For decades, doctors have known that leg and foot pain is often caused by spinal discs pressing on nerves that extend into the lower body.
Known as sciatica - because the symptoms travel the path of the sciatic nerve - this often-debilitating ailment is common, striking as many as 4 million Americans a year.
But some doctors now believe that many sciatica sufferers don't actually have back problems. Instead, their symptoms may be caused by the piriformis, a little-known muscle that extends from the hip to the bottom of the spine.
In recent years, this small strip of tissue has provoked a heated medical debate. Proponents say piriformis syndrome remains woefully underdiagnosed, and may affect millions of Americans, many of whom have suffered for years.
"It is probably a significant problem for at least a million patients a year," says Dr. Aaron Filler, a neurosurgeon at Cedars-Sinai Institute for Spinal Disorders in Los Angeles. He estimates that the problem accounts for about 10 percent of sciatic pain.
He and others typically treat piriformis symptoms with therapies that can include exercise, Botox injections and, in some cases, surgery.
But many doctors, particularly spine experts, scoff at the idea, arguing that piriformis syndrome is an unusual condition that is, if anything, overdiagnosed.
"It is a very infrequent cause of pain," says Dr. Vincent Pellegrini, a spinal surgeon who chairs the orthopedics department at the University of Maryland School of Medicine. "It is not an epidemic cause of sciatica."
Most doctors stand somewhere in between. "It is a controversy," says Dr. Scott Fishman, a pain specialist at the University of California, Davis in Sacramento. "We just don't know how common it is."
For patients, the disagreement is not trivial. Studies show that almost half of those diagnosed with sciatica get little or no relief from first-line treatments - which typically include physical therapy and cortisone injections.
Of the 300,000 patients who undergo expensive and potentially risky back surgery for the ailment each year, as many as 40 percent get little or no relief, according to a study in the 1990s.
Filler and other researchers say that many of the nonresponders may actually have piriformis syndrome. For example, Dr. Loren Fishman, a physical medicine specialist in New York City, says he has treated hundreds of piriformis patients who had previously undergone "needless and fruitless" back surgeries.
Nestled deep inside the pelvis, beneath much larger and better-known muscles such as the gluteus maximus, the piriformis is a narrow ribbon 5 to 6 inches long and 1 inch wide. Along with several other small muscles, it helps rotate the hip and leg outward.
Through an evolutionary quirk, it sits just above the sciatic nerve, which transmits sensation from the legs to the brain, and movement commands from the brain to the legs.
Because this nerve is the largest in the body - typically half an inch thick in the pelvis - it is particularly vulnerable to being pinched by surrounding body structures.
Some researchers believe a tight piriformis can spasm, irritating the nerve in many patients. In some people, the sciatic nerve actually passes through the center of the muscle, increasing the likelihood of chafing and pain.
Loren Fishman (who is not related to Scott Fishman) says that evolution and modern living have combined to make us vulnerable to piriformis problems. The muscle, which has likely existed in animals since dinosaur times, functions best in creatures that walk on four legs.
"We walk on two legs and we sit in chairs. The good Lord didn't intend for us to do that," Fishman says, joking.
In the early decades of the 20th century, doctors regularly diagnosed a version of piriformis syndrome. But in 1934, two Harvard University spine experts proved that damaged vertebral discs - the squishy padding between backbones - could press on spinal nerves, causing referred pain in the legs.
The researchers, Dr. William Mixter and Dr. Joseph Barr, also developed a solution: trimming or removing damaged disc tissue, which relieved pressure on the nerves.
The discovery of disc-related sciatica created a sensation, and the new approach spread quickly. Piriformis syndrome receded into medical history, leaving most doctors unfamiliar with the phenomenon.
"There are probably a fair percentage of people with piriformis syndrome who are not diagnosed. A lot of doctors are not that familiar with it," says Northwestern University pain specialist Dr. Honorio T. Benzon, who has studied the condition.
But others doubt that the syndrome even exists. "We don't understand what's causing this pain," says Dr. Robert L. Tiel, chairman of neurosurgery at the Louisiana State University Health and Science Center in New Orleans. "I don't think that this muscle is responsible for [the problem]."