Metastatic Burst Fracture Risk Assessment Based on Complex Loading of the Thoracic Spine

The role of bone metastases on the mechanical competence of human vertebrae

Spine is the most common site for bone metastases. The evaluation of the mechanical competence and failure location in metastatic vertebrae is a biomechanical and clinical challenge. Little is known about the failure behaviour of vertebrae with metastatic lesions. The aim of this study was to use combined micro-Computed Tomography (microCT) and time-lapsed mechanical testing to reveal the failure location in metastatic vertebrae. Fifteen spine segments, each including a metastatic and a radiologically healthy vertebra, were tested in compression up to failure within a microCT. Volumetric strains were measured using Digital Volume Correlation. The images of undeformed and deformed specimens were overlapped to identify the failure location. Vertebrae with lytic metastases experienced the largest average compressive strains (median ± standard deviation: -8506 ± 4748microstrain), followed by the vertebrae with mixed metastases (-7035 ± 15605microstrain), the radiologically healthy vertebrae (-5743 ± 5697microstrain), and the vertebrae with blastic metastases (-3150 ± 4641microstrain). Strain peaks were localised within and nearby the lytic lesions or around the blastic tissue. Failure between the endplate and the metastasis was identified in vertebrae with lytic metastases, whereas failure localised around the metastasis in vertebrae with blastic lesions. This study showed for the first time the role of metastases on the vertebral internal deformations. While lytic lesions lead to failure of the metastatic vertebra, vertebrae with blastic metastases are more likely to induce failure in the adjacent vertebrae. Nevertheless, every metastatic lesion affects the vertebral deformation differently, making it essential to assess how the lesion affects the bone microstructure. These results suggest that the properties of the lesion (type, size, location within the vertebral body) should be considered when developing clinical tools to predict the risk of fracture in patients with metastatic lesions.

Large Lytic Defects Produce Kinematic Instability and Loss of Compressive Strength in Human Spines: An in Vitro Study

BACKGROUND

In patients with spinal metastases, kinematic instability is postulated to be a predictor of pathologic vertebral fractures. However, the relationship between this kinematic instability and the loss of spinal strength remains unknown.

METHODS

Twenty-four 3-level thoracic and lumbar segments from 8 cadaver spines from female donors aged 47 to 69 years were kinematically assessed in axial compression (180 N) and axial compression with a flexion or extension moment (7.5 Nm). Two patterns of lytic defects were mechanically simulated: (1) a vertebral body defect, corresponding to Taneichi model C (n = 13); and (2) the model-C defect plus destruction of the ipsilateral pedicle and facet joint, corresponding to Taneichi model E (n = 11). The kinematic response was retested, and compression strength was measured. Two-way repeated-measures analysis of variance was used to test the effect of each model on the kinematic response of the segment. Multivariable linear regression was used to test the association between the kinematic parameters and compressive strength of the segment.

RESULTS

Under a flexion moment, and for both models C and E, the lesioned spines exhibited greater flexion range of motion (ROM) and axial translation than the control spines. Both models C and E caused lower extension ROM and greater axial, sagittal, and transverse translation under an extension moment compared with the control spines. Two-way repeated-measures analysis revealed that model E, compared with model C, caused significantly greater changes in extension and torsional ROM under an extension moment, and greater sagittal translation under a flexion moment. For both models C and E, greater differences in flexion ROM and sagittal translation under a flexion moment, and greater differences in extension ROM and in axial and transverse translation under an extension moment, were associated with lower compressive strength of the lesioned spines.

CONCLUSIONS

Critical spinal lytic defects result in kinematic abnormalities and lower the compressive strength of the spine.

CLINICAL RELEVANCE

This experimental study demonstrates that lytic foci degrade the kinematic stability and compressive strength of the spine. Understanding the mechanisms for this degradation will help to guide treatment decisions that address inferred instability and fracture risk in patients with metastatic spinal disease.

Conventional finite element models estimate the strength of metastatic human vertebrae despite alterations of the bone's tissue and structure

INTRODUCTION

Pathologic vertebral fractures are a major clinical concern in the management of cancer patients with metastatic spine disease. These fractures are a direct consequence of the effect of bone metastases on the anatomy and structure of the vertebral bone. The goals of this study were twofold. First, we evaluated the effect of lytic, blastic and mixed (both lytic and blastic) metastases on the bone structure, on its material properties, and on the overall vertebral strength. Second, we tested the ability of bone mineral content (BMC) measurements and standard FE methodologies to predict the strength of real metastatic vertebral bodies.

