Surgical Techniques: Posterior Lumbar Interbody Fusion

Fig. 13.1

PLIF at the L4–5 level. (a) A laminectomy is performed. The interspinous ligament between L4 and L5 spinous processes are removed and the caudal part of the spinous process of L4 and the cephalad half of the spinous process of L5 is removed. A lamina spreader may be placed in between L4 and L5 spinous processes to facilitate ligamentum flavum removal and lateral recess and foraminal decompression. Part of the pars of L5 and facet joints of L4–5 are preserved to promote stability and midline direct posterior interbody fusion. (b) The dural sac is retracted and an annuolotomy is performed. A total discectomy is then performed. In most situations, this is repeated bilaterally to ensure complete discectomy and bilateral interbody placement for greater biomechanical stability and greater surface area to promote fusion. After complete cartilaginous removal of the end plates and trialing, an interbody graft is tamped into place. The interbody should sit anteriorly in the disc space to promote restoration of lordosis and limit the likelihood of posterior cage migration into the spinal canal. We typically use a cage device although a piece of structural iliac crest bone graft can also be used

An arterial line is used in interbody fusion procedures as it allows for instantaneous blood pressure monitoring. An acute drop in blood pressure not accounted for by anything else and not responding to fluids suggests a possible disruption of the anterior longitudinal and retroperitoneal great vessel rupture. In this catastrophic situation, the wound should be quickly closed and the patient flipped supine at which time an emergent exploratory laparatomy to repair the injured vessel versus a CT angiogram if the patient is stable enough to have this test obtained. This is a rare complication of the PLIF but due to the catastrophic nature of the event, it should always be a concern during the procedure [28–30].

We typically utilize neuromonitoring including SSEPs and EMGs during all lumbar surgical procedures involving pedicle screw instrumentation or interbody fusion. While there is little high level evidence to support the use of neuromonitoring in the lumbar spine, the medicolegal implications of not utilizing are high.

For the open PLIF procedure, a standard midline incision is used. We always perform bilateral pedicle screw instrumentation and therefore both sides of the spine should be exposed. The facet joint of the most proximal pedicle being exposed should not be violated in order to decrease the likelihood of iatrogenic destabilization and ASD. All levels to be fused should be exposed out to the transverse processes. It is helpful to coordinate with the anesthesiologist to provide relaxation during the exposure as this allows for easier stripping and lateral retraction of the paraspinal muscles off of the posterior elements.

The order and specific of the decompression, instrumentation, and interbody fusion can vary based on surgeon preference and training. We typically cannulate the pedicles for instrumentation first using anatomic landmarks. However, if preferred in patients with significant deformity or small pedicles, the pedicles can be cannulated using lateral fluoroscopy. Alternatively, the medial wall of the pedicles can be palpated from within in the spinal canal and then cannulated once the laminectomy has been performed.

The decompression is begun with removal of the interspinous ligament and full or partial laminectomy depending on the pathology seen on MRI. Part of the spinous processes at the level of the PLIF can be preserved at least temporarily. By preserving them, a laminar spreader can be used to distract across the desired disc space of the PLIF in order to facilitate decompression of the spinal canal, lateral recess, and foramina. The distraction from the lamina spreader also allows for distraction across the disc space which helps with placement of a larger interbody and restoration of the collapsed disc space height.

In the PLIF procedure, at least part of the pars interarticularis is preserved and up to half of the medial facet joints can be removed to obtain adequate decompression. The discectomy and subsequent PLIF are performed from a directly posterior direction. The dural sac and the traversing caudal nerve root are retracted medially in order to allow for passage of scalpel for annulotomy, curettes, distracters, shavers, trial, and finally the interbody fusion device itself. With a significant degree of stenosis or a large disc herniation often present in PLIF procedures, it is important to identify and protect the traversing nerve root prior to beginning the discectomy because the nerve root can often be crushed and mistaken for an epidural vessel or disc space. If this is the case, the surgeon may inadvertently injure the nerve root with bipolar cuatery or by cutting it believing it to the pathologic disc.

