Measurement of laxity in the anterior cruciate ligament-deficient knee: a comparison of three different methods in vitro

The aim of this study was to compare in-vitro measurements of anteroposterior laxity in the anterior cruciate ligament (ACL)-deficient knee using three different methods: an Instron materials-testing machine, then a KT-2000 arthrometer, and finally by Roentgen stereophotogrammetric analysis (RSA). Eight ACL-deficient human cadaver knees were used. Total displacement was measured between 90 N anterior and 90 N posterior tibiofemoral drawer forces at both 20 degrees and 90 degrees knee flexion. Laxity ranged from 11.5 to 27.6 mm at 20 degrees and from 8.7 to 23.9 mm at 90degrees. A statistically significant difference was not found between the mean RSA and KT-2000 measurements. However, the mean Instron measurements of laxity were significantly (3-4 mm) higher than both RSA and KT-2000 measurements. The clinical methods of RSA and the KT-2000 measurements agreed well but appeared to underestimate tibiofemoral anteroposterior laxity compared with the materials-testing machine. These findings may be helpful in the future comparison of different studies.     

Objective measurement of knee laxity and stiffness with reference to knee injury diagnosis. Part 1: Design considerations and apparatus

Apparatus capable of objectively evaluating the laxity of the knee in vivo has been developed. The equipment consisted of a microcomputer-controlled machine, into which the leg was firmly clamped. The mechanical properties of the knee were measured by slowly applying a load to the tibia, while the femur was held stationary, and monitoring the resulting displacement of the tibia. Three separate tests could be performed: anterior-posterior drawing, varus-valgus rotation and tibial rotation. The tests were carried out on both legs of each subject, making six tests in all. The forces versus displacement (or torque versus rotation) took the form of a hysteresis loop. From these a total of 24 variables describing the stiffness, laxity and visco-elastic properties of the knee were calculated.     

A multivariate gait data analysis technique: application to knee osteoarthritis

Modern gait analysis is a powerful non-invasive tool for calculating the mechanical factors involved in pathological processes such as knee osteoarthritis (OA). Although very accurate measurements can be made, the clinical applicability and widespread use of gait analysis have been hindered by a lack of appropriate data analysis techniques for reducing and analysing the resulting large volumes of highly correlated gait data. This paper introduces a multidimensional gait data analysis technique that simultaneously considers multiple time-varying and discrete measures, exploiting the correlation structure between and within the measures. The multidimensional analysis technique was used to detect discriminatory mechanical features of knee OA gait patterns that involved interacting changes in several gait measures, at specific time portions of the gait cycle. The two most discriminatory features described a dynamic alignment difference and a loading response difference with knee OA.     

Effect of twisting hamstring tendon grafts on strength and joint laxity through the range of knee movement

Twisting or braiding of hamstring tendon grafts for use in anterior cruciate ligament reconstructions has been advocated to increase their strength and stiffness under load. In this study, a two-dimensional model was used to determine the failure strength of twisted and parallel grafts and associated knee laxity under simulated physiological loading conditions. For validation, mechanical tests of tendon grafts were also simulated with these models. The simulated physiological loading of the graft models showed that knees with twisted grafts had greater laxity than knees with parallel grafts, although there was little difference in failure load between the two graft configurations. The tensile loading of the graft models showed little difference in failure load when the tendons were modelled using line segments. When the tendons were considered as three-dimensional helical elements, which more accurately describe the tendon structure, the failure load of the twisted graft decreased significantly. This research provided no evidence to support the belief that a twisted tendon graft is a superior graft configuration relative to a parallel tendon graft.     

Muscle-ligament interactions at the human knee (II)

In part I of this paper, a three-dimensional model of the human knee joint was incorporated into a detailed human musculoskeletal lower extremity model. The model was used to determine the muscle, ligament, and articular contact forces transmitted at the knee as humans extend/flex in an isometric state. Part II investigates the sensitivity of the model calculations to changes in the parameters which describe the mechanical behavior of cartilage and the ligaments of the knee. The ligament function in the real knee was most sensitive to changes in ligament reference lengths or strains, less sensitive to changes in cartilage stiffness, and least sensitive to changes in ligament stiffness.     

