AXISYMMETRICAL FINITE-ELEMENT ANALYSIS OF A DEBONDED TOTAL HIP STEM WITH AN UNSUPPORTED DISTAL TIP

A tapered femoral total hip stern with a debonded stem-cement interfac e and an unsupported distal tip subjected to constant axial load was e valuated using two-dimensional (2D) axisymmetric finite element analys is. The analysis was performed to test if the mechanical condition sug gest that a ''taper-lock'' with a debonded viscoelastic bone cement mi ght be an alternative approach to cement fixation of stem type cemente d hip prosthesis. Effect of stem-cement interface conditions (bonded, debonded with and without friction) and viscoelastic response (creep a nd relaxation) of acrylic bone cement on cement mantle stresses and ax ial displacement of the stem was also investigated. Stem debonding wit h friction increased maximum cement von Mises stress by approximately 50 percent when compared to the bonded stem. Of the stress components in the cement mantle, radial stresses were compressive and hoop stress es were tensile and were indicative of mechanical taper-lock. Cement m antle stress, creep and stress relaxation and stern displacement incre ased with increasing load level and with decreasing stem-cement interf ace friction. Stress relaxation occur predominately in tensile hoop st ress and decreased from 1 to 46 percent over the conditions considered . Stem displacement due to cement mantle creep ranged from 614 mu m to 1.3 mu m in 24 hours depending upon interface conditions and loan lev el.