Biomechanical and Finite Element Analyses of Alternative Cements for use in Vertebral Kyphoplasty

A vertebral compression fracture occurs when the vertebral body of the spine collapses due to osteoporosis or trauma. Depending on the degree of osteoporosis, the forces required to induce vertebral compression fracture can vary while the forces required to induce fractures via trauma are large. However, the vast majority of vertebral compression fractures occur in elderly people or in individuals with osteoporotic disorders. These fractures, which tend to cause a collapse of the anterior wall of the vertebral body, result in the vertebra forming a wedge shape causing pain and altering the biomechanics of the spine. Vertebroplasty and kyphoplasty are two minimally invasive surgical procedures that inject cement into the fractured vertebra to relieve pain and provide stability. While highly successful, there are a few limitations that can alter the long term outcome of the patient.Kyphoplasty/vertebroplasty procedures traditionally use PMMA to treat the fractured vertebrae due to its mechanical properties. However, with time the bone erodes around the cement due to osteoporosis and inhibited bone remodeling due to the cyotoxicity of PMMA [1, 2]. Acrylic monomers left over from the polymerization of PMMA are responsible for the foreign body reactions observed in augmented vertebrae [3]. The exothermic reaction of PMMA is also responsible for thermal necrosis and can cause complications in cases of extravasation [4, 5]. Severe complications due to extravasation can include paraplegia from cement in the spinal canal and pulmonary embolisms due to cement migration [6, 7]. Lastly, PMMA is not bioactive and will not be reabsorbed, causing the cyotoxic cement to remain in the body permanently. Thus, alternative cements with similar mechanical strengths are being explored. Calcium phosphate cements (CaP) have been a material of high interest due to their bioactive and thermal properties. They have a similar chemical composition as that of the mineral components of natural bone [8]. As such, they promote bone growth into the cement and with time are completely replaced by natural bone [9]. CaP cements also cure via crystallization and have been shown to have reduced curing temperatures, removing the risk for thermal necrosis [1, 10]. Despite these advantages, there are reservations of traditional CaP cements due to deficiencies in mechanical properties. As CaP cements are ceramics instead of polymers, such as PMMA, they have different fracture mechanisms and tend to be very brittle. The purpose of this study was to evaluate two different CaP cements, one of which is polymerized, and compare their biomechanical compression properties to PMMA via cadaveric experimentation and finite element modeling. Cadaveric experiments were carried out to investigate the failure loads of individual osteoporotic thoracolumbar vertebrae. Fractures were artificially created and specimens were restored with one of the cements of interest via Osseoplasty. Restored samples were tested for static and cyclic loading failure.A three dimensional, non-linear osteoporotic L1-S1 finite element model that had been previously validated was used to study the effects of cement on adjacent vertebrae. A compression fracture was simulated in the L3 vertebral body by using a modulus reduction criterion. Vertebral augmentation was then simulated using the material properties of CaP, polymerized CaP (pCaP), and PMMA bone cements to compare with the unaltered osteoporotic model. Mechanical properties of the bone were altered to reflect various combinations of level specific osteoporotic degeneration. All motion was conducted via load control protocols. Maximum von Mises stresses were recorded for the endplates of the augmented and adjacent vertebrae. Stress profiles were also captured for the same endplates. Cadaveric testing results showed that PMMA significantly increased the maximum compression strength of the osteoporotic bone. CaP and pCaP restored the strength to its intact condition. Further cadaveric testing is needed with simulated physiological loads on a motion segment augmented with pCaP. It is also recommend that animal testing be done in further studies to compare bone remodeling and integration between cement types.Finite element modeling of the three cements showed that osteoporosis was the driving force behind increased endplate stresses. There were no significant differences between the three cements modeled. All three cements changed the biomechanical loading placed on L3, the augmented vertebra, which became apparent under certain physiological loading conditions. While the stress contours of the endplates did change for L3 due to cement, this change in loading was not evident on the adjacent vertebral endplates. The FE results suggest that cement augmentation does alter spine biomechanics but osteoporosis and spinal deformities are the main factors behind increased adjacent vertebral endplate loads. This model does not account for deformities or degenerated discs. This is important to note as a healthy disc may be able to account for the altered loading of the augmented vertebra whereas a degenerated disc may not. As vertebroplasty does not correct for deformities and kyphoplasty does not fully restore vertebral height, the deformity created by the initial VCF still influences the biomechanics of the spine. It is unclear how significant of an influence osteoporosis, spinal deformities, and cement augmentation are in increasing the rate of adjacent vertebral fractures.

Name thesis:degree_name

Master of Science in Bioengineering

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Bioengineering

Grantor dc:publisher

University of Toledo

Year dc:date

2013

Author dc:creator

• Jones, Andrew D.

Contributors dc:contributor

• Goel, Vijay

• Biomechanics
• Biomedical Research
• Materials Science

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=toledo1364828764

OAI identifier oai:identifier

oai:etd.ohiolink.edu:toledo1364828764