Background and Objective. Contralateral oblique (CLO) angle view has been a useful addition to standard views in fluoroscopically guided interlaminar epidural injections. Determination of the appropriate CLO angle is paramount in the usefulness of this technique. Using MRI laminar angle measurements as a pre-procedural guide for the intra-procedural fluoroscopic CLO angle has been proposed. The purpose of this study was to help determine if using axial MRI laminar measurements prior to a cervical or thoracic epidural steroid injection would be useful in predicting the appropriate fluoroscopic CLO angle.
Study Design. A retrospective review was performed for patients who underwent cervical or thoracic interlaminar injections. In the performance of interlaminar injections, the authors had routinely determined the true fluoroscopic contra-lateral oblique angle after epidural access was confirmed, for use during any potential future injections. The fluoroscopic CLO angle measurements were obtained from a chart review and compared blindly to each patient’s MRI axial laminar angle measurements.
Results. 34 injections were included. Inter-rater reliability comparing the two authors’ MRI angle measurements was considered fair, ICC = 0.395. Accuracy was only 57% comparing MRI laminar angle measurements to within five degrees of the true fluoroscopic CLO angle as determined during the injection procedure. Accuracy by ICC showed only fair agreement, 0.47 and 0.22, for the two authors.
Conclusions. The findings of this study indicate fair inter-rater reliability in manual measurements of laminar angle on axial MRI images. MRI laminar angle measurements do not appear to be highly accurate in determining the appropriate fluoroscopic CLO angle.
Epidural steroid injections are often used in the treatment of cervicalgia and cervical radicular pain [ 1 ]. To gain access to the cervical epidural space an interlaminar approach is often utilized [ 1 ]. In contrast to the historical “blind” approach, fluoroscopy is now typically employed for the purpose of improved safety and accuracy. However, iatrogenic spinal cord injuries have occurred even with the use of fluoroscopic imaging [ 2 , 3 ].
Determining needle depth appropriately during the performance of an interlaminar approach is paramount in avoiding the catastrophic complication of spinal cord injury through needle puncture or through direct injection into the cord [ 1 , 2 ]. Although obtaining loss-of-resistance (LOR) through the ligamentum flavum remains the final step of needle placement into the epidural space, determining the needle depth on imaging has two important roles. First, the LOR portion of the procedure must be initiated near but not through the ligamentum flavum. Although some practitioners favor contacting the lamina to help establish an initial depth [ 1 ], others prefer a more direct approach. Regardless, the depth of the needle tip prior to beginning the LOR portion of the procedure should be accurately determined with the assistance of fluoroscopic imaging. Secondly, as the needle tip is advanced during the LOR portion, remaining cognizant of the needle depth anticipated to achieve epidural access serves as a safety measure against false negative LOR. This is especially important with a midline approach in light of the high frequency of non-fusion of the ligamentum flavum in the midline at the cervical and proximal thoracic levels [ 4 ].
Traditionally, the lateral view has been utilized to determine needle depth. When a midline approach is used the lateral fluoroscopic image should demonstrate accurate needle tip position at or just beyond the ventral margins of the lamina [ 1 ], often referred to as the spinolaminar line on lateral view. As there is usually a paucity of dorsal epidural adipose at the mid and proximal cervical levels, and consistent high cervical flow can be achieved with access from the cervicothoracic junction [ 5 , 6 ], it is recommended that the epidural approach to the cervical region should be obtained at C7/T1 or T1/T2 [ 1 ]. Unfortunately, visualization of the C7/T1 and T1/T2 levels on lateral view is often difficult depending on patient positioning, body habitus, and anatomy [ 7 ]. Some practitioners prefer to use a non-midline lateral interlaminar approach dependent upon the patient symptoms. With a non-midline needle position, the lateral view can no longer provide an accurate needle tip depth. The needle will appear to be more anterior than actual position secondary to the vertebral geometry [ 1 , 7 , 8 ].
Although anterior-posterior and lateral imaging has been standard practice, more recent attention has been directed toward the use of a contralateral oblique (CLO) view for determining needle depth during interlaminar epidural steroid injections [ 1 , 7–10 ]. Obtaining a view in which the x-ray beam is parallel to the obliquity of the lamina in which the needle is passing provides a very accurate image of the needle depth ( Figure 1 ). To obtain this view, the image intensifier is moved to a contralateral oblique position (contralateral from the lamina the needle tip is passing beyond). For example, during a right sided C7/T1 interlaminar injection, the image intensifier is rotated to a left oblique position to “look down” the right C7 and T1 laminae. This concept is very well illustrated in Landers et al. as well as the work of Furman et al. [ 7 , 8 ]. Ideally, LOR should occur at or just beyond the ventral edge of the lamina, referred to as the ventral interlaminar line (VILL), when using this CLO approach.
