The Sciatic Nerve

The following course is an interactive teaching module on the sciatic nerve. At the end of this module you should be able to confidently describe the anatomy and function of the sciatic nerve. In addition to this, you should be able to apply this knowledge to multiple choice and problem solving questions.

 

Introduction to the sciatic nerve

Course introduction

The following course is an interactive teaching module on the sciatic nerve. Complete the six sections of the module at your own pace. On completing each section you will be directed to the section to complete next. 

At the end of this module you should be able to confidently describe the anatomy and function of the sciatic nerve. In addition to this, you should be able to apply this knowledge to multiple choice and problem solving questions.

Roots of the sciatic nerve

Figure 1: Formation of sciatic nerve form the rami of L4, L5, S1, S2, S3.

The sciatic nerve is the largest nerve in the human body .

It is a thickened and flattened nerve of approximately 2cm in width.

It is formed by the ventral rami of L4, L5, S1, S2, S3 (Figure 1).

The sciatic nerve provides a direct nerve supply to the posterior thigh and an indirect supply to all compartments of the leg and foot.

Overview of the anatomical course of the sciatic nerve

Figure 2: Simplified diagram of the lower limb illustrating the course of the sciatic nerve in the lower limb.

The sciatic nerve is a continuation of the lumbosacral plexus.

It divides into tibial (L4, L5, S1-3) and common peroneal (L4, L5, S1-2) in the posterior thigh (figure 2).

The sciatic nerve normally divides into tibial and common peroneal nerves at the junctions of the middle and lower thirds of the thigh, proximal to the apex of the popliteal fossa.

The tibial nerve then continues in the posterior compartment of the leg giving rise to its terminal branches: medial and lateral plantar nerves.

The common peroneal nerve wraps round the head of the fibula before dividing into superficial and deep peroneal nerves.

Surface markings of the sciatic nerve

Figure 3: Posterior aspect of hemisected bony pelvis, illustrating the relations of the sciatic nerve to bony features of the pelvis.

Knowledge of the surface markings of the sciatic nerve are important in a clinical setting to ensure safe intramuscular injection and successful sciatic nerve block.

Descriptions of the surface markings of the sciatic nerve vary between anatomical textbooks.

Gray's anatomy describe that the sciatic nerve can be marked on the skin by a point midway between the posterior superior iliac spine and the ischial tuberosity. It then courses outwards and downwards to be marked by a point half way between the greater trochanter and ischial tuberosity (figure 3).

The sciatic nerve is formed by the ventral rami of which spinal nerves?

  • L3, L4, L5, S1, S2
  • L4, L5, S1, S2
  • L4, L5, S1, S2, S3
  • L3, L4, L5, S1
Answer the following question to assess your understanding of the sciatic nerve so far. 

The sciatic nerve supplies which compartments in the lower limb?

  • Posterior thigh, posterior leg and foot
  • Anterior and posterior thigh
  • Anterior thigh and all compartments of the leg
  • Posterior thigh, all compartments of the leg and foot
Answer the following question to assess your understanding of the sciatic nerve so far.

The sciatic nerve divides into which two named nerves?

  • Femoral nerve
  • Tibial nerve
  • Superficial peroneal nerve
  • Deep peroneal nerve
  • Common peroneal nerve
Answer the following question to assess your understanding of the sciatic nerve so far.

When performing a sciatic nerve block,  a surface marking for the sciatic nerve can be made between which two bony features? (See figure 4 to aid your answer)

Figure 4: Posterior aspect of hemisected bony pelvis. Labels for two important bony features for the surface marking of the sciatic nerve have been removed.

  • Ischial tuberosity
  • Lesser trochanter
  • Greater trochanter
  • Anterior superior iliac spine
Answer the following question to assess your understanding of the sciatic nerve so far.

Congratulations

Congratulations! You've successfully completed "Introduction to the sciatic nerve". Move on to "Anatomical relations of the sciatic nerve" to learn more about the anatomy of the sciatic nerve.

