an accurate placement of spinal needle is crucial while injecting drugs . apart from knowledge of anatomy and technical skill , a pre - puncture estimate of skin to subarachnoid space depth ( ssd ) may guide spinal needle placement . a failure in obtaining cerebrospinal fluid despite spinal needle being inserted further than the estimated depth suggests it is offline and needs to be withdrawn and redirected . . a conventional spinal needle may be too long for a lean patient while it may fall short of length in the obese patient resulting in multiple punctures , unsuccessful attempts and increased patient discomfort . while there are several studies on distance from skin to epidural space , studies on ssd are relatively few . of these , most focused on ssd in the paediatric population , considering spinal anaesthesia to be more challenging in children . studies determining ssd in the adult population are few and have not evaluated differences , if any , based on gender . our study hypothesis was that there would be significant differences in ssd between males , females and parturients . we conducted this study ( 1 ) to determine the ssd in indian males , females , parturients and in the overall population ( 2 ) to derive formulae for predicting ssd and ( 3 ) to determine which of the previously suggested formulae ( abe 's , bonadio 's , craig 's , stocker 's and chong 's modified formula ) for predicting ssd best suited our population in terms of both accuracy and ease of application . after obtaining approval from the local ethics committee and informed consent , 800 adult indian patients of either gender , belonging to asa physical status i - iii , scheduled for elective surgical procedures under spinal anaesthesia were included in this prospective , observational study . patients with neurological disorder , history of seizures , spinal anomaly , low back pain , prior back surgery , skin infection at puncture site , sepsis , drug allergies , coagulation defects and other concurrent medical illness where spinal anaesthesia would otherwise be a relative contraindication were excluded from the study . pregnant patients with hypertensive disorder , less than term pregnancy or those scheduled for surgery other than caesarean delivery were also excluded . the procedure of spinal anaesthesia was explained to the patients during their pre - anaesthetic visit and a written informed consent was obtained . all patients received premedication as per our hospital protocol . in the operation room , standard monitoring ( electrocardiogram , pulse oximetry and non - invasive blood pressure ) all patients were co - loaded with 10 ml / kg ringer 's lactate solution . lumbar puncture was performed by an anaesthesiologist with at least 3 years experience in performing lumbar puncture . all procedures were either performed or supervised by a consultant anaesthesiologist . depending on patient 's choice and convenience or requirement for the surgical procedure , the patients were placed either in the lateral recumbent or in the sitting position with their back fully flexed . under aseptic precautions , the l3 - 4 inter - vertebral space was identified , guided by the tuffiers line . dural puncture was performed with a 25/26 gauge quincke ( 3.5 inch/8.9 cm ) spinal needle using the midline approach . the needle was advanced until loss of resistance was obtained , signifying entry into the subarachnoid space and confirmed by free flow of cerebrospinal fluid . patients with traumatic lumbar puncture and those in whom either the angle of spinal needle was altered or the approach was changed from midline to paramedian were eliminated from analysis of ssd . the dose of intrathecal local anaesthetic was decided by the anaesthesiologist conducting the case based on surgical requirement and patient characteristics . following intrathecal injection , the spinal needle was grasped firmly between the thumb and the index finger abutting the patient 's back and removed . demographic and anthropometric data included age , gender , height , weight , body surface area ( bsa ) and body mass index ( bmi ) . the study population was divided into three groups : males ( group m ) , females ( group f ) and parturients ( group pf ) . for all patients , bsa was calculated using the mosteller formula bsa ( m ) = ( [ height ( cm )  weight ( kg)]/3600 ) and bmi using quatelet index bmi = weight ( kg)/height ( m ) . abe 's , bonadio 's , craig 's , stocker 's and chong 's modified formulae were applied individually to all the patients to determine predicted ssd in the overall population . for purpose of comparison the formulae by previous investigators are as follows : abe 's formula : ssd ( cm ) = 17 weight ( kg)/height ( cm ) + 1 bonadio 's formula : ssd ( cm ) = 0.77 cm + 2.56  bsa ( m ) craig 's formula : ssd ( cm ) = 0.03 cm  height ( cm ) stocker 's formula : ssd ( mm ) = 0.5  weight ( kg ) + 18 chong 's modified formula : ssd ( cm ) = 10 [ weight ( kg)/height ( cm ) ] + 1 descriptive statistics for overall population and group - wise ( group m , group f and group pf ) were calculated for all the variables . one way anova with post hoc ( bonferroni correction factor ) analysis was applied to see significant differences among the three groups . forward step wise multivariate regression analysis was performed to see important covariates influencing ssd for each group separately . paired t tests were performed to see significant mean differences between the predicted depth of spinal needle ( after applying formula by bonadio , craig , abe , stocker and chong 's modified ) and observed depth of spinal needle separately . p value 0.05 ( two - tailed ) was considered as statistically significant level . statistical package for social sciences ( spss 19.0 , chicago , il ) a total of 800 adult indian patients were enrolled in the study ; 117 patients were excluded because of protocol violation . thus , data analysis was performed in 683 patients of whom 288 were males , 205 were females who were not pregnant and 190 were parturients . surgical procedure distribution was as follows : gynaecological 149 ( 21.8% ) ; orthopaedic , 208 ( 30.5% ) ; caesarean section , 190 ( 27.8% ) ; general surgery , 76 ( 11.1% ) ; urosurgery , 58 ( 8.5% ) ; cancer surgery , 2 ( 0.3% ) . in 513 ( 75% ) cases the procedure was performed in the lateral recumbent position and in 170 ( 25% ) cases it was performed in the sitting position . the patient characteristics were age 18 to 90 years ; weight 33 to 118 kg ; and height 125 to 186 cm . patient characteristics and ssd , both observed and predicted using various formulae , in the overall study population is presented in table 1 . the observed ssd in the overall study population was 4.71  0.70 cm ( range 2 to 8 cm ) . table 2 shows mean difference between the predicted ssd using various formulae and the observed ssd in the overall study population . the mean difference was least ( 0.01 cm ) and not significant ( p = 0.59 ) when stocker 's formula was applied to our population . the mean difference was statistically significant when abe 's , craig 's , bonadio 's and chong 's modified formulae were applied to our study population . amongst these craig 's formula had the lowest mean difference ( 0.08 cm ) . thus , in terms of accuracy it was next to stocker 's formula [ table 2 ] . patient characteristics and skin to subarachnoid space depth in the overall study population mean difference between the predicted skin to subarachnoid space depth using various formulae and the observed ssd in the overall study population patient characteristics and ssd ( both observed and predicted by formulae ) in the three groups are presented in table 3 . patient characteristics such as age , height and body surface area in group m were significantly different from those in group f and pf [ table 3 ] . bmi values in group m were comparable with those in group f but significantly different from group pf . a statistically significant difference was observed in age , height , weight and bmi values when group f was compared to pf . while applying post hoc bonferroni correction factor comparisons after anova analysis , the observed ssd in group m was significantly different from group f ( p < 0.05 ) but was comparable with group pf ( p = ns ) [ table 3 ] . patient characteristics and skin to subarachnoid space depth , observed and predicted , using various formulae in the three study groups table 4 presents forward step wise multivariate regression analysis performed to determine covariates ( age , weight , height , bmi , bsa ) that influence ssd . in group m , ssd was seen to correlate with weight and age of the patients . in group f correlation was seen with height , bsa and weight of the patients . in group the formulae derived from our study for predicting ssd in the overall population and in the three study groups ( males , not - pregnant females and parturients ) are shown in table 5 . forward stepwise multivariate regression analysis to determine covariates that influence skin to subarachnoid space depth in the three study groups formulae for predicting skin to subarachnoid space depth derived from our study the mean ssd in our overall study population was 4.7  0.70 cm ( range 2 to 8 cm ) . we found gender - based differences in ssd , with ssd in males ( 4.81  0.68 cm ) being significantly greater than that in the female population ( 4.55  0.66 cm ) . ssd in the parturient population ( 4.73  0.73 cm ) was significantly greater than that in the female non - pregnant population ( 4.55  0.66 cm ) . a positive correlation was observed between ssd and the body mass index ( bmi ) in the parturient and the overall population . amongst the previously proposed formulae , the ssd in our subjects ( indian population ) is comparatively shorter than that observed in the western population . reported mean ssd of 5.40  0.66 cm while that in our population is 4.71  0.70 cm . vassiliadis et al . found mean ssd of 5.4  0.7 cm in male patients which is 0.6 cm longer than that found in our male population . found ssd in parturients to be 6.5  1.2 cm which is 1.8 cm longer than that observed in our parturients . the shorter ssd in our population is possibly because of anthropometric