METHODS

Fifty-seven vertebral bodies from eleven cadaver spines containing lytic, blastic, and mixed metastatic lesions from donors with breast, esophageal, kidney, lung, or prostate cancer were scanned using micro-computed tomography (μCT). Based on radiographic review, twelve vertebrae were selected for nanoindentation testing, while the remaining forty-five vertebrae were used for assessing their compressive strength. The μCT reconstruction was exploited to measure the vertebral BMC and to establish two finite element models. 1) a micro finite element (μFE) model derived at an image resolution of 24.5 μm and 2) homogenized FE (hFE) model derived at a resolution of 0.98 mm. Statistical analyses were conducted to measure the effect of the bone metastases on BV/TV, indentation modulus (E_it), ratio of plastic/total work (W_Pl/W_tot), and in vitro vertebral strength (F_exp). The predictive value of BMC, μFE stiffness, and hFE strength were evaluated against the in vitro measurements.

RESULTS

Blastic vertebral bodies exhibit significantly higher BV/TV compared to the mixed (p = 0.0205) and lytic (p = 0.0216) vertebral bodies. No significant differences were found between lytic and mixed vertebrae (p = 0.7584). Blastic bone tissue exhibited a 5.8% lower median E_it (p< 0.001) and a 3.3% lower median W_pl/W_tot (p<0.001) compared to non-involved bone tissue. No significant differences were measured between lytic and non-involved bone tissues. F_exp ranged from 1.9 to 13.8 kN, was strongly associated with hFE strength (R²=0.78, p< 0.001) and moderately associated with BMC (R²=0.66, p< 0.001) and μFE stiffness (R²=0.66, p< 0.001), independently of the lesion type.

DISCUSSION

Our findings show that tumour-induced osteoblastic metastases lead to slightly, but significantly lower bone tissue properties compared to controls, while osteolytic lesions appear to have a negligible impact. These effects may be attributed to the lower mineralization and woven nature of bone forming in blastic lesions whilst the material properties of bone in osteolytic vertebrae appeared little changed. The moderate association between BMC- and FE-based predictions to fracture strength suggest that vertebral strength is affected by the changes of bone mass induced by the metastatic lesions, rather than altered tissue properties. In a broader context, standard hFE approaches generated from CTs at clinical resolution are robust to the lesion type when predicting vertebral strength. These findings open the door for the development of FE-based prediction tools that overcomes the limitations of BMC in accounting for shape and size of the metastatic lesions. Such tools may help clinicians to decide whether a patient needs the prophylactic fixation of an impending fracture.

The effects of metastatic lesion on the structural determinants of bone: Current clinical and experimental approaches

Metastatic bone disease is incurable with an associated increase in skeletal-related events, particularly a 17-50% risk of pathologic fractures. Current surgical and oncological treatments are palliative, do not reduce overall mortality, and therefore optimal management of adults at risk of pathologic fractures presents an unmet medical need. Plain radiography lacks specificity and may result in unnecessary prophylactic fixation. Radionuclide imaging techniques primarily supply information on the metabolic activity of the tumor or the bone itself. Magnetic resonance imaging and computed tomography provide excellent anatomical and structural information but do not quantitatively assess bone matrix. Research has now shifted to developing unbiased data-driven tools that can predict risk of impending fractures and guide individualized treatment decisions. This review discusses the state-of-the-art in clinical and experimental approaches for prediction of pathologic fractures with bone metastases. Alterations in bone matrix quality are associated with an age-related increase in skeletal fragility but the impact of metastases on the intrinsic material properties of bone is unclear. Engineering-based analyses are non-invasive with the capability to evaluate oncological treatments and predict failure due to the progression of metastasis. The combination of these approaches may improve our understanding of the underlying deterioration in mechanical performance.

Instability and impending instability in patients with vertebral metastatic disease

Metastatic disease commonly involves the spine with an increasing incidence due to a worldwide rise of cancer incidence and a longer survival of patients with osseous metastases. Metastases compromise the mechanical integrity of the vertebra and make it susceptible to fracture. Patients with pathological vertebral fracture often become symptomatic, with mechanical pain generally due to intervertebral instability, and may develop spinal cord compression and neurological deficits. Advances in imaging, radiotherapy, as well as in spinal surgery techniques, have allowed the evolution from conventional palliative external beam radiotherapy to modern stereotactic radiosurgery and from traditional open surgery to less-invasive, and sometimes prophylactic stabilization surgical treatments. It is therefore clear that fracture risk prediction, and maintenance or restoration of intervertebral stability, are important objectives in the management of these patients. Correlation between imaging findings and clinical manifestations is crucial, and a common knowledge base for treatment team members rather than a compartmentalized view is very important. This article reviews the literature on the imaging and clinical diagnosis of intervertebral instability and impending instability in the setting of spine metastatic disease, including the spinal instability neoplastic score, which is a reliable tool for diagnosing unstable or potentially unstable metastatic spinal lesions, and on the different elements considered for treatment.