Some controversy does exist, however, in determining the appropriate contralateral oblique angle. Different authors have proposed various angles ranging from 30 to 60 degrees [ 7–10 ]. Guidelines from the International Spine Intervention Society propose a starting angle of 45 degrees [ 1 ]. It is then recommended to adjust the angle accordingly to obtain an appropriate laminar view with a characteristically wide foramen and longitudinally co-aligned superior articular process and lamina [ 1 ]. Gill et al. [ 11 ] expressed that the previously described method to determine the appropriate contralateral oblique angle is somewhat ambiguous and recommended angular measurements of the lamina on axial MRI. Furman et al. [ 12 ] replied that “measurement [of angle of the lamina on the MRI] is not precise, practical, nor fluoroscopically reproducible.” If an appropriate contralateral oblique angle can be accurately predicted by MRI measurements as described by Gill et al. [ 11 ], it would certainly increase practitioners’ confidence that an accurate needle depth is being visualized.
The purpose of this study is to evaluate the usefulness of pre-procedural MRI laminar angle measurements in determining the appropriate fluoroscopic contralateral oblique angle to be used during cervical or thoracic interlaminar epidural steroid injections.
The study was approved by an independent IRB, Sterling ID#4516-001.
At the time of this study, the authors (DL and SH, both fellowship trained interventional physiatrists), performed cervical interlaminar epidural approach with AP and lateral views and had added the contralateral oblique angle view with some apprehension. Seemingly appropriate contralateral oblique angles had, often times, failed to be close to the correct angle. As confidence in the contralateral oblique view was not strong, the authors typically continued to use a lateral view to determine depth with the understanding that the further lateral from midline the needle tip, the more ventral the position would be perceived on the lateral image. A CLO view was also often employed using the International Spine Intervention Society technique for angle determination [ 1 ]. After the epidural space was accessed by LOR, the true contralateral oblique angle was determined by needle tip position and recorded. The true angle was defined as the fluoroscopic angle in which the needle tip lies within the epidural space and at or just beyond the VILL. The true CLO angle was determined after contrast confirmation of epidural access. If the needle tip lies superficial or deep to the VILL, the fluoroscope was rotated until an oblique angle was such that the needle tip appeared to lie at or just beyond the contralateral oblique ventral laminar line ( Figures 1 and 2 ). The fluoroscopic angle measurement was adjusted appropriately if the patient’s true A–P angle was not zero. This true CLO angle was then recorded in the procedure report. The purpose of routinely determining the true CLO angle measurement was primarily to increase the safety and efficiency of a possible subsequent injection. This also helped to confirm appropriate epidural flow pattern including the cephalad and caudal extent of the contrast spread.
A chart review was performed, retrospectively, through the authors’ practice electronic medical record (EMR). Patients undergoing cervical or thoracic epidural steroid injections were included from December 1, 2012 through October 15, 2013. Patients were excluded if a true contralateral oblique angle was not recorded.
For each patient found to have a documented CLO angle recorded in the procedure note, the patient’s MRI was then blindly reviewed to determine the laminar angle measurement. The measurements were performed independently by two of the authors (DL and SH). MRI measurements were not performed prior the injection procedure, but only completed as part of this review. One patient had a CT only, which was used in place of the MRI.
Technique for MRI laminar measurements: The MRI was blindly reviewed by two of the authors independently (DL and SH) to determine the laminar angle measurement. The authors agreed to use the axial MRI T2 sequence at the level superior to the needle insertion (i.e., for a C7/T1 injection the axial image best demonstrating the C7 lamina was used). As some of the MRIs reviewed had no software to include angular measurements, all MRI angles were determined manually to maintain consistency.
( Figure 3 ) A line was drawn bisecting the spinous process and central portion of the vertebral body or disc (segment A). A second line (segment B) was drawn through the mid portion of the long axis of the left lamina (for a left sided procedure) and extended to cross beyond “segment A.” The angle between segments A and B (the contralateral oblique angle) was determined. Note that opposite angles formed by two intersecting lines are equal per the vertical angles theorem.
Main outcome measures:
MRI angle measurement inter-rater reliability by Intra-class correlation coefficient (ICC).
