Superficial Necrosis

In superficial necrosis, the inflammation that takes place in the bone around leads to an exudation, which afterwards ossifies upon its surface under the periosteum; and as the inflammation extends more deeply, reaching through the whole thickness of the bone to the inner surface of the medullary canal, and to the spongy diploetic tissue, bone is deposited in those parts also. At the same time, pus and granulations are produced beneath the dead lamella, and the latter form a basis for a new layer of bone. The peripheral exudations first mentioned very frequently grow to considerable dimensions. The earlier the sequestrum is completely removed, so much the sooner does the suppuration cease, and with it the formation of granulations. The latter unite with the cicatrix of the soft parts, and as only a thin layer of them ossifies within the cavity, the scar is generally depressed, and is rendered still more so by the heaping up of exudation upon the healthy bone around.

The muscles remain connected with the old bone for some time after it is dead, reaching it through the openings which are left in the new: but they gradually separate from it, and become implanted in the newly-formed bone (Meckel).

In internal necrosis, new osseous matter, generally in considerable quantity, is furnished, chiefly by the outer surface of the bone, and deposited beneath the periosteum, while that membrane becomes closely adherent to the inflamed soft parts around. In the interior of the bone, the separation of the dead piece from the living proceeds in the manner already described, and at length the sequestrum is found enclosed in a shell which consists of a layer of the old bone and a stratum of newly formed osseous substance, and is lined by granulations. This is the sequestral capsule. If this form of necrosis, occurring in a long bone, should involve only a portion of its shaft, the rest of the medullary canal becomes filled up with new bone: should it extend the whole length of the shaft, the exudation then occupies the adjoining spongy tissue of the epiphysis.

The sequestral capsule, so long as the sequestrum remains in it, is perforfted by openings, which vary in number, size, and form, and are named cloacoe by Weidmann, and by Troja, foramina grandia.

Their usual form is round or oval; in size they equal a pea or a bean; and they lead into the cavity of the sequestral capsule directly, or somewhat obliquely through a short funnel-shaped canal: the inner opening of the cloaca is the narrower, and the outer, which is the wider and the more dependent, runs out, in well-marked cases, into a low rim, the lips of which are rounded off. Besides these, there are sometimes other irregular gaps in the new bone, which may be small or very extensive, and the margins of which are sinuous. Most of these openings communicate with abscesses in the soft parts which open externally, whilst others are continuous immediately with fistulous canals that lead outwards through the soft parts.

The openings last named are situated at parts where the necrosis, as it extended from within outwards, has advanced to the outermost lamella of the bone, and where consequently no new bone has been deposited; whilst those first described are formed in connection with the suppuration, and they serve for the discharge of the matter, and for the escape of the sequestrum.

All these openings are lined with granulations which secrete pus, and are continuous with the inner membrane of the sequestral capsule.

As soon as the sequestrum is removed from its shell, the granulations rise from the inner surface of the cavity and fill it, and the secretion of matter ceases. This mass then gradually ossifies, and instead of a medullary tube there is produced a solid cylinder of bone. At a later period, a gradual enlargement which takes place in the Haversian canals of the new bone, changes its structure into cellular, and incompletely supplies the place of a medullary tube.

Thus, when the process of regeneration is completed, the bone consists, from without inwards, of the bone exuded beneath the periosteum, of a layer of the old bone, and lastly, of the central mass of bone which has been produced in the granulations, and which fills up the medullary tube.

At first it has a coarse exterior, is misshapen, thick, and uneven, and a marked boundary is clearly distinguishable between its outermost layer and the surface of the old bone: but as the outer layer becomes more dense and homogeneous with the old bone, this boundary line gradually disappears, the whole cylinder gradually loses its misshapen, thick, and coarse look, and acquires the natural form. This, however, is far from being invariably the case, for sometimes at irregular spots adjacent or superjacent to one another, the tissue of the outer new lamella, as well as of the old compact bone, becomes loose and expanded, and its Haversian canals widened: sometimes its actual mass is increased by internal deposition (hyperostosis interna), the bone retains its coarse appearance externally, becomes very dense in its texture, and is uncommonly heavy. It frequently happens, that the two states are found together, and the latter appears as if it had been developed from the former. In J. Muller's opinion, the so-called new bone, which is produced after internal necrosis, proceeds for the most part from the old bone, the outer layer of which becomes swollen, and grows uninterruptedly during the continuance of the suppuration by the dilatation of its Haversian canals and the formation of new bone in their interstices. In any case, this increase of bulk or swelling of the old bone is to be distinguished from that expansion and swelling upon which Scarpa grounds his theory of regeneration (Miescher).

The mode in which regeneration takes place, when the whole thickness of a bone is dead, corresponds with what has been already mentioned. An osseous exudation takes place upon the outer surface, as well as in the medullary canal of the healthy bone; and after the sequestrum has been removed, granulations continue to rise from the surfaces of the wound, and new bone is formed in them. The regeneration is in most cases incomplete; the growths from the two ends unite too soon, and the bone is diminished in length, and frequently a false joint is formed.

Necrosis of a whole bone is an extremely rare occurrence.

From what has been said, it appears that the regeneration is accomplished by that part of the old bone which has remained healthy; but there can be no question that the periosteum and other surrounding soft parts, and even newly-formed vascular tissue, are capable of furnishing an exudation which will become bone. This is clearly proved by the few cases in which spontaneous necrosis of a whole bone has occurred; by the fact that reproduction ensues in animals when a bone has been entirely removed (Heine); by the appearance of bony substance in the middle of openings made in the skull by necrosis, or by injury, as well as by the occurrence of growths of bone in periosteum and especially in the dura mater.

The foregoing remarks have been directed especially to the long bones, but they apply to necrosis and the process of regeneration in other bones also: only it must be observed, that there is very seldom complete regeneration of any part of a cranial bone lost by necrosis; and when it does occur, it always takes place very slowly (p. 125).

The suppurative process, which ensues upon the death of a bone, may, under various unfavorable local and general conditions, degenerate into an inflammation, attended with an ichorous product, and into caries.

A form of necrosis, differing from that to which the foregoing observations have been especially applicable, is a gangrenous ulceration of spongy bones corresponding to humid gangrene in the soft parts: it is met with chiefly in hospital gangrene, in bed sloughs, etc. The bone is soft and brittle, is filled with dirty greenish shreds of soft parts in a state of slough, and is saturated with a similar sanious fluid.