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Section 9, Chapter 6: Anterior and Lateral Interbody Fusion: Indications and Techniques in Lumbar Scoliosis

Comron Saifi, Joseph M. Lombardi, and Frank M. Phillips

INTRODUCTION

Degenerative scoliosis is an increasingly common pathology in the elderly population, posing clinical challenges for the surgeon and patient alike. Approximately 68% of adults over the age of 60 have some degree of degenerative scoliosis, defined as coronal curvature of the spine greater than 10 degrees.1-2 As the American population continues to age, this condition will carry an even greater societal impact with current estimates at $3.7 billion dollars spent annually on the treatment of degenerative scoliosis.3 Novel techniques for the treatment of this disease state, such as using anterior and lateral approaches to achieve interbody fusion, offer the surgeon powerful tools to treat and correct this three-dimensional deformity.

Degenerative or de novo scoliosis is a developed deformity not present in childhood or adolescence. It is the byproduct of chronic spinal degeneration and subsequent asymmetric spinal collapse. This results in asymmetric biomechanical loading of the spine and eventual progression to scoliosis, kyphosis and/or lateral listhesis.4-5 Facet arthrosis, disc degeneration, spinal stenosis, instability and deformity account for major pain generators in this population. Common segmental patterns of deformity emerge as a result of degenerative scoliosis. These include localization to the lumbar spine, lateral listhesis commonly seen at the L3-L4 segment and progressive loss of lumbar lordosis resulting in sagittal malalignment.6 The Scoliosis Research Society (SRS) published a classification system for adult degenerative scoliosis in 2006 with the aim of providing a comprehensive and reproducible system to classify curves and guide treatment.6 This classification identifies major curves based on location, Cobb angle and vertebral body translation. This classification system was later modified with sagittal parameters from Schwab et al.,1 which have been demonstrated to correlate with patient-reported outcomes.

CLINICAL PRESENTATION AND EVALUATION

Of patients with degenerative scoliosis roughly 90% will present with complaints of axial back pain.7-9 Identification of the back pain generators is essential to guide clinical treatment. With progressive degeneration resulting in spinal stenosis, patients may also present with neurogenic claudication symptoms. As the curve progresses, neuroforaminal collapse on the curve concavity can result in radicular pain. In fact, 85% of patients presenting for evaluation of degenerative scoliosis were noted to experience neurological symptoms.10 Despite this, frank sensory or motor deficits are uncommon in this population, occurring in only 5-8% of cases. Symptoms commonly result in loss of quality of life and ability to perform activities of daily living.11 The sagittal malalignment that often accompanies degenerative scoliosis has also been shown to impact quality of life.12

Physical examination should consist of observation for an asymmetric waistline, thoracic hump, and shoulder tilt and rib cage abutment to the iliac wing. When pelvic obliquity is noted, the clinician must evaluate for leg length discrepancy as a potential etiology. A clinical assessment of sagittal malalignment is also essential as this is a major cause of patient reported disability.12 This malalignment is typically described as early fatigue and back pain with prolonged periods of standing or activity. Compensatory hip extension and knee flexion is often observed in patients who are no longer able to compensate through their spine alone.

RADIOGRAPHIC EVALUATION AND ADVANCED IMAGING

To properly assess scoliotic deformity, full-length radiographs must be taken on all patients. This entails standing AP and lateral radiographs which at the minimum should include the base of the skull to the femoral heads to properly judge sagittal alignment. Full body EOS films are preferred to better gauge overall global alignment, including hip and knee compensation. Flexion and extension images will elucidate the presence of dynamic instability. Lastly, push-prone images are helpful in determining the overall flexibility of the deformity, which is vital for operative planning. Important radiographic measurements of sagittal alignment that correlate closely with patient-reported outcomes include the sagittal vertical axis (SVA), which is the distance from a vertical line bisecting C7 to the posterior-superior aspect of S1. Thoracic kyphosis (T5-T12 Cobb angle) and lumbar lordosis (L1-S1 Cobb angle) should also be measured. Emerging literature demonstrates the importance of pelvic parameters as they relate to overall global alignment.13 Sacral slope (SS), pelvic tilt (PT) and pelvic incidence (PI) should be measured pre-operatively. Post-operative goals for correction should include an age-dependent SVA distance, pelvic tilt less than 20 degrees and lumbar lordosis within 9 degrees of pelvic incidence.5

