Cement Microcracks in Thin-Mantle Regions After In Vitro Fatigue Loading Kenneth A. Mann, PhD,* Sameer Gupta, BS,* Amos Race, PhD,* Mark A. Miller, MS,* Richard J. Cleary, PhD,† and David C. Ayers, MD* Abstract: An in vitro study of cemented femoral hip components was conducted to determine if microcracks in the cement mantle would preferentially form in thin- mantle regions as a result of cyclic fatigue loading via stair-climbing. Overall, there was not an increased amount of microcracks in thin-mantle (2 mm) regions (number found/number expected = 0.59, P.03). However, through cracks that extended between the stem to the bone were more prevalent in thin-mantle regions (number found/number expected = 2.93, P.03). Although cracks form throughout the cement mantle and appear to grow at the same rate, thin-mantle regions are most likely to have through cracks after fatigue loading. This is consistent with results from at-autopsy studies of well-fixed femoral components and supports the general guideline that thin-mantle regions should be avoided in the cementing of the femoral stem. Key words: cement, fatigue, biomechanical, cracks, loosening. © 2004 Elsevier Inc. All rights reserved. Thin cement mantles are thought to be associated with increased risk of aseptic loosening of the fem- oral component of cemented total hip arthroplas- ties [1–3]. Areas of thin mantles also have been associated with regions of osteolysis [4]. This sug- gests that fracture of the cement between the stem and bone may allow debris to elicit an osteolytic response. At-autopsy retrieval studies of well-functioning femoral constructs support the concept that cracks are more likely to occur in thin-mantle regions. In a retrieval study of 8 hips, Kawate et al. [5] found most (92 of 101) cracks in thin-mantle regions of 1 mm thick. Similarly, Kadakia [6] reported that in 11 hips, 96% of complete wall fractures were found in cement mantles that were 2 mm thick. The cracks appeared to be open, with a gap between the crack surfaces. This could allow direct commu- nication of fluid between the stem and bone. How these cracks develop as a result of loading remains unclear. In vitro studies of idealized stem- cement-bone constructs have shown that some mi- crocracks exist at the time of implantation and that these grow in length and number with cyclic load- ing [7]. There is also some evidence that thin- mantle regions incur higher stresses upon loading and thus would be expected to have cracks grow more quickly [8,9]. However, the experimental and computational models are relatively simplistic and do not directly address the entire cemented stem construct. From the *Department of Orthopedic Surgery, Upstate Medical University, Syracuse, New York; and the †Department of Mathematical Sciences, Bentley College, Waltham, Massachusetts. Submitted December 31, 2002; accepted December 15, 2003. Benefits or funds were received in partial or total support of the research material described in this article from the National Institutes of Health (NIH AR42017). Femoral stems were pro- vided by J&J-DePuy, Inc., Warsaw, Indiana; and Simplex cement was donated by Stryker-Howmedica-Osteonics, Mahwah, New Jersey. Reprint requests: Kenneth A. Mann, PhD, 3216 Institute for Human Performance, Upstate Medical University, 750 East Ad- ams Street, Syracuse, NY 13210. © 2004 Elsevier Inc. All rights reserved. 0883-5403/04/1905-0012$30.00/0 doi:10.1016/j.arth.2003.12.080 The Journal of Arthroplasty Vol. 19 No. 5 2004 605