The changes in the bones in arthritis deformans fall under three heads, viz. : 1. Osteophyte formation; 2. Eburnation; 3. Abrasion.
• In contrast to the destruction of the central portion of the articular cartilages already described is the heaping up or lipping of their edges, which is thus explained by Cornil and Ranvier.
The edges of the cartilaginous surfaces are to some extent over lapped and covered by the synovial membrane, and their synovial covering prevents the escape of the contents of the ruptured capsules into the synovial cavity, with the result that the cellular elements collect under it and lead to the formation of outgrowths of newly formed. cartilage which are known as chondrophytes. As time goes on these chondrophytes become ossified and then constitute the familiar bony outgrowths or osteophytes which have so important a share in the production of the deformities characteristic of the dis ease. In some parts, instead of definite outgrowths, a rim of new bone is formed which gives rise to the condition recognized clinically as lipping.
The process of ossification commences, as "Volkmanu" showed, in the parts in the immediate neighborhood of the original bone, and progresses at a slow rate, so that even at a period when the proc ess is far advanced the osteophytes still retain a covering of carti lage. As a result of the outgrowth of osteophytes the bulk of the articular ends of the bones may be very greatly increased, and the mobility of the joint may be thereby seriously interfered with.
Turning to the consideration of the more central portions of the articular surfaces, the bone in these situations also is found to have undergone a remarkable change. The superficial layer has become extremely hard and compact, so that it resembles ivory in appearance, a resemblance which is further increased by the polishing which it has undergone in consequence of the friction of the articular surfaces against each other. On this account the change in question has re ceived the name of eburnation. The smoothness of the eburnated sur faces is broken by minute perforations with which they are riddled, and which correspond to Haversian canals which have been laid open.
liburuation may be regarded as, to smile extent, a conservative process, seeing that the hardness of the ivory-like layer tends to impede the process of abrasion to which the exposed bony surfaces are subjected.
Various explanations of the process of eburnation have been sug gested. Thus Ziegler" attributes this change to a softening of the subjacent bone simultaneously with the changes in the cartilage which lead to fibrillation, which softening leads to the production of cavi ties, into which a growth of vascular medullary substance extends, and in this way the formation of a layer of new bone is brought about.
Cornil and Ranvier, on the other hand, ascribe the formation of the compact new bone to the discharge into the adjacent medullary spaces of the cartilage capsules of the deepest layers, which as they increase in size exert pressure upon the intervening bqne and so bring about its absorption; but they further admit that inflammatory changes may have a share in the process.
Even the eburnated surfaces often show conspicuous evidence of the abrasion to which they are subjected, and are apt to be scored by deep grooves running parallel with each other and following the direction in which the bones rub upon each other in the ordinary movements of the joint.
By the combined action of the two processes of osteophyte forma tion and abrasion most profound alterations in the mechanism of the affected joints may be produced. What was originally a convex articular surface may be reduced by abrasion to a concave form, and the abnormal cavity may be rendered much deeper by osteophytic outgrowths surrounding it. There is a specimen in the museum of St. Bartholomew's Hospital (London) which affords a most striking illustration of such changes. In this specimen the shoulder joint is that which is affected, and what was once the glenoid cavity of the scapula has become converted into a highly polished convex boss ; whereas the head of the humerus has been changed into a cavity sur rounded by bulky osteophytic outgrowths, which have themselves undergone extensive abrasion adapting their form to the requirements of the reversed mechanism of the joint.