Full Polyethylene or Metal Back?

The development of different types of tibial components has led to the development of two major groups: full-polyethylene (FPE) implants and metal-back implants fixed or mobile. UKA procedure is sacrificing the underlying subchondral bone layer to leave room for a suitable thickness of polyethylene. The surface area of support provided is less than half of what is available for TKR. UKA is a demanding tibial implant due to its small surface area, low homogeneity and low-density bone. It is important to choose the most appropriate implant and fixation for this specific context. Metal-back implants enable better distribution of stresses on the surface area of the tibia and up to 3 cm below the tibial implant, leading to better results in clinical studies. Several teams have studied the stress exerted on PE and its distribution based on the type of tibial plateau. Simpson et al. found that mobile metal-back implants had peaks of contact forces below the PE level of resistance, and von Mises found that peaks of stress varied inversely with the thickness of the PE. Mobile metal-back implants have a greater risk of aseptic loosening than fixed metal-back implants due to higher wear rate and higher generation of PE debris. Manson et al. identified seven types of wear for three types of uni replacements, with full-PE and fixed metal-back implants showing higher wear scores than mobile metal-back implants. Kretzer et al. found burnishing processes as the least harmful wear on the upper sides of the mobile metal-back and fixed metal-back implants. The type of implant (full PE or metal back) in contact with bone will govern the transfer of stresses. Full PE implants generate more abnormal stress than metal-back implants and have a higher risk of revision surgery and implant loosening in heavy patients. Metal-back implants provide a rigid base that distributes load over a bone surface inhomogeneously, but the transmission of stresses is optimised by the metal plate. Mobile implants are marked by PE solidity with the aim of decreasing wear, and have two technical specificities: ligament balance and better visibility during tibial sealing. Clinical studies comparing full-PE implants to metal-back implants show discrepant results. Clinical studies have compared full-PE to metal-back implants, with no definitive clinical evidence supporting one type over another. Survival rates range from 82% to 98% at 10 years, with mobile metal-back implants superior over the long term. PE wear and creep are found mainly in fixed plateau implants. Mobile metal-back implants are more technically demanding and require a steeper learning curve than full-PE implants, but the literature remains difficult to use. Metal-back implants have a lower rate of revision surgery due to loosening, but transient upper metaphyseal pain has been reported. Metal-back implants are better for heavy and active subjects, while full-PE implants are better for older, less active patients. UKA is a good procedure with the right indication and surgical procedure.