A modification of the standard midline posterior approach to the intertransverse area of the lumbar spine

Abstract

A midline approach to the lumbar region is most frequently used for posterior lumbar spine surgery. The exposure of the deeper layer of muscles, however, is imprecise and can entail substantial tissue damage and blood loss. During 10 years of operative surgical experience, we have developed an improved and less traumatic technique for exposure of the lumbar transverse processes and intertransverse region in which the tendons of multifidus and longissimus muscles are isolated at every level and divided laterally to the facet joints. This method eases identification and accurate cauterisation of the subjacent arteries, thereby reducing tissue damage and blood loss. It takes no more time and clarifies the exposure of the lumbar transverse processes and intertransverse region. Cadaveric dissection confirms the muscular and arterial anatomy of the region. We recommend use of this modified approach to improve standard practice.

A mid-line approach to the lumbar region is the most common approach for posterior lumbar spine surgery and is standard for posterior scoliosis surgery. Posterolateral fusions undertaken with this approach need division of the deep muscle attachments lateral to the line of the facet joints. Present descriptions of this approach to the lateral gutter are not precise, and unnecessary tissue damage and blood loss occur as a result. Control of blood loss is important and can be crucial in young children.

We propose a technique that reduces tissue damage and blood loss and has been defined by cadaveric dissection. It has been our standard practice for more than 10 years.

Materials and Methods

Development of new technique

The standard posterior approach to the lumbar spine is made through a longitudinal incision. The thoracolumbar fascia is incised lateral to the supraspinous ligament, and the paraspinal muscles are raised laterally off the spinous processes, laminae, and facet joints with a Cobb elevator and a gauze swab. To expose the transverse processes for a posterolateral fusion, the dissection continues down the lateral side of the superior facet and onto the transverse process.¹ We have always found that the subperiostal dissection out to the facet joints was fairly avascular, but that the dissection lateral to the facet joint was less precise and associated with more bleeding, the sources of which were difficult to identify. Wiltse and Spencer² noted similar difficulty with haemostasis in this area with a paraspinal approach. They did not define the precise muscular anatomy in the depth of the exposure.

We noted that the muscle attachments lateral to the facet joint were not diffuse or random but, in the exposure, were mainly organised into two bundles passing in rostrolateral and caudolateral directions and with musculotendinous attachments to bone immediately lateral to the facet joint capsule. A Cobb dissector placed between these two bundles and withdrawn laterally exposes the transverse process (Fig. 1). No bleeding occurred when we isolated and divided these two attachments close to the bone, and we were able to expose the arteries beneath easily. The vessel beneath the rostral attachment is the main source of bleeding with the standard approach. With our method of exposure, these vessels on the lateral and rostral side of the superior articular process and the dorsum of the base of the transverse process could be cauterised precisely and accurately, with long, angled forceps. A Cobb dissector on a gauze swab then exposed the rest of the dorsum of the transverse process out to its tip, with minimum bleeding.

Figure 1.

Operative photograph of the L4/5 level of the lumbar spine on the left side. A curved forceps passes beneath each of the two muscle bundles, multifidus (M) and longissimus (L), which attach to the bone just lateral to the facet joint capsule (C). The transverse process (TP) of L5 lies between them. The midline structures (S) comprise the spinous processes and interspinousand supraspinous ligaments.

Assessment of muscular anatomy by cadaveric dissection

In the embalmed cadaver, we identified the two muscle bundles exposed during the operative procedure, which attach to bone laterally to the capsule of the facet joint, as being part of multifidus and longissimus, respectively. The multifidus and longissimus muscles occupy the gutter between the spinous processes and the transverse processes in the lumbar region (Fig. 2). The undisturbed fibres of both muscles run in a caudolateral direction from the mid-line. The multifidus is the largest and most medial of the deep lumbar back muscles. It consists of a repeating series of fascicles, radiating from the spinous processes and laminae of the lumbar vertebrae to attach to the mamillary process (the inferolateral part of the superior articular process) of the vertebra two or more levels caudad. The longissimus muscle is lateral to the multifidus and has deeper attachments. It consists of five fascicles, each of which arises from the accessory process (situated close to the mamillary process but slightly more inferior, lateral and deeper). The upper four fascicles diverge to form the lumbar intermuscular aponeurosis, which attaches to the pos-teromedial part of the ilium.³ Thus, both the multifidus and the longissimus have bony attachments close to the capsule. The multifidus attaches to the mamillary process and the longissimus to the accessory process (Fig. 3). The longissimus lies in a deeper plane, and the orientation of its fascicles is not changed by the operative approach. The natural orientation of the multifidus fascicles, however, is changed: the subperiostal separation of the muscle attachments of the multifidus to the spinous processes and laminae changes the orientation of its fascicles at operation from caudolateral to caudomedial. This process is shown in the peri-operative photograph (Fig. 1) and clarified diagram-matically in Figure 4.