Case-specific non-linear finite element models to predict failure behavior in two functional spinal units

Current finite element (FE) models predicting failure behavior comprise single vertebrae, thereby neglecting the role of the posterior elements and intervertebral discs. Therefore, this study aimed to develop a more clinically relevant, case-specific non-linear FE model of two functional spinal units able to predict failure behavior in terms of (i) the vertebra predicted to fail; (ii) deformation of the specimens; (iii) stiffness; and (iv) load to failure. For this purpose, we also studied the effect of different bone density-mechanical properties relationships (material models) on the prediction of failure behavior. Twelve two functional spinal units (T6-T8, T9-T11, T12-L2, and L3-L5) with and without artificial metastases were destructively tested in axial compression. These experiments were simulated using CT-based case-specific non-linear FE models. Bone mechanical properties were assigned using four commonly used material models. In 10 of the 11 specimens our FE model was able to correctly indicate which vertebrae failed during the experiments. However, predictions of the three-dimensional deformations of the specimens were less promising. Whereas stiffness of the whole construct could be strongly predicted (R²  = 0.637-0.688, p < 0.01), we obtained weak correlations between FE predicted and experimentally determined load to failure, as defined by the total reaction force exhibiting a drop in force (R²  = 0.219-0.247, p > 0.05). Additionally, we found that the correlation between predicted and experimental fracture loads did not strongly depend on the material model implemented, but the stiffness predictions did. In conclusion, this work showed that, in its current state, our FE models may be used to identify the weakest vertebra, but that substantial improvements are required in order to quantify in vivo failure loads. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodical, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 36:3208-3218, 2018.

Female Human Spines with Simulated Osteolytic Defects: CT-based Structural Analysis of Vertebral Body Strength

Purpose To evaluate a CT structural analysis protocol (SAP) for estimating the strength of human female cadaveric spines with lytic lesions. Materials and Methods Osteolytic foci was created in the middle vertebra of 44 thoracic and lumbar three-level segments from 11 female cadavers (age range, 50-70 years). The segments underwent CT by using standard clinical protocol and their failure strength was assessed at CT SAP. The spines were mechanically tested to failure in pure axial compression or in compression with torsion. The relationships of defect size, bone mineral density, and predicted failure load (at CT SAP) with measured vertebral strength were assessed with linear regression. Analysis of variance and Tukey test were used to evaluate the effect of region and mechanical test on spine strength. Results With axial compression, CT SAP predictions of vertebral strength correlated with the thoracic (r = 0.84; P < .001) and lumbar (r = 0.85; P < .001) segment-measured strength. Bone mineral density correlated with the lumbar (r = 0.64; P = .003) and thoracic (r, 0.51; P = .050) strength. At compression with torsion, CT SAP predictions of strength were moderately correlated with vertebral strength (r = 0.66; P = .018). At compression with torsion, bone mineral density was not correlated with spinal strength (thoracic and lumbar: r = 0.31 and r = 0.26, respectively; P = .539 and .610, respectively). The lytic focus size (range, 28%-41%) was not associated with vertebral strength. Conclusion CT SAP assessment of strength in vertebrae with lytic lesions correlated with the measured strength of female vertebral bodies. ^© RSNA, 2018 Online supplemental material is available for this article.

Biomechanical effects of metastasis in the osteoporotic lumbar spine: A Finite Element Analysis

BACKGROUND

Cancer patients are likely to undergo osteoporosis as consequence of hormone manipulation and/or chemotherapy. Little is known about possible increased risk of fracture in this population. The aim of this study was to describe the biomechanical effect of a metastatic lesion in an osteoporotic lumbar spine model.

METHODS

A finite element model of two spinal motion segments (L3-L5) was extracted from a previously developed L3-Sacrum model and used to analyze the effect of metastasis size and bone mineral density (BMD) on Vertebral bulge (VB) and Vertebral height (VH). VB and VH represent respectively radial and axial displacement and they have been correlated to burst fracture. A total of 6 scenarios were evaluated combining three metastasis sizes (no metastasis, 15% and 30% of the vertebral body) and two BMD conditions (normal BMD and osteoporosis).