Anatomical relations of the sciatic nerve

Sciatic nerve and the lumbosacral plexus

Figure 5: The lumbosacral plexus and the named nerves which originate from the lumbosacral plexus.

The lumbosacral plexus is located on the posterior wall of the pelvis in close relation to piriformis muscle.

The lumbosacral plexus is formed by the ventral rami of the fourth lumbar to fourth sacral nerves.

It gives rise to a number of named nerves, illustrated in figure 5.

The sciatic nerve is formed by the ventral rami of L4, L5, S1, S2, S3.

Sciatic nerve in the gluteal region and pelvis

Figure 6: Posterior aspect of a hemisected pelvis illustrating the anatomical relations of the sciatic nerve in the gluteal region. Gluteal muscles and gemelli have been removed.

Ventral rami of the lumbosacral plexus converge to form the sciatic nerve at the inferior border of piriformis (figure 6).

The sciatic nerve leaves the pelvis via the greater sciatic foramen passing inferior to piriformis.

The sciatic nerve descends inferolaterally, deep to gluteus maximus and midway between the greater trochanter and ischial tuberosity (figure 6).

The sciatic nerve does not supply any structures in the gluteal region.

Sciatic nerve and piriformis

Figure 7: Posterior aspect of hemisected pelvis illustrating the bifurcation patterns of the sciatic nerve seen in 12% of the population.

Rami of nerves forming the sciatic nerve normally converge at the inferior border of piriformis.

The sciatic nerve normally bifurcates in the posterior thigh near the apex of the popliteal fossa.

However, in 12% of people, the sciatic nerve will bifurcate before leaving the pelvis. 

In this small group of people, following bifurcation of the sciatic nerve, the tibial nerve will pass inferior to piriformis whilst the common peroneal will either pass superior (A) to piriformis or pierce piriformis (B), see figure 7.

Sciatic nerve in the posterior thigh

Figure 8:  The sciatic nerve in relation to the muscles of the posterior thigh.

Emerging from the inferior border of piriformis, the sciatic nerve travels inferolaterally covered by gluteus maximus.

The sciatic nerve then descends in the posterior thigh where it lies deep to the long head of biceps femoris (figure 8).

 

In most individuals the sciatic nerve will bifurcate at the junctions of the middle and lower thirds of the thigh close to the apex of the popliteal fossa (figure 9).

The sciatic nerve bifurcates into the tibial and common peroneal nerves.

Figure 9: Bifurcation of the sciatic nerve into tibial and common peroneal nerves close to the apex of the popliteal fossa.

Blood supply to the sciatic nerve

Due to the size of the sciatic nerve, it receives its own blood supply via a named  branch of the inferior gluteal artery.

The sciatic nerve gains its blood supply from the artery to the sciatic nerve.

What are the names given to nerves A and B in this diagram of the lumbosacral plexus?

Figure 10: Diagram of the lumbosacral plexus and named nerves. The names of nerves A and B which make up the sciatic nerve have been removed.

  • Tibial nerve
  • Femoral nerve
  • Sural nerve
  • Common peroneal nerve
Answer the following question to assess your knowledge so far (select two responses using figure 10 to aid your answer).

The sciatic nerve emerges under the inferior border of which muscle?

  • Gluteus maximus
  • Quadratus femoris
  • Inferior gemelli
  • Piriformis
Answer the following question to assess your knowledge so far.

The sciatic nerve provides nerve supply to structures of the gluteal region?

  • True
  • False
Answer the following question to assess your knowledge so far.

The sciatic nerve exits the pelvis through which structure?

  • Greater sciatic foramen
  • Obturator foramen
  • Lesser sciatic foramen
Answer the following question to assess your knowledge so far.

The sciatic nerve runs under the cover of which muscle in the posterior thigh?

  • Semitendinosus
  • Semimembranosus
  • Long head of biceps femoris
  • Short head of biceps femoris
Answer the following question to assess your knowledge so far.

Congratulations!

