The process of femoral impaction grafting requires vigorous impaction to obtain adequate stability but the force of impaction has not been determined. This process has been reported to result in femoral fractures with rates reaching 16%. The aims of this study were to determine the threshold force required for femoral impaction grafting, to determine the affect cortical thickness, canal diameter and bone mineral density (BMD) have on this threshold force and to measure subsidence of an Exeter prosthesis following impaction at the threshold force.

Adult sow femurs were prepared and placed through a DEXA scanner and the BMD and canal diameter measured. Thirty five femurs were impacted with morsellised bone chips and an increasing force of 0.5kN was applied until the femur fractured. Using callipers the cortical thickness of the bone was measured along the fracture line. Once the threshold force was determined 5 femurs were impacted to this threshold force and an Exeter stem was cemented into the neomedullary canal and a 28mm Exeter head attached. Axial cyclic loading was performed between 440N (swing phase of gait) and 1320N (stance phase of gait) for 150,000 cycles at a frequency of 3Hz. The position sensor of the hydraulic testing machine measured the subsidence.

29 tests were successfully completed. The threshold force was found to be 4kN. There was no significant correlation between the load at fracture and the cortex: canal ratio or the bone mineral density. Following impaction with the maximum force of 4kN the average subsidence for the 5 femurs was 0.276mm (range 0.235 – 0.325mm). In this animal study the threshold force was 4kN. Minimal axial subsidence of the implant occurred when impacting the graft with this threshold force. We therefore achieved a stable construct without fracture which is the ultimate goal for the revision hip surgeon.

The main mode of failure of the acetabular component in total hip arthroplasty is aseptic loosening. Successive generations of cementation techniques have evolved to alleviate this problem.

This paper evaluates one such method, Negative Pressure Intrusion cementation. Two groups of machined bovine cancellous bone samples were created; experimental (n = 26) and control (n = 26). The experimental group was cemented using the negative pressure technique and control group was cemented in the absence of negative pressure. The relative cement intrusion depths were then assessed for each group using MicroCT. These samples were then further machined and tested to failure in torsion to estimate their mechanical properties.

Results show mean cement intrusion depth for the negative pressure group to be 8676μm and 6042 μm for the control group (p = 0.078). Mechanical testing also revealed a greater mean torque in the negative pressure group (1.6223Nm vs 1.2063Nm) (p = 0.095).

This work quantifies the effect of negative intra-osseous pressure on cement intrusion depth in cancellous bone and for the first time relates this to increased mechanical strength.

The cement-in-cement femoral revision is a possible method of reducing complications. During recent research on this revision it was observed that a number of the inner cement contained macropores. It was hypothesized that porosity of the mantle influenced the subsidence and inducible displacement of the revision stems. The aim was to calculate the porosity and assess its relationship to the above factors.

Primary cement mantles were formed by cementing a stem into sections of tubular steel. At this stage, the specimen was chosen to be in a test or a control group. If in the test group, it underwent a fatigue of 1 million cycles. This was carried out in a fatigue machine mounted with a specifically designed rig. If in the control group, no such fatigue was undertaken. Into these fatigued and unfatigued mantles, the cement-in-cement procedure was performed. Both groups underwent a fatigue of again 1 million cycles. Subsidence and inducible displacement was recorded. The composites were then sectioned and photographed. The images underwent image analysis to calculate the porosity.

Multiple regression and a general linear model showed subsidence was inversely correlated to the porosity of the “fresh cement” in Gruen zones 3 and 5 (p = 0.021, R2 = 0.36). This relationship was not expected. The reason could be related to the fact that the migration of the stems in each separate direction was not monitored. Inducible displacement was inversely correlated to porosity of the inner cement, again in Gruen zones 3 and 5 (p = 0.001, R2 = 0.61). A possible explanation is that the stem was able to subside more due to the higher porosity and find a more stable position.

The subsidence and inducible displacement of these stems is influenced by porosity, specifically by the porosity of the distal inner cement.

One method of reducing intra-operative complications in revision hip surgery is the cement-in-cement technique. Some concern exists regarding the retention of the existing fatigued cement mantle. It was hypothesised that leaving the existing fatigued cement mantle does not degrade the mechanical properties of the cement in cement revision construct. The aim of this research was to test this hypothesis using in vitro fatigue testing of analogue cement in cement constructs.

Primary cement mantles were formed by cementing a large polished stem into sections of tubular stainless steel using polymethylmethacrylate with Gentamicin. At this stage, the specimen was chosen to be in the test group or the control group. If in the test group, it underwent a fatigue of 1 million cycles. This was carried out in a specifically designed rig and a fatigue testing machine. Into these fatigued and unfatigued primary mantles, the cement in cement procedure was carried out. Both groups underwent a fatigue of again 1 million cycles. Subsidence of the stems and their inducible displacement was recorded. A power calculation preceded testing.

Completion of a Mann Whitney test on the endpoints of the subsidence curves revealed that there is no statistical difference between the data sets (means 0.51, 0.46, n=10 + 10, p = 0.496). This data was also calculated for the inducible displacement. Again, there was no statistical difference in the separate groups for this parameter (means 0.38, 0.36, p = 0.96). This methodology produces a complex 3 dimensional reconstruction of the cement in cement revision which replicates the in vivo structure. This reconstruction has undergone fatigue testing. Neither of these two aspects has been produced for the study of cement in cement revision before.

A fatigued primary cement mantle does not appear to degrade the mechanical properties of the cement in cement revision construct

Computational modelling has the potential of becoming a useful tool in assessing revision risk on a patient-specific basis. However, there are many difficulties encountered in generating subject-specific computational models that have unknown influences on such predictions, e.g. accuracy of the anatomical geometry and material properties of the patient. This study compares the influence of these two patient-specific parameters on predictions of revision risk due to aseptic loosening.

First, X-rays from seventeen patients were processed using previously developed technique utilising rigid scaling of a generic femur to match selected dimensions from each patient’s post-operative X-ray and, then, the same set of 3D models was obtained by using an automated technique that generates 3D extra-cortical geometries from planar X-rays using a combination of 2D contour extraction and 3D warping of a generic model to match the extracted contour.

A cement and cement-metal interfacial damage accumulation algorithm developed previously was used. For each geometric set two types of simulations were performed. First, constant cortical and cancellous bone apparent Young’s moduli were assumed. A second set of simulations used age-dependent Young’s moduli for each bone type. Walking and stair-climbing activities were simulated. Resultant migration of the prostheses was used to indicate revision risk.

Factorial analysis has shown that the geometry has a larger influence on resultant migration magnitude for each case; however, unexpectedly, using more realistic geometry weakened the strength of predictions. This is most likely to be due ongoing mesh-induced contact problems.

Aseptic loosening can be considered as a combination of both mechanical and biological failure scenarios. This study investigated the influence of including bone remodelling in the simulation of aseptic loosening of cemented hip prostheses.

A combined strain and damage stimulated bone adaptation algorithm (Mulvihill et al., Proc. ESB Summer Workshop, p.114–115, 2007) was modified for use on an apparent tissue level. Constant rate resorption or deposition occurs if local strain falls outside a quiescent reference strain range. Furthermore, damage accumulates as a function of tensile stress. Resorption and simultaneous repair is activated above a critical damage level. Model parameters are related to specific surface area expressed as a function of apparent tissue density. Elastic modulus was also a function of accumulated damage. This algorithm was applied in conjunction with a bone cement and cement-metal interfacial damage accumulation algorithm to simulate aseptic loosening for a retrospective dataset of early revision and long-term-unrevised patients (Lennon et al. JOR, 779-88, 2007). One year of walking activity was simulated and resultant migrations of the prostheses were used to indicate revision risk.

The current implementation demonstrated increased migration for simulations with bone remodelling (p= 0.01). Variability was increased but mean predicted migration for early revisions was significantly higher than for the unrevised group (p= 0.03). Bulk bone remodelling was predicted primarily in the proximal regions. Interfacial bone remodelling demonstrated oscillation in damage at the interface due to alternate resorption-repair and deposition cycles. Interfacial bone density changes were more prominent in proximal regions but some models did show small amounts of resorption in more distal Gruen zones.

We conclude that bone remodelling has potential to predict more realistic migration patterns but further development and assessment is needed to identify the correct parameters for the bone adaptation algorithm.