Creep, bone cements

Bone cements used to fill the void and improve adhesion between implants and the host bone tissue have been reinforced with various fibers to prevent loosening and enhance shear strength. The typical bone cement is PMMA powder mixed with a methacrylate-type monomer that is polymerized during fixation. Low volume fractions of graphite, carbon, and Kevlar fibers have been added to PMMA matrices to increase fatigue life and r uce creep deformation. ... 

Migliaresi C, Fambri L, Kolarik J (1994) Polymerization kinetics, glass transition temperature and creep of acrylic bone cements. Biomaterials 15 875—881... 

The other acrylate bone cement is based on poly-ethylmethacrylate (PEMA) and n-butylmethacrylate ( -BMA) monomer [61], Comparing to PMMA cement, less heat is produced during polymerization of the PEMA-n-BMA cement, and the polymer has a relatively low modulus and high ductility to reduce the issue of fracture. The biocompatibility of the PEMA-n-BMA cement has been excellent [62]. But these bone cements have been found to be susceptible to creep. To improve creep resistance, bioactive HA particles were incorporated [63]. Although HA improved bioactivity and creep behavior of the cement, the cement failed at lower number of cycles. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Verdonschot, N., and Huiskes, R., 1997, Acrylic cement creeps but does not allow much subsidence of femoral stems. J. Bone Joint Surg. 79-B 665-669. 

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