What happens when a segment of bone is missing after a severe fracture or removal of a tumor? The bone ends try to grow into one another as they do after routine fractures, but the gap and lack of immobilization are usually insurmountable despite how hard the world’s best building material tries to repair itself. The gap instead fills with fibrous scar tissue; this gristle-like material does not restore stability, and a false joint forms. Orthopedists can overcome this situation by bringing in new bone from afar. To help you understand bone grafts, consider the analogy of borrowing money.
If you find yourself just a couple of dollars short, searching under the sofa cushions and raiding the piggy bank may produce the necessary cash. Nobody misses the money, and there is no pressing need to pay off the loan. If you need a substantial amount of money, perhaps you can borrow from your retirement account or from your kid’s educational fund. Doing so solves the immediate gap in your finances but leaves a deficit elsewhere, which may or may not recover over time. Finally, if you cannot fund it yourself, you could go to the bank, and as strange as it might seem, ask for a gift.
The concepts are the same for bone grafting. If surgeons only need some hearty bone cells to stimulate new bone formation, maybe to supplement local bone for spinal fusion, then they can temporarily open the hard outer surface of the pelvis and scrape spongy bone from the pelvis’s interior. Several tablespoons of bone can be obtained in this manner without changing the outer contour of the pelvis. The crumbly graft offers no mechanical stability, but it is full of bone-forming osteoblasts that quickly overcome the small debit in the recipient bone. At the same time, the donor site fills with new bone and could be re-excavated later if necessary.
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Sometimes a surgeon needs a short section of structurally sound bone to fill a gap at the site of a nonhealing fracture or to span a gap after removal of a bone tumor. In these instances, a chunk of full-thickness bone from the patient’s own pelvis works well.
It comes from the rim of the pelvis, near where your thumb rests when you put your hands on your hips. Unless the owner is quite thin or the chunk is bigger than an inch square, the withdrawal is harmless and invisible. Otherwise there is permanent, but manageable evidence of the loan.
If a long, straight bone graft is needed, attention often turns to the leg. Of the two bones between the knee and the ankle, the sturdy one that transmits weight is the robust shinbone, aka tibia. Just to its outside is the half-inch-in-diameter fibula. Except for a short portion near the ankle, the fibula is expendable because it is mainly an attachment site for ankle and toe muscles, which are fully functional even if the fibula had been borrowed. Hence the fibula is a mainstay in the orthopedist’s “long-bone lumberyard.” Depending on the patient’s height, a six-to-ten-inch strut is available to span a large gap in a critical bone.
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The fibula strut is far more slender than most of the bones it replaces, so it will need support from a sturdy internal plate and an external brace for at least a year. This bridging graft is “seed money” and will grow stronger over time allowing it to resist the limb’s normal bending, twisting, and compression forces. The complete conversion of a grafted fibula from scrawny to robust and from muscle anchor to body-weight resister usually takes several years.
In the next post I will describe jumbo loans.
