All polyethylene tibial components (APT) for total knee joint replacement have been recently reintroduced due to their past success and cost savings with respect to knee designs with a metal backed tibial tray (MBT). However, isolated cases of collapse of the medial bone in APT designs have been observed by the authors prompting this investigation. The objective of this study was to investigate the stress/strain distribution within the cancellous bone for the APT and MBT systems, particularly looking at the effects of coverage of the tray over the proximal tibia in each design. A three-dimensional finite element model of the proximal tibia implanted with a tibia tray was generated. An elliptical cylindrical tibia tray with a peg was modeled as being perfectly bonded to a PMMA layer on the superior surface of the cancellous and cortical bone. Gap size between the edge of the tray and outer of the cancellous bone, was introduced in the medial direction. Load was applied on the superior surface of the tibial insert in the medial side. Two lift-off loading cases were used, a low load of 800N (1 body-weight) and a high load of 3200N (4 x BW), both on the medial side. Permanent plastic deformation and collapse was allowed only in the cancellous bone, while all other materials were modeled elastically. Under low load conditions within the elastic limit, introducing a gap between the tray and the cortical bone produced a stress/strain intensity in the cancellous bone beneath the edge of the tray. The strain in the cancellous bone within the APT design was generally 3 times greater than the MBT design, however, peak strain values were similar at the edge of the tray. Whilst the strain increased with the introduction of a gap the resulting strain was not sensitive to the gap size for both designs. Under high load conditions, permanent plastic deformation and bone collapse were observed in the cancellous bone at the edge of the tibial tray in both designs where a gap was introduced. The maximum strain in the cancellous bone was found to be more sensitive to the gap size for the APT design than the MBT design. This can be contributed to the difference in the load transfer through the cancellous bone in the two designs. The MBT design with the more rigid tibial tray transfered higher load through the outer cortical bone than the APT design. The less rigid APT design resulted in progressive collapse of the cancellous bone beneath the tray. Particularly significant was the volume of highly stressed cancellous bone which was 4 times greater in the APT design compared to the MBT design. The results suggest that coverage may be a more important parameter for the APT design than the MBT design. The APT design may, therefore, be more suited to patients with better bone quality.
Viscosupplementation is the current treatment modality for early stage arthritis and in some cases for delaying joint replacement procedures. Rheological properties similar to that of synovial fluid and high molecular weight have been recognized as the determining factors in hyaluronic acids (HA) therapeutic and analgesic value (
There is currently much interest in the wear of metal-on-metal THRs and potential concerns about elevated metal ion levels. Generally, wear of metal-on-metal THR’s has been low in simulator studies. Slightly higher and more variable wear has been found clinically. Variations in surgical approach, technique and fixation method may influence the level of force applied across the prosthesis during gait. It is hypothesised that increased joint tensioning may increase loading of THR’s during the swing-phase; leading to elevated wear and friction due to depleted fluid film lubrication. This study aimed to assess the effect of swing-phase load on the friction, lubrication and wear of metal-on-metal THR’s. Cobalt-chrome 28mm metal-on-metal THR’s were tested in a physiological hip simulator, loading was modified to provide; (1) ISO swing-phase load (280N, as per ISO 14242-1) and (2) low swing-phase load (<
100N). Friction testing was conducted using a pendulum friction simulator, with 280N and 100N swing-phase loads. Theoretical lubrication modelling was carried out using elastohydrodynamic lubrication theory. The overall mean volumetric wear rates was 10-times greater in THR’s tested with an ISO swing-phase load in comparison to THR’s tested with low swing-phase loads (0.58±0.49 compared to 0.06±0.039mm3/million cycles). The friction factors were 0.129 and 0.173 respectively under low and ISO swing-phase conditions. A decrease in the predicted lubricant film thickness when the swing-phase load was increased was observed; at the start of stance phase this was 0.12microns and 0.07microns under low and ISO swing-phase conditions respectively. The results demonstrate that the performance of metal-on-metal THR’s is highly dependent on swing-phase load conditions. It is postulated that fixation method and surgical technique can affect the swing-phase load. This study has demonstrated that over-tensioning of the tissues may also accelerate wear. These observations may explain some of the variations reported clinically.
