Abstract
Osteoarthritis (OA) of the knee joint is mostlyso-called primary OA. Secondary OA due to meniscus injury, tibia plateau fracture, femoral condyle fracture, patella fracture, angular deformity of femur and/or lower leg, rheumatoid arthritis and so on is of relatively low incidence. The‘primary’ OA of the knee joint is frequent in aged people. Because of this, it is attributed to primary senile degeneration of the articular cartilage, but is this correct? If it is, primary OA should be seen equally often in all other joints of the body. But‘primary’ OA occurs predominantly in the knee joint and is associated with a varus deformity of the joint. The pathogenesis of‘primary’ OA should be sought elsewhere.
I think that so-called primary OA of the knee joint is really secondary to neuro-muscular dysfunction. If the muscle strength of the body decreases and reflex control of standing posture attenuates, the body may be apt to fail down. To prevent the falling down, it becomes necessary to stand with the legs open to expand the base area of the body. If a person walks with the legs open, the body sways from side to side. Recently I have analyzed influence of this sway on the knee by a biomechanical method.
The following description concerns a person whose left lower extremity is watched from behind during a half step including a left single support phase and the subsequent double support phase.
(1) When a left single support begins, the left acetabulum is pushed upward by a force (f1) . The line of action passes left of the center of gravity of the body segment composed of head, arms, trunk and right lower extremity (HATRE) . The HATRE begins to rotate clockwise around the center. Then the center moves upward and also leftward or rightward according to whether the left lower extremity is adducted or abducted. If the gravity center of HATRE, origin and attachment of the hip abductors, abduction angle of the functional axis of the left lower extremity, and the size of (f1) are known, the force (f2) of the hip abductors necessary to arrest the rotation of HATRE, vertical shifting force (g1) and horizontal shifting force (i1) can be calculated. As the left foot is fixed on the floor f2also pushes the body to the right.
(2) The body segment HATRE sinks during the single support period (t1) by the force of gravity. An additional force (f3) of the left hip abductors is necessary to arrest the gravitational sinking of HATRE. The force f3can be calculated if the mass (M kg) of HATRE is known. As the left foot is fixed on the floor f3also pushes the body to the right.
(3) Even if the rotation of HATRE due to f1, and the sinking of HATRE due to (M kgf) are arrested by f3the body as a whore will fall to the right during the left single support period (t1), if the left foot is located left of the perpendicular line through the center of gravity of the whole body. This resembles the motion of an inverted physical pendulum, though the human body is not so rigid as a statue. The angular velocity at the end of ti can be calculated if the moment of inertia around the gravity center of the whole body, the distance between the left foot and the gravity center, and the initial inclination of the line connecting the left foot and the gravity center are known. The revolution of the whole body around the left foot has vertical and horizontal components of momentum. The sum of the horizontal component of the momentum and the impulse of the above described horizontal forces is approximately the total horizontal momentum of the whole body moving to the right. The horizontal momentum must be checked by the force f4of the right hip abductors in a very short duration of double support phase, otherwise the whole body must actually fall to the right.