RESULTS

15% metastasis increased VB and VH by 178% and 248%, respectively in normal BMD model; while VB and VH increased by 134% and 174% in osteoporotic model. 30% metastasis increased VB and VH by 88% and 109%, respectively, when compared to 15% metastasis in normal BMD model; while VB and VH increased by 59% and 74% in osteoporotic model.

CONCLUSION

A metastasis in the osteoporotic lumbar spine always leads to a higher risk of vertebral fracture. This risk increases with the size of the metastasis. Unexpectedly, an increment in metastasis size in the normal BMD spine produces a greater impact on vertebral stability compared to the osteoporotic spine.

Biomechanical responses due to discitis infection of a juvenile thoracolumbar spine using finite element modeling

Growth modulation changes occur in pediatric spines and lead to kyphotic deformity during discitis infection from mechanical forces. The present study was done to understand the consequences of discitis by simulating inflammatory puss at the T12/L1 disc space using a validated eight-year-old thoracolumbar spine finite element model. Changes in the biomechanical responses of the bone, disc and ligaments were determined under physiological compression and flexion loads in the intact and discitis models. During flexion, the angular-displacement increased by 3.33 times the intact spine and localized at the infected junction (IJ). The IJ became a virtual hinge. During compression loading, higher stresses occurred in the growth plate superior to the IJ. The components of the principal stresses in the growth plates at the T12/L1 junction indicated differential stresses. The strain increased by 143% during flexion loading in the posterior ligaments. The study indicates that the flexible pediatric spine increases the motion of the infected spine during physiological loadings. Understanding intrinsic responses around growth plates is important within the context of growth modulation in children. These results are clinically relevant as it might help surgeons to come up with better decisions while developing treatment protocols or performing surgeries.

Vertebral compression fracture after stereotactic body radiotherapy for spinal metastases

The use of stereotactic body radiotherapy for metastatic spinal tumours is increasing. Serious adverse events for this treatment include vertebral compression fracture (VCF) and radiation myelopathy. Although VCF is a fairly low-risk adverse event (approximately 5% risk) after conventional radiotherapy, crude risk estimates for VCF after spinal SBRT range from 11% to 39%. In this Review, we summarise the evidence and predictive factors for VCF induced by spinal SBRT, review the pathophysiology of VCF in the metastatic spine, and discuss strategies used to prevent and manage this potentially disabling complication.

Quantification of the effect of osteolytic metastases on bone strain within whole vertebrae using image registration

The vertebral column is the most frequent site of metastatic involvement of the skeleton with up to 1/3 of all cancer patients developing spinal metastases. Longer survival times for patients, particularly secondary to breast cancer, have increased the need for better understanding the impact of skeletal metastases on structural stability. This study aims to apply image registration to calculate strain distributions in metastatically involved rodent vertebrae utilizing µCT imaging. Osteolytic vertebral lesions were developed in five rnu/rnu rats 2-3 weeks post intracardiac injection with MT-1 human breast cancer cells. An image registration algorithm was used to calculate and compare strain fields due to axial compressive loading in metastatically involved and control vertebrae. Tumor-bearing vertebrae had greatly increased compressive strains, double the magnitude of strain compared to control vertebrae (p=0.01). Qualitatively strain concentrated within the growth plates in both tumor bearing and control vertebrae. Most interesting was the presence of strain concentrations at the dorsal wall in metastatically involved vertebrae, suggesting structural instability. Strain distributions, quantified by image registration were consistent with known consequences of lytic involvement. Metastatically involved vertebrae had greater strain magnitude than control vertebrae. Strain concentrations at the dorsal wall in only the metastatic vertebrae, were consistent with higher incidence of burst fracture secondary to this pathology. Future use of image registration of whole vertebrae will allow focused examination of the efficacy of targeted and systemic treatments in reducing strains and the related risk of fracture in pathologic bones under simple and complex loading.

A novel classification system for spinal instability in neoplastic disease: an evidence-based approach and expert consensus from the Spine Oncology Study Group

STUDY DESIGN

Systematic review and modified Delphi technique.

OBJECTIVE

To use an evidence-based medicine process using the best available literature and expert opinion consensus to develop a comprehensive classification system to diagnose neoplastic spinal instability.

