Theodore W. Rudy

The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL

This study investigated the effect of hamstring co-contraction with quadriceps on the kinematics of the human knee joint and the in-situ forces in the anterior cruciate ligament (ACL) during a simulated isometric extension motion of the knee. Cadaveric human knee specimens (n = 10) were tested using the robotic universal force moment sensor (UFS) system and measurements of knee kinematics and in-situ forces in the ACL were based on reference positions on the path of passive flexion/extension motion of the knee. With an isolated 200 N quadriceps load, the knee underwent anterior and lateral tibial translation as well as internal tibial rotation with respect to the femur. Both translation and rotation increased when the knee was flexed from full extension to 30 of flexion; with further flexion, these motion decreased. The addition of 80 N antagonistic hamstrings load significantly reduced both anterior and lateral tibial translation as well as internal tibial rotation at knee flexion angles tested except at full extension. At 30 of flexion, the anterior tibial translation, lateral tibial translation, and internal tibial rotation were significantly reduced by 18, 46, and 30%, respectively (p<0.05). The in-situ forces in the ACL under the quadriceps load were found to increase from 27.8+/-9.3 N at full extension to a maximum of 44.9+/-13.8 N at 15 of flexion and then decrease to 10 N beyond 60 of flexion. The in-situ force at 15 was significantly higher than that at other flexion angles (p<0.05). The addition of the hamstring load of 80 N significantly reduced the in-situ forces in the ACL at 15, 30 and 60 of flexion by 30, 43, and 44%, respectively (p<0.05). These data demonstrate that maximum knee motion may not necessarily correspond to the highest in-situ forces in the ACL. The data also suggest that hamstring co-contraction with quadriceps is effective in reducing excessive forces in the ACL particularly between 15 and 60 of knee flexion.

The effect of anterior cruciate ligament graft fixation site at the tibia on knee stability: Evaluation using a robotic testing system

Despite its current popularity and relative success, endoscopic reconstruction of the anterior cruciate ligament (ACL) using a bone-patellar tendon-bone (BPTB) graft has not yet been perfected. Using a recently developed robotic/UFS testing system, we assessed the overall stability of porcine knees following ACL reconstruction with different sites of tibial graft fixation--proximal, central, and distal. Testing of the intact knee was performed first to determine the normal anterior-posterior (A-P) displacements and in situ forces of the ACL under 110 N of anterior tibial loading of 30 degrees, 60 degrees, and 90 degrees of knee flexion. The knee was then reconstructed with a BPTB autograft, and the distal end of the graft was fixed sequentially at three different locations in each specimen--proximal, central, distal. A-P testing was repeated for each fixation site, and the resulting knee kinematics and the in situ forces of the grafts were compared to the intact case. The site of tibial fixation was demonstrated to have a significant effect on the resulting anterior displacement and internal rotation of the tibia as well as the in situ forces of the graft. Proximal fixation produced the most stable knee (A-P displacements reduced to 120% of intact at 30 degrees and 170% at 90 degrees), becoming significantly less stable with more distal fixation. These results suggest that proximal graft fixation may provide the most acute stability of the reconstructed knee.

A combined robotic/universal force sensor approach to determine in situ forces of knee ligaments.

We developed a system that uses a 6-degree-of-freedom (6-DOF) robotic manipulator combined with a 6-DOF force-moment sensor and a control system. The system is used to find and record the passive knee flexion path for controlling the knee flexion positions. It is also used to strain a knee structure by finding a multiple-DOF path in response to specific joint loading, e.g. anterior-posterior tibial force application. It is additionally used to measure in-situ forces in ligaments by recording differences in forces and moments when repeating a prerecorded path, both before and after removal of the ligament of interest. Example applications are included in the study.

Relative contribution of the ACL, MCL, and bony contact to the anterior stability of the knee.

Abstract Ligaments and other soft tissues, as well as bony contact, all contribute to anterior stability of the knee joint. This study was designed to measure the in situ force in the medial collateral ligament (MCL), anterior cruciate ligament (ACL), posterolateral structures (PLS), and posterior cruciate ligament (PCL) in response to 110 N anterior tibial loading. The changes in knee kinematics associated with ACL deficiency and combined MCL+ACL deficiency were also evaluated. Utilizing a robotic/universal force-moment sensor system, ten human cadaveric knee joints were tested between 0° and 90° of knee flexion. This unique testing system is designed to determine the in situ forces in structures of interest without making mechanical contact with the tissue. More importantly, data for individual structures can be obtained from the same knee specimen since the robotic manipulator can reproduce the motion of the intact knee. The in situ forces in the ACL under anterior tibial loading to 110 N were highest at 15° flexion, 103 ± 14 N (mean ± SD), decreasing to 59.2 ± 30 N at 90° flexion. For the MCL, these forces were 8.0 ± 3.5 N and 38.1 ± 25 N, respectively. Forces due to bony contact were as high as 34.1 ± 23 N at 30° flexion, while those in the PLS were relatively small at all flexion angles. Combined MCL+ACL deficiency was found to significantly increase anterior tibial translation relative to the ACL-deficient knee only above 60° of knee flexion. These findings confirm the hypothesis that there is significant load sharing between various ligaments and bony contact during anterior tibial loading of the knee. For this reason, the MCL and osteochondral surfaces may also be at significant risk during ACL injury.