A three-dimensional dynamic finite element model of the prosthetic knee joint: simulation of joint laxity and kinematics

For testing purposes of prostheses at a preclinical stage, it is very valuable to have a generic modelling tool, which can be used to optimize implant features and to avoid poor designs being launched on to the market. The modelling tool should be fast, efficient, and multi-purpose in nature; a finite element model is well suited to the purpose. The question posed in this study was whether it was possible to develop a mathematically fast and stable dynamic finite element model of a knee joint after total knee arthroplasty that would predict data comparable with published data in terms of (a) laxities and ligament behaviour, and (b) joint kinematics. The soft tissue structures were modelled using a relatively simple, but very stable, composite model consisting of a band reinforced with fibres. Ligament recruitment and balancing was tested with laxity simulations. The tibial and patellar kinematics were simulated during flexion-extension. An implicit mathematical formulation was used. Joint kinematics, joint laxities, and ligament recruitment patterns were predicted realistically. The kinematics were very reproducible and stable during consecutive flexion-extension cycles. Hence, the model is suitable for the evaluation of prosthesis design, prosthesis alignment, ligament behaviour, and surgical parameters with respect to the biomechanical behaviour of the knee.     

The effect of cartilage deformation on the laxity of the knee joint

In this paper, deformation of the articular cartilage layers is incorporated into an existing two-dimensional quasi-static model of the knee joint. The new model relates the applied force and the joint displacement, as measured in the Lachmann drawer test, and allows the effect of cartilage deformation on the knee joint laxity to be determined. The new model augments the previous knee model by calculating the tibio-femoral contact force subject to an approximate 'thin-layer' constitutive equation, and a method is described for finding the configuration of the knee under a specified load, in terms of a displacement from a zero-load reference configuration. The results show that inclusion of deformable cartilage layers can cause a reduction of between 10 and 35 per cent in the force required to produce a given tibial displacement, over the range of flexion angles considered. The presence of cartilage deformation was found to be an important modifier of the loading response but is secondary to the effect of ligamentous extension. The flexion angle dependence of passive joint laxity is much more strongly influenced by fibre recruitment in the ligaments than by cartilage deformation.     

Effect of ultra-high molecular weight polyethylene thickness on contact mechanics in total knee replacement

One of the important design parameters in current knee joint replacements is the thickness of the ultra-high molecular weight polyethylene (UHMWPE) tibial insert, yet there is no clear definition of the upper limit of the 'thick' polyethylene insert. Using one design knee implant and subjecting it to the physiological loads encountered throughout the gait cycle, measurements of the lengths of contact imprints generated were compared with the corresponding theoretical predictions for different insert thicknesses under the same applied load. Multiple regression analysis was applied to test whether the dimensions of contact imprints are influenced by UHMWPE thickness. Good agreement was obtained between the theoretical predictions and the experimental measurements of the dimensions of contact imprints when the knee was at 60 degrees flexion. Therefore, it was possible to estimate the contact pressure at the articulating surface using the theoretical model. Contact imprint dimensions increased with increasing applied load. Statistical analysis of the experimental data revealed that, at 0 degree flexion, the overall imprint dimensions increased as the UHMWPE thickness increased from 8 to 20 mm. However, the increment was not significant when the thickness subinterval 10-15 mm was considered. Furthermore, at 60 degrees flexion, thickness was not a significant factor for the overall imprint dimensions. No evidence was found from the data to suggest that an increment in polyethylene thickness over 10 mm would significantly reduce the contact imprint dimensions. These findings suggest that thicker inserts can be avoided, as they require unnecessary bone resection.     

