Introduction: Finite element (FE) simulation of damage accumulation in the femoral cement mantle is widely used to predict failure of hip prostheses. It is often assumed that the stem-cement interface remains bonded, although debonding is thought to affect cement stress and damage. Rough stems may reduce subsidence, but have been reported to have a detrimental effect on implant survival. Other factors thought to influence cement damage include stem design and orientation and cement thickness. This study investigates the effect of cement mantle thickness and stem malpositioning on cement damage around a smooth, collared implant, and the extent to which this is affected by debonding of the stem-cement interface.

Method: Three FE meshes were built to represent proximal femora with Stanmore Hip prostheses implanted into a thick (2.5 mm) and a thin (1.0 mm) cement mantle, and another thin (1.0 mm) mantle with the implant tilted in varus to achieve a minimal thickness of 0.1 mm laterally. Each model consisted of 4304 eight-noded brick elements with frictional contact at the stem-cement interface. Two analyses were run for each model, in which the stem-cement interface was (a) fully bonded, and (b) fully debonded, with Coulomb frictional contact using a friction coefficient of 0.5. Standardised femur geometry and elastic properties were used. Creep and non-linear damage accumulation in the cement mantle under cyclic loading was modelled using subroutines developed by Stolk et al. (2003). Boundary conditions were applied representing a peak stair-climbing load.

Results: Bonded cases showed extensive cracking around the tip in all cases. Debonded cases had 4–8 times less cracking, which was much more focused at the tip; only the poorly-centralised mantle showed extensive damage elsewhere, in the very thin lateral region. When bonded, the thick mantle had least cracks and the poorly-centralised mantle had most; in the debonded cases, there was no major difference between thick, thin, and poorly-centralised mantles. For each cement mantle geometry, peak maximum principal cement stress was consistently lower in the debonded case than in the bonded case.

Discussion: Our results show greater, more widely distributed cracking in bonded than debonded cement mantles, in contrast with previous studies involving collarless implants. For a collared stem, calcar contact prevents subsidence, allowing cement stress relaxation. A possible explanation for our result is that debonding enhances the stress relaxation process, reducing and redistributing interfacial and shear stresses; thus reducing damage rates. In contrast, a debonded collarless stem subsides continuously, sustaining high cement stress levels and damage rates. These results may explain the disappointing clinical performance of some rough-surfaced prostheses. Our results suggest that bonding might increase both cement damage and its sensitivity to cement thickness. Similar results for all debonded cement mantles indicate that cement thickness may be less critical than previously thought for smooth, collared prostheses. Bonding should not be assumed in FE studies of smooth stems which clinically are likely to debond; cement damage simulation should be extended to incorporate the debonding process.

Introduction: Surgeons in the UK and Europe generally use a thinner cement mantle than their counterparts in the USA for the femoral component in total hip replacement (THR). The aim of this study was to compare the performance of different thicknesses of cement mantle using finite element analysis. The measures by which comparison might be made include cement cracking, subsidence, migration and stress shielding. In this study, we use a linear-elastic model of the implanted femur to give a prediction of the stresses in the cement mantle and in the femoral cortex. These measures give an indication of the relative rates of cement cracking and loss of bone stock due to stress shielding. To assess the reliability of our model in representing patients with different bone densities, we use a range of cancellous bone stiffnesses.

Method: Two cadaveric femora from the same donor were sized, reamed and implanted with identical plastic replica femoral components following standard surgical technique for the Stanmore Hip system. One was prepared using UK rasps, over-reaming by ~2mm, the other using US rasps, over-reaming by ~5mm. Serial CT-scans were used to create three-dimensional geometric models of the implanted femora. Two finite element meshes were hand-built in MSC. Marc finite element software, incorporating cortical and cancellous bone, bone cement and prosthesis. Each model consisted of 10,000 eight-noded brick elements, with a fully bonded stem-cement interface. The thick and thin cement mantles had thicknesses of 2.5mm and 1.0mm respectively, in regions where thickness is affected by rasp size. Models were identical in the distal medullary canal. Cortical bone was modelled as transversely isotropic, with longitudinal and transverse moduli of 17.0 and 11.5 GPa. Bone cement was given a modulus of 2.7 GPa. Loading conditions were chosen to represent the heel-strike phase of gait. In order to assess the impact of variability in patient bone density, cancellous bone modulus was varied between 0.06 and 2.90 GPa.

