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The synovial membrane is sometimes studded on its inner aspect with loose fleshy or semicartilaginous substances projecting into the cavity of the joint. The entire surface of the membrane is occasionally covered with these bodies, which are of a white or yellowish colour, and very variable in size and shape; the smallest presenting the form of villi not much larger than those of the jejunum, the largest having somewhat the magnitude and appearance of the appendices epiploicæ of the large intestine, while many of an intermediate size approach in appearance to a lemon-seed. In some instances the membrane is only partially pervaded by them, and they are not unfrequently arranged like a fringe around the edge of the articular surfaces. They are generally very smooth on the surface, which appears to be perfectly continuous, if not identical, with the synovial membrane. Their attachment is frequently broad, and occasionally very narrow and pedunculated, often merely filamentous, so that a little further thinning of the part, or slight force acting on the body, would remove it from the capsule, and throw it loose into the cavity of the articulation. The disease has been most frequently seen in the knee, and sometimes in the elbow.

The affection is obscure in its nature; it is slow in its progress; the joint is the seat of pain after and during exercise, probably from the morbid processes interfering with the motions of the articular surfaces. As the disease advances, the joint becomes swelled and elastic, unattended generally by ulceration of the tissues within or around it. In examining the part, when the articular surfaces are moved on each other, it will be found that their motion is more or less interfered with; and considerable irregularity in their action may be felt by the hand placed firmly on the joint during the procedure.

Loose bodies may be found in the articulations, particularly in the knee and elbow, under the circumstances just described; but we often find others of a different structure loose within the capsule, which may be throughout smooth and apparently healthy. These bodies are extremely variable in size, generally rounded or oval, with a polished surface; many of the smallest present the character of fibro-cartilage or cartilage; the larger are often more or less modulated, very firm and gristly, and sometimes contain a considerable nucleus of bony matter. The articular ends of the bones are occasionally more or less misshapen by the formation around them of knobs of adventitious osseous substance.

The disease is common in the knee; and patients frequently go about for years, who will tell you that they have something rolling within the joint, which will often be found on examination to be actually the case. It is obvious that loose bodies in the joint must prove an obstacle to easy progression; and the smaller ones seem to be particularly so, probably from becoming easily entangled by different parts of the articular apparatus.

However difficult it may be to account for the cause of these formations, the means by which they may get within the articulation seems obvious enough. If a small knob of coagulated lymph should form on the inner aspect of the membrane, the point of attachment, as has already been noticed, might gradually become narrower, from the motions of the joint or other causes, till the connection were severed. But there is reason to believe that many of the substances found loose in the joints are formed altogether in the cellular tissue without the capsule; for it is difficult to conceive that they can increase in size, or take on the ossific action in their centre, by an act of independent vitality, after they have been separated from the surrounding parts. Accordingly, dissection has occasionally disclosed loose bodies within the knee-joint, and others of just the same structure outside the synovial membrane, which was however protruded by their pressure, and appeared to connect some of them by a pedicle only, ready to break and allow the foreign body, with its covering of the membrane, to fall into the cavity of the articulation.