The effect of kyphoplasty parameters on the dynamic load transfer within the lumbar spine considering the response of a bio-realistic spine segment

doi:10.1016/j.clinbiomech.2013.09.013

Clinical Biomechanics

Volume 28, Issues 9–10, November–December 2013, Pages 949-955

https://doi.org/10.1016/j.clinbiomech.2013.09.013 Get rights and content

Abstract

Background

With an increasing prevalence of osteoporosis, physicians have to optimize treatment of relevant vertebral compression fractures, which have significant impact on the quality of life in the elder population. Retrospective clinical studies suggest that kyphoplasty, despite being a procedure with promising potential, may be related to an increased fracture risk of the adjacent untreated vertebrae.

Methods

A bio-realistic model of a lumbar spine is introduced to determine the morbidity of cemented augmentation. The model was verified and validated for the purpose of the study and subjected to a dynamic finite element analysis. Anisotropic bone properties and solid ligamentous tissue were considered along with α time varying loading scenario.

Findings

The yielded results merit high clinical interest. Bi-pedicular filling stimulated a symmetrically developing stress field, thus comparing favourably to uni-pedicular augmentation which resulted in a non-uniform loading of the spine segment. An enslavement of the load transfer was also found to both patient bone mineral density and reinforcement–nucleous pulpous superimposition.

Interpretation

The investigation presented refined insight into the dynamic biomechanical response of a reinforced spine segment. The increase in the calculated occurring stresses was considered as non-critical in most cases, suggesting that prevalent fractures are a symptomatic condition of osteoporosis rather than a sequel of efficiently preformed kyphoplasty.

Introduction

Vertebral compression fractures (VCFs) caused by osteoporosis are an increasingly common occurrence. The annual incidents of vertebral fractures in Europe among elderly (50–79) are estimated at 1.1% for women and 0.6% for men, while epidemiologic studies foresee an increase of these numbers in the future (EPOS, 2002). Approximately 85% of these fractures are due to primary osteoporosis and the remainder due to secondary osteoporosis or malignancies (Cooper et al., 1992). These VCFs lead to progressive sagittal spinal deformity and changes in spine biomechanics, as the adopted kyphotic posture of the patients displaces their centre of gravity reducing their overall stability (Overstall et al., 1977).

Balloon kyphoplasty is a minimally invasive surgical treatment for osteoporotic and osteolytic VCF with promising clinical potential, during which a filler material is percutaneously injected into a cavity of a degenerated vertebral body, created by an inflatable tamp. Next to reversing kyphosis, cemented augmentation also results in high local rigidity within a functional spine unite (FSU) and retrospective clinical studies have indicated new VCFs as a potential late sequela of the reinforcement procedures (Fribourg et al., 2004, Heaney, 1992). It remains however elusive whether this is the aetiology (Lindsay et al., 2001) or a symptomatic condition of the gradual loss of bone mineral density (BMD) due to evolving osteoporosis (Uppin et al., 2003).

The pathogenesis of fractures at adjacent non-treated spine levels has been heuristically investigated both in vitro (Berlemann et al., 2002, Boger et al., 2007) and in vivo (Fournol et al., 2007). Experimental studies are however conducted on FSU’s originating from different spine levels, age groups and varying surgical approaches and have thus been indicated as methodologically flawed (Hardouin et al., 2001). This hinders a collective evaluation of the existing literature, as different hypothesis and conclusions render it unclear whether these trends will hold true, once deducted to other patients. Finite element analysis (FEA) has been used to determine the in situ effect of cemented augmentation on the load transfer within a FSU (Polikeit et al., 2003), indicating increased pressure in the intervertebral disc (IVD) and deflection of the vicinal endplate, which could provoke subsequent fractures. The biomechanical alterations of ligaments however could not be reflected, as these were simulated by cable elements, capable of enduring tension only. Recent FEA continue to focus on the response of the adjacent vertebra considering motion segments of 3–5 vertebral bodies (Rohlermann et al., 2006) with ligamentous tissue either modelled by two nodal elements or neglected at all.