TREATMENT OPTIONS

Goals for treatment of any patient with degenerative scoliosis are to address back pain, spinal misalignment as well as neurogenic claudication and/or radiculopathy. This entails a wide spectrum of modalities ranging from conservative therapies to open surgical correction. Initial treatment might consist of physical therapy, focusing on core and paraspinal muscular strengthening, and medical modalities including non-steroidal anti-inflammatories and muscle relaxants. Narcotic medications are not recommended in the treatment of chronic back pain given the potential for therapeutic tolerance, dependence, addiction and respiratory compromise. Cooper et al. demonstrated the efficacy of epidural or selective nerve root corticosteroid injections for radicular pain with nearly 60% successful outcomes at one week after injection.14 However, the long-term efficacy of corticosteroid injections for radiculopathy in patients with degenerative scoliosis was shown to decrease substantially to only 27.3% at the 2-year follow-up period.14

Operative treatment should aim to address the etiology of patient pain and disability for those who have failed medical management. For patients with radiculopathy or neurogenic claudication, the goals of treatment include direct or indirect neurologic decompression. For patients with painful, advanced or progressive deformity, the goals of treatment may include correction of coronal and sagittal malalignment and fusion of the affected levels. Kelleher et al. demonstrated the efficacy of MIS decompression alone for the treatment of stable degenerative scoliosis.15 In this study, patients with lateral listhesis accounted for 75% of revisions, prompting the study group to recommend against decompression alone with translational deformity.15 Transfeldt et al. demonstrated the efficacy of selective posterior fusion/decompression versus full curve stabilization with decompression.16 Patients in groups with decompression alone and selective fusion/decompression showed a 20% and 22% improvement in ODI score respectively versus 0.4% in complete curve stabilization with decompression at 2-year follow-up. 16 Additionally, revision rates were highest in patients who underwent complete curve stabilization with decompression. These findings support decompression or a combination of decompression/selective stabilization when possible for the treatment of degenerative scoliosis. However, patients who demonstrate multi-level instability and significant sagittal plane deformity often require correction of the deformity with instrumentation and fusion of the entire curve. Although traditionally surgeons have utilized an open direct decompression and instrumentation with or without osteotomies and interbody augmentation from a posterior approach, these procedures have reported complication rates as high as 80%.17-21 These complications include pseudarthrosis (24%), painful hardware (22%) and implant failure (18%).22 Berven et al. also reported on high complication rates resulting from an anterior approach with up to 40% of patients experiencing perioperative or late complications.23 As a result of the morbidity associated with the surgical treatment of degenerative scoliosis from an open posterior approach, novel less invasive techniques have been implemented to provide stabilization and decompression. Operative treatment of degenerative scoliosis with anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF) and posterior percutaneous instrumentation provides a minimally invasive solution with less blood loss, a shorter hospital stay and less postoperative pain than traditional open techniques.24

Anterior Interbody Fusion for Degenerative Scoliosis

Historical descriptions of access to the spinal column by means of an anterior approach date back nearly 200 years with modern techniques reappearing in the literature in the 1930’s.25,26 In 1956, Hodgson and Stock advocated for this approach in the treatment of Pott’s disease.27 Since that time, significant advancements have been made including the development of bone grafting substitutes, improved surgical techniques, and surgical instruments and implants.

Multiple theoretical benefits for anterior interbody fusion exist, most notably the biomechanical advantages. The anterior and middle columns of the spine bear roughly 80% of the axial forces through the spinal column.28 Anterior interbody fusion promotes bony arthrodesis by placing the graft in the area of greatest compression in accordance with Woolf’s law. Additionally, the anterior and middle column account for roughly 90% of the vascular osseous surface, making fusion potential greatest at this area.29 Anterior lumbar interbody fusion (ALIF) reestablishes native spinal biomechanics and alignment by restoring disc height and load distribution to the anterior column. The lower lumbar spine, notably L4-S1, accounts for roughly 50% of lumbar lordosis.30 As a result, restoration of disc height at these levels is paramount in achieving desired sagittal correction. Multiple studies have demonstrated the superiority of an anterior interbody to other interbody modalities in restoration of lumbar lordosis.31,32 Additionally, use of anterior interbodies allows for a more harmonious correction of sagittal balance through the lumbar spine versus the sharp, angular, localized corrections obtained with posterior-based osteotomies. Saville et al. demonstrated how use of 30-degree hyperlordotic ALIF cages produce correction of a similar magnitude to pedicle subtraction osteotomy, but with a lower complication rate.33 Other theoretical benefits include sparing the lumbar paraspinal musculature resulting in less blood loss, improved post-operative mobility and decreased chronic muscle pain described after posterior approaches.

