Scott A. Banks

In vivo kinematics of cruciate-retaining and -substituting knee arthroplasties

A fluoroscopic measurement technique has been used to provide detailed three-dimensional kinematic assessment of knee arthroplasty function during a step-up activity. Three groups of knee arthroplasty subjects with excellent clinical outcomes and similar ranges of motion were evaluated. Each group received different prosthetic components and surgical treatments of the posterior cruciate ligament (PCL). Group 1 had relatively flat articular surfaces with retention of the bony insertion of the PCL, group 2 had similar articular geometry but recessed the PCL without retaining the bony insertion, and group 3 had prostheses with greater sagittal conformity and post/cam substitution of the sacrificed PCL. Although none of the knees exhibited normal knee kinematics, the ranges of axial rotation and condylar translation for group 1 were similar to ranges previously reported for normal and anterior cruciate-deficient knees. Axial rotations and condylar translations decreased when the PCL was surgically recessed or substituted. The smallest kinematic ranges were observed in group 3. The results indicate that both prosthetic component selection and surgical technique have a significant effect on prosthetic knee kinematics during functional activities.

Grand Challenge Competition to Predict In Vivo Knee Loads

Impairment of the human neuromusculoskeletal system can lead to significant mobility limitations and decreased quality of life. Computational models that accurately represent the musculoskeletal systems of individual patients could be used to explore different treatment options and optimize clinical outcome. The most significant barrier to model‐based treatment design is validation of model‐based estimates of in vivo contact and muscle forces. This paper introduces an annual “Grand Challenge Competition to Predict In Vivo Knee Loads” based on a series of comprehensive publicly available in vivo data sets for evaluating musculoskeletal model predictions of contact and muscle forces in the knee. The data sets come from patients implanted with force‐measuring tibial prostheses. Following a historical review of musculoskeletal modeling methods used for estimating knee muscle and contact forces, we describe the first two data sets used for the first two competitions and summarize four subsequent data sets to be used for future competitions. These data sets include tibial contact force, video motion, ground reaction, muscle EMG, muscle strength, static and dynamic imaging, and implant geometry data. Competition participants create musculoskeletal models to predict tibial contact forces without having access to the corresponding in vivo measurements. These blinded predictions provide an unbiased evaluation of the capabilities and limitations of musculoskeletal modeling methods. The paper concludes with a discussion of how these unique data sets can be used by the musculoskeletal modeling research community to improve the estimation of in vivo muscle and contact forces and ultimately to help make musculoskeletal models clinically useful.

Knee motions during maximum flexion in fixed and mobile-bearing arthroplasties.

Full flexion is a critical performance requirement for patients in Asia and the Middle East, and increasingly for patients in Europe and North America who have total knee arthroplasty. There has been considerable work characterizing maximum flexion in terms of clinical, surgical, and preoperative factors, but less in vivo experimental work after rehabilitation. The purpose of the current investigation was to determine whether anteroposterior tibiofemoral translation influenced maximum weightbearing knee flexion in patients with good or excellent clinical and functional outcomes. One hundred twenty-one knees in 93 subjects, including 16 different articular surface designs, were studied using fluoroscopy and shape matching to determine knee kinematics in a weightbearing deep flexion activity. A relatively posterior position of the femur on the tibia was significantly correlated with greater maximum knee flexion. Posterior-stabilized arthroplasties had significantly more posterior femoral position and maximum flexion than posterior cruciate-retaining fixed-bearing arthroplasties, which had more posterior femoral position and greater maximum flexion than mobile-bearing arthroplasties. Posterior bone-implant impingement was observed in 28% of knees. Tibiofemoral motions influence the mechanics of weightbearing deep flexion in well-functioning knee arthroplasties.

The influence of tibial slope on maximal flexion after total knee arthroplasty

Many surgeons believe that increasing the tibial slope in total knee arthroplasty (TKA) is beneficial with regard to maximal postoperative flexion. Review of the clinical literature, however, does not confirm this hypothesis, neither does it give an answer to the question of how much flexion gain can be expected per degree extra tibial slope. The purpose of this study was, therefore, to evaluate and quantify the influence of tibial slope on maximal postoperative flexion in contemporary posterior cruciate ligament (PCL)-retaining TKA. Twenty-one cadaver simulations of a standard PCL-retaining TKA were studied while reproducing identical deep flexion femorotibial kinematics as documented by three-dimensional computer-aided videofluoroscopy from patients with well-functioning TKAs of the same design. In each knee the tibial component was consecutively implanted with 0° posterior slope, 4° posterior slope, and 7° posterior slope. Maximal flexion was recorded for each configuration. Average maximal flexion at 0° tibial slope was 104°, and increased significantly to 112° when the same knees were implanted with 4° tibial slope. Increasing the slope further to 7° again significantly improved average maximal flexion to 120°. When postoperative radiographic tibial slope was compared to maximal flexion, an average gain of 1.7° flexion for every degree extra tibial slope was noted. Increasing the tibial slope in PCL-retaining TKA does indeed improve maximal flexion before tibial insert impingement occurs against the femoral bone. The surgeon can expect an average gain of 1.7° flexion for every degree extra tibial slope.

