Mechanical Properties of Femoral Cortical Bone Following Cemented Hip Replacement G.X. Ni, 1,2 W.W. Lu, 2 P.K.Y. Chiu, 2 Y. Wang, 3 Z.Y. Li, 2 Y.G. Zhang, 3 B. Xu, 4 L.F. Deng, 5 K.D.K. Luk 2 1 Department of Rehabilitation Medicine, Fujian Medical University, Fuzhou, China 2 Department of Orthopaedics and Traumatology, The University of Hong Kong, Hong Kong 3 Department of Orthopaedic Surgery, General Hospital of PLA, Beijing, China 4 Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong 5 Department of Orthopaedic Surgery, Shanghai 2nd Medical University, Shanghai, China Received 20 September 2006; revised 16 January 2007; accepted 25 January 2007 Published online 16 May 2007 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20388 ABSTRACT: Femoral bone remodeling following total hip replacement is a big concern and has never been examined mechanically. In this study, six goats underwent unilateral cemented hip hemiarthroplasty with polymethyl methacrylate (PMMA) bone cement. Nine months later animals were sacrificed, and the femoral cortical bone slices at different levels were analysed using microhardness testing and microcomputed tomography (micro-CT) scanning. Implanted femurs were compared to contralateral nonimplanted femurs. Extensive bone remodeling was demon- strated at both the proximal and middle levels, but not at the distal level. Compared with the nonimplanted side, significant decreases were found in the implanted femur in cortical bone area, bone mineral density, and cortical bone hardness at the proximal level, as well as in bone mineral density and bone hardness at the middle level. However, no significant difference was observed in either variable for the distal level. In addition, similar proximal-to-distal gradient changes were revealed both in cortical bone microhardness and bone mineral density. From the mechanical point of view, the results of the present study suggested that stress shielding is an important mechanical factor associated with bone adaptation following total hip replacement. ß 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:1408–1414, 2007 Keywords: microhardness; total hip replacement; bone remodeling; stress shielding INTRODUCTION Total hip replacement (THR) is among the most successful and cost-effective surgical procedures and remains the best treatment for long-term pain relief and restoration of function for patients suffering with diseased or damaged hips. How- ever, femoral bone remodeling following THR is a big concern because it can compromise the out- come of arthroplasty and may be predisposed to loosening and migration of the prosthesis, periprosthetic fracture, and problems in revision arthroplasty, if required. A fundamental under- standing of the skeletal response to THR is necessary to improve the outcome and avoid postsurgical complications. Much of the information as it relates to bone remodeling comes from clinical radiographic studies. 1–3 However, the three-dimensional (3D) geometry of the femur and the limitation of plain radiographs make it difficult to accurately assess quantitative bone remodeling. Dual energy X-ray absorptiometry has been proven to be accurate for the determination of periprosthetic bone mineral density, with an error below 1%. 4 However, this technique has mainly been used in patients with uncemented THR 5–8 because it is limited in its application as it relates to cemented components. With the presence of cement, it is difficult to accurately assess the boundaries between cement and bone. 9 Recently, direct examination was adapted in several autopsy retrievals and animal experiments recently to quantify cortical bone remodeling in the femur after cemented femoral replacement. 10–13 Autopsy retrievals and animal experiments pro- vide valuable tools leading to marked improvement in the knowledge of the skeletal response to 1408 JOURNAL OF ORTHOPAEDIC RESEARCH NOVEMBER 2007 Correspondence to: P.K.Y. Chiu (Tel.: 852-2855-4259, Fax: 852-2817-4392.; E-mail: pkychiu@hkucc.hku.hk) ß 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.