Congratulations! You've successfully completed "Anatomical relations of the sciatic nerve". Move on to "Function of the sciatic nerve" to learn more about the role of the sciatic nerve in the lower limb.

Function of the sciatic nerve

Muscular supply

The sciatic nerve provides motor supply to the muscles of the posterior compartment of the thigh (figure 11).

The tibial division of the sciatic nerve supplies semimembranosus, semitendinosus and the long head of biceps femoris

In addition to these muscles, the tibial division of the sciatic nerve also supplies the hamstring portion of adductor magnus.

The short head of biceps femoris is supplied by the common peroneal division of the sciatic nerve.

The sciatic nerve also provides indirect motor supply to all of the muscles of the leg and foot via the tibial and common peroneal nerves.

Figure 11: Muscles of the posterior compartment of the thigh. Those muscles labelled in red are supplied by the tibial division of the sciatic nerve.

Cutaneous supply

The sciatic nerve does not provide cutaneous sensation to the thigh.

The sciatic nerve supplies cutaneous sensation to the leg and foot via the tibial and common peroneal nerves and their branches.

Articular branches

Figure 12: Joints of the lower limb in relation to the sciatic nerve.

The sciatic nerve supplies articular branches to the joints of the lower limb (figure 12).

The hip joint is supplied by the sciatic nerve.

The knee joint is supplied by articular branches of the tibial nerve and common peroneal nerve.

Before bifurcating into medial and lateral plantar nerves, the ankle joint is supplied by articular branches of the tibial nerve. The common peroneal nerve also gives articular branches to the ankle joint.

Which of the following muscles is NOT supplied by the tibial division of the sciatic nerve?

  • Semimembranosus
  • Semitendinosus
  • Hamstring portion of adductor magnus
  • Short head of biceps femoris

The sciatic nerve provides sensation to the anterior thigh, true or false?

  • True
  • False

The only joint of the lower limb supplied by direct articular branches of the sciatic nerve is the hip joint, true or false?

  • True
  • False

Congratulations!

Congratulations! You've successfully completed "Function of the sciatic nerve". Move on to "Tibial nerve" to learn more about the individual divisions of the sciatic nerve.

Tibial Nerve

Anatomy

The tibial nerve is the largest component of the sciatic nerve formed by the L4, L5, S1-3 nerve roots.

Following bifurcation of the sciatic nerve, the tibial nerve descends through the posterior thigh and popliteal fossa to supply the posterior compartment of the leg.

In the posterior compartment of the leg it descends with the posterior tibial vessels to lie between the heel and the medial malleolus, figure 13.

The tibial nerve then passes deep to the flexor retinaculum where it bifurcates into medial and lateral plantar nerves.

Figure 13: Tibial nerve in the posterior compartment of the leg accompanied by the posterior tibial vessels.

Function

Motor  Innervation:

The tibial nerve following bifurcation from the sciatic nerve provides motor nerve supply to the muscles of the posterior compartment of the leg.

At the popliteal fossa, the tibial nerve provides motor innervation to both heads of gastrocnemius, soleus, plantaris, popliteus and tibialis posterior.

Within the leg the tibial nerve also provides motor supply to flexor digitorum longus and flexor hallucis longus.

The terminal branches of the tibial nerve are the medial and lateral plantar nerves which provide motor innervation to the muscles of the foot.

Articular branches:

The tibial nerve provides articular branches to the knee joint, superior tibiofibular joint and ankle joint.

Cutaneous branches:

The tibial nerve does not directly supply sensation to the skin of the leg.

The sural nerve formed from branches of the tibial and common fibular nerves provides sensation to the skin on the posterior and lateral aspects of the leg and lateral side of the foot. (figure 14) 

The medial and lateral plantar nerves also supply sensation to the plantar surface of the foot. Whilst a medial calcaneal branch of the tibial nerve supplies the skin overlying the calcaneous. 

Figure 14: Sensory nerve supply by named nerves on the posterior leg and plantar surface of the foot.

The tibial nerve is formed by which nerve roots?