In order to most safely retract the dural sac and nerve root, as much ligamentum flavum and adhesions as necessary should be removed. The inferior pedicle should be palpated with a woodson or other blunt instrument. The disc space should be superior to the pedicle in close proximity to it. Often there is an epidural leash of vessels or other fibrotic adhesions which should be cauterized with bipolar cuatery. A penfield 4 or other small blunt instrument should be used to free the adhesions and bipolar cautery used to coagulate any epidural vessels overlying the disc space. This is important as it allows mobilization and excursion of the dura and nerve root, which makes it less likely for excessive dural and nerve root retraction, which can cause durotomy or radiculitis. A blunt nerve root retractor is then used to gently retract the dural sac and inferior nerve root.

At this point, with adequate visualization of the disc space and distraction provided by the laminar spreader, the discectomy and interbody placement is performed. A 15 blade scalpel is used to perform an annulotomy over the disc space. The cut should be from medial to lateral in order to avoid cutting the dura or nerve root. A small pituitary is then used to remove the disc followed by curettes, shavers, and other instruments. Throughout the procedure, the surgeon and assistant who is retracting the dural sac should take care to retract gently while at the same time preventing any instrument from injuring the dural sac or nerve root.

Trials are used and the proper height decided based upon the fit of the trial. It is at this time that the lamina spreader may be temporarily removed to get a better sense of the fit of the trial. It can be placed back into position prior to final placement of the interbody device. By trialing and placing the interbody device with the lamina spreader in place, we feel that proper disc height is more closely restored while at the same time providing an indirect decompression of the formina. However, one must be careful so as not to overdistract the disc space, which may result in overstuffing the disc space with an interbody too large longitudinally, resulting in either a traction injury to the traversing nerve roots and/or point loading and graft subsidence into the end plate. Typically, modern interbody cages have a threaded hole which allows an insertion device to impact them into the disc space with a mallet, and then can be unscrewed to easily disengage from the impacted interbody without disrupting the tight fit obtained with impaction.

Once the PLIF procedure has been performed, pedicle screws are placed through the previously cannulated pedicle screw holes. Alternatively, pedicle screws can be placed prior to the PLIF procedure and can be used for distraction of the disc space. However, in older and osteoporotic patients, we avoid using pedicle screws for distraction of the disc space or for reduction of a spondylolisthesis out of concern for loosening of the screws and weakening their pullout strength with these maneuvers [31].

Reduction of Spondylolisthesis and Deformity Correction Using the PLIF

Spondylolisthesis, if significant, can be reduced or fused in situ. No definitive evidence exists for better functional outcomes with reduction versus fusion in situ and in most situations, a grade 1 spondylolisthesis can be left in situ. However, if the surgeon desires reduction of a grade 2 degenerative or high grade isthmic spondylolisthesis, which may benefit the overall sagittal balance of the patient, various reduction maneuvers have been described. Reduction can be obtained with simple prone positioning on the table. It can also be obtained by various pedicle screw construct maneuvers with rods or plates. The reduction can also be obtained by performing the PLIF procedure with an insert and rotate method of distraction [32, 33] (Fig. 13.2).

Fig. 13.2

A 55-year-old male presented with neuroclaudication in the legs and severe back pain after having failed nonoperative treatment. (a, b) Flexion and extension radiographs showed an L4–5 degenerative spondylolisthesis with kyphotic collapse as well as a slight spondylolisthesis at L3–4 with fluid in the L3–4 facet joints on preoperative MRI. (c) The patient underwent posterior lumbar decompression and fusion from L3–5 with PLIF at L4–5. PLIF trialing and insertion successfully reduced the L4–5 spondylolisthesis and restored lordosis. His back pain and leg symptoms resolved subsequently

In terms of coronal deformity correction, we typically elect to perform a TLIF over a PLIF, as removing the entire inferior facet of the cepahalad vertebrae helps to mobilize the spinal motion segment best. If a unilateral PLIF is performed, it should be performed on the side of the concavity of the curve. Techniques which can be used to distract a collapsed disc space include using a lamina spreader across the spinous processes; temporary rod placement on the pedicle screws to hold the disc space distracted while the PLIF is performed; custom distracters which attach to the pedicle screws and are available with most modern pedicle screw systems; disc space distracters once the annulotomy and discectomy is performed [22]. Caution should be used when using pedicle screws for distraction or reduction purposes in osteoporotic individuals [31].

