Motion of the knee after condylar resurfacing--an in vivo study.

The purpose of this study was determine the amounts of rotation and displacement occurring in a relatively unconstrained condylar replacement knee, and to compare the motion with a matched group of normal knees. The motion was measured using an electromagnetic device, the 3-Space Tracker, for various common activities, for 25 normal volunteers and 25 total knee patients. The main variables studied were internal-external rotation of the tibia about its long axis and anterior-posterior displacement of the femoral origin with respect to the tibia, as a function of flexion angle or per cent of gait cycle. The motion of the total knees was very similar to that of the normals. For sitting, standing and free-swing, the knee rotated internally by 5-10 degrees and the femur displaced posteriorly by 9-14 mm, as the knee was flexed from 0 to 90 degrees. For walking and going upstairs and downstairs the absolute values of the rotations and displacements were similar to the above. However, the mean values of rotation were less due to greater variation in the rotation patterns, due probably to the increased laxity of the knee during the swing phases. From these data, taking the mean motions and one standard deviation (S.D.), it is suggested that a knee prosthesis should allow a rotation from minus to plus 12 degrees (a total range of 24 degrees) and an anterior-posterior displacement of 13 mm. It is suggested that these motions be subject to progressive restraint from the neutral position in the manner of the natural knee.     

Coupled motions under compressive load in intact and ACL-deficient knees: a cadaveric study

Knowledge of the coupled motions, which develop under compressive loading of the knee, is useful to determine which degrees of freedom should be included in the study of tibiofemoral contact and also to understand the role of the anterior cruciate ligament (ACL) in coupled motions. The objectives of this study were to measure the coupled motions of the intact knee and ACL-deficient knee under compression and to compare the coupled motions of the ACL-deficient knee with those of the intact knee. Ten intact cadaveric knees were tested by applying a 1600 N compressive load and measuring coupled internal-external and varus-valgus rotations and anterior-posterior and medial-lateral translations at 0 deg, 15 deg, and 30 deg of flexion. Compressive loads were applied along the functional axis of axial rotation, which coincides approximately with the mechanical axis of the tibia. The ACL was excised and the knees were tested again. In the intact knee, the peak coupled motions were 3.8 deg internal rotation at 0 deg flexion changing to -4.9 deg external rotation at 30 deg of flexion, 1.4 deg of varus rotation at 0 deg flexion changing to -1.9 deg valgus rotation at 30 deg of flexion, 1.4 mm of medial translation at 0 deg flexion increasing to 2.3 mm at 30 deg of flexion, and 5.3 mm of anterior translation at 0 deg flexion increasing to 10.2 mm at 30 deg of flexion. All changes in the peak coupled motions from 0 deg to 30 deg flexion were statistically significant (p<0.05). In ACL-deficient knees, there was a strong trend (marginally not significant, p=0.07) toward greater anterior translation (12.7 mm) than that in intact knees (8.0 mm), whereas coupled motions in the other degrees of freedom were comparable. Because the coupled motions in all four degrees of freedom in the intact knee and ACL-deficient knee are sufficiently large to substantially affect the tibiofemoral contact area, all degrees of freedom should be included when either developing mathematical models or designing mechanical testing equipment for study of tibiofemoral contact. The increase in coupled anterior translation in ACL-deficient knees indicates the important role played by the ACL in constraining anterior translation during compressive loading.     

Validation of a non-invasive fluoroscopic imaging technique for the measurement of dynamic knee joint motion

