Background

Established hip and knee arthroplasty registers exist in many countries but this is not the case with spinal implants. Moreover, in the case of a rod intended to guide spinal growth in a child and then be removed, the definition of ‘failure’ (revision) used for hip or knee arthroplasty is inappropriate. How can the performance of such spinal implants be judged?

Objectives

We have encountered patients who developed large joint fluid collections with massive elevations in chromium (Cr) and cobalt (Co) concentrations following metal-on-metal (MoM) hip arthroplasties. In some cases, retrieval analysis determined that these ion concentrations could not be explained simply by the wear rates of the components. We hypothesized that these effects may be associated with aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL).

Objectives

The high revision rates of the DePuy Articular Surface Replacement (ASR) and the DePuy ASR XL (the total hip arthroplasty (THA) version) have led to questions over the viability of metal-on-metal (MoM) hip joints. Some designs of MoM hip joint do, however, have reasonable mid-term performance when implanted in appropriate patients. Investigations into the reasons for implant failure are important to offer help with the choice of implants and direction for future implant designs. One way to assess the performance of explanted hip prostheses is to measure the wear (in terms of material loss) on the joint surfaces.

INTRODUCTION

Deformation of modular acetabular press-fit shells is a topic of much interest for surgeons and manufacturer. Such modular components utilise a titanium shell with a liner manufactured from metal, polyethylene or ceramic. Initial fixation is achieved through a press-fit between shell and acetabulum with the shell mechanically deforming upon insertion. Shell deformation may disrupt the assembly process of inserting the bearing liner into the acetabular shell for modular systems. This may adversely affect the integrity and durability of the components and the tribology of the bearing.

Objectives

Wear debris released from bearing surfaces has been shown to provoke negative immune responses in the recipient. Excessive wear has been linked to early failure of prostheses. Analysis using coordinate measuring machines (CMMs) can provide estimates of total volumetric material loss of explanted prostheses and can help to understand device failure. The accuracy of volumetric testing has been debated, with some investigators stating that only protocols involving hundreds of thousands of measurement points are sufficient. We looked to examine this assumption and to apply the findings to the clinical arena.

Objectives

The aims of this piece of work were to: 1) record the background concentrations of blood chromium (Cr) and cobalt (Co) concentrations in a large group of subjects; 2) to compare blood/serum Cr and Co concentrations with retrieved metal-on-metal (MoM) hip resurfacings; 3) to examine the distribution of Co and Cr in the serum and whole blood of patients with MoM hip arthroplasties; and 4) to further understand the partitioning of metal ions between the serum and whole blood fractions.

Objectives

An ongoing prospective study to investigate failing metal-on-metal hip prostheses was commenced at our centre in 2008. We report on the results of the analysis of the first consecutive 126 failed mated total hip prostheses from a single manufacturer.

Introduction: One of the latest groups of replacement hip joints are known as hip resurfacings and they consist of a relatively large diameter femoral head articulating within a thin acetabular cup. Many of these devices show good short to medium term clinical results. However there are concerns over such implants including fracture of the femur and possible wear debris related reactions. Much valuable data can be learnt from explanted prostheses which have ‘failed’ and then been removed from patients. As hip resurfacing prostheses have only recently been introduced, there are relatively few such retrieval studies.

Methods and materials: Nineteen femoral and acetabular components from metal-on-metal hip resurfacing prostheses were obtained at revision operations. There were eight patients who had femoral fractures and the remainder experienced worsening groin pain and a characteristic sterile effusion. There were eleven head components and four pairs of matching heads and cups. Each of these was examined using a Zeiss TSK Rond-com60A roundness measuring machine and a Mitutoyo LEGEX co-ordinate measuring machine (CMM). Out of roundness measurements were taken on three planes for each acetabular and femoral component. The CMM was used to obtain 12 traces at 30° intervals for each acetabular and femoral component, allowing areas of localised wear to be identified and the maximum wear depth to be quantified.

Results: The maximum out of roundness values for the nineteen components ranged from 1.8 to 91.8 microns. A similar range of values was obtained from the CMM results. From the paired components, out of roundness was greater in the head than in the cup. All eight femoral heads which were retrieved after fracture of the femur showed out of roundness of less than 5 microns.

