A More Fun Way to Pass Kidney Stones?

According to research published in 2016 in The Journal of the American Osteopathic Association, a trip to your local amusement park might be just what the doctor should order, at least if you have kidney stones. Following up on patient anecdotes, the researchers went to Walt Disney World, anatomically correct model of a kidney in tow (complete with kidney stones), to systematically test whether riding a roller coaster can dislodge a kidney stone.

About one in ten people in the United States will experience kidney stones, solid objects formed when minerals band together in the urine. Passing them can be immensely painful and is often compared to childbirth. Each year more than a million people see their health care provider for kidney stones, and around 300,000 go to the emergency room. Most kidney stones pass before they cause any damage, but large stones can lead to infection and kidney damage if left untreated.

Stones 6 mm or larger are usually too big to pass spontaneously and require more aggressive treatments to break them apart or remove them surgically. Drinking a lot of fluids, positional inversion (like on an inversion table), and applying an external force may help smaller kidney stones pass more quickly and keep them from growing into a bigger problem. As this new research shows, ridding a roller coaster can be effective, entertaining option to encourage passage.

The researchers, Marc Mitchell from the Doctors Clinic in Poulsbo, Washington and David Wartinger from Michigan State University, chose Big Thunder Mountain Railroad as the scene of the experiment. This choice was inspired by patient stories, such as one person who told Wartinger that he passed a kidney stone after each of three consecutive rides on the coaster.

The roller coaster features speeds up to 35 mph, sharp turns, and quick drops. The ride lasts about 2.5 minutes and doesn’t have any loops. For purposes of the experiment it’s important to note that passengers ride in a “train” composed of five cars, each with three rows. There is also a front “engine” that doesn’t have any seats.

Had you been on the coaster during the experiment, you would have seen a padded backpack riding in a middle seat, with a researcher on either side. The backpack contained a model of a kidney made out of clear silicone and based on CT scans of a patient. Three actual kidney stones were suspended in urine inside—the stones were passed spontaneously by the person on whom the CT scans were taken.

After 20 runs, during which the position of the stones was documented before and after each run, the researchers analyzed the data according to the volume of the stone, location of the stone, position on the roller coaster (in a front or back seat), and whether the stone passed. The results vary according to size and location of the kidney stone, but overall the passage rate was about 64% for stones in the back seat (car 5) and 17% for stones in the front seat (cars 1-3).

Since you’re reading this on a physics blog, the natural questions are: What forces are at work that cause the stones to pass? Can we isolate and recreate these forces for an even better passage rate? Are other roller coasters as effective, or even more effective?

According to Wartinger, the key seems to lie in the random quick motion of a moderate-intensity coaster (like those that don’t require a harness). The dislodging is a mechanical process, a rattling around that leads to the stone passing. Since each person has uniquely shaped kidneys and stones differ in size and location, there’s not one ideal roller coaster for everyone. To put this another way, it’s possible to design an ideal force pattern to dislodge a specific kidney stone, but that pattern won’t work for everyone.

So, should insurance companies start covering admission to Disney World? At this point, the researchers recommend discussing your particular case with your doctor first, since the results vary with size and location of the kidney stone. But if you have a kidney stone less than 4 mm in size, riding a few moderate-intensity roller coasters may be worthwhile.

A trip to the amusement park is also a good option for patients with larger stones that have undergone something called extracorporeal shock wave lithotripsy, in which the stone is broken into smaller pieces with sound waves. If they remain in the kidney, these fragments can grow or rejoin. Similarly, roller coasters can also help women with a history of kidney stones that are planning to become pregnant. Passing any remaining fragments before starting a prenatal vitamin regime can significantly reduce the risk of kidney stone complications during pregnancy.

This is a quirky piece of research, but it’s also much more than that. The results have a tremendous potential to help people, according to Wartinger. “We can reduce the need for future surgery, we can reduce trips to the emergency room, and reduce a lot of human suffering.” That’s not bad for something built to entertain.

—Kendra Redmond