U.P. Wyss

The Influence of Head Size on Corrosion and Fretting Behaviour at the Head-Neck Interface of Artificial Hip Joints

The primary goal of this study was to determine if head size affects corrosion and fretting behaviour at the head-neck taper interface of modular hip prostheses. Seventy-four implants were retrieved that featured either a 28 mm or a 36 mm head with a metal-on-polyethylene articulation. The bore of the heads and the neck of the stems were divided into eight regions each and graded by three observers for corrosion and fretting damage separately using modified criteria as reported in the literature. The 36 mm head size featured a significant difference in the corrosion head scores (p=0.022) in comparison to the 28 mm heads. This may be attributed to a greater torque acting along the taper interface due to activities of daily living.

Accuracy of single-plane fluoroscopy in determining relative position and orientation of total knee replacement components

The accuracy of estimating the relative pose between knee replacement components, in terms of clinical motion, is important in the study of knee joint kinematics. The objective of this study was to determine the accuracy of the single-plane fluoroscopy method in calculating the relative pose between the femoral component and the tibial component, along knee motion axes, while the components were in motion relative to one another. The kinematics of total knee replacement components were determined in vitro using two simultaneous methods: single-plane fluoroscopic shape matching and an optoelectronic motion tracking system. The largest mean differences in relative pose between the two methods for any testing condition were 2.1°, 0.3°, and 1.1° in extension, abduction, and internal rotation respectively, and 1.3, 0.9, and 1.9 mm in anterior, distal, and lateral translations, respectively. For the optimized position of the components during dynamic trials, the limits of agreement, between which 95% of differences can be expected to fall, were -2.9 to 4.5° in flexion, -0.9 to 1.5° in abduction, -2.4 to 2.1° in external rotation, -2.0 to 3.9 mm in anterior-posterior translation, -2.2 to 0.4mm in distal-proximal translation and -7.2 to 8.6mm in medial-lateral translation. These mean accuracy values and limits of agreement can be used to determine whether the shape-matching approach using single-plane fluoroscopic images is sufficiently accurate for an intended motion tracking application.

Knee Kinematics of High-Flexion Activities of Daily Living Performed by Male Muslims in the Middle East

Full flexion is critical for total knee arthroplasty (TKA) patients in the Middle East, where daily activities require a high range of motion in the lower limb. This study aimed to increase understanding of the knee kinematics of normal Muslim subjects during high-flexion activities of daily living, such as kneeling, Muslim prayer, sitting cross-legged, and squatting. The early postoperative kinematics for a select group of Muslim, high-flexion TKA patients are also reported. Mean curves were compared between the normal group and the TKA group. During kneeling, the average maximum flexion was 141.6° for the normal group and 140.2° for the TKA group. The normal groups maximum and minimum knee angles (flexion, abduction, external rotation) were reported and, with the exception of maximum extension, were not significantly different from the TKA group, despite short postoperative times.

Biochemical analyses of human osteoarthritic and periprosthetic synovial fluid

Biochemical analyses were performed on osteoarthritic and periprosthetic synovial fluid in order to propose changes to lubricant specifications currently outlined in orthopaedic wear testing standards. Osteoarthritic and periprosthetic synovial fluid samples were obtained from the hip and knee joints of 40 patients. The samples in each group were analysed and compared in order to identify differences between the protein concentration, constituent fractions, osmolality, thermal stability and the hyaluronic acid concentration and molecular weight distribution of osteoarthritic and periprosthetic synovial fluid. The average total protein concentration was approximately 30 g/L, which was much higher than the 20 g/L currently specified in the knee wear testing standard; however, the 30 g/L protein concentration matched the recently revised standard for hip simulator wear testing. No significant difference was found between the protein concentration, osmolality, thermal stability, and hyaluronic acid concentration of osteoarthritic and periprosthetic synovial fluid. The clinical data provided should be used to better define the composition of a more clinically relevant lubricant for orthopaedic wear testing.

Wear testing of crosslinked polyethylene: Wear rate variability and microbial contamination

The wear performance of two types of crosslinked polyethylene (Marathon™ and XLK™, DePuy Synthes Inc., Warsaw, IN) was evaluated in a pin-on-disc wear tester, a hip wear simulator, and a knee wear simulator. Sodium azide was used as the microbial inhibitor in the calf serum-based lubricant. In the pin-on-disc wear tester, the Marathon wear rate of 5.33±0.54mm(3)/Mc was significantly lower (p=0.002) than the wear rate of 6.43±0.60mm(3)/Mc for XLK. Inversely, the Marathon wear rate of 15.07±1.03mm(3)/Mc from the hip wear simulator was 2.2-times greater than the XLK wear rate of 6.71±1.03mm(3)/Mc from the knee wear simulator. Differences in implant design, conformity, GUR type, and kinematic test conditions were suggested to account for the difference between the wear rates generated in the different types of wear testing apparati. In all wear tests, sodium azide was ineffective at inhibiting microbial growth in the lubricant. Eight different organisms were identified in the lubricant samples from the wear tests, which suggested the necessity of using an alternative, more effective microbial inhibitor. Careful sample preparation and thorough cleaning has shown to improve the consistency of the wear results. The wear rates generated in the hip and knee wear simulators closely reflected the wear behaviour of Marathon and XLK reported in published data that were tested under similar conditions.

