The effect of surgical alignment and soft tissue conditions on the kinematics and wear of a fixed bearing total knee replacement

As life expectancy and activity levels of patients increase so does the demand on total knee replacements (TKRs). Abnormal mechanics and wear of TKRs can lead to implant loosening and revision. Component alignment after surgery varies due to the presurgical alignment, the accuracy of the surgical instrumentation and due to patient factors, such as the soft tissue balance. This study experimentally investigated the effect of variation in component alignment and the soft tissue conditions on the kinematics and wear of a fixed bearing TKR. DePuy Sigma fixed bearing TKRs with moderately cross-linked UHMWPE were used. Different alignment conditions were simulated in the coronal, sagittal and transverse planes in an ISO force-controlled simulation system. Three different soft tissue conditions were simulated using virtual springs to represent a stiff knee, a preserved PCL and a resected PCL. Four different alignment conditions were studied; ideal alignment, 4° tibial and femoral varus joint line, 14° rotational mismatch and 10° posterior tibial slope. The varus joint line alignment resulted in similar kinematics and lower wear rate compared to ideal alignment. The rotational mismatch alignment resulted in significantly higher tibial rotation and abduction-adduction as well as a significantly higher wear rate than ideal alignment. The posterior tibial slope alignment resulted in significantly higher wear than the ideal alignment and dislocated under the lower tension soft tissue conditions. Component alignment and the soft tissue conditions had a significant effect on the kinematics and wear of the TKR investigated in this study. The surgical alignment of the TKR is an important factor in the clinical outcome of the joint as factors such as increased tibial rotation can lead to anterior knee pain and instability and increased wear can lead to aseptic loosening and early failure resulting in revision

Quantification of the effect of cross-shear and applied nominal contact pressure on the wear of moderately cross-linked polyethylene

Polyethylene wear is a great concern in total joint replacement. It is now considered a major limiting factor to the long life of such prostheses. Cross-linking has been introduced to reduce the wear of ultra-high-molecular-weight polyethylene (UHMWPE). Computational models have been used extensively for wear prediction and optimization of artificial knee designs. However, in order to be independent and have general applicability and predictability, computational wear models should be based on inputs from independent experimentally determined wear parameters (wear factors or wear coefficients). The objective of this study was to investigate moderately cross-linked UHMWPE, using a multidirectional pin-on-plate wear test machine, under a wide range of applied nominal contact pressure (from 1 to 11 MPa) and under five different kinematic inputs, varying from a purely linear track to a maximum rotation of ±55°. A computational model, based on a direct simulation of the multidirectional pin-on-plate wear tester, was developed to quantify the degree of cross-shear (CS) of the polyethylene pins articulating against the metallic plates. The moderately cross-linked UHMWPE showed wear factors less than half of that reported in the literature for the conventional UHMWPE, under the same loading and kinematic inputs. In addition, under high applied nominal contact stress, the moderately cross-linked UHMWPE wear showed lower dependence on the degree of CS compared to that under low applied nominal contact stress. The calculated wear coefficients were found to be independent of the applied nominal contact stress, in contrast to the wear factors that were shown to be highly pressure dependent. This study provided independent wear data for inputs into computational models for moderately cross-linked polyethylene and supported the application of wear coefficient–based computational wear models

Comparison of the biomechanical tensile and compressive properties of decellularised and natural porcine meniscus

