The middle layer of lumbar fascia and attachments to lumbar transverse processes: implications for segmental control and fracture

Abstract

The anatomy of the middle layer of lumbar fascia (MLF) is of biomechanical interest and potential clinical relevance, yet it has been inconsistently described. Avulsion fractures of the lumbar transverse processes (LxTP's) are traditionally attributed to traction from psoas major or quadratus lumborum (QL), rather than transversus abdominis (TrA) acting via the MLF. This attachment is also absent from many biomechanical models of segmental control. The aims of this study were to document: (1) the morphology and attachments of the MLF and (2) the attachments of psoas and QL to the LxTP's. Eighteen embalmed cadavers were dissected, measuring the thickness, fibre angle and width of the MLF and documenting the attachments of MLF, psoas and QL. The MLF was thicker at the level of the LxTP's than between them (mean 0.62: 0.40 mm). Psoas attached to the anteromedial surface of each process and QL and TrA to its lateral border; QL at its upper and lower corners and TrA (via the MLF) to its tip. In three cadavers, tension applied to the MLF fractured a transverse process. The MLF has a substantial and thickened attachment to the tips of the LxTP's which supports the involvement of TrA in lumbar segmental control and/ or avulsion fracture of the LxTP's.

Fig. 1

Arrangement of the lumbar fasciae at L4 level. Adapted from Barker et al. [3]. ALF, MLF, PLF anterior, middle and posterior layers of lumbar fascia, Ps psoas, QL quadratus lumborum, TrA transversus abdominis, IO and EO internal and external oblique, ES erector spinae

Fig. 2

Attachments to transverse processes at L3. a The attachment of the ALF divides the attachments psoas (medially) and QL (laterally). QL has tendinous attachments to each corner (QL₁ and QL₂). b The MLF attaches to the lateral edge and tip of the transverse process and is more extensive posteriorly. Legends as for Fig. 1

Fig. 3

Middle layer of lumbar fascia. Note the MLF is continuous with TrA and the borders and tips of the transverse processes have been marked and pinned (respectively). Legends as for Fig. 1

Fig. 4

Mean MLF thickness at and between transverse processes. Note greater mean thickness of MLF at the level of the transverse processes compared to between the processes. Error bars on the histogram indicate standard deviations

Fig. 5

Dissection of transverse process attachments. Note the relation of psoas (Ps) and quadratus lumborum (QL) attachments to the L2–L4 transverse processes (pinned at their tips). QL’s attachments are in two layers (QL₁ vertical and QL₂ oblique) to the corners of the TP’s, while psoas’ attachments lie medial to the tips