A sequentially-defined stiffness model of the knee

A new procedure is proposed to obtain a stiffness model of the knee, i.e. a model of the joint when static external loads are applied to the tibia and femur. A sequential approach is used to generalise a kinematic model of the passive motion of the articulation previously presented by the authors. The procedure is devised in such a way that the restraining function of the articular components which guide the passive motion of the joint is preserved after generalisation. As a result, the new stiffness model can replicate the passive motion with the same accuracy as the previous kinematic model, and it can also replicate the relative motion of the tibia and femur when the knee is loaded by static external loads. The proposed procedure is applied to a specimen and the relevant stiffness model is devised. The motion of the model under several loading conditions is then compared with the original motion of the specimen and with data obtained from the literature, in order to show the accuracy of the model and the potential of the proposed procedure.     

KIN-Nav navigation system for kinematic assessment in anterior cruciate ligament reconstruction: features, use, and perspectives

In this paper a new navigation system, KIN-Nav, developed for research and used during 80 anterior cruciate ligament (ACL) reconstructions is described. KIN-Nav is a user-friendly navigation system for flexible intraoperative acquisitions of anatomical and kinematic data, suitable for validation of biomechanical hypotheses. It performs real-time quantitative evaluation of antero-posterior, internal-external, and varus-valgus knee laxity at any degree of flexion and provides a new interface for this task, suitable also for comparison of pre-operative and post-operative knee laxity and surgical documentation. In this paper the concept and features of KIN-Nav, which represents a new approach to navigation and allows the investigation of new quantitative measurements in ACL reconstruction, are described. Two clinical studies are reported, as examples of clinical potentiality and correct use of this methodology. In this paper a preliminary analysis of KIN-Nav's reliability and clinical efficacy, performed during blinded repeated measures by three independent examiners, is also given. This analysis is the first assessment of the potential of navigation systems for evaluating knee kinematics.     

Analysis of the assistance characteristics for the knee extension motion of knee orthosis using muscular stiffness force feedback

In this paper, we are interested in the characteristics of a knee joint when the knee extension motion was assisted by a powered knee orthosis using a muscular stiffness force feedback. For this purpose, we developed the powered knee orthosis with an artificial pneumatic actuator, which is intended for the assistance and the enhancement of muscular activities of lower limbs. The objective of this study was to confirm the effectiveness of the powered knee orthosis that generated a knee extension torque in the motion related to a knee joint. Twenty healthy subjects participated in this study and their lower limb muscular activities were measured to identify the effectiveness of the powered knee orthosis during sit-to-stand (STS) and squat motion. The muscular activities between with and without assistance of knee extension motion were compared and analyzed for the assistance characteristics of the powered knee orthosis. To generate the knee extension torque, the knee orthosis was controlled using muscular stiffness force (MSF) feedback that is controlled by muscular activities of the vastus intermedius muscle that mainly related to the knee extension motion. For analysis of muscular activities, the surface electromyography of the muscles related to the knee extension motion, i.e., RF, vastus lateralis, vastus medialis and vastus intermedius muscles in lower limbs of the right side were recorded and biodex dynamometer was used to measure the maximal concentric isokinetic strength of the knee extensors. The experimental result showed that muscular activities in lower limbs with the assistance of the powered knee orthosis was reduced by 25.62% in rectus femoris muscle and 29.82% in biceps femoris muscle, respectively and knee extension torque of an knee joint wearing knee orthosis was increased by 17.68% in averaged peak torque. Based on the effectiveness of the powered knee orthosis, weaken elder people may have benefited from the knee extension motion augmented by the powered knee orthosis during activity of daily living, e.g., stair ascent.     

