B. H. van Duren

Combined anterior cruciate reconstruction and Oxford unicompartmental knee arthroplasty: In vivo kinematics

The in vivo kinematics of 10 patients after combined anterior cruciate ligament reconstruction (ACLR group) and Oxford unicompartmental knee arthroplasty (UKA) was compared to those of 10 Oxford UKA patients with an intact ACL (ACLI group) and a group of 22 normal knees. The kinematics during a step-up exercise and a deep knee bend exercise was measured using a fluoroscopic technique. The patellar tendon angle (PTA) to knee flexion angle relationship during both exercises was similar for all three groups of subjects. For the UKA groups the pattern of mobile bearing movement during both exercises was similar. This study demonstrates that normal knee kinematics is achieved in the ACL deficient arthritic knee following ACLR and UKA. As a result these patients, who tend to be young and high demand, have excellent outcome and achieve high levels of function. As the relative position of the components and thus component loading are similar to the ACLI UKA, we would expect similar long term survival.

Accuracy evaluation of fluoroscopy-based 2D and 3D pose reconstruction with unicompartmental knee arthroplasty.

The recent development in Oxford lateral unicompartmental knee arthroplasty (UKA) design requires a valid method of assessing its kinematics. In particular, the use of single plane fluoroscopy to reconstruct the 3D kinematics of the implanted knee. The method has been used previously to investigate the kinematics of UKA, but mostly it has been used in conjunction with total knee arthroplasty (TKA). However, no accuracy assessment of the method when used for UKA has previously been reported. In this study we performed computer simulation tests to investigate the effect of the different geometry of the unicompartmental implant has on the accuracy of the method in comparison to the total knee implants. A phantom was built to perform in vitro tests to determine the accuracy of the method for UKA. The computer simulations suggested that the use of the method for UKA would prove less accurate than for TKAs. The rotational degrees of freedom for the femur showed greatest disparity between the UKA and TKA. The phantom tests showed that the in-plane translations were accurate to <0.5mm RMS and the out-of-plane translations were less accurate with 4.1mm RMS. The rotational accuracies were between 0.6 degrees and 2.3 degrees which are less accurate than those reported in the literature for TKA, however, the method is sufficient for studying overall knee kinematics.

Use your phone to build a simple laparoscopic trainer

Simulation is becoming increasingly integral to surgical training with progressive restrictions on working hours. This paper describes a unique, cable free, laparoscopic trainer that can be constructed using items readily available to the average surgical trainee. The trainer described is not a substitute for surgical practice but, nonetheless, a useful tool in developing skills such as hand-eye co-ordination, triangulation and depth queuing.Simulation is becoming increasingly integral to surgical training with progressive restrictions on working hours. This paper describes a unique, cable free, laparoscopic trainer that can be constructed using items readily available to the average surgical trainee. The trainer described is not a substitute for surgical practice but, nonetheless, a useful tool in developing skills such as hand-eye co-ordination, triangulation and depth queuing.

A novel flexible capacitive load sensor for use in a mobile unicompartmental knee replacement bearing: An in vitro proof of concept study

Instrumented knee replacements can provide in vivo data quantifying physiological loads acting on the knee. To date instrumented mobile unicompartmental knee replacements (UKR) have not been realised. Ideally instrumentation would be embedded within the polyethylene bearing. This study investigated the feasibility of an embedded flexible capacitive load sensor. A novel flexible capacitive load sensor was developed which could be incorporated into standard manufacturing of compression moulded polyethylene bearings. Dynamic experiments were performed to determine the characteristics of the sensor on a uniaxial servo-hydraulic material testing machine. The instrumented bearing was measured at sinusoidal frequencies between 0.1 and 10Hz, allowing for measurement of typical gait load magnitudes and frequencies. These correspond to frequencies of interest in physiological loading. The loads that were applied were a static load of 390N, corresponding to an equivalent body weight load for UKR, and a dynamic load of ±293N. The frequency transfer response of the sensor suggests a low pass filter response with a -3dB frequency of 10Hz. The proposed embedded capacitive load sensor was shown to be applicable for measuring in vivo loads within a polyethylene mobile UKR bearing.

