Peter S. Walker

Wear in retrieved condylar knee arthroplasties

The plastic components of 280 retrieved unicondylar and total knee arthroplasties were studied. Wear was visually scored using a relative ranked data method. Although wear on the components was highly variable, several conclusions could be drawn regarding the nature and causes. Wear was associated more with the medial than the lateral condyle. Delamination was the most severe type of wear and occurred in short (< 5 year)-, medium (5-10 years)-, and long (> 10 years)-term retrievals. In the short term, delamination wear was associated with hot pressing of the tibial plastic or with fracture of the tibial baseplate. For a single design, a significant difference in the amount of delamination on hot-pressed and non-hot-pressed tibial components was observed. In medium- and long-term retrieved specimens of the designs with moderately high conformity, delamination wear was associated with restriction of rotational movement of the femoral component or with abrupt changes in the radius of the tibial component. In flatter, less conforming designs, wear was associated with laxity, such that the polyethylene delaminated toward the edges of the tibial component. Wear attributed to cement abrasion or entrapment occurred on the more conforming designs. Delamination was associated with the presence of fusion defects in the polyethylene but could also occur in the absence of such defects. That delamination was the principal were type and that this is caused by a fatigue mechanism mean that the incidence of failure could accelerate considerably over follow-up periods beyond 10 years. Designs of moderate conformity without abrupt changes in radii may prolong the duration of plastic tibial components before serious delamination occurs.

A knee simulating machine for performance evaluation of total knee replacements

A knee simulating machine is required for the design and evaluation of total knee replacements, the kinematics and the long-term wear being aspects of particular importance. There are no generally agreed design criteria, such that existing designs of simulator have a wide variety of input and constraint conditions. In this study, it was postulated that in order to reproduce physiological wear patterns, the correct kinematics is required, on the basis that the wear will be a direct function of the sliding, rolling and tractive rolling conditions at the joint surfaces. In turn, the correct kinematics would only be achieved by the input of physiological forces, by the appropriate constraints on the fixtures holding the components, and by simulating the soft tissue restraints. A knee simulating machine based on these principles was constructed, and used to test the kinematics of a range of contemporary condylar replacement knees. The displacements and rotations varied over a range of almost two times, even with the soft tissue restraints. Without the restraints, the low constraint designs would have dislocated or moved unrealistically. It was concluded that a simulating machine should be based on the input of forces and moments, rather than on displacements and rotations, in order to provide data of kinematics and wear.

The use of a force-controlled dynamic knee simulator to quantify the mechanical performance of total knee replacement designs during functional activity

The experimental evaluation of any total knee replacement (TKR) design should include the pre-implantation quantification of its mechanical performance during tests that simulate the common activities of daily living. To date, few dynamic TKR simulation studies have been conducted before implantation. Once in vivo, the accurate and reproducible assessment of TKR design mechanics is exceedingly difficult, with the secondary variables of the patient and the surgical technique hindering research. The current study utilizes a 6-degree-of-freedom force-controlled knee simulator to quantify the effect of TKR design alone on TKR mechanics during a simulated walking cycle. Results show that all eight TKR designs tested elicited statistically different measures of tibial/femoral kinematics, simulated soft tissue loading, and implant geometric restraint loading during an identical simulated gait cycle, and that these differences were a direct result of TKR design alone. Maximum ranges of tibial kinematics over the eight designs tested were from 0.8mm anterior to 6.4mm posterior tibial displacement, and 14.1 degrees internal to 6.0 degrees external tibial rotation during the walking cycle. Soft tissue and implant reaction forces ranged from 106 and 222N anteriorly to 19 and 127N posteriorly, and from 1.6 and 1.8Nm internally to 3.5 and 5.9Nm externally, respectively. These measures provide valuable experimental insight into the effect of TKR design alone on simulated in vivo TKR kinematics, bone interface loading and soft tissue loading. Future studies utilizing this methodology should investigate the effect of experimentally controlled variations in surgical and patient factors on TKR performance during simulated dynamic activity.

