Minimally Invasive Surgery (MIS) and Fusion of the Lumbosacral Junction (L5-S1)
Ajit Jada
Sertac Kirnaz
Mauricio J. Avila
Connor Berlin
Roger Härtl
As the evolution of spinal surgery continues, there has been a transition from open approaches to more minimally invasive spine (MIS) techniques. The open, or macrosurgical, exposures are associated with increased bleeding, muscular denervation, increased scar tissue formation, and local pain syndromes.1 MIS techniques are readily applied to the lumbar spine,1 whereas the lumbosacral junction (L5-S1) presents challenges specific to this region which sometimes limits the use of MIS procedures.
The mechanics and load-bearing nature of L5-S1 differs from the rest of the lumbar spine, and transitional vertebrae, C7-T1, T12-L1, and L5-S1, all have unique characteristics and pathology associated with them. For example, isthmic spondylolisthesis most commonly occurs at the L5-S1 level of the lumbar spine.
There are several fusion options available for L5-S1, and the challenge for the surgeon is not only to perform the operation but to select the best surgical approach for a given patient. Techniques available for MIS fusion of the lumbosacral junction include anterior lumbar interbody fusion (ALIF) with anterior and/or posterior fixation, transforaminal lumbar interbody fusion (TLIF), axial lumbar interbody fusion (AxiaLIF), oblique lumbar interbody fusion (OLIF), and posterior lumbar interbody fusion (PLIF). Each technique has benefits and contraindications but the decision on which fusion to employ ultimately depends on the clinical and radiographic characteristics of the patient.
ANATOMY OF L5-S1
The intricate structures present at L5-S1 make any surgery at this level challenging. Before deciding on either an anterior or posterior approach, several anatomical considerations should be addressed, such as the internal iliac artery and vein, middle sacral artery and vein, sympathetic chain, lumbosacral trunk, and sigmoid colon and rectum, all of which lie directly over the sacrum. These structures should be evaluated on preoperative imaging prior to determining a surgical approach.2
The sacrum unites the two pelvic halves, and it has distinctive biomechanical features which enable the transition of axial load from the lumbar spine to the pelvis.3 The widest anterior-posterior diameter of the sacrum is 47 to 50 mm at S1 and it narrows 28 to 30 mm at the S2 level.2,4
The sacrum is mostly cancellous bone composed of five fused vertebrae, except the sacral alae and promontory which contain primarily cortical bone. The poor bony architecture of the sacrum makes instrumentation challenging. Instrumentation in this region may result in pseudarthrosis or hardware failure, which makes the lumbosacral junction more difficult to fuse than the rest of the lumbar spine.5, 6, 7
When comparing other aspects of the lumbar spine, the disk heights are similar between L4-L5 and L5-S1; usually, the anterior disk heights are 14 mm at L4-L5 and 13 mm at L5-S1, the posterior disk heights are 5.5 mm at L4-L5 and 4.5 mm at the L5-S1 level.8 Additionally, the width of the interlaminar space is the largest at the L5-S1 level, with an average of 31 mm (range: 21 to 40 mm).9
Finally, it is crucial to consider the different angles and the balance of curvature in the spine, as an adequate restoration of balance may further improve pain and function after surgery.10,11
Of the different parameters, pelvic incidence is the most relevant.10 Pelvic incidence is a fixed anatomical parameter defined as “the angle between the line perpendicular to the sacral endplate at its midpoint and the line connecting this point to the midpoint of femoral heads axis.”10 Additionally, the pelvic incidence is the sum of the sacral slope and the pelvic tilt.10,12 This parameter is closely correlated with lumbar lordosis and pelvic orientation, and is therefore an indicator for sagittal balance of the spine.12 The most important parameter for spinopelvic balance is that the lumbar lordosis should be within 10 degrees of the pelvic incidence. The spine surgeon should aim to restore the normal pelvic incidence of each particular patient in order to avoid an unbalanced spine after any surgical procedure at the lumbopelvic junction. An excellent review by Mehta et al. 10 illustrates further the importance of pelvic incidence. Surgery at L5/S1 offers a unique opportunity to restore the patient’s balance if needed, and this should always be taken into consideration as some approaches allow better restoration than others.