An energy dissipation and cross shear time dependent computational wear model for the analysis of polyethylene wear in total knee replacements

The cost and time efficiency of computational polyethylene wear simulations may enable the optimization of total knee replacements for the reduction of polyethylene wear. The present study proposes an energy dissipation wear model for polyethylene which considers the time dependent molecular behavior of polyethylene, aspects of tractive rolling and contact pressure. This time dependent - energy dissipation wear model was evaluated, along with several other wear models, by comparison to pin-on-disk results, knee simulator wear test results under various kinematic conditions and knee simulator wear test results that were performed following the ISO 14243-3 standard. The proposed time dependent - energy dissipation wear model resulted in improved accuracy for the prediction of pin-on-disk and knee simulator wear test results compared with several previously published wear models.

Corrosion on the Acetabular Liner Taper from Retrieved Modular Metal-on-Metal Total Hip Replacements

Eight retrieved metal-on-metal total hip replacements displayed corrosion damage along the cobalt-chromium alloy liner taper junction with the Ti alloy acetabular shell. Scanning electron microscopy indicated the primary mechanism of corrosion to be grain boundary and associated crevice corrosion, which was likely accelerated through mechanical micromotion and galvanic corrosion resulting from dissimilar alloys. Coordinate measurements revealed up to 4.3mm(3) of the cobalt-chromium alloy taper surface was removed due to corrosion, which is comparable to previous reports of corrosion damage on head-neck tapers. The acetabular liner-shell taper appears to be an additional source of metal corrosion products in modular total hip replacements. Patients with these prostheses should be closely monitored for signs of adverse reaction towards corrosion by-products.

Design and virtual evaluation of a customized surface-guided knee implant.

Although total knee arthroplasty is generally a successful operation, many studies have shown that it results in significant alterations in the kinematics of the joint, which cause limitations in performing the activities of daily living. This study aimed to define the design features for a customized surface-guided total knee replacement and to evaluate the kinematic outcomes. Magnetic resonance imaging data of the knee joint are used to generate the design features as they relate to the functionality of the implant. The motion is guided by considering a partial ball and socket configuration on the medial condyle and varying radii of curvature on the lateral articulating surface. A virtual simulation of the behavior of the surface-guided total knee replacement was performed to investigate the motion patterns of this total knee replacement under gait and squatting loading conditions. Results of the virtual simulation show that flexion and extension of the knee make the center of the lateral condyle move more naturally in the posterior and anterior directions, in comparison to the center of the medial condyle. Such guidance is achieved as a result of the novel customized designed contact between the articulating surfaces. The proposed customized surface-guided total knee replacement provides patterns of motion close to the expected more natural target, not only during a gait cycle but also as the knee flexes to higher degrees during squatting. Major design features include location and orientation of the flexion and pivoting axes, the trace of the contact points on the tibia, and the radii of the guiding arcs on the lateral condyle.

Customized surface-guided knee implant: Contact analysis and experimental test

Contact pressure and stresses on the articulating surface of the tibial component of a total knee replacement are directly related to the joint contact forces and the contact area. These stresses can result in wear and fatigue damage of the ultra-high-molecular-weight polyethylene. Therefore, conducting stress analysis on a newly designed surface-guided knee implant is necessary to evaluate the design with respect to the polyethylene wear. Finite element modeling is used to analyze the design’s performance in level walking, stair ascending and squatting. Two different constitutive material models have been used for the tibia component to evaluate the effect of material properties on the stress distribution. The contact pressure results of the finite element analysis are compared with the results of contact pressure using pressure-sensitive film tests. In both analyses, the average contact pressure remains below the material limits of ultra-high-molecular-weight polyethylene insert. The peak von Mises stresses in 90° of flexion and 120° of flexion (squatting) are 16.28 and 29.55 MPa, respectively. All the peak stresses are less than the fatigue failure limit of ultra-high-molecular-weight polyethylene which is 32 MPa. The average contact pressure during 90° and 120° of flexion in squatting are 5.51 and 5.46 MPa according to finite element analysis and 5.67 and 8.14 MPa according to pressure-sensitive film experiment. Surface-guided knee implants are aimed to resolve the limitations in activities of daily living after total knee replacement by providing close to normal kinematics. The proposed knee implant model provides patterns of motion much closer to the natural target, especially as the knee flexes to higher degrees during squatting.

Kinematic behavior of a customized surface-guided knee implant during simulated knee-bending

Different designs of total knee replacements (TKRs) aim to enhance the satisfaction of the patients by providing close to normal kinematics. In the surface-guided TKRs, the guidance of the motion in a normal pattern should be achieved through specially shaped articulating geometries. This study used virtual simulation along with a load-controlled knee wear simulator to evaluate the kinematic performance of a customized surface-guided TKR under weight-bearing conditions of lunging and squatting activities. The outcome pattern of TKR motion almost agreed with the predefined design target. The tibial insert rotated internally through a maximum angle of 10.6° and 19.94° for the experimentally simulated lunging and squatting cycles, respectively. This rotation occurred around a medial center, as indicated by a small amount of posterior translation of the medial condyle (maximum of 2.5mm and 6.4mm for lunging and squatting) versus the posterior translation of the lateral condyle (maximum of 12mm and 24.2mm for lunging and squatting). The contact forces mainly provided the guidance of the motion at the tibiofemoral articulating surfaces.The normalized root mean square error between outcomes of the virtual simulations and tests for the angle of internal-external rotation of the tibial insert was less than 8% for one cycle of lunging and squatting. These measures confirm the validity of the virtual simulation for future evaluations of the customized surface-guided TKRs.