thus , the high prevalence of olfactory dysfunction after surgery or head trauma has a great effect on overall quality of life . following head injury with a fracture across the cribriform plate ,
a minor blow to the occipital area may result in anosmia due to tearing or shearing of olfactory nerve filaments .
the olfactory nerve and tracts are also at risk during intradural explorations using the interhemispheric approach or the frontotemporal approach [ 10 , 11 ] .
both approaches require some degree of frontal lobe retraction , which may result in temporary or permanent olfactory dysfunction because of nerve avulsion or mechanical compression [ 3 , 4 , 6 , 12 ] .
furthermore , it is well known that bilateral anosmia is highly likely to occur when bilateral olfactory nerves are pulled out from the frontal base .
it is not always true that preservation of the anatomical structure will result in preservation of function .
however , preservation of anatomical structure may be an important factor in the preservation of olfactory function . based on this background
, strength tests of the human olfactory nerve at the frontal skull base were performed using cadavers , and the microsurgical anatomical features of this nerve were reviewed in order to identify operative nuances that may contribute to reducing the rate of postoperative olfactory dysfunction .
all procedures were conducted in accordance with the institutional guideline of jikei university and the tokyo medical examiner s office .
ninety brains were obtained from autopsies of 68 men and 22 women , aged 2480  years at the time of death .
they were within 12  h after death and had no intracranial disease or trauma .
the calvaria was opened , and the brain was removed en bloc after the olfactory nerves were severed 1015  mm from the skull base with minimal retraction ( figs .  1 and 2a ) .
the calvaria was positioned so that the frontal base was perpendicular to the ground without rotation .
then , the cut edges of the olfactory nerves were clipped and connected to a light , small rubber bag by a monofilament string ( fig . 
2b ) . in the first set of 30 cases ( group 1 ) , each right olfactory nerve
was pulled 0 laterally ( from the axis of the olfactory nerve ) and 0 upward ( from the anterior skull base ) ( 1 ) ) , and each left olfactory nerve was pulled 0 laterally and 15 upward ( 2 ) ) . in the second set of 30 cases ( group 2 ) , each right olfactory nerve
was pulled 0 laterally and 15 upward ( 3 ) ) , and each left olfactory nerve was pulled 15 laterally and 15 upward ( 4 ) ) . in the third set of 30 cases ( group 3 ) ,
each right olfactory nerve was pulled 15 laterally and 15 upward ( 5 ) ) , and each left olfactory nerve was pulled 30 laterally and 15 upward ( 6 ) ) .
the angle of pulling against the frontal skull base and the axis of the olfactory nerve was regulated by pulling .
the olfactory nerves were pulled by a rubber bag through a string while water was poured into the bag at a rate of 1  ml per 10  s until the nerve was pulled out from the cribriform plate .
then , the string , clip , and the rubber bag filled with water were weighed , and this value was used as the strength of the olfactory nerve ( fig . 
b the cut edges of the olfactory nerve are clipped and connected to a light , small rubber bag by a monofilament string . whole view ( c ) and schematic drawing view ( d ) of this study .
the angle of pulling of the olfactory nerve from the frontal skull base and from the axis of the olfactory nerve is regulated by pulling .
the olfactory nerve was pulled by a rubber bag through a string during which water was poured into the bag at 1  ml per 10  s until the nerve was pulled out from the cribriform platefig . 3the
supine ( a ) and sagittal ( b ) schematic drawing views of the anterior skull base .
numbers show the direction of pulling of the olfactory nerves in each group schematic drawing view of the technique of olfactory nerve dissection a superior view of the anterior skull base .
b the cut edges of the olfactory nerve are clipped and connected to a light , small rubber bag by a monofilament string . whole view ( c ) and schematic drawing view ( d ) of this study .
the angle of pulling of the olfactory nerve from the frontal skull base and from the axis of the olfactory nerve is regulated by pulling .
the olfactory nerve was pulled by a rubber bag through a string during which water was poured into the bag at 1  ml per 10  s until the nerve was pulled out from the cribriform plate the supine ( a ) and sagittal ( b ) schematic drawing views of the anterior skull base .
numbers show the direction of pulling of the olfactory nerves in each group statistical analysis was performed using the mann 
whitney u - test for comparisons of the nonparametric variables with a probability value of 0.05 indicating significance .
in group 1 , the strengths ( mean  standard deviation [ sd ] ) of the olfactory nerve for ( 1 ) and ( 2 ) were 3.681.74 and 3.091.60  g , respectively . the difference between the strengths of ( 1 ) and ( 2 ) was not significant ( p  =  0.191 , mann 
whitney u - test for comparisons of the nonparametric variables ) . in group 2 , the strengths for ( 3 ) and ( 4 ) were 3.141.87 and 4.051.70  g , respectively .
the difference between the strengths of ( 3 ) and ( 4 ) was almost significant ( p  =  0.068 , mann 
whitney u - test for comparisons of the nonparametric variables ) . in group 3 , the strengths for ( 5 ) and ( 6 ) were 4.001.44 and 4.201.65  g , respectively .
the difference between the strengths for ( 5 ) and ( 6 ) was not significant ( p  =  0.684 , mann 
whitney u - test for comparisons of the nonparametric variables ) ( fig .  4 ; table  1).fig .
