A treatise on orthopedic surgery . eformities may be classed assecondary. Secondary Deformities.—Of these the most common is outwardrotation of the tibia upon the femur. When the limb is fullyextended the tibia is fixed, but when it is flexed lateral motionis possible, and in the attitude of flexion the traction of thebiceps upon the head of the fibula tends to rotate it upon thefemur. This deformity is also favored by the use of the limbin the attitude of outward rotation, which is always assumedwhen the weakness or stiffness of the knee-joint is present, andby the secondary knock-knee that-
A treatise on orthopedic surgery . eformities may be classed assecondary. Secondary Deformities.—Of these the most common is outwardrotation of the tibia upon the femur. When the limb is fullyextended the tibia is fixed, but when it is flexed lateral motionis possible, and in the attitude of flexion the traction of thebiceps upon the head of the fibula tends to rotate it upon thefemur. This deformity is also favored by the use of the limbin the attitude of outward rotation, which is always assumedwhen the weakness or stiffness of the knee-joint is present, andby the secondary knock-knee that- often accompanies the disease. 424 OETHOPEDIC SUBGEBY. Subluxation or backward displacement of the tibia upon thefemur is anotheriecondary deformity. WFen tEe leg is flexedupon the thigh the articulating surface of the tibia glides back-ward upon the condyles of the femur. Here it becomes fixed bymuscular contraction, and later by the secondary changes withinthe joint. If muscular spasm is extreme, this alone may cause Fig. Untreated disease of the knee-joint involving tlie shaft of the femur, illus-trating lengthening and the hypertrophy of the femur, the subluxation and out-ward rotation of the tibia, the atrophy and the characteristic deformity. the subluxation; but there are other factors: one is the destruc-tive action of the disease, which is usually most marked at the TUBEECUL0U8 DISEASE OF THE KNEE-JOINT. 425 point at which the bones are in contact, and the other is theleverage exerted upon the joint. This is exemplified by theincrease of the displacement that is often observed when anattempt is made to straighten the limb by force, against theresistance offered by the contracted tissues on the flexor same leverage, in slighter degree, is exerted when the weightof the distorted limb is supported on the heel in the recumbentposture, or when the limb is extended in the act of walking, orif the upper extremity of the tibia is not supported during theperi
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