The present study supports the hypothesis that BMI correlates with the depth of transforaminal ESI. A 3.5-in (8.89 cm) needle will suffice for patients with BMI of less than 25 kg/m2. A 5-in (12.7 cm) needle will be required for some patients with a BMI of 25–30 kg/m2 (preobese) and most patients with a BMI of more than 30 kg/m2 (obese classes I, II, and III). Occasionally, a 7-in needle is necessary for patients with a BMI of greater than 30 kg/m2. The largest patient studied had a BMI of 66.8 kg/m2, and the epidural space was reached at 16.5 cm. This left an extra 1.3 cm of available needle length on a 7-in needle, indicating that the transforaminal epidural space can usually be accessed even on patients with extreme obesity. The power analysis conducted was intended to detect a difference between patients with a BMI of less than 25 kg/m2 and greater than 30 kg/m2 and calculated group sizes of at least 16 patients per group for each of these categories. There may be intergroup differences between neighboring groups (ie, obese classes I and II) that were not detected in this study.
There was no association between BMI and degree of oblique angulation, which is not surprising. The degree of angulation is based on the fluoroscopic view of the relationship between the facet joint and vertebral body. The difference in body habitus between the BMI groups would not be expected to affect the bony landmarks used for fluoroscopic positioning.
D’Alonzo et al16
found an association between ethnicity and epidural depth. Although the present study was not powered to detect a difference between different ethnic groups, there was no association between ethnicity and epidural depth when BMI was controlled. The predicted difference if BMI is equal to 30 kg/m2
found by D’Alonzo et al was the greatest when comparing African Americans and Asians and was only 0.7 cm (5.9 vs 5.2 cm, respectively). The present study did not include enough Asian patients to compare with the African American group in a subgroup analysis. Future studies with larger sample sizes may detect a difference between ethnic groups. However, given the small difference noted in the interlaminar study, the difference may not be clinically significant.
Although it is not surprising that there is a positive association between BMI and transforaminal epidural depth, the results of this study hold clinical significance. The available data allow physicians to select an appropriately sized needle and can lead to improved patient comfort and practice efficiency. Longer needles are more expensive and can be more difficult to direct. Therefore, they are not recommended in all patients. Previous interlaminar epidural depth studies do provide interesting data; however, they do not impact clinical practice in the same manner. Clinkscales et al13
showed in obstetric patients that the mean depth to the interlaminar epidural space plus 1 SD in all of the BMI groups was below the 9-cm length of a standard Tuohy needle (9 cm). In the largest group studied, the mean depth in the BMI >50 group (n = 27) was 7.5 ± 1.2 cm.13
In interventional pain medicine, however, there is a greater selection in needle length, thereby making these data important clinically.
The oblique angle used in the present study is greater than that which has been described in some textbooks. In the Atlas of Image-Guided Intervention in Regional Anesthesia and Pain Medicine
the author recommends that the C-arm be “rotated obliquely 20 to 30 degrees until the facet joint and pars interarticularis are clearly visualized,” and in the corresponding figure, the zygapophyseal joint is approximately 30% of the distance from the lateral borders of the corresponding vertebral bodies.24
The International Spine Intervention Society does not specify an average degree of fluoroscopic obliquity; however, the fluoroscopic images provided do show a lesser degree of obliquity.25
In the present study, the fluoroscope was angled until the facet joint was at the midpoint of the vertebral body at the targeted level. This is the preferred technique of the authors as it allows for more medial direction of the needle and a true transforaminal injection. Lesser angles direct the needle in a more anterior direction and can lead to a selective nerve root block instead of the desired transforaminal injection. In the present study, the median needle depth was 9.3 cm, and the mean oblique angle was 31.5 ± 8.9 degrees. Assuming that the transforaminal depth represents the hypotenuse of a right triangle, the midline depth would be projected to be 7.9 cm. If the midline angle is fixed at 7.9 cm and the oblique angle is reduced to the lower 20-degree angle describe by Rathmell,24
the estimated transforaminal depth would be estimated to be 8.4 cm, representing a 9.7% decrease in estimated needle depth (). A less than 10% decrease in depth would be well within the interquartile range in each of the BMI groups, thereby indicating that the present data are relevant regardless of the preferred approach to the transforaminal epidural space.
FIGURE 3 The estimated transforaminal epidural depth using a lesser oblique angle would be close to the depth found in the present study. The median depth in the present study was 9.3 cm with an oblique angle of 31.5 degrees. The midline length, assuming a right-angled (more ...)
One could postulate that the transforaminal depth could be estimated using the previously established data on interlaminar lumbar epidurals, but our data show this not to be the case. Using the above trigonometric approach and interlaminar depth data published by Clinkscales et al13
(obese class I, 5.3 ± 0.9 cm), the transforaminal depth in the obese class I population would be predicted to be 6.4 cm, if assuming an oblique fluoroscopic angle of 33.8 degrees (oblique angle for obese class I; ). The median transforaminal depth in this study was 10 cm, however, representing a difference of 3.6 cm. This differential can be explained by the difference in muscle and fat distribution. The paraspinous muscles and adipose tissue in the low back elevate the transforaminal entry point above that of the interlaminar insertion site. Clinkscales et al13
showed that the mean interlaminar epidural depth difference between the obese class II and normal-BMI patients was only 1.4 cm (6.2 vs 4.8 cm, respectively; 29% increased depth), whereas the difference in the median between the same groups in the present study was 2.9 cm (10.4 vs 7.5 cm, respectively; 38.6% increased depth). This again indicates that the difference in BMI has a greater impact when a transforaminal approach is selected.
There is a great deal of variability in the length and type of needles used for transforaminal ESIs. In our clinic, 3.5-, 5-, and 7-in, 22-gauge straight Quincke-tip needles are used. The data in and are presented such that they can be applied to all clinical practices regardless of the needle size used.