost of acute hospitalization was calculated for each patient based on administrative data. Patients operated on within 24 h of the trauma were compared with patients operated on later than 24 h after the trauma. The effects of surgical timing on complication rate and cost of hospitalization were adjusted for potential confounding variables using multiple regression analyses. Fifty-five patients were operated on ≤ 24 h from injury and 142 were operated on >24 h from injury. Baseline demographic and clinical variables were comparable between the two groups. Pneumonia, UTI, and the presence of any complications were significantly higher in the group operated on >24 h post-trauma. Cost of hospitalization was higher among patients operated >24h post-trauma (≤ 24 h: 22,828$ vs. >24 h: 29,714$). Surgical timing >24 h was a predictor of pneumonia, UTI, total complications. and higher cost of hospitalization after controlling for other confounding variables. This study shows that surgical decompression and stabilization ≤ 24 h following a complete SCI may be a cost-effective strategy to reduce the postoperative complication rate.

Biomechanical Assessment of Reduction Forces Measured During Scoliotic Instrumentation Using Two Different Screw Designs.

STUDY DESIGN: Biomechanical finite element models simulated deformity correction using pedicle screw instrumentation and measured forces at the screw-vertebra interface. OBJECTIVES: Compare 2 different screw designs with respect to reaction forces at screw-vertebra interfaces during scoliosis correction maneuvers. SUMMARY OF BACKGROUND DATA: Pedicle screw developments strive to enhance surgical techniques and improve patient safety. It is believed that a screw with increased lateral angulation and reduction tabs enables a more gradual correction, more effectively distributes corrective forces over multiple levels, and reduces forces at screw-vertebra interfaces compared with standard polyaxial screws. METHODS: We selected 3 scoliotic patients and reconstructed their preoperative spinal profiles as finite element models using radiographic clinical measures. The osteoligamentous models were programmed and validated with mechanical properties from published literature. We used postoperative radiographs to determine instrumented levels and calibrate disc properties to corroborate simulated results with clinical data. We alternatively examined favored angle (FA) screws and polyaxial (PA) screws using correction steps characteristic to their design. We also explored sensitivity of screw forces consequent to misalignment with adjacent screws. RESULTS: Simulated postoperative spinal profiles on average adhered to clinical measures within 5°. We observed no significant differences in simulated corrective profiles between screw types (5° or less). Compared with PA screws, FA screws reduced peak pullout and lateral forces by 27% and 35%, respectively, and correspondingly reduced mean pullout and lateral forces by 48% and 40%, respectively. Changes in peak and average pullout forces resulting from screw misalignment were 56% and 82% less, respectively, with FA screws. CONCLUSIONS: This analysis demonstrated reduced screw-vertebra peak and mean forces when using a pedicle screw with a favored angle bias and reduction tabs to correct scoliosis. Compared with PA screws, FA screws provide similar correction, decrease forces applied at the screw-vertebra interface, and are more forgiving if misaligned.

Does timing of surgery affect hospitalization costs and length of stay for acute care following a traumatic spinal cord injury?

Although there is a trend toward performing early surgery for traumatic spinal cord injury (SCI), it remains unclear whether this tendency leads to decreased costs and length of stay (LOS) for acute care. This study determined the impact of surgical timing on costs and LOS after a traumatic SCI. A total of 477 consecutive patients sustaining an acute traumatic SCI and receiving surgery at a level I trauma center were included. A general linear model was used to assess the relationship among costs, LOS, and surgical delay, while accounting for various sociodemographic and clinical covariables. The analysis was also repeated with surgical delay dichotomized within 24 h or later after the trauma. Mean costs and LOS for all patients were respectively 24,156 ± 17,244 $CAD and 35.0 ± 39.4 days. The costs of acute care hospitalization were related to the surgical delay between the trauma and the surgery, in addition to age, injury severity score (ISS), American Spinal Injury Association (ASIA) grade, and neurological level. LOS was associated with the surgical delay dichotomized into two groups (<24 vs. ≥24 h), as well as with age, ISS, ASIA grade, and neurological level. This study suggests that resource utilization in terms of costs and LOS for the acute hospitalization is decreased with early surgery after an acute traumatic SCI, particularly if the procedure is performed within 24 h following the trauma. Performing the surgery as early as possible when the patient is cleared for surgery could lower the financial burden on the healthcare system, while optimizing the neurological recovery.