  • L4, L5, S1, S2, S3
  • L4, L5, S1, S2
  • L3, L4, L5, S1, S2
  • L3, L4, L5, S1

Which two nerves are the terminal branches of the tibial nerve supplying motor and sensory supply to the foot?

  • Medial plantar nerve
  • Sural nerve
  • Lateral plantar nerve
  • Saphenous nerve

The tibial nerve provides direct motor innervation to...

  • Anterior compartment of the leg
  • Anterior and lateral compartments of the leg
  • Posterior compartment of the leg
  • Posterior and lateral compartments of the leg

Tibial nerve fibres contribute to the formation of a nerve which supplies sensation to the posterior and lateral aspect of the leg and lateral aspect of the foot. What is this nerve called?

  • Saphenous nerve
  • Sural nerve
  • Anterior cutaneous nerve of the thigh
  • Posterior cutaneous nerve of the thigh

Congratulations!

Congratulations! You've successfully completed "Tibial nerve". Move on to "Common peroneal nerve" to learn more about the individual divisions of the sciatic nerve.

Common peroneal nerve

Anatomy

Figure 15: Simplified diagram of the bifurcation of the common peroneal nerve in the lower limb.

The common peroneal nerve is half the size of the tibial nerve and is formed by the posterior divisions of L4, L5, S1 and S2.

The common peroneal nerve descends in the posterior thigh, then along the lateral side of the popliteal fossa to wrap round the neck of the fibula

After wrapping round the neck of the fibular the common peroneal nerve divides into superficial and deep peroneal nerves. (figure 15)

The common peroneal nerve is the nerve supply to the anterior and lateral compartments of the leg.

Function

Motor  Innervation:

The common peroneal nerve provides motor supply to the anterior and lateral compartments of the leg.

The superficial peroneal nerve provides motor supply to the muscles of the lateral compartment of the leg. Whilst the deep fibular nerve provides motor supply to the muscles of the anterior compartment of the leg.

Articular branches:

​The common peroneal nerve provides articular branches to the knee joint and superiortibiofibular joint.

The deep peroneal nerve provides articular branches to the inferior tibiofibular joint and ankle joint.

Cutaneous branches:

The common peroneal nerve also contributes fibres to the formation of the sural nerve. The sural nerve is the cutaneous nerve supply to the skin on the posterior and lateral aspects of the leg and lateral side of the foot.

The superficial peroneal nerve provides sensation to the skin of the anterolateral leg and dorsum of the foot, except the web space between the great toe and 2nd toe (figure 16). 

The deep peroneal nerve provides sensation to the web space between the great toe and 2nd toe

Figure 16: Sensory nerve supply by named nerves on the anterior aspect of the leg and dorsum of the foot.

From which nerve roots is the common peroneal nerve formed?

  • L3, L4, L5, S1 and S2
  • L4, L5, S1 and S2
  • L4, L5, S1, S2 and S3
  • L3, L4, L5 andS1

The common peroneal nerve bifurcates into which two named nerves?

Figure 17: Simplified diagram of the bifurcation of the common peroneal nerve in the lower limb. The names of the nerves formed have been removed.

  • Saphenous nerve
  • Sural nerve
  • Superficial peroneal nerve
  • Deep peroneal nerve
(Select two options using figure 17 )

The superficial peroneal nerve provides motor nerve supply to which compartment (s) of the leg?

  • Medial compartment of the leg
  • Anterior and lateral compartments of the leg
  • Lateral compartment of the leg
  • Posterior and lateral compartments of the leg

Area B in figure 18 receives sensory supply by which named nerve?

Figure 18: Sensory nerve supply by named nerves on the anterior aspect of the leg and dorsum of the foot. The names of the nerves supplying areas A and B have been removed.

  • Deep peroneal nerve
  • Common peroneal nerve
  • Sural nerve
  • Superficial peroneal nerve

Congratulations!

Congratulations! You've successfully completed "Common peroneal nerve". You should now have an understanding of the anatomy and function of the sciatic nerve and it's named divisions. The next section will allow you to apply your knowledge to a problem solving scenario.

