What exactly is a positive straight leg raise (SLR) ? Keep in mind that a negative straight leg raise doesn't exclude a symptomatic lumbar disc protrusion or internal derangement. And, a positive SLR doesn't necessarily mean there's a sciatica.
Important : this test is performed in a passive way and not in an active way, first on the painfree side, because we would like to find out which range of movement is normal for this particular patient.
Secondly, the patient can make a difference between a "normal" stretching of muscles-feeling and a different sort of pain, the pain which he actually complains of.
A painful arc on straight leg raise is quite common, therefore on performing the passive movement we stop the movement whenever it is painful, we stay in this position, we ask the patient where he feels the symptoms and…we try to continue the movement !
Only in this way we can discover a painful arc. The diagnostic difference between e.g. -40° ROM of SLR versus a painful arc is significant (big limitation is most lileky caused by a big internal derangement ; a painful arc always points in the direction of a small lumbar internal derangement which is easily reducible).
We perform the SLR as far as we can, perhaps pain is produced (where exactly ?), then we add an accessory neck flexion. If the added neck flexion affects the pain (i.e. it can worsen the pain, abolish it or produce it (where ?), then this is a clear dural sign excluding the sacroiliac joints, the facet joints or muscles as the cause of the pain.
Remark : it is possible that accessory neck flexion, at the end of SLR, eases the pain instead of increasing it.
There probably is an anatomical explanation for this : in that case the protrusion probably lies medial to the nerve root. Neck flexion also exerts a cranial force on the dura, possibly resulting in less compression of the nerve root.
We consider the straight leg raise as being a dural sign if it provokes a dural pain (i.e. backache with or without radiation). Just like neck flexion stretches the dura cephally, SLR stretches it caudally. The dura also moves in an anterior direction on neck flexion and SLR.
Actually, we can state that any considerable limitation of dural mobility results in a positive SLR.
In an acute lumbago (big posterocentral internal derangement), the limitation of SLR is bilateral. Unilateral lumbago causes more a unilateral limitation/pain of SLR, or a limitation which is greater on one side. When the compression of the dura ceases, the range of movement becomes normal again.
The SLR is a very useful criterion during a treatment session by manipulation/traction or by any other active methods. Interpreting the evolution of the limitation and/or pain during the SLR, we can easily interpret the gradual reduction of the internal derangement.
Example : suppose a patient has backache and 20° limitation of SLR. We start manipulation (if indicated) using the SLR as a test-retest criterion inbetween the manoeuvres. After a first manipulation, the test shows 10° limitation is left. After the next maneouvre, SLR becomes negative, allthough there is no full reduction yet ; some movements in standing may still be painful. We continue manipulation, but from now on the positive movements in standing become our new criterion.
The SLR clearly shows us whether the treatment we deliver is clinically effective.
Example : a three week old sciatica with 30° limitation of SLR. The treatment is ,e.g. daily continous mechanical traction. The SLR is tested before every traction session.
At the end of the first week, only 10° limitation remains. In the week-end, the patient makes an excursion with his family, sits in his car for hours and hours, and when he comes back after the week-end he again has 30° limitation of SLR. What does that mean ? Indeed, it is worse again, but not worse than the beginning ; we can still continue with our treatment. If SLR would be e.g. 60° limited, then probably we would have to change the treatment strategy.
Small internal derangement : think of one the first three options.
Bigger internal derangement : think of options 4-6.
A past lumbar internal derangement causing an episode of sciatica has resolved but the repair process has left some tethering or adherence that now inhibits full movement of the nerve root/dural complex. Flexion in standing is limited and the patients feels end-range pain in the back or the leg (this is the only dysfunction that can cause peripheral pain ; all others cause spinal pain only).
The treatment of an adherent nerve root will consist of an intense stretching program.
Keep one important message in mind : one single positive test has no clinical meaning at all ; focus on the complete image : history, inspection and a cluster of positive and negative tests. For more detailed analysis and recent scientific background of the clinical images described above, I refer to the ETGOM training programs in Modern Orthopaedic Medicine Cyriax and to our publications.