The modes and degrees in which the formation of callus may be arrested, are very various. The quantity formed may be insufficient for its purpose, or there may be none at all: it may undergo the change which is incident to it very tardily, and not be ossified till a very late period; or may be imperfectly, or but partially, ossified: or the exudation, instead of becoming cartilaginous and bony, may assume and retain an apparently ligamentous structure. Such arrests may take place on both fragments, or predominate on one: and further, the anomaly may extend to both the first and second growths of callus, but usually it occurs in the latter only. In such cases, days and even weeks after the occurrence of a simple fracture, neither the bone nor the soft parts around it exhibit well-marked, or indeed any traces of reaction; and if the fracture should at last unite, it does so by the way of suppuration. In other cases, the growth of callus may be insufficient, the masses first formed may meet each other only at a few points, or may even not meet at all; as is the case when there is great displacement of the fragments. The secondary callus may not unite completely with the primary, and may remain soft and cartilaginous, or there may be too little of it formed, especially at the fractured extremities of the bone; in which case, its metamorphosis may be arrested or anomalous; or there may be no callus formed at that part at all.

It is in the last-mentioned circumstances that the cause is found for the formation of new, or false joints. (Articulus novus, spurius, praeternaturalis; pseudarthrosis; articulation surnuméraire of Béclard).

The unnatural joints, which result from fracture, are of two different kinds: one more or less resembles a synchondrosis; the other is like a diarthrosis, and is accordingly, in its proper sense, a new joint.

In the former case, the fractured ends of the bone are held together by a ligamentous tissue. Either a disc of ligament, the thickness of which may vary, is interposed between them, and allows of but little movement; or, as occurs when there has been loss of substance, either from injury, from considerable absorption of the fractured ends, or otherwise, ligamentous bands connect the fragments, and allow them to move freely on each other. The connecting substance appears to be nothing but the substantia intermedia mentioned above, which, as the formation of the secondary callus has failed, or been insufficient, remains in its first state.

In the second case, a ligamentous articular capsule is formed, and is lined by a smooth membrane, which secretes synovia: the fractured surfaces adapt themselves to each other, and become covered with a layer of tissue, which is fibro-ligamentous, or more or less fibrocartilaginous, or which resembles, and sometimes (Howship) really is, cartilage: they may articulate immediately with one another, or may have between them an intervening layer of ligament, which corresponds to an interarticular cartilage; and their movement upon each other is more or less free, according to the size of the articular capsule, and the form of the articulating surfaces. These last are sometimes horizontal and smooth; they glide over each other, and allow of restricted motion: sometimes one surface becomes convex, and the other concave: sometimes both are rounded off, and lying within a capacious articular capsule far apart, they come in contact only during particular movements. The articular capsule is the product of the inflammation of the soft parts: the cartila-giniform layer, which covers the ends of the bone, is secondary callus arrested in its metamorphosis and converted into a fibroid tissue: the other ligamentous cords, which are sometimes present, and the structures resembling an interarticular cartilage, are remnants of the substantia intermedia.

Both forms of new joint, but more particularly the synchondrosal form, have an analogue in the lateral new joints sometimes found between the masses of callus which are thrown out around two adjoining fractured bones. In the forearm and leg, and between the ribs, for instance, new joints are sometimes met with between the masses of callus after fractures have united.

Between that kind of new joint which constitutes a firm synchondrosis, and that which as nearly as possible resembles a natural diarthrosis, there are numerous gradations.

The circumstances which arrest the growth of callus generally, and give rise to the formation of false joints, are as follow: advanced age and senile atrophy of the bones; emaciation in consequence of disease, or loss of the fluids; cachexia generally; diseases of the bones in particular, such as rickets, osteomalacia, too severe inflammation, suppuration, caries, and necrosis at the broken spot; paralysis, and similar affections (on which subject reference may be made to Roechling's experiments on animals); pregnancy; any improper bandage which stops the access of the fluids to the part (a circumstance which is explained by Brodie's experiments of tying the crural artery in animals, in which the femur was broken); inquietude of the limb; considerable displacement of the fragments, and the occurrence of the fracture within the capsule of a joint.

But these conditions do not always put forth their power of arresting the formation of callus and the repair of fractures; certainly they do not always act equally. Thus the fractures of bones affected with rickets or osteomalacia are not very unfrequently united by a mass of callus, which not only is sufficient to repair the fracture, but even has a more perfect internal structure than the other bones: fractures very often unite at every period of pregnancy; and the neglect of quietude is unquestionably rather a grave impediment to the reunion of fragments in proper position than to the formation of callus and the repair of a fracture generally.

Fractures within the capsule of a joint require more particular notice. It is a fact ascertained from much experience, and now established as a rule, that fractures within the capsules of joints very rarely unite completely: the fragments become bound either firmly or loosely together by a ligamentous apparatus; or, their surfaces becoming eburnated or covered with membrane, they form an articulation with each other within the old joint. Many reasons have been assigned for the great deficiency in the formation of primary callus around these fractures, as well as for the arrest of the growth of secondary callus; but which of them is to be regarded as the true and the universally applicable one is not ascertained. As fracture of the neck of the femur is the principal instance of the kind, and is that which has led to all the investigations that have been instituted on the subject, I shall enter more particularly into the consideration of it when treating of Diseases of the Bones of the Lower Extremities.