PNS: Sciatic Nerve

Anatomy

• The sciatic nerve is the largest nerve of the lumbosacral plexus and the peripheral nervous system (PNS).
• It comprises both the tibial and common fibular nerves.
• The sciatic originates from nerve roots L4-S3, although some sources incorrectly state L4-S1.

Palpation of the Nerve

• The sciatic nerve includes the tibial nerve, which arises from the anterior portion of the sacral plexus (L4-S3), and the common fibular nerve, originating from the posterior portion (L4-S2).
• It exits the sacral plexus through the greater sciatic foramen, formed partially by the sacrotuberous ligament, and runs between the piriformis and superior gemellus muscles.
• In about 13% of individuals, the sciatic nerve passes through the piriformis muscle.
• The nerve runs approximately halfway between the ischial tuberosity and the greater trochanter, either directly in the middle of these structures or slightly closer to the ischial tuberosity.
• The sciatic nerve is large and easily palpable through the overlying tissue by applying pressure through the mid-belly of the piriformis.
• Distal to the deep rotators, the sciatic nerve is located just lateral to the proximal attachment of the biceps femoris.
• It then runs down the middle of the posterior thigh between the medial and lateral hamstrings (semitendinosis and biceps femoris).
• This nerve ends in the distal thigh, proximal to the popliteal fossa.
• Proximal to the popliteal fossa, the sciatic nerve splits into common fibular and tibial nerves, both running through the popliteal fossa.
• The tibial nerve continues straight down the deep posterior compartment to the medial malleolus/tarsal tunnel.
• The common fibular nerve is superficial as it wraps around the head of the fibula and is prone to compressive mononeuropathy over the neck of the fibula.
• Dysfunction in these nerves can occur due to compression or injury to the sciatic nerve fibers.

Palpation of Affected Tissue

• Tone and tissue health should be assessed throughout the lower extremity: posterior thigh, superficial and deep posterior compartments, lateral and anterior compartments, and intrinsic foot muscles.
• Tissue health/trophic changes are possible due to the high proportion of autonomic fibers in the tibial nerve.
• These changes can manifest as edema, nail changes, skin fragility or discoloration, and altered sweating, especially in distal segments.

Function

• Motor functions include control of the hamstrings and the posterior portion of the adductor magnus.
• The tibial nerve controls the superficial and deep posterior compartments of the lower leg, as well as the intrinsic foot muscles (plantar).
• The common fibular nerve branches into the superficial fibular nerve, controlling the lateral compartment of the leg.
• A branch into the deep fibular nerve controls the anterior compartment.
• It also innervates the extensor digitorum brevis and extensor hallucis brevis muscles.
• Sensory function of the sciatic nerve is minimal.
• The tibial nerve divides into medial and lateral plantar nerves, serving sensory stimulation from the plantar surface of the foot and gives off branches to the sural nerve.
• The sural nerve supplies the skin over the medial posterior lower leg and sends small sensory branches to the calcaneus.
• The common fibular nerve branches into the superficial fibular nerve, providing sensory input to the lateral lower leg and dorsum of the foot, except between digits 1 and 2.
• The deep fibular nerve provides sensory input to the dorsum of the web space between digits 1 and 2.
• The common fibular nerve also gives off the lateral sural cutaneous nerve, supplying the area around the head and proximal fibula.

Sensory Innervation of the Sciatic Nerve

• The sural nerve is formed by branches of the common fibular and tibial nerves.
• The deep fibular nerve is a branch of the common fibular nerve.
• The superficial fibular nerve is a branch of the common fibular nerve.
• Medial calcaneal branches are branches of the tibial nerve.