Real time noninvasive assessment of external trunk geometry during surgical correction of adolescent idiopathic scoliosis.

BACKGROUND: The correction of trunk deformity is crucial in scoliosis surgery, especially for the patient's self-image. However, direct visualization of external scoliotic trunk deformity during surgical correction is difficult due to the covering draping sheets. METHODS: An optoelectronic camera system with 10 passive markers is used to track the trunk geometry of 5 scoliotic patients during corrective surgery. The position of 10 anatomical landmarks and 5 trunk indices computed from the position of the passive markers are compared during and after instrumentation of the spine. RESULTS: Internal validation of the accuracy of tracking was evaluated at 0.41 +/- 0.05 mm RMS. Intra operative tracking during surgical maneuvers shows improvement of the shoulder balance during and after correction of the spine. Improvement of the overall patient balance is observed. At last, a minor increase of the spinal length can be noticed. CONCLUSION: Tracking of the external geometry of the trunk during surgical correction is useful to monitor changes occurring under the sterile draping sheets. Moreover, this technique can used be used to reach the optimal configuration on the operating frame before proceeding to surgery. The current tracking technique was able to detect significant changes in trunk geometry caused by posterior instrumentation of the spine despite significant correction of the spinal curvature. It could therefore become relevant for computer-assisted guidance of surgical maneuvers when performing posterior instrumentation of the scoliotic spine, provide important insights during positioning of patients.

Assessment of lumbosacral kyphosis in spondylolisthesis: a computer-assisted reliability study of six measurement techniques.

Although recognized as an important aspect in the management of spondylolisthesis, there is no consensus on the most reliable and optimal measure of lumbosacral kyphosis (LSK). Using a custom computer software, four raters evaluated 60 standing lateral radiographs of the lumbosacral spine during two sessions at a 1-week interval. The sample size consisted of 20 normal, 20 low and 20 high grade spondylolisthetic subjects. Six parameters were included for analysis: Boxall's slip angle, Dubousset's lumbosacral angle (LSA), the Spinal Deformity Study Group's (SDSG) LSA, dysplastic SDSG LSA, sagittal rotation (SR), kyphotic Cobb angle (k-Cobb). Intra- and inter-rater reliability for all parameters was assessed using intra-class correlation coefficients (ICC). Correlations between parameters and slip percentage were evaluated with Pearson coefficients. The intra-rater ICC's for all the parameters ranged between 0.81 and 0.97 and the inter-rater ICC's were between 0.74 and 0.98. All parameters except sagittal rotation showed a medium to large correlation with slip percentage. Dubousset's LSA and the k-Cobb showed the largest correlations (r= -0.78 and r= -0.50, respectively). SR was associated with the weakest correlation (r= -0.10). All other parameters had medium correlations with percent slip (r= 0.31-0.43). All measurement techniques provided excellent inter- and intra-rater reliability. Dubousset's LSA showed the strongest correlation with slip grade. This parameter can be used in the clinical setting with PACS software capabilities to assess LSK. A computer-assisted technique is recommended in order to increase the reliability of the measurement of LSK in spondylolisthesis.

Assessment of sacral doming in lumbosacral spondylolisthesis.

