The Bone as a Compression Member in a Cable Tensioning Device: The Example of the Hip
Clinical Application of Our Model
Regarding the knee, we have already said that the size of the axis kink between tibia and femur is predetermined by the development of the lateral muscles of the lower legs. In the hip area this role is obviously played by the rotators. Thus, the neck attachment becomes the critical node in the load-bearing member. The respective cycle of forces is shown in figure 6: The femoral force pushes to the left and upwards on the node, while under the pressure of the tractus, the trochanter acts almost horizontally to the right. Being the strongest force, the neck shifts to the left and downwards (case a, broken line). The cycle is closed by the pull of the rotators to the right.
|Figure 6: Cycle of forces for the node "neck attachment" with varying tension of the rotators
a) Normal case; b) tension halved: steep neck; c) tension doubled: flat neck
Let us now assume that the rotators can produce only half of the required force; we divide the arrow for the rotator force in half (case b). The arrow for the neck force thus becomes relatively steep, and its length decreases a little. As a contrasting possibility, we double the tension of the rotators (case c). The arrow for the neck force becomes relatively flat and considerably longer. To a great extent, the latter example also applies to the other elements of the pelvis. The response of the bone depends on whether it is still growing or not. In children, the neck will grow according to the predetermined direction of force (Fig. 7). If the neck is steep (coxa valga) the acetabulum can follow to only a limited extent, and in an emergency it is not able to hold the femoral head (congenital dislocation of the hip joint).
|Figure 7: Anatomical relationships within the pelvis according to variations of forces shown in fig. 6|
As a remedy, the femoral neck is operatively flattened. In the years that follow it straightens up again, since the relationship of forces has not changed. If surgery is performed again shortly before growth stops, or even later, the neck will remain in the position imposed. The femoral head moves against the upper edge of the acetabulum and is worn down when the ability to regenerate tissue slackens with increasing age ("arthrosis of the upper edge"). This applies as well when the tension of the rotators becomes weaker later in life, or when the muscles react more slowly. In addition, bending comes into play, so that the medial condyles of the knee are more strongly stressed. Their detrition leads to bowlegs.
The reverse case, when the rotators pull very strongly and the femoral neck is accordingly flat, seems to present no problem to children. However, if these changes begin in later life, two possibilities arise: The head rubs against the lower edge of the acetabulum ("arthrosis of the lower edge"), or it penetrates the floor of the acetabulum due to the increased force of the neck (protrusion). The lateral condyles are stressed more at the knees.
The Structure of Trabeculae
Thus far we have assumed the femur, because of its walking-stick shape, to be a crane that stands bent. The neck should be braced from the inside by its trabeculae, whereby the upper trabeculae should take up tension and the lower pressure.
Our model considers the neck and shaft to have two rigidly coupled compression rods instead, being the center of a larger crane system which is braced from the outside. The trabeculae first form a dome in the head of the joint and with their continuing ends support the articular facet. This should apply to the cancellous mass in all joints. Thus, trabeculae are generally stressed by pressure.
Comment by the editor:
 It seems that the relevant facts of compression and tension in the bones are interpreted quite differently by engineers and osteologists. The osteologist will point out that in a biological structure only the bending stress could evoke an adaption effect. (This view is also shared by the electro-physiological scientists; see p. 171). The engineer will note - with the help of computer modelling and by thought - that the effect of all relevant muscles and ligaments will show a macroscopic dominance of compression as the typical stress of bone (mixed with biological significant bending stress and tension stress in the bone structure (see piezo-activity)).
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