Effect of weight-bearing on bone-bonding behavior of strontium-containing hydroxyapatite bone cement G.X. Ni, 1 W.W. Lu, 1 B. Tang, 2 A.H.W. Ngan, 2 K.Y. Chiu, 1 K.M.C. Cheung, 1 Z.Y. Li, 1 K.D.K. Luk 1 1 Department of Orthopaedics and Traumatology, The University of Hong Kong, Hong Kong, Hong Kong 2 Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, Hong Kong Received 22 June 2006; revised 3 November 2006; accepted 10 January 2007 Published online 2 July 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31294 Abstract: The purpose of this study was to investigate and compare the chemical composition and nanomechanical properties at the bone-cement interface under non-weight- bearing and weight-bearing conditions, in order to under- stand the effect of weight-bearing on the bone-bonding behavior of strontium-containing hydroxyapatite (Sr-HA) cement. In one group, Sr-HA cement was injected into rabbit ilium (under non-weight-bearing conditions). Unilateral hip replacement was performed with Sr-HA cement (under weight-bearing conditions) in the other group. Six months later, scanning electron microscopy (SEM) with energy-dis- persive X-ray (EDX) analysis and nanoindentation tests were conducted on the interfaces between cancellous bone and the Sr-HA cement. The nanoindentation results revealed two different transitional behaviors under different conditions. nder weight-bearing conditions, both the Young modulus and hardness at the interface were considerably higher than those at either the Sr-HA cement or cancellous bone. On the contrary, under non-weight-bearing conditions, both the Young modulus and hardness values at the interface were lower than those at the cancellous bone, but were higher than the Sr-HA cement. In addition, EDX results showed that the calcium and phosphorus contents at the interface under weight-bearing conditions were considerably higher than those under non-weight-bearing conditions. The differ- ences in chemical composition and nanomechanical proper- ties at the cement-bone interface under two different condi- tions indicate that weight-bearing produces significant effects on the bone-bonding behavior of the Sr-HA cement. Ó 2007 Wiley Periodicals, Inc. J Biomed Mater Res 83A: 570–576, 2007 Key words: weight-bearing; strontium-containing hydroxy- apatite; interface; nanoindentation; bioactive bone cement INTRODUCTION Recently, a large number of bioactive bone cements have been developed to overcome the disadvantages of PMMA bone cement, and these include surface-active ceramics (e.g., bioglass, ceravital, hydroxyapatite (HA), apatite- and wollastonite-containing glass-ceramic (A-W GC)) and resorbable ceramics (e.g., b-tricalcium phos- phate (b-TCP), calcite). 1–5 All these materials bond to bone tightly, but their bone-bonding behaviors are not identical. The bone-bonding behavior of a bioactive ma- terial depends not only on the chemical composition, crystallinity, and solubility of the material, 6–9 but also on the type (cancellous bone or cortical bone) 10,11 and the quality 12,13 of bone to which it bonds. Despite our increasingly improved understanding of the way in which bioactive bone cements bond to bone, little is known about how mechanical forces modulate their bone-bonding behavior. This is surpris- ing for two reasons. First, there is a wide range of clini- cal applications for bone cement in orthopaedics, both under weight-bearing and non-weight-bearing condi- tions. It is therefore necessary to understand the effect of weight-bearing on the bone-bone cement interface. Second, bone is inherently mechanosensitive, that is, it responds and adapts to its mechanical environment. 14 Controlled (nonextensive) bone loading has been found to be related to increased bone mineral content, in- creased bone density, or to controlled bone loss. 15–17 Furthermore, the effect of loading on bone-implant bonding has been extensively examined. 18–23 A signifi- cant increase of bone-to-implant contact was demon- strated under various loading conditions when com- pared with nonloaded conditions. Biomechanical forces can stimulate the proliferation of osteoblasts as well as their activation to produce more and denser peri- implant bone. As a result, bone metabolism can be sig- nificantly increased with a resulting increase in bone- implant contact. Therefore, it is hypothesized that weight-bearing should play an important role in the bone-bonding behavior of bioactive bone cements. Correspondence to: W.W. Lu; e-mail: wwlu@hkusua. hku.hk Contract grant sponsor: University of Hong Kong ' 2007 Wiley Periodicals, Inc.