How Do Bones Heal? Living Tissue Rebuilding Its Own Scaffolding

Even without injury, bone is broken down and rebuilt throughout life by two cell types working in opposition. Osteoclasts dissolve mineral and digest the protein matrix, excavating small cavities, and osteoblasts follow behind depositing new matrix that subsequently mineralises. The whole adult skeleton is replaced over a period of years by this process, which serves two purposes: it repairs accumulated microscopic damage before it can propagate into a fracture, and it allows the skeleton to act as a mineral reserve, releasing calcium when blood levels require it. Crucially, the process responds to mechanical loading, laying down bone where stresses are high and removing it where they are not, which is why bone is thick exactly where forces travel and why weightlessness and prolonged bed rest cause rapid loss. That responsiveness is the reason fracture healing works as well as it does, since the repair tissue is shaped by the loads applied to it, and it is also why the balance between the two cell types shifting with age produces osteoporosis.

Healing of a typical fracture proceeds through overlapping phases over weeks to months:

• •Haematoma formation, as bleeding from the broken bone and surrounding tissue clots and releases signalling molecules that recruit repair cells
• •Inflammation, bringing in cells that clear debris and further signal the response, which is why suppressing it aggressively may not be helpful
• •Soft callus, in which cartilage and fibrous tissue bridge the gap and provide initial stability, forming over roughly one to three weeks
• •Hard callus, as that cartilage is progressively replaced by woven bone, which is disorganised and weak compared with mature bone but mechanically sufficient
• •Remodelling, lasting months to years, in which the woven bone is gradually replaced by organised lamellar bone aligned with the loads it carries, and the excess bulk of the callus is resorbed
• •Direct healing without visible callus, which occurs only when the fragments are held rigidly together with compression, as with a plate and screws, and proceeds by cells tunnelling straight across the fracture line

The body does the healing and the surgeon's contribution is chiefly alignment and stability. Fragments must be positioned acceptably, because a bone that unites in a bad position may work poorly or load a joint unevenly, and they must be held still enough for the repair tissue to bridge the gap, since excessive movement disrupts the developing callus repeatedly and can result in a false joint forming instead of union. Casting achieves this externally and remains appropriate for many fractures. Internal fixation with plates, screws, rods and wires achieves it directly and allows earlier movement of neighbouring joints, which matters because prolonged immobilisation causes stiffness, muscle loss and other complications that can be worse than the original injury. External fixators hold fragments through pins entering the skin and are used where the soft tissue is too damaged for internal fixation. A small amount of movement at the fracture actually promotes callus formation, which is why rigid fixation produces a different healing pathway than a cast does, and modern implant design deliberately provides the degree of stiffness the situation calls for.

Union fails or delays for identifiable reasons, and knowing them explains most of the clinical advice given after a fracture. Poor blood supply is the leading one, and certain bones are notorious because their blood enters from one direction, so a break in the wrong place cuts off the fragment, which is the problem with the scaphoid in the wrist and with the femoral head. Smoking substantially impairs healing and is one of the most consistent modifiable factors identified. Diabetes, poor nutrition, low vitamin D, certain drugs and infection all delay it. Excessive motion or an unstable fixation prevents bridging. A gap too large for callus to cross may need bone grafting. Children heal far faster than adults and remodel far more effectively, tolerating degrees of angulation that would require correction in an adult, because their growth plates continue reshaping the bone. Age, the specific bone, the pattern of the break and the state of the surrounding soft tissue together determine a timeline that ranges from a few weeks to many months.

A common belief is that a bone which has healed becomes stronger at the break than it was originally. There is a grain of truth in it: for a while after union, the callus makes the repaired section thicker than the surrounding shaft, and during that period the old break is often not where the bone would fail next. But remodelling steadily removes the extra bulk, and once the repair has been reshaped to match the loads it carries, the bone is roughly as strong as before, and no stronger. The healed region is not a reinforced weld that protects the limb for life.

Stress fractures show the other side of continuous remodelling. When a bone is loaded heavily and repeatedly, as in a runner who increases distance too quickly or a recruit starting military training, microscopic cracks form faster than the osteoclasts and osteoblasts can repair them. Early remodelling can even make the bone briefly weaker, since cavities are dug out before new bone fills them, and the cracks can join into a hairline fracture, often in the shin or the long bones of the foot. The usual treatment is simply reduced load for some weeks, and the usual prevention is building up training gradually so the skeleton has time to adapt.

Bone's ability to fill a gap has been turned into a way of growing new bone deliberately. In the 1950s Gavriil Ilizarov, a surgeon in the Siberian town of Kurgan, developed a frame of metal rings fixed around a limb by thin wires passing through the bone. The surgeon cuts the bone cleanly, waits several days for the early repair to begin, and then turns screws on the frame to draw the two ends apart very slowly. If the gap widens gradually enough, the healing tissue never quite finishes bridging it, and new bone keeps forming in the space, while muscle, nerves and skin stretch to match.

The standard rate is about a millimetre a day, usually split into several small adjustments, because faster pulling leaves a gap that fills with fibrous tissue, while slower pulling risks the ends fusing too soon. At that pace, lengthening a leg by five centimetres takes around fifty days, and the new bone then needs a longer spell with the frame held still to harden. The method, called distraction osteogenesis, spread to the West in the 1980s after Ilizarov treated an Italian explorer whose broken leg had failed to heal for years. It is now used for unequal limbs, bone lost to injury and some jaw deformities.

Bone constantly adjusts to the loads placed on it, and spaceflight shows how quickly it responds when those loads disappear. In weightlessness, the skeleton no longer has to support the body, and astronauts lose bone mineral from their hips and spine at a rate of roughly one to one and a half per cent per month, far faster than the slow loss of normal ageing.

To fight this, crews on the International Space Station exercise for around two hours a day, using a treadmill with harnesses, an exercise bicycle and a resistance machine that simulates weightlifting. Even so, recovery after a long mission can take months or years, and some loss may be permanent. Studying astronauts has helped researchers understand osteoporosis on Earth, where too little load on bones also weakens them.

Bone is rebuilt continuously by cells that dissolve it and cells that lay it down, responding to the loads it carries, which is why a fracture is reconstructed rather than scarred over. Repair runs from clot to cartilage bridge to woven bone to remodelled lamellar bone over months. Treatment mostly supplies alignment and stability while the body does the work, and failures cluster around poor blood supply, smoking and instability.