differences between the study subjects , our patients being shorter and less heavy compared with the western population . the longer ssd in the parturient population ( 4.73  0.73 cm ) compared with the female non - pregnant population ( 4.55  0.66 cm ) could be attributed to the hormonal effects of pregnancy such as weight gain , softening of tissues and ligaments , and collection of fat in the subcutaneous tissue . based on physical and anthropometric parameters , the mathematical model derived for determining ssd in the overall population is 2.71 + 0.09  bmi . derived a formula for predicting ssd from lumbar puncture depth measured on the computed tomographic scan in 175 patients ( age range 25 days to 80 years ) . in terms of accuracy , stocker 's formula when applied to our study population was found to be the most accurate ; it predicted ssd only 0.01 cm less than the actual observed value . craig 's formula is simple , easy to remember and has height as the only variable that can be easily measured even in bedridden patients ; however , it over - predicted ssd by 0.08 cm . reported that use of their formula resulted in needle selection that was too short in 6% and too long in 31% cases . in our study , ssd predicted using abe 's formula significantly overestimated by 2.44 cm . chong et al . also found that abe 's formula over - predicted ssd by 1.2 cm . therefore , ssd predicted using abe 's formula could result in selecting a relatively longer spinal needle . excess projection of spinal needle beyond the skin may cause difficulty in controlling the needle while injecting the drug , thus increasing the technical difficulty . it could also prompt the clinician to insert the needle too far anteriorly , increasing the risk of traumatic tap or nerve injury . previous investigators have correlated ssd with different demographic and anthropometric parameters . a positive correlation of ssd with age has been reported . the subcutaneous tissue is the most variable layer that is related to weight . in our study , bassiakou et al . also reported correlation between ssd , bmi and body weight in parturients . in male subjects , ssd could be determined as a function of weight and age . vassiliadis et al . found a positive correlation of ssd with age but not with weight , height and bmi in male patients . in females , ssd correlated with weight , height and bsa . reported that an accurate estimation of the depth to reach intrathecal space is possible using ultrasound . however , inaccessibility to this expensive equipment and lack of skill in performing neuraxial ultrasound may limit its role in determining the ssd . the applicability of formulae derived from our study is pertinent only when midline approach is used with spinal needle insertion perpendicular to the skin , in patients with no spinal anomaly . if needle insertion is 30 to the skin surface , skin to epidural distance may be increased up to 1.5 mm for every 10 mm perpendicular distance . a paramedian approach would also increase the needle insertion distance by varying degrees depending on the inclination of the needle . ssd in adult males was significantly greater than that in females who were not pregnant but was comparable with ssd in parturients . in the overall population and parturients , the ssd depended on bmi as the only variable . amongst the various formulae ,
background and aims : a pre - puncture estimate of skin to subarachnoid space depth ( ssd ) may guide spinal needle placement and reduce complications associated with lumbar puncture . our aim was to determine ( 1 ) the ssd in indian males , females , parturients and the overall population ; ( 2 ) to derive formulae for predicting ssd and ( 3 ) to determine which previously suggested formula best suited our population.methods:in this prospective , observational study , 800 adult indian patients undergoing surgery under spinal anaesthesia were divided into three groups : males ( group m ) , females ( group f ) and parturients ( group pf ) . 
 ssd was measured after lumbar puncture . 
 the relationship between ssd and patient characteristics was studied and statistical models were used to derive formula for predicting ssd . 
 statistical analysis included one - way anova with post hoc analysis , forward stepwise multivariate regression analysis and paired t-tests.results:mean ssd was 4.71  0.70 cm in the overall population . 
 ssd in adult males ( 4.81  0.68 cm ) was significantly longer than that observed in females ( 4.55  0.66 cm ) but was comparable with ssd in parturients ( 4.73  0.73 cm ) . 
 formula for predicting ssd in the overall population was 2.71 + 0.09  body mass index ( bmi ) . 
 stocker 's formula when applied correlated best with the observed ssd . 
 formulae were derived for the three groups.conclusions:we found gender - based differences in ssd , with ssd in males being significantly greater than that observed in the female population . 
 ssd correlated with bmi in the parturient and the overall population . amongst the previously proposed formulae , 
 stocker 's formula was most accurate in predicting ssd in our population .