Meniscal repair is widely used as a treatment for meniscus injury. However, where meniscal damage has progressed such that repair is not possible, approaches for partial meniscus replacement are now being developed which have the potential to restore the functional role of the meniscus, in stabilising the knee joint, absorbing and distributing stress during loading, and prevent early degenerative joint disease. One attractive potential solution to the current lack of meniscal replacements is the use of decellularised natural biological scaffolds, derived from xenogeneic tissues, which are produced by treating the native tissue to remove the immunogenic cells. The current study investigated the effect of decellularisation on the biomechanical tensile and compressive (indentation and unconfined) properties of the porcine medial meniscus through an experimental-computational approach. The results showed that decellularised medial porcine meniscus maintained the tensile biomechanical properties of the native meniscus, but had lower tensile initial elastic modulus. In compression, decellularised medial porcine meniscus generally showed lower elastic modulus and higher permeability compared to that of the native meniscus. These changes in the biomechanical properties, which ranged from less than 1% to 40%, may be due to the reduction of glycosaminoglycans (GAG) content during the decellularisation process. The predicted biomechanical properties for the decellularised medial porcine meniscus were within the reported range for the human meniscus, making it an appropriate biological scaffold for consideration as a partial meniscus replacement

Representing the effect of variation in soft tissue constraints in experimental simulation of total knee replacements

As life expectancy and activity levels of patients increase so does the demand on total knee replacements (TKRs). Abnormal mechanics and wear of TKRs can lead to implant loosening and early failure. Polyethylene inserts of varying design and conformity have been introduced in the past decade to improve stability and patient's confidence in the replaced knee, particularly in cases where soft tissue support around the knee is sub optimal. This study experimentally investigated the effect of variation in the soft tissues on the kinematics and wear of a TKR on three different tibial insert designs. DePuy Sigma fixed bearing TKRs with moderately cross-linked UHMWPE and the ISO force control inputs were used. Different soft tissue constraints were simulated using virtual springs in an ISO force controlled simulation system. The spring gaps and stiffness' were varied and their effect on the output kinematics and wear rates assessed. The lower conformity inserts resulted in significantly higher displacements and more variation between the stations on the simulator. They were also more sensitive to changes in the soft tissue constraints than the high conformity insert. The wear rate for the high tension springs was significantly lower than for the lower tension springs tested. Tibial insert geometry and soft tissue constraints significantly affected kinematics and wear in these experimental simulations. Soft tissue constraints and the variability in patients are important considerations in the stratified design of TKRs and approach to patient selection

A comparison between electromechanical and pneumatic-controlled knee simulators for the investigation of wear of total knee replacements

More robust preclinical experimental wear simulation methods are required in order to simulate a wider range of activities, observed in different patient populations such as younger more active patients, as well as to fully meet and be capable of going well beyond the existing requirements of the relevant international standards. A new six-station electromechanically driven simulator (Simulation Solutions, UK) with five fully independently controlled axes of articulation for each station, capable of replicating deep knee bending as well as other adverse conditions, which can be operated in either force or displacement control with improved input kinematic following, has been developed to meet these requirements. This study investigated the wear of a fixed-bearing total knee replacement using this electromechanically driven fully independent knee simulator and compared it to previous data from a predominantly pneumatically controlled simulator in which each station was not fully independently controlled. In addition, the kinematic performance and the repeatability of the simulators have been investigated and compared to the international standard requirements. The wear rates from the electromechanical and pneumatic knee simulators were not significantly different, with wear rates of 2.6 ± 0.9 and 2.7 ± 0.9 mm3/million cycles (MC; mean ± 95% confidence interval, p = 0.99) and 5.4 ± 1.4 and 6.7 ± 1.5 mm3/MC (mean ± 95 confidence interval, p = 0.54) from the electromechanical and pneumatic simulators under intermediate levels (maximum 5 mm) and high levels (maximum 10 mm) of anterior–posterior displacements, respectively. However, the output kinematic profiles of the control system, which drive the motion of the simulator, followed the input kinematic profiles more closely on the electromechanical simulator than the pneumatic simulator. In addition, the electromechanical simulator was capable of following kinematic and loading input cycles within the tolerances of the international standard requirements (ISO 14243-3). The new-generation electromechanical knee simulator with fully independent control has the potential to be used for a much wider range of kinematic conditions, including high-flexion and other severe conditions, due to its improved capability and performance in comparison to the previously used pneumatic-controlled simulators

Influence of contact pressure, cross-shear and counterface material on the wear of PEEK and CFR-PEEK for orthopaedic applications

