INTRODCUTION: In Lumbar Multifidus (LM) studies, previously, inconsistent descriptions of morphology were identified, especially in ultrasonography and electromyography research, hampering its clinical applicability with regard to diagnosis and therapy (1). The aim of this study was to describe the lumbar multifidus (LM) sonoanatomy by comparing high-resolution reconstructions from a 3-D digital spine with a healthy participants’ standard LM ultrasonography and set a basis for evidence-based research methodology.

METHODS: An observational study was carried out and comparisons with high impact literature were made. From three deeply frozen human tissue blocks of the lumbosacral spine a large series of consecutive high-resolution photographs with 78 μm interval were acquired that were reformatted into 3-D tissue blocks (voxel size 78 μm) (2). This enabled the reconstruction of (semi-)oblique cross-sections that could match standard ultrasound (US) images obtained from a healthy volunteer. The volunteer was matched to the specimen based on gender and age. Transverse and oblique short-axis views were compared from the most caudal insertion of LM to L1.

RESULTS: Based upon the anatomical reconstructions we could clearly distinguish LM from the adjacent erector spinae (ES) in the standard US imaging of the lower spine (Fig_1A;B). At the lumbosacral junction, LM is the only dorsal muscle facing the surface. From L5 upwards, ES progresses from lateral to medial, to completely cover LM at about L2. A clear distinction between deep and superficial LM could not be discerned. Only in caudal anatomical cross-sections, but not in the standard US images, we could identify five separate bands between every lumbar spinous process and the dorsal part of the sacrum, with a medio-lateral and superficial-deep orientation.

DISCUSSION: The detailed cross-sectional LM anatomy and reconstructions facilitate the interpretations of standard LM US imaging, position of the separate LM-bands, details of deep interspinal muscles, and demarcation of LM versus ES. Clear identification of deep versus superficial versus lateral LM as described earlier (3) could not be verified. Limitation of the study is that only two human specimens were used to construct the High-Resolution 3-D tissue blocks, which may limit external validity. The use of a more detailed sonoanatomy improves interpretation of standard LM US-imaging. In this manner guidelines for studies using ultrasonography and electromyography can be developed, which eventually should also be implemented in evidence-based research and therapy.

Figure 1. Transversal cross-sections (A, C) and matched ultrasonographic (US) views (B, D) at the level of the posterior superior iliac spine (PSIS). ES originates from the PSIS and from the dorsal ligaments of the sacro-iliac joint (SI-J). (A) S1= dorsal spine S1. (B) Matched US view in human volunteer. At this level, LM is the only superficial muscle (demarcated by white dotted line). Deep to it the origin fibers of ES (*) can be discerned at the dorsal part of the sacro-iliac joint (SI-J) as separate structure. (C) S1= dorsal spine S1. (D) Same curvilinear view as in Fig. B showing the different ultrasonographic composition of the ES fibers (demarcated by white dotted line), deep and lateral  to LM.