The accurate measurement of the in vivo knee joint kinematics in six degrees-of-freedom (6DOF) remains a challenge in biomedical engineering. We have adapted a dual fluoroscopic imaging system (DFIS) to investigate the various in vivo dynamic knee joint motions. This paper presents a thorough validation of the accuracy and repeatability of the DFIS system when used to measure 6DOF dynamic knee kinematics. First, the validation utilized standard geometric spheres made from different materials to demonstrate the capability of the DFIS technique to determine the object positions under changing speeds. The translational pose of the spheres could be recreated to less than 0.15±0.09 mm for velocities below 300 mm/s. Next, tantalum beads were inserted into the femur and tibia of two fresh frozen cadaver knees to compare the dynamic kinematics measured by matching knee models to the kinematics from the tantalum bead matching—a technique similar to Roentgen stereophotogrammetric analysis (RSA). Each cadaveric knee was attached to the crosshead of a tensile testing machine and vertically translated at a rate of 16.66 mm/s while images were captured with the DFIS. Subsequently, the tibia was held fixed and the femur manually flexed from full extension to 90° of flexion, as the DFIS acquired images. In vitro translation of the cadaver knee using the tensile testing machine deviated from predicted values by 0.08±0.14 mm for the matched knee models. The difference between matching the knee and tantalum bead models during the dynamic flexion–extension motion of the knee was 0.1±0.65°/s in flexion speed; 0.24±0.16 mm in posterior femoral translation; and 0.16±0.61° in internal–external tibial rotation. Finally, we applied the method to investigate the knee kinematics of a living subject during a step ascent and treadmill gait. High repeatability was demonstrated for the in vivo application. Thus, the DFIS provides an easy and powerful tool for accurately determining 6DOF positions of the knee when performing daily functional activities.     

A method for quantifying the anterior load–displacement behavior of the human knee in both the low and high stiffness regions

The anterior load–displacement behavior of the human knee with an intact ACL is characterized by a very low stiffness region initially and a high stiffness region that develops as anterior load is increased. Although this behavior has been well recognized for some time, a method for quantitatively describing the behavior in these two regions based on limits of motion at specific values of anterior/posterior force has not yet been developed. Thus, the purposes of this study were to describe and justify such a method for measuring the laxity and stiffness in both of these regions in the intact knee.

Unique to this study, low stiffness and high stiffness laxities were computed based on three limits of motion for seven cadaveric knees tested at flexion angles ranging from 0° to 90°. Defining the reference position of the tibia relative to the femur, one limit was the 0 N posterior limit which was determined using a specially designed load cycle to reduce uncertainty in establishing a reference position. Defining the upper bound of the load–displacement curve, a second limit was the 225 N anterior limit. A third intermediate limit was the 45 N anterior limit, which was the load that represented the transition from the low stiffness to the high stiffness region. Stiffnesses corresponding to each of the two regions were computed using regression analysis and also estimated based on the laxities. Comparison between the computed and estimated stiffnesses demonstrated that the stiffnesses in both the low and high stiffness regions can be estimated reasonably accurately based on the laxities. Therefore, the 0 N posterior limit and the two laxities are the three quantities needed to describe the load–displacement behavior of the normal knee.     

In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3°) than is prescribed by the displacement standard (5.7°). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.     

A new method to measure post-traumatic joint contractures in the rabbit knee

A new device and method to measure rabbit knee joint angles are described. The method was used to measure rabbit knee joint angles in normal specimens and in knee joints with obvious contractures. The custom-designed and manufactured gripping device has two clamps. The femoral clamp sits on a pinion gear that is driven by a rack attached to a materials testing system. A 100 N load cell in series with the rack gives force feedback. The tibial clamp is attached to a rotatory potentiometer. The system allows the knee joint multiple degrees-of-freedom (DOF). There are two independent DOF (compression-distraction and internal-external rotation) and two coupled motions (medial-lateral translation coupled with varus-valgus rotation; anterior-posterior translation coupled with flexion-extension rotation). Knee joint extension-flexion motion is measured, which is a combination of the materials testing system displacement (converted to degrees of motion) and the potentiometer values (calibrated to degrees). Internal frictional forces were determined to be at maximum 2% of measured loading. Two separate experiments were performed to evaluate rabbit knees. First, normal right and left pairs of knees from four New Zealand White (NZW) rabbits were subjected to cyclic loading. An extension torque of 0.2 Nm was applied to each knee. The average change in knee joint extension from the first to the fifth cycle was 1.9 deg +/- 1.5 deg (mean +/- sd) with a total of 49 tests of these eight knees. The maximum extension of the four left knees (tested 23 times) was 14.6 deg +/- 7.1 deg, and of the four right knees (tested 26 times) was 12.0 deg +/- 10.9 deg. There was no significant difference in the maximum extension between normal left and right knees. In the second experiment, nine skeletally mature NZW rabbits had stable fractures of the femoral condyles of the right knee that were immobilized for five, six or 10 weeks. The left knee served as an unoperated control. Loss of knee joint extension (flexion contracture) was demonstrated for the experimental knees using the new methodology where the maximum extension was 35 deg +/- 9 deg, compared to the unoperated knee maximum extension of 11 deg +/- 7 deg, 10 or 12 weeks after the immobilization was discontinued. The custom gripping device coupled to a materials testing machine will serve as a measurement test for future studies characterizing a rabbit knee model of post-traumatic joint contractures.     

