Stem surface roughness alters creep induced subsidence and 'taper-lock' ina cemented femoral hip prosthesis

The clinical success of polished tapered steins has been widely reported in numerous long term Studies. The mechanical environment that exists for pol ished tapered stems. however. is not fully understood. In this investigatio n, a collarless, tapered femoral total hip stem with an unsupported distill tip was evaluated using a 'physiological' three-dimensional (3D) finite el ement analysis. It was hypothesized that stem-cement interface friction. wh ich alters the magnitude and orientation of the cement mantle stress, would subsequently influence stem 'taper-lock' and viscoelastic relaxation of bo ne cement stresses. The hypothesis that creep-induced subsidence would resu lt in increases to stem-cement normal (radial) interface stresses was also examined. Utilizing a viscoelastic material model for the bone cement in th e analysis, three different stem-cement interface conditions were considere d: debonded stem with zero friction coefficient (mu = 0) (frictionless), de bonded stem with stem-cement interface friction (mu = 0.22) ('smooth' or po lished) and a completely bonded stem ('rough'). Stem roughness had a profou nd influence on cement mantle stress, stem subsidence and cement mantle str ess relaxation over the 24-h test period. The frictionless and smooth taper ed stems generated compressive normal stress at the stem-cement interface c reating a mechanical environment indicative of 'taper-lock'. The normal str ess increased with decreasing stem-cement interface friction but decreased proximally with time and stem subsidence. Stem subsidence also increased wi th decreasing stem-cement interface friction. We conclude that polished ste ms have a greater potential to develop 'taper-lock' fixation than do rough stems. However, subsidence is not an important determinant of the maintenan ce of 'taper-lock'. Rather subsidence is a function of stem-cement interfac e friction and bone cement creep. (C) 2001 Elsevier Science Ltd. All rights reserved.