In this investigation a FEA of a bio-realistic lumbar (L1–L5) spine is introduced to compare the biomechanical response of its preoperative state to the postsurgical cemented augmentation, both for bony and connecting soft tissue. This approach is based on the preliminary hypotheses that cement injection exaggerates force transmission to the adjacent vertebral bodies, thereby predisposing those levels to future fractures. The effect of uni- and bi-pedicular filling with polymethyl-methacrylate (PMMA) was examined for loads encountered in diurnal activities.

Section snippets

Model development

A lumbar spine was scanned in its entirety by high resolution computed tomography (CT). Upon segmentation and reconstruction of the vertebral bodies, the interposing IVDs were reverse engineered based on the superior and inferior surfaces of the connecting vertebrae. The resulting model represents an evolution of a previously published FSU (Tsouknidas et al., 2012) simulated here with full solid ligamentous tissue.

The mesh grid was generated in ANSA (by BETA CAE Systems S.A.) in order to ease

Results

The analysis revealed that the load transfer to the non-augmented vertebrae was influenced by the PMMA injection only to one adjacent spine level and any effect to further apart vertebrae can be considered as insignificant. Therefore and in order to increase computational efficiency only L5–L3 were considered to determine the qualitative influence of cemented augmentation (in the L4) to one superior and one inferior spinal level.

In all cases the highest stress was recorded mid-stance,

Discussion

Based on the computed results a slight increase in the load transfer was observed in all treated models regardless of the patient’s BMD, as the stiffness characteristics of the healthy vertebra (untreated model) compare favourably to the osteoporotic reinforced ones. This can be attributed to the capacity of an intact vertebra to uniformly distribute developing stress over its entire volume (Wilson et al., 2000). Stating, however, that kyphoplasty increases the risk of fragility fractures in

Conclusions

The results of our study suggest that the biomechanical changes of the dynamic load transfer to the adjacent non-treated vertebral bodies after kyphoplasty are insignificant when compared to a healthy spine segment. This however does not consider the unfavorable effect of kyphosis itself and presumes, in our approach, an anatomy fully reversed to its physiological state.

With respect to literature data, kyphoplasty might predispose adjacent spine level to subsequent fractures, but it is likely

Acknowledgements

The authors would like to acknowledge that this investigation was partially funded by the General Secretariat for Research and Technology of Greece under grant PE8(3227).

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The effect of kyphoplasty parameters on the dynamic load transfer within the lumbar spine considering the response of a bio-realistic spine segment

Abstract

Background

Methods

Findings

Interpretation

Introduction

Section snippets

Model development

Results

Discussion

Conclusions

Acknowledgements

Clin. Biomech.

Clin. Biomech.

J. Radiol.

Clin. Biomech.

Joint Bone Spine

Wear

Gait Posture

Clin. Biomech.

Theor. Appl.

Clin. Biomech.

Adjacent vertebral failure after vertebroplasty

J. Bone Joint Surg. Br.

Adjacent vertebral failure after vertebroplasty: a biomechanical study of low-modulus PMMA cement

Eur. Spine J.

Cement distribution, volume, and compliance in vertebroplasty: some answers from an anatomy-based nonlinear finite element study

Spine

Incidence of clinically diagnosed vertebral fractures: a population-based study in Rochester, Minnesota, 1985–1989

J. Bone Miner. Res.

J. Bone Miner. Res.

Evaluation of adjacent segment failure following vertebroplasty

Incidence of subsequent vertebral fracture after kyphoplasty

Spine

New technologies in spine: kyphoplasty and vertebroplasty for the treatment of painful osteoporotic compression fractures

Spine

Long-term observations of vertebral osteoporotic fractures treated by percutaneous vertebroplasty

Rheumatology