longed disability and feeling of
insecurity in the knee, is to be found in the wasting and loss of tone
in the quadriceps extensor muscle; the feeling of insecurity is most
marked in coming down stairs. The instability of the joint is often
added to by stretching of the ligaments and lateral mobility. As a
result of both of these factors the joint is liable to repeated
slight strains or jars which irritate the synovial membrane and tend
to keep up the effusion and excite the overgrowth of its tissue
elements.

TUBERCULOUS DISEASE

While tuberculous disease of the knee is specially common in childhood
and youth, it may occur at any period of life, and is not uncommon in
patients over fifty. The disease originates in the synovial membrane
and in the bones respectively with about equal frequency.

When the synovial membrane is diseased, it tends to grow inwards over
the articular surfaces (Fig. 122), shutting off the supra-patellar
pouch and fixing the knee-cap to the femur, and diminishing the area
of the articular surfaces. The ingrowth of synovial membrane may fill
up the cavity of the joint, or may divide it up into compartments.
Ulceration of the cartilage and caries of the articular surfaces are
common accompaniments.

[Illustration: FIG. 122.--Tuberculous Synovial Membrane of Knee,
spreading over articular surface of femur.]

The femur and tibia are affected with about equal frequency, and the
nature and seat of the bone lesions are subject to wide variations.
Multiple small foci may be found beneath the articular cartilage of
the tibia, or along the margins of the femoral condyles--especially
the medial. Caseating foci are comparatively rare, but they sometimes
attain a considerable size--especially in the head of the tibia, where
they may take the form of a caseous abscess. Sclerosed foci, which
form sequestra, are comparatively common (Fig. 123).

[Illustration: FIG. 123.--Lower End of Femur from an advanced case of
Tuberculous Arthritis of the Knee. Towards the posterior aspect of the
medial condyle there is a wedge-shaped sequestrum, of which the
surface exposed to the joint is polished like porcelain.

(Anatomical Museum, University of Edinburgh.)]

#Clinical Types.#--(1) _Hydrops_ usually arises from a purely synovial
lesion, but the joint may suddenly become distended with fluid when an
osseous focus ruptures into the synovial cavity.

It is met with chiefly in young adults. As the fluid accumul