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We combined mobile biplane X‐ray imaging and magnetic resonance imaging to measure the regions of articular cartilage contact and cartilage thickness at the tibiofemoral and patellofemoral joints during six functional activities: standing, level walking, downhill walking, stair ascent, stair descent, and open‐chain (non‐weight‐bearing) knee flexion. The contact centers traced similar paths on the medial and lateral femoral condyles, femoral trochlea, and patellar facet in all activities while their locations on the tibial plateau were more varied. The translations of the contact centers on the femur and patella were tightly coupled to the tibiofemoral flexion angle in all activities (r2 > 0.95) whereas those on the tibia were only moderately related to the flexion angle (r2 > 0.62). The regions of contacting cartilage were significantly thicker than the regions of non‐contacting cartilage on the patella, femoral trochlea, and the medial and lateral tibial plateaus in all activities (p < 0.001). There were no significant differences in thickness between contacting and non‐contacting cartilage on the medial and lateral femoral condyles in all activities, except open‐chain knee flexion. Our results provide partial support for the proposition that cartilage thickness is adapted to joint load and do not exclude the possibility that other factors, such as joint congruence, also play a role in regulating the structure and organization of healthy cartilage. The data obtained in this study may serve as a guide when evaluating articular contact motion in osteoarthritic and reconstructed knees. [ABSTRACT FROM AUTHOR]

Accurate knowledge of knee kinematics is important for a better understanding of normal joint function and for improving patient outcomes subsequent to joint reconstructive surgery. Limited information is available that accurately describes the relative movements of the bones at the knee in vivo, even for the most common of all activities: walking. We used a mobile X‐ray imaging system to measure the three‐dimensional motion of the entire knee‐joint complex—femur, tibia, and patella—when humans walk over ground at their natural speeds. Data were recorded from 15 healthy individuals (9 males, 6 females; age 30.5 ± 6.2 years). The most pronounced rotational motion of the tibia was flexion‐extension followed by internal‐external rotation and abduction‐adduction (peak‐to‐peak displacements: 70.7°, 9.2°, and 1.9°, respectively). Maximum anterior translation of the tibia was 6.5 mm and occurred in early swing, coinciding with peak knee flexion and peak internal rotation. The most prominent rotational motion of the patella was flexion‐extension (peak‐to‐peak displacement: 50.5°). The tibia pivoted about the medial compartment of the tibiofemoral joint, conferring greater movements of the contact centers in the lateral compartment than the medial compartment (15.4 and 9.7 mm, respectively). Internal‐external rotation, anterior‐posterior translation and medial‐lateral shift of the tibia as well as flexion‐extension and anterior‐posterior translation of the patella were each coupled to the knee flexion angle, as were movements of the contact centers at each joint. These fundamental data serve as a valuable resource for evaluating knee joint function in normal and pathological gait. The data are available in Supplementary_Material_Data.xlsx. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res [ABSTRACT FROM AUTHOR]

We measured the moment arm of the knee-extensor mechanism as ten healthy young individuals performed six functional activities: level walking, downhill walking, stair ascent, stair descent, open-chain (non-weight-bearing) knee flexion, and open-chain knee extension. The moment arm of the knee-extensor mechanism was described by the moment arm of the patellar-tendon force, which acts to rotate the tibia about the instantaneous axis of rotation (screw axis) of the knee. A mobile biplane X-ray imaging system enabled simultaneous measurements of the three-dimensional movements of the femur, tibia and patella during each activity, from which the position and orientation of the screw axis and the patellar-tendon moment arm (PTMA) were determined. Mean PTMA across all activities and all participants remained nearly constant (~46 mm) from 0° to 70° of knee flexion and decreased by no more than 20% at higher flexion angles. The magnitude of the PTMA varied more substantially across individuals than across activities, indicating that the moment arm is more heavily influenced by differences in knee-joint geometry than muscle loading. Hence, PTMA measurements obtained for a given activity performed by one individual may be used with good confidence to describe the PTMA for any other activity performed by the same individual. Caution is advised when using PTMA measurements obtained from one individual to describe the moment arm in another individual even once the data are normalized by knee bone size, as the PTMA varied by as much as 13% from the mean across individuals. [ABSTRACT FROM AUTHOR]

Background: The ability of the quadriceps muscles to extend the knee depends on the moment arm of the knee-extensor mechanism, which is described by the moment arm of the patellar tendon at the knee. The knee-extensor moment may be altered by a change in quadriceps force, a change in the patellar tendon moment arm (PTMA), or both. A change in quadriceps muscle strength after anterior-cruciate-ligament-reconstruction (ACLR) surgery is well documented, however, there is limited knowledge about how this procedure affects the PTMA.Research Question: Does ACLR surgery alter the moment arm of the knee-extensor mechanism during gait?Methods: We measured the PTMA in both the ACLR knee and the uninjured contralateral knee in 10 young active individuals after unilateral ACLR surgery. Mobile biplane X-ray imaging was used to measure the three-dimensional positions of the femur, tibia and patella during level walking and downhill walking over ground. The PTMA was found from the location of the instantaneous axis of rotation at the knee and the line-of-action of the patellar tendon.Results: There was a small but statistically significant difference in the mean PTMA calculated over one cycle of level walking between the ACLR knee and the contralateral knee, with the mean PTMA in the ACLR knee being 1.5 mm larger (p < 0.01). In downhill walking, statistically significant differences were found in the range 15°- 25° of knee flexion, where the PTMA was 4.7 mm larger in the ACLR knee compared to the contralateral knee (p < 0.01).Significance: Significant differences were evident in the mean PTMA between the ACLR knee and the contralateral knee in both activities, however, the magnitudes of these differences were relatively small (range: 3-10%), indicating that ACLR surgery successfully restores the moment arm of the knee-extensor mechanism during dynamic activity. [ABSTRACT FROM AUTHOR]