H. Gray

The incidence of physiological radiolucency following Oxford unicompartmental knee replacement and its relationship to outcome

Narrow, well-defined radiolucent lines commonly observed at the bone-implant interface of unicompartmental knee replacement tibial components have been referred to as physiological radiolucencies. These should be distinguished from pathological radiolucencies, which are poorly defined, wide and progressive, and associated with loosening and infection. We studied the incidence and clinical significance of tibial radiolucent lines in 161 Oxford unicondylar knee replacements five years after surgery. All the radiographs were aligned with fluoroscopic control to obtain views parallel to the tibial tray to reveal the tibial bone-implant interface. We found that 49 knees (30%) had complete, 52 (32%) had partial and 60 (37%) had no radiolucent lines. There was no relationship between the incidence of radiolucent lines and patient factors such as gender, body mass index and activity, or operative factors including the status of the anterior cruciate ligament and residual varus deformity. Nor was any statistical relationship established between the presence of radiolucent lines and clinical outcome, particularly pain, assessed by the Oxford Knee score and the American Knee Society score. We conclude that radiolucent lines are common after Oxford unicompartmental knee replacement but that their aetiology remains unclear. Radiolucent lines were not a source of adverse symptoms or pain. Therefore, when attempting to identify a source of postoperative pain after Oxford unicompartmental knee replacement the presence of a physiological radiolucency should be ignored.

Experimental Validation of a Finite Element Model of a Human Cadaveric Tibia

Finite element (FE) models of long bones are widely used to analyze implant designs. Experimental validation has been used to examine the accuracy of FE models of cadaveric femurs; however, although convergence tests have been carried out, no FE models of an intact and implanted human cadaveric tibia have been validated using a range of experimental loading conditions. The aim of the current study was to create FE models of a human cadaveric tibia, both intact and implanted with a unicompartmental knee replacement, and to validate the models against results obtained from a comprehensive set of experiments. Seventeen strain rosettes were attached to a human cadaveric tibia. Surface strains and displacements were measured under 17 loading conditions, which consisted of axial, torsional, and bending loads. The tibia was tested both before and after implantation of the knee replacement. FE models were created based on computed tomography (CT) scans of the cadaveric tibia. The models consisted of ten-node tetrahedral elements and used 600 material properties derived from the CT scans. The experiments were simulated on the models and the results compared to experimental results. Experimental strain measurements were highly repeatable and the measured stiffnesses compared well to published results. For the intact tibia under axial loading, the regression line through a plot of strains predicted by the FE model versus experimentally measured strains had a slope of 1.15, an intercept of 5.5 microstrain, and an R(2) value of 0.98. For the implanted tibia, the comparable regression line had a slope of 1.25, an intercept of 12.3 microstrain, and an R(2) value of 0.97. The root mean square errors were 6.0% and 8.8% for the intact and implanted models under axial loads, respectively. The model produced by the current study provides a tool for simulating mechanical test conditions on a human tibia. This has considerable value in reducing the costs of physical testing by pre-selecting the most appropriate test conditions or most favorable prosthetic designs for final mechanical testing. It can also be used to gain insight into the results of physical testing, by allowing the prediction of those variables difficult or impossible to measure directly.

Thermal Effects of Cement Mantle Thickness for Hip Resurfacing

Hybrid hip resurfacing arthroplasty with uncemented acetabular and cemented femoral fixation is increasingly becoming popular as an alternative to total hip arthroplasty. There is concern about femoral neck fractures, and long-term survival has not yet been demonstrated. Thermal necrosis may be an important factor for neck fracture and will affect the viability of the femoral bone. This cadaveric study investigated the thermal effect of thick (1.5 mm, n = 3) and thin (0.5 mm, n = 3) cement mantles; 5 thermocouples were used to record temperature at the femoral bone/cement interface during hip resurfacing arthroplasty. The highest recorded temperatures were significantly higher when a thick cement mantle is used (45.4 degrees C), compared to a thin cement mantle (32.7 degrees C). To reduce the potential for thermal necrosis, the thin cement mantle technique is recommended.

The effect of bearing congruency, thickness and alignment on the stresses in unicompartmental knee replacements.

