A so-called "herniated disc" may compress a nerve root and cause radicular complaints/findings.
What are the classic motor and sensory findings of involvement of L4? What are the classic sensory and motor findings of involvement of L5? What are the classic findings for S1?
Why, given that an MRI can localize the disc herniation, if present, should/must a student know the motor and sensory findings of each nerve root level?
An L4 disc herniation often presents with quadriceps weakness (if any), medial knee and shin sensory loss and pain distributed down the anterior thigh.
An L5 disc herniation classically presents with weakness in extension of the big toe (EHL), sensory loss in the big toe, and pain distributed down the back of the thigh and lateral calf.
An S1 disc herniation classically presents with weakness of the gastocnemius causing impaired ankle plantar flexion, sensory loss of the lateral foot and pain distributed down the back of the calf. The motor findings are more reliable than the sensory.
We need to be able to match the likely "positive" findings on MRI with the findings on exam.
A Penn med grad, Scott Boden among others has shown that many asymptomatic people have positive MRIs hence the radiologist will not say that all findings are necessarily pathological; rather he or she will say "clinical correlation suggested". By knowing what each lesion might cause you can make that correlation.
Thus, there is a strong medical rationale for not obtaining an MRI in a patient with low back pain and no neurological features: MRIs of healthy patients can lead to false positives and incorrect diagnoses that cause unnecessary stress, psychosocial difficulties and even psychiatric morbidity AND MOST ESPECIALLY OVERTREATMENT.
As noted above, many asymptomatic people have positive MRIs. Accordingly, the purpose of the MRI is not to screen (for it will fail on that account by picking up too much) but rather to plan the next step in treatment.
MRI ABOVE: Does this person have pain? If so, does it match?
MRI is needed primarily as surgical (or injection) planning investigation, a "gateway test", one might say, to many perhaps unnecessary (one also might say) procedures.
If all patients with back pain were to get an MRI there is a great chance that more (unnecessary) back surgery will be done.
The interested student is encouraged to consult the medical literature on the following points: first, the success rates of surgical treatment of back pain; the variability of the rates of back surgery by region of the country (suggesting non-scientific indications, one might say); and the cost of such surgical treatment. The answers are "low", "high" and "high" in case you could not guess…
(In the scheme of things, MRI is almost free; that is, the dollar cost of the test itself is a trivial to the cost of the treatment it could invoke. Don't be fooled by the list price--what is charged--but concentrate on what is actually paid.)