Indications

Various indications for ALIF have been supported by the literature including discogenic disease, spondylolisthesis, degenerative scoliosis and revision surgery. The efficacy of anterior interbody use for degenerative disc disease was demonstrated by Burkus et al. in a series of 279 cases with a clinic success rate of 81%.34 Other studies have demonstrated similar outcomes with clinical success rates ranging from 71%-100%.35-37 Prashanth et al. demonstrated an 89% clinical success rate in the treatment of low grade spondylolisthesis with a 95% fusion rate based on radiographs utilizing stand-alone ALIF procedures.38 However, for isthmic spondylolisthesis surgeons should consider augmentation of ALIF with posterior instrumentation given the inherent instability.39

ALIF has multiple advantages in its application for the treatment of degenerative scoliosis. These include soft tissue release, thorough discectomy, and restoration of vertebral height and lordosis through placement of a large interbody cages.40 In the treatment of degenerative scoliosis, ALIF is most commonly used as part of a combined anterior-posterior strategy in order to achieve optimal lumbar lordosis, improve fusion rates and provide load-sharing to longer posterior instrumented constructs. Pateder et al. published the largest series of ALIF for use of degenerative scoliosis, which showed a clinical success rate of 88% that correlated to radiographic rates of fusion.41 However, major complications were high in this series, with 24% seen on same day surgery and a 45% complication rate for staged anterior-posterior procedures. A smaller series performed by Crandall and Revella demonstrated similar complication rates.40 Flouzat-Lachaniette et al. showed significantly improved functional outcomes with high fusion rates when using standalone one or two level ALIF’s for degenerative scoliosis with Cobb angle of at least 10 degrees on AP radiograph and lateral listhesis at the deformity apex of at least 3 mm.42 Contraindications to ALIF include prior abdominal surgery, aberrant vascular anatomy, severe peripheral vascular disease, solitary kidney on operative side, spinal infection and high-grade spondylolisthesis.43

Patient selection and complications

Vertebral level should be taken into consideration when deciding whether to utilize an ALIF in the setting of lumbar degenerative scoliosis. This approach is ideal for the L4/5 and L5/S1 disc levels due to the bifurcation of the iliac vessels which typically occurs at the level of the L4 vertebral body. While not an absolute contraindication, performing an ALIF at L2/3 and L3/4 has a relatively higher risk of vascular and viscera injury through retraction of peritoneum, blood vessels and kidneys. Specifically, patients undergoing an upper lumbar ALIF are at risk for superior mesenteric artery thrombosis. The published rates of vascular injury associated with ALIF range from 1.9-24%.44,45 Vascular laceration most commonly occurs at the left iliac vein, whereas rates of arterial injury to the left iliac artery occur far less frequently at a reported rate of 0.9%.46 Other sites of vascular injury reported in the literature include the iliolumbar vein, avulsion of the median sacral and ascending lumbar vein, inferior vena cava, and aorta. Also specifically reported in the literature are vascular complications such as retroperitoneal hematomas, thrombosis of the left iliac artery and deep venous thrombosis, which have been reported in up to 12.2% of cases.47 While uncommon, visceral injuries include inadvertent enterotomy and ureteric injury. Retrograde ejaculation in males occurs in anywhere from 0.42-5.9% of ALIF approaches with other neurologic injuries including femoral nerve palsy and sympathetic dysfunction resulting in lower limb temperature dysregulation and unilateral limb edema.48

Lateral Interbody Fusion for Degenerative Scoliosis

The lateral interbody approach has gained considerable traction in the spinal community since the early 2000s as a less invasive interbody fusion technique, sparing the morbidity of the anterior or posterior approach. Originally published by Pimenta and later described in detail by Ozgur and colleagues, the lateral lumbar interbody fusion (LLIF) has benefitted from advancements in spinal implants and equipment.24 The use of LLIF was expanded to the treatment of degenerative scoliosis by Phillips and Pimenta in 2005.49 This approach can be performed in isolation or in conjunction with open or percutaneous pedicle screw placement. The theoretical benefits of the lateral approach aim to avoid the vascular complications of the direct anterior approach and the extensive soft tissue dissection seen during a posterior approach.