A computer model with surface friction for the prediction of total knee kinematics

A computer model was generated which modelled the bearing surfaces of total knees, and predicted the kinematics for a set of input forces and moments. The model included friction at the bearing surfaces and soft tissue restraint forces, including the effect of cruciate resection. Predictions from the model were compared with data from a Knee Simulating Machine. There was close agreement in the shapes of the curves and in the magnitudes of the displacements and rotations under most conditions. The model predicted major differences in kinematics when friction between metal and polyethylene was included, the differences being even greater at the friction levels associated with small embedded acrylic particles. Soft tissue restraint was shown to reduce the displacements and rotations for tibial surfaces of low constraint but for moderate to high constraint, the soft tissues affected the kinematics only slightly. When the model was used to predict the motions for different condylar geometries, widely different contact paths on the tibial surface were determined. This suggested that condylar geometries which appeared to be generally similar, could have important differences in kinematics, function and wear.

Telemetry of forces from proximal femoral replacements and relevance to fixation

Two proximal femoral replacements were instrumented to enable axial forces to be determined at two sites within the prosthesis: in the main shaft and near the tip of the intramedullary stem. The goal was to measure the changes in force distribution over time, as indicated by the ratio of the two forces. Inductive coupling between a coil worn around the leg and a small implanted coil was used, both to supply power to electronic circuits sealed into a welded cavity in the prosthesis and to telemeter data from the prosthesis. Data from both subjects were recorded over the first two years following surgery. For the first subject, there was an increase in mean shaft force excursions (peak force minus resting force) during level walking from 0.53 x BW after 1 week 2.77 x BW after 23 months. The corresponding mean tip force excursions were 0.13 x BW and 1.74 x BW, respectively. The ratio of mean tip force excursions to shaft force excursions steadily increased over the same period from 25 to 63%. Similar increases over time in the tip/shaft ratio were found during treadmill walking, stair climbing and stair descending. Data from the second subject were obtained for the shaft forces only, and were consistent with those from the first subject. The progressive transfer of axial load from the proximal to the distal part of the IM stem recorded telemetrically, together with radiographic observations, suggested that bone remodelling had taken place together with a less stable interface around the proximal part of the stem. This process evidently began soon after implantation.

Effect of oxidation on delamination of ultrahigh-molecular-weight polyethylene tibial components

Whether oxidation of ultrahigh-molecular-weight polyethylene (UHMWPE) causes delamination of the plastic in total knee arthroplasties (TKAs) was investigated. Examination of retrieved TKAs has shown that oxidation of UHMWPE can be caused by postirradiation damage leading to a subsurface band or, to a lesser extent, by mechanical forces during use leading to surface oxidation. Delamination cracks propagated through the subsurface oxidized band. In wear tests, delamination occurred in artificially aged UHMWPE where only subsurface oxidized bands had formed. Increased surface wear predominated where oxidation was associated with the surface of the plastic. Similarly, in tensile and fatigue tests of oxidized UHMWPE, there was a significant reduction in the ultimate tensile strength and in the fatigue resistance of specimens that had developed a subsurface band. Oxidation increased fatigue crack growth rate. It was observed that UHMWPE from different manufacturers varied in its resistance to oxidation. This study demonstrates that the effect of oxidation, which results in the development of a subsurface white band, combined with high subsurface shear forces observed in TKAs, is to enhance delamination wear.

Optimization of the bearing surface geometry of total knees

Various design criteria were examined in combination to find the ideal geometry for a condylar knee replacement. The criteria were the contact stresses on the plastic, femoral-tibial size interchangeability, patella lever arm, laxity and stability and the amount of bone resection required. The variables were the radii of curvature of the femoral and tibial bearing surfaces in the sagittal and frontal planes. Metal toroidal indentors were loaded onto dished surfaces of UHMWPE covering a range of radii and the contact areas measured. Using elasticity equations, the apparent elastic modulus of UHMWPE ranged from 400 to 600 MPa for less conforming to closely conforming surfaces. Using a value of 600 MPa, contact stresses were predicted for a complete spectrum of radii of curvature. Finite element analysis was used to determine the stresses beneath the contact patches when different femoral-tibial sizes were interchanged. A computergraphics program was written to analyse the effects of flexion, rotation and femoral roll-back on the contact point locations. An influential variable was the sagittal curvature of the femoral component, notably the point of transition between the posterior curve of small radius and the distal curve of larger radius. This affected the patella lever arm, the stability, and the bone resection. Interchangeability was primarily dependent upon the relative frontal radii. Contact stresses and contact locations depended upon the combination of sagittal and frontal radii. The most suitable geometrical combinations overall were discussed.

