Short Communication Relationship of design features of stemmed tibial knee prosthesis with stress shielding and end-of-stem pain A. Completo a , P. Talaia a , F. Fonseca b , J.A. Simões a, * a Departamento de Engenharia Mecânica, Universidade de Aveiro, 3810-193 Aveiro, Portugal b Faculdade de Ciências da Saúde, Universidade da Beira Interior, 6201-001 Covilhã, Portugal article info Article history: Received 19 January 2007 Accepted 28 June 2008 Available online 22 July 2008 abstract Materials and design are important issues within general product development and even more in bio- medical product. Knee prosthesis is a product that still lacks of adequate design solutions. Stress shielding and stress concentrations related to the end-of-stem pain are frequent clinical evidences that are inti- mately related to the design of the prosthesis and stems used in revision total knee arthroplasty, to the femoral and tibial components. Stress shielding and stress concentrations were assessed through finite element analyses of different tibial stem designs. Stress shielding was determined considering the relative minimal principal cancel- lous bone stresses between implanted and intact tibiae models. Scintigraphy and radiographs of painful implanted knees were assessed to correlate clinical findings with the finite element stresses. This study showed that stem design, geometry and material change the mechanical behavior of bone around tibial tray and stem in revision TKA. For the analyzed bone regions, underneath tibial tray and around the stem, the geometry of stem had a more pronounced bone effect comparatively to the stem material. The results of this study support that short stems produce a minor effect in bone relatively to long stem in terms of stress shielding and stress concentration at tip region. No significant stress shielding differences was observed between Co–Cr and titanium stems. Overall, all stems provoked high stress concentrations in bone at the tip of the stem. Scintigraphy of a painful implanted knee showed local osteoblastic activity and radiographs evidence bone ossification at the distal tip of the stem due the stress concentration. The results show that long stems induce very high distal stresses and are more prone to induce pain. A new concept of long stem with a distal polymeric (flexible material) tip reduces the magnitude of stresses at the end of the stem and therefore can theoretically contribute to reduce pain. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction One of the less successful aspects of knee arthroplasties is re- lated to pain experienced by patients in the short and/or long term. There are a myriad of causes of pain after total knee arthroplasty (TKA) [5]. Common causes of pain after TKA can be due to infection or prosthetic loosening, or may be present without either of these factors. Stress shielding is also a major concern, since problem of bone loss due to this effect have become apparent. Several authors have addressed this problem because an inadequate bone stock is often found in revision surgery of total knee replacements. Revision total knee replacements or primary total knee replace- ments with large amounts of bone loss frequently require stemmed prostheses. Stems allow additional fixation and ensure better alignment of the tibial and femoral components [1]. Debates are ongoing about the best way to fix these stems: cemented or press-fit. A fixation strategy that seems to provide good results are a hybrid solution, in which the surface of the tibial plateau and/or femoral component is cemented while the stem not [7]. The most cited disadvantage of this type of fixation is the presence of pain at the end of the stem [3,2,7] due to stress concentrations. A clinically-based reported study demonstrated the incidence of pain in around 32% of cases involving stems in Co–Cr, of which 63% were localized on the tibia, 20% on the femur, and 17% on both tibia and femur [2]. Theoretical concerns about the use of cemented stemmed revi- sion total knee arthroplasty include stress shielding with adverse effects on prosthesis fixation [13]. Dual-energy X-ray absorptiom- etry scanning has shown that stress shielding occurs from altered mechanical loading. Lonner et al. [10] determined stem design af- fects over bone mineral density (BMD) in the long-term. Results of their study showed that there was a significantly reduced density of bone in the tibial metaphysis in the cemented stemmed group but not in the pegged group and no changes were observed distally in the diaphyseal bone. The use of a cemented stem reduces prox- imal stresses and may result in proximal bone resorption due to osteoclastic activity. Bourne and Finlay [4] refer that the use of 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.06.071 * Corresponding author. Tel.: +351 234 370830; fax: +351 234 370953. E-mail address: simoes@mec.ua.pt (J.A. Simões). Materials and Design 30 (2009) 1391–1397 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes