Femoral stem wear in cemented total hip replacement

The great success of cemented total hip replacement to treat patients with endstage osteoarthritis and osteonecrosis has been well documented. However, its long-term survivorship has been compromised by progressive development of aseptic loosening, and few hip prostheses could survive beyond 25 years. Aseptic loosening is mainly attributed to bone resorption which is activated by an in-vivo macrophage response to particulate debris generated by wear of the hip prosthesis. Theoretically, wear can occur not only at the articulating head-cup interface but also at other load-bearing surfaces, such as the stem-cement interface. Recently, great progress has been made in reducing wear at the head-cup interface through the introduction of new materials and improved manufacture; consequently femoral stem wear is considered to be playing an increasingly significant role in the overall wear of cemented total hip replacement. In this review article, the clinical incidences of femoral stem wear are comprehensively introduced, and its significance is highlighted as a source of generation of wear debris and corrosion products. Additionally, the relationship between femoral stem surface finish and femoral stem wear is discussed and the primary attempts to reproduce femoral stem wear through in-vitro wear testing are summarized. Furthermore, the initiation and propagation processes of femoral stem wear are also proposed and a better understanding of the issue is considered to be essential to reduce femoral stem wear and to improve the functionality of cemented total hip replacement.     

Three-body wear of UHMWPE acetabular cups by PMMA particles against CoCr, alumina and zirconia heads in a hip joint simulator

Three-body abrasive wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by loose polymethyl methacrylate (PMMA) bone cement particles is an important mechanism responsible for elevated wear debris generation in total hip arthroplasty. The resistance of the femoral head material to third-body damage has been considered critical for the wear performance of the polyethylene component. This study examines the effect of loose bone cement particles on the wear rate of UHMWPE acetabular cups against both metal and ceramic counterfaces in a hip joint simulator. Against the CoCr head, the UHMWPE cup showed a strong dependence of wear rate on the concentration of the PMMA particles in the lubricant. At a concentration less than 5 g/l, the presence of the PMMA particles had no detrimental effect on the wear rate; higher concentrations of the PMMA particles greater than 5 g/l led to an accelerated wear of the acetabular cups. Mild scratching damage was observed on the CoCr heads after testing with all PMMA-containing lubricants. However, no increased UHMWPE wear rate was found against these damaged femoral heads in a fresh lubricant without PMMA particles, indicating that femoral head scratching was not a major cause for the elevated wear observed under the three-body abrasive conditions. Against both alumina and zirconia ceramic heads, the wear rate of the UHMWPE was independent of the concentration of the PMMA particles. It was observed that a significant portion of the CoCr heads was covered with loose patches of PMMA particles. The higher the concentration of the PMMA particles, the greater the area of the head covered with PMMA particles. The attachment of PMMA particles to the ceramic heads was much reduced compared to the CoCr heads. It is therefore concluded that ceramic femoral heads are effective against potential run-away wear of the UHMWPE acetabular cups when an excessive amount of loose PMMA particles are present in the lubricant.     

Scratch and wear performance of prosthetic femoral head components against crosslinked UHMWPE sockets

Total hip arthroplasty is a highly successful procedure where the hip joint is replaced by an artificial ball and socket joint. Bearing wear continues to be a contributing factor to implant failure. Prosthetic femoral heads roughen in vivo which leads to increased wear. Along with the introduction of improved polyethylene which reduces wear by up to 99%, improved femoral head materials have been introduced to improve resistance to abrasion. The abrasion resistance of two of these improved femoral heads was assessed in this study and compared to a cobalt chromium (CoCr) femoral head. The resulting wear performance against a polyethylene acetabular component was assessed. The bulk ceramic (zirconia toughened alumina) femoral head exhibited superior abrasion resistance compared to CoCr (97% reduction in damage) as well as reduced wear after abrasion (97% reduction in wear). The oxide coated zirconium niobium femoral head showed inferior abrasion resistance compared to CoCr (99% increase in damage) as well as increased wear after abrasion (161% increase in wear). Both femoral head surfaces utilize hard ceramic materials, however, the thin ceramic coating on top of a softer metallic substrate of the oxide coated bearing was unable to withstand aggressive abrasion.     

Histochemical evidence of the initial chondrogenesis and osteogenesis in the periosteum of a rib fractured model: implications of osteocyte involvement in periosteal chondrogenesis

We have examined cellular events at the early stages of periosteal chondrogenesis and osteogenesis induced by bone fracture, using a well-standardized rib fracture model of the mouse. The initial cellular event was recognized as considerable proliferation in the deeper layer referred to as the "cambium layer" of the periosteum, as evidenced by numerous proliferating cell nuclear antigen-positive cells. The periosteal cartilage and bone were then regenerated directly from the region of the most-differentiated cell, i.e., mature osteoblasts of the cambium layer both close to and distant from the fracture site. Therefore, periosteal osteoblasts appeared to have the potential to differentiate into chondrogenic and osteoblastic lineages. CD31-positive blood vessels were uniformly localized along the periosteum that was regenerating cartilage and bone, being therefore indicative of less influence on the initiation of osteochondrogenesis. In contrast, however, the regenerated periosteal cartilage or bone extended from the cortical bones included dead or living osteocytes, respectively. Empty lacunae and lacunae embedded with amorphous materials were found close to the regenerated cartilage, while intact osteocytes persisted adjacent to the regenerated bone. The embedded lacunae with amorphous materials would render the tissue fluid, nutrients, oxygen, and several secretory factors such as dentin matrix protein-1 impossible to be delivered to the periosteal osteoblasts that interconnect osteocytes via gap junctions. Our study thus provides two major clues on initial cellular events in response to bone fracture: the potentiality of periosteal osteoblastic differentiation into a chondrogenic lineage, and a putative involvement of osteocytes in periosteal cartilage and bone regeneration.     

Hip resurfacing arthroplasty: the evolution of contemporary designs

Metal-on-metal hip resurfacing is considered by many as the most significant recent development in hip arthroplasty. It preserves proximal femoral bone stock, optimizes stress transfer to the proximal femur, and offers inherent stability and optimal range of movement. The early results of hip resurfacing in the 1970s and 1980s were poor and the procedure was largely abandoned by the mid-1980s. The expectation that these prostheses would be easy to revise was not often fulfilled. The large diameter of the articulation combined with thin polyethylene cups or liners resulted in accelerated wear and the production of large volumes of biologically active particulate debris, leading to bone loss and implant loosening. Failure has been attributed to other factors, mainly avascular necrosis of the femoral head. However, this concern has not been confirmed by retrieval studies. The failure of early hip resurfacings was essentially a consequence of the use of inappropriate materials, poor implant design, inadequate instrumentation, and crude surgical technique. It was not an inherent problem with the procedure itself. The renaissance of metal-on-metal articulations for total hip arthroplasty enabled the introduction of new hip resurfacings and most of the major implant manufacturers have already introduced such systems. Early results are encouraging and complications commonly seen in the 1970s and 1980s, such as early implant loosening and femoral neck fracture, now appear to be rare. Whilst early results should be regarded with caution, modern metal-on-metal hip resurfacing potentially offers the ultimate bone preservation and restoration of function in appropriately selected young patients.     

Investigations on the wear behaviour of the temporary PMMA-based hip Spacer-G

Total hip replacement has become one of the most successful orthopaedic procedures. However, complications due to infections may give serious problems and have devastating consequences for the hip implant. The use of a temporary three-dimensional polymethylmethacrylate (PMMA) cement spacer may be an alternative to solve infections in hip implants, improving the lives of patients awaiting reimplantation. In order to evaluate their wear behaviour, five PMMA Spacer-G femoral heads were tested against five post-mortem pelves in a hip joint simulator with bovine calf serum as lubricant. The surface of the worn spacers was characterized by scanning electron microscopy (SEM) analysis; all the samples revealed a similar morphology, showing areas characterized by different degrees of wear. Particle debris was isolated from the lubricant and PMMA particles and bone fractions were quantified. The amount of debris was found to be higher than where no-temporary prostheses were used. However, this result is acceptable since wear debris is removed by lavage irrigation when the Spacer-G is explanted. On the basis of these data, it is considered that the use of the cement Spacer-G could be a promising approach to the treatment of complicated infections of the hip joint. Therefore, Spacer-G is worthy of further research.     

Shock machine for the mechanical behaviour of hip prostheses: a description of performance capabilities

The aim of this study was to describe the behaviour of a shock machine designed for testing hip prostheses. A microseparation between head and cup occurs inducing a shock of several times the body weight, leading to fracture of ceramic femoral components. Femoral heads and cups of diameter 32 mm manufactured from alumina were tested in dry and wet conditions. Implants were subjected to shocks with a load profile of 9 kN load at 2 Hz and various microseparations. Position is monitored and force is measured with two acquisition systems. The working range and the device capabilities were investigated. Only cups tested in dry conditions failed. Observations by scanning electron microscopy revealed intergranular and transgranular fractures. Two wear stripes were observed on the heads. Three‐dimensional roughness of wear stripes was measured. Since experimental results are in good agreement with retrieved femoral heads, the shocks machine reproduces the in vivo degradations. Copyright © 2011 John Wiley & Sons, Ltd.     

Properties of composite specimens of old and new bone cement

The most common cause of failure of a total hip replacement is aseptic loosening of an implant. In a number of cases, the cement-bone interface of at least one component is not compromised. In cases of aseptic cup loosening, removal of a well-fixed femoral stem may be undertaken to facilitate exposure of the acetabulum for cup revision, and the surgeon may choose to leave the functional cement-bone interfaces in the femur undisturbed. After cup revision, new cement is pressurized within the old cement mantle and a stem is cemented into this 'old-new cement' composite. Retaining the old cement mantle is an attractive option as it reduces the duration of surgery, minimizes bleeding, and preserves the bone stock. Excellent results have been shown with this technique of 'in-cement femoral revision' using a double-tapered polished stem. While considerable literature is available on the short- and long-term properties of PMMA bone cement, very little is known about the mechanical properties of old-new composite cement specimens where the old cement is more than a few days old. This paper tests the properties of such old-new composite specimens where the 'old' cement is aged between 3.3 and 17.7 years, better reflecting clinical situations.     

A new method for three-dimensional skeleton graph analysis of porous media: application to trabecular bone microarchitecture

This paper introduces a new three-dimensional analysis of complex disordered porous media. Skeleton graph analysis is described and applied to trabecular bone images obtained by high resolution magnetic resonance imaging. This technique was developed bearing in mind topological considerations. The correspondence between vertices and branches of the skeleton graph and trabeculae is used in order to get local information on trabecular bone microarchitecture. In addition to real topological parameters, local structural information about trabeculae, such as length and volume distributions, are obtained. This method is applied to two sets of samples: six osteoporosis and six osteoarthritis bone samples. We demonstrate that skeleton graph analysis is a powerful technique to describe trabecular bone microarchitecture.     

The stability of the femoral component of a minimal invasive total hip replacement system

In this study, the initial stability of the femoral component of a minimal invasive total hip replacement was biomechanically evaluated during simulated normal walking and chair rising. A 20 mm diameter canal was created in the femoral necks of five fresh frozen human cadaver bones and the femoral heads were resected at the smallest cross-sectional area of the neck. The relatively short, polished, taper-shaped prostheses were cemented centrally in this canal according to a standardized procedure. A servohydraulic testing machine was used to apply dynamic loads to the prosthetic head. Radiostereophotogrammetric analysis was used to measure rotations and translations between the prosthesis and bone. In addition, the reconstructions were loaded until failure in a static, displacement-controlled test. During the dynamic experiments, the femoral necks did not fail and no macroscopical damage was detected. Maximal values were found for normal walking with a mean rotation of about 0.2 degrees and a mean translation of about 120 microm. These motions stabilized during testing. The mean static failure load was 4714 N. The results obtained in this study are promising and warrant further development of this type of minimal invasive hip prosthesis.     

Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model

Total hip arthroplasty (THA) enjoys excellent rates of success in older patients, but younger patients are still at risk of aseptic loosening and bone resorption from stress shielding. One solution to the stress shielding problem is to use a hip stem with mechanical properties matching those of cortical bone. The objective of the present study was to investigate numerically the biomechanical performance of such a biomimetic hip stem based on a hydroxyapatite (HA)-coated carbon fibre composite. A finite element model (FEM) of the biomimetic stem was constructed. Contact elements were studied to model the bone-implant interface in a non-osseointegrated and osseointegrated state in the best way. Three static load cases representing slow walking, stair climbing, and gait in a healthy individual were considered. Stress shielding and bone-implant interface micromotions were evaluated and compared with the results of a similar FEM based on titanium alloy (Ti-6Al-4V). The composite stems allowed for reduced stress shielding when compared with a traditional Ti-6Al-4V stem. Micromotions were slightly higher with the composite stem, but remained below 40 microm on most of the HA-coated surface. It is concluded that a biomimetic composite stem might offer a better compromise between stress shielding and micromotions than the Ti-6Al-4V stem with the same external geometry.     

On the tribological behavior of retrieved hip femoral heads affected by metallic debris. A comparative investigation by stylus and optical profilometer for a new roughness measurement protocol

In this study, 35 retrieved ceramics femoral heads with evidence of metallic dark lines on the surface were analyzed. The surface roughness of these femoral heads was acquired using both a stylus contact profiler and an optical non-contact profilometer. The metallic deposition on hip ball surface, also known as Metal Transfer (MT), appeared as dark metallic and can occur during the surgery or during the reduction of a dislocated prosthesis. In this study, we validate a new protocol to measure surface roughness on retrieved femoral heads, by using the aforementioned two different acquisition techniques and also to investigate the hypothesis that such metal transfer retrieved ceramic femoral head is associated with increased surface roughness.All femoral components of this investigation showed remarkable differences in roughness values between MT-affected and MT-unaffected areas. The two acquisition procedures – conventional stylus and 3D confocal profilometer – confirmed a satisfying agreement, even considering the obvious resolution difference.     

Wear and creep of highly crosslinked polyethylene against cobalt chrome and ceramic femoral heads

The wear and creep characteristics of highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against large-diameter (36mm) ceramic and cobalt chrome femoral heads have been investigated in a physiological anatomical hip joint simulator for 10 million cycles. The crosslinked UHMWPE/ceramic combination showed higher volume deformation due to creep plus wear during the first 2 million cycles, and a steady-state wear rate 40 per cent lower than that of the crosslinked UHMWPE/cobalt chrome combination. Wear particles were isolated and characterized from the hip simulator lubricants. The wear particles were similar in size and morphology for both head materials. The particle isolation methodology used could not detect a statistically significant difference between the particles produced by the cobalt chrome and alumina ceramic femoral heads.     

A comparison between gravimetric and volumetric techniques of wear measurement of UHMWPE acetabular cups against zirconia and cobalt-chromium-molybdenum femoral heads in a hip simulator

Five cobalt-chromium-molybdenum (CoCrMo) and five zirconia femoral head components have been wear tested against 28 mm diameter ultra high molecular weight polyethylene (UHMWPE) acetabular cups for 5 million cycles in the Durham hip joint wear simulator using bovine serum as a lubricant. Wear measurements used gravimetric and volumetric techniques and no statistically significant difference was found between the measurement methods. The wear rates of the acetabular cups against both femoral heads are presented for both measurement methods. The UHMWPE acetabular cups showed a statistically significant higher linear wear rate for the first 2 million cycles than the lower linear wear rate from 2 million cycles to the end of the test, against both femoral head materials. Over the full duration of the wear test, the wear rates of acetabular cups articulating against zirconia femoral heads were lower than against CoCrMo femoral heads. The wear rates up to 2 million cycles and from 2 to 5 million cycles for both femoral head materials were consistent with other studies.     

Biotribocorrosion: Some electrochemical observations from an instrumented hip joint simulator

36 mm Metal-on-Metal heads and cups were tested in a hip joint simulator instrumented with a 3-electrode electrochemical cell. This facilitated in situ monitoring of the tribological depassivation of the bearing surface. Subsurface analysis of the different regions of the femoral head was analysed using transmission electron microscopy. It was found that tribological depassivation increased the rate of corrosion, but subsequently decreased at around 450,000 cycles. Observations of the subsurface microstructure revealed the formation of nano-crystallites and the presence of micro-scale subsurface cracks.     

The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel

Studies of explanted hip prostheses have shown high wear rates of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups and roughening of the surface of the metallic femoral head. Bone and bone cement particles have also been found in the articulating surfaces of some joints. It has been proposed that bone or bone cement particles may cause scratching and deterioration in the surface finish of metallic femoral heads, thus producing increased wear rates and excessive amounts of wear debris. Sliding wear tests of UHMWPE pins on stainless steel have been performed with particles of different types of bone and bone cement added. Damage to the stainless steel counterface and the motion of particles through the interface have been studied. Particles of bone cement with zirconium and barium sulphate additives and particles of cortical bone scratched the stainless steel counterface. The cement particles with zirconium additive produced significantly greater surface damage. The number of particles entering the contact and embedding in the UHMWPE pin was dependent on particle size and geometry, surface roughness and contact stress. Particles are likely to cause surface roughening and increased wear rates in artificial joints.     

Hip simulator wear testing according to the newly introduced standard ISO 14242.

Ultra-high molecular weight polyethylene (UHMWPE) acetabular cups were tested against alumina-ceramic femoral heads using a new type of hip joint simulator according to ISO/FDIS 14242-1. Bovine serum as well as newborn calf serum were used as test fluids. Total polyethylene wear was determined by weight loss of the cups. In addition. wear depth and its distribution were recorded by means of a coordinate measurement system. Wear particle analysis and inspection of the worn polyethylene surfaces using light and scanning electron microscopy (SEM) were performed to analyse damage and identify the acting wear mechanisms. The total wear rate was determined to be 22.07 +/- 1.75 mg/10(6) cycles for the bovine serum group and 26.57 + 3.55 mg/10(6) cycles for the calf serum group. Unexpectedly, the formation of two wear vectors corresponding to recent clinical findings was detected. Retrieved polyethylene wear debris was comparable in size and shape with clinical findings. The test method described by ISO/FDIS 14242-1 produced reliable and reproducible wear data using UHMWPE acetabular cups articulating against alumina-ceramic heads. In the authors' opinion, the lubricant composition should be described in more detail, since the protein and additive content seem to have a high impact on the wear results. It needs to be emphasized that the findings of this study cannot be regarded as a general validation of hip wear tests according to ISO/FDIS 14242-1 but are limited to the material combinations investigated herein. Further testing of other clinically relevant materials and interlaboratory ring tests must follow.     

Comparison of wear,wear debris and functional biological activity of moderately crosslinked and non-crosslinked polyethylenes in hip prostheses

The wear, wear debris and functional biological activity of non-crosslinked and moderately crosslinked ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups have been com pared when articulating against smooth and intentionally scratched femoral heads. Volumetric wear rates were determined in a hip joint simulator and the debris was isolated from the lubricant and characterized by the percentage number and volumetric concentration as a function of particle size. The volumetric concentration was integrated with the biological activity function determined from in vitro cell culture studies to predict an index of specific biological activity (SBA). The product of specific biological activity and volumetric wear rate was used to determine the index of functional biological activity (FBA). On smooth femoral heads the crosslinked UHMWPE had a 30 per cent lower wear rate, but it had a greater percentage volume of smaller, more biologically active particles, which resulted in a similar index of FBA compared with the non-crosslinked material. On the scratched femoral heads the volumetric wear rate was three times higher for the moderately crosslinked UHMWPE and two times higher for the non-crosslinked UHMWPE compared with the smooth femoral heads. This resulted in a higher wear rate for the moderately crosslinked material on the scratched femoral heads. All the differences in wear rate were statistically significant. There were only small differences in particle volume concentration distributions, and this resulted in similar indices of FBA which were approximately twice the values of those found on the smooth femoral heads. Both materials showed lower wear and FBA than for previously studied aged and oxidized UHMWPE gamma irradiated in air. However, this study did not reveal any advantage in terms of predicted FBA for moderately crosslinked UHMWPE compared with non-crosslinked UHMWPE.     

An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Although the wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/10(6) cycles) is much lower than the more widely used polyethylene-on-metal bearings, there are concerns about the toxicity of metal wear particles and elevated metal ion levels, both locally and systemically, in the human body. The aim of this study was to investigate the possibility of reducing the volume of wear, the concentration of metal debris and the level of metal ion release through using surface-engineered femoral heads. Three thick (8-12 microm) coatings (TiN, CrN and CrCN) and one thin (2 microm) coating (diamond-like carbon, DLC), were evaluated on the femoral heads when articulating against high carbon content cobalt-chromium alloy acetabular inserts (HC CoCrMo) and compared with a clinically used MOM cobalt-chromium alloy bearing couple using a physiological anatomical hip joint simulator (Leeds Mark II). This study showed that CrN, CrCN and DLC coatings produced substantially lower wear volumes for both the coated femoral heads and the HC CoCrMo inserts. The TiN coating itself had little wear, but it caused relatively high wear of the HC CoCrMo inserts compared with the other coatings. The majority of the wear debris for all half-coated couples comprised small, 30 nm or less, CoCrMo metal particles. The Co, Cr and Mo ion concentrations released from the bearing couples of CrN-, CrCN- and DLC-coated heads articulating against HC CoCrMo inserts were at least 7 times lower than those released from the clinical MOM prostheses. These surface-engineered femoral heads articulating on HC CoCrMo acetabular inserts produced significantly lower wear volumes and rates, and hence lower volumetric concentrations of wear particles, compared with the clinical MOM prosthesis. The substantially lower ion concentration released by these surface-engineered components provides important evidence to support the clinical application of this technology.     

The effect of acetabular cup size on the short-term stability of revision hip arthroplasty: a finite element investigation

The study uses idealized two-dimensional finite element models to examine the behaviour of the acetabular construct following revision hip arthroplasty, carried out using the Slooff-Ling impaction grafting technique. The behaviour of bone graft was considered in detail, with non-linear elasticity and non-associated plasticity being adopted. Load was applied to the acetabular construct through a femoral head using smooth sliding surfaces. In particular, four models were subjected to two idealized cyclic load cases to investigate the effect of acetabular cup size on the short-term stability of the acetabular construct. The study suggests that benefits may be gained by using the largest practical size of acetabular cup.