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Mechanism of Knee Flexion Differs Between Morphological and Physiological Measurements

Key findings

  • This study analyzed the physiological contact point motions on the medial and lateral femoral condyles and compared the results with the corresponding morphological femoral condyle translations at different flexion intervals along the knee's flexion path
  • The morphological and physiological methods revealed different kinematics features of the knee and suggested different mechanisms of axial femoral condyle/tibial rotation
  • Specifically, morphological data indicated a larger posterior translation of the lateral femoral condyle than the medial condyle, implying a medial pivoting rotation
  • In contrast, physiological articular contact motion showed larger articular contact distances on the medial femoral condyle, implying a lateral pivoting rotation
  • These data should be useful for improvement of surgical techniques intended to restore normal articular motion of the knee

Most studies of knee flexion have concluded that the characteristic movement pattern is the medial-pivoting axial rotation of the femoral condyle. Various total knee arthroplasty (TKA) systems have been developed based on this conclusion. However, medial-pivoting axial rotation of the femur has not been noted in all studies of knee movement.

Massachusetts General Hospital researchers previously compared morphological measures of knee motion (femoral condyle motion on the tibial plane) with physiological measurements (tibial articular contact kinematics). They reported in the Journal of Biomechanics that morphological femoral condyle translations and axial rotations varied with different flexion axes and didn't represent the physiological articular contact kinematics.

Extending that research, authors from the Orthopedic Bioengineering Research Center of Newton-Wellesley Hospital, including Guoan Li, PhD, director of the Center, Chaochao Zhou, PhD, and Zhenming Zhang, PhD, research fellows, and Hany Bedair, MD, chair of the Center for Hip & Knee Replacement in the Department of Orthopaedic Surgery at Mass General, and colleagues analyzed femoral condyle articular contact motion and investigated factors that could be associated with its axial rotation.

In the Journal of Biomechanics, they report that asymmetric articular contact motions on the femoral condyles represented the physiological motion characteristics of the femur, which were different from those observed from translations of morphological femoral condyle axes during knee flexion.


In the previous study, 20 healthy patients (14 men, six women, mean age 33) had one knee imaged with MRI and dual fluoroscopy during weight-bearing flexion from 0° to 120° (15° increments). A 3D model was constructed for each knee.

In this analysis, tibiofemoral cartilage contact points were determined at each flexion position to represent physiological knee motion. The distances between consecutive contact points were measured on each condyle. Physiological axial rotation of the knee was calculated.

Anteroposterior condyle motion was measured using two popular morphological flexion axes, the trans-epicondyle axis and the geometric center axis. The differences between medial and lateral condyle motions were used to calculate the morphological axial rotations of the knee.


The key findings were that:

  • When morphological flexion axes were used, smaller posterior translations were measured for the medial condyle than the lateral condyle during knee flexion. This suggests a medial-pivoting axial rotation of the condyle
  • When physiological contact motion measurements were used, contact distances on the medial condyle surface were larger than on the lateral condyle surface. This implies a lateral-pivoting axial rotation

Applying the Findings

The specific measurements given in the paper could be useful references for planning TKA and other surgeries intended to restore normal articular motion of the knee and for evaluating patients afterward. They could also inform the development of new TKA systems.

In addition, since the data on physiological articular contact motion of the knee is coordinate system–independent, it could provide a consistent reference for basic and clinical research into physiological knee kinematics, such as robotic-assisted knee surgeries.

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