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General Orthopaedics


Current Concepts in Joint Replacement (CCJR) – Spring 2015


Two big problems exist with the all-polyethylene cemented tibial component—the polyethylene and the cement. The polyethylene is too weak and flexible to bear tibial load, so it deforms and loosens. Isoelastic material has never worked, and it never will. The interface stresses are too high when two flexible structures are poorly bonded and heavily loaded. Critical review of the literature fails to make a convincing case for use of cement in TKA. Many studies demonstrate clinical, mechanical, and biological failure when cement is used for fixation. Work by Ryd et al. has shown that initial migration within the first few months diminished rapidly after the first 6 months with virtually no additional movement for years after. They also found that cemented components do not remain rigidly fixed to bone long-term, but loosen enough to move 0.2 to 2.1 mm at the bone-cement interface with provocative testing. Although bone-ingrowth tibial components migrate slightly more initially than cemented ones do, they stabilise and do not sink progressively. Screw fixation adds rigidity, but does not seem to improve results. Rigidity of initial fixation is the most important feature after alignment to ensure pain-free function after arthroplasty, and can be achieved with press-fit techniques in TKA. Several early reports of bone-ingrowth TKA had inferior results because the tibial component had no stem, peg, or screw fixation, leading to implant migration and loosening. An effective stem has been shown to greatly improve tibial component fixation. The cut upper surface of the prepared tibia has areas that are too weak to withstand the forces that are applied to the surface, and failure in compression is likely unless fixation is augmented. An effective stem also reduces the shear and tensile loads at the bone-prosthesis interface. The effectiveness of compression or compaction of the tibial cancellous bone with an appropriately sized tibial metaphyseal stem has been shown, and probably was a major factor in the long-term success of fixation in our series.

Clinical results of TKA with osteointegration techniques for fixation of the femoral and tibial components in our series are comparable with the best series reported with cemented fixation. Many recent studies show significant advantages of osteointegration over cement fixation in TKA. Fixation of implants with PMMA pressed into cancellous bone eventually loosens, and fixation of a metal component to bone cement also is tenuous in most cases. Cement is disappearing rapidly from use in total hip, ankle, and shoulder arthroplasty, and soon will be replaced with osteointegration technique in the knee. Perhaps the most appealing aspect of bone-ingrowth TKA is bone preservation. The ease of revisability because of good bone was encouraging in the components that wore, loosened, or became infected in the current series of TKA. These knees are functioning as well as knees with primary TKA. Should these knees develop additional problems, progressive destruction of bone is unlikely to occur, even if repeated revision is necessary.