Depth

The depth of the PLIF should be carefully followed. The distance of most lumbar disc endplates from posterior to anterior is approximately 30 mm. However, it is always a good idea to measure the distance of the disc space from posterior to anterior on preoperative axial MRI or CT scan. Modern interbody instrumentation systems have etchings which allow the surgeon to be aware of the depth of the instrumentation at all times during the procedure. While as much disc material as possible should be removed to promote a solid fusion, depth penetration of greater than 30 mm from the posterior longitudinal line to anterior risks violation of the anterior longitudinal ligament (ALL) and retroperitoneal great vessel injury.

While not wanting to violate the ALL, the goal should be to place the interbody as anteriorly as possible within the disc space in order to restore the sagittal alignment and normal lordosis of the intervertebral level [15]. Placing the interbody anteriorly also decreases the likelihood of the graft migrating posteriorly into the spinal canal.

Unilateral Versus Bilateral PLIF

Depending on the clinical scenario, bilateral PLIF procedures may be indicated. If the patient has a degenerative scoliosis due to asymmetric disc space collapse, the surgeon may elect to place a unilateral PLIF on the collapsed side, which acts as a shim to prop open the collapsed space and correct the curve due to it.

If the disc space is completely and symmetrically collapsed, there may be a benefit to performing bilateral PLIF procedures. By performing bilateral PLIFs, a more thorough discectomy can be performed as the disc is removed from both sides of the posterior canal. Also bilateral PLIFs provide greater end plate interbody surface area. Using bilateral PLIFs theoretically increases the likelihood of fusion and lessens the likelihood of graft subsidence due to point loading; however, there is very little evidence from the literature to support this view. A retrospective study comparing 88 cases of unilateral PLIF to 99 cases of bilateral PLIF found no significant differences in visual analog scale, Oswestry disability index, lumbar lordosis, lumbar scoliotic angles, fusion level scoliotic angles, or fixation stabilities [34]. However, the unilateral PLIF group had a significantly lower operative time than the bilateral PLIF group. Molinari et al. retrospectively compared unilateral to bilateral PLIF in a military population and found no difference in hospital stay, fusion rates, pain levels, functional outcomes, or patient satisfaction [35]. However, the bilateral procedure resulted in a higher incidence of dural tears and an average increased cost of $1,728 per patient.

When electing to perform a unilateral PLIF, we chose to perform the PLIF on the most symptomatic side in terms of leg pain, which should correlate to the more stenotic side on MRI. Also, if there is asymmetric disc space collapse, the PLIF is placed on the collapsed side. When the patient has severe bilateral leg symptoms and the disc space is symmetrically collapsed, a bilateral procedure with interbody cages may be elected.

Cage Selection and the Use of RhBMP2

The choice of the type of interbody graft to use is largely surgeon dependent. Various materials have been used for the interbody device to promote fusion. These include iliac crest bone graft, other forms of autograft including the spinous process and lamina, allograft bone, titanium cages, threaded cages (e.g., Bagby and Kuslich [BAK]), polymeric rectangular cages (e.g. Brantigan cage), and various synthetic ceramic and polymeric cages. While autologous structural iliac crest is considered the gold standard, it is typically no longer used except in the case of revision or infection due to the donor site morbidity. Studies have shown varying success with multiple different materials without one definitively better bone graft alternative [36]. More studies are needed to elucidate the best form of interbody graft; however, we feel that proper discectomy and endplate preparation may be more important in promoting fusion than the type of bone graft or bone graft equivalent used.

The use of RhBMP-2 has become common in spinal fusion surgery based largely on industry sponsored studies. However, higher levels of evidence and systematic critical reviews of the existing literature have failed to show a benefit of RhBMP-2 and increased complication rates with its use. Complications associated with RhBMP-2 include endplate osteolysis, radiculitis, ectopic bone formation, and carcinogenic risk when used in high doses [37, 38]. Therefore, we do not recommend the use of RhBMP2 in the cage construct. Instead we pack local decompressed cancellous autologous bone graft into the interbody cage.

In addition to the type of interbody, the shape of the interbody is important to consider. A wedge-shaped interbody has been shown to restore lumbar lordosis and sagittal alignment better than a rectangular-shaped alternative [39]. In terms of other dimensions, bullet-shaped and banana-shaped cages are also available. A biomechanical study comparing different cage shapes in the setting of TLIF did not find a difference in construct stability when used in conjunction with pedicle screw fixation [40]. This would seem to apply to cage shapes for PLIF as well. Another biomechanical study comparing two small posterolateral cages, one small anterior banana cage, and one central rectangular cage found no significant differences in failure forces across the endplates or the stiffness of the motion segment in compression [41]. However, banana-shaped cages typically have a greater surface area and are better suited for a TLIF as they can span the disc space when inserted from a posterolateral position and allow for greater surface area to promoted fusion. When performing a PLIF, a straight cage is used typically and if greater surface area to promote fusion is desired, then consideration should be given to performing bilateral PLIF procedures.

