Study On The Distribution Of Intramuscular Nerves And Muscle Spindles Of Extraocular Muscles And Its Significance

Objective: To reveal the distribution of intramuscular nerves and muscle spindles of extraocular muscles,and explore the division of neuromuscular compartment.According to the distribution relationship between intramuscular nerve dense region and muscle spindle,to explore the optimal target position and depth of botulinum toxin A injection to correct strabismus,and provide potential applied anatomical basis for extraocular muscle transplantation and botulinum toxin A injection for strabismus correction.Methods: 1.Specimens: Thirty heads of Chinese adult cadavers aged 35-70 years old without history of neuromuscular diseases and cephalic and ocular malformation(Half male and half female).Among them,20 specimens were fixed with formalin and 10 were freshly collected.2.Design of reference line and puncture angle: The medial canthus was designed as point a and the lateral canthus was designed as point b.The arc line connecting a-b with the skin close to the orbital border(orbital border line)was designed as the reference line.The included angle between the puncture needle and the skin is θ(Acute angle).3.Sihler’s staining: The overall distribution pattern of intramuscular nerves in extraocular muscles of 10 cadavers fixed with formalin were shown by Sihler’s staining.4.HE staining: The extraocular muscles of 10 formalin fixed cadavers were removed.The muscle belly was divided into three parts along the horizontal direction: the origin,the middle and the insertion,the middle part is divided into two compartments parts longitudinally.HE staining was performed,counting and comparing the abundance of muscle spindles in each part.5.Block target localization: The ten fresh cadavers were thawed,the extraocular muscles were dissected and exposed.The corresponding position of the center of intramuscular nerve dense region was labelled with barium sulfate,and spiral CT scanning and three-dimensional reconstruction were performed.Measurement under Syngo system: Percentage position of body surface projection point(P point)of the center of intramuscular nerve dense region on a-b reference line,the included angle between the puncture needle through point P penetrating to the center of intramuscular nerve dense region and the skin(θ)and the depth of the center of intramuscular nerve dense region were measured.6.Statistical analyses.Results: 1.Gross anatomical findings: The superior branch of the oculomotor nerve obliquely extended forward and upward and diverged into the superior rectus branch at 1/3of the deep surface near the origin of the superior rectus.The medial rectus,inferior rectus,and inferior oblique branches,which entered into the muscle laterally from the origin of the muscle belly,deep from the origin of the muscle belly,and laterally from the middle part of the muscle belly,respectively,all stemmed from the inferior branch of the oculomotor nerve.The abducens nerve was near the medial surface of the lateral rectus and entered the muscle at 1/3 of the length of the muscle’s origin.The trochlear nerve passed over the superior rectus and the levator palpebrae superioris,traveled medially anteriorly,and entered the muscle on the lateral surface at 1/3 of the origin of the superior oblique muscle,and the entry muscle no following blood vessels were found.2.Sihler’s staining findings: We observed an intramuscular nerve dense region formed by the intramuscular arborized branches in the semitransparent levator palpebrae superioris,superior rectus,inferior rectus,medial rectus,lateral rectus,superior oblique and inferior oblique muscles with Sihler’s staining technique.The seven extraocular muscles can each be divided into two neuromuscular compartments.The intramuscular nerve dense regions of the levator palpebrae superioris,superior rectus,inferior rectus,medial rectus,lateral rectus,superior oblique and inferior oblique muscles were positioned at(27.72～66.07)%,(33.50～72.72)%,(40.21～66.79)%,(37.92～64.51)%,(31.69～56.01)%,(26.35～64.98)% and(40.46～73.20)%of the lengths of the muscle bellies,respectively,and the centers of intramuscular nerve dense regions were located at 50.67%,59.45%,54.30%,51.50%,50.05%,48.31%,and56.14% of the length of each muscle belly,respectively.The aforementioned values are the means of the actual values.3.Muscle spindle abundance: The abundance of muscle spindle in the origin,middle and insertion of the muscle belly of seven extraocular muscles were compared,and the middle of the muscle belly was the highest.That is,the abundance of muscle spindles in the intramuscular nerve dense region is the highest.The spindle abundance of intramuscular nerve dense region of seven extraocular muscles were inferior rectus,superior oblique,lateral rectus,medial rectus,superior rectus,inferior oblique and levator palpebrae superioris.Comparison of muscle spindle abundance between the two compartment of intramuscular nerve dense region of each extraocular muscle,P>0.05.no statistical difference were found.4.Spiral CT localization of the center of intramuscular nerve dense region: The body surface projection point(P point)of the center of intramuscular nerve dense region of levator palpebrae superioris,superior rectus,inferior rectus,medial rectus,lateral rectus,superior oblique and inferior oblique was located on68.42%,53.43%,53.21%,11.76%,98.56%,30.15% and 43.73% of a-b line(orbital margin line),respectively.The angles θ of the puncture needle and the skin were 83.54°,69.75°,74.15°,82.98°,80.58°,83.69° and 71.52°,respectively.The depth of the center of intramuscular nerve dense region was 3.05 cm,2.71 cm,2.82 cm,3.02 cm,3.00 cm,3.09 cm and 2.18 cm,respectively.The body surface projection point(P point)of the center of intramuscular nerve dense region of levator palpebrae superioris,superior rectus,medial rectus,lateral rectus and superior oblique was located on supraorbital rim;inferior rectus and inferior oblique located on infraorbital rim.The data of aforementioned results were all mean value.5.The comparison between left and right and men and women,no statistical difference were found(P>0.05).Conclusions:(1)Each of the seven extraocular muscles has an intramuscular nerve dense region in the middle of the muscle belly,and each muscle can be divided into two neuromuscular compartments,which suggests that the compartmentalized muscle transplantation can be considered when the extraocular muscle transplantation is used to correct strabismus,which is beneficial to give consideration to the function of donor and recipient muscles at the same time.(2)The middle part of the extraocular muscle(i.e.intramuscular nerve dense region)has the highest muscle spindle abundance,and the center of intramuscular nerve dense region should be recommended as the optimal target for botulinum toxin A injection in the treatment of strabismus;(3)The accurate definition of the body surface puncture position,needle-puncturingangles and depth of center of intramuscular nerve dense region of the these extraocular muscle may provide morphological guidance for injecting botulinum toxin A to correct strabismus.