Wear and wear debris induced osteolysis is recognised as a major cause of long term failure in hip prostheses. Historically ultra high molecular weight polyethylene acetabular cups produced micron and submicron wear particles which accumulated in peri prosthetic tissues, and stimulated macrophages to generate wear debris induced osteolysis. Acceleration of wear and osteolysis was caused in historical materials by oxidative degradation of the polyethylene following gamma irradiation in air, and by third body damage and scratching of metallic femoral heads. Current conventional ultra high molecular weight polyethylene cups are irradiated in an inert atmosphere to reduce oxidative degradation and are articulated against ceramic femoral heads to reduce third body wear. More recently modified highly cross linked polyethylene has been developed, and while these materials produce a four to five fold reduction in wear volume the wear particles have been found to be more reactive, resulting in only a two fold reduction in functional osteolytic potential. The question remains as to whether this performance is adequate for high demand patients, particularly if larger diameter femoral heads are to be used. Recent interest in improved function, stability and reducing dislocations has generated interest in using larger diameter heads and hard on hard bearings. Alumina ceramic on ceramic bearings have shown a one hundred fold decrease in wear compared to highly cross linked polyethylene materials, and cell culture studies have shown the wear particles to be more bio-compatible and less osteolytic potential. Metal on metal bearings also produced very low wear rates compared to polyethylene. The wear particles are very small, 10 to 50 nanometers in size, some concern remains about the systematic release of metallic ions. These are lubrication sensitive bearings, and they unlike polyethylene wear decreases as the head size increases due to improved lubrication. Size 36 mm metal bearings are now commonplace for total joint replacements with even larger head sizes being used for surface replacement solutions. The demand for increased function and improved stability is leading to increased use of hard on hard bearings with larger diameter heads.
Low positive swing phase load (<
100N) Positive swing phase load (300N, as per standard ISO 14242–1) Negative swing phase load, leading to microseparation and joint laxity. All tests were carried out in 25% (v/v) new-born bovine serum, with gravimetric wear measurements completed every million cycles.
Osteolysis and subsequent mechanical loosening often occurs in hip arthroplasties using polyethylene-on-ceramic (POC) bearings. This has prompted an ongoing search for alternative bearing surfaces. Ceramic-on-ceramic (COC) and metal-on-metal (MOM) prostheses are widely used, with good clinical results. Using hip simulator studies, we compared ceramic-on-metal (COM) and MOM prostheses. We found COM pairings had 100-fold lower wear rates than MOM. The wear particles from both articulations were oval to round in shape and in the nanometer size range, with the COM producing smaller particles than the MOM. In both pairings, particle size decreased as the bearings bedded in. The volumetric particle loads were far smaller with COM bearing-surfaces than in currently-used MOM prostheses. These findings have encouraged us to investigate the use of these novel bearing surfaces. Ethical approval has been obtained, and a prospective randomised clinical trial comparing POC, MOM, COC and COM bearing surfaces has started.
Ultra high molecular weight polyethylene (UHMWPE) wear debris induced osteolysis is a major cause of long term failure of total hip replacements. Particles in the 0.1–1.0_m size range are believed to have greater osteolytic potential than larger wear debris. Crosslinked polyethylenes have been shown to have improved wear resistance compared to non-crosslinked materials on smooth counterfaces, however wear debris from cross-linked UHMWPE has been shown to be smaller than that produced from non-crosslinked materials. The aim of this study was to compare the wear, wear debris and biological activity of non-crosslinked and crosslinked polyethylenes when worn against smooth and scratched counterfaces.
Reduction of ultra high molecular weight polyethylene (UHMWPE) surface wear in total knee replacements (TKR) may delay the onset of osteolysis and loosening of components. This study examined the wear of fixed bearing and rotating platform (RP) mobile bearing TKR with two different bearing materials. Testing was completed on a Leeds ProSim six-station knee simulator under ‘high’ kinematics [ The 1020 GVF fixed bearings exhibited a wear rate of 16.4 ± 4 mm3 per million cycles (MC). This was significantly greater (p <
0.05) than wear of the same bearing material in the rotating platform mobile bearing TKR (10.85 ± 2.39 mm3/MC). Similarly, when uncross-linked 1020 UHMWPE was introduced as the bearing material, a significant (p <
0.05) reduction in wear was observed between the fixed bearing (16 ± 7 mm3/MC) and the RP knee designs (5.85 ± 2.05 mm3/MC). The RP design decouples the motions between the femoral-insert and tray-insert articulating surfaces. This translates complex knee motions into more unidirectional motions at two interfaces, thus reducing wear under high kinematics compared with fixed bearing TKR. This significant reduction in wear was observed with uncross-linked and moderately cross-linked bearing materials. Design of TKR is an important factor that influences UHMWPE surface wear and may affect long-term success of knee replacements in highly active patients.
