Tl. Norman et al., Stem surface roughness alters creep induced subsidence and 'taper-lock' ina cemented femoral hip prosthesis, J BIOMECHAN, 34(10), 2001, pp. 1325-1333
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.