The conflicting requirements of laxity and conformity in total knee replacement

Bearing surfaces of total condylar knees which are designed with a high degree of conformity to produce low stresses in the polyethylene tibial insert may be overconstrained. This study determines femoral and tibial bearing surface geometries which will induce the least destructive fatigue mechanisms in the polyethylene whilst conserving the laxity of the natural knee. Sixteen knee designs were generated by varying four parameters systematically to cover the range of contemporary knee designs. The parameters were the femoral frontal radius (30 or 70 mm), the difference between the femoral and tibial frontal radii (2 or 10 mm), the tibial sagittal radius (56 or 80 mm) and the posterior-distal transition angle (-8 or -20 degrees), which is the angle at which the small posterior arc of the sagittal profile transfers to the larger distal arc. Rigid body analyses determined the anterior-posterior and rotational motions as well as the contact points during the stance phase of gait for the different designs. In addition, a damage function which accumulated the fluctuating maximum shear stresses was used to predict the susceptibility to delamination wear of the polyethylene (damage score). This study predicted that of the 16 designs, the knee with a frontal radius of 70 mm, a difference in femoral and tibial frontal radii of 2 mm, a tibial sagittal radius of 80 mm and a posterior distal transition angle of -20 degrees would satisfy the conflicting needs of both resistance to delamination wear and natural kinematics.

The effect of a lateral flare feature on uncemented hip stems

Ideal goals for a primary uncemented femoral stem prosthesis are to transmit the loads to the femur proximally, and to minimise the interface migration. It has been proposed that the addition of a lateral flare which loads the lower region of the greater trochanter will contribute positively to these goals. Analytical and radiographic studies were used to study the load transfer between the stem and the bone, and the migration. A comparison was made between a straight stem, and a straight stem with the addition of a lateral flare. The finite element study showed that the straight stem migrated down the canal approximately four millimeters before stabilisation was reached. The forces were transmitted on to the proximal-medial femur and around the lower half of the stem. When the lateral flare was added, there was only one millimeter of migration to reach stability. The loads were transferred by a wedging effect between the proximal-medial femur and the around the lateral flare, with little force transfer from the stem. In a radiographic follow-up of an HA-coated lateral flare stem, trabeculae could be seen attaching to the lateral flare. The axial migration was significantly less for this stem design compared with that from a series of previously reported cemented stems. This study suggested that the lateral flare contributed positively to the goals of uncemented stem design, and that the stems could be made shorter than designs not incorporating the lateral flare feature.

Inherent differences in the laxity and stability between the intact knee and total knee replacements

Abstract To compare the laxity and stability characteristics between the natural knee and condylar replacements, tests were carried out in a knee simulating machine. The tests consisted of applying compressive forces and then applying cyclic AP force and cyclic torque. The magnitudes were similar to those of functional conditions. For the natural knee, the laxities were only reduced modestly by increases in compressive force, especially internal-external rotation. For a low conformity TKR soft tissue restraint was required under low compression in order to avoid anterior tibial subluxation and internal or external rotation in excess of 20 ° . As compression was increased, the rapidly increasing effect of the dishing and the frictional effects provided sufficient inherent stability, although on average, soft tissue restraint reduced the laxity by about 30%. A high conformity TKR still required soft tissue restraint at low compression, but as the compressive force was increased, the surfaces reduced the laxity to the point where, the laxities were only a few millimetres and a few degrees, and the soft tissues contributed little. This phenomenon, where the femoral component is constrained to be close to the bottom of the tibial dish, may not be fully recognised at surgery and may result in excessive PCL tensions and contact forces in function, as well as reduced mobility, especially when a deep-dished tibial component was used. Relatively shallow posterior tibial curvature and a steep anterior curvature were concluded to provide the most satisfactory combination of laxity and stability.