Multiple-Level Interbody Fusion: How Do Two-Level Fusion Techniques Compare between Open and Minimally Invasive Surgery?

7 Multiple-Level Interbody Fusion: How Do Two-Level Fusion Techniques Compare between Open and Minimally Invasive Surgery?

MIS: Christopher Clayton Hills and Robert E. Isaacs
Open: P. Justin Tortolani

7.1 Introduction

Lumbar interbody fusion, whether performed via the PLIF (posterior lumbar interbody fusion) technique as originally described by Cloward 1 in 1952 or via the TLIF (transforaminal lumbar interbody fusion) method as described by Harms,² is a technically demanding surgical technique which has a long learning curve. Epidural bleeding can be extensive, and numerous passes into the disc space are required in order to prepare the end plates appropriately. In addition, regardless of the technique employed (PLIF or TLIF, open or minimally invasive surgery [MIS]), some degree of nerve root retraction is required. While PLIF and TLIF techniques have been shown to enhance fusion rates when compared to posterolateral intertransverse fusion (PLF),^3,4,5 the risk of nerve root injury, dural tear, and the so-called battered root syndrome is higher.⁶ Furthermore, Rampersaud et al⁷ have elegantly demonstrated that the lifetime incremental cost-utility ratio (ICUR) for traditional PLF for focal stenosis due to grade I degenerative spondylolisthesis is $7,153 per quality-adjusted life year (QALY),⁸ whereas the ICUR for total hip arthroplasty is $5,321 per QALY and the ICUR for total knee arthroplasty is $11,275 per QALY.^7,9 As the traditional benchmark for cost-effective treatment is $50,000 per QALY, lumbar decompression and PLF without interbody fusion is a justified treatment for spinal stenosis in the setting of spondylolisthesis. Considering these data as well as the additional costs associated with interbody implants, it strongly suggests that interbody fusions (whether single or multilevel) are not cost-effective for routine grade I degenerative spondylolisthesis cases¹⁰ and, therefore, should be reserved for more complex surgical scenarios in which our current PLF methods will not predictably achieve the desired surgical outcomes. The focus of this chapter will be to compare and contrast the effectiveness of open vs MIS for two-level lumbar interbody fusion in appropriately selected individuals.

7.2 Indications of Two-Level Interbody Fusion

Evidence-based guidelines regarding the indications for single-level or multiple-level lumbar interbody versus standard post-erolateral intertransverse (PL) arthrodesis have not been clearly defined. In general, lumbar arthrodesis following decompression for spinal stenosis is advocated for patients with gross instability (i.e., grade I spondylolisthesis with greater than 3 mm of translation on lateral flexion/extension radiographs or grade II or higher spondylolisthesis) or scoliosis. Lumbar arthrodesis has also been indicated in patients at risk for iatrogenic instability as may occur when more than 50% of the facet joint complex has been resected during the decompression.4 Severe mechanical back pain, due to facet arthritis and/or degenerative disc disease, is another potential indication for posterior spinal fusion. Nondegenerative conditions such as pyogenic vertebral osteomyelitis or pathologic (or impending) fracture due to infiltrative tumors represent additional indications for posterior spinal fusion.^11,12

Interbody fusions, which employ the interbody application of structural bone graft or synthetic devices, have the added benefit of distracting the interbody space, which increases foraminal volume and facilitates sagittal and coronal plane deformity correction. By accessing the subchondral interbody region, open PLIF/TLIF procedures have been shown to increase fusion rates via increased surface area for bone healing, compression-loading of the graft, and exposure of the graft to greater numbers of osteoprogenitor cells.¹³ Two-level interbody fusions are therefore indicated in degenerative conditions when restoration of foraminal height is needed to alleviate neurologic compression over two continuous levels.¹⁴ Such clinical scenarios are often associated with some combination of spondylolisthesis, severe disc degeneration or herniation, lumbar flatback (as can occur with degenerative scoliosis, degenerative disc disease, postlaminectomy kyphosis, or spondylolisthesis), and scoliosis. Two-level (or greater) interbody device placement is also a powerful tool in restoring lumbar lordosis as a primary indication in association with posterior column osteotomies in patients with fixed sagittal plane imbalance.