Kinematic motion of the anterior cruciate ligament deficient knee during functionally high and low demanding tasks

The purpose of this study was to determine whether mechanical adaptations were present in patients with anterior cruciate ligament (ACL)-deficient knees during high-demand activities. Twenty-two subjects with unilateral ACL deficiency (11 males and 11 females, 19.6 months after injury) performed five different activities at a comfortable speed (level walking, ascending and descending steps, jogging, jogging to a 90-degree side cutting toward the opposite direction of the tested side). Three-dimensional knee kinematics for the ACL-deficient knees and uninjured contralateral knees were evaluated using the Point Cluster Technique. There was no significant difference in knee flexion angle, but an offset toward the knee in less valgus and more external tibial rotation was observed in the ACL-deficient knee. The tendency was more obvious in high demand motions, and a significant difference was clearly observed in the side cutting motions. These motion patterns, with the knee in less valgus and more external tibial rotation, are proposed to be an adaptive movement to avoid pivot shift dynamically, and reveal evidence in support of a dynamic adaptive motion occurring in ACL-deficient knees.     

The load-bearing area in the knee joint

Measurements were made of the location and size of the contact areas in cadaver knee joints, for a load of 150 Kgf applied for 5 sec down the long axis of the tibia. Results were obtained from a total of 4 knees, considering flexion angles from 0 to 120°. The methods used were to measure directly from castings of the joint cavity; and to calculate from measurements of radii of curvature and joint deflection. Average contact areas for lateral and medial condyles were 1·4 and 1·8 cm² respectively. Areas for the medial condyle were greater than for the lateral condyle and also the areas diminished as flexion angle increased. The implications of the results to contact stresses, joint lubrication and ‘condylar replacement’ knee prosthesis design were discussed.     

Limited rotation of the mobile-bearing in a rotating platform total knee prosthesis

The hypothesis of this study was that the polyethylene bearing in a rotating platform total knee prosthesis shows axial rotation during a step-up motion, thereby facilitating the theoretical advantages of mobile-bearing knee prostheses. We examined 10 patients with rheumatoid arthritis who had a rotating platform total knee arthroplasty (NexGen LPS mobile, Zimmer Inc. Warsaw, USA). Fluoroscopic data was collected during a step-up motion six months postoperatively. A 3D-2D model fitting technique was used to reconstruct the in vivo 3D kinematics. The femoral component showed more axial rotation than the polyethylene mobile-bearing insert compared to the tibia during extension. In eight knees, the femoral component rotated internally with respect to the tibia during extension. In the other two knees the femoral component rotated externally with respect to the tibia. In all 10 patients, the femur showed more axial rotation than the mobile-bearing insert indicating the femoral component was sliding on the polyethylene of the rotating platform during the step-up motion. Possible explanations are a too limited conformity between femoral component and insert, the anterior located pivot location of the investigated rotating platform design, polyethylene on metal impingement and fibrous tissue formation between the mobile-bearing insert and the tibial plateau.     