Pathophysiology: Sciatic Nerve Injuries

• The sciatic nerve is susceptible to various types of nerve injuries, including:
  • Neuropraxia
  • Axonotmesis
  • Neurotmesis
• Determining the type of treatment depends on the context of the injury.
• Injury to the tibial nerve, due to its high proportion of autonomic fibers, carries a higher risk of causalgia or Reflex Sympathetic Dystrophy.
• Mechanisms of Injury (MOI):
  • Fractures of the pelvis, femur, tibia, or fibula
  • Dislocations of the hip, knee, or ankle
  • Trauma
  • Iatrogenic causes such as injections into the gluteal region or hip surgery
  • Internal compression from piriformis (neuropraxia), tarsal tunnel (tibial nerve), or Morton's neuroma (distal interdigital branch)
  • External compression, especially around the head of the fibula
• With nerve injuries or compression, symptoms often manifest distal to the site of the lesion; thus, Morton's neuroma is not a sciatic nerve injury.
• Signs and Symptoms (degeneration):
  • Foot drop that arises from a complete lesion
  • Paralysis of dorsiflexors and evertors (common fibular fibers), resulting in a foot hanging limp in plantar flexion and inversion, leading to a steppage gait
  • Claw toe, equivalent to ulnar and median claw hand
  • Claw toe presenting as extension at the MCP and flexion at interphalangeal PIP joint due to paralysis of intrinsic foot muscles (tibial fibers)
  • Weakness and atrophy of various muscles depending on the injury.
• Similar to the median nerve, the tibial nerve is rich in autonomic fibers, so injury to the fibers carries an increased risk of developing edema, trophic changes, sweating abnormalities, causalgia, or reflex sympathetic dystrophy.

Piriformis Syndrome

• Piriformis syndrome is a neuropraxia condition without nerve degeneration that occurs through compression of the sciatic nerve by the piriformis muscle which attaches to the anterior surface of the sacrum and the medial superior aspect of the greater trochanter.
• The sciatic nerve typically passes under the piriformis muscle, but some nerves pass around or through the muscle.
• Abnormal tone or TrP in this muscle compress the nerve, resulting in neuropraxia.
• Contributing factors:
  • Trauma: Muscle spasm, scar tissue, TrP
  • Internal rotation of the thigh (postural or while walking, which lengthens the piriformis).
  • Pelvis or SI instability
  • Pregnancy
  • Prolonged sitting, especially on a wallet or while driving
• Manifestations:
  • Pain in the buttock, posterior thigh, calf, and sole of foot caused by nerve irritation and/or piriformis TrPs
  • Compression causes paresthesia in the foot
  • Severe compression causes weakness and possible foot drop
  • SI joint dysfunction may arise from tight & short piriformis
  • Aggravation upon sit-to-stand transitions, prolonged standing, or sitting
  • Relief with hip external rotation which slackens the muscle and reduces tension

Differential Diagnosis

• Radiculopathy characterized by MOI, myotome deficits, and dermatome symptoms
• Pudendal or perforating cutaneous nerve irritation which causes perineum and dyspareunia
• Cluneal nerves: Strictly buttock pain without distal radiation
• Trigger points involving the piriformis, glute max, glute med, glute min, TFL
• Sacroiliitis

Precaution

• Piriformis syndrome usually involves a simple entrapment/neuropraxia, so treat with the goal to alleviate compression.
• Informed consent should include the possibility of temporarily recreating symptoms and the discomfort may last a few days.
• Alleviate compression given external rotation and aggravated by internal rotation, stretching the piriformis muscle is not indicated.
• Refer to Rattray/Ludwig p861 concerning precautions post cortisone injections, during pregnancy, or with medication use.

Treatment

• Position the patient in prone or sidelying position
• If prone, utilize a pillow under the thigh to create slight external rotation to slacken tissue.
• If sidelying, ensure the thigh is at least level and not below it
• Apply deep & moist heat to the glutes while working on the low back
• Release the piriformis attachment, focusing on the trochanter while avoiding the middle of the belly where the nerve runs. This approach is wise in all cases
• Nerve glides (modify the SLR)
• Work along the full sciatic nerve pathway
• Thoracolumbar fascia & low back
• Glutes
• SI joint play and/or Hip PROM/joint play have an inhibitory effect on the piriformis
• PIR or agonist contract to internal rotators
• Work both the superficial and deep hamstrings.
• Calves
• Address trigger points in the QL, Glute max, glute med, glute min, piriformis (if present)
• Sacrotuberous ligament loading with superomedial pressure at the ischial tuberosity, if adverse tension or pelvic alignment are factors.

Home Care

• Address Nerve glides.
• Improve Glute med strengthening.

Sciatic Nerve Assessment

• Straight Leg Raise
• Pace Abduction Test
• Piriformis Length Test
• Piriformis Test
• RROM & MMT lateral rotators
• Assess strength or recreation of TrP or pain upon resisted contraction