Complications: Prevention and Management

Great Vessel Injury

Great vessel injury is an extremely rare but devastating injury that the surgeon should be aware of during the PLIF procedure. The incidence of great vessel injury during lumbar discectomy is unknown due to underreporting of complications and specifically those resulting in death, but it is estimated to 0.01 and 2.4 % of lumbar discectomies [28–30]. Case reports exist of uncontrolled hemorrhage and death with great vessel injury specifically during the PLIF procedure [42]. If anterior penetration of the ALL occurs, there is a risk of injury to the great vessels depending on the level being fused. This is why it is extremely important to be aware of the depth of the instruments being used to perform the discectomy and modern day instrumentation systems have markings on them to better judge their depth. As a general rule, one should not penetrate further than 30 mm from the posterior to anterior disc space to avoid ALL disruption.

The key to patient survival and recovery when a great vessel injury is early recognition of the injury and emergent vascular surgery evaluation. In some situations, injury to a retroperitoneal great vessel may be somewhat obvious, recognizable by hemorrhage in the disc space or acute sudden hypotension intraoperatively. It also may be recognized immediately postoperatively when the patient is flipped and becomes hypotensive with a distended abdomen. In other situations, the injury may result in an arteriovenous fistula or pseudoaneurysm not recognized until months later. Treatment of vascular injury involves either open repair via a retroperitoneal approach or angiography and endovascular repair, depending on the preferences of the vascular team and stability of the patient [43].

Adjacent Segment Degeneration

Whether or not ASD is a complication of interbody fusion and spinal arthrodesis in general or is rather part of the natural history of disc degeneration and arthritis has yet to be fully elucidated from the literature [44]. The overall incidence of ASD after a PLIF procedure was shown in one study to be 33 % at 2 years postoperatively with a 29 % incidence of radiographic progression and a 4 % incidence of both radiologic and clinical ASD [45]. Multiple studies have shown that ASD is due to multiple factors including age-related disc degeneration as well as disruption of surrounding segmental stabilizing structures during the decompression and fusion procedure. Cadaveric biomechanical studies have shown that the ALIF, which is typically larger and has more surface area in contact with the endplates, better restores the natural stress distribution pattern of adjacent levels than the PLIF [46]. One clinical study found a statistically significant higher incidence of ASD after the PLIF procedure than the ALIF procedure [23]. Total disc arthroplasty was developed as an alternative to fusion with this in mind in order to preserve or lessen the degree of stiffness and adjacent level stresses. A systematic review of the literature found weak evidence to support the effectiveness of total disc arthroplasty compared to arthrodesis to prevent ASD; however, increasing patient age also had a strong effect on ASD [47]. We do not view total disc arthroplasty in the lumbar spine as a viable alternative to arthrodesis based on the evidence from the literature.

One study retrospectively examined risk factors for ASD after the PLIF procedure in 87 patients with a history of L4–5 degenerative spondylolisis at 2 years postoperatively [45]. In 58 patients (67 %), there was no progression of ASD. In 25 patients (29 %), there was progression of ASD at L3–4 but no neurologic decline. In four patients (4 %), there was progression of ASD and neurologic decline at that level and subsequent surgery. No preoperative radiologic signs could be identified as risk factors for radiologic ASD. L3 laminar horizontalization and L3/4 facet tropism were identified as risk factors for clinically significant ASD, although it is difficult to draw any conclusions from a group of only four patients. While ASD may be accelerated by the use of arthrodesis and interbody fusion, it may also be part of the natural history and should not prevent the surgeon and patient from considering an interbody fusion or PLIF specifically. The best surgeon-controlled technique of preventing ASD is to not violate the adjacent level facet joints, which are not part of the planned fusion levels.

