Vertebral compression model and comparison of augmentation agents

ABSTRACT

Study design: Biomechanical study.

Objectives: To evaluate the compression strengths of various bone fillers used in treating vertebral compression fractures using a third-generation sawbone model and to evaluate the viability of this novel model as an alternative to actual human or animal vertebrae for biomechanical testing of vertebral-filling materials.

Methods: Cavities were created in the osteoporotic vertebral body sawbone models and filled with PMMA, SRS, MIIGX3 HiVisc, and BoneSource fillers. These were cured according to manufacturers’ recommendations and then tested to failure in the compression model. Elastic modulus was calculated and compared with the control group which was not augmented.

Results: The mean modulus of elasticity for the control group vertebrae was 92.44 ± 19.28 MPa. The mean modulus of elasticity was highest in the polymethylmethacrylate (PMMA) group (195.47 ± 2.33 MPa) and lowest in the MIIG group (25.79 ± 4.77 MPa). The results for the SRS-tricalcium phosphate group (79.14 ± 20.20 MPa) were closest to the control group, followed by the BoneSource group (57.49 ± 8.35 MPa). Statistical analysis, for comparison of individual group means, identified significant differences between the control group and all other groups (P < .05), with the exception of the SRS-tricalcium phosphate group (P = .65, versus control). The modulus of elasticity for the PMMA group was significantly higher than all other groups (P < .001).

Conclusion: The third-generation osteoporotic sawbones model simulates in vitro physiological specimen function. It was effective for comparing which osteoconductive agents may provide adequate strength while minimizing potential adjacent level fracture. Increased stiffness was seen with PMMA compared with the unaugmented control as well as with calcium phosphate or calcium sulfate cements suggesting that these may reduce adjacent segment fractures.

STUDY RATIONALE AND CONTEXT Despite documented pain relief following kyphoplasty and vertebroplasty for the treatment of vertebral compression fractures, one potential complication of these procedures is adjacent segment fracture in the osteoporotic spine. Substances used to reinforce the osteoporotic or diseased vertebrae include polymethylmethacrylate (PMMA) and others, such as calcium phosphate bone cements. Standardized evaluation of the biomechanical properties of different vertebral augmentation agents in a consistently reproducible vertebral body model which mimics physiological conditions in a reproducible fashion and, moreover, is readily available would enhance our ability to draw conclusions on the biomechanical effects of vertebral augmentation on the surrounding spinal column through comparative analysis. OBJECTIVES To compare the compression strengths of four different filler materials used in vertebral augmentation and to confirm the feasibility of the third-generation osteoporotic sawbone model as a viable alternative to human or animal vertebrae for biomechanical testing. METHODS Study design: Biomechanical study. Materials and procedures An anatomical, third-generation sawbone vertebral model (Pacific International) was utilized. Twenty upper lumbar replica vertebrae were allocated to five groups, with four vertebrae in each group. The first group, control, was tested before any augmentation and the remaining four groups were tested after augmentation with one of the four materials below. To prepare the 16 vertebrae for augmentation, a drill press was used to drill a 5 mm hole through the pedicle into the anterior vertebral body, stopping approximately 5–8 mm before reaching the anterior wall. A Kyphon inflatable bone tamp system was used to create a defect within the cancellous portion of the vertebrae. To ensure that all vertebrae were augmented in a similar fashion, radiopaque dye was used in the insufflation liquid and x-rays of each augmented vertebrae were taken with the inflated balloons in place (Fig 1). Once the bony defect had been created within the vertebral body, each side of the vertebrae was filled with one of the four different filler materials (Fig 2). After filling with the appropriate material, all of the vertebrae were placed in a water bath at 37 °C for 24 hours to allow for complete polymerization. The materials were allowed adequate time to harden and were removed from the water bath. Four different filler materials were used: (1) Simplex P, polymethylmethacrylate (PMMA) from Stryker Orthopaedics; (2) SRS-tricalcium phosphate cement from Norian Corp; (3) MIIG X3 HiVisc, calcium sulfate cement from Wright Medical; and (4) BoneSource, calcium phosphate cement from Stryker Orthopaedics. The same amount of liquid (5 mL) was used in each augmentation and each vertebra was augmented bilaterally via injection of two 2.5 mL volumes of the investigated material. The vertebrae were then tested using specially fabricated end plates and the Instron screw-driven mechanical testing load frame (Fig 3). Each vertebra was compressed under a stroke-controlled loading regime to measure the construct’s elastic modulus in compression. Mechanical testing of each sample was stopped when either a compressive displacement of 6 mm was achieved or mechanical failure was detected, as denoted by a rapid decrease in the measured compressive load that was greater than 15 % of the current load. Outcomes: The modulus of elasticity (in megapascals) under compressive loading was obtained via standard stress/strain calculations, wherein stress is the applied compressive load divided by the surface area of the load-applying custom metallic discs and strain is the recorded axial compression divided by the initial vertical height of the sample. Analysis: Load deformation and stress / strain curves were calculated using SPSS statistical software. The means across all groups were compared using one-way analysis of variance (ANOVA) and Tukey post hoc testing was used to compare all possible pairs of experimentally derived means, based on a studentized range distribution. Differences between groups were considered significant if P < .05. Additional methodological and technical details are provided in the web appendices at www.aospine.org / ebsj.

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EDITORIAL STAFF PERSPECTIVE

The reviewers found that this is a well-executed study, methodologically, in examining and comparing the compressive stiffness of vertebrae augmented with various bone-filler substances. The reviewers identified the following issues:

• This study uses a third-generation sawbone model for greater consistency in mechanical testing. However, the study itself does not validate this model and there is no comparison of the model used to allow the reader to scale it a given severity of osteoporosis.

• The actual desirable stiffness of a vertebral body to be augmented remains elusive. If it is made too stiff, which polymethylmethacrylate (PMMA) injected into a solid mold such as done with kyphoplasty may lead to, increased adjacent segment fractures may occur. If not stiff enough the resultant construct may not be sufficient in providing adequate support and inadequate pain relief. Fig 4 indeed suggests that augmentation with PMMA substantially increases the stiffness of the vertebra compared with the control. However, it also suggests that Norian, BoneSource, and MIIG augmentation result in a vertebra that is less stiff than a nonaugmented vertebra. Why would a physician want to inject a substance that (at least by this report) results in a less stiff vertebra?

• Finally, the notion of a „super stiff” vertebra after cement augmentation potentially resulting in adjacent segment fracture is certainly reasonable and widely believe to be the case, though it is difficult to definitively prove. However, there is clinical concerns with „less stiff” bone fillers. There remains uncertainty that cements, which are injected as a not yet set suspension, may leak into blood circulation before curing, and may subsequently result in undesirable systemic effects. Specifically, Norian XR has not been approved for treatment of vertebral fractures in the United States and has been removed from the market for this indication [1]. Clearly, the potential advantages of a „less stiff” construct (potentially reducing adjacent level fracture) must be balanced against the safety of the index procedure. As many of the cements are injected into a vulnerable and elderly population with damaged vertebral bodies, improved mechanical, rheological, and physiological understanding is desirable to assure patient safety prior to routine clinical implementation.

1.         www.fda.gov/ICECI/CriminalInvestigations / ucm228273.htm