7.3 Advantages of Minimally Invasive Surgery

McAfee et al in a summary statement defined MI spine surgery as “one that by virtue of the extent and means of surgical technique results in less collateral tissue damage, resulting in measurable decrease in morbidity and more rapid functional recovery than traditional exposures, without differentiation in the intended surgical goal.”15 Conventional open lumbar fusion techniques are associated with significant muscle stripping and retraction that can adversely affect both short- and long-term patient outcomes.^{16,17,18,19,20,21,22,23} Recognizing the need for minimizing approach-related morbidities associated with open TLIF, Foley and colleagues introduced the MITLIF procedure, which utilizes a muscle-dilating approach and significantly diminishes the amount of soft-tissue disruption.^24,25 Since its introduction, a growing body of evidence has been reported on the advantages that include, but are not limited to, less intra-operative blood loss, less postoperative pain, decreased postoperative narcotic usage, early ambulation, and decreased length of stay in hospital.^{24,25,26,27,28,29,30,31}

A steep learning curve, including the complexity of the MITLIF decompression via a unilateral approach, is argued as a disadvantage compared to the open procedure. There is no question that an open midline approach affords the surgeon a virtually limitless exposure for decompression utilizing familiar techniques and instrumentation. However, this advantage for the surgeon becomes a disadvantage to the patient, given that iatrogenic soft-tissue injury incurred from subperiosteal dissection and retraction leads to subsequent denervation and atrophy of back muscles, which has objectively been shown to adversely affect clinical outcomes.16,18,19,20,22,23,28 The learning curve can be significantly shortened by combining the techniques utilized in open interbody fusion (OIF), percutaneous pedicle screw fixation, and MI microdiscectomy at hands-on didactic courses that are readily available, thus ensuring that patients are not subjects in the learning curve as inexperienced surgeons adopt this new technology. In addition, by utilizing this approach to gain experience with this MI technique, other factors debated as disadvantages such as operative time and radiation exposure will be improved upon. Furthermore, the risk of additional radiation exposure to the surgeon and operating room (OR) staff can be mitigated with the incorporation of image-guidance technology.

It was previously stated that MI screws are approximately 20% more expensive than traditional open screws, thus resulting in increasing implant costs for MI multiple-level fusion. Although this is a slight disadvantage of newer technology, implant cost difference is easily offset by the cost savings accrued by decreased length of hospital stay by MITLIF patients compared to open patients. In fact, a financial analysis performed by Singh et al evaluating single-level open versus MITLIF found lower total hospital direct costs over a 60-day perioperative period in the MIS versus the open group ($19,512 vs. $23,550).³² In addition, McGirt and colleagues showed direct costs were greater in the OIF cases when compared to two-level minimally invasive interbody fusion (MIIF) due to increased prevalence of SSI in the open group Table 7.1).

7.4 Advantages of Open Surgery

According to Ockham’s razor or “the principle of parsimony,” the simplest solution to any given problem is the best solution.33 At the most basic level, the main advantage of two-level OIF versus MIIF lies in its relative simplicity. OIF requires far less radiation exposure for the patient, provides better exposure and visualization of the relevant anatomy, is more versatile, and has a shorter learning curve. Extrapolating from single-level data, the radiation exposure for two-level OIF versus MIIF is less than 20 seconds versus 3 to 4 minutes.³⁴ Whether this 10 × increase in exposure translates into greater risk for the patient or surgical staff in MIS cases has yet to be determined.

