Prolapse of intervertebral discs occurs most frequently at the L4/5 or L5/SI level, producing compression of the L5 and SI nerve roots respectively. Tension can be applied to these nerve roots by flexing the hip with the knee straight -the so-called straight leg raising test. Normally, about 90 degrees of hip flexion should be possible but this varies considerably (70-120 degrees). When the root is stretched over a prolapsed disc, straight leg raising will be restricted
and pain will usually be felt in the lumbar region, not just in the leg (Fig, 8,1 IB).
Less frequently, disc prolapse at high levels may occur, involving the L2, L3 or L4 roots of the femoral nerve. This may cause weakness of the quadriceps muscles, resulting in reduced power of extension at the knee, sometimes associated with a reduced knee jerk (L3/4). There may also be altered sensation over the anteromedial aspect of the thigh. The femoral nerve passes into the thigh anterior to the pubic ramus, ant! hip flexion or straight leg raising will relieve any tension on these roots. The roots can be stretched by extending the hip with the knee flexed (femoral nerve stretch test) (Fig. 8.12).
Additional tests are sometimes useful. The 'bowstring sign' is another useful test and may be tised to confirm nerve root irritalion. If there is still doubt as to whether there is sciatic nerve root irritation, for example where malingering is suspected, other manoeuvres may be tried, such as the 'flip test' (Fig. 8.13).
Straight leg raising
□ Examine the patient lying supine (Fig. 8.11 A).
□ With the knee flexed, check that passive hip flexion is normal.
□ With the knee extended, raise the leg on the unaffected side by lifting the heel with one hand while preventing knee flexion with the other. Note the range of movement.
□ Repeat this on the affected side, asking the patient to report as soon as it becomes painful and where the pain or paresthesia is felt (Fig. 8.11B).
□ When this limit is reached, gently dorsitlex the ankle, thereby applying further tension to the nerve root (Bragaardtest) (Fig. 8.11C).
□ Perform straight leg raising and at the limit flex the knee, reducing tension on the sciatic roots and hamstrings (Fig. 8.11D).
□ Gently extend the knee until the pain is reproduced once again (Lasegite's sign) (Fig. 8.11 F.).
□ The posterior tibial nerve is now stretched like a bowstring across the popliteal fossa. Firm pressure is then applied with the thumb, first over the hamstring nearest the examiner, then over ihe nerve in the middle of the popliteal fossa and finally over the other hamstring tendon. Ask the patient which manoeuvre exacerbated the pain (Fig. 8.1 IF).
□ The test is positive if the second manoeuvre is painful and if the resultant pain radiates from the knee to the back.
Fig. B.11 Stretch tests - sciatic nerve roots. ® Neutral; nerve roots slack. BD Straight leg raising limited by tension ol root over prolapsed disc, [c] Tension increased by dorsillexion ol foot (Bragaard test). El Root tension relieved by flexion at the knee. [H With knee extension over prolapsed disc causing pain radiating to the back (Laseguetest), El Pressure over centre of popliteal fossa bears on posterior tibial nerve which is 'bowstrlnging' across the fossa causing pain locally and radiation into the back.
□ Ask the patient to sit on the edge of the couch with the hips and knees flexed to 90 degrees and test the knee reflexes (Fig. 8.13A).
1 Then, extend the knee, ostensibly to examine the ankle jerk.
□ When there is root irritation, the patient will 'Hip' backwards to relieve the sudden increase in tension on the nerve roof (Fig. 8.13BJ.
□ In the absence of significant nerve root irritation, the patient, distracted hy attention to the ankle jerk test, may permit full extension of the knee, which is the equivalent of full 90 degrees straight leg raising (Fig. 8.13C).
□ Ask the patient to lie prone or on the unaffectcd side if there is a painful flexion deformity of the hip.
□ Flex the knee slowly, asking the patient to report onset of pain.
□ If this fails to produce pain, in the thigh or back, gently extend the hip with the knee still flexed (Fig. 8.12).
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