Results: Equivalent stress was examined on the external surface of the cortex and the internal surface of the cement mantle. The lowest cortical bone stresses were proximal and the highest cement stresses around the distal tip of the prosthesis. In the proximal cortex, higher equivalent stresses were observed medially and laterally with a thick cement mantle. Distally, lower cement stresses were observed in the thick cement mantle. With the highest cancellous modulus, there was little difference between the two models. As this modulus was reduced, stress differences between the models became more apparent. For all cancellous bone moduli, peak distal cement stresses were lower and minimum proximal calcar stresses higher in the thick cement mantle.

Discussion: Proximal stress shielding was greatest in the calcar, in agreement with clinical findings. The thicker cement mantle led to less stress shielding in this region. Cement stresses, highest around the distal tip of the prosthesis, were larger in the thin cement mantle. This suggests a higher rate of both cracking and bone resorption in thin cement mantles. Although observed over a range of cancellous bone stiffness, this finding applies particularly to patients with low bone density.

Introduction: Aseptic loosening remains the commonest causes of failure of total hip arthroplasty. Cement mantle defects are associated with aseptic loosening. This study aimed to determine a correlation between surgical approach and cement mantle defects in the Stanmore Hip. The Stanmore total hip replacement was chosen because it has greater than an eighty-five percent survivorship over 25 years and unlike other prostheses with comparable results such as the Charnley total hip, it remains essentially unchanged to date.

Method: This was a retrospective review of all Stanmore hips. AP and lateral radiographs were available for 62 patients operated via the posterior approach and 100 patients operated via the anterolateral approach. The mean cement thickness in all fourteen Gruen zones was estimated for each patient. Gruen zones IV and XI, representing the stem tip, were removed from data relating to mantle thickness. Mantles were graded as less than 2mm, 2–5mm, 5–10mm and more than 10mm. Alignment was also measured.

Results: Fifty-nine percent (32/54) of cement mantle defects are seen in Gruen zones VIII to XIV. The mean cement mantle thickness in A-L approach was 3.11mm compared to 4.23mm with the posterior approach. This corresponds with the frequency of cement mantle defects occurrence. No cement defects were seen in Gruen zones IV or XI. Using the anterolateral approach, defects were observed in 49 out of 1200 zones (4.08%) and using the posterior approach in 6 out of 744 zones (0.81%). With the anterolateral approach, 19 out of 100 cement mantles (19%) had defects, compared to only 3 out of 62 (4.84%) with the posterior approach. Defects were most commonly seen in zones I, V, VIII and XII, which corresponds to valgus and posterior orientation of the stem.

Discussion: The posterior approach does generate a more uniform cement mantle. Several studies suggest that a cement mantle smaller than 2mm or greater than 10mm can be detrimental to the survivorship of the arthroplasty. This study suggests that a deficient cement mantle is more likely using an anterolateral approach.

Introduction A consensus exists regarding the optimal range of femoral cement mantle thickness in hip replacement. However, within this range surgical preferences differ, surgeons in Europe generally preferring thinner cement mantles whilst those in the US prefer a thicker mantle. For a given implant size, the rasps provided in the US for use with the Stanmore Hip are larger than those used in Europe, producing a thicker cement mantle. The integrity of the femoral cement is considered to be crucial to the long-term survival of cemented hip replacements. Previous studies have used cement cracking under fatigue loading as a comparative measure of implant survival. Damage accumulation levels between different implants are associated with clinical failure rates. The aim of this study was to compare the cracking behaviour of cement mantles of different thicknesses around Stanmore Hip replacements. We hypothesised that a thicker cement mantle would lead to reduced cement cracking.

