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Effect of Bone Porosity on the Mechanical Integrity of the Bone-Cement Interface

Investigation performed at the Departments of Bioengineering and Mechanical Engineering, University of California, Berkeley, and the Department of Orthopaedic Surgery, University of California, San Francisco, California

Jove Graham, PhD
Lisa Pruitt, PhD
Department of Bioengineering, University of California, Berkeley, 5134 Etcheverry Hall, Berkeley, CA 94720. E-mail address: jove{at}socrates.berkeley.edu

Michael Ries, MD
Department of Orthopaedic Surgery, University of California, San Francisco, 500 Parnassus Avenue, MU-320-W, San Francisco, CA 94143

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: Osteopenia is one factor that may influence the decision about the type of implant fixation to use in total hip arthroplasty. However, clinical studies generally do not associate the outcome of an arthroplasty with the degree of osteopenia. The mechanical integrity of the cement fixation of an implant may be affected by the relative degree of osteopenia, which could account for some of the variable long-term results after total hip arthroplasty performed with cement. The purpose of this study was to determine the effects of bone porosity, trabecular orientation, cement pressure, and cement penetration depth on fracture toughness at the bone-cement interface.

Methods: Trabecular bone from the proximal part of bovine femora was used with a single brand of commercial acrylic bone cement to form compact-tension interface specimens representing a range of bone porosities, orientations, and cement pressures within a clinically achievable range. All specimens were loaded to failure with use of a servohydraulic testing machine, and fracture toughness at the interface was calculated. After testing, images of a representative sample of specimens were made with use of computed tomography to measure the penetration depth of the cement into the bone.

Results: Significant correlations were found between fracture toughness and bone porosity, trabecular orientation, and cement pressure, with bone porosity having the strongest effect (p < 0.000015). Examination of the computed tomographic images also showed a significant correlation between fracture toughness and maximum cement penetration depth (p < 0.033), as well as significant partial correlations between maximum and mean penetration depth and bone porosity (p < 0.0037 and p < 0.0028).

Conclusion: The fracture resistance of the bone-cement interface is greatly improved when the ability of the cement to flow into the intertrabecular spaces is enhanced.

Clinical Relevance: The results in the present study identify a significant effect of bone porosity on fracture toughness at the interface surrounding a cemented implant. The results of mechanical testing support the clinical recommendation that osteoporosis be considered a relative indication for cement fixation. Our results emphasize the fact that bone quality is particularly important and should be considered a primary concern when deciding whether to use acrylic bone cement.

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