294 THE ARTICULATIONS OR JOINTS. 
ending, a certain amount of screw movement or rotation. Flexion and rotation occur at the 
femoro-tibial sections of the joint, whereas the movement at the femoro-patellar portion produces 
a regulating and controlling influence upon flexion. 
Taking these factors separately, we observe that each condyle adapts itself to a shallow cup 
formed by the head of the tibia and the corresponding semilunar cartilage, and as the two 
condyles move simultaneously and parallel to each other, there is more than the characteristic 
hinge-joint action, for each condyle glides and rolls in its cup “like a wheel restrained by a 
drag” (Goodsir) when the movement of bending occurs. Thus the different parts of the condyles 
are successively brought into relation with the transverse axis of the joimt while it passes from 
extension to flexion and vice versa. From the fact that the internal condyle is longer than the 
external, it is believed that extension is completed by a movement of rotation whereby the joint 
becomes locked, and the anterior crucial, the posterior and the lateral ligaments, become tense. A 
similar rotation initiates the movement of flexion, and unlocks the joint by relaxing the liga- 
ments just mentioned. 
Since the tibia and foot are fixed in the act of walking, it is the femur which rotates upon 
the tibia in passing from extension to flexion and vice versa ; and as relaxation of the ilio-femoral 
ligament is essential for this rotation, some observers are of opinion that the body weight falls 
behind the transverse axis of the knee-joint, as in the case of the hip-joint, and consequently that 
extension of the knee-joint is maintained by the ilio-femoral hgament, as it is not possible to 
bend the knee without first having bent the hip-joint. 
During flexion and extension the semilunar cartilages glide along with the condyles, so as to 
maintain their close adaptation and preserve their value as packing agents. When the movement 
of flexion is completed, the condyles are retained upon the tibia, and prevented from slipping off 
by the tension of the posterior crucial ligament. In this position a small degree of rotation of 
the tibia, both inwards and outwards, is also permissible. 
The regulating and controlling influence of the femoro-patellar portion of the articulation is 
brought into play during the movements of flexion and extension. In the latter position the 
inferior pair of patellar facets is in apposition with the upper part of the femoral trochlea. As 
flexion Pieanese the middle pair of facets adapt themselves to a deeper area of the trochlea, into 
which the patellar keel fits. When flexion is still further advanced, the upper pair of patellar 
facets will be found fitting into that part of the trochlea adjoining the intercondyloid notch ; 
and finally, when flexion is complete, the patella lies opposite the intercondyloid notch, while 
the forward thrust of the longer internal condyle brings its semilunar facet (Goodsir) into 
apposition with the somewhat vertical facet at the inner border of the patella. The wedge-like 
influence of the patella is most marked, for it is only in the position of extension that it can be 
moved from side to side. The movements of the patella may be described as ghding and 
co-aptation, as it slips or rocks from one pair of facets to another in its progress along the trough * 
of the femoral trochlea. 
THE TIBIO-FIBULAR JOINTS. 
The upper and lower ends of the fibula articulate with the tibia. Primarily, 
the fibula is required to form a strong lateral support for the ankle-joint, and 
therefore its articulations are so arranged as to provide a certain amount of 
elasticity without any sacrifice of the rigidity necessary for security. Hence the 
amount of movement is very small, but what there is, enables these joints to be 
classified as arthrodial diarthroses. 
The superior tibio-fibular joint (articulatio tibio-fibularis) is formed, on the 
one hand by a flat oval or circular facet which 1s situated upon the postero-external 
aspect of the outer tuberosity of the head of the tibia, and is directed downwards 
and backwards; on the other, by a similar facet on the upper surface of the head 
of the fibula in front of the styloid process. 
A fibrous capsule (capsula articularis, Fig. 224) invests the joint, and it may 
be regarded as holding the articular surfaces in apposition, although certain 
special bands receive separate designations. Occasionally there is an opening in the 
capsule by which communication is established between the joint-cavity and the knee- 
joint through the intermediation of the synovial prolongation, subjacent to the 
tendon of the popliteus muscle. 
The anterior superior tibio-fibular ligament (lig. capituli fibule anterius, Fig. 223) 
is a strong flat band whose fibres extend from the anterior aspect of the fibular 
head, upwards and inwards, to the adjoining part of the tuberosity of the tibia. 
The posterior superior tibio-fibular ligament (lig. capituli fibule anterius, 
Fig. 224) is a similar, but weaker band, passing upwards and inwards from the 
posterior aspect of the fibular head to the posterior aspect of the outer tuberosity 
of the tibia, where they are attached immediately below the opening in the 
capsule of the knee-joint, from which the tendon of the popliteus muscle escapes. 
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