Neurologic Injury

Neurologic injury from a PLIF procedure is an uncommon but serious adverse complication. Often referred to as a “battered nerve root syndrome,” it is believed to result from excessive or prolonged retraction on the nerve root during discectomy procedures, including PLIF [48]. One argument for the use of TLIF and other interbody techniques over the PLIF procedure is that minimal to no retraction of the dural sac or nerve roots is necessary with these alternatives. Evidence from the literature does show a trend toward a higher incidence of neurologic injury with the PLIF compared to other interbody procedures although it is equivocal. For instance, in one retrospective comparative study comparing 39 patients who underwent ALIF to 35 patients who underwent PLIF, one patient who underwent the ALIF suffered a neurologic injury due to the type of interbody used, while no patients in the PLIF group suffered neurologic injuries [49]. The XLIF has its own concerns for neurologic injury not to nerve roots but the lumbar plexus.

Many argue that the TLIF is a better procedure than the PLIF based on a lower incidence of neurologic injury, and the evidence from the literature, while not a high level, points to this being true. In a retrospective study comparing 40 patients who underwent TLIF to 34 patients who underwent PLIF, there were four cases (11.8 %) of neurologic injury in the PLIF group versus no cases of neurologic injury in the TLIF group [50]. The authors did not report on whether or not this was a statistically significant difference or whether the neurologic injuries were transient or permanent [50]. In another retrospective comparative study, 76 patients who underwent PLIF were compared to 43 patients who underwent TLIF. While there was a trend toward a higher incidence of iatrogenic nerve root dysfunction in the PLIF group versus the TLIF group (6 [7.8 %] versus 1 [2 %] respectively), this difference did not reach statistical significance [51]. Furthermore, in all patients who suffered nerve root dysfunction, the morbidity was transient and resolved by 3 months postoperatively.

It is believed that neurologic injury during the procedure may be a result of excessive and/or prolonged retraction on the nerve root. In a study of 31 patients who underwent posterior lumbar discectomy, a pressure transducer was used to measure the amount of retraction on the traversing nerve root as well as the length of retraction [52]. In the four of 31 patients with the highest retraction pressure, all had transient sensory changes postoperatively in the distribution of the retracted nerve root. The time of retraction was also longer by greater than 4 min on average in this group than in the rest of the patients. The study supports the assertion that nerve root dysfunction is likely the result of excessive and/or prolonged retraction on the nerve root during the procedure. Therefore, in order to prevent neurologic injury, the discectomy and interbody placement should be performed as quickly but as safely as possible to limit the amount of nerve root retraction.

Durotomy

Durotomy during the PLIF procedure can occur with either the decompressive laminectomy and medial facetectomy or during the discectomy and interbody placement. It is not at all certain that a well-repaired durotomy has any short- or long-term clinically adverse effects. In an analysis of 389 patients with degenerative spondylolisthesis from the spine patient outcomes research trial (SPORT) who underwent decompressive laminectomy with or without fusion, there was a 10.5 % incidence of durotomy [53]. The authors found no difference in incidence of nerve root injury, mortality, additional surgeries, SF-36 body pain and physical function, or Oswestry Disability Index at 1, 2, 3, and 4 years postoperatively. While the clinical consequences of a repaired durotomy are equivocal, the medicolegal ramifications are not: incidental durotomy was reported as the second leading cause of lawsuits in spine surgery [54].

Much like neurologic injury, it is believed durotomy is more common with the PLIF procedure than other interbody procedures, including the TLIF, because less dural sac and nerve root manipulation is required with the TLIF procedure. However, high levels of evidence from the literature are sparse on this topic. In a retrospective study comparing 40 patients who underwent TLIF to 34 patients who underwent PLIF, there was a higher incidence of durotomy in the PLIF group compared to TLIF group (13 [17 %] versus 4 [9 %]), though this was not a statistically significant difference [51].

While durotomy is often attributable to multiple factors, including epidural fibrosis, revision surgery, ossification of the ligamentum flavum, and synovial cysts, meticulous surgical technique should help minimize the risk of durotomy [55]. The dural sac should be as freely mobile as possible prior to retraction. That is, the epidural leash that is typically present ventrally in the spinal canal should be carefully cauterized with bipolar cuatery without cauterizing the dura or nerve roots, and then gently and bluntly swept away with a penfield 4 or other blunt instrument. This should be freed over the disc space as well as cephalad and cuadad to it. If performed appropriately, the dural sac should be more easily mobilized for safe retraction, otherwise there is a risk of tearing the dural sac either dorsally or ventrally, which is extremely challenging to repair. This careful but thorough cauterization also helps to control hemostasis from epidural bleeding during the discectomy and interbody placement.

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