In the previous post, I began drawing an analogy between borrowing money and borrowing bone to repair/replace a skeletal defect produced by trauma or tumor removal. As with borrowing money, choosing the source of the bone loan depends on its proposed use, how much is needed, and the terms of repayment. I also noted that small bone loans have minimal obligations and are available from several local sources (pelvis, wrist, fibula). Small grafts, however, may not fund a major project quickly enough for the loan to achieve its ultimate purpose. What else is possible?
Orthopedists have several ways to coax the fibula into early service. If the bone gap to be bridged is less than half the fibula’s length, the fibular strut can be cut in half and doubled. It will still take many months for the grafts to thicken enough to provide adequate support, but it will take less time than using a single segment. In other words, it doubles the loan’s productivity without increasing the debt.
Whatever its size, getting a loaner part into service quickly also improves its effectiveness. An orthopedic surgeon can kickstart a fibula graft into service by meticulously harvesting the bone along with the blood vessels that supply it. After the bone graft is secured in the gap, the surgeon connects the fibula’s artery and vein to nearby vessels. Blood then flows through the fibula just the same as it did in the bone’s original location. With the immediate restoration of circulation, the grafted fibula can heal and enlarge far more quickly to make the loan pay off.
The bone loans described so far all come from the patient’s own body, so the patient’s immune system raises no concern, and there is no risk of rejection. At times, however, huge segments of bone are required and constitute loans too large for patients to make. Consequently, the surgeon may turn to a gift from an organ donor. These bones are taken from the donor immediately after removal of the recently deceased’s heart, liver, and kidneys, which require placement on ice, immediate transplantation into a grateful recipient, and life-long protection against immune rejection with powerful, rather risky anti-rejection drugs. By contrast, the bones undergo a leisurely cleaning to remove all the blood and protein they contain. Then they are dried, sealed in plastic bags, sterilized, cataloged, and shelved until needed. Absent their proteins, bones generate no immune response when implanted into a different person, so they can provide grafts of the needed size and shape without any risk of rejection. This is a wonderful gift, but it comes with some strings attached. Since the grafted cadaver bone has no blood supply and no cells, the recipient site has to supply these, which it does, but ever so slowly. In the meantime, the cadaver bone graft can crack, crumble, or dissolve. For that reason, grafting cadaver bone is not undertaken lightly.
If you happen to have an identical twin, you could borrow living bone filled with cells and do so without risk of rejection, since you both have the same immune system. Careful though, your twin might need one of your kidneys sometime—payback for the bone loan.
Just as you weigh the options before borrowing money, orthopedists weigh with patients the pros and cons of one type of bone graft over another and discuss the relative merits of each. For instance, spongy bone is readily available without causing any permanent skeletal defect, and it heals faster than compact bone; but compact bone is immediately stronger than spongy bone. The surgery is much longer and more difficult when moving bone around with its blood supply intact, but healing time can be markedly shorter. Cadaver bone comes in any desired size and shape but is slow to heal. Sometimes the trade-offs are reduced by using two types of bone graft for the same debt—taking out a home loan and accepting a unique gift.