The medial branch of the lateral branch of the posterior ramus of the spinal nerve
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- Saito, T., Yoshimoto, M., Yamamoto, Y. et al. Surg Radiol Anat (2006) 28: 228. doi:10.1007/s00276-006-0090-3
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In the needle insertion of epidural anesthesia with the paramedian approach, the needle can pass through the longissimus muscle in the dorsum of the patients. When the needle touches a nerve in the muscles, the patients may experience pain in the back. Obviously, the needle should avoid the nerve tract. To provide better anesthetic service, analysis of the structure and where the concerned nerves lie in that region is inevitable. Material and method: We studied five cadavers in this study. Two cadavers were fixed with Thiel’s method. With these cadavers, we studied the nerve running of the posterior rami of the spinal nerve from the nerve root to the distal portion. Three of them were used for the study of transparent specimen, with which we studied the course and size of the nerve inside the longissimus muscle. Results: We observed there were three branches at the stem of the posterior rami of the spinal nerves between the body segment T3 and L5, i.e. medial branch, medial branch of the lateral branch and lateral branch of the lateral branch. The medial branch of the lateral branch supplied to the longissimus muscle. With the transparent specimen, we found that there were different nerve layouts between the upper thoracic, lower thoracic, upper lumbar, and lower lumbar segments in the medial branch of the lateral branch in the longissimus muscle. In the lower thoracic and upper lumbar segments, the medial branch of the lateral branch of the upper lumbar segments produced layers nerve network in the longissimus muscle. L1 and L2 nerves were large in size in the muscle. Conclusion: In the upper lumbar segments the medial branch of the lateral branch of the posterior rami of the spinal nerve produced dense network in the longissimus muscle, where the epidural needle has high possibility to touch the nerve. Anesthetists have to consider the existence of the medial branch of the lateral branch of the posterior rami of the spinal nerve when they insert the needle in the paramedical approach to the spinal column.
KeywordsSpinal nervePosterior ramusMedial branchTransparent specimenLongissimus muscleAnesthesia
After Cathelin FM started caudal anesthesia in 1901 and Dogliotti AM tried lumbar epidural anesthesia in 1933, epidural anesthesia has become very popular in the management of surgical interventions. During the needle insertion in the epidural anesthesia, patients sometimes complain of pain in the back at the needle insertion. This is because the needle touches or pierces a nerve in the longissimus and spinous muscles. Although the textbooks require topical anesthesia in advance of the insertion of the needle, they do not describe even where to anesthetize [1,2,5,9].
In the paramedian approach in the epidural anesthesia where the needle can aim the epidural space through the narrow spatium intervertebrale independent from the shape of spinous process, the needle may pass through the longissimus muscle. Therefore, we think that it is worthwhile examining the nerve layout in the longissimus muscle.
Materials and methods
Demographic data of the cadavers
Cadavers for transparent specimens
Cadavers fixed with Thiel’s method
Step 1: study of the posterior rami of the spinal nerve with fixed cadavers
The first step in this study was to investigate the proximal portion of the posterior rami. Two cadavers were allocated to this study. The cadavers were fixed by Thiel’s methods [12,13]. After performing the Thiel’s methods, their vertebral columns between T3 and L5 with dorsal muscles were taken from the rest of the cadaver body. The limit of the specimen was decided by the range of the actual epidural anesthesia. We had enclosed both sides of the latissimus dorsi and the dorsal muscles. We had cut out the ribs bilaterally at the lateral border of the latissimus dorsi correspondingly. To take out the specimen from the cadavers the quadrates lumborum and the psoas muscles were dissected. The aorta, the heart and the lungs were removed from the specimen. The corpus vertebrae, spinal cord and the processus transverses were removed. On these blocks, we studied the tract of the posterior rami of the spinal nerve from the anterior region toward the posterior, i.e. from the proximal region toward the distal. This study enabled us to examine the branching pattern of the posterior rami near the stem to the rami.
Step 2: study with transparent specimens
Three cadavers contributed to this study. We took the musculus longissimus between T2 and L5 to use for the study. This muscle bloc was processed to make a transparent specimen with the modified Spalteholz’s technique [8,11]. With the transparent specimens, we studied the course and the size of the posterior branches in the longissimus muscle.
For the first specimen, we removed the back muscle of a 76-year-old male together with the skin from the posterior surface of the vertebras and the ribs. That is, the longissimus and a part of the spinous muscles were dissected at the posterior surface of the rib, of the intercostal muscles, of the transverse process, of the intertransversal ligament, of the arcus vertebrae, of the ligamentum flavum, of the spinous process, and of the interspinous ligament. The skin, subcutaneous tissue and the iliocostalis muscle were removed from the specimen. The specimen was made of the longissimus and the part of the spinous muscles. To stain the nerves we used toluidine blue . After that we bleached the specimen and performed a freeze substitution . However, this specimen was not used for further anatomical analysis because the staining was not satisfactory. Toluidine caused rapid browning of the whole specimen, which let us abandon the specimen.
