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The Aequos G1 Knee

  For more than 120 years it has been known that the joint surfaces of the human knee femur predominantly roll while flexing from full extension to about 40° of flexion; and, for the remaining degree of flexion a combination of rolling and gliding occurs.  In the stance phase, the femur rolls backwards over the tibia plateau, while fur ther flexion causes increased gliding. If a prosthesis was designed where both, the medial and lateral tibial surfaces have concave shapes (picture A), then the femur would necessarily rotate around its own centre of rotation and also around the centre of rotation of the tibial joint surfaces. The resulting instantaneous centre of

rotation of the joint does not allow a predominant rolling. However, this design would have the benefit of high flexion angles. Bilateral convexity (picture B) with pure rolling as a consequence would, however, limit the range of motion to small flexion angles because of the limited space. Therefore in the human knee rolling and gliding is combined. The design of the Aequos G1 Knee Endoprosthesis combines both principles of construction. Medially the tibia is concave and laterally is convex.This kinematics is based on the principle of a quadruple joint. The four morphological axes of rotation are the midpoints of the curva tures of the medial and lateral femoral con dyles and the medial and lateral tibia plateau.  In addition, the medial and lateral compart ments are shifted a few millimeters in a sagittal direction, the medial tibia plateau being concave and the lateral plateau convex.
In most knee arthroplasties these factors are not taken into account; instead they are equipped with symmetrical medial and lateral joint surfaces. Within the framework of the authorization tests, the endoprosthesis was examined in a knee simulator according to ISO/WC 14243 (standards).  The abrasion rates were,thereby lower than, or at least as good as, those of conventional endoprostheses.  In principal, nature uses the same mechanism to reduce wear and load of the cartilage during walking or running.
At the same time, it makes use of the anterior posterior translation of the femur (“roll back”) to increase leverage forces for the extension mechanism and, to minimize contact forces of the patellar femoral joint.  The Aequos G1 Knee Endoprosthesis imitates the natural roll-glide mechanism by optimizing surface radii while positioning the joint compartments in the appropriate way.The Aequos G1 Knee Endoprosthesis has been developed to reconstruct the motion of the natural knee.

Dr. Hans Nägerl, MD
University Hospital Göttingen
Göttingen (Germany)

 

Karl-Heinz Frosch, MD
Asklepios Clinic St. Georg
Hamburg  (Germany)

 



 

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