Indications

The efficacy of a lateral lumbar interbody fusion for the treatment of degenerative scoliosis has been well demonstrated in the literature. Akbarnia et al. showed significant improvement in outcome measurements including the VAS, ODI and SRS-22 when using a standalone lateral interbody fusion for degenerative scoliosis. Additionally, coronal Cobb angles improved by 64% and lordosis improved by an average of 13 degrees.50 Dakwar et al. showed improvement in VAS by an average 5.7 points, and in ODI by 23.7%.51 These findings were echoed by Diaz et al., who reported improvement in the VAS and ODI by 4.5 points and 26%, respectively, in a series of 39 patients who underwent standalone XLIF procedures.52 Coronal Cobb angles improved by 10 degrees with lumbar lordosis changing from 34 to 41 degrees on average. In a multicenter, prospective study, Phillips et al. examined long-term (2 year) outcomes for 82 patients who underwent XLIF for degenerative scoliosis. His group demonstrated coronal correction of 35%, with significant improvement seen in all outcome measures (ODI, SF-36, VAS) at 24 months. Eighty-five percent of patients reported satisfaction with the results.53

Lateral interbody fusion also plays a role in the treatment of central and/or neuroforaminal stenosis through indirect decompression as demonstrated by Oliveira et al.54 In this study, a post-operative MRI was performed on patients treated with standalone lateral interbody fusion for degenerative scoliosis and concomitant lumbar stenosis. Following the procedure, there was a 41% improvement in height of disc space, 33% increase in central canal diameter, 25% increase in foraminal area as well as 14% increase in foraminal height.54

Patient selection and complications

Although the efficacy of lateral lumbar interbody fusion has been well demonstrated for coronal and sagittal correction as well as indirect decompression, the true advantages lie in the decreased morbidity versus traditional open procedures. Diaz et al. demonstrated an average operative time of 125 minutes and blood loss of 50 mL in a series of 39 patients treated with LLIF for degenerative scoliosis.52 Philips et al. demonstrated a mean blood loss between 50-100 mL for an average of 4.4 levels fused in his prospective series of LLIF in the treatment of scoliosis. Average length of stay was 2.9 days for standalone interbody fusions.53

Multiple factors should be taken into consideration when choosing the optimal candidate for LLIF. On rare occasions L4/5 may be obstructed by the iliac crest, precluding a lateral approach. It is essential for the surgeon to have a comprehensive understanding of the neurovascular anatomy and how it relates to each vertebral level prior to surgery. In particular, variation of the relative positioning of both major vessels and the lumbar plexus is more common in patients with adult spinal deformity. Benglis et al. performed a cadaveric study demonstrating the progressive anterior course of the lumbosacral plexus as one moves inferiorly down the spinal column.55 Whereas the plexus is initially located posteriorly relative to the vertebral body in the upper lumbar segments, at the L4-5 level the plexus could be found anywhere in the posterior two-thirds of the disc space. Park et al. measured the distance of the nerve roots from the center of the discs where an annulotomy would typically be performed for placement of an XLIF.56 The average distance was 16.4 mm at L2-3 levels versus 10.6 mm at L4-5. These findings are often exaggerated in the setting of lumbar degenerative scoliosis, which tends to displace the nerve roots further anteriorly and vascular structures posteriorly, particularly on the concave aspect of the curve.57

The most common complication from LLIF is thigh (psoas muscle) pain and lower extremity sensory changes. A series by Anand et al. found transient sensory paresthesia rates up to 61% in patients undergoing lateral interbody fusion without using real time neural monitoring.58 A more concerning complication, however, is the presence of motor palsies after LLIF. Although temporary, quadriceps weakness has been reported in up to 7% of patients with most having resolution of symptoms at 3-6 months.58 Isaacs et al. noted a 27.1% incidence of ipsilateral hip flexor weakness for which 82.1% had complete resolutions.59 In these instances, it was hypothesized that this deficit was due to violation of the psoas muscle. More recent literature has demonstrated lower incidences of femoral nerve injury at 1.7%, achieved through use of continuous neuromonitoring.60 Lehmen and Gerber performed a recent systematic review which found significant differences in outcomes between minimally invasive lateral interbody approaches with and without the use of continuous neuromonitoring.61 These findings included post-operative thigh sensory changes (16.4% vs. 35.6%, respectively) as well as rates of new onset distal weakness (1.6% vs. 5.1%, respectively). More uncommon complications have been reported, including large intestine perforation and persistent pleural effusion.50,62

