Our studies have permitted us to arrive at very definite conclusions with respect to the evaluation of cancellous and cortical bone when employed as graft material. At the very outset, it should be said that in regard to the behavior of the fully matured bony elements, whether they be derived from cancellous or compact bone, there is no great difference. The mature bony elements once transplanted do not, for the most part, survive. They constitute a mass which is so inert that it does not provide even a foreign-body reaction, and sooner or later will be replaced by that process known as creeping substitution. This process is exceedingly slow and extends over many months, with a gradually subsiding impetus. In any graft, whether it be of cancellous or cortical origin, the only elements which survive and which possess osteogenetic power in any degree, are the cells which form the so-called endosteal layer. This is also true, but to a lesser extent, of the elements of the cambium layer of the periosteum. Once these two fundamental features are appreciated, the relative merits of cancellous over cortical bone grafts become immediately apparent.

In the case of the cortical bone, the greater bulk of the graft is made up entirely of fully mature osseous elements. The endosteal layer is often absent or, when present, is exceedingly limited in amount. As for the most part the mature elements fail to survive, the graft itself exists, therefore, as an inert body which can only serve for a limited length of time as an internal splint. Before such a graft can be wholly replaced by living bone, its mass must be entirely removed. This can only occur by revascularization, the leaching out of the mineral elements, and autolysis of the collagenous scaffolding. Unfortunately, the very physical nature of cortical bone prohibits rapid substitution. The only pathways which a cortical graft presents for the ingress of new blood vessels are the small Haversian canals and the immediate surfaces in contact with the graft bed. Furthermore, since after the trauma of operation, the formation of new vessels reaches its critical point within a few days, and thereafter, with changes in biochemical conditions, starts to decline, the progress of penetration is choked off, and many regions of the graft remote from the vascular bed may never obtain a vascular supply. The cellular elements of the capillary tufts possess, as is well known, considerable potentiality in the direction of osteogenesis; but for osteogenesis to occur, an accommodation space must be available. Such space is not available in the case of cortical bone, until the original collagenous elements have been removed to provide for it. The graft, therefore, is in a sense a deterrent to the formation of new bone. New bone is laid down only on the surfaces of the graft, and, therefore, we frequently find cortical grafts becoming incorporated in, rather than replacing, new bone.

In the case of cancellous bone, conditions are far different. The mature elements of the trabeculae behave in much the same manner as that described for cortical bone, but each and every one of these trabeculae is provided with an endosteal surface possessing great osteogenetic power. In addition, there are the numerous marrow spaces. The bone marrow cells themselves undergo rapid degeneration, and provide wide channels for the invasion of new vessels and eventually for the accommodation of newly formed bone. The cells of the endosteum rapidly come into relationship with a vascular bed, which accounts not only for their survival, but for the early proliferation and establishment of new bone. New trabeculae and the incorporation of the old is the characteristic picture in cancellous grafts. The old elements being small, and lying in such close relationship to vital elements, are removed and replaced more rapidly, although they, too, may remain for many months before complete substitution. In this connection, it should be pointed out that there is some difference between cancellous bone in which the marrow is highly fatty and that in which red marrow predominates. Fat has, as is well known, an inhibitory effect on the formation of new vascular tufts and where present to excess, the process of bone formation is retarded. This would account for the superiority of iliac bone, or bone obtained from other sources containing red marrow, over the fatty cancellous bone found, for example, in the lower end of the femur or the upper end of the tibia. In a word, then, the superiority of cancellous bone over cortical bone lies for the most part in the fact that the cancellous bone presents a very extensive endosteal surface and numerous accommodation spaces which allow for osteogenesis.

These distinctions are most exquisitely seen in the case of grafts derived from ribs where both elements—namely, compact and cancellous bone—are present. In such grafts there is the additional advantage that both processes may be observed in the same subject under identical conditions. The compact tissue of the rib differs slightly from that obtained from other sources, in that its Haversian spaces are more numerous and larger, thus allowing a more ready access to new vessels. The cancellous element is like that of other bones. In such grafts the observer is struck by the rapid formation of new osteogenic tissue about the trabeculated elements, in comparison to the slow and restricted process of creeping substitution occurring in the cortical portion.