Participation in an Advanced Anatomy Capstone Project Facilitates Student Involvement in the Development of an Instructional Tool for Novel Dissection

Table 1

Dorsal primary ramus dissection guide.

General approach. When planning this dissection, it is important to recognize that the branches of the dorsal primary rami (DPR) pass not just dorsally, but quite inferiorly as they descend several vertebral levels from the bifurcation of the DPR and the ventral primary rami (VPR). To ensure preservation of the superficial nerve branches contained in the muscles of the low back, the approach detailed below begins inferiorly and superficially and follows the nerve branches back to their deeper, more superior origins.
Dissection step	Instructions
(1) Landmark identification and skin removal	Basic dissection: place the cadaver in prone. Remove the skin but preserve the subcutaneous fat in the lower thoracic, lumbar and upper gluteal regions. Advanced dissection: the skin should initially be preserved in order to identify the superior cluneal nerves (SCNs). Begin by labelling with marker key landmarks in the region of interest which will be useful as the dissection progresses. These include the spinous processes of T10 to L5, the posterior superior iliac spine (PSIS), the twelfth rib, and the iliac crest from the PSIS to the anterior superior iliac spine (ASIS). The SCNs typically pierce the thoracolumbar fascia (TLF) just superior to the rim of the iliac crest and continue inferiorly in the adipose tissue until they reach their target tissues in the skin of the upper gluteal region. You will, therefore, look for the SCNs in the subcutaneous fat and skin of the upper gluteal region beginning just below the iliac crest, in a zone between the PSIS medially and the tubercle of the iliac crest laterally.
(2) Identification and cleaning of superior cluneal nerves	Advanced dissection (Figure 1): make a vertical cut through the skin running from the PSIS superiorly to the level of the L4 spinous process and make a second parallel vertical cut laterally at the midaxillary line. Connect these two cuts with shallow horizontal scalpel cuts, which will result in a rectangular region in which the terminal branches of the SCNs can be found. Begin to peel the skin downwards while using a probe and forceps to identify the cutaneous branches of the SCNs passing through the subcutaneous fat. After you have identified the SCN branches, carefully clean the fat that surrounds each one. Making sure to preserve this region of skin and subcutaneous fat supplied by the SCNs, you can now remove all other proximal skin and subcutaneous fat up to the thoracolumbar junction (TLJ) exposing the TLF and superficial back muscles.
(3) Identification of lateral branches of dorsal primary rami piercing thoracolumbar fascia	Basic and advanced dissection (Figure 1): identify how the SCNs merge into the lateral branches of the T12 and L1 DPR, which penetrate the TLF and then pass deep into the iliocostalis (IC) lumborum muscle. Continue to trace the lateral branches of the DPR as they pass through the deeper layers of posterior spinal musculature proximally to their origin at the TLJ.
(4) Removal of thoracolumbar fascia	Basic and advanced dissection: to clear the dissection field, remove the TLF, except for the inferior portion through which the SCNs pass. Remove the serratus posterior inferior muscle and all superficial fascia from the surface of the ES muscles. Use your fingers and probe to define the borders between the IC, longissimus (LG), and spinalis muscles.
(5) Transection and reflection of erector spinae muscles	Basic and advanced dissection: the next part of the dissection involves cutting and reflecting the ES muscles inferiorly in order to identify the lateral and medial branches of T12 and L1. Begin by making a transverse cut through the IC and LG muscles at the level of T10. You do not need to transect or reflect the spinalis or the multifidus muscles. Then, carefully reflect IC and LG inferiorly using your hands and scissors as needed (Figures 2 and 3). You will need to cut LG away from spinalis in order to reflect it inferiorly. Palpate for the floating ribs in order to orient yourself to the correct spinal level.
(6) Cleaning and following the T12 lateral branch from the L1 transverse process distally towards the iliac crest	Basic and advanced dissection: as you peel back the IC and LG muscles inferiorly, you will see lateral branches of the higher spinal nerves (T9–T11) penetrating these muscles. Cut these higher branches in order to facilitate the downward reflection of these muscles. Use a probe and blunt scissors to hollow out the muscle tissue surrounding the lateral branches of T12 and L1 so that the nerve will not be stretched and torn as you reflect the muscles. Carefully clean and follow-up to 3 levels of lateral branches from where they exit the lateral foramen, pass over the transverse process of the vertebrae below, and continue into the IC muscle.
(7) Identification of the medial branch of T12	Basic and advanced dissection: identify the medial branch of the T12 DPR as it branches medially off of the DPR at about an 80-degree angle (Figures 2 and 4). Careful removal of connective tissue surrounding the nerve, using scissors and mosquito forceps, is required in order to expose the course of this delicate branch. Advanced dissection: however, before attempting the full dissection of the medial branch, identify and highlight the borders of the mamillo-accessory ligament (MAL).
(8) Cleaning and opening of tunnel under mamillo-accessory ligament	Advanced dissection (Figure 4): the MAL passes over the T12 medial branch running from the mammillary process of the superior articular facet of L1 to the accessory process of the transverse process (TP) of L1. Use the tip of the mosquito forceps to open the tunnel under the MAL and carefully clean and expose the medial branch on both sides of the MAL.
(9) Following the medial branch to its targets	Basic and advanced dissection: medial to the MAL, you can follow the medial branch as it passes into the multifidus (MF) muscle (Figure 5). In a very careful dissection, you may be able to identify the articular branches from the medial branch that pass into the superior and the inferior facet joints (Figure 5).
(10) Finishing cleaning of the T12 dorsal primary ramus	Basic and advanced dissection: follow the nerve superiorly, if possible cleaning the DPR up to its bifurcation with the ventral primary ramus (VPR) and the spinal nerve of T12. Although it is not strictly required for this dissection, removal of some portions of the multifidus muscle may aid in exposing more of the spinal nerve, as well as portions of the medial branch.
(11) Repetition of dissection with L1 dorsal primary ramus	Basic dissection: repeat the above steps for the L1 DPR so that you cleanly demonstrate DPR and medial and lateral branches. Advanced dissection: expose the superior cluneal nerves, associated with this spinal nerve.
(12) Exposure and cleaning of TPs and facet joints of T12 and L1	Basic and advanced dissection: clean tissues from the TPs of L1 and L2 and the T12/L1 and L1/L2 facet joints to clearly demonstrate their relationships to the nerve branches.