How does a cast make broken bones heal? - Cheyanne, aged 7
A cast doesn't actually make bones heal, it just helps hold them in place so they can heal on their own. Bones are amazing things - they can bend (a little), they can break, and they can heal themselves again. They're super strong when they're holding up weight, but not very strong when they're pulled or twisted.
We may think of them as unchanging, but they're actually always changing: mineralising and demineralising. As fast as calcium gets laid down in one part of the bone, it's coming off some place else. It's like someone building a brick wall, while another person is dismantling it at the same time.
As long as they're working at the same speed, the wall will stay the same size and strength. This process of rebuilding is going on constantly in our bones. It's what allows our bones to grow, or even to respond to stresses and get stronger.
As an example, weightlifters don't just grow bigger muscles, but stronger bones, too. Their bones know they need to get stronger to deal with the extra weight, and so their bodies lay down more calcium just where it's needed, along lines of stress.
Unfortunately, the opposite happens too. When we stop walking, exercising, or using our muscles, our bones know it. They lose calcium and get weaker.
If we become couch potatoes, or get too sick to exercise, or too old to move around much, our bones lose calcium and get softer. You've seen some older folks hunch over and get a lot shorter as they age, right? Much of that is because of the bones in their spine getting softer and being crushed down by the weight of their bodies.
A little weightlifting and physical activity can go a long way to reducing weak bones, whether you're a weightlifter or a 90-year-old granny. Calcium from dairy foods and vitamin D from supplements or 20 minutes of sun a day also helps build strong bones.
But getting back to your cast question: bones want to heal, they just need to be touching or nearly touching to knit together. If there's a lot of wiggling, the bone ends can't heal together. That's where a cast comes in. The cast acts like a bug's exoskeleton or a scaffold, providing support to the broken limb from the outside.
And just like our bones, a cast is made mostly of calcium. The calcium comes from gypsum, a soft mineral-like talc that can be baked, ground up into a powder, and used to coat bandages.
When those bandages are wrapped around your arm or leg, and wet with water, they heat up and turn back into gypsum crystals, getting very hard in the process. Your cast is basically turning back into what it originally was: a stone (albeit a soft one).
In most cases, after about four to six weeks, it's ready to come off. New bone has formed a thick cuff at the fracture site, with calcium being laid down across the crack like cement. The surrounding bone, which demineralised and weakened in the cast along with the muscles, takes a while to get back up to normal strength. But once it does, you're good to go.
And even though people worry that they may have a permanent weak spot at a fracture site, studies don't bear this out. Healed bones are no more likely than unbroken bones to fracture again, assuming they healed correctly the first time.
The fracture will continue to remodel, getting smoother, straighter, and stronger over the course of months. Pretty amazing.
Gary Payinda, MD, is an emergency medicine consultant in Whangarei.
If you have a science or health topic question you'd like addressed, email drpayinda@gmail.com.
(This column provides general information and is not a substitute for the advice of your doctor.)