Open surgery enables the surgeon to decompress both sides of the spinal canal with similar techniques and instruments. MIIF generally incorporates a unilateral exposure in which the surgeon removes bone toward himself/herself for ipsilateral decompression and away from himself/herself for the contralateral decompression. This requires greater time to learn and increases the complexity of the procedure. Furthermore, in open techniques, the surgeon can employ virtually limitless lines of view for decompression and instrumentation, whereas lines of vision are constrained by the tubular retractor system in MIIF procedures. While this may not translate directly into greater complications for MIIF, it increases the time to learn and the complexity of the technique. In addition, open surgery provides greater flexibility especially in cases with added complexity. In revision cases, for example, the epidural space is often scarred, making access to the interbody space difficult or impossible. In unilateral MIIF, the surgeon would have to abandon the technique on one side and make a separate contralateral exposure in such circumstances. Especially in two-level cases, this will add additional time and radiation exposure. This same rationale applies to cases of aberrant anatomy such as a conjoined nerve root or extensive epidural venous plexuses, which cannot be predicted preoperatively. Open surgery allows the surgeon to easily move to the opposite side for transforaminal disc space access without adding complexity (additional incision/incisions, radiation exposure, surgical time). In some cases, intraoperative circumstances such as ossification of the posterior annulus, rigid disc degeneration, and massive durotomy, the surgeon may elect to abort interbody graft placement. In open surgery, the intertransverse region can be easily accessed bilaterally, thereby salvaging many of the surgical goals, whereas MIIF exposure suboptimally exposes the ipsilateral intertransverse region and requires an additional incision and radiation exposure to expose the contralateral intertransverse region. Another advantage of OIF occurs in the treatment of deformity, in which the soft-tissue releases, which are routine components of the exposure, in addition to transfacet osteotomies, improve the potential for sagittal and coronal plane correction.

Another major advantage of open versus MIIF relates to implant costs. MI screws cost on average 20% more than open screws from the same manufacturer (personal communication). While cost-effectiveness studies have not been performed comparing two-level OIF versus MIIF, with increasing numbers of levels fused the direct implant costs become increasingly greater for MI cases.

Finally, in revision surgery, a midline incision as performed with an open approach provides unlimited access for handling numerous complications. Thorough lumbar wound irrigation and debridement for surgical-site infection (SSI), for example, cannot be adequately performed through an MIS portal.

7.5 Case Illustration

A 66-year-old man presents with 1-year history of progressive neurogenic claudication and weakness in the left quadriceps, left anterior tibialis, and left extensor hallucis longus refractory to multiple nonoperative treatment methods including physical therapy and epidural injections. Ten years prior, he underwent a successful L5/S1 microdiscectomy for severe sciatica. Currently, his walking tolerance was 50 yards and standing tolerance was 5 minutes before he felt as though he needed to sit down in order alleviate his leg symptoms. He rates leg pain to back pain as 80:20. After thorough discussion about the risks and benefits of continued nonoperative versus operative treatment methods, informed consent was obtained and the patient was consented for L3–L5 lumbar decompression with instrumentation and interbody fusion. Anteroposterior (AP) and lateral plain radiographs demonstrated multilevel lumbar degenerative disc disease with a lateral listhesis at L3/L4 and right-sided disc collapse at L4/L5 in the setting of a mild degenerative scoliosis ( Fig. 7.1). T2-weighted sagittal images in the midsagittal ( Fig. 7.2a) and left foraminal plane ( Fig. 7.2b) demonstrate severe central and lateral recess stenosis at L3/L4 and moderate stenosis at L4/L5, with severe narrowing in left L3 and L4 neuroforamina. T2-weighted axial image at L3/L4 ( Fig. 7.3a) demonstrates severe circumferential stenosis. T2-weighted axial image at L4/L5 ( Fig. 7.3b) demonstrates moderate circumferential stenosis.

7.6 Surgical Technique in Minimally Invasive Surgery

Positioning of the patient is no different than for an open procedure. Prior to prepping and draping the patient, fluoroscopy is brought in to obtain Ferguson’s angles and marked these upon the Fluoro machine for later use in the procedure. We also mark the projection of the upper end plates of each level upon the patient’s skin, and after obtaining 15-degree off-angle oblique images, we draw vertical lines on the skin corresponding to the midpedicle line. These lines can help gauge the correct location of the incisions for the procedure, as they correspond to the correct trajectory for the screws and a TLIF approach. At this point, we prep and drape the patient.

Fig. 7.1 (a) Anteroposterior and (b) lateral plain radiographs demonstrated multilevel lumbar degenerative disc disease with a lateral listhesis at L3/L4 and right-sided disc collapse at L4/L5 in the setting of a mild degenerative scoliosis.