Following total hip replacement surgery, fluroscopy studies have shown that a mean separation of 2 mm can occur between CoCr femoral heads and UHMWPE acetabular cups during the swing phase of gait [ A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. The alumina ceramic heads and polyethylene cups were 28 mm in diameter and were tested for 5 million cycles in 25% new born calf serum at 1 Hertz. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, where the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation. The volumetric wear of the polyethylene inserts was approximately four times less under microseparation conditions (5.6 ± 5.3 mm3 per million cycles), in comparison to standard conditions (25.6 ± 5.3 mm3 per million cycles). Deformation of the cup rim was observed, but some of this was attributed to creep. It is postulated that this reduction in wear was due to the separation of the components in swing phase improving the entrapment of lubricant, hence reducing wear via a squeeze film lubrication mechanism. In conclusion, surgical procedures that produce a small and controlled amount of joint laxity and microseparation may lead to a reduction in wear of the polyethylene acetabular cups.
A physiological hip simulator was used, loads and motions were applied to approximate
Bone cement with an antibiotic additive is currently widely available, gentamicin being the most common type. However, the high resistance of such organisms as staphylococci to gentamicin has popularised the practice of adding additional antibiotic powders to the cement mix. This study aims to quantify the effects of adding 1g active of seven antibiotics on the viscoelastic properties of the cement from mixing to set time using a robust rheometer, developed at the University of Leeds. CMW1 Radiopaque cement was the base cement selected for its widespread familiarity. Viscosity and elasticity were recorded at two rates of shear until the cements set. Viscosity was found to decrease with shear rate, but the cements were found to have a significant elastic component that greatly increased with shear rate. This indicates that for maximum cement penetration, maintained pressure would be more effective than “hammering”. It should be noted that the effects described above are small compared to other theatre variables, especially temperature and humidity.
Traditional hip prostheses, which involve metal on poly-ethylene articulations, have shown good survivorship at ten years, but in the long term, wear debris induced osteolysis has been found to cause loosening and failure. Specifically, micron and submicron size polyethylene wear particles generated at the articulating surfaces enter the periprosthetic tissues, activate the macrophages causing adverse cellular reactions and bone resorption. Recent laboratory, retrieval and clinical studies have shown that oxidation of the traditional polyethylene irradiated in air, causes wear to increase by a factor of three following either storage on the shelf for five years or following implantation in vivo for 15 years. Furthermore, damage or scratching of metallic femoral heads has been shown to increase wear by a factor of two. In vitro cell culture studies with real polyethylene wear particles, have shown that the intensity of the adverse cellular reactions is critically dependent on the size of the polyethylene wear particle with the smallest particles 0.1 to 1 mm being the most active. A novel model has been developed to predict functional biological activity and osteolytic potential, by integrating wear rates, particle analysis and cell culture studies. Stabilised and crosslinked polyethylenes have been investigated and been found to reduce wear rates by a factor of three compared to oxidised and aged materials. A moderate level of crosslinking reduced wear from 50 to 35 mm3 per million cycles compared to non crosslinked materials. However, against scratched femoral heads, the wear rate of both stabilised and cross-linked polyethylene was elevated to levels where the functional biological activity remains a concern in the long term. Alternative bearing surfaces, metal on metal, and alumina ceramic on ceramic provide potential to substantially reduce wear. Metal on metal bearings have shown mean wear rates of 1.5 mm3/year in the hip joint simulator, with very small, 30 nm size particles. Alumina ceramic ceramic have also shown very low wear rates of approximately 1 mm3/year, even in the presence of microseparation and rim contact, with small 10 nm size wear particles and larger particles up to 1 mm in size caused by grain boundary fracture. The functional biological activity and osteolytic potential of the alumina ceramic couple is predicted to be at least ten times lower than crosslinked polyethylene. New ceramic materials (zirconia toughened alumina) have been shown to further reduce ceramic ceramic wear. Furthermore, novel differential hardness ceramic on metal bearings have shown even lower wear rates. The currently available hard on hard bearings and the recent further improvements of these bearing couples, indicate that osteolysis free lifetimes well beyond 20 years are now possible.
When considering all the subjects, no correlation was found between the response of their cells to polyethylene particles and LPS stimulation. However the cells of four subjects gave a much higher response to LPS than the rest and when these where excluded the correlation between the response to LPS and PE particles was significant with an R2 value of 0.9076.
TNFα
levels Particle stimulation
LPS stimulation
Control
0.043–0.059
0.097–0.208
Patients
0–1.1
0.03–17.693