After a severe fracture or removal of a tumor, what happens if a segment of bone is missing? The bone ends will try to grow into one another just as they do after routine fractures, but the lack of immobilization and the gap may be insurmountable despite how hard the bone tries to repair itself. The void instead fills with gristle-like scar tissue, which cannot restore stability. A wobbly false joint ensues. Orthopedic surgeons can correct this situation by bringing in additional bone from elsewhere. To help understand bone grafting, I will compare it to borrowing money.
If you need just a couple of dollars, raiding the piggy bank and looking under the sofa cushions may produce the necessary cash. Nobody misses the money, and there is no urgent need to repay the loan. When more money is needed, people may borrow from their retirement fund or from their kid’s educational fund. Doing so solves the immediate shortage in their finances but leaves a gap elsewhere, which may or may not recover over time. Finally, if borrowers cannot fund the loan themselves, they might go to the bank, and as strange as it might seem, ask for a gift.
The same concepts apply to bone grafting. Consider, for instance, when a spinal fusion procedure requires some supplemental bone. Here, only hearty bone cells are needed to stimulate new bone formation, and the surgeon can temporarily open the hard exterior of the pelvis and scrape several tablespoons full of spongy bone from the interior. This does not change the outer contour of the pelvis; and although the crunchy graft in its new location offers no mechanical stability, it is full of bone-forming cells that hasten the healing in the recipient bone. Simultaneously, the donor site fills with new bone.
Sometimes a surgeon needs a short piece of solid bone to fill a gap and provide structural support. Here, a chunk of full-thickness bone from the rim of the patient’s own pelvis works well. Unless the owner is skinny or the graft is larger than an inch square, the withdrawal is invisible and harmless. If the graft is larger, the loan is permanently evident but does not affect function.
When a long, straight bone graft is needed, the leg is a capable lender. The one-half-inch diameter fibula is the smaller of the two bones between the knee and ankle and resides on the outside of the robust tibia. Except for a short portion near the ankle, the fibula is expendable because it mainly serves as an attachment site for ankle and toe muscles, which remain fully functional even if the fibula had been loaned. According to a person’s height, a six-to-ten-inch fibular strut can be loaned to span a gap in a more important bone.
The fibula strut is skimpy compared to most of the bones it replaces, so an internal plate and an external brace must protect it for at least a year. This bridging graft is “seed money” and will grow stronger over time to eventually resist the limb’s normal compression, twisting, and bending forces. Over several years, a grafted fibula completely converts from muscle anchor to body-weight resister and from spindly to robust.
Orthopedic surgeons can kick start a fibula graft into earlier service by meticulously harvesting the bone along with the blood vessels that supply it. After a plate and screws have secured the bone graft in its new location, connecting the fibula’s artery and vein to nearby vessels promptly restores its circulation. With immediate nourishment ensured, the bone-forming cells in the grafted fibula can quickly make the loan return a profit.
The patient’s own body supplies all the loans described so far, which eliminates any risk of an immune rejection. At times, however, huge segments of bone are required and can only come from an organ donor — an incredible gift. Immediately after an organ donor’s heart, liver, and kidneys are removed and cooled, they are transplanted into appreciative recipients. Powerful, rather risky anti-rejection drugs then aim to provide lifelong protection against immune rejection. The organ donor’s bones, by contrast, require no urgent cooling or transplantation. Rather they are processed to remove all their contained blood and proteins, dried, sealed in plastic bags, sterilized, and shelved. Cadaver bones can provide a graft of any desired shape and size and do so without any risk of rejection. This wonderful gift, however, comes with caveats. Since the graft has no bone-forming cells or blood circulation, the recipient site must supply these, which it does, but over many months. In the meantime, the donor bone in its new location can crack, crumble, or dissolve. Therefore, grafting cadaver bone can be considered a jumbo loan — monumental potential but with considerable risks.
Just as one considers the alternatives before taking out a monetary loan, orthopedists help patients understand the relative merits of the different types of bone grafts. Accessing spongy bone, for instance, causes no permanent skeletal defect, it heals faster than compact bone, but it offers no structural support. When grafting a fibula with immediate restoration of its blood supply, the surgery is long and tedious, but healing time is much shorter. Cadaver bone comes in the exact size and shape required, but it heals slowly. Sometimes using two types of bone graft to meet the same debt, for instance, taking out a home loan and accepting a unique gift, can minimize the tradeoffs.
Roy A. Meals is an orthopedic surgeon who blogs at About Bone.
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