Caries and NecrosisCaries is a word which appears to be used with a considerable amount of looseness. In addition to the meaning implied by necrosis (namely, 'death' of the part), caries is generally used to indicate that there is also a condition of rottenness, decay, and stench. It is particularly applied, in fact, when the death of the bone is slowly progressive, and is due to the inroads made upon it by putrefactive or septic matter.
Necrosis of bone may be the result of any injury, such as severe blows, or pricks and stabs. In such cases it would appear that it is loss of a portion of periosteum that is the starting-point. With death of a portion of this membrane the vascular supply to a portion of the bone is cut off, and necrosis ensues.
It may also result from the extension of inflammatory affections of the structures adjoining it, as, for instance, the spread of the infective material in severe tread, or the encroaches made by pus in cases of quittor, suppurating corn, or complicated sand-crack.
When the necrosed portion of bone is small, and is free from infective properties, it is quite possible that it may, as is the case with small spots of necrosis in softer tissues, be removed by a process of absorption. It must be remembered, however, that where the necrosis has occurred as a result of septic invasion this cannot be looked for, for in every case such reparative changes are worked solely by healthy tissue.
If the tissues around the necrosis are engaged in dealing with organismal invasion and the poisonous products thus poured into their working area, their state of health is so weakened that they are unable to successfully combat with the two conditions simultaneously. As a consequence, the necrotic piece of bone persists, and acts as a permanent source of irritation.
It must be remembered, too, that if the dead portion of bone—even though it be free from septic matter — is very large, that it may itself act as a continual irritant, in which case it again persists, and cannot by natural means be removed.
In our cases necrosis of bone may be met with in punctured foot, in severe cases of tread, in cases of complicated crack, and in suppurating corn. It is met with, too, in navicular disease, in the extension of irritating discharges in cases of quittor, and in cases of chronic laminitis where the solar margin of the os pedis has penetrated the sole. In this latter case the protruding portion of bone is quickly denuded of its periosteum. Its blood-supply is destroyed, and necrosis follows.