Hip flexion and knee extension serve as the foundation for a number of radicular and sciatic tests, the most common being straight leg raising. SLR is part of almost every examination for patients with lower back and leg complaints. Of course, the test involves lifting the straightened leg (the hip in neutral and the knee extended) of the patient in a supine position.
Positive findings are posterior leg pain and/or the knee flexing (buckling sign). Negative findings include the ability to raise the leg without producing posterior leg pain and/or causing the knee to flex.
While the purpose of SLR is to test the nerve roots that supply the lower extremities and the sciatic nerves, other structures, most commonly the hamstring muscles, are influenced during the test. This makes differential diagnosis necessary – a factor that will be discussed shortly.
Significant Diagnostic Findings
During SLR, important diagnostic findings occur along the course the leg travels. Between 0-30 degrees, there is minimal tension in the nerve roots, sciatic nerve or hamstrings. The likelihood of positive neurological findings is rare. However, there is an exception. Positive radicular findings can appear in this range when a nerve root is tethered.
From 30-70 degrees, tension in all structures increases, with nerve root and sciatic nerve tension peaking during the upper portion of this range.
Beyond 70 degrees, the nerve roots and sciatic nerve decrease in significance. If the tension between 30-70 degrees does not produce positive results, there is little chance the nerve roots or sciatic nerve are pathological. Emphasis shifts to the hamstrings for differential diagnosis.
If posterior leg pain occurs above the 70 degree point and/or the knee flexes, it is due to tight hamstrings. This is not a positive SLR result.
There are two additional points to consider for differentiation. Nerve symptoms are usually unilateral, while hamstring symptoms are usually bilateral; and nerve symptoms can extend below the knee, while hamstring symptoms stop at the knee.
Five Tests for Confirmation
Confirmation of positive SLR results is most commonly obtained by performing Bragard's test. Bragard's involves lowering the leg being tested until symptoms are relieved and/or the knee can be extended. The foot is then dorsiflexed. The test is positive if posterior leg pain and/or knee flexion recur.
Additional confirmation can be obtained by performing other tests for nerve root / sciatic tension that are also based upon hip flexion and knee extension. Bechterew's, Lasegue's and slump tests apply here, as does the flip sign.
There are only subtle differences in these tests. All five tests involve hip flexion and knee extension. With that said, SLR and Lasegue's are performed supine, while Bechterew's, slump and flip are performed seated.
Using Lasegue's as an example, it is often considered the same test as SLR and the names are sometimes used interchangeably. While both are performed supine and involve hip flexion and knee extension, the order of the two movements is reversed. SLR begins with the knee extended and the hip is then flexed. Lasegue's begins with the hip flexed and the knee is then extended. Bechterew's, slump and flip all begin with the hip flexed. The knee is then extended as in Lasegue's test.
When using the tests in this group, it is best to perform at least one supine and one seated test. The results from the two testing positions should be consistent. If the results vary significantly, it should raise questions. Inconsistent results between the two testing positions can be a sign of a non-organic condition or malingering.
It could be said that all of these tests are "six of one and half a dozen of the other." The key point here is to understand the two primary mechanisms, hip flexion and knee extension, in combination.
Bechterew's, Lasegue's, slump and flip can also identify hamstring tightness. Positive findings are the same as those that apply to SLR testing with one additional indicator. A positive indicator for the flip sign occurs when the patient leans back (flips back) and rests his / her hands on the examination table, attempting to relieve tension / leg pain. The hands and buttocks form the three legs of a tripod, which is why the test is alternately known as the tripod sign.
Differential Diagnosis
Differential diagnosis of SLR results can be accomplished through understanding symptoms of other pathologies that can be induced through hip flexion and knee extension.
There are instances in which patients report symptoms consistent with nerve root / sciatic pathology, yet SLR testing of the symptomatic leg is negative; while testing of the asymptomatic leg produces positive findings in the symptomatic leg. This crossover effect is given the name well-leg raising or crossed straight leg raising (CSLR).
It is traditionally thought that the CSLR test result is due to a lesion, usually a disc pathology, lying medial to a nerve root. This is in contrast to SLR findings that are thought to be associated with a lesion, usually a disc pathology, lying lateral to a nerve root.
Confirmation of CSLR results can be accomplished through Fajersztajn's test. Fajersztajn's is performed in the same manner as Bragard's test, but like CSLR is performed on the asymptomatic leg.
Continued confirmation can be sought through the use of Bechterew's, Lasegue's and slump tests, as well as the flip sign, just as with SLR testing. For each of these tests, testing the asymptomatic leg reproduces symptoms in the opposite / symptomatic leg.
Hip flexion and knee extension also cause tension in the meninges. This is the basis of Kernig's test for meningitis. The test can be performed with the patient in either a supine or a sitting position. It is replicated during the performance of each hip flexion / knee extension test described above. Positive Kernig's results include leg, spine and/or head pain in addition to bilateral knee flexion. Positive findings can occur regardless of which leg is tested. (It should be noted that these findings are rarely seen in a chiropractic office.)
Take-Home Points
I stated initially that hip flexion and knee extension serve as the basis for a number of radicular and sciatic nerve tests. I then highlighted many of these tests through their relationship to the most common test, the SLR. This leads to my final point: Orthopedic tests are not as individual as they are often depicted. A comparison of how they relate to other tests in performance, mechanism, interpretation, diagnosis, confirmation and differential diagnosis is always essential.
Resources
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