'Equivalent geometry' of the knee and the prediction of tensions along thecruciates: an experimental study

In this paper we introduce the concept of the functional (or equivalent) ge ometry of the knee, which is an attempt to reduce the natural knee with its complex geometry, frictional resistance and deformable cartilage into a tw o-dimensional joint comprising rigid femur and tibia in frictionless contac t. An apparatus and method are described to measure the slope of the tangen t to the surfaces of the 'equivalent' bones at their 'point' of contact. An antero-posterior force of +/-300-500 N and axial compressive load of twice body weight were applied on cadaveric knee joints. The corresponding displ acement of the tibia in the saggital plane was measured firstly with both c ruciates intact and then when each was severed in turn. From the data obtai ned both the slope of the tangent mentioned above and the tensions develope d along the cruciates under the influence of the forces applied were calcul ated. The results showed that the functional geometry of the knee in the sa ggital plane can be represented by a convex femur and a concave tibia. The tensions along the cruciates calculated on the basis of the experimental me asurements were nearly always lower than the antero-posterior force applied , and although this corroborated the trend demonstrated in a previous theor etical analysis, they were lower still. The reason for this may be the defo rmation of the cartilage under load, thus modifying the geometry of contact resulting in a more concave tibia of the 'equivalent' knee joint, than tha t of the rigid model used in the theoretical analysis. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.