The second specimen was a longissimus muscle of a 71-year-old female. For the second specimen, we adopted iron alum for staining. We bleached and dehydrated the specimen to obtain bright tissue . In the process of impregnation, we cut the muscle block into about four segments, so that each block can get the maximum effect of benzyl benzoate. The transparent specimens were mounted in glass cubes, and we examined the nerve running inside the specimen without any traumatic damage to the specimen.
The last specimen was sectioned from the backside of an 89-year-old male. We used the Thiel nerve fermentation methods [12,13] combined with bleaching and dehydration  and tanning with iron alum and made the specimen transparent, thereafter .
The longissimus muscle covers the spinous muscle and the vertebral column. The needle for anesthesia may pass through the longissimus muscle before the needle enters into the spinous muscles. When the needle touches any nerve, it is anticipated that the nerve is damaged to some extent. There is a recent report of the nerve damage of the medial branch of the posterior rami of the spinal nerve. Boelderl et al. reported the danger of damaging the medial branch of the posterior rami of spinal nerve during orthopedic surgical intervention . But there is no report as yet about the structural analysis and the possibility of the nerve damage in the longissimus muscle.
The longissimus muscle is supplied by the medial branch of the lateral branch of the posterior rami. After we extensively examined the branching pattern of the posterior rami of the spinal nerve and the layout of the medial branch of the lateral branch, these factors were found different according to each body segment. Between T3 and T5, the medial branch of the lateral branch originated from the lateral branch of the posterior rami at a point distant from the branching point of the posterior rami of the spinal nerve. The medial branch of the lateral branch went farther lateral to enter into the longissimus muscle from the medial side of the muscle. The size of the nerve in diameter was almost the same for both. At these body segments, the longissimus muscle is located the most laterally, farthest from the vertebral column. Therefore, it is quite natural that the nerve enters into the longissimus muscle from the medial side. In the anesthetic practice, because the longissimus muscle is located very laterally and relatively far from the vertebral column, it is not likely that the needle punctures the longissimus muscle and touches the nerves inside.
At T6, we did not identify (see) the lateral branch of the posterior ramus in any cadaver studied regardless of either for the study with fixed cadaver or that with transparent specimen. We do not know the reason of the non-existence of the lateral branch at T6. Future investigations are necessary.
In the lower thoracic and the upper lumbar regions between T7 and L2, we found that branching pattern of the posterior rami was more like triple branches, which supplied to the posterolateral region of the trunk, i.e. the medial branch, the medial branch of the lateral branch, which is more like an intermediate branch, and the lateral branch of the lateral branch. Branching points of the medial branch from the lateral branch were very close to the point where the posterior rami divides into the medial branch and lateral branch. Carlson reported this branching pattern as three branches in the posterior rami in the lumbar region in cats . He reported that the lateral, intermediate and medial branches are specifically distributed to the iliocostalis, longissimus, and multifidus, respectively. Our finding between T7 and L2 coincides with his report. In the study with transparent specimen, the medial branch of the lateral branch of the posterior rami did not show their trace at these segments. We think that this is because the nerve enters the longissimus muscle from the anterior edge and branches extensively after the nerve enters into the muscle. Between the T10 and L2 segments, the medial branch of the lateral branch was shown to have entered into the muscle from the lateral side. This is because the longissimus muscle changed its position. The muscle came close to the midline as the body segment came caudal. Therefore, it is very natural that the nerve entered into the muscle from the lateral side after it originated at the upper edge of the tip of the transverse process.
Between T10 and L2 segment, the medial branch of the lateral branch descends in the longissimus muscle for about three segments within the longissimus muscle. Therefore, the nerve of the upper, middle, and lower segments produce layers within the muscle. At L1 and L2 body segments, the medial branch of the lateral branch of the posterior rami is the biggest in size, which goes medially. Therefore, the needle for epidural anesthesia may pierce or touch the nerve at L1 and L2 body segments. The layout of the nerve of this region should be better examined to avoid pain caused by the nerve damage by needles. The layout of the nerve in this region is thought to be complicated by the medial branches of the lateral branch and the medial branch of the posterior rami of the spinal nerve.
We feel a three dimensional study about the relation of the layout of the medial branch of the lateral branch of the posterior rami in the longissimus muscle and needle path is necessary to know the exact possibility of the nerve damage in the longissimus muscle.