BACKGROUND Unicompartmental knee replacement offers an effective treatment for patients with single compartment knee disease and is becoming an increasingly popular alternative to total knee replacement. An important cause of failure in a unicompartmental knee replacement implant is polyethylene wear. Significant contributory factors to the amount of polyethylene wear are contact stress, bearing alignment, congruency and thickness. METHODS Four different unicompartmental knee replacement implant designs (Fully-Congruent; Partially-Congruent; Non-Congruent-metal-backed; Non-Congruent-all-polyethylene) were inserted into a validated finite element model of a proximal tibia. The effect that bearing congruency, alignment and thickness had on the polyethylene stresses during a simulated step-up activity for each design was investigated. Additionally, contact pressures were compared to those calculated from Hertz elastic theory. FINDINGS Only the Fully-Congruent bearing experienced peak von Mises and contact stresses below the lower fatigue limit for polyethylene during the step-up activity. The highest polyethylene contact stresses were observed for the Partially-Congruent and Non-Congruent-metal-backed designs, which experienced approximately three times the polyethylene lower fatigue limit. Increasing the bearing thickness from 3.5mm to 8.5mm of the Non-Congruent design decreased the contact stresses in the bearing; however they did not fall below the lower fatigue limit for polyethylene. Good agreement between finite element and Hertz contact pressures was found. INTERPRETATION Fully congruent unicompartmental knee replacement bearings can be markedly thinner without approaching the material failure limit, have a greater potential to preserve bone stock and are less likely to fail mechanically.

Experimental validation of a finite element model of a composite tibia.

Abstract Composite bones are synthetic models made to simulate the mechanical behaviour of human bones. Finite element (FE) models of composite bone can be used to evaluate new and modified designs of joint prostheses and fixation devices. The aim of the current study was to create an FE model of a composite tibia and to validate it against results obtained from a comprehensive set of experiments. For this, 17 strain rosettes were attached to a composite tibia (model 3101, Pacific Research Laboratories, Vashon, Washington, USA). Surface strains and displacements were measured under 13 loading conditions. Two FE models were created on the basis of computed tomography scans. The models differed from each other in the mesh and material properties assigned. The experiments were simulated on them and the results compared with experimental results. The more accurate model was selected on the basis of regression analysis. In general, experimental strain measurements were highly repeatable and compared well with published results. The more accurate model, in which the inner elements representing the foam were assigned isotropic material properties and the elements representing the epoxy layer were assigned transversely isotropic material properties, was able to simulate the mechanical behaviour of the tibia with acceptable accuracy. The regression line for all axial loads combined had a slope of 0.999, an intercept of -6.24 microstrain, and an R2 value of 0.962. The root mean square error as a percentage was 5 per cent.

Mobile Biplane X-Ray Imaging System for Measuring 3D Dynamic Joint Motion During Overground Gait

Most X-ray fluoroscopy systems are stationary and impose restrictions on the measurement of dynamic joint motion; for example, knee-joint kinematics during gait is usually measured with the subject ambulating on a treadmill. We developed a computer-controlled, mobile, biplane, X-ray fluoroscopy system to track human body movement for high-speed imaging of 3D joint motion during overground gait. A robotic gantry mechanism translates the two X-ray units alongside the subject, tracking and imaging the joint of interest as the subject moves. The main aim of the present study was to determine the accuracy with which the mobile imaging system measures 3D knee-joint kinematics during walking. In vitro experiments were performed to measure the relative positions of the tibia and femur in an intact human cadaver knee and of the tibial and femoral components of a total knee arthroplasty (TKA) implant during simulated overground gait. Accuracy was determined by calculating mean, standard deviation and root-mean-squared errors from differences between kinematic measurements obtained using volumetric models of the bones and TKA components and reference measurements obtained from metal beads embedded in the bones. Measurement accuracy was enhanced by the ability to track and image the joint concurrently. Maximum root-mean-squared errors were 0.33 mm and 0.65° for translations and rotations of the TKA knee and 0.78 mm and 0.77° for translations and rotations of the intact knee, which are comparable to results reported for treadmill walking using stationary biplane systems. System capability for in vivo joint motion measurement was also demonstrated for overground gait.

Androgen deprivation causes selective deficits in the biomechanical leg muscle function of men during walking: a prospective case–control study

Although muscle mass declines with testosterone deficiency in men, previous studies of muscle function have not demonstrated consistent deficits, likely due to relatively insensitive methodology. Our objective was to determine the effects of testosterone deprivation on the biomechanical function of individual lower‐limb muscles.