Total joint replacement is a successful surgical intervention for the treatment of the degeneration of many joints, particularly the hip and knee. As the demand for joint replacement grows, and the life expectancy of the population increases, the performance requirements of these implants also changes. New materials, to improve longevity and enhance performance have been explored including PEEK and CFR-PEEK. This study investigated whether CFR-PEEK and PEEK were appropriate materials for total joint replacement by examining wear performance in simple configuration studies articulating against cobalt chrome under a range of cross-shear and contact pressure conditions. Simple geometry pin on plate studies were conducted for one million cycles for each test condition, with the contact pressure and cross-shear conditions representing a range in which the material may need to operate in-vivo. The wear factor for PEEK was significantly higher than CFR-PEEK and conventional polyethylene under all test conditions. Both PEEK and CFR-PEEK wear were influenced by contact pressure, with the highest wear factors for both materials measured at the highest pressure conditions. PEEK appeared to have a cross-shear dependent wear response, but this was not observed for the CFR-PEEK material. This study has further characterised the wear performance of two materials that are gaining interest for total joint replacement. The wear performance of the PEEK material showed poorer wear performance compared to polyethylene when articulating with a metal counterface, but the performance of the CFR-PEEK material suggested it may provide a suitable alternative to polyethylene in some applications. The wear performance of CFRPEEK was poorer than polyethylene when it was used as the plate, when there was translation of the contact zone over the surface of the CFR-PEEK plate. This has implications for applications in low conforming contacts, such as lower conformity knee replacement

The influence of simulator input conditions on the wear of total knee replacements: an experimental and computational study

Advancements in knee replacement design, material and sterilisation processes have provided improved clinical results. However, surface wear of the polyethylene leading to osteolysis is still considered the longer-term risk factor. Experimental wear simulation is an established method for evaluating the wear performance of total joint replacements. The aim of this study was to investigate the influence of simulation input conditions, specifically input kinematic magnitudes, waveforms and directions of motion and position of the femoral centre of rotation, on the wear performance of a fixed-bearing total knee replacement through a combined experimental and computational approach. Studies were completed using conventional and moderately cross-linked polyethylene to determine whether the influence of these simulation input conditions varied with material. The position of the femoral centre of rotation and the input kinematics were shown to have a significant influence on the wear rates. Similar trends were shown for both the conventional and moderately cross-linked polyethylene materials, although lower wear rates were found for the moderately cross-linked polyethylene due to the higher level of cross-linking. The most important factor influencing the wear was the position of the relative contact point at the femoral component and tibial insert interface. This was dependent on the combination of input displacement magnitudes, waveforms, direction of motion and femoral centre of rotation. This study provides further evidence that in order to study variables such as design and material in total knee replacement, it is important to carefully control knee simulation conditions. This can be more effectively achieved through the use of displacement control simulation

The effect of insert conformity and material on total knee replacement wear

The mean average life is increasing; therefore, there is a need to increase the lifetime of the prostheses. To fulfil this requirement, new prosthetic designs and materials are being introduced. Two of the design parameters that may affect wear of total knee replacements, and hence the expected lifetime, are the insert conformity and material. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. The objective of the present study was to use a previously validated non-dimensional wear coefficient-based computational wear model to investigate the effect of insert conformity and material on the predicted wear in total knee replacements. Four different inserts (curved, lipped, partial flat and custom flat), with different conformity levels, were tested against the same femoral and under two different kinematic inputs (intermediate and high), with different levels of cross-shear. The insert bearing materials were either conventional or moderately cross-linked ultra-high molecular weight polyethylene (UHMWPE). Wear predictions were validated against the experimental data from Leeds knee simulation tests. The predicted wear rates for the curved insert (most conformed) were more than three times those for the flat insert (least conformed). In addition, the computationally predicted average volumetric wear rates for moderately cross-linked UHMWPE bearings were less than half of their corresponding conventional UHMWPE bearings. Moreover, the wear of the moderately cross-linked UHMWPE was shown to be less dependent on the degree of cross-shear, compared to conventional UHMWPE. These results along with supporting experimental studies provide insight into the design variables, which may reduce wear in knee replacements