Accuracy by percentage of the MRI measurement compared with the true CLO angle determined during the fluoroscopic procedure within five degrees. Five degrees error was determined by the authors’ experience in that beyond this amount (plus or minus), the needle tip appeared clearly too anterior or posterior to be considered the true CLO angle.
Thirty-four injections were included. Twenty-five were paramedian, and nine were midline. Thirty-one were performed at C7/T1, two were at T1/2, and one was at T9/10.
The average true fluoroscopic angle was 53 degrees (range 44–61 degrees). The average measured angle on MRI was 48.5 degrees (range 28–59 degrees).
Inter-rater reliability comparing the two authors MRI (one CT) angle measurements was considered fair, ICC = 0.395.
Accuracy of the MRI (one CT) angle measurement to predict, within five degrees, the “true” contralateral oblique angle as determined during the injection procedure was overall, 57% (CI 73.6–40.4).
Individual accuracy rates within five degrees for each of the two authors were 65% and 50%.
Accuracy within five degrees measured by ICC also showed only fair agreement, 0.47, and poor agreement, 0.22, for each of the two authors ( Figures 4–6 ).
The purpose of this study was to help determine if using axial MRI laminar measurements prior to a cervical or thoracic epidural steroid injection would be useful in predicting the appropriate fluoroscopic contralateral oblique angle. This retrospective review provides evidence against using a pre-procedural laminar measurement. The accuracy of the MRI measured contralateral oblique angle allowing five degrees of error was poor. Stated in a different manner, had the MRI measurements been performed prior to the procedure and utilized to determine the fluoroscopic CLO angle (plus or minus up to five degrees), the needle tip would have been at or just beyond the ventral portion of the lamina only 57% (CI 73.6–40.4) of the time.
Our findings are in concordance with an outstanding prospective study by Gill et al. [ 13 ] investigating the appropriate angle for the contralateral oblique view. The authors used 30, 40, 45, and 50 degree angles, an isointense angle with dorsal and ventral laminar lines of equal intensity as well as the MRI laminar measured angle prior to the injection. The MRI measurement technique was similar to that of our study except Gill et al. used computer software rather than manual angular measurements. They described foraminal zones as anterior, middle, and ventral thirds of the foramen and determined needle zone location at each of the above angles once epidural access was achieved. Similar to the findings in the current study, Gill et al. [ 13 ] found that loss of resistance at the ventral interlaminar line occurred in only 14/24 patients (58%) when using the MRI measured angle. However, when using the MRI measured angle, or 50 degrees, the needle tip position was typically within the dorsal third of the foramen and always within the dorsal half of the foramen when loss of resistance occurred. There was no statistical difference between the measured MRI laminar angle and 50 degrees in their study [ 13 ]. Although falling short of providing an exact measurement at which LOR is likely to occur at the VILL, the work of Gill et al. does provide rationale for using 50 degrees CLO angle with the understanding that the needle tip may appear to traverse up to half of the foramen [ 13 ].
As previously mentioned, our intention in recording the true fluoroscopic CLO angle was for use in possible subsequent injections. The average angle was 53 degrees with a wide range of 44–61 degrees. As far as we are aware, no other study has documented the angle at which epidural access was achieved on CLO fluoroscopic view. Although no formal study has yet to be undertaken, anecdotally, using the recorded true fluoroscopic CLO from an initial injection appears to be very accurate for subsequent injections. This is an area for future research.
There are several limitations of our study. The measurements of the MRI laminar angle were performed manually as several of the MRIs did not have angle measurement on the viewing software. However, our average MRI laminar angular measurement was similar to foramen and laminar angle measurements of other studies [ 13 , 14 ]. In addition, the laminar line is rarely perfectly straight, introducing error whether the angle is measured manually or electronically. The authors also did include a single CT in place of the MRI but this was not expected to have altered the results. Although fluoroscopic angle measurements were felt to be accurate, the angle demarcation lines on the c-arm are only at five degree increments and this may have introduced some measurement error as well.
The findings of this study indicate only fair inter-rater reliability in manual measurements of laminar angle on axial MRI images. MRI laminar angle measurements do not appear to be highly accurate in determining the appropriate fluoroscopic contralateral oblique angle. This study does not support the use of pre-procedural laminar angle measurements as a predictive tool to determine the appropriate fluoroscopic CLO angle for cervical or thoracic interlaminar injections.
Funding sources: No funding or other forms of support were provided for this study.
Disclosure and conflicts of interest: There are no conflicts of interest to report.