Biomechanics of the ACL: Measurements of in situ force in the ACL and knee kinematics

Injury of the anterior cruciate ligament ACL can lead to knee instability associated with damage to other knee structures and the increased risk of degenerative joint disease. This has led to the frequent use of intra-articular tissue grafts for ACL reconstruction in an attempt to restore normal knee function. Despite moderate success, the continued failure of ACL reconstruction to restore normal knee kinematics has led many investigators to study the role played by the ACL in normal knee motion. At our research center, we have focused on the development of a new and innovative approach to measure . multiple degree of freedom DOF knee kinematics and to determine the in situ forces within the ACL. A unique testing . system utilizing a 6-DOF robotic manipulator and universal force)moment sensor UFS has been developed such that these measurements can be made in a non-contact fashion while allowing a series of experiments to be performed on the same knee. In this manuscript, we will describe the functional and mathematical development of the roboticrUFS system and its use in a series of studies designed to give insight into the function of the ACL and ACL grafts. Our first study investigated the effect of constrained vs. unconstrained knee motion on anterior tibial translation and on the in situ force in the ACL. We found that unconstrained multiple-DOF knee motion significantly increased anterior tibial translation. While the magnitude of the in situ force remained similar to the more constrained condition, its direction, point of application and distribution between the . . anteromedial AM and posterolateral PL bundles were found to significantly change. These findings led us to investigate the effect of knee flexion angle and magnitude of anterior applied tibial load on the in situ force in the ACL and its bundles during unconstrained knee motion. We found the PL bundle of the human ACL to carry a greater proportion of the in situ force than the AM bundle near knee extension. Also, the change in magnitude of the in situ force in the PL bundle with changing knee flexion angle was similar to that of the entire ACL. This led us to conclude that the PL bundle must play a significant role in ACL function and in resisting anterior tibial load and that it should receive more serious consideration during ACL reconstruction. Lastly, we used our new testing system to examine two popular ACL reconstruction techniques: . . bone)patellar tendon)bone BPTB and quadruple semitendinosusrgracilis hamstring QSTrG grafts. We compared them in terms of restoration of anterior tibial translation and reproduction of the in situ force in the intact ACL. Each reconstruction was performed on the same knee, allowing us to minimize interspecimen variability and take advantage of paired statistical analysis. We found that while both reconstructions effectively reduced anterior tibial translation secondary to anterior tibial loading to a level not significantly different from the ACL intact knee, use of a QSTrG graft may be advantageous, as it reproduced the in situ forces of the intact ACL more closely. This series of three studies has garnered quantitative data to further our understanding of how the ACL functions and has yielded new concepts to help improve ACL reconstruction. Through the use of a robotic manipulator, our testing system has the capability to reproduce complex physiologic loading conditions that allow us to evaluate the efficacy of various ACL reconstruction techniques and

The effect of the point of application of anterior tibial loads on human knee kinematics.

Coupled axial tibial rotation in response to an anterior tibial load has been used as a common diagnostic measurement and as a means to load the ligamentous structures during laboratory tests. However, the exact location of the point of application of these loads as well as the corresponding sensitivity of the coupled tibial rotation to this point can have an effect on the function of the soft tissues at the joint. Therefore, the purpose of this study was to determine the effects of four different points of application of the anterior tibial load on the anterior tibial translation and coupled axial tibial rotation. The four points include: (1) geometric point - midway between the collateral ligament insertion sites on the tibia, (2) clinical point - a position that attempts to simulate clinical diagnostic tests, (3) medial point - a position medial to the geometric point and (4) lateral point - a position lateral to the clinical point. A robotic/universal force-moment sensor testing system was used to apply the anterior tibial load at the four points of application and to record the resulting joint motion. Anterior tibial translation in response to an anterior tibial load of 100N was found not to vary between the four points of application of the anterior tibial load at all flexion angles examined. However, internal tibial rotation was found for the lateral point (13+/-10 degrees at 30 degrees of knee flexion) in all specimens and clinical point (8+/-10 degrees at 30 degrees of knee flexion) while external rotation resulted when the load was applied at the medial point (-8+/-7 degrees at 30 degrees of knee flexion). Both internal and external tibial rotations occurred throughout the range of flexion when the tibial load was applied at the geometric point. The results suggest that the clinical point should be used as the point of application of the anterior tibial load whenever clinical examinations are simulated and multi-degree-of-freedom joint and soft tissue function are examined.

Improvement of Accuracy in a High-Capacity, Six Degree-of-freedom Load Cell: Application to Robotic Testing of Musculoskeletal Joints

AbstractThis study investigated a previously unaccounted for source of error in a high-capacity, six degree-of-freedom load cell used in multi-degree-of-freedom robotic testing of musculoskeletal joints, an application requiring a load cell with high accuracy in addition to high load capacity. A method of calibration is presented for reducing the error caused by changes in universal force-moment sensor (UFS) orientation within a gravitational field. Uncorrected, this error can exceed a magnitude of 1% of the full-scale load capacity—the manufacturer-stated accuracy of the UFS. Implementation of the calibration protocol reduced this error by approximately 75% for a variety of loading conditions. This improvement in load cell accuracy (while maintaining full load capacity) should improve both the measurement and control of specimen kinetics by robotic/UFS and other biomechanical testing systems.