A description of the lumbar interfascial triangle and its relation with the lateral raphe: anatomical constituents of load transfer through the lateral margin of the thoracolumbar fascia

Abstract

Movement and stability of the lumbosacral region is contingent on the balance of forces distributed through the myofascial planes associated with the thoracolumbar fascia (TLF). This structure is located at the common intersection of several extremity muscles (e.g. latissimus dorsi and gluteus maximus), as well as hypaxial (e.g. ventral trunk muscles) and epaxial (paraspinal) muscles. The mechanical properties of the fascial constituents establish the parameters guiding the dynamic interaction of muscle groups that stabilize the lumbosacral spine. Understanding the construction of this complex myofascial junction is fundamental to biomechanical analysis and implementation of effective rehabilitation in individuals with low back and pelvic girdle pain. Therefore, the main objectives of this study were to describe the anatomy of the lateral margin of the TLF, and specifically the interface between the fascial sheath surrounding the paraspinal muscles and the aponeurosis of the transversus abdominis (TA) and internal oblique (IO) muscles. The lateral margin of the TLF was exposed via serial reduction dissections from anterior and posterior approaches. Axial sections (cadaveric and magnetic resonance imaging) were examined to characterize the region between the TA and IO aponeurosis and the paraspinal muscles. It is confirmed that the paraspinal muscles are enveloped by a continuous paraspinal retinacular sheath (PRS), formed by the deep lamina of the posterior layer of the TLF. The PRS extends from the spinous process to transverse process, and is distinct from both the superficial lamina of the posterior layer and middle layer of the TLF. As the aponeurosis approaches the lateral border of the PRS, it appears to separate into two distinct laminae, which join the anterior and posterior walls of the PRS. This configuration creates a previously undescribed fat-filled lumbar interfascial triangle situated along the lateral border of the paraspinal muscles from the 12th rib to the iliac crest. This triangle results in the unification of different fascial sheaths along the lateral border of the TLF, creating a ridged-union of dense connective tissue that has been termed the lateral raphe (Spine, 9,1984, 163). This triangle may function in the distribution of laterally mediated tension to balance different viscoelastic moduli, along either the middle or posterior layers of the TLF.

Fig. 1

Modified with permission from Willard Vleeming Schuenke: fig. 9 in: The thoracolumbar fascia: anatomy function and clinical considerations. Submitted to the Journal of Anatomy, 2012. This is a transverse section of the posterior (PLF) and middle layer (MLF) of the thoracolumbar fascia (TLF) and related muscles at the L3 level. Fascial structures are represented such that individual layers are visible, but not necessarily presented to scale. Please note that the serratus posterior inferior (SPI) often is not present caudal to the L3 level. The transversus abdominis (TrA) muscle is covered with a dashed line on the peritoneal surface illustrating the transversalis fascia (TF). This fascia continues medially covering the anterior side of the investing fascia of the QL. Anteriorly and medially, the transversalis fascia (TF) also fuses with the psoas muscle fascia (not drawn). The internal (IO) and external oblique (EO) are seen external to TrA. SPI is highly variable in thickness and, more often than not, absent on the L4 level. Latissimus dorsi (LD) forms the superficial lamina of the PLF together with the SPI, when present. The three paraspinal muscles, multifidus (Mu), longissimus (Lo) and iliocostalis (Il) are contained within the paraspinal retinacular sheath (PRS). The aponeurosis (tendon) of the paraspinal muscles (4) is indicated by stippling. Please note that the epimysium of the individual spinal muscles is very thin and follows the contours of each separate muscle within the PRS. The epimysium is not indicated in the present figure but lies anteriorly to the aponeurosis (4). The upper circle shows a magnified view of the different fascial layers contributing to the MLF. The picture shows that MLF is made up of three different structures: (1) this dashed line depicts the investing fascia of QL; (3) this dashed line represents the PRS, also termed the deep lamina of the PLF encapsulating the paraspinal muscles; (2) the thick dark line between the two dashed lines 1 and 3 represents the aponeurosis of the abdominal muscles especially deriving from TrA. Numbers 1, 2 and 3 form the MLF. The lower circle shows a magnified view of the different fascial layers constituting the PLF. The picture shows that on the L3 level the PLF is also made up of three layers, as the fascia of SPI is normally present on this level. (5) This dashed line depicts the PRS or deep lamina of the PLF encapsulating the paraspinal muscles; (6) the investing fascia of SPI is seen blending medially into the gray line marked (7) and representing the aponeurosis of SPI- posteriorly to the PRS; (8) this dark line represents the investing fascia of LD blending medially into the black line representing the LD aponeurosis (9) posteriorly to the SPI aponeurosis. Numbers 5, 7 and 9 form the PLF. Numbers 7 and 9 form the superficial lamina of the posterior layer (sPLF).

Fig. 2

A comparatively large lumbar interfascial triangle (LIFT) at the L3 vertebral level. Note the fatty composition of the LIFT. The right pincer is pulling the junction of the anterior and posterior laminae of the TA aponeurosis (small dashed outline). The left pincer is pulling on the PRS (large dashed outline). Inset: magnified view of LIFT without dashed lines. ALF, anterior layer of thoracolumbar fascia (transversalis fascia); ATA, anterior lamina of TA aponeurosis; LIFT, lumbar interfascial triangle; LR, lateral raphe; MLF, middle layer of thoracolumbar fascia; PRS, paraspinal retinacular sheath; PTA, posterior lamina of TA aponeurosis; QL, quadratus lumborum; TAAPO, transversus abdominis aponeurosis.

Fig. 3

T1 MRI tracing demonstrating the relationship of the paraspinal retinacular sheath (PRS) and the aponeurosis of the transversus abdominis (TAAPO). TAAPO splits into anterior and posterior laminae, which separately join the PRS. The lumbar interfascial triangle (LIFT, indicated by ‘*’) resides in between the laminae of the TAAPO. The anterior lamina of the TAAPO blends with the investing fascia of quadratus lumborum (QL) and the PRS to form a thickened middle layer of thoracolumbar fascia (MLF). (Inset A) Magnified view of LIFT from the left side of image, without labels. (Inset B) Magnified view of LIFT from the right side of image, without labels. ESA, erector spinae aponeurosis; PM, psoas major; SLPLF, superficial lamina of posterior layer of thoracolumbar fascia.

Fig. 4

A schematic and simplified view of the bifurcation of the TA and IO aponeurosis and the paraspinal retinacular sheath (PRS), creating the lumbar interfascial triangle (LIFT). A represents the empty space normally occupied by the paraspinal muscles and enclosed by the PRS. The aponeurosis of the transversus abdominis (TA) and internal oblique (IO) bifurcates into anterior and posterior laminae. The anterior lamina contributes to the middle layer of the thoracolumbar fascia (MLF). The posterior lamina contributes to the deep lamina of the posterior layer of the thoracolumbar fascia (PLF). The lateral raphe (LR) represents a thickened complex of dense connective tissue at the lateral border of the PRS, from the iliac crest caudally to the 12th rib cranially. The junction of the TA aponeuroses with the PRS creates the LIFT, which is at the core of the LR. Thus, the raphe is formed at the location where abdominal myofascial structures join the PRS surrounding the paraspinal muscles. sPLF, superficial lamina of PLF.