Figure 2.

Diagramatic cross-section of lumbar spine to show the arrangement of the deep back muscles.

Figure 3.

Diagram of posterior view of a lumbar vertebra to show the position of the superior articular process (SAP), the mamillary process (MP) the accessory process (AP) and the transverse process (TP).

Figure 4.

Diagram of muscle attachments at the L4/5 and L5/S1 levels. The separate muscle bundles at L4/5 are replaced at the L5/S1 level by a fascial sheet or aponeurosis that extends laterally over the sacral ala and is separated from t by a fat pad. The positions of the arterial branches are indicated.

The attachments of the multifidus and longissimus remain the same at every level of the lumbar spine, but we have noted that the anatomy is different at the lumbosacral junction. Here, the separate muscle attachments are replaced by a fascial sheet, extending laterally from the lateral surface of the superior articular process of S1 (Fig. 4). Division of this fascial sheet close to its attachment releases fat from a fat pad, invariably separating the fascia from the ala sacri. This arrangement is consistent and changed only with sacralisation of the fifth lumbar vertebra when the fascial sheet is attached to the lateral surface of the superior articular process of the sacralised L5 vertebra.

Arterial pattern and haemostasis

Macnab and Dall⁴ have described the arterial pattern lateral to the superior articular process and at the base of the transverse process. The vessels are the continuation of the lumbar segmental arteries, passing posteriorly on each side around the waist of each vertebra. The location of the vessels is constant, although their identification at operation is difficult because of their placement in the depth of the exposure with the overhang of the facet joints and, lateral to the capsule, the muscle attachments of the multifidus to the mamillary process and longissimus to the accessory process (Fig. 4). Division of the fascicles of these muscles close to bone gives access to the deeper part of the exposure, enabling cauterisation of the underlying vessels before they bleed. The cauterisation is eased by the use of long, angled forceps to compensate for the overhang.

Discussion

Text-book descriptions of the approach to the intertransverse area of the lumbar spine do not provide much detail. Watkins,⁵ in his description of the approach to the transverse process for an intertransverse fusion, advises extension of the electro cautery cutting of the facet joint capsule down to the base of the transverse process. Weinstein¹ proposes continuing the dissection down the lateral side of the superior facet and onto the transverse process. Both descriptions guide the surgeon to the intertransverse plane, but blood loss and tissue damage will be reduced with an improved understanding of the anatomy of the deeper layers of the approach.

We have consistently found that there are two principal and discrete muscle attachments to bony landmarks – the mamillary process and the accessory process. Precise confirmation of the muscle groups from which the bundles arose was not easy in the embalmed cadaver. Furthermore, the initial identification of the upper bundle (multifidus) was not obvious because its orientation was changed by the operative exposure. Our experience has shown that this pattern at operation is consistent throughout the lumbar spine down to the L5/S1 junction.

The finding that the above configuration is replaced at the L5/S1 level by a fascial layer or aponeurosis extending from the capsule and lateral surface of the superior articular process is another constant. We have found no previous description of this fascia. We did not feel that to widen the operative exposure for anatomical identification alone was acceptable. That it lies at the same depth and has a similar medial attachment to bone suggests that the fascial layer could represent a fusion of the lowermost fascicles of multifidus and longissimus. The anatomical difference at this level might be an indicator of the relative lack of movement at the lumbosacral level. Alternatively, based partly on the plane in which the exposed fascia lies, it could represent a superficial component of the iliolumbar ligament complex.

Appearances at cadaveric dissection favoured insufficient movement at the lumbosacral level. Whatever its exact anatomical identity, the fascial sheet is a constant finding and a landmark to identification of the ala. The consistent finding that this arrangement moves cephalad by one level with sacralisation of the fifth lumbar vertebra suggests that the definition of a transitional segment should apply not only to the bony structures but also to the soft tissues.

Conclusions

We believe that the modification that we use of the standard posterior midline approach to the intertransverse area leads to reduced tissue damage and blood loss, and provides a clearer operative field. Furthermore, it adds no extra time to the operation. We therefore recommend the use of this method.