STUDY DESIGN: Quantitative versus subjective evaluation of sacral doming in lumbosacral spondylolisthesis. OBJECTIVES: To evaluate the relevance of the Spinal Deformity Study Group (SDSG) index in the assessment of sacral doming and to propose a quantitative criterion to differentiate between significant and nonsignificant doming. SUMMARY OF BACKGROUND DATA: There is no consensus on the optimal technique to assess sacral doming, although it is an important feature in spondylolisthesis. METHODS: Five spinal surgeons subjectively assessed the sacral endplate of 100 subjects (34 high-grade spondylolisthesis, 50 low-grade spondylolisthesis, 16 controls) from lateral radiographs. Subjects were classified by each surgeon as having significant or nonsignificant sacral doming. An independent observer quantitatively evaluated sacral doming for all subjects using the SDSG index. A criterion to differentiate significant from nonsignificant sacral doming was sought, based on the comparison between the subjective assessment of surgeons and the quantitative evaluation by the independent observer. Intrarater and interrater reliability of the SDSG index was evaluated using intraclass correlation coefficient (ICC). RESULTS: Intrarater and interrater ICCs for the SDSG index were excellent at 0.91 and 0.88, respectively. Sacral doming evaluated with the SDSG index was 11.6% +/- 5.0% (range, 1.5%-18.9%), 16.4% +/- 6.3% (range, 3.7%-35.6%), and 27.9% +/- 10.9% (range, 5.7%-56.9%) for controls, low-grade, and high-grade cases, respectively. Overall intersurgeon agreement on the significance of sacral doming was substantial at 88% (kappa = 0.72). With a threshold value of 25% for the SDSG index, 93% of concordance was found between the quantitative evaluation using the SDSG index and the multisurgeons subjective assessment. CONCLUSION: This study confirms the relevance of the SDSG index to assess sacral doming in lumbosacral spondylolisthesis. The authors propose a criterion of 25% to differentiate significant from nonsignificant sacral doming using the SDSG index. Such a criterion will allow more accurate assessment of sacral remodeling, especially for borderline cases, and facilitate comparisons between studies.

Comparison between constrained and non-constrained Cobb techniques for the assessment of thoracic kyphosis and lumbar lordosis.

Sagittal curvatures of the spine can be assessed using the constrained or non-constrained Cobb techniques. However, there is no study that specifically compares these two techniques. The objective of this study is to assess the reproducibility and clinical relevance of the non-constrained Cobb technique (non-constrained limit vertebrae) compared to the constrained Cobb technique (constrained limit vertebrae). Standing sagittal radiographs of the spine of ten adolescents with idiopathic scoliosis, ten adolescents with spondylolisthesis and ten normal adolescents were selected. Thoracic kyphosis (TK) and lumbar lordosis (LL) were measured twice by three observers using both constrained and non-constrained Cobb techniques. Pearson's correlation coefficients, as well as intra- and inter-observer intra-class correlation coefficients (ICC) were calculated. Inter-observer ICCs were similar for TK and LL with both techniques, ranging from 0.84 to 0.89. Intra-observer ICCs for both techniques were between 0.74 and 0.92 for TK, while they were between 0.87 and 0.97 for LL. The two techniques were highly correlated for the measurement of the TK (r = 0.96) and LL (r = 0.94). Computer-assisted assessment of the sagittal profile using the non-constrained Cobb technique provides excellent reproducibility. As opposed to the constrained Cobb technique, the non-constrained Cobb technique takes into account the variability in the level of transition between the TK and LL. However, adequate use of this technique requires accurate identification of the limit vertebrae in the thoracolumbar spine. Consequently, a computer-assisted technique is recommended when using the non-constrained Cobb technique.

Computerized assessment of sagittal curvatures of the spine: comparison between Cobb and tangent circles techniques.

OBJECTIVE: The tangent circles technique has been proposed as an alternative to the Cobb angle technique to assess sagittal curves of the spine. However, it has never been compared directly to the Cobb technique. This study compares the reproducibility and clinical relevance of the maximum Cobb angle and tangent circles techniques. METHOD: Standing sagittal radiographs of the spine of 10 adolescents with idiopathic scoliosis, 10 adolescents with spondylolisthesis, and 10 healthy adolescents were used. Thoracic kyphosis (TK) and lumbar lordosis (LL) were measured by 3 observers using the maximum Cobb angle and the tangent circles techniques. Intra- and interobserver intraclass correlation coefficients (ICCs) were calculated. RESULT: Intra- and interobserver ICCs for TK were 0.88 and 0.85, respectively, for the maximum Cobb angle technique, and 0.94 and 0.83, respectively, for the tangent circles technique. Intra- and interobserver ICCs for LL were 0.97 and 0.77, respectively, for the maximum Cobb angle technique, and 0.88 and 0.94, respectively, for the tangent circles technique. The 2 techniques were highly correlated for the measurement of the TK (r=0.93) and LL (r=0.88). CONCLUSION: Both techniques provide excellent intra- and interobserver reproducibility. Tangent circles technique may be a good alternative to the Cobb angle technique because it allows the evaluation of the global geometry of sagittal spinal curves, especially when there is limited visibility of bony structures on radiographs.