The influence of implant articular thickness and glenohumeral conformity on stability of an all-metal glenoid component Ryan T. Bicknell, MD, MSc, FRCSC, a Allan S. L. Liew, MD, FRCSC, a Matthew R. Danter, MD, c Stuart D. Patterson, MBChB, FRCSC, d Graham J. W. King, MD, MSc, FRCSC, a,b,c,e David G. Chess, MD, FRCSC, a,b,c,e and James A. Johnson, PhD, a,b,c,e London, Ontario, Canada, and Lakeland, FL The objective of this study was to determine the effect of implant thickness and glenohumeral conformity on fixation of an all-metal glenoid component. A stainless steel glenoid component was designed and implanted in 10 cadaveric scapulae. A testing apparatus capa- ble of producing a loading vector at various angles, magnitudes, and directions was used. The indepen- dent variables included 6 directions and 3 angles of joint load, 3 implant thicknesses, and 4 glenohumeral conformities. Implant micromotion relative to bone was measured by use of 4 displacement transducers at the superior, inferior, anterior, and posterior sites. The components displayed a consistent response to load- ing of ipsilateral compression and contralateral distrac- tion. Stability decreased as the load application angle increased (P  .05). A decrease in the implant thick- ness and glenohumeral conformity resulted in in- creased implant stability (P  .05). Decreasing implant thickness and glenohumeral conformity reduce the ec- centric component of loading and may improve the durability of glenoid implants. (J Shoulder Elbow Surg 2007;16:631-639.) L oosening of the glenoid component remains a fac- tor limiting the long-term success of total shoulder arthroplasty. 7,16,18,27,36,41,43,44,46 Furthermore, gle- noid loosening has been shown to be significantly correlated with deteriorating functional results and increasing pain. 7,24,39,43 This is thought to be a result of several factors, including inadequate implant-bone fixation and eccentric loading. Implant-bone fixation has been investigated extensively. Various designs have attempted to achieve fixation to bone through the use of anchoring systems on the undersurface of the glenoid component. In vitro studies have been conducted to assess the strength of fixation and the stability or micromotion of various implant designs relative to bone. 1,6,8,31,32 Theoretic finite-element studies have also analyzed the state of stress in the implant-bone structure. 15,23,32,38 However, these studies have not conclusively defined the optimal methods by which to maximize the stability between the glenoid component and bone. Previous studies have also indicated that the gle- noid component is very sensitive to eccentric loading (ie, loading that occurs nonperpendicular to the gle- noid surface). 6,9,25 Distraction or liftoff of the implant from the bone surface, potentially inhibiting bone ingrowth or producing interface tension, is a con- cern with eccentric loading. In extreme arm posi- tions or with certain pathologic conditions, sublux- ation of the humeral head may lead to rim loading of the glenoid. 3,14,20,22 It has been shown that ad- vances in bone preparation and cementing tech- niques can reduce the effects of eccentric loading that act to cause glenoid component distraction and loosening. 9,30 Reducing the eccentricity of the ap- plied load (and also rim loading) by using a thinner implant has also been shown to minimize the distrac- tion of the glenoid component. 25 In addition, reduc- ing glenohumeral conformity may also reduce eccen- tric loading by minimizing rim loading. 21,22,37 The objectives of this study were to determine the effects of implant articular thickness and glenohu- meral conformity on the stability of uncemented all- metal glenoid components, as quantified by the mag- nitude of micromotion between implant and bone. We hypothesized that decreasing implant articular From the Departments of a Surgery, b Mechanical and Materials Engineering, and e Medical Biophysics, University of Western Ontario, and c Hand and Upper Limb Centre, St Joseph’s Health Centre, London, and d Central Florida Orthopaedic Surgery As- sociates, Lakeland. Reprint requests: Ryan T. Bicknell, MD, MSc, FRCSC, Hand and Upper Limb Centre, St Joseph’s Health Centre, 268 Grosvenor St, London, Ontario, Canada, N6A 4L6 (E-mail: rtbickne@ yahoo.ca). Copyright © 2007 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2007/$32.00 doi:10.1016/j.jse.2006.10.014 631