Discussion: Both out of roundness measurements and those from the CMM provided information about the wear of the implants. For a new component, a typical out of roundness value would be no greater than 5 microns. Therefore, from the out of roundness values it was seen that the eight ‘fracture’ components showed minimal distortion or wear after removal. In contrast the remaining components, which had a minimum out of roundness of 15 microns, showed much greater wear, thus suggesting that the groin pain was associated with relatively large volumes of wear debris. CMM scans helped to identify localised areas of wear and maximum wear depths. Values in the range of < 2 microns to 164 microns have been reported previously and show good agreement with the findings of this study. Retrieved components which had been implanted at high angles of inclination and anteversion tended to show the greatest wear, implying that correct positioning in vivo is crucial to the longevity of hip resurfacing prostheses.

Introduction First metatarsophalangeal (MTP) arthroplasty is a relatively uncommon procedure compared with hip and knee joint replacement. A range of different designs of first MTP prostheses have been proposed including metal hemi-arthroplasties, single-piece double-stem silicone designs, and multi-component designs. Of the latter group, a cobalt chrome-on-cobalt chrome prosthesis, which had a diamond like carbon (DLC) coating applied to its articulating faces and hydroxyapatite-coated stems, was implanted. However, due to poor clinical results the cohort of implants were removed and one was obtained for ex vivo analysis. In addition, calculation of predicted lubrication regimes applicable to this implant design was undertaken.

Materials and Methods The ex vivo MTP implant was examined using standard microscopy as well as by using an environmental scanning electron microscope and a non-contacting profilometer. The latter device also allowed values of surface roughness to be determined while the radii of the articulating faces were measured using a co-ordinate measuring machine. Modelling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory [1] allowed the minimum film thickness to be calculated and in turn the lambda value to indicate the lubrication regime [2]. These calculations were undertaken for a 0 to 800N range of loading values, and a 0 to 50mm/s range of entraining velocities. The viscosity of the synovial fluid lubricant was taken to be 0.01Pa s, while for the cobalt chrome a Young’s modulus of 210GPa and a Poisson’s ratio of 0.3 were assumed.

Results and Discussion The implant was measured to have a nominal radius of 10mm and a radial clearance of 0.1mm. Calculations showed that, for the range of entraining velocities and loads considered, the implant would almost always operate in the boundary lubrication regime. Therefore surface to surface contact would most frequently take place, with little if any separation between the articulating surfaces. This result is in contrast to resurfacing designs of hip prosthesis which can operate in the fluid film lubrication mode [3]. This outcome is due to their larger radii, greater entraining velocity and reduced surface roughness values compared with the MTP implant considered here. It is felt that these design differences, inherent in different joints around the body, should be appreciated by those concerned with such implants. The presence of scratches on the articulating faces of the ex vivo sample further implied boundary lubrication. The DLC coating had been removed from the entire face of the phalangeal component and from most of the face of the metatarsal component. From the latter it appeared as if the coating had been scratched and then flaked away parallel to the scratches. In turn this suggested a corrosion based failure of the interface between the DLC coating and the cobalt chrome subsurface, a result noted recently elsewhere [4].

Introduction The clinical use of an all-polymer knee which articulated a polyacetal femoral component against an ultra high molecular weight polyethylene (UHMWPE) tibial component has been reported [1]. A ‘polyacetal group’ of 63 total knee replacements were followed for at least ten years and no instances of femoral component fracture or failure due to wear occurred [1]. Such results are remarkable for an all-polymer pros-thesis in such a heavily loaded joint as the knee. Recently a wear screening device has been described which reproduced in vitro the clinical wear rates reported for three biopolymers which have been employed as the acetabular cup material in hip prostheses [2]. Given this validated rig, the objective of the work reported here was to undertake wear tests of polyacetal against UHMWPE.

