Summary

Using MRI images from 112 patients, the optimal method (entry point, direction, and depth) for intraoperative adductor canal block injection for postoperative pain control following total knee arthroplasty was suggested to involve inserting at the most concave point of the medial femoral cortex, with a medial angulation of 15.4 degrees and a depth of 31 mm.

Abstract

Background

Intra-operative adductor canal block (ACB) is an efficient method for immediate pain management after TKA, with lower medical resources such as an anesthesiologist, instruments, and time. Previous studies have presented various injection methods, but the number of subjects was limited and there were no examinations utilizing MRI that could be referenced for the actual injection method. The purpose of this study is to 1) measure the angles and lengths to the saphenous nerve according to various entry points and 2) analyze whether practical application of the results to the actual injection method is possible

Methods

This retrospective study was conducted using knee MRI images of 112 patients from our institution. Three injection methods were assumed; (A) entry at the adductor tubercle and injection in the same transverse plane, (B) entry at the most concave site of the medial cortex and injection in the same transverse plane, (C) entry at the adductor tubercle and injection directed towards the saphenous nerve located in the same transverse plane as the most concave site of the medial cortex. Using Picture Archiving and Communication System software, the length of the trans-epicondylar axis, and the angle and distance from the entry point to the saphenous nerve were measured to the 0.1 unit.

Results

The mean age of the subjects in this study was 61.31 ± 10.3 years (range: 24–82), with 47 males and 65 females. The mean trans-epicondylar axis length was 81.8 ± 6.2 mm (range: 70.4–98.3), and there was a significant difference between genders (86.8 vs. 78.2 for males and females, respectively). When using method A, the mean angle was 3.1 ± 11.8 degrees laterally, with a distance of 31.4 ± 7.6 mm; for method B, the mean angle was 15.4 ± 13.3 degrees medially, with a distance of 31.0 ± 7.7 mm; for method C, the mean angle was 7.7 ± 13.2 degrees laterally and 22.6 ± 6.8 degrees proximally, with a distance of 32.8 ± 8.4 mm. There were no significant differences between males and females in all measurements for methods A, B, and C.

Conclusion

When performing intraoperative ACB using the representative landmark on the femur's medial side, targeting the most concave site as the entry point towards the medial direction is considered the most appropriate injection method. When using a 1.5-inch needle, it is recommended to insert it only to a depth leaving approximately 7 mm.