In vivo six-degree-of-freedom knee-joint kinematics in overground and treadmill walking following total knee arthroplasty.

No data are available to describe six‐degree‐of‐freedom (6‐DOF) knee‐joint kinematics for one complete cycle of overground walking following total knee arthroplasty (TKA). The aims of this study were firstly, to measure 6‐DOF knee‐joint kinematics and condylar motion for overground walking following TKA; and secondly, to determine whether such data differed between overground and treadmill gait when participants walked at the same speed during both tasks. A unique mobile biplane X‐ray imaging system enabled accurate measurement of 6‐DOF TKA knee kinematics during overground walking by simultaneously tracking and imaging the joint. The largest rotations occurred for flexion‐extension and internal‐external rotation whereas the largest translations were associated with joint distraction and anterior‐posterior drawer. Strong associations were found between flexion‐extension and adduction‐abduction (R2 = 0.92), joint distraction (R2 = 1.00), and anterior‐posterior translation (R2 = 0.77), providing evidence of kinematic coupling in the TKA knee. Although the measured kinematic profiles for overground walking were grossly similar to those for treadmill walking, several statistically significant differences were observed between the two conditions with respect to temporo‐spatial parameters, 6‐DOF knee‐joint kinematics, and condylar contact locations and sliding. Thus, caution is advised when making recommendations regarding knee implant performance based on treadmill‐measured knee‐joint kinematic data.

Radiographic evaluation of factors affecting bearing dislocation in the domed lateral Oxford unicompartmental knee replacement

BACKGROUND The rate of bearing dislocation with the domed lateral Oxford Unicompartmental Knee Replacement (OUKR) in different series varies from 1% to 6% suggesting that dislocation is influenced by surgical technique. The aim of this study was to identify surgical factors associated with dislocation. METHODS Aligned post-operative antero-posterior knee radiographs of seven knees that had dislocated and 87 control knees were compared. Component alignment and position and the alignment of the knee were assessed. All bearing dislocations occurred medially over the tibial wall. RESULTS Knees that dislocated tended to be overcorrected: Compared with those that did not dislocate, they were in 2° less valgus (p=0.019) and the tibial components were positioned 2 mm more proximal (p<0.01). Although the relative position of the centre of the femoral component and the tibial component was the same (p=0.8), in the dislocating group the gap between the edge of the femoral component and the top of the wall in flexion was 3mm greater (p=0.019) suggesting that the components were internally rotated. CONCLUSIONS To minimise the risk of dislocation it is recommended that the knee should not be overstuffed. This is best achieved by selecting the bearing thickness that just tightens the ligaments in full extension, and re-cutting the tibia if necessary. In addition to minimise the gap between the femoral and tibial components through which the bearing dislocates, the femoral component should be implanted in neutral rotation and should not be internally rotated. LEVEL OF EVIDENCE Level IV.

Accuracy of mobile biplane X-ray imaging in measuring 6-degree-of-freedom patellofemoral kinematics during overground gait

The aim of this study was to evaluate the accuracy with which mobile biplane X-ray imaging can be used to measure patellofemoral kinematics of the intact knee during overground gait. A unique mobile X-ray imaging system tracked and recorded biplane fluoroscopic images of two human cadaver knees during simulated overground walking at a speed of 0.7m/s. Six-degree-of-freedom patellofemoral kinematics were calculated using a bone volumetric model-based method and the results then compared against those derived from a gold-standard bead-based method. RMS errors for patellar anterior translation, superior translation and lateral shift were 0.19mm, 0.34mm and 0.37mm, respectively. RMS errors for patellar flexion, lateral tilt and lateral rotation were 1.08°, 1.15° and 1.46°, respectively. The maximum RMS error for patellofemoral translations was approximately one-half that reported previously for tibiofemoral translations using the same mobile X-ray imaging system while the maximum RMS error for patellofemoral rotations was nearly two times larger than corresponding errors reported for tibiofemoral rotations. The lower accuracy in measuring patellofemoral rotational motion is likely explained by the symmetric nature of the patellar geometry and the smaller size of the patella compared to the tibia.