4graphs showing the strength of pulling for different directions in each grouptable 1olfactory nerve strength(1 ) g(2 ) g(3 ) g(4 ) g(5 ) g(6 ) gmean3.683.093.144.054.004.20sd1.741.601.871.701.441.65min.1.700.460.120.510.691.41max.10.006.357.466.798.207.72sd standard deviation , min .
maximumnumbers in parentheses indicate the strength of the olfactory nerve measured as defined in fig .  3 : ( 1 ) 
pulled 0 laterally ( from the axis of the olfactory nerve ) and 0 upward ( from anterior skull base ) ; ( 2 ) , ( 3 )  pulled 0 laterally and 15 upward ; ( 4 ) , ( 5 )  pulled 15 laterally and 15 upward ; ( 6 )  pulled 30 laterally and 15 upward graphs showing the strength of pulling for different directions in each group olfactory nerve strength sd standard deviation , min .
maximum numbers in parentheses indicate the strength of the olfactory nerve measured as defined in fig .  3 : ( 1 ) 
pulled 0 laterally ( from the axis of the olfactory nerve ) and 0 upward ( from anterior skull base ) ; ( 2 ) , ( 3 )  pulled 0 laterally and 15 upward ; ( 4 ) , ( 5 )  pulled 15 laterally and 15 upward ; ( 6 )  pulled 30 laterally and 15 upward and in each subgroups of pulling for different directions from ( 1 ) to ( 6 ) , there seemed to be no relationship between age and olfactory strength ( fig .  5 )
moreover , there were no statistical differences of olfactory strength between male and female in each subgroup.fig .
5age ( x - axis ) and strength of pulling for different directions from ( 1 ) to ( 6 ) ( the y - axis ) age ( x - axis ) and strength of pulling for different directions from ( 1 ) to ( 6 ) ( the y - axis )
the results of this study indicated that frontal lobe retraction in a posterior direction from the olfactory bulb , regardless of angle from the skull base posteriorly , is associated with the risk of pulling out the olfactory nerve , while retraction in a lateral direction appears to be relatively safe .
this was based on the fact that that there was no significant difference in the strength of the olfactory nerves pulling them in a postero - upward direction between 0 and 15 upward ( each , 0 laterally ) . furthermore , the difference in the strengths of the olfactory nerves when they were pulled in a postero - lateral direction between 0 and 15 laterally ( each , 15 upward ) was almost significant .
it had been thought that only serious injuries of the head with fractures of the anterior cranial fossa resulting in anatomical disruption of the olfactory filaments or tract cause permanent anosmia , but permanent anosmia can result from even trivial injuries .
previous studies have reported anosmia after surgical treatment of anterior communicating artery ( acoa ) aneurysms and suggest that its prevalence depends on the mode of approach to the aneurysm .
it has been mentioned that there are three mechanisms of olfactory nerve damage during frontal lobe retraction ( depression and elevation ) : partial or total avulsion of the olfactory nerve from the cribriform plate [ 14 , 6 , 9 ] ; injury during nerve dissection ; and direct pressure damage [ 2 , 9 ] .
suggested that maintaining the anatomic integrity of the nerve is important to preserve olfactory function .
even a mild retractive pressure on the nerve can lead to temporary or permanent lesions [ 2 , 6 ] .
it is said that this is because the microvasculature lying on the dorsal surface of the nerve , which is constituted of microscopic branches of the ethmoidal arteries nourishing the nerve , is fragile to retraction and compression of the frontal lobe .
suzuki at al .   reported that 42% of patients with bilateral preservation of the olfactory nerves in bifrontal craniotomy for anterior communicating artery aneurysms developed anosmia .
they also reported that one - third of the patients who had both olfactory tracts damaged or severed during surgery had normal olfactory function .
invisible factors affecting the olfactory nerve , such as heat injury to the nerve by drilling the bone surrounding the nerve or direct pressure damage to the medial frontal lobe ( septal area ) by retraction , might cause anosmia .
furthermore , vries at al .   reported no improvement of olfactory nerve function at follow - up after having olfactory nerve damage after the operation , which indicates that the regenerative capacity of the olfactory nerve is limited .
thus , in cases in which a mechanical disruption ( neurotmesis ) of the fila olfactoria has occurred , regeneration would not be expected . on the other hand , rouit and
murali   noted that traumatic anosmia may recover at any time from a few days up to 5  years .
however , in our research , we have focused only on maintaining the anatomic integrity of the olfactory nerve .
anatomically , the olfactory bulb is located on the cribriform plate of the olfactory fossa , which is a couple of millimeters lower than the surrounding cranial base .
the depth of the olfactory fossa is 4.8 ( 0.611.7 ) mm in the anterior part , 5.0 ( 015.5 ) mm in the middle part , and 3.2 ( 010.0 ) mm in the posterior part .
