425 - Micromotion at the tibial plateau in total knee arthroplasty

MicroMotion at the tibial plateau in priMary and revision total knee arthroplasty bone & Joint research 123 with excellent long-term survivorship in several different device designs. 1-9 polyethylene wear resulting in osteoly- sis and aseptic loosening is a common, multifactorial, late-term failure mechanism in cemented arthroplasty. Micromotion at the cement-implant and cement-bone interfaces may contribute to this failure mechanism through predisposition to focal osteolysis and the ena- bling of polyethylene wear debris to migrate into the bone distally. 10,11 Because adequate initial stability of cemented TKa tibial prostheses contributes to long-term success of the implant, several prior studies focused on increased stability through cementing technique and sur- face preparation. 11-16 Fewer studies have investigated the role of device design in improving implant stability in cemented tibial trays. Tibial component micromotion is a mechanical result of the combination of shear forces and moments at the tibiofemoral joint generated during the complex biome- chanics at the knee during gait. Mobile-bearing tibial components were introduced in the late 1970s in order to decouple the shear moments generated by rotation between femoral and tibial components during knee flex- ion. 17 These designs enable high articular conformity and decreased polyethylene contact stress, reducing articular wear in addition to reducing post damage in constrained condylar and posterior stabilised designs. 18-22 previous studies have documented reduced torque transfer from femoral component rotation and malalignment at the tibiofemoral articulation with mobile bearing designs, subsequently reducing torsion-induced strain across the proximal tibia. 23,24 The reduction of torsion-induced strain during knee loading following mobile-bearing TKa may decrease excessive loading at the bone-cement and cement-prosthesis interface, thus reducing the rate of interface fatigue. Understanding the mechanical implications of rota- tion at the knee as a result of flexion or rotational mala- lignment is crucial for maximising component stability and promoting durable implant-cement-bone interfaces, particularly during revision TKa. prior studies have described reduced torque transfer and cortical tibial strain in tibias implanted with rotating platform (Rp) mobile-bearing tray designs. 23,24 We therefore asked if the Rp TKa designs result in improved tibial tray stability and decreased component micromotion in both the pri- mary and revision settings. Materials and Methods in this study, relative micromotion was measured within tibial specimens implanted with one of four prosthesis designs: fixed-bearing, posterior stabilised primary com- ponents (pFC Sigma, Depuy orthopaedics inc., Warsaw, indiana); Rp, posterior stabilised primary components (pFC Sigma, Depuy orthopaedics, inc.); fixed-bearing, posterior stabilised revision components with 115 mm × 14 mm press-fit distal stem and reduced intercondylar width (pFC TC3, Depuy orthopaedics inc.); and Rp, poste- rior stabilised revision components with 75 mm × 14 mm press-fit distal stem and reduced intercondylar width (pFC TC3, Depuy orthopaedics inc.). all tibial trays were manu- facturer ‘Size 3’ with 10 mm thick polyethylene bearings. Fourth generation composite tibial specimens (Medium, left, Model 3401, pacific Research laboratories, vashon, Washington) were chosen as the test specimen for their reduced interspecimen variability as compared with cadaveric tissue. 25,26 all tibial components were implanted by a board certified orthopaedic surgeon (RaM) using standard instrumentation and high-viscosity polymethyl- methacrylate (pMMC) bone cement (SmartSet Hv, Depuy orthopaedics inc.) following the manufacturer’s sug- gested surgical technique of 0° of posterior slope and sur- geon preference of neutral varus-valgus alignment based on the long axis of the composite tibia. primary tibial components were fully cemented along the baseplate and stem, while revision components uti- lised press-fit fixation in the distal stem, with proximal cementing limited to the underside of the tibial tray. Cement was finger-packed to both the component and tibial plateau and manually impacted. Full tibial tray seat- ing was attained in all specimens and visually verified by the implanting surgeon. a total of six tibial specimens were implanted per experimental group. Biomechanical testing was conducted on a biaxial elec- trodynamic materials testingmachine (Electropuls E10,000 a/T, instron, Norwood, Massachusetts). Specimens were incorporated into the materials testing machine via a custom fixture allowing free medial/lateral and anterior/ posterior translation of the potted base. Femoral compo- nents selected to match tibial component size and design (pFC Sigma and pFC T3 Size 3, Depuy orthopaedics, inc.), were integrated into the upper testing grips to allow for repeatable load application through the femoral compo- nent onto the polyethylene bearing surface at the lowest preferred dwell point. a silicon-based lubricant (DM-Fluid- 350CS, Shin-Etsu Chemical Co, Tokyo, Japan) was applied between all articulating surfaces to lubricate the otherwise dry in vitro test environment. Testing was conducted in two phases: compressive loading ramped at a rate of 60 N/s to a peak load of 2.5 kN, followed by femoral com- ponent rotation at a rate of 0.5 degrees per second to a final position of 10° external tibiofemoral malalignment. in all instances the femoral component was positioned in 90° of flexion. a total of five trials were repeated for each of four digital image correlation (DiC) viewing angles (ante- rior, medial, posterior, and lateral) in all 24 specimens, with micromotion data collected as the change in position of each data point between the start and completion of each testing phase. DiC was used in this study to provide non-contact micromotion analysis with greater precision than alterna- tive techniques, namely linear variable displacement transducers (lvDTs). a set of two calibrated, high | The Surgical Technologist | MAY 2019 204