Morphological Observation Of The Fascial Sheath Of The Sacral Plexus, Sciatic Nerve And Its Main Branches

Objectives To study the fascial sheath of the sciatic nerve and its main branches and toreveal the relationship between the sheath and the surrounding structures, using acombination of microdissection, epoxy sheet plastination, ultrasound, and MRI.Methods A total of16cadaveric specimens（14adult,2new-born infants?）and22adultliving subjects were used for cadaveric examination and magnetic resonance imaging(MRI) or ultrasound observation, respectively. The cadaveric specimens wereinvestigated by the local latex injection method (10), sectional anatomy (2) andhistological staining methods (4). The MR images were collected and analyzed fromtwenty patients without obvious disorders. Two other normal adults were performedwith ultrasound scans. To standardize observation on cadavers and living subjects, thesciatic nerve was divided as4segments: popliteal segment which was5-7cm above thepopliteal crease where the sciatic nerve gave off tibial and common peroneal nerves，posterior thigh segment which was from the inferior border of the gluteus maximusmuscle to the5-7cm above the popliteal crease, posterior gluteal segment which wasbetween the inferior borders of the piriformis and gluteus maximus muscles, and pelvicsegment which was from the sacral plexus nerve root to the inferior border of thepiriformis.(1) Latex injection: Ten cadaveric specimens were injected with latex usinga step-by-step injection method from2-3cm above the popliteal crease to the sacralplexus roots. The latex-injected specimens were dissected for topographic anatomyexamination.(2) Plastination: An adult and two new-born infant cadavers were deepfreezed at-35C and cut into a series of transverse sections. The sections wereplastinated via procedures of dehydration, defatting, impregnation and hardening.(3) Histology: The sciatic nerve and its surrounding tissue were collected from two adultcadavers and cut into small segments. The popliteal segment of the sciatic nerve wascollected from two new-born infant cadavers. The sampled tissue was embedded inparaffin and sectioned. The HE or VG stain and immunohistochemical methods wereapplied..(4) Medical imaging: Observation of the ultrasound images of the sciatic nerveand its surrounding tissues was made from the popliteal fossa to the inferior border ofthe piriformis. Twenty MR images were collected from twenty patients and analyzed bycomparing with the cross-sectional and plastinational specimens.Result1.At the popliteal fossa and posterior thigh region, the injected latex spread upwith the sciatic nerve, indicating that there was a fascia-like structure around thepopliteal segment of the sciatic nerve. The results of sectional anatomy, histologicalstaining, ultrasound and Plastination specimens examination were in conformity withthe above observation.2. Compared with the adult and new-born infant results ofhistological staining, there was a fascia-like structure around the sciatic nerve at thepopliteal segment in fetus, but there was more fat around the nerve. The fetalimmunohistochemical results demonstrated that the fascial sheath at the popliteal fossawas not derived from smooth muscles.3. Along the posterior gluteal segment betweenthe ischial tuberosity and the greater trochanter, latex diffused along the sciatic nerve upto the piriformis, indicating that there was a fascia-like sheath. However, along the otherparts of the nerve, the sheath was not complete.4. The injected latex did not diffusealong the sciatic nerve at the pelvic segment. An intact circular sheath around the nervewas rarely observed. There was no fascia barrier between the obturator and sciaticnerves.5. Ultrasound scans showed that the sciatic nerve appeared as an elongated orfusiformis area of white high echo signals, and the fascial sheath appeared as lines-likewhite high echo signals. The anatomical relationship between the sciatic nerve and its surrounding connective tissues were clearly showed by the T2W1of MRI, which wasconsistent with the cross-sectional and plastinational specimen observation.Conclusion1.There was no multiple ‘‘common epineurial sheath’’ layer that surroundsthe sciatic nerve from its origin to its divisions of tibial nerve (TN) and commonperoneal nerve (CPN).2. The sciatic nerve from the sacral plexus to the piriformissegment has no surrounded fascia-like structure. At the popliteal fossa and posteriorthigh region, the nerve did exist an epineurial sheath layer.3. No intact fascial sheathexisted along the pelvic segment of the sciatic nerve, thus parasacral sciatic anesthesiamay block the obturator nerve.4.At the back of thigh and popliteal fossa, there is a fullfascia sheath around the sciatic nerve, thus anesthetic can spread along this “catheter”which may reduce the dosage of anesthetic drugs and improve the success rate ofanesthesia.5. The sheet plastination displays the macro and micro organizationalstructures and is applicable for the fascia study.