Validation of the soft tissue restraints in a force-controlled knee simulator

In vitro testing of total knee replacements (TKRs) is important both at the design stage and after the production of the final components. It can predict long-term in vivo wear of TKRs. The two philosophies for knee testing are to drive the motion by displacement or to drive the motion by force. Both methods have advantages and disadvantages. For force control an accurate simulation of soft tissue restraints is required. This study was devised to assess the accuracy of the soft tissue restraints of the force-controlled Stanmore knee simulator in simulating the restraining forces of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). In order to do this, human cadaver knee joints were subjected to the ISO Standard Walking Cycle. The resulting kinematics were monitored when the soft tissue structures were intact, when the ACL and PCL were resected, and when they were simulated by springs positioned anteriorly and posteriorly. The stiffness of the springs was determined from the literature. Two different stiffnesses of springs were used which were 7.24 N/mm (designated as soft springs) and 33.8 N/mm (designated as hard springs). All the intact knees showed displacements that were within the range of the machine. Cutting the ACL and PCL resulted in anterior and posterior motion and internal external rotation that were significantly greater than the intact knee. Results showed that when the ACL and PCL were cut hard springs positioned anterior and posterior to the knee returned the knee to near normal anterior-posterior (AP) motion. Using hard springs in the posterior position in any condition reduced rotational displacements. Therefore using springs in a force-controlled simulator is a compromise. More accuracy may be obtained using springs that are of intermediate stiffness.     

A computerized bioskills system for surgical skills training in total knee replacement

Although all agree that the results of total knee replacement (TKR) are primarily determined by surgical skill, there are few satisfactory alternatives to the 'apprenticeship' model of surgical training. A system capable of evaluating errors of instrument alignment in TKR has been developed and demonstrated. This system also makes it possible quantitatively to assess the source of errors in final component position and limb alignment. This study demonstrates the use of a computer-based system to analyse the surgical skills in TKR through detailed quantitative analysis of the technical accuracy of each step of the procedure. Twelve surgeons implanted a posterior-stabilized TKR in 12 fresh cadavers using the same set of surgical instruments. During each procedure, the position and orientation of the femur, tibia, each surgical instrument, and the trial components were measured with an infrared coordinate measurement system. Through analysis of these data, the sources and relative magnitudes of errors in position and alignment of each instrument were determined, as well as its contribution to the final limb alignment, component positioning and ligament balance. Perfect balancing of the flexion and extension gaps was uncommon (0/15). Under standardized loading, the opening of the joint laterally exceeded the opening medially by an average of approximately 4 mm in both extension (4.1 +/- 2.1 mm) and flexion (3.8 +/- 3.4 mm). In addition, the overall separation of the femur and the tibia was greater in flexion than extension by an average of 4.6 mm. The most significant errors occurred in locating the anterior/posterior position of the entry point in the distal femur (SD = 8.4 mm) and the correct rotational alignment of the tibial tray (SD = 13.2 degrees). On a case-by-case basis, the relative contributions of errors in individual instrument alignments to the final limb alignment and soft tissue balancing were identified. The results indicate that discrete steps in the surgical procedure make the largest contributions to the ultimate alignment and laxity of the prosthetic knee. Utilization of this method of analysis and feedback in orthopaedic training is expected rapidly to enhance surgical skills without the risks of patient exposure.     

The biomechanics of a validated finite element model of stress shielding in a novel hybrid total knee replacement

This study proposes a novel hybrid total knee replacement (TKR) design to improve stress transfer to bone in the distal femur and, thereby, reduce stress shielding and consequent bone loss. Three-dimensional finite element (FE) models were developed for a standard and a hybrid TKR and validated experimentally. The Duracon knee system (Stryker Canada) was the standard TKR used for the FE models and for the experimental tests. The FE hybrid device was identical to the standard TKR, except that it had an interposing layer of carbon fibre-reinforced polyamide 12 lining the back of the metallic femoral component. A series of experimental surface strain measurements were then taken to validate the FE model of the standard TKR at 3000 N of axial compression and at 0 degreeof knee flexion. Comparison of surface strain values from FE analysis with experiments demonstrated good agreement, yielding a high Pearson correlation coefficient of R(2)= 0.94. Under a 3000N axial load and knee flexion angles simulating full stance (0O degree, heel strike (200 degrees, and toe off (600 degrees during normal walking gait, the FE model showed considerable changes in maximum Von Mises stress in the region most susceptible to stress shielding (i.e. the anterior region, just behind the flange of the femoral implant). Specifically, going from a standard to a hybrid TKR caused an increase in maximum stress of 87.4 per cent (O0 degree from 0.15 to 0.28 MPa), 68.3 per cent (200 degrees from 1.02 to 1.71 MPa), and 12.6 per cent (600 degrees from 2.96 to 3.33 MPa). This can potentially decrease stress shielding and subsequent bone loss and knee implant loosening. This is the first report to propose and biomechanically to assess a novel hybrid TKR design that uses a layer of carbon fibrereinforced polyamide 12 to reduce stress shielding.     