OPERATIVE TECHNIQUE

The author’s preferred technique for minimally invasive surgical (MIS) treatment of degenerative scoliosis requiring pan-lumbar fusion often involves ALIF initially at L5/S1 followed by LLIF at the involved levels cephalad to L5/S1 and pedicle screw instrumentation of the involved levels. Initially the patient is positioned in a supine position on Jackson operating room (OR) table to access the lower lumbar spine via a direct anterior approach (as described above). A lateral fluoroscopic image is used to localize the L5-S1 and L4-5 interspace as necessary. A left-sided retroperitoneal approach is utilized. Following placement of the abdominal retractors the appropriate level is localized on a lateral fluoroscopic image.

After localization of the correct disc space, an annulotomy is made with a scalpel. A Cobb is then used along the superior and inferior endplates to detach the disc from the vertebral bodies. Then a combination of straight and curved curettes is used to remove the disk material and cartilaginous endplates. Trial cages are placed and checked under fluoroscopy. The authors prefer the use of an appropriately lordotic intervertebral cage. The intervertebral cage may be secured using anterior spinal instrumentation with a screw placed into adjacent vertebral bodies or plate fixation. By not fixing the cage to both vertebra, additional lordosis can be obtained during posterior instrumentation if necessary.

Next the patient is placed in the lateral decubitus position. In cases of lumbar coronal deformity, the patient is typically positioned with the concave side up to facilitate access to the lower lumbar vertebral levels above the iliac crest. Concave-side up positioning also allows for correction of the scoliotic deformity with the table bend. In addition, when approaching on the concave side, more disc levels can be reached through fewer lateral incisions.

The iliac crest is placed at the hinge of the OR table. The table is bent with the apex towards the ceiling to increase room between iliac crest and the inferior rib. The hips and knees are flexed to relax the psoas. The legs, pelvis, chest and arms are secured with two-inch silk tape. The table is then adjusted until an accurate AP fluoroscopic image is obtained with the spinous process midline and equidistant from the pedicles. The table should be adjusted so that a true lateral image is obtained with the fluoroscope perpendicular to the floor. This image should demonstrate co-linear vertebral body endplates and pedicles. Proper fluoroscopic imaging is a requirement of safe and effective surgery.

A straight radiopaque instrument is superimposed over the disc space on the perfect lateral and the incision is then marked. This is followed by prepping and draping the surgical site. A 2 cm longitudinal incision is also marked at the lateral border of the erector spinae muscles. Through this incision, the transversus muscle is bluntly dissected in order to reach the retroperitoneal space. Finger palpation is utilized to ensure that the retroperitoneal contents are displaced anteriorly and that the path of the dilator from the lateral incision to the psoas is clear. This incision also allows finger palpation of the inner table of the iliac crest, the anterior aspect of the transverse processes, and the diaphragm and 12th rib. Next the primary incision overlying the operative disc space is incised. The incision is continued through the external oblique, internal oblique and transversus abdominis muscles. Blunt dissection is performed parallel to the direction of the muscle fibers at this stage to prevent injury to the iliohypogastric and ilioinguinal nerves. Additionally, we avoid the use of electrocautery through the abdominal wall musculature. Once the transversus abdominis muscle and its underlying transversalis fascia are bluntly spread the retroperitoneal space is then reached.

The first dilator is inserted through the lateral incision guided by a finger from the posterior incision through the retroperitoneal space onto the psoas muscles. This is directed towards the center of the disc on the lateral fluoroscopic image. The psoas muscle is then slowly traversed with the dilator under real-time directional neural monitoring. The genitofemoral nerve runs on the anterior surface of the psoas. The femoral branch supplies the anterior skin in the femoral triangle (below the midportion of the inguinal ligament). The genital branch of the genitofemoral nerve supplies the cremaster muscle and the scrotal skin in males, whereas in females it supplies sensation to the mons pubis and labia majora. The lumbar plexus is located within the psoas muscle and migrates anteriorly as it travels caudally from the posterior quarter of the vertebral body cephalad and can approach the midline of the vertebral body at L4/L5. Hence, docking at L4/L5 can be challenging given the relatively small safe window. Following proper positioning, appropriate EMG signals are checked through the dilator as it passes through the psoas muscle down to the level of the disc space. If EMG activity is identified, indicating close proximity of the lumbar plexus, the dilator will likely need to be repositioned more anteriorly. Some surgeons prefer using a single incision approach laterally to perform LLJF.