Applied knowledge scenario

Introduction

In the pervious sections, you improved your understanding of the anatomy and function of the sciatic nerve. A problem at any level of the sciatic nerve can result in a radiating pain down the entire lower limb. 

In this section, you will be able to put your new found knowledge into practice!

Patient presentation

Figure 19: Dr Brown consulting with a patient in her GP clinic.

Dr Brown is a general practitioner in clinic. A patient has come into Dr Brown's clinic complaining of pain radiating down the back of their thigh, leg and into their foot (Figure 19). Dr Brown has not revised their anatomy since first year of medical school and is struggling to work out the source of this patient's pain. She needs your help to locate the anatomical structure responsible for the pain.

Examination of the patient

Figure 20: Dr Brown examining the patient's leg.

Dr Brown goes on to examine the patient's leg as shown. When lifting the patient's leg off the bed, the patient's pain prevents Dr Brown lifting the leg any greater than 90 degrees from the bed. 

Dr Brown lowers the patient's leg towards the bed and dorsiflexes the patient's foot  (see figure 20). This movement also makes the patient's pain worse.

Dr Brown also notes the patient has numbness over the plantar surfaces of the the foot. Dr Brown informs you that numbness is a clinical sign for loss of sensation.

Your turn...

Now you have read the case presented to Dr Brown, go on and answer the following questions to see if you can work out the anatomical structure resulting in this patients pain.

Loss of sensation over the plantar surface of the foot is most likely to be caused by a problem with which nerves?

  • Superficial peroneal nerve
  • Medial plantar nerve
  • Deep peroneal nerve
  • Lateral plantar nerve
(Select two options)

The medial and lateral plantar nerves originate from which named nerve?

  • Tibial nerve
  • Common peroneal nerve
  • Superficial peroneal nerve
  • Deep peroneal nerve

The tibial nerve is formed by the bifurcation of which named nerve?

  • Femoral nerve
  • Obturator nerve
  • Sciatic nerve
  • Common peroneal nerve

From your anatomy knowledge and the information presented, a problem in which anatomical structure would result in pain radiating down the back of the thigh, leg and into the foot accompanied with loss of sensation?

  • Biceps femoris
  • Femoral nerve
  • Sciatic nerve
  • Semimembranosus

Congratulations!

Dr Brown agreed with your choice that the sciatic nerve is the most likely source of this patients pain. Thanks to your help she was able to give the patient this information and continue with her clinic.

Congratulations! You have successfully completed the "Applied knowledge scenario" and therefore this teaching module on the sciatic nerve. You should now have be confident in your understanding of the anatomy and function of the sciatic nerve.

References

References

Agur AMR, Dalley AF. (2005). Grant’s atlas of anatomy. (11th ed). Lippincott, Williams and Wilkins, Philadelphia.

 

Currin SS, Mirjalili SA, Meikle, G, Stringer, MD. (2015). Revisiting the surface anatomy of the sciatic nerve in the gluteal region. Clinical anatomy: 28: pp: 144-149.

 

Douglas G, Nicol, F, Roberston, C (eds). (2009). Macleod’s clinical examination. (12th ed). Churchill Livingstone Elsevier, Edinburgh

 

Kumar P, Clark M (eds). (2012). Kumar & Clark’s clinical medicine. (8th ed). Saunders Elseveir, London.

 

Moore KL, Dalley AF, Agur AMR. (2014). Clinically oriented anatomy. (7th ed). Lippincott, Williams and Wilkins, Philadelphia.

 

Palastanga N, Soames R. (2012). Anatomy and Human Movement Structure and Function. (6th ed). Churchill Livingstone Elsevier, Edinburgh.

 

Standring S, Borley NR, Collins P, Crossman AR, Gatzoulis MA, Healy JC, Johnson D, Mahadevan V, Newell RLM, Wigley CB (eds). (2008). Gray’s anatomy: the anatomical basis of clinical practice. (40th ed). Churchill Livingstone Elsevier, Edinburgh.