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British Medical Books Health Issues Computerized Tomography

Sciatic Nerve

Anatomy

  • arises from LS plexus - L4,5 S1,2,3
  • emerges from pelvis below piriformis & enters thigh between ischial tuberosity & GT
  • in 10% of patients - sciatic N separated in greater sciatic foramen by the piriformis
  • accompanied by PFCN (post.fem.cut.N) & inferior gluteal artery
  • enters thigh beneath lower border of Gmax.
  • descends near middle of thigh, lying on adD magnus & crossed obliquely by long head of biceps femoris
  • usually separates in upper part of popliteal space

Tibal Nerve Branch

  • from anterior branches of LS plexus L4,5 & S1,2,3
  • 2 branches from tibial division - below quadratus femoris
    • upper branch to long head of biceps fem. & upper portion of semiT
    • lower branch innervates lower portion of semiT & semiM. & portion of adD magnus

Common Peroneal Nerve

  • from posterior branches - L4,5 & S1,2
  • nerve to short head of biceps fem.

Sciatic N. in THR

  • injured by excessive tension when extremity lengthened significantly, especially in pts with DDH
  • peroneal division most often affected

Incidence

  • 1-2% of primary THR, 3-4% after revision & 5-6% in THR for CDH
    • majority of these nerve deficits partial & many will resolve
    • females seem to be at significantly higher risk
  • sciatic & sup. gluteal N. and vessels course opposite the posterior superior quadrant
  • inferior gluteal and pudendal structures are opposite the postero-inferior quadrant
    • with EMG evaluation -> 75% incidence of subclinical injury to superior & inf. glut. muscles with use of posterior & lateral approaches

Kocher-Langenbeck Approach

  • injury prevented by monitoring of amount of tension applied by assistants retracting the N

Risk factors

  • revision THR
  • limb lengthening
  • female gender
  • anticoagulation
  • broken trochanteric osteotomy wires
  • vascular insufficiency

Prognosis

  • recovery from mild injury may occur in days to wks - neuropraxia
  • axonal damage - recovery may not occur at all or may be incomplete after 1 to 2 yrs
  • consider EMG

Management

  • if traction injury -> keep pts leg flexed, N. may recover in the relaxed position
  • early treatment consists of AFO
  • sciatic N recovery may occur over a 3 yr period
  • tendon transfers usually not performed until 3 yr post op

Tibial Nerve

Anatomy

  • from anterior branches of LS plexus L4,5 & S1,2,3
  • supplies muscles of posterior thigh (except short head of biceps which supplied by peroneal N.)
  • in popliteal space
    • branches to popliteus muscle, two heads of gastrocnemius, soleus & plantaris muscles
  • nerve passes into posterior compartment of the leg
    • supplies tib. post., FHL & FDL
  • passes behind medial malleolus to plantar side of foot & divides into medial & lateral plantar Nn
    • medial branch - counterpart of median N in hand
    • lateral branch - counterpart of ulnar N in the hand

Tibial Nerve Palsy

  • with tibial nerve palsy foot develops
    • sensory deficit of the plantar surface
    • cavus deformity d/t plantar fascia contracture
    • atrophy of intrinsic muscles
    • lengthening of Achilles tendon rotating calcaneus into dorsiflexion
      • only tib. ant. strong enough to produce active plantar flexion in presence of paralyzed triceps surae

Peroneal nerve

Anatomy

  • common peroneal N. derived from posterior branches - L4,5 & S1,2 as a part of sciatic N
  • supplies short head of biceps fem. in thigh
  • crosses posterior to lateral head of gastrocnemius & becomes subcutaneous behind head of fibula
  • penetrates the posterior inter-muscular septum & then divides into superficial & deep peroneal nerves
  • also gives off a lateral sural cutaneous branch which joins with the medial sural cutaneous nerve (from tibial N) to form the sural N

Deep Peroneal Nerve

  • courses anteriorly around fibula to enter the anterior compartment of leg
    • immediately below the fibular head lies on the anterior cortex of the fibula for a distance of 3-4 cm
    • supplies muscles (tib.ant., EHL & EDL) as it travels with anterior tibial artery between tib.ant. & EHL
    • sends a sensory branch to 1st webspace

Superficial Peroneal Nerve

  • supplies lateral compartment of the leg
    • on the lateral cortex of the fibula passes between peroneus longus & brevis
    • subcutaneous superficial sensory branch lies between peroneus brevis and EDL
    • about 10-12 cm above the tip of the lateral malleolus pierces the fascia
    • about 6-7 cm distal to the fibula, the superficial peroneal nerve divides into intermediate and medial dorsal cutaneous Nn.
    • branches of the superficial peroneal N or the sural N may be injured during ORIF of ankle #s

Peroneal Nerve Palsy

  • lead to severe disability with foot drop & paraesthesias

Traumatic peroneal palsy

  • may result from supracondylar #, knee dislocation & proximal tibial #

Atraumatic peroneal nerve palsy

  • may result from a large fabella which impinges on peroneal nerve behind the knee or from a proximal tibio-fibular synovial cyst

Examination

  • always consider lumbar radiculopathy
  • there may be an obvious foot drop
  • sensory loss may be difficult to determine - variable & small autonomous zone of sensation
  • Tinel’s sign over the fibular neck, helps localize the site of nerve compression
  • always check for a fabella and check to see if direct compression reproduces nerve symptoms
  • in knee dislocation -> test for function of tibial branch of sciatic N as well
  • in some cases of peroneal nerve avulsion, there will also be a sciatic nerve traction injury;

EMG

  • useful to document conduction block
  • should be performed within one month of injury
  • amplitude of the sensory potential & decreases in N conduction velocities - confirm sensory & motor deficits, respectively

Prognosis

  • with partial nerve palsy -> 80% recover completely
  • with complete palsy - < 40% have complete recovery

Treatment

Nerve in continuity - neuropraxia

  • observe first
  • if no neurologic improvement after 2-3 months - operative decompression
    • external neurolysis of peroneal N at the level of the fibular head
    • may be entrapped by thick fibrous bands which arch over the N as it crosses the fibular neck

nerve not in continuity - neurotmesis

  • nerve repair
    • in knee dislocation - may be concomitant tibial N division palsy
  • tendon transfer - tib.post. to dorsum of the foot