The Bone as a Compression Member in a Cable Tensioning Device: The Example of the Hip
Introduction of the Hip Joint
A joint is introduced into the weight jib at approximately two fifths of its length, which corresponds in its position with the space between the femoral head and the acetabulum (Fig. 2a). In order to stabilize the joint like a node, it has to be braced from above and from below. This is accomplished by the ischium (or ischii) and the wing of the ilium (ala ossis ilii), which in turn must also be braced: The lower tension begins at the base of the neck attachment and leads first to the ischium. This "lower chord" is provided by the rotator muscles, mainly the m. quadratus femoris and the m. obturator externus. The sacrotuberal ligament stretches from the ischium to the sacrum. From there the iliolumbar ligament stretches as an "upper chord" to the iliac crest (crista iliaca). The pelvis as a whole is stretched back by the tractus.
The central part of the weight jib now consists of an osseous cross, formed by the femoral neck (collum femoris), the iliac crest, the thickened lower edge of the ilium (corpus ossis ilii), and the ischium. The cross is braced in a circular manner. The upper area of the tractus corresponds with what we know as the tractus iliotibialis.
Introduction of the Iliosacral Joint
The iliosacral joint is the second joint in the weight jib (Fig. 2b). Its introduction takes into consideration that the sacrum is relatively broad and shows practically rib stumps (pars lateralis). They are only axially loadable. The iliosacral joint is held from above by ligaments in the sacrum and the ilium (ll. sacroiliaca) which, however, require only a little energy. In turn, the tension downward is stronger from the l. sacrotuberale.
|Figure 2: Latticing of the pelvis by introduction of (a) the hip joint and (b) the iliosacral joint; osseous cross with circular bracing|
Introduction of the Knee Joint
|Figure 3: Latticing of leg by introduction of the knee joint.
a) bowlegs; b) genu valgum; c) physiological genu valgum; d) reduced system
To understand the mechanics of the hip joint it suffices to simplify the conditions in the leg, as we have here. Since the hip joint and the knee joint affect one another in case of disease, an explanation of strength relationships in the knee is desirable as far as the frontal plane is concerned. To this point, our interest has generally been limited to the sagittal plane.
In the first step, the lateral condyles of the femur and tibia are combined into a rod that is coupled at right angles to the retaining cable (Fig. 3a). Thus its load approaches zero ("zero rod"), and the existing force values remain unchanged. The tractus now ends aside the knee as the true tractus iliotibialis. The tension is extended down to the foot by the lateral muscles of the lower leg, i. e., the m. tibialis anterior et posterior and the m. peronei. These muscles have no equivalents on the inside of the lower leg.
Since the lateral muscles of the lower leg have to transmit the full tension of the tractus - according to our presentation - they must be very strong. This makes the legs appear as though they are bent outwards (bowed), although femur and tibia are really on one line. Now we will construct the other extreme, putting the knee directly beneath the center of gravity, thus producing knock-knees (genu valgum, fig. 3b). The knee itself thus takes over the support. Correspondingly, the retaining cable ends there. This requires that the tractus dispose of a tension that leads to the center of the knee. This is done basically by the tractus suprapatellaris. The whole system balances on the vertical tibia. Since it no longer needs lateral bracing, the corresponding muscles are only weakly developed, which enhances the impression of knock-knees. Thus, if the lateral muscles of the lower legs are weakly developed the result is knock-knees; if they are strongly developed the results is bowlegs.
Normally, there is a slight bend of some 6% between the tibia and the femur; this is called physiological genu valgum (Fig. 3c). The tractus has to begin both at the center as well as on the side of the knee. This stresses the first tension, the tractus suprapatellaris, more. The lower leg as a whole is moderately stressed.
We cannot say anything detailed about the tractus in its entirety, except that it stretches from the wing of the ilium down to the knee, which is why we introduce the term "tractus iliogenualis". The division downwards into the lateral and medial branches begins at the trochanter.
If it were necessary to show the exact angle between the neck of the femur (CCD angle) and the shaft, the whole leg would have to be drawn. Here, a reduced system is advisable (shown in Fig. 3d). The femur is drawn up to the line of force of the body weight. The intersection will be somewhere between the knee and the foot in a normal leg; it forms the new support. From there a line is drawn to the trochanter, the resulting tractus.
The Anterior (Ventral) Pelvic System
|Figure 4: Anterior pelvic system with inguinal ligament, pubic bone, and m. obturator internus|
In the system we have developed so far, the ilium was secured at its upper edge basically by the l. iliolumbale, which attaches there far dorsally. But the tractus covers the whole edge, so that the ilium ought to be turned outwards. This is prevented by the inguinal ligament, which stretches diagonally down to the pubic bone (Fig. 4). In turn, this is stretched back to the neck attachment by the m. obturator internus, acting as a lower chord.
Since this lower chord is formed here by just one muscle, the lower chord of the posterior system by two, however (m. quadratus femoris and m. obturator externus), the loading of these two systems should be in the same ration, namely P = 0.5 G'. All further considerations therefore refer to the dorsal system.
The M. gluteus maximus as Stretcher of the Tractus
We have developed a scheme of forces for the one-leg stance thus far on the basis of a passive stretching of the tractus. The human being achieves this by allowing his pelvis to tip down on the side of his trailing leg, which makes it rise on the side of his standing leg. This results when one takes the so-called resting position ("at ease").
|Figure 5: The tractus in side-tensioning by the m. gluteus maximus, which stretches to the sacrum and works against a torque that results from the tilting of the trunk|
If the pelvis is to be kept in a straight line the tractus must be shortened. In this position a tensing of the m. gluteus maximus - more precisely, of its upper part - can be felt on the side of the standing leg. Additionally, the back muscles on the trailing leg side become taut, which indicates that the upper body is inclined over the standing foot. Since the tractus has no open end, the gluteal muscle can affect it only with side tension. We define the place for this halfway between the trochanter and the iliac crest. As a counterreaction, the m. gluteus maximus works on the lower area of the sacrum. The tension is transmitted via the sacroiliac ligaments and the iliosacral joint on the trailing leg side to the lateral back muscles. These work against a displacement of the body's center of gravity. Thus, the whole system straightens up a little – the femur in Fig. 5 is now steeper by 1.5°. We did not undertake the erection in the hip joint, but rather in the neck attachment, in order to maintain the geometrical relations established to this point in the pelvis.
The CCD angle is thus greater by 1.5°. The weight jib shortens by 15%, and the stress as a whole decreases. When one is walking, the m. gluteus maximus works cyclically against the inertia mass of the upper body. With women, the relative shifting of the body weight is realized via a shifting of the pelvis (a rolling gait); men swing more with the upper body.
The principle of side-tensioning (stretching) of the tractus gears down the m. gluteus maximus; i. e., it requires only a small amount of energy. It is thus able to act as an erector in the sagittal plane, which differentiates man from animals.
The one-leg stance (with upright pelvis) therefore activates the following muscles, beginning at the bottom: the lateral muscles of the lower leg, the rotators, the m. gluteus maximus, and the back muscles on the side of the trailing leg. The main work is done by the rotators. The many interconnected ligaments (including the tractus) develop a strong spring reaction through which the invested energy is maintained to a great extent for alternating the standing leg.