Skeletal scaffold: X-rays reveal how metal screws, rods and plates help mend America's broken bones.

Picture this: You're rushing to catch a plane. Passing through the airport metal detector, you set off a chorus of alarms. A pack of hefty security guards surrounds you. "Wait!" you shout, whipping out an X-ray. "It's just a metal plate and a few screws in my leg."

Too far-fetched? It could happen, says Todd Smith, research director at DePuy, Inc., a company that manufactures "body hardware," such as steel plates, screws, and rods. Orthopedists (bone doctors) surgically insert these metal odds and ends into more than 400,000 broken-boned Americans each year.

The metal holds together severely crushed bones, which can't be rejoined with plaster casts, says orthopedist Vietta Johnson. "Essentially we're trying to hold the bone still enough" so that [bone-building] cells can begin to make repairs, she says. To insert the metal parts, doctors drill holds near the fracture and screw the parts right into the bone.

SW's editor, Karen McNulty, underwent this surgery eight years ago, when doctors implanted four metal screws in her left ankle to fix a smash-up softball injury. McNulty has never set off an airport metal detector, though. She'd need a lot more metal, says Depuy's Smith--perhaps as much as the skeleton we "patched" together with X-rays on pages 10-11.

Doctors use X-rays like these to detect whether metal parts are inserted correctly, and whether healing is under way. For example, to tell if our cracked-up skeleton's tibia (large shin bone) was healing properly, a doctor would look at X-rays of the rod inserted through the bone.

Notice that the rod shows up darker than the bone. That's because metal is denser, or more compact, than bone, explains radiologist Ruth Ramsey. When X-rays--high-energy beams of electromagnetic radiation--pass through a body, dense tissues (and metal objects) absorb the rays. Because these blocked rays don't hit the X-ray film to expose it (turn it black), bones and metal parts appear light gray or white, Dr. Ramsey says. Dark regions show where X-rays pass right through the body.

SKELETAL SAFETY

The penetrating power of X-rays makes them useful or bone-viewing. But doctors try to limit patients' X-ray exposure; high doses (no one's sure how much) can burn tissue and even cause cancer, Ramsey says.

But don't be afraid to walk through an airport metal detector. These devices don't even use X-rays, says Bryan Allman at EG&G Astrophysics, a company that manufactures the machines. Instead, the detectors produce a weak electromagnetic field--a force field given off by all electrical devices. When meal disrupts the detector's field, it sets off an alarm.

To avoid setting off a metal detector with internal hardware, you can have the metal parts removed--after the bones heal. Most people do, says orthopedist Johnson. "Hardware tends to bother people in cold weather," she says. "It's metal. It gets cold, so [patients] feel achiness in their bones." Plus, she adds, "in young people, we usually remove the metal because [the patients] grow, but the products don't."

INNER AND OUTER WEAR

We humans have a skeleton inside our bodies--an endoskeleton. Some other animals, such as cockroaches wear an exoskeleton, a hard shell, outside their bodies. What are the pros and cons of each? This activity will help you design experiments to find out.

WHAT YOU NEED:

drinking straws * soft modeling clay or Play-Doh * scissors * a balance

WHAT TO DO:

1 . Cut drinking straws into 5cm lengths. These will be pieces of skeleton.

2. Pack one straw piece with clay until no more fits inside. How big can the organism inside this exoskeleton "grow?" Measure and record the clay-packed straw's mass.

3. Using a second piece of straw, mold clay around the outside. How much mass can you add on to this endoskeleton? Record the mass of this "organism."

CONCLUSIONS:

Which type of skeleton allows an organism to grow bigger? How is that an advantage/disadvantage for each?

Consider: As people and other creatures with endoskeletons grow, their bones grow with them. Bones are living tissue. Insect shells and other exoskeletons are not alive. These nonliving shells must be shed for the organism to grow. How will this shedding, or molting, affect the exoskeleton-enclosed organism's ability to grow? To protect itself? Experiment with the clay and straw pieces to find out.

DON'T STOP NOW!

List other pros and cons of endo- and exoskeletons. For example, how do the two compare when it comes to protection, movement, supporting a body, and so on? Design experiments to study these effects.