WHAT HAPPENS WHEN POLISHED TAPER-SLIP STEMS REACH THEIR SUBSIDENCE LIMIT? A FINITE ELEMENT ANALYSIS STUDY

Cemented polished taper-slip (PTS) stems are the most frequently used design for primary total hip arthroplasty (THA) in the UK. Periprosthetic fracture is a significant contributor to reoperations post-PTS implantation. Stem centralisers help create an even cement mantle and accommodate subsidence. The allowance for subsidence is integral to the PTS implant, a “force-closed” design, to maintain fixation. However, if subsidence exceeds the centraliser's limit the implant becomes “shape-closed” potentially introducing stress rises within the cement mantle. This study used finite element analysis (FEA) to assess how stress distributions changed when further subsidence was no longer possible.

A high-accuracy 3-D scanner and computer-aided design (CAD) software were used to create geometric models of three commonly used PTS designs: C-Stem, Exeter, and CPT. The stems were then modelled with a 4mm cement mantle and stem centraliser gap. FEA simulated loading conditions (applied forces of 6,900 N) with and without a distal space, modelling scenarios with/without a subsidence gap. Variations in stem materials (SS316L and Co-Cr alloys) were also modelled. Distributions of von Mises stress and cement deformation under load were evaluated.

Maximum von Mises stresses in the cement mantle with space to subside were predominantly in Gruen zone 7. The CPT design showed the highest stress at ∼45 MPa. Without space to subside, maximum stress shifted to the distal tip (zone 4). The increase in stress at zone 4 increased by 49%-102% depending on stem design and material modelled. In all models, irrespective of stem design, Co-Cr further increased the percentage increase in stress at the distal tip when compared to SS316L.

This study shows that when PTS stems reach the subsidence limit allowed by the centraliser, the resulting stress concentrations in the distal cement mantle increase significantly. Factors like stem design, material, and friction at the implant-cement interface influence stress distribution and subsidence within the cement mantle. These findings highlight the complex interaction of the stem and cement highlighting factors that may contribute, in part, to cement failure and in turn risk of periprosthetic fracture.