Kinematics of posterior cruciate ligament-retaining and -substituting total knee arthroplasty: a prospective randomised outcome study

We performed a prospective, randomised trial of 44 patients to compare the functional outcomes of a posterior-cruciate-ligament-retaining and posterior-cruciate-ligament-substituting total knee arthroplasty, and to gain a better understanding of the in vivo kinematic behaviour of both devices. At follow-up at five years, no statistically significant differences were found in the clinical outcome measurements for either design. The prevalence of radiolucent lines and the survivorship were the same. In a subgroup of 15 knees, additional image-intensifier analysis in the horizontal and sagittal planes was performed during step-up and lunge activity. Our analysis revealed striking differences. Lunge activity showed a mean posterior displacement of both medial and lateral tibiofemoral contact areas (roll-back) which was greater and more consistent in the cruciate-substituting than in the cruciate-retaining group (medial p < 0.0001, lateral p = 0.011). The amount of posterior displacement could predict the maximum flexion which could be achieved (p = 0.018). Forward displacement of the tibiofemoral contact area in flexion during stair activity was seen more in the cruciate-retaining than in the cruciate-substituting group. This was attributed mainly to insufficiency of the posterior cruciate ligament and partially to that of the anterior cruciate ligament. We concluded that, despite similar clinical outcomes, there are significant kinematic differences between cruciate-retaining and cruciate-substituting arthroplasties.

Theoretical accuracy of model-based shape matching for measuring natural knee kinematics with single-plane fluoroscopy

Quantification of knee motion under dynamic, in vivo loaded conditions is necessary to understand how knee kinematics influence joint injury, disease, and rehabilitation. Though recent studies have measured three-dimensional knee kinematics by matching geometric bone models to single-plane fluoroscopic images, factors limiting the accuracy of this approach have not been thoroughly investigated. This study used a three-step computational approach to evaluate theoretical accuracy limitations due to the shape matching process alone. First, cortical bone models of the femur tibia/fibula, and patella were created from CT data. Next, synthetic (i.e., computer generated) fluoroscopic images were created by ray tracing the bone models in known poses. Finally, an automated matching algorithm utilizing edge detection methods was developed to align flat-shaded bone models to the synthetic images. Accuracy of the recovered pose parameters was assessed in terms of measurement bias and precision. Under these ideal conditions where other sources of error were eliminated, tibiofemoral poses were within 2 mm for sagittal plane translations and 1.5 deg for all rotations while patellofemoral poses were within 2 mm and 3 deg. However, statistically significant bias was found in most relative pose parameters. Bias disappeared and precision improved by a factor of two when the synthetic images were regenerated using flat shading (i.e., sharp bone edges) instead of ray tracing (i.e., attenuated bone edges). Analysis of absolute pose parameter errors revealed that the automated matching algorithm systematically pushed the flat-shaded bone models too far into the image plane to match the attenuated edges of the synthetic ray-traced images. These results suggest that biased edge detection is the primary factor limiting the theoretical accuracy of this single-plane shape matching procedure.

Non-invasive assessment of soft-tissue artifact and its effect on knee joint kinematics during functional activity

The soft-tissue interface between skin-mounted markers and the underlying bones poses a major limitation to accurate, non-invasive measurement of joint kinematics. The aim of this study was twofold: first, to quantify lower limb soft-tissue artifact in young healthy subjects during functional activity; and second, to determine the effect of soft-tissue artifact on the calculation of knee joint kinematics. Subject-specific bone models generated from magnetic resonance imaging (MRI) were used in conjunction with X-ray images obtained from single-plane fluoroscopy to determine three-dimensional knee joint kinematics for four separate tasks: open-chain knee flexion, hip axial rotation, level walking, and a step-up. Knee joint kinematics was derived using the anatomical frames from the MRI-based, 3D bone models together with the data from video motion capture and X-ray fluoroscopy. Soft-tissue artifact was defined as the degree of movement of each marker in the anteroposterior, proximodistal and mediolateral directions of the corresponding anatomical frame. A number of different skin-marker clusters (total of 180) were used to calculate knee joint rotations, and the results were compared against those obtained from fluoroscopy. Although a consistent pattern of soft-tissue artifact was found for each task across all subjects, the magnitudes of soft-tissue artifact were subject-, task- and location-dependent. Soft-tissue artifact for the thigh markers was substantially greater than that for the shank markers. Markers positioned in the vicinity of the knee joint showed considerable movement, with root mean square errors as high as 29.3mm. The maximum root mean square errors for calculating knee joint rotations occurred for the open-chain knee flexion task and were 24.3 degrees , 17.8 degrees and 14.5 degrees for flexion, internal-external rotation and abduction-adduction, respectively. The present results on soft-tissue artifact, based on fluoroscopic measurements in healthy adult subjects, may be helpful in developing location- and direction-specific weighting factors for use in global optimization algorithms aimed at minimizing the effects of soft-tissue artifact on calculations of knee joint rotations.