Materials and Methods The polyacetal and UHMWPE couples were tested using a modified, four-station, pin-on-plate wear test rig [2]. The modification entailed the addition of rotational motion to the test pins, in addition to the standard reciprocating motion, to give multi-directional motion. In the wear tests, two stations had reciprocation-only and two applied multi-directional motion. Investigating the influence of both types of motion permitted a fuller tribological analysis to be undertaken. Control pins and control plates were included to account for any weight change due to lubricant uptake. A load of 40N was employed and reciprocating and rotating speeds of 1Hz were chosen. The lubricant consisted of 25% bovine calf serum and 75% distilled water, which was heated to 37°C during testing. A standardised cleaning and weighing protocol was followed, and the pins and plates were weighed on a balance sensitive to 0.1mg.

Results and Discussion After an average of 1.4 million cycles of sliding, the mean wear factors were: UHMWPE pins rubbing against polyacetal plates, 1.5 x 10-6mm3/ Nm under reciprocation, and 4.1 x 10-6mm3/Nm under multi-directional motion. For polyacetal pins rubbing against UHMWPE plates they were 0.7 x 10-6mm3/ Nm under reciprocation, and 2.8 x 10-6mm3/Nm under multi-directional motion. As can be seen, the wear factors depended on both the orientation of the material, whether it was a pin or a plate, and the motion it was subjected to. The increase in weight of the polyacetal control components due to lubricant uptake was many times that of the UHMWPE components. For example the UHMWPE control plate showed an increase of 0.2mg compared with 33.4mg for the polyacetal control plate. Using the same wear screening rig, the wear factors for UHMWPE articulating against stainless steel were measured to be 0.1 x10-6mm3/Nm under reciprocating motion and 1.1 x10-6mm3/Nm under multi-directional motion [2]. Though greater than this latter value, the all-polymer wear factors were not excessively high and were less under reciprocation-only. How much multi-directional motion, or cross-shear, it is appropriate to apply to a wear simulation of an artificial knee joint is worth further investigation, as it may be much less than in the hip joint.

Introduction Finger prostheses lack the long-term clinical success associated with hip and knee replacements. The most commonly implanted type of finger prostheses consists of single-piece silicone designs such as the Swanson, the Sutter and the NeuFlex [1]. Such designs act as flexible spacers around which a process of encapsulation can occur. A recent long-term study stated that, at an average of 14 years after surgery, Swanson meta-carpophalangeal (MCP) prostheses showed a fracture rate of 67% compared with 52% for Sutter MCP prostheses [2]. A 2005 paper reported that, at 2 years follow-up, the fracture rates were 13% and 20% respectively for these two designs [3]. Perhaps such high rates could be reduced if a better understanding of the nature of fracture of these implants was attained.

Materials and Methods Twelve Sutter MCP prostheses were obtained from three hands (two dominant) of two women and one man who were aged 56–66 years at time of surgery [4]. They were retrieved at a mean of 42 (range 32–53) months following implantation. All patients had rheumatoid arthritis. Of the twelve explanted prostheses, eleven had fractured, ten completely. These fractured prostheses were visually examined and were then sliced so that, after washing and gold-coating, the two fracture faces of each prosthesis could be examined using a Hitachi S-4700 scanning electron microscope (SEM).

Results and Discussion All of the ten total fractures occurred at the junction of the distal stem and the hinge of the implant. Visual inspection showed that the initial point of fracture was on the dorsal aspect of the prosthesis, indicating that fracture is due to the subluxing forces seen in rheumatoid MCP joints. Also, the fracture began distally and travelled in a slightly proximal direction as well as in the dominant dorsal to palmar direction. For the prostheses removed from a right hand, it appeared that the crack direction was also from ulnar to radial. When all of the fracture faces were examined by SEM, significant variation was seen. Some fracture faces appeared to show surface gouging of the material, which may have been caused by bone after fracture had taken place, therefore indicating that fracture had occurred long before the prostheses were removed. In contrast another fracture face showed what appeared to be a region of gradual abrasion, perhaps caused by osteophytes, next to a relatively smooth zone which could have indicated an area of rapid fracture or tearing. The author is not aware of any similar topographical analysis having been undertaken elsewhere on fractured, ex-vivo silicone MCP prostheses. While the time span between fracture and removal of the implant can never be known precisely, so that the ‘virgin’ fracture face could have been damaged post-fracture, it is still hoped that such ex-vivo analysis can contribute to improved finger prostheses.