INDICATIONS
TLIF at L5-S1
Harms and Rolinger, in 1982, first described TLIF.13 The MIS TLIF has since become one of the common MIS fusion procedures.1
Intractable low back pain due to degenerative disk disease (DDD)
Symptomatic spondylolisthesis (grades I and II)
Failed back syndrome
Treatment of pseudarthrosis
Spinal stenosis with instability
Foraminal stenosis that requires facetectomy for decompression
Recurrent disk herniations or lateral disk herniations which require wide facetectomies resulting in spinal instability
There are few contraindications to the MIS TLIF procedure at L5-S1. Mainly, pelvic incidence may present a challenge for the use of MIS TLIF at L5-S1. High pelvic incidence makes it challenging to place a tubular retractor at an angle parallel to the L5-S1 endplate.1 In the authors’ experience, we have been able to overcome the steep angle and advocate for the MIS approach over the open TLIF even with a steep sacral slope.1 Furthermore, if the patient has significant osteoporosis, adequate placement of an interbody graft may be difficult or impossible secondary to the risk of subsidence.1,18 In patients with previous back surgeries, scar tissue may present a challenge, so it is advisable to consider an anterior approach of the L5-S1 disk space. Also if there is significant disk space collapse, a graft may not be able to be inserted into the disk space. Finally, high-grade spondylolisthesis is another contraindication to MIS or open TLIF because interbody graft placement is not possible (Fig. 38.1).
ALIF at L5-S1
The ALIF procedure for spondylolisthesis was first described by Capener in 1932,19 and the MIS techniques for ALIF began in 1991 with Obenchain.20
The ALIF at L5-S1 is an important procedure secondary to the dynamics and particular anatomy at this level. The lower lumbar spine contributes to roughly 60% of lumbar lordosis, and sagittal imbalance is a prime consideration for degenerative lower spine disease.21 The ALIF procedure offers the greatest restoration of lumbar lordosis. Finally, due to the possibility to fully visualize the disk space in this approach, a more complete diskectomy can be done.22 Indications include23:
Intractable low back pain due to DDD
Symptomatic spondylolisthesis (grades I and II)
Failed back syndrome
Spondylitis, spondylodiscitis
Treatment of post-laminectomy kyphosis
Treatment of pseudarthrosis
Post-diskectomy collapse with neural foraminal stenosis
AxiaLIF at L5-S1
Originally described by Cragg et al.25 the AxiaLIF has not gained wide acceptance. Due to the anatomic corridor used (presacral space), the main benefits of this approach are its ability to preserve the dorsal musculature and the ligaments surrounding the spine, thus resulting in a stronger construct25, 26, 27 with less surgical trauma. During this procedure, the diskectomy is performed in the middle of the disk preserving the annulus and anterior longitudinal ligament, which can also indirectly decompress the neural elements via ligamentotaxis.28,29
Indications include30:
One level instrumentation for DDD with intractable pain where31:
Restoration of disk height, correction of lordosis, foraminal decompression or restoration of spinopelvic balance are not important goals of the operation
Revision surgery (pseudarthrosis)
Contraindications include32:
Previous pelvic or retroperitoneal surgery
Aberrant midline vasculature anterior to S1
Sacral agenesis
High sacral lordosis
Grade II or higher spondylolisthesis
Severe osteoporosis
Revision of the AxiaLIF can be challenging and we have published our experience with revision surgery.33 Our revision techniques are described in the AxiaLIF Complications Section. Furthermore, the reported fusion rates have been generally low and our series corroborates these findings.31 This is further discussed in the Patient Outcomes Section below.
For this reason, and because of possible complications such as vascular and bowel injury, the procedure is currently not widely performed.
OLIF at L5-S1
Oblique lumbar interbody fusion was first described by Mayer in 1997.34 A recent modification of the original technique by Silvestre et al.35 gave this technique a new role in lumbar fusion. However, due to the iliac vessels and the iliac wing, access to the L5-S1 disk may be limited.35
Intractable low back pain due to DDD
Failed back syndrome
Symptomatic spondylolisthesis grade I
Foraminal stenosis, unilateral or bilateral
Revision surgery for prior instrumentation
Pseudarthrosis
Grade II or higher spondylolisthesis
Previous anterior surgery
Severe osteoporosis
Sacral agenesis
High sacral lordosis
PLIF at L5-S1
First described by Cloward in 1940,14 the PLIF became popular after the modifications proposed by Lin.37 Since then it has been widely used for different pathologies in the lumbar spine. One
benefit of the PLIF over the TLIF is that the surgeon can potentially reduce a spondylolisthesis with a PLIF procedure.
benefit of the PLIF over the TLIF is that the surgeon can potentially reduce a spondylolisthesis with a PLIF procedure.
Intractable low back pain due to DDD
Failed back syndrome
Spinal stenosis with instability
Symptomatic spondylolisthesis grade I or II that requires bilateral direct decompression
Reoperation for pseudarthrosis
Recurrent disk herniation with low back pain, or radiculopathy
Bilateral foraminal stenosis that requires open bilateral decompression
Contraindications include38: