A numerical study of failure mechanisms in the cemented resurfaced femur: effects of interface characteristics and bone remodelling.

Failure mechanisms of the resurfaced femoral head include femoral neck fracture in the short-term and stress shielding and implant loosening in the long-term. In this study, finite element simulations of the resurfaced femur considering a debonded implant-cement interface, variable stem-bone interface conditions, and bone remodelling were used to study load transfer within the resurfaced femur and to investigate its relationship with known failure mechanisms. Realistic three-dimensional finite element models of an intact and resurfaced femur were used. Various conditions at the interface between the stem of the prosthesis and the bone were considered. Loading conditions included normal walking and stair climbing. For all stem-bone contact conditions, the tensile stresses in the cement mantle varied between 1 MPa and 5.4 MPa, except near the distal rim of the resurfacing component where they reached 5.4-7MPa. In the case of full stem-bone contact, high von Mises stresses (114-121MPa) were generated in the implant at the stem-cup junction. These stresses were considerably reduced (maximum von Mises stress, 76 MPa) where a gap was present at the stem-bone interface. Resurfacing led to strain shielding of the bone of the femoral head (20-75 per cent strain reductions) and periprosthetic bone resorption (50-80 per cent bone density reductions) for all interface stem-bone contact conditions. In the lateral femoral head and the proximal femoral shaft around the trochantric region, bone density reductions varied between 10 per cent and 50 per cent. Bone apposition was observed in the inferior-medial part of the femoral head and proximal femoral neck region. For full stem-bone contact, more load was transferred through the stem to the surrounding bone, exacerbating strain shielding. Although femoral hip resurfacing conserves bone stock at the primary operation, strain shielding and periprosthetic bone resorption might lead to eventual loosening over time. Post-operatively, the resurfacing procedure generated elevated strains (0.50-0.75 per cent strain) in the proximal femoral neck-component junction irrespective of the variation in interface conditions, indicating an initial risk of femoral neck fracture. Subsequent to bone remodelling, this strain concentration was considerably reduced (0.35-0.50 per cent strain), lowering the risk of neck fracture. In order to reduce the potential risk of neck fracture, patients should avoid activities which might induce high loading of the hip during the early post-operative period to allow the bone around the proximal femoral neck to remodel and heal.

The use of complementary non-destructive evaluation methods to evaluate the integrity of the cement-bone interface.

The integrity of the cement-bone interface is vital to the long-term stability of cemented hip arthroplasty. Most of the previous studies investigating the interface have been confined to the continuum level, neglecting the effects of microstructure. Microscopic damage at the interface may eventually lead to macroscopic loosening of the implant. However, as the strength of the interface depends on the interlock of the cement with bone and because the properties of cancellous bone depend on its microstructure, the study of the behaviour of the interface at the microstructural level may help to gain an understanding of the factors governing initiation of loosening. In this study, two complementary non-destructive methods, acoustic emission (AE) and computed tomography (CT), have been implemented to study the initiation and progression of damage of an analogue cement-bone interface sample under four-point bending. Early failure was detected, localized, and characterized using AE. CT images of the sample before and after loading were used to visualize damage in three dimensions. Damage initiated at the interface and was found to be related to stress-raising microstructural features in the cement. These were caused by irregularities in the geometry of the bone analogue and recesses and notches formed by the flow of cement.

Smooth surface micro finite element modelling of a cancellous bone analogue material.

Tetrahedral finite element meshes with smooth surfaces can be created from computed tomography scans of cancellous bone in order to evaluate its mechanical properties. Image processing before creation of the mesh can affect the accuracy of determined mechanical properties. For a cancellous bone analogue, threshold, mesh density and surface smoothing parameters used in mesh generation were varied and the mechanical properties predicted by the resulting meshes were compared to experimental results. This study has shown that threshold selection is vital for accurate determination of volume fraction and resulting mechanical properties.

Vibration-assisted bone-graft compaction in impaction bone grafting of the femur.

The complications of impaction bone grafting in revision hip replacement includes fracture of the femur and subsidence of the prosthesis. In this in vitro study we aimed to investigate whether the use of vibration, combined with a perforated tamp during the compaction of morsellised allograft would reduce peak loads and hoop strains in the femur as a surrogate marker of the risk of fracture and whether it would also improve graft compaction and prosthetic stability. We found that the peak loads and hoop strains transmitted to the femoral cortex during graft compaction and subsidence of the stem in subsequent mechanical testing were reduced. This innovative technique has the potential to reduce the risk of intra-operative fracture and to improve graft compaction and therefore prosthetic stability.

Cementing techniques in hip resurfacing.

The subject of the cementing technique in hip resurfacing has been poorly studied to date. The hip resurfacing prosthesis is unique in the family of cemented prostheses because the cement mantle is blind (hidden underneath the implant) and is radiographically obscured. This presents an immediate challenge to the surgeon at the time of surgery, but also has a longer-term implication in terms of lack of post-operative clinical observation. This should be compared with total hip replacement or total knee replacement where the cement mantle can at least be partially observed both intra- and post-operatively. With this in mind, the objective of this review is, firstly, to understand the cement mantles typically achieved in current clinical practice and, secondly, to identify those factors affecting the cement mantle and to consolidate them into an improved and reproducible cementing technique. The outcome of this work shows that the low-viscosity technique can commonly lead to excessive cement penetration in the proximal femoral head and an incompletely seated component, whereas a more consistent controlled cement mantle can be achieved with a high-viscosity cementing technique. Consequently, it is recommended that a high-viscosity technique should be used to minimize the build-up of excessive cement, to reduce the temperature created by the exothermic polymerization, and to help to ensure correct seating of the prosthesis. A combination of these factors is potentially critical to the clinical success of some articular surface replacement (ASR) procedures. It is important to note that we specifically studied the DePuy ASR system; therefore only the general principles (and not the specifics) of the cementing technique may apply to other resurfacing prostheses, because of differences in internal geometry, clearance, and surgical technique.

Effect of hip stem taper on cement stresses.

The aim of this study was to use finite element models to investigate the effect of the design of the taper of polished, collarless, total hip replacement femoral components on stresses in the cement mantle surrounding the component. A single-taper prosthesis, double-taper prosthesis, and triple-taper prosthesis were compared. Peak stresses and stress distributions in the cement mantle were found to be a function of taper design, although the differences between designs were minor. Using a probability of failure technique based on the initial cement stress distribution, a triple-taper prosthesis was predicted to cause less cement mantle damage (0.15% of the volume of the cement mantle failing after 20 million loading cycles) than a double-taper prosthesis (0.74%) or a single-taper prosthesis (1.50%). Further research is required to confirm this finding.

Effect of bone material properties on the initial stability of a cementless hip stem: a finite element study.

In previous finite element studies of cementless hip stems reported in the literature, the effect of bone quality on the initial micromotion and interface bone strain has been rarely reported. In this study, the effect of varying cortical and cancellous bone modulus on initial stem micromotion and interface bone strain was examined and the potential consequence of these changes on bone ingrowth and implant migration was reported. A finite element (FE) model of a total hip replacement (THR) was created and the Young's moduli of cortical and cancellous bone were systematically varied to study the relative effect of the quality of both types of bone on the initial stability of a cementless THR. It was found that the initial micromotion and interface bone strain in a THR was significantly affected by the overall stiffness of the femur. In other words, both the reduction of the modulus of cortical and cancellous bone caused an increase in the initial micromotion and interface bone strain. This suggests that for FE studies to be truly predictive, a range of bone quality must be examined to study the performance envelope of a particular stem and to allow comparison with clinical results.

In vivo measurement of acetabular cement pressurization using a simple new design of cement pressurizer.

Aseptic loosening of the acetabular component remains a limiting factor in the long-term success of total hip replacement. An instrumented pressurizer has been designed to allow the intraoperative measurement of acetabular cement pressurization, which is known to contribute to implant fixation. Average intraoperative cement pressures in 16 operations performed by 2 surgeons were 49 +/- 17 kPa (6.4 +/- 2.3 psi) and 47 +/- 17 kPa (6.2 +/- 2.2 psi), and peak pressures were 76 +/- 5 kPa (10.0 +/- 0.6 psi) and 93 +/- 15.kPa (12.2 +/- 1.9 psi), comparable to previous work in vitro. The pressurization required for optimal cement penetration into cleaned low-density cancellous bone is reported to be of the order of 35 to 50 kPa (4.6-6.6 psi) for 30 to 60 seconds, and the present data show that this is attainable in vivo using a simple device.

Malunion in the lower limb. A nomogram to predict the effects of osteotomy.

Nomograms derived from mathematical analysis indicate that the level of malunion is the most important determinant of changes in the moment arm of the knee, the plane of the ankle and alterations in limb length. Testing in five patients undergoing reconstruction showed a mean error of postoperative limb length of 2.2 mm (SD 0.8 mm), knee moment arm of 4.7 mm (SD 3.3 mm) and ankle angle of 2.6 degrees (SD 2.3 degrees). These nomograms provide the information required when assessing whether a particular degree of angulation may be accepted.

Proximal femoral osteotomy as the primary operation for young adults who have osteoarthrosis of the hip.

We report the results of proximal femoral osteotomy that was performed to treat osteoarthrosis in twenty-three consecutive young adults (twenty-five hips) who had a mean age of thirty-eight years (range, eighteen to fifty-three years). The mean duration of follow-up was seven years (range, two to twelve years). With conversion to a total hip replacement as the end point, the rate of survival at twelve years was 67 per cent (95 per cent confidence interval, 37 to 88 per cent). Four hips (16 per cent) were converted to a total hip replacement at a mean of eight years after the osteotomy. For the patients who did not have conversion to a total hip replacement, the mean score for pain, according to the system of Merle d'Aubigné and Postel as modified by Charnley, improved from 3.4 points preoperatively to 5.1 points postoperatively, the mean score for walking ability improved from 3.9 to 4.7 points, and the mean score for range of motion improved from 3.2 to 4.2 points. These results compare favorably with those following other forms of operative treatment of osteoarthrosis of the hip in young adults. In addition, the osteotomy does not preclude subsequent replacement arthroplasty if one is necessary.

An in vitro study of a new design of acetabular cement pressurizer.

Aseptic loosening of the acetabular component remains one of the limiting factors in the long-term success of total hip arthroplasty. Cement pressurization has been shown to improve fixation. A new pressurizer has been designed that seals around the rim of the acetabulum and covers the transverse ligament notch with a flap. The results of in vitro testing of this device are presented and compared with those of pressure generated by insertion of an acetabular cup. The pressurizer allowed sustained, uniform cement pressurization. Peak pressures with the new pressurizer were 180 kPa at both the iliac region of the rim and the pole of an instrumented model acetabulum, compared with 55 kPa at the rim and 120 kPa at the pole on cup insertion. Pressures were maintained in the range of 80-90 kPa. The flap was effective in preventing cement leakage from the notch, and pressures were higher than when the flap was absent. Cup insertion alone gave only transient pressurization, substantially less near the rim of the acetabulum than at the pole. Peripheral pressurization may be significant in producing secure local fixation at the rim of the acetabulum, in particular in the region of the ilium (Charnley zone 1), where radiolucencies are most commonly observed and where stresses in the implanted acetabulum are highest. Improved rim fixation may also play a role in preventing the ingress of wear debris.