Implant design affects knee arthroplasty kinematics during stair-stepping.

Knee implant motions have a direct influence on patient function and implant longevity. The purpose of this study was to determine if there were consistent differences in knee motions among three groups of knee implants. Two hundred thirteen knees in 173 patients, with 25 implant designs, were studied using fluoroscopy during stair-stepping. All knee implants were assigned to one of three groups based on the design: fixed-bearing posterior-stabilized, fixed-bearing posterior cruciate-retaining, and mobile-bearing. All types of implants had the same pattern of internal/external rotations, but different designs had different anteroposterior translations. Seventy-five percent of posterior-stabilized knee implants had a medial center of rotation, indicating posterior femoral translation with flexion. Sixty-three percent of cruciate-retaining fixed-bearing knee implants had a lateral center of rotation. Eighty-six percent of mobile-bearing knee implants had a lateral center of rotation, indicating anterior femoral translation with flexion. Knee motion in patients with successful total knee arthroplasties is related directly to the constraints of the implant design.

Simultaneous prediction of muscle and contact forces in the knee during gait

Musculoskeletal models are currently the primary means for estimating in vivo muscle and contact forces in the knee during gait. These models typically couple a dynamic skeletal model with individual muscle models but rarely include articular contact models due to their high computational cost. This study evaluates a novel method for predicting muscle and contact forces simultaneously in the knee during gait. The method utilizes a 12 degree-of-freedom knee model (femur, tibia, and patella) combining muscle, articular contact, and dynamic skeletal models. Eight static optimization problems were formulated using two cost functions (one based on muscle activations and one based on contact forces) and four constraints sets (each composed of different combinations of inverse dynamic loads). The estimated muscle and contact forces were evaluated using in vivo tibial contact force data collected from a patient with a force-measuring knee implant. When the eight optimization problems were solved with added constraints to match the in vivo contact force measurements, root-mean-square errors in predicted contact forces were less than 10 N. Furthermore, muscle and patellar contact forces predicted by the two cost functions became more similar as more inverse dynamic loads were used as constraints. When the contact force constraints were removed, estimated medial contact forces were similar and lateral contact forces lower in magnitude compared to measured contact forces, with estimated muscle forces being sensitive and estimated patellar contact forces relatively insensitive to the choice of cost function and constraint set. These results suggest that optimization problem formulation coupled with knee model complexity can significantly affect predicted muscle and contact forces in the knee during gait. Further research using a complete lower limb model is needed to assess the importance of this finding to the muscle and contact force estimation process.

A kinematic comparison of fixed- and mobile-bearing knee replacements

Mobile-bearing posterior-stabilised knee replacements have been developed as an alternative to the standard fixed- and mobile-bearing designs. However, little is known about the in vivo kinematics of this new group of implants. We investigated 31 patients who had undergone a total knee replacement with a similar prosthetic design but with three different options: fixed-bearing posterior cruciate ligament-retaining, fixed-bearing posterior-stabilised and mobile-bearing posterior-stabilised. To do this we used a three-dimensional to two-dimensional model registration technique. Both the fixed- and mobile-bearing posterior-stabilised configurations used the same femoral component. We found that fixed-bearing posterior stabilised and mobile-bearing posterior-stabilised knee replacements demonstrated similar kinematic patterns, with consistent femoral roll-back during flexion. Mobile-bearing posterior-stabilised knee replacements demonstrated greater and more natural internal rotation of the tibia during flexion than fixed-bearing posterior-stabilised designs. Such rotation occurred at the interface between the insert and tibial tray for mobile-bearing posterior-stabilised designs. However, for fixed-bearing posterior-stabilised designs, rotation occurred at the proximal surface of the bearing. Posterior cruciate ligament-retaining knee replacements demonstrated paradoxical sliding forward of the femur. We conclude that mobile-bearing posterior-stabilised knee replacements reproduce internal rotation of the tibia more closely during flexion than fixed-bearing posterior-stabilised designs. Furthermore, mobile-bearing posterior-stabilised knee replacements demonstrate a unidirectional movement which occurs at the upper and lower sides of the mobile insert. The femur moves in an anteroposterior direction on the upper surface of the insert, whereas the movement at the lower surface is pure rotation. Such unidirectional movement may lead to less wear when compared with the multidirectional movement seen in fixed-bearing posterior-stabilised knee replacements, and should be associated with more evenly applied cam-post stresses.