THE DERIVATION OF KNEE-JOINT TYPES FROM THE GEOMETRY OF THE CRUCIATE LIGAMENT 4-BAR SYSTEM

The system of the anterior (a) and posterior (p) cruciate ligaments an d their distances between attachments to femur (f) and tibia (t) as fo und in the knee joint of tetrapods is considered as a planar crossed f our-bar linkage. The shape of the femoral articulating surfaces (condy les) can be calculated starting from a flat or curved tibial articulat ing surface and known bar-lengths (Menschik, 1974 Z. Orthop. 112, 481- 495; Huson, 1974 Orthopade 3, 119-126). Regression analysis of the dim ensions of the cruciate ligament four-bar system of 11 species of mamm al and one species of bird revealed a general ratio of (a):(t):(p):(f) = (7.1):(7.9):(10.0):(6.1). These data differ from the results obtain ed by Badoux (1984,Acta Anat. 119, 60-64) who examined only dog and ho rse. Our data of the dog agree with those of Badoux, i.e. (a):(t):(p): (f) approximate to (10):(8):(10):(4). Based on these ratios between ba r-lengths, two types of knee joint shapes were distinguished. The shap e of the dog's joint (''type A'') has a very large femoral condyle com pared with the tibial articulating surface. Maximum knee angulation is 170-180 degrees. Sliding between the articulating surfaces of this jo int is distributed approximately uniformly over the whole angulation r ange. The general shape obtained from the regression analysis (''type R'') has a relatively small femoral condyle and an angulation range of about 174 degrees. Uniformly distributed sliding occurs within this r ange over an angle less than 90 degrees. Theoretically derived, limiti ng requirements concerning maximum angulation range (Delta gamma(max) less than or equal to 180 degrees), stabilization (e.g. avoidance of a perpendicular position of the cruciate ligaments to the articulating surfaces; Delta gamma(78.5) greater than or equal to 90 degrees) and u niformly distributed sliding (Delta gamma(s) greater than or equal to 30 degrees) lead to at least two different possible knee joint shapes. These shapes closely correspond to the two real knee joint shapes fou nd from the statistical analysis mentioned above. This was verified by studying quantitative characteristics obtained from the derivation of knee joint shapes from the bar lengths and vice versa. The bird (Arde a) possessed a knee joint shape, very different from the shapes descri bed above (i.e. f > t, type Dr). (C) 1998 Academic Press.