thus , the olfactory filaments are fairly well protected in the cribriform plate against the shearing stresses of the brain that often accompany even minor injuries to the head . from our research ,
the olfactory nerve could easily be pulled out irrespective of the degrees of pulling from the frontal base towards the posterior direction .
however , the olfactory nerve could withstand more force when pulled laterally against the axis of the olfactory nerve .
this is because there is only a tiny bone in the posterior wall ( planum sphenoidale , frontal edge ) compared to the anterior wall of the olfactory fossa . on the other hand
, there is a large lateral wall bilaterally along the entire olfactory fossa ( fig . 
thus , it might hold the nerve in the cribriform plate when it is pulled laterally .
there is a possibility that the pulling force is absorbed by the lateral wall of the olfactory fossa when the nerve is pulled laterally .
however , there was no significant difference in the strength of the olfactory nerve between pulling the nerve 15 and 30 laterally .
therefore , with frontal lobe retraction in a posterior direction from the olfactory bulb , the olfactory nerve is at high risk of being pulled out , but in a lateral direction , retraction is relatively safe from the perspective of maintaining the anatomical integrity of the olfactory nerve .
however , if the nerve is pulled more laterally , it is pressed harder against the bone of the lateral wall of the olfactory fossa and is twisted against the axis of this nerve .
but these data should be considered in realistic surgical settings like bifrontal approach and pterional approach , and one should try to measure the strength of olfactory nerve drawn by spatula pressure and of the own weight of the brain shifting to get a better clinical impact.fig . 6histological study of the olfactory fossa in the coronal view demonstrating that there is a large lateral wall bilaterally at any part of the olfactory fossa .
hematoxylin and eosin ( h&e ) stain , original magnification 20 histological study of the olfactory fossa in the coronal view demonstrating that there is a large lateral wall bilaterally at any part of the olfactory fossa .
hematoxylin and eosin ( h&e ) stain , original magnification 20 several techniques have been reported to prevent perioperative olfactory nerve injury [ 1 , 2 , 4 , 9 , 12 ] .
most of them are techniques that minimize the effect on the olfactory nerve from frontal lobe retraction during surgery .
nakayama   recommended that frontal lobe retraction should be done towards the superficial direction to put less tension on the olfactory nerve .
suzuki at al .   reported that , to prevent nerve injury on the operated side , dissection of the nerve at the beginning of the microsurgical procedures is recommended , and the key to successful separation of the olfactory tract from the brain is never to apply pressure in a downward direction posteriorly from the olfactory bulb , but to continually apply upward pressure ( toward the olfactory bulb ) on the frontal lobe to which the tract is adherent .
aydin et al .   reported that olfactory nerve function could be preserved at a relatively high rate of 85% .
this high rate probably resulted from the microtechnique used during the relatively cautious frontal lobe retraction , which was less than 1.5  cm .
in addition , the nerve is pulled out by a strength of only 34  g. the present study had some limitations because , during the actual operation , the nerves are pulled while retracting the brain .
thus , these data of strength alone do not provide definitive information that would be useful during operation .
if preoperative coronal ct scanning is done , and the surrounding bone structure of the cribriform plate is investigated , that is , the depth of the olfactory fossa from the surrounding anterior skull base in every direction , which direction the olfactory nerve can be pulled to minimize damage to the nerve as much as possible can be identified . in each group , the absolute value of the strength of the olfactory nerves varied widely .
this difference may come from not only the direction of pulling the nerve , but also from gender , aging , the length of time after death , and other factors .
this study found that frontal lobe retraction in a posterior direction may be dangerous from the point of view of the postoperative prevalence of anosmia , but retraction in a lateral direction may be relatively safe .
 olfactory dysfunction may influence the quality of life tremendously . 
 this study investigated the strength of the human olfactory nerve at the frontal skull base using cadavers . 
 a total of 180 olfactory nerves were examined in 90 human cadaveric heads . 
 the cut edges of the olfactory nerves were pulled until they were pulled out from the skull base . in the first set of 30 cases , each right olfactory nerve 
 was pulled 0 laterally and 0 upward , and each left olfactory nerve was pulled 0 laterally and 15 upward . in the second set of 30 cases , each right olfactory nerve was pulled 0 laterally and 15 upward , and each left olfactory nerve was pulled 15 laterally and 15 upward . in the third set of 30 cases , 
 each right olfactory nerve was pulled 15 laterally and 15 upward , and each left olfactory nerve was pulled 30 laterally and 15 upward . 
 the strength of the olfactory nerve was measured when pulled in each direction . 
 there was no significant difference in the strength of the olfactory nerves when pulling them in the postero - upward direction between 0 and 15 upward . 
 the strengths of the olfactory nerves when pulling them in the postero - lateral direction 0 and 15 laterally were 3.141.87 and 4.051.70  g ( mean  standard deviation [ sd ] ) , respectively ; the difference was almost significant . 
 the olfactory nerve could be pulled more laterally than posteriorly because the retraction force is absorbed by the lateral wall of the olfactory fossa . 
Introduction
Methods
Results
Discussion
Conclusion