Excessive compression of the human tibio-femoral joint causes ACL rupture

The knee is one of the most frequently injured joints in the human body. A recent study suggests that axial compressive loads on the knee may play a role in injury to the anterior cruciate ligament (ACL) for the flexed knee, because of an approximate 10 degrees posterior tilt in the tibial plateau (J. Orthop. Res. 16 (1998) 122-127). The hypothesis of the current study was that excessive axial compressive loads in the human tibio-femoral (TF) joint would cause relative displacement and rotation of the tibia with respect to the femur, and result in isolated injury to the ACL when the knee is flexed to 60 degrees , 90 degrees or 120 degrees . Sixteen isolated knees from eleven fresh cadaver donors (74.3+/-10.5 yr) were exposed to repetitive TF compressive loads increasing in intensity until catastrophic injury. ACL rupture was documented in 14/16 cases. The maximum TF joint compressive force for ACL failure was 5.1+/-2.1 kN for all flexion angles combined. For the 90 degrees flexed knee, the injury occurred with a relative anterior displacement of 5.4+/-3.8mm, a lateral displacement of 4.1+/-1.4mm, and a 7.8+/-7.0 degrees internal rotation of the tibia with respect to the femur.     

Anterior cruciate ligament injury induced by internal tibial torsion or tibiofemoral compression

The knee is one of the most frequently injured joints in the human body. Approximately 91% of ACL injuries occur during sporting activities, usually from a non-contact event. The most common kinetic scenarios related with ACL injuries are internal twisting of the tibia relative to the femur or combined torque and compression during a hard landing. The hypothesis of this study was that the magnitudes and types of motion observed after ACL rupture would significantly change from the relative joint displacements present just before ACL injury. Compression or torsion experiments were conducted on 7 pairs of knee joints with repetitive tests at increasing intensity until catastrophic failure. ACL injury was documented in all cases at 5.4±2 kN of TF compression or 33±13 Nm of internal tibial torque. The femur displaced posteriorly relative to the tibia in pre-failure and with a higher magnitude in failure tests under both loading conditions. In compression experiments there was internal rotation of the tibia in pre-failure tests, but external rotation of the tibia after the ACL failed. In torsion experiments, failure occurred at 58±19° of internal tibial rotation, and valgus rotation of the femur increased significantly after ACL injury. These new data show that the joint motions can vary in magnitude and direction before and after failure of the ACL. Video-based studies consistently document external rotation of the tibia combined with valgus knee bending as the mechanism of ACL injury although these motions could be occurring after ACL rupture.     

The coupled motion of the femur and patella during in vivo weightbearing knee flexion

The movement of the knee joint consists of a coupled motion between the tibiofemoral and patellofemoral articulations. This study measured the six degrees-of-freedom kinematics of the tibia, femur, and patella using dual-orthogonal fluoroscopy and magnetic resonance imaging. Ten normal knees from ten living subjects were investigated during weightbearing flexion from full extension to maximum flexion. The femoral and the patellar motions were measured relative to the tibia. The femur externally rotated by 12.9 deg and the patella tilted laterally by 16.3 deg during the full range of knee flexion. Knee flexion was strongly correlated with patellar flexion (R(2)=0.91), posterior femoral translation was strongly correlated to the posterior patellar translation (R(2)=0.87), and internal-external rotation of the femur was correlated to patellar tilt (R(2)=0.73) and medial-lateral patellar translation (R(2)=0.63). These data quantitatively indicate a kinematic coupling between the tibia, femur, and patella, and provide base line information on normal knee joint kinematics throughout the full range of weightbearing flexion. The data also suggest that the kinematic coupling of tibia, femur, and patella should be considered when investigating patellar pathologies and when developing surgical techniques to treat knee joint diseases.     

The effects of knee joint kinematics on anterior cruciate ligament injury and articular cartilage damage

This study determined which knee joint motions lead to anterior cruciate ligament (ACL) rupture with the knee at 25° of flexion. The knee was subjected to internal and external rotations, as well as varus and valgus motions. A failure locus representing the relationship between these motions and ACL rupture was established using finite element simulations. This study also considered possible concomitant injuries to the tibial articular cartilage prior to ACL injury. The posterolateral bundle of the ACL demonstrated higher rupture susceptibility than the anteromedial bundle. The average varus angular displacement required for ACL failure was 46.6% lower compared to the average valgus angular displacement. Femoral external rotation decreased the frontal plane angle required for ACL failure by 27.5% compared to internal rotation. Tibial articular cartilage damage initiated prior to ACL failure in all valgus simulations. The results from this investigation agreed well with other experimental and analytical investigations. This study provides a greater understanding of the various knee joint motion combinations leading to ACL injury and articular cartilage damage.     