An objective tool for assessing the outcome of total knee replacement surgery

The need for an objective tool to assess the outcome of total knee replacement (TKR) surgery is widely recognized. This study investigates the potential of an objective diagnostic tool for assessing the outcome of TKR surgery based on motion analysis techniques. The diagnostic tool has two main elements: collection of data using motion analysis, and the assessment of knee function using a classifier that is based around the Dempster-Shafer theory of evidence. The tool was used to analyse the knee function of nine TKR subjects preoperatively and at three stages post-operatively. Using important measurable characteristics of the knee, the tool was able to establish the level of benefit achieved by surgery and to enable a comparison of subjects. No subject recovered normal knee function following TKR surgery. This has important implications for knee implant designs.     

Long-term results for Kinemax and Kinematic knee bearings on a six-station knee wear simulator

A long-term wear test was performed on Kinemax and Kinematic (Howmedica Inc.) knee bearings on the Durham six-station knee wear simulator. The bearings were subjected to flexion/extension of 65-0 degrees, anterior-posterior translation of between 4.5 and 8.5 mm and a maximum axial load of 3 kN. Passive abduction/adduction and internal/external rotation were also permitted, however, two of the stations had a linkage system which produced +/- 5 degrees active internal/external rotation. The bearings were tested at 37 degrees C in a 30 per cent bovine serum solution and the test was run to 5.6 x 10(6) cycles. The bearings from stations 2 and 3, and stations 4 and 5 were swapped during the test to investigate the effects of interstation variability. The average wear rate and standard error was 3.00 +/- 0.98 mg/10(6) cycles (range 1.33-4.72 mg/10(6) cycles) for the Kinemax bearings and 3.78 +/- 1.04 mg/10(6) cycles (range 1.87-4.89 mg/10(6) cycles) for the Kinematic bearings. There were no significant differences in wear rates between the different bearing designs, the addition of active internal/external rotation or a change of stations. However, the wear tracks were different for the two types of bearings and with active internal/external rotation. The wear rates and factors were generally lower than previously published in vitro wear results; however, this may have been due to a difference in the axial loads and lubricants used. The appearance of the wear tracks with active internal/external rotation was comparable with those seen on explanted knee bearings.     

Knee and hip joint forces--sensitivity to the degrees of freedom classification at the knee

Previous research has demonstrated that the number of degrees of freedom (DOF) modelled at a given joint affects the antagonistic muscle activity predicted by inverse dynamics optimization techniques. This higher level of muscle activity in turn results in greater joint contact forces. For instance, modelling the knee as a 3 DOF joint has been shown to result in higher hip and knee joint forces commensurate with a higher level of muscular activity than when the knee is modelled with 1 DOF. In this study, a previously described musculoskeletal model of the lower limb was used to evaluate the sensitivity of the knee and hip joint contact forces to the DOF at the knee during vertical jumping in both a 1 and a 3 DOF knee model. The 3 DOF knee was found to predict higher tibiofemoral and hip joint contact forces and lower patellofemoral joint contact forces. The magnitude of the difference in hip contact force was at least as significant as that found in previous research exploring the effect of subject-specific hip geometry on hip contact force. This study therefore demonstrates a key sensitivity of knee and hip joint contact force calculations to the DOF at the knee. Finally, it is argued that the results of this study highlight an important physiological question with practical implications for the loading of the structures of the knee; that is, the relative interaction of muscular, ligamentous, and articular structures in creating moment equilibrium at the knee.     

