Osteogenic stimulation

This is the ability to stimulate local bone-forming cells, osteoblasts, to produce new bone. Osteoblasts are present in fresh bone autograft taken from the same patient. Osteoblasts can also form from primitive stem cells, in bone marrow aspirated from a patient, when subjected to an osteoinductive stimulus (see below). Bone marrow can be injected into an autograft or allograft site or mixed with bone substitutes before being used in that patient.

Osteoconductive stimulation

Osteoconduction is the ability of material to serve as a scaffold on which bone cells can attach, migrate, and divide. Osteoconductive materials improve the ability of bone cells to fill the gap between two-bone ends. They also serve as a spacer that reduces the ability of fibrous tissue around the graft site from growing into the site. Bone grafts are the best osteoconductive material. Other naturally occurring and synthetic osteoconductive materials are also available.

Osteoinductive stimulation

Osteoinduction is the ability to stimulate primitive nondifferentiated bone cells to grow and mature into bone-forming osteoblasts. A number of growth factors present in normal human bone have been demonstrated to be osteoinductive. These are proteins which were first discovered in 1965 by Marshall Urist who coined the term “bone morphogenetic protein” (BMP). Subsequent studies have identified many different BMPs as well as other naturally occurring factors that stimulate bone growth.

Autografts

Autografts are bone taken from and inserted into the same individual. They are osteoconductive, osteoinductive, and osteogenic and have become the standard by which all other biological methods of bone stimulation are measured. Autograft is usually harvested as fragments of cancellous or corticocancellous bone from the anterior or posterior iliac crest. Smaller amounts can be obtained from the proximal tibia, the greater trochanter, the distal radius, and the olecranon. In spinal surgery, spinous processes and ribs can be used. As dry air kills cells, autografts are best harvested shortly before they are required for use. The graft may be temporarily covered and stored with saline or blood-soaked gauze for up to 2 hours.

Occasionally a large segment of bone with its nutrient blood vessels and with or without its soft-tissue attachments can be transferred as a free-vascularized autograft to fill a large-bone defect. The vessels are reanastomosed to local vessels to provide the graft with a blood supply. This procedure is demanding and is usually performed by plastic surgeons.

Cortical and strut bones, usually the fibula, may be used when a graft requires additional strength for load bearing. Cancellous bone incorporates faster than cortical bone but cannot be expected to take much load. Corticocancellous block grafts are useful when there is a need to provide a structural support that heals quickly, usually filling defects in and around joints, particularly when there are missing pieces of bone or joint surface. Most of the time it is harvested by cutting out a suitably shaped section of the anterior iliac crest.

The advantages of autografts include their availability in all patients, the fact that there is no risk of disease transmission or graft rejection, and that it requires no special processing technique. Disadvantages include the variability of graft quality (particularly poor in osteoporotic patients), and the limited quantity of the donor-site bone. Harvesting autograft tends to increase operating time and graft donor sites have complications of varying severity occurring in about 10–35% of patients.