Inferolateral collagen bundles as inseparable elements in the posterior layer of the thoracolumbar fascia

The posterior layer of the thoracolumbar fascia, which includes the aponeuroses of several skeletal muscles among its components, has long been of interest in relation to low back pain. This layer has been described as consisting of two laminae, a superficial and deep. These laminae are known to fuse tightly at a lower thoracic level, forming a unified posterior layer. However, the precise anatomical continuity of these components, particularly in the lower lumbar and sacral regions, remains unclear. Therefore, we performed a comprehensive macroscopic analysis of the posterior layer, focusing on the composition and orientation of collagen bundles (CB) within the aponeurosis of its component muscles, while taking anatomical variations into account. We examined 17 sides from 11 cadavers, focusing on the lumbar to the upper sacral vertebral region. In all specimens, no distinct retinacular sheet could be separated from the aponeurotic complex of the posterior layer, even in the lower lumbar and sacral regions where previous studies illustrated a deep lamina. In the upper lumbar region, the aponeuroses of the latissimus dorsi (LD) and serratus inferior muscles contributed to the posterior layer, forming a continuous structure that could not be divided into distinct layers. Meanwhile, in the lower lumbar region, the posterior layer comprised the aponeuroses of the LD and internal oblique muscles, along with a bundle of fibers extending from the periosteum of the iliac crest. In the upper sacral region, the posterior layer comprised the aponeurosis of the LD, erector spinae, and gluteus maximus muscles, together with the bundle of fibers from the periosteum of the iliac crest. Despite some variations in the site of muscle attachment, the posterior layer consistently contained CB that are inferolaterally oriented from the lumbar to iliac crest spinous processes between the lumbar and upper sacral vertebrae, in a direction independent of the orientation of muscular aponeuroses. These consistent directional patterns may suggest a functional integration of muscular and fascial components, regardless of anatomical variation. Considering the structural features of the posterior layer, further studies are needed to clarify how dynamic changes in this layer during trunk movement may contribute to clinical conditions, including low back pain.