The Effect of Rotating Platform TKA on Strain Distribution and Torque Transmission on the Proximal Tibia Robert A. Malinzak, MD b , Scott R. Small, MS b , Renee D. Rogge, PhD a , Derek B. Archer, BS a , Jordan W. Oja, BS a , Michael E. Berend, MD b , Merrill A. Ritter, MD b a Department of Applied Biology and Biomedical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana b Joint Replacement Surgeons of Indiana Foundation, Inc., Mooresville, Indiana abstract article info Article history: Received 31 May 2013 Accepted 26 August 2013 Keywords: mobile bearing TKA rotating platform digital image correlation torque strain Limited experimental data exist comparing the mechanical response of the tibial cortex between fixed and rotating platform (RP) total knee arthroplasty (TKA), particularly in the revision setting. We asked if RP- TKA significantly affects tibiofemoral torque and cortical stain response in both the primary and revision settings. Fixed and RP tibial trays were implanted into analogue tibias and biomechanically tested under axial and torsional loading. Torque and strain response were analyzed using digital image correlation. Fixed bearing designs exhibited 13.8 times greater torque (P b 0.01), and 69% (P b 0.01) higher cortical strain than RP designs. Strain response was similar in the primary and revision cohorts. The decrease in torque transfer could act as a safeguard to reduce stress, micromotion and torsional fatigue in scenario of poor bone stock. © 2014 Elsevier Inc. All rights reserved. Total knee arthroplasty (TKA) has demonstrated favorable clinical results with high survivorship reported into the third decade across a variety of implant designs [1–9]. Aseptic loosening and osteolysis attributed to polyethylene wear have historical mechanisms associ- ated with late failure and TKA revision [10–12]. In an attempt to address these failure mechanisms, mobile-bearing tibial components were developed in the late 1970s. The mobile-bearing construct, initially available in the form of a rotating platform or meniscal bearing design, allows for high articular conformity for decreased polyethylene contact stresses, while decoupling shear generated by femoral component rotation during flexion. This decoupled articu- lation has been shown to reduce tibial component wear in vitro, and may additionally serve to lessen post damage in posterior stabilized or constrained condylar designs, an additional source of wear [13–17]. Additionally, a rotating platform design should theoretically inhibit the transfer of torsional moments generated by a rotated or malaligned femoral component, subsequently lessening excessive fatigue loads at the bone-implant or cement-implant interface, thus leading to long-term loosening [18,19]. While a theoretical advantage of mobile-bearing TKA designs, few biomechanical studies have evaluated the comparative mechanical response of the tibia to femoral component rotation in the implanted tibia, particularly in the case of revision arthroplasty, which can encompass significant biomechanical differences such as increased constraint. Revision arthroplasty has historically poorer survivorship than primary surgery [20]. In the scenario of revision TKA, poor bone quality may lead to increased risk for subsequent loosening. A single previous biomechanical study has compared the effect of fixed and mobile-bearing TKA on cortical strain in the tibia [19]. That study, however, utilized only primary TKA components and ob- served strain gradients in a small region of the anterior tibia. Current literature is lacking assessment of the tibial response to relative component rotation in the revision setting where additional con- straint and distal stem fixation may play a factor into torque gene- ration and stress transfer. Evaluation of the full-field biomechanical response of the tibia to the rotating platform (RP) design is critical in the understanding of load transfer mechanisms during relative component rotation or rotational malalignment. We therefore asked if a rotating platform design gives rise to a significant reduction of torque at the tibiofemo- ral joint, if torque transfer at the tibial component results in a sig- nificant change in strain response across the tibial cortex, and if these trends are comparable between the primary and revision settings. Materials and Methods In order to compare the mechanical response to torsional loading at the knee in fixed and rotating platform mobile-bearing TKA com- ponents, torque and cortical strain was measured in tibial specimens implanted with one of four prosthesis designs: 1) Fixed-bearing, posterior stabilized primary components (PFC Sigma, DePuy, The Journal of Arthroplasty 29 (2014) 541–547 The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.08.024. Reprint requests: Robert A. Malinzak, MD, Joint Replacement Surgeons of Indiana Foundation, Inc., Mooresville, IN 46158. 0883-5403/2903-0019$36.00/0 – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.08.024 Contents lists available at ScienceDirect The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org