PATELLA, or KNEE-CAP, is a sesamoid bone (q.v.), the single tendon of the reetus, vastus externus, and rastus interims muscles—the greater extensor muscles of the leg. It is hNtrt-shaped in form, the broad end being directed upwards and the apex downwards. The anterior or external surface is convex, perforated by small apertures for the entrance of vessels, and marked by rough longitudinal.strire, while the posterior or internal surface is smooth and divided into two facets by a vertical ridge, which cor responds and fits into the groove on the lower articulating surface of the femur or thigh bone, while the two facets (of which tie outer is the broader and deeper) corresponds to the articular surface of the two condyles. • This bone is liable both to dislocation and fracture. Dislocation may occur either inwards or outwards; but it is most frequent in the outward direction. The displace ment may be caused either by mechanical violence, or by too sudden contraction of the i extensor muscles in whose conjoined tendon it lies; and is most liable to occur in knock kneed, flabby persons. It may be readily deteCted by the impossibility of bending the knee, and by the bone being felt in its new position•, and, except in one rare variety, the dislocation is capable of being reduced without any difficulty. Fracture of the patella
may (like dislocation) be caused either by muscular action or by mechanical violence.
Fracture by muscular action is the more common of the two forms, and occurs thus: A person in danger of falling forwards attempts to recover himself by throwing the body backwards, and the violent action of the extensors (chiefly the reetus) snaps the patella across, the upper fragment being drawn up the thigh,while the lower portion is retained in situ by that portion of the common tendon which is continued from the patella to the tuberel3 of the tibia, and which is called the ligamentum patellm. The treatment con sists in relaxing the opposing muscles by raising the trunk, and slightly efevating the limb, which should be kept a straight position. In consequence of the great diffi culty of bringing the broken surfaces into exact apposition, it is very difficult to obtain bony reunion of the parts, and the case generally results either in mere ligamentous unioa or in no true union at.all.