Validity of the Flip Test as a Sign of Nerve Root Tension

Generations of orthopedic surgeons, physical therapists, and sports specialists have used the flip test (also known as the sitting straight leg raise) to confirm sciatic nerve root compression. The technique has been presented in texts books, articles, and other publications for the past 50 years.

The sciatic nerve is a large nerve that starts in the low back and goes down the back of the leg from hip to foot. As it travels through the buttocks area, it passes out of the pelvis through the a hole called the greater sciatic foramen. Nerve compression often comes from disc disease, especially disc protrusion. A disc herniation may also put pressure against a spinal nerve. Pressure on an irritated or damaged sciatic nerve can produce pain that radiates along the nerve. True sciatic nerve pain from disc protrusion or herniation radiates down the leg past the knee, even going as far as the foot sometimes.

The authors of this study suggested it was time to test the validity of the flip test. Does it really measure nerve root tension? Exactly how should the test be administered? Descriptions of the test vary from book to book and article to article.

The basic flip test is done in the sitting position with the patient's legs dangling off the edge of the table. The examiner holds under the patient's heel and straightens the knee as far as it will go without putting so much tension on the nerve that it causes shooting pain down the leg. This position and movement place tension on the sciatic nerve. If it is compromised in any way, the test will be painful. The original flip test was considered positive when the patient fell backwards (flipped back) or had to brace him or herself to keep from falling backwards as the leg was straightened.

But here's where there are some questions. Does it matter how soon the person feels the pain? Is it a positive flip test if the patient can straighten the knee all the way but reports sciatic pain? Is it more positive if the pain begins earlier in the leg straightening process? Is this test the same as doing a straight leg raise from a supine (lying flat on your back) position?

To find out, they tested 67 adults with known sciatica from a disc protrusion that was putting pressure on the spinal nerve root. The diagnosis was confirmed using MRI scans. They all had pain that traveled from the low back/buttock area down the leg to below the knee that was brought on or made worse by doing a straight leg raise in the supine position.

Everyone was tested in two ways. First, they were placed supine and the examiner performed a straight leg raise test. The straight leg raise test was done by the examiner with the patient lying on the table. The examiner lifted the leg off the table by supporting a hand under the heel. The knee remained straight as the leg was lifted as far off the table as possible without pain.

The patient's range-of-motion, pain response, and report of sciatica being reproduced were all recorded. Then the patient sat up and the test was repeated as a sitting straight-leg-raise test (i.e., the flip test). With the knee in a 90-degree position of flexion to start, the examiner lifted the lower leg by the heel until the knee was as straight as possible before painful symptoms were reproduced.

The results were compared and analyzed and here's what they found. The flip test didn't really flip patients backwards when it was least not with the force or sudden response that was described when this test was first developed. All of the tested patients had a known sciatic nerve compression, so in theory, this test should have been positive 100 per cent of the time. But it was only positive (patient moved backward) in about one-third of the patients. One third of the group showed no reaction at all to this test. That tells us the test isn't reliable or valid as a dependable sign of nerve compression.

So, the authors suggested using the name sitting straight-leg raise test (instead of flip test) as a more accurate descriptor of the test. Second, the test was only useful as a check of nerve root tension when the patient's response was pain before the leg was raised 45 degrees. Test validity was much stronger when this modification was made in determining a positive test result.

The authors described various reactions that could be interpreted as a positive flip test. For example, some patients placed their hands beside or behind the buttocks; others leaned the trunk back 10 to 20 degrees. Some merely rolled the pelvis back slightly, but all of these reactions were in response to tension being placed on the sciatic nerve from hip flexion and knee extension.

The authors conclude that the flip test should really be called the sitting leg raise. It can still be used to test for nerve root tension but only to confirm a passive supine straight-raise leg test that is positive between zero and 45 degrees of hip flexion. The definition of a positive test in the sitting position should also be expanded to include back and/or leg pain when the leg is raised or the knee is straightened. Pain (not movement of the trunk backwards) is the defining standard.

Bruce Summers, FRCS, et al. The Flip Test. A Reappraisal. In Spine. July 1, 2009. Vol. 34. No. 15. Pp. 1585-1589.

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