Lumbar anatomy may differ considerably from common understanding. Netter’s Atlas of Human Anatomy has several mistakes in the lumbar region.
Figure 1: MRI axial section through the body of L4. Note the position and size of the labeled structures
Multifidus: The multifidusmuscle fibers make up the medial portion of the “spinal erectors” as shown above.
- The small deep laminar fibers attach superiorly to the inferior portion of the lamina of each lumbar vertebra and travel two segments caudal to the mamillaryprocess. The L5fibers cross one segment and attach to the sacrum. These fibers can be considered homologous to the thoracic rotatores. There are no lumbar rotatores.
- The bulk of the multifidusattaches superiorly to the spinousprocess of each lumbar vertebra and runs inferiorly2-5 segments attaching caudally to the mamillaryprocess, sacrum and iliac crest. See L2and L3example image below (Figure 2).
- The multifidusdoes notattach to the transverse process and stays within the yellow highlighted area above (Figure 1), until the iliac crest and sacral fibers travel more lateral below L5.
- The multifidusis approximately 2 inches deep and 3/4 inch wide.
- Netters anatomy and ART have this completely wrong!
Each lumbar veterbra has multifidus fibers with similar orientation. It should become clear how all of these fibers create layers that make up the medial portion of the “erector” group as seen in Figure 1 above.
Contrary to Netter’s and many technique systems, there are no lumbar muscles that travel from transverse process to spinous process. See Bogduk’s Clinical Anatomy of the Lumbar Spine for an in depth review (interesting but not a necessary level of detail).
The primary function of the multifidusis to stabilize the lumbar spine; specifically to prevent flexion when the abdominals and obliques contract. This also generates significant compressive (stabilizing) force on the lumbar spine. This is why the lumbar multifidus quickly becomes atrophied with lumbar disc problems. The disuse atrophy is secondary to protective weakness to prevent further compressive load on the disc.
The lumbar portion of the longissimus makes up the “erector group” immediately lateral to the multifidus. Figure 1 green shading.
- The longissimus attaches to the accessory process (on the posterior medial portion of the transverse process) superiorly on each lumbar vertebra and attaches to the iliac crest just lateral to the PSIS inferiorly.
- The longissimusis approximately 2 inches deep and 3/4 inch wide.
- The upper lumbar segments run almost purely vertical while the lower lumbar segments have a significant anterior to posterior orientation. See figure 3 below.
It should become clear how these fibers make up the middle portion of the “erector” group as highlighted green in figure 1.
The longissimus provides compressive and extension force through the lumbar spine. The lower lumbar fibers are particularly helpful in helping to prevent anterior shear, as occurs with standing flexion.
The Iliocostalis makes up the lateral portion of the “erector group” immediately lateral to the longissimus. Figure 1 purple shading.
- Iliocostalis attaches superiorly to the tip of transverse process and approximately 1 inch lateral along the middle layer of the thoracolumbar fascia of L1-L4. Notice how at least half of the muscle lies lateral to the transverse process in figure 1.
- Iliocostalis attaches caudally along the iliac crest lateral to the PSIS.
- These fibers are almost identical in orientation to the longissimus just further lateral.
- Iliocostalis provides compression of the lumbar spine, extension and contributes to lateral flexion and rotation (given the attachment on the lateral portion of the transverse process).
- Iliocostalisis is approximately 1 1/4 inches wide and 2 inches deep.
Immediately superficial to the multifidus, longissimus and iliocostalis lies the erector spinae aponeurosis (ESA). The ESAis made up of the caudal tendons of the thoracic fibers of the longissimus and iliocostalis. The point is, there is a thin aponeurosis covering the multifidus, longissimus and iliocostalis running in a superior to inferior direction. Figure 4 below.
Immediately superficial to the ESA lies the thoracolumbar fascia. This attaches to the spinous processess at midline and runs to the lateral edge of the iliocostalis (lumbar portion described above). At this lateral attachment it meets with additional fascia layers to form the lateral raphe.
When it comes to treatment it is critical to know exactly what structures are between your contact and the adhesion.
If you place your thumb immediately lateral to the L4spinousprocess you will palpate the following structures from superficial to deep:
- Subcutaneous fat
- Posterior layer of thoracolumbarfascia
- Erector spinaeaponeurosis
- Lamina of vertebra