The influence of design, materials and kinematics on the in vitro wear of total knee replacements

Debris-induced osteolysis due to surface wear of ultra high molecular weight polyethylene (UHMWPE) bearings is a potential long-term failure mechanism of total knee replacements (TKR). This study investigated the effect of prosthesis design, kinematics and bearing material on the wear of UHMWPE bearings using a physiological knee simulator. The use of a curved fixed bearing design with stabilised polyethylene bearings reduced wear in comparison to more flat-on-flat components which were sterilised by gamma irradiation in air. Medium levels of crosslinking further improved the wear resistance of fixed bearing TKR due to resistance to strain softening when subjected to multidirectional motion at the femoral-insert articulating interface. Backside motion was shown to be a contributing factor to the overall rate of UHMWPE wear in fixed bearing components. Wear of fixed bearing prostheses was reduced significantly when anterior-posterior displacement and internal-external rotation kinematics were reduced due to decreased cross shear on the articulating surface and a reduction in AP displacement. Rotating platform mobile bearing prostheses exhibited reduced wear rates in comparison to fixed bearing components in these simulator studies due to redistribution of knee motion to two articulating interfaces with more linear motions at each interface. This was observed in two rotating platform designs with different UHMWPE bearing materials. In knee simulator studies, wear of TKR bearings was dependent on kinematics at the articulating surfaces and the prosthesis design, as well as the type of material.     

Tibio-femoral movement in the living knee. A study of weight bearing and non-weight bearing knee kinematics using 'interventional' MRI

The aim of this study was to image tibio-femoral movement during flexion in the living knee. Ten loaded male Caucasian knees were initially studied using MRI, and the relative tibio-femoral motions, through the full flexion arc in neutral tibial rotation, were measured. On knee flexion from hyperextension to 120 degrees , the lateral femoral condyle moved posteriorly 22 mm. From 120 degrees to full squatting there was another 10 mm of posterior translation, with the lateral femoral condyle appearing almost to sublux posteriorly. The medial femoral condyle demonstrated minimal posterior translation until 120 degrees . Thereafter, it moved 9 mm posteriorly to lie on the superior surface of the medial meniscal posterior horn. Thus, during flexion of the knee to 120 degrees , the femur rotated externally through an angle of 20 degrees . However, on flexion beyond 120 degrees , both femoral condyles moved posteriorly to a similar degree. The second part of this study investigated the effect of gender, side, load and longitudinal rotation. The pattern of relative tibio-femoral movement during knee flexion appears to be independent of gender and side. Femoral external rotation (or tibial internal rotation) occurs with knee flexion under loaded and unloaded conditions, but the magnitude of rotation is greater and occurs earlier on weight bearing. With flexion plus tibial internal rotation, the pattern of movement follows that in neutral. With flexion in tibial external rotation, the lateral femoral condyle adopts a more anterior position relative to the tibia and, particularly in the non-weight bearing knee, much of the femoral external rotation that occurs with flexion is reversed.     

In vivo determination of normal and anterior cruciate ligament-deficient knee kinematics