Once the dilator is advanced through the psoas and onto the lateral disc, a guide wire is passed through the first dilator and into the disc space. Proper positioning of guide wire is confirmed on biplanar fluoroscopy. Next the sequentially larger dilators are placed under EMG stimulation. Retractor blades are placed over the final dilator and secured into place. The operative field within and posterior to the retractors should be explored with an EMG probe to verify that no nerves are within the field and that the lumbar plexus is posterior to the retractors. After inspecting the operative field, an annulotomy is performed. The discectomy should be performed under direct visualization. Free disc material is removed with a pituitary rongeur. A Cobb elevator is used to disrupt the far-side annulus. The Cobb is gently malleted through the contralateral annulus under AP fluoroscopy. Curettes are utilized for the discectomy and endplate preparation. It is critical to remove all of the cartilaginous endplate to minimize the risk of pseudarthrosis; however, overly aggressive endplate preparation can lead to endplate violations, which increase the risk for subsidence. Interbody cage size is then determined by trialing spacers of increasing size. Once an appropriate size is determined the actual implantable interbody cage is filled with bone graft material and then placed under fluoroscopy. After irrigation and retractor removal, the wound is closed. The transversalis fascia is closed with sutures, followed by skin closure.

Posterior instrumentation may be performed in the same sitting as the anterior surgery or may be staged. In cases of significant deformity or severe stenosis, the primary author will stage the surgery by 3 to 4 days to allow for long cassette films to be obtained prior to deciding on whether additional deformity correction is required, usually necessitating an open posterior approach. If adequate alignment has been achieved, the primary author will typically back-up the construct with percutaneous pedicle screw fixation. In addition, having the patient mobilize for a few days after the surgery allows for assessment of whether indirect neural decompression has been achieved and relieved the neurogenic symptoms, thereby avoiding the need for direct posterior decompression (using MIS or open techniques).

For posterior percutaneous screw fixation, the patient is positioned prone on a Jackson spinal frame. Two orthogonal C-Arms are utilized for simultaneous AP and lateral fluoroscopy to aid in pedicle cannulation. Endplate views are first obtained for the pedicles immediately cephalad to the most superior LLIF cage. An incision is localized and marked under the AP image at the lateral aspect of the pedicle. The area is then infiltrated with Marcaine. The skin and fascia are then incised large enough to accommodate the dilator and pedicle screw instrumentation.

With an AP fluoroscopic image, the Jamshidi needle is placed on the junction of the superior facet and transverse process (TP). An AP image is taken again to confirm placement at the lateral edge of the pedicle. Additionally, a lateral image is taken to ensure that the Jamshidi needle is in line with the pedicle. The Jamshidi needle is then advanced into the pedicle under bi-planar imaging to confirm the appropriate trajectory. Once in the vertebral body, the center needle is removed and a stilette is malleted through the Jamshidi needle into the vertebral body. The stilette is then removed, and a guide wire is placed. The guidewires are then snapped to the drapes and out of the field. Guidewire placement is done at the remaining levels in an identical manner.

Once all the desired pedicles are cannulated and have guidewires placed, pedicle screw placement is initiated. Sequential insulating plastic dilators are placed over each guidewire that allow for direct EMG monitoring while tapping the pedicle and placing the screw. An appropriately sized pedicle screw is placed under AP and lateral fluoroscopy. Lastly, the guidewire is removed. This sequence is repeated in an identical manner for all of the remaining pedicle screws.

Once all screws are placed, the rods are positioned into the reduction tabs of the pedicle screws. The set screw reducer is used to ensure that the rods are properly seated within the tulips. Next the set screws are placed. Lastly, final tightening of the set screws is performed with a counter-torque.

CONCLUSION

ALIF and LLIF play an important role in the restoration of disc height, sagittal balance and decompression of neural elements in patients with lumbar spinal deformity. While significant complications are possible as with any spine surgery, minimizing psoas retraction time and the use of directional EMG monitoring are two significant factors to mitigate risk. Although these procedures can be technically challenging in lumbar deformity patients, a reproducible and standardized approach has been demonstrated to have excellent results in the literature.

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