SUMMARY OF BACKGROUND DATA

Spinal instability is poorly defined in the literature and presently there is a lack of guidelines available to aid in defining the degree of spinal instability in the setting of neoplastic spinal disease. The concept of spinal instability remains important in the clinical decision-making process for patients with spine tumors.

METHODS

We have integrated the evidence provided by systematic reviews through a modified Delphi technique to generate a consensus of best evidence and expert opinion to develop a classification system to define neoplastic spinal instability.

RESULTS

A comprehensive classification system based on patient symptoms and radiographic criteria of the spine was developed to aid in predicting spine stability of neoplastic lesions. The classification system includes global spinal location of the tumor, type and presence of pain, bone lesion quality, spinal alignment, extent of vertebral body collapse, and posterolateral spinal element involvement. Qualitative scores were assigned based on relative importance of particular factors gleaned from the literature and refined by expert consensus.

CONCLUSION

The Spine Instability Neoplastic Score is a comprehensive classification system with content validity that can guide clinicians in identifying when patients with neoplastic disease of the spine may benefit from surgical consultation. It can also aid surgeons in assessing the key components of spinal instability due to neoplasia and may become a prognostic tool for surgical decision-making when put in context with other key elements such as neurologic symptoms, extent of disease, prognosis, patient health factors, oncologic subtype, and radiosensitivity of the tumor.

Comparison of computed tomography based parametric and patient-specific finite element models of the healthy and metastatic spine using a mesh-morphing algorithm

STUDY DESIGN

A comparative analysis of parametric and patient-specific finite element (FE) modeling of spinal motion segments.

OBJECTIVES

To develop patient-specific FE models of spinal motion segments using mesh-morphing methods applied to a parametric FE model. To compare strain and displacement patterns in parametric and morphed models for both healthy and metastatically involved vertebrae.

SUMMARY OF BACKGROUND DATA

Parametric FE models may be limited in their ability to fully represent patient-specific geometries and material property distributions. Generation of multiple patient-specific FE models has been limited because of computational expense. Morphing methods have been successfully used to generate multiple specimen-specific FE models of caudal rat vertebrae.

METHODS

FE models of a healthy and a metastatic T6-T8 spinal motion segment were analyzed with and without patient-specific material properties. Parametric and morphed models were compared using a landmark-based morphing algorithm.

RESULTS

Morphing of the parametric FE model and including patient-specific material properties both had a strong impact on magnitudes and patterns of vertebral strain and displacement.

CONCLUSION

Small but important geometric differences can be represented through morphing of parametric FE models. The mesh-morphing algorithm developed provides a rapid method for generating patient-specific FE models of spinal motion segments.

Immediate biomechanical effects of lumbar posterior dynamic stabilization above a circumferential fusion

STUDY DESIGN

Biomechanical in vitro human cadaveric lumbar flexibility testing with 6 sequential treatments.

OBJECTIVE

To compare the range of motion (ROM) of dynamic one-level posterior stabilization constructs to one-level rigid rod fixation constructs and to study the effects of extending the posterior construct to the adjacent superior level.

SUMMARY OF BACKGROUND DATA

Patients experiencing pain and biomechanical instability at one level may also present with radiographic or other indicators of early degeneration at an adjacent level. Clearly, fusion would be warranted at the symptomatic level, but the treatment plan for the adjacent level remains controversial. Additionally, the effects on adjacent motion segments above a fusion level are currently not well understood.

METHODS

Thirteen fresh frozen human cadaveric lumbar spines (L1-L5) were tested in 6 modes of loading: 3 were randomized to dynamic posterior stabilization constructs and 7 to a rigid rod pedicle screw system. Each group was subjected to 6 treatments.

RESULTS

When comparing the instrumented treatments, only Treatment 6, two-level hybrid constructs, exhibited a statistically significant effect in flexion-extension bending at L2-L3 between the posterior dynamic system and rigid rod fixation (P = 0.014).

CONCLUSION

ROM at the superior adjacent level (L2-L3) demonstrated no significant difference between intact, destabilized, one-level posterior fixation, and one-level circumferential fusion at the index level (L3-L4) when comparing posterior dynamic stabilization to rigid rod fixation. However, ROM at the superior adjacent level (L2-L3) was significantly greater for lateral bending and axial rotation when both levels (L2-L3 and L3-L4) were stabilized with a dynamic stabilization system. When the functional spinal units were instrumented with a two-level hybrid construct, two-level posterior instrumentation (L2-L3 and L3-L4) with a cage at the index level (L3-L4), all bending modes generated significantly greater ROM for the dynamic stabilization group at L2-L3 compared with rigid rod fixation.