The objective of the current study was to use fluoroscopy to accurately determine the three-dimensional (3D), in vivo, weight-bearing kinematics of 10 normal and five anterior cruciate ligament deficient (ACLD) knees. Patient-specific bone models were derived from computed tomography (CT) data. 3D computer bone models of each subject's femur, tibia, and fibula were recreated from the CT 3D bone density data. Using a model-based 3D-to-2D imaging technique registered CT images were precisely fit onto fluoroscopic images, the full six degrees of freedom motion of the bones was measured from the images. The computer-generated 3D models of each subject's femur and tibia were precisely registered to the 2D digital fluoroscopic images using an optimization algorithm that automatically adjusts the pose of the model at various flexion/extension angles. Each subject performed a weight-bearing deep knee bend while under dynamic fluoroscopic surveillance. All 10 normal knees experienced posterior femoral translation of the lateral condyle and minimal change in position of the medial condyle with progressive knee flexion. The average amount of posterior femoral translation of the lateral condyle was 21.07 mm, whereas the average medial condyle translation was 1.94 mm, in the posterior direction. In contrast, all five ACLD knees experienced considerable change in the position of the medial condyle. The average amount of posterior femoral translation of the lateral condyle was 17.00 mm, while the medial condyle translation was 4.65 mm, in the posterior direction. In addition, the helical axis of motion was determined between maximum flexion and extension. A considerable difference was found between the center of rotation locations of the normal and ACLD subjects, with ACLD subjects exhibiting substantially higher variance in kinematic patterns.     

Anterior laxity,lateral tibial slope,and in situ ACL force differentiate knees exhibiting distinct patterns of motion during a pivoting event: A human cadaveric study

Knee instability following anterior cruciate ligament (ACL) rupture compromises function and increases risk of injury to the cartilage and menisci. To understand the biomechanical function of the ACL, previous studies have primarily reported the net change in tibial position in response to multiplanar torques, which generate knee instability. In contrast, we retrospectively analyzed a cohort of 13 consecutively tested cadaveric knees and found distinct motion patterns, defined as the motion of the tibia as it translates and rotates from its unloaded, initial position to its loaded, final position. Specifically, ACL-sectioned knees either subluxated anteriorly under valgus torque (VL-subluxating) (5 knees) or under a combination of valgus and internal rotational torques (VL/IR-subluxating) (8 knees), which were applied at 15 and 30° flexion using a robotic manipulator. The purpose of this study was to identify differences between these knees that could be driving the two distinct motion patterns. Therefore, we asked whether parameters of bony geometry and tibiofemoral laxity (known risk factors of non-contact ACL injury) as well as in situ ACL force, when it was intact, differentiate knees in these two groups. VL-subluxating knees exhibited greater sagittal slope of the lateral tibia by 3.6 ± 2.4° (p = 0.003); less change in anterior laxity after ACL-sectioning during a simulated Lachman test by 3.2 ± 3.2 mm (p = 0.006); and, at the peak applied valgus torque (no internal rotation torque), higher posteriorly directed, in situ ACL force by 13.4 ± 11.3 N and 12.0 ± 11.6 N at 15° and 30° of flexion, respectively (both p ≤ 0.03). These results may suggest that subgroups of knees depend more on their ACL to control lateral tibial subluxation in response to uniplanar valgus and multiplanar valgus and internal rotation torques as mediated by anterior laxity and bony morphology.     

OpenSim as a preliminary kinematic testing platform for the development of total knee arthroplasty implants

The design of a total knee replacement implant needs to take account the complex surfaces of the knee which it is replacing. Ensuring design performance of the implant requires in vitro testing of the implant. A considerable amount of time is required to produce components and evaluate them inside an experimental setting. Numerous adjustments in the design of an implant and testing each individual design can be time consuming and expensive.Our solution is to use the OpenSim simulation software to rapidly test multiple design configurations of implants. This study modeled a testing rig which characterized the motion and laxity of knee implants. Three different knee implant designs were used to test and validate the accuracy of the simulation: symmetrical, asymmetric, and anatomic. Kinematics were described as distances measured from the center of each femoral condyle to a plane intersecting the most posterior points of the tibial condyles between 0 and 135° of flexion with 15° increments. Excluding the initial flexion measurement (∼0°) results, the absolute differences between all experimental and simulation results (neutral path, anterior-posterior shear, internal-external torque) for the symmetric, asymmetric, and anatomical designs were 1.98 mm ± 1.15, 1.17 mm ± 0.89, and 1.24 mm ± 0.97, respectively. Considering all designs, the accuracy of the simulation across all tests was 1.46 mm ± 1.07. It was concluded that the results of the simulation were an acceptable representation of the testing rig and hence applicable as a design tool for new total knees.