Methods Ten synthetic femurs (Sawbones) were prepared following standard surgical practice for the Stan-more Hip. Five of these were rasped using the larger US rasp, and five using the European version. Stanmore Hip femoral components were then cemented into the femurs with Palacos-R cement and using a custom insertion rig to ensure good alignment and centralisation, confirmed by radiographs. The femurs were then cyclically loaded with an aggressive 4 kN stair-climbing load for 4 million cycles at 3 Hz. The femurs were sectioned at 5 mm intervals and dye penetrant used to highlight cement cracks. Image analysis software was used to measure cement thickness and crack lengths under light microscopy.

Results The minimum cement mantle thickness per section was found to average 0.8 mm and 2.0 mm for the thin and thick mantle groups respectively, measured around the proximal half of the implant. This was significantly different (p< 0.05). Cracks in the cement mantle were irregularly distributed along the length of the prostheses. We found no significant difference in either the total number or total length of cracks found in each group. These were investigated over the whole mantle and by Gruen Zone.

Discussion The geometric and mechanical properties of human femurs vary considerably, which might be expected to increase dramatically the scatter in any clinical trend relating cement thickness to cracking. Our study, using identical synthetic femurs and well-centralised prostheses to minimise experimental variability, found no difference in cracking. Given this experimental consistency, it is thought that there would be no clinically significant difference in cracking rates between different cement thicknesses within the normal range for the Stanmore Hip replacement. The Stanmore Hip is designed to minimise cement stress. A collar prevents subsidence-related hoop stresses, and smooth corners minimise stress concentration in the cement. It is likely that, for a sub-optimal implant design with higher stress risers, cement thickness might have a more noticeable effect on crack propagation.

Introduction: Surgeons in the UK and Europe generally use a thinner cement mantle than their counterparts in the USA for the femoral component in total hip replacement. The aim of this study was to compare the performance of different thicknesses of cement mantle using finite element analysis. A linear-elastic model of the implanted femur is used to give a prediction of the stresses in the cement mantle and in the femoral cortex. These measures give an indication of cement cracking rates and stress shielding. To assess the reliability of our model in representing patients with different bone densities, we use a range of cancellous bone stiffnesses.

Method: Two cadaveric femora from the same donor were sized, reamed and implanted with identical Stanmore Hips. One was prepared using UK rasps, over-reaming by 2mm, the other using US rasps, over-reaming by 5mm. Serial CT-scans were used to create three-dimensional geometric models of the implanted femora. Two finite element meshes were hand-built in MSC.Marc finite element software, incorporating cortical and cancellous bone, bone cement and prosthesis, with a bonded stem-cement interface. Loading conditions were chosen to represent the heel-strike phase of gait. In order to assess the impact of variability in patient bone density, cancellous bone modulus was varied between 0.06 and 2.90 GPa.

Results: Equivalent stress was examined on the external surface of the cortex and the internal surface of the cement mantle. The lowest cortical bone stresses were proximal and the highest cement stresses around the distal tip of the prosthesis. In the proximal cortex, higher equivalent stresses were observed medially and laterally with a thick cement mantle. Distally, lower cement stresses were observed in the thick cement mantle. With the highest cancellous modulus, there was little difference between the two models. As this modulus was reduced, stress differences between the models became more apparent.

Discussion: Proximal stress shielding was greatest in the calcar, in agreement with clinical findings. The thicker cement mantle led to less stress shielding in this region. Cement stresses, highest around the distal tip of the prosthesis, were larger in the thin cement mantle. This suggests a higher rate of both cracking and bone resorption with thin cement mantles, particularly in patients with low bone density.