Performance improvement of PV panel using water cooling technology under Egyptian conditions

This paper presented the effect of water cooling technique on the performance of PV panels under Egyptian condition. In this study three different positions of the cooling technique was studied (front side, backside and both sides simultaneously). The results of this study was compared to the conventional PV model without cooling to obtain the optimal technology of cooling system. The performance of the system was studied experimentally during the period 10.00AM: 2.00PM. The experimental results show that the effective power produced for the PV panel increased by 15.5% (effective 10.0%), 13.97% (effective 8.4%) and 17.4% (effective 11.9%) in case of front side, back side and both sides simultaneously cooling technique respectively in comparison with the non-cooled panel. Furthermore, it was also possible to decrease panel temperature from an average 55 °C (non-cooled PV panel) to 27 °C, 32 °C and 24 °C in the case of front side, backside and simultaneous front and backside PV panel cooling respectively

Understanding the differences in wear testing method standards for total knee replacement

Preclinical evaluation of the wear of total knee replacements (TKR) is usually undertaken using International Standards Organization (ISO) test methods. Two international standards for the preclinical wear simulation of TKRs have been developed; using either force or displacement control. In addition, based on previously published measured kinematics of healthy subjects, a gait cycle (displacement control) was also developed at the University of Leeds, which pre-dates the ISO displacement control standard. Furthermore, different test methods have adopted different approaches to defining the centres of rotation and polarity (direction of application) of motions. However, the effects of using these different control regimes and input conditions on the kinematics, contact mechanics, and wear of any one TKR have not been fully investigated previously. The current study investigated the kinematics, contact mechanics, and wear performance of a TKR when running under ISO force and displacement control test methods as well as the Leeds gait cycle inputs using experimental and computational simulation methods, with the aim of understanding the mechanical and tribological outcomes predicted by the different test method standard conditions. Three ISO wear testing standards were investigated using a mid-size Sigma curved TKR (DePuy, UK), with moderately cross-linked UHMWPE curved inserts; ISO-14243-3-2004, ISO-14243-3-2014 and ISO-14243-1-2009. In addition, the Leeds displacement control gait cycle was also investigated. According to the computational simulation predictions, reversing the anterior-posterior (AP) displacement and tibial rotation polarities in the displacement control ISO-2014 standard compared to the ISO-2004 standard resulted in high stress, of more than 65 MPa, at the posterior edge of the inserts with more than 10% increase in wear rate for this TKR design. Although Leeds gait input kinematics produced femoral rollback, it did not result in high stress edge loading on the posterior lip of the insert. This was attributed to different test input kinematics and different centres of rotation of the femoral component adopted in the displacement control standard ISO-2014 and Leeds gait test methods. The predicted AP displacement and tibial rotation from the force control ISO-2009 had different polarities and magnitudes to the corresponding displacement control profiles. In addition, the predicted wear rate, from the computational model, under the force control ISO-2009 standard was more than double that predicted under displacement control ISO standards due to the increased AP displacement and tibial rotation motions predicted under the force control standard. These major differences, in the mechanics and wear, between different test methods imply that each standard must therefore be used with its own predicate control results from a device with proven clinical history and results across different standards should never be compared, as the choice of test method standard may well be dependent on the design solution for the knee. Clinically, the kinematics in the population are extremely variable, which results in highly variable wear rates. While a standard method is necessary, on its own it is not adequate and needs to be supported by tests under a portfolio of representative conditions with different kinematic conditions, different soft tissue constraints, as well as with different alignments, so that the variability and range of wear rates expected clinically might be determined. This study enables further progress towards the definition of such a portfolio of representative conditions, by deepening the understanding of the relationships between currently used input conditions and the resulting mechanical and wear outputs