KNEE-JOINT (normal anatomy of the). Gr. .yeia ; Lat. gene; Fr. genou ; Germ.
Kniegelenk ; Ital. ginocchio. The knee-joint, the largest joint in the body, results from the articulation of the os femoris with the tibia below and the patella anteriorly. It admits of extensive motion as a ginglymus, to which is added an arthrodial motion, or a small degree of rotation of the leg and foot, when the joint is partly flexed. The articular surfaces are large and complicated, the ligaments numerous, and the joint chiefly superficial ; circumstances necessary to the freedom, stability, and sym metry of the limb, but exposing this important articulation to frequent accident and disease. It is intended here to describe so much of, a, the bones entering into the formation of this joint, and, b, the cartilages, ligaments, &c., as may be necessary to the elucidation of, c, the mechanical functions.
( a.) Bones. The shaft of the os femoris, which in the middle of the thigh is triangular, becomes of a four-sided form as it approaches the knee, .in consequence of the bifurcation of the linea aspera. This rough ridge, which in the middle of the bone forms a prominent pos terior angle, divides on entering its inferior third into two diverging lines which terminate at the convex articulating eminences called condyles; a flat triangular surface of bone is thus left, where the popliteal vessels lie. The outer line is most strongly marked, and gives origin to the vastus extermis and short head of the biceps flexor cruris : the inner line is deficient near the upper part, over which the femoral vessels pass into the ham; it gives attachment below to the vastus internus and adductor magnus. The internal condyle is narrower and more pro jecting behind than the external; and in rela tion to the shaft of the bone, it appears to extend further downwards ; but the natural oblique position of the os femoris brings the condyles nearly horizontal. The greater width of the 1 pelvis in women gives, ceteris paribus, a greater obliquity to the os femoris than in men. The condyles are separated behind by a deep fossa, out of which the crucial ligaments take their origin ; their articulating surfaces are con vex both in the transverse and in the antero posterior directions, until in front they coalesce into one pulley-like surface over which the patella plays in the motions of the joint : this trochlea is convex from above downwards, but concave from side to side; its outer half is more prominent than the inner, and extends higher up the corresponding condyle. Above the
trochlea there is a flattened or slightly depressed surface, upon which the patella partly rests during complete extension of the joint. The thickness of the os femoris from front to hack undergoes little change till the condyles sud denly jut out behind, and the edges of the trochlea rise up in front ; but from side to side the shaft of the bone increases in breadth as it i approaches the knee, the two postero-lateral surfaces winding gradually round to become antero-lateral, at the same time diverging ra pidly to form a smooth slope on the side of each condyle. Towards the posterior part of each of these sloping surfaces, there is an irregular prominence called the tuberosity, for the attach ment of the lateral ligaments ; below which, on the outer condyle, there is a pit for the origin of the popliteus tendon, and a fossa leading upwards and backwards from it which lodges the tendon when the joint is fully flexed. In the lateral aspect of the bone we best see the peculiar curvature of the articulating surface. In two adult, but rather small, specimens before me the inferior part of the outer condyle is a segment of a circle of fourteen lines radius, while the radius of the posterior portion is only seven lines ; similar measurements of the curves of the internal condyle give radii of six and twelve lines respectively : the centre of the smaller circle coincides precisely with the point of attachment of the lateral ligament on each side, and the advantages of this arrangement will appear when we come to consider the functions of the joint. The smooth articulating surface of the trochlea and condyles is, in the recent state, covered with cartilage. Above this surface and in the fossa between the con dyles are numerous foramina for the transmis sion of the nutrient vessels of the bone, the internal structure of which is here made up of minute cancelli. The lower extremity of the os femoris is cartilaginous at birth, becomes ossified from a separate centre, and long conti nues to form an epiphysis ; but it is ultimately joined to the shaft by perfect bony union.