The Influence of Stem Design on Critical Squeaking Friction With Ceramic Bearings Na Fan, 1,2 Michael M. Morlock, 2 Nicholas E. Bishop, 2 Gerd Huber, 2 Norbert Hoffmann, 3 Michele Ciavarella, 2,4 Guang X. Chen, 1 Arne Hothan, 2 Florian Witt 2 1 Tribology Research Institute, Southwest Jiaotong University, Chengdu, 610031, China, 2 Biomechanics Section, TUHH Hamburg University of Technology, Hamburg, 21073, Germany, 3 Institute of Mechanics and Ocean Engineering, TUHH Hamburg University of Technology, 21073, Hamburg, Germany, 4 Politecnico di Bari. V.le Gentile 182, 70125, Bari, Italy Received 6 November 2012; accepted 28 May 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jor.22413 ABSTRACT: Ceramic-on-ceramic hip joints have been reported to squeak, a phenomenon that may occur in compromised lubrication conditions. One factor related to the incidence of in vivo squeaking is the stem design. However, it has not yet been possible to relate stem design to squeaking in deteriorating lubrication conditions. The purpose of this study was to determine critical friction factors for different stem designs. A hip simulator was used to measure the friction factor of a ceramic bearing with different stem designs and gradually deteriorating lubrication represented by evaporation of a volatile fluid lubricant. The critical squeaking friction factor was measured at the onset of squeaking for each stem. Critical friction was higher for the long cobalt chrome (0.32  0.02) and short titanium stems (0.39  0.02) in comparison with a long titanium stem (0.29  0.02). The onset of squeaking occurred at a friction factor lower than that measured for dry conditions, in which squeaking is usually investigated experimentally. The results suggest that shorter or heavier stems might limit the possibility of squeaking as lubrication deteriorates. The method developed can be used to investigate the influence of design parameters on squeaking probability. ß 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res Keywords: ceramic bearing; squeaking; stem design; critical friction Today’s generation of alumina ceramics have demon- strated excellent tribological properties, high-wear resistance, and biocompatibility. 1,2 However, the occurrence of squeaking in ceramic-on-ceramic hip joints presents social and psychological problems for patients. The reported prevalence of squeaking in ceramic bearings ranges from <1% to >10%. 3–6 Vari- ous causes have been identified, including surgical implantation, 7–9 friction-induced vibration 9–12 and im- plant design. 13,14 Restrepo et al. 13 observed a seven times greater incidence of squeaking for one of two stem designs (28 cases) in a consecutive series of 304 hips, and noted that it was a more flexible material than the other. In the laboratory, squeaking has been replicated in dry conditions, 15–17 or in seriously compromised lubri- cation conditions by introducing ceramic chips into the joint space. 18,19 Under dry conditions, Hothan et al. 16 demonstrated that stem design influences the squeak- ing frequency as well as sonic pressure. The resonant frequency of stems was measured to lie within the audible squeaking range, while the natural frequency of an acetabular cup with a ceramic insert lies above the audible range due to its higher stiffness and is not thought to influence squeaking directly. 10,16 Since squeaking is excited by relative motion be- tween the joint surfaces, it is reasonable to assume that it is related to friction. The friction factor was related to the incidence of squeaking in numerical studies, 11,20 and it was also shown that the friction factor can increase by an order of magnitude in conditions of highly compromised lubrication. 18,19 However, the actual level of friction related to the onset of squeaking, as lubrication conditions deterio- rate, has not been demonstrated. The aim of this study was to determine critical friction factors for different stem designs. MATERIALS AND METHODS Materials Three commercially available uncemented stem designs (S1– S3) were investigated (Fig. 1, Table 1). S1 is a titanium stem with plasma sprayed proximal surface (Bicontact, Aesculap, Germany). S2 is a cobalt chrome stem with trabecular-like structured surface (G1, Eska, Germany). S3 is a short titanium stem with a beaded porocoat surface (Proxima, DePuy, UK). These implants represent a broad range of mass, geometry and first eigenfrequency. The same 32 mm ceramic head (Biolox1 forte, CeramTec, Plochingen, Germany) with corresponding liner (Biolox1 delta, Ceram- Correspondence to: Florian Witt (T: 0049-0-40-42878-4361; F: 0049 (0)40 42878 2996; E-mail: witt@tu-harburg.de) # 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Figure 1. Three stem designs (S1, S2, and S3) were investigated. JOURNAL OF ORTHOPAEDIC RESEARCH MONTH 2013 1