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]."
While he doesn't question that patients feel leg pain, he says the condition is all but impossible to diagnose - and may be caused by nerves irritated elsewhere in the body, "anywhere from the toes to the brain."
Filler and others say that when back treatments and back surgery don't work, practitioners sometimes label patients as neurotic malingerers whose pain is psychosomatic. "Doctors, and in particular surgeons, like to think that they are all-knowing and all-capable," he says.
Michael Edelman went through some of this frustration. In the mid-1990s, he began to suffer "excruciating" pain in his right hip that radiated down the side of his leg to his calf. He went to doctor after doctor, and underwent physical therapy, back injections and massage. Nothing helped.
"I was very frustrated," says Edelman, a 58-year-old personal injury lawyer and political consultant who lives in a suburb north of New York City. "I was walking around in constant pain."
After several years, he heard about Loren Fishman and made an appointment. The doctor diagnosed piriformis syndrome, and prescribed stretching and injections to relax the muscle.
Edelman says the pain lessened dramatically. He still gets occasional treatment when his piriformis tightens up, but he now plays golf three times a week. Before seeing Fishman, he had all but given up the game.
Fishman injects many of his piriformis patients, including Edelman, with Botulinum toxin, or Botox. In recent years, many doctors have used Botox for a variety of muscle- and nerve-related pain problems. It blocks nerve signals, relaxes muscles and decreases pain.
In piriformis cases, physical therapy and injections don't work. Some patients undergo surgery to pare or remove the muscle, reducing the strain on the sciatic nerve. Filler, who has developed a minimally invasive version of the surgery, says that about 90 percent of patients improve after the procedure.
Tiel argues that the surgery remains unproven, and suggests that post-treatment improvement in those who undergo it may be because of the placebo effect - when patients improve just because they believe they have been treated.
"I always like to understand the problem before I treat it," he says.
Filler disagrees, referring to a study he published last year that found many subjects who underwent successful piriformis treatment had previously had unsuccessful back therapy.
"How does the patient get no placebo effect from spinal treatment, but get a placebo effect from my piriformis injections?" he asks.
With so much ideological distance between the sides, the debate is not likely to be resolved soon. But all agree that the dispute has at least raised awareness of the issue. Scott Fishman says that more doctors around the country are considering piriformis syndrome as a potential diagnosis.
"It is real and it does occur," he says. "We need to look for it and treat it, because it is treatable."