Approximation of the functional kinematics of posterior stabilised total knee replacements using a two-dimensional sagittal plane patello-femoral model: comparing model approximation to in vivo measurement.

Previous in vivo studies have observed that current designs of posterior stabilised (PS) total knee replacements (TKRs) may be ineffective in restoring normal kinematics in Late flexion. Computer-based models can prove a useful tool in improving PS knee replacement designs. This study investigates the accuracy of a two-dimensional (2D) sagittal plane model capable of predicting the functional sagittal plane kinematics of PS TKR implanted knees against direct in vivo measurement. Implant constraints are often used as determinants of anterior–posterior tibio-femoral positioning. This allowed the use of a patello-femoral modelling approach to determine the effect of implant constraints. The model was executed using motion simulation software which uses the constraint force algorithm to achieve a solution. A group of 10 patients implanted with Scorpio PS implants were recruited and underwent fluoroscopic imaging of their knees. The fluoroscopic images were used to determine relative implant orientation using a three-dimensional reconstruction method. The determined relative tibio-femoral orientations were then input to the model. The model calculated the patella tendon angles (PTAs) which were then compared with those measured from the in vivo fluoroscopic images. There were no significant differences between the measured and calculated PTAs. The average root mean square error between measured and modelled ranged from 1.17° to 2.10° over the flexion range. A sagittal plane patello-femoral model could conceivably be used to predict the functional 2D kinematics of an implanted knee joint. This may prove particularly useful in optimising PS designs.

A Validated Two-Dimensional Model to Predict TKR Functional Kinematics

INTRODUCTION Posterior Stabilized (PS) Total Knee Replacement (TKR) design utilises a cam and post mechanism to generate femoral rollback in higher flexion. The construct is designed to replicate the function of the posterior cruciate ligament (PCL). There is much interest in obtaining optimum post cam engagement to achieve normal femoral rollback and improved outcome after TKR. Previous studies [1] have observed that the PS knee may be ineffective in restoring normal kinematics in high flexion. However, such patient based fluoroscopic studies are demanding in terms of resources and time. Ideally, computer based modelling methodologies can offer a more suitable adjunct. The aims of this study were to develop a validated model capable of predicting the functional kinematics of PS TKR implanted knees, and compare results obtained using this model to those measured in vivo as validation. METHODS Functional kinematics were assessed using the Patella Tendon Angle (PTA) (the angle subtended between the patella tendon and the long axis of the tibia). PTA provides a valid assessment of relative tibial-femoral position [1] and as such is taken as the output of the model. The input to the model is the relative tibial femoral position. This allows the use of a patella-femoral modelling approach similar to that developed by Gill & O’Connor [2]. The model was executed using motion simulation software which uses the constraint force algorithm [3] to achieve a solution, the output parameter being the PTA. A group of ten patients implanted with Scorpio PS implants, at least one year previously for an underlying diagnosis of osteoarthritis, were recruited. The patients were asked to perform a step-up exercise and a deep knee bend whilst undergoing fluoroscopic imaging. The fluoroscopic images were subsequently corrected for distortion and the relative implant orientation was achieved using a 3D reconstruction method. The determined relative tibial femoral orientations were then input to the model. The mathematically obtained PTAs were then compared to those measured from in vivo fluoroscopy images using an established protocol [1]. The mean PTA values for each each data set were compared using paired t tests at 10 degree flexion intervals. Variation between model and in vivo data was further explored using root mean square analysis RESULTS The mean PTA measured in vivo ranged from 11 in extension to -1 at 100 flexion (fig.1). The mean PTA obtained using the model for the same patients was similar ranging from 10 in extension to 0 at 100 flexion. There was no significant difference between the PTAs for the data sets at any point in the range. A large standard deviation is present for both mathematically derived and in vivo data sets. The average root mean square error (RMSE) ranged between 1.17 and 2.10 over the flexion range. Table 1 shows the average RMSE and its standard deviation as well as the significance at 10 intervals.