Lumbar spine, sacrum (pelvis) and hip joints work synchronously in statics and dynamics. So one can imagine that nervous structures (i.e. ischiadic nerve) are "at ease" when the lubar lordosis (incl. sacrum tilt) ranges between 30-50 degrees and the hips are at the neutral null position. If there is an entrappment of nerve roots the situation changes. Hip flexion is more favorable.
I would say that rather hip extension leads to decreased tension of the sciatic nerve as the lumbo-sacral roots have their origin and the nerve runs through the sciatic notch posterior to the hip center of rotation. increased pelvic tilt and hip flexion (with the knee straight) increase sciatic nerve tension. this is actually what you assess by testing for Lasègue's sign during a clinical examination.
Dear Georg,
Ok for the Lasègue's sign and the extension, but a lot of patients tend to compensate this test by automatically tilting their pelvis...
Dear colleagues, I do insist that the ischiadic nerve is put at a lesser tension when the hips are flexed provided that the knees and ankles are also in flexion. This can be proved by the pain relief when Lasegue's sign is interrupted by flexing of the knee and putting the foot in plantar flexion. The same effect could be achieved by posterior pelvic tilt by reduction of lumbar lordosis. The classic Perl device for conservative treatment of herniated discs with engagement of lumbosacral nerve roots is based on this principle.
Totally agree with both of you. Yes, lots of patients compensate Lasègue's test by tilting their pelvis (confirms what I wrote, does it?) and , yes, knee flexion decreases sciatic nerve tension (at least theoretically, there are no studies showing this).
This does not change the fact that the sciatic nerve runs posterior to the hip rotation center and that therefore flexion (or anterior pelvic tilt) will increase local tension and extension will release the nerve.
In the elderly, you will see anterior plumb line shift secondary to loss of lordosis secondary to lumbar disc degeneration. This is compensated for by posterior pelvic tilting to shift back the center of gravity. The extent of compansatory pelvic tilting is limited by the degree of possible hip extension (see papers on pelvic incidence from the groups around Le Huec and Roussoly in Tour and Lyon). If the hips are maximally extended and further compensatory posterior tilting of the pelvis is not possible, some patients will go into compensatory knee flexion. All these mechanisms reduce tension on the sciatic nerve. However, if any surgical intervention (spine or hip) is planned, that involves realignment of the lumbo-pelvic axis, one has to be aware that reversing of the compensatory mechanisms mentioned above might lead to increased tension of the now shortened soft-tissues (including the sciatic nerve).
Often an anterior pelvic tilt causes Sciatica.
Now regarding the relation between hip flexor:........ Increased Anterior pelvic tilt could be due to Short hip flexor muscles. When there is increased anterior pelvic tilt, the gluteal muscles tighten in an attempt to stabilize the pelvis. This attempt may lead to compression of the sciatic nerve against the bone.
Ishchiadic nerve or the Sciatic nerve runs along the course of the sciatic notches of the pelvis. Any movements may the nerve do be stressed and produces pain. Especially the forward tilting puts additional stress on the nerve whereas posterior tilting would try to reduce the tension. Slight hip and knee flexion in supine position would comfort the pain.