Mechanics of the passive knee joint. Part 2: interaction between the ligaments and the articular surfaces in guiding the joint motion

The aim of this study was to examine how the interaction between ligament tensions and contact forces guides the knee joint through its specific pattern of passive motion. A computer model was built based on cadaver data. The passive motion and the ligament lengthening and force patterns predicted by the model were verified with data from the literature. The contribution of each ligament and contact force was measured in terms of the rotational moment that it produced about the tibial medial plateau and the anterior-posterior (AP) force that it exerted on the tibia. The high tension of the anterior cruciate ligament (ACL) and the geometric constraints of the anterior horns of the menisci were found to be key features that stabilized the knee at full extension. The mutual effect of the cruciates was found as the reason for the screw-home mechanism at early flexion. Past 300, the AP component of contact force on the convex geometry of the lateral tibial plateau and tension of the lateral collateral ligament (LCL) were identified as elements that control the joint motion. From 60 degrees to 90 degrees, reduction in the tension of the ACL was determined as a reason for continuation of the tibial anterior translation. From 90 degrees to 120 degrees, increase in the tension of the posterior cruciate ligament and the AP component of the contact force on the convex geometry of the lateral tibial plateau pushed the tibia more anteriorly. This anterior translation was limited by the constraining effects of the ACL tension and the AP component of the contact force on the medial meniscus. The important guiding role observed for the LCL suggests that it should not be overlooked in knee models.     

Determination of representative dimension parameter values of Korean knee joints for knee joint implant design

Knee joint implants developed by western companies have been imported to Korea and used for Korean patients. However, many clinical problems occur in knee joints of Korean patients after total knee joint replacement owing to the geometric mismatch between the western implants and Korean knee joint structures. To solve these problems, a method to determine the representative dimension parameter values of Korean knee joints is introduced to aid in the design of knee joint implants appropriate for Korean patients. Measurements of the dimension parameters of 88 male Korean knee joint subjects were carried out. The distribution of the subjects versus each measured parameter value was investigated. The measured dimension parameter values of each parameter were grouped by suitable intervals called the "size group," and average values of the size groups were calculated. The knee joint subjects were grouped as the "patient group" based on "size group numbers" of each parameter. From the iterative calculations to decrease the errors between the average dimension parameter values of each "patient group" and the dimension parameter values of the subjects, the average dimension parameter values that give less than the error criterion were determined to be the representative dimension parameter values for designing knee joint implants for Korean patients.     

Muscle-ligament interactions at the knee during walking.

A two-dimensional mathematical model of the knee is used with gait analysis to calculate muscle, cruciate ligament and tibio-femoral contact forces developed at the knee during normal level walking. Ten normal adult subjects--four females and six males--participated. The knee model is based upon a four-bar linkage comprising the femur, tibia and two cruciate ligaments. It takes account of the rolling and sliding of the femur on the tibia during flexion/extension and the changes in direction of the ligaments and muscle tendons. We considered forces transmitted by six elements: quadriceps, hamstrings, gastrocnemius, anterior and posterior cruciate ligaments, and tibio-femoral contact. The equations of mechanics can be used to determine the absolute values of only three of the knee forces simultaneously, so that twenty limiting solutions of three of the six forces were considered. A limiting solution was rejected if any of the three forces were negative, corresponding to compressive muscle or ligament forces, or tensile contact forces. These constraints always reduced and at times removed the redundancy of the knee structures. The high incidence of predicted single muscle activity, supported by electromyography, suggested that the ligaments play a significant role in load transmission during gait. The temporal patterns of muscle and ligament activity and ligament force magnitudes were sensitive to the choice of model parameters. The analysis showed that each of four possible minimum principles of muscle selection--minimal muscle force, muscle stress, ligament force and contact force--was unlikely to be valid throughout the walking cycle.