(a) Full-length scoliosis AP x-ray revealing significant coronal (75°) and rotational deformity, along with rotatory subluxation at L3–L4 (yellow arrow). Lateral x-ray demonstrating loss of the normal lumbar lordosis and a positive sagittal vertical axis (SVA) of +7 cm
Additionally, there can be a loss of the normal lumbar lordosis, and patients can become sagittally imbalanced and lean forward. To compensate and regain balance in order to stand erect and bring their head over their pelvis, patients will attempt to maximize hip extension and retrovert their pelvis (tucking their buttocks). In severe cases of positive sagittal imbalance, patients will also flex their knees to stand erect. Patients with >5 cm of positive sagittal imbalance often report a significant functional decline due to the energy expenditure required to maintain spinal sagittal homeostasis. Patients can experience early fatigue, intolerance of standing, and walking with compensation through other joints. This constant hip extensor and quadriceps eccentric contraction can lead to muscle fatigue and intolerance of most activities.
Understanding the etiology, anatomic features, and clinical presentation of the adult patient with scoliosis is important in determining an appropriate surgical strategy.
However, not every adult patient with spinal deformity can be managed with MIS techniques. Prior reports have shown that MIS techniques have limitations in the ability to adequately correct and restore sagittal spinal parameters, along with increased pseudarthrosis rates if interbody fusion is not performed at every lumbar level [28–31, 38, 39]. Appropriate patient selection for less invasive correction techniques is critical to optimize successful patient outcomes in the treatment of adult deformity. Relative contraindications for this approach include previous retroperitoneal dissection, previous pyogenic kidney infection, or retroperitoneal infection. This may result in adhesions of the kidneys, peritoneum and bowel, and vasculature. Additionally, unfavorable anatomy may restrict access to the planned operative level.
Full-length scoliosis x-rays capturing the base of the skull down to the femoral heads, along with magnetic resonance imaging (MRI) and computed tomography (CT) scans of the planned instrumented areas, should be obtained for preoperative planning. Careful review of the imaging should be performed to identify any obstacles to the planned surgical levels. High-riding iliac crests may block access to L4–L5 and can usually be detected on preoperative x-rays, but may also be identified intraoperatively with fluoroscopic x-ray. Ankylosed facets can be identified on computed tomography (CT) scans and may limit the ability of the LLIF technique to restore disk height, lordosis, and coronal Cobb angle due to facet hypertrophy, osteophytes, or fusion. Additionally, unrecognized ankylosed facet joints may result in endplate and ring apophysis violation when performing the disk preparation or implant insertion resulting in suboptimal deformity correction and indirect decompression (Fig. 20.2).
(a) Preoperative standing x-ray revealing the iliac crest (red line) blocking access to the L4–L5 disk space (yellow line). (b) Intraoperative fluoroscopic x-ray with a flexible guide wire held against the patient’s skin outlining the iliac crest, which is blocking access to L4–L5. (c) CT scan of the lumbar spine reveals hypertrophic, osteophytic, ankylosed facet joints, which could limit the restoration of disk height, lordosis, and deformity correction, (Arrows) reveal hypertrophic, osteophytic, ankylosed facet joints, which could limit restoration if disc height, lordosis, and deformity correction utilizing the LLIF technique
The rotational component of the deformity may alter the normal anatomy placing nerves, visceral organs, and vascular structures in direct line with the surgical approach placing them at greater risk for injury (Fig. 20.3). An axial MRI of the planned surgical levels can identify any anomalous anatomy and assist in surgical decision-making (Fig. 20.4). The anatomic variations, stiffness, rotary olisthesis, and extensive osteophyte formation make attention to detail extremely important in surgical planning. This approach relies on excellent preoperative imaging studies to make appropriate technical decisions and to ensure safety when approaching the spine. In cases where the lumbar plexus cannot be clearly defined, particularly in rotatory scoliosis, magnetic resonance neurography (MRN) can better delineate the anatomic location of the nerves by optimizing selectivity for their unique MRI water properties (Fig. 20.5).
(a) Axial CT scan of the lumbar spine revealing a rotational deformity. (b) Axial MRI of the lumbar spine reveals the great vessels in the operative field adjacent to the optimal docking site at the anterior 1/3 of the disk as a result of the rotational deformity of the lumbar spine (Arrow) reveals the great vessels in the operative field adjacent to the optimal docking site at the anterior 1/3 of the disc space as a result of the rotational deformity of the lumbar spine.
(a) Axial MRI of the L4–L5 level revealing a more anterior L4 nerve in the middle of the disk space, which may prevent safe access to this level. (b) Axial MRI of the L4–L5 level demonstrates a rising psoas sign or Mickey Mouse ears. The psoas muscle (white arrows) is rising away from the vertebral column as opposed to its typical location immediately lateral to it. This finding is consistent with the trend of progressive ventral migration of the lumbar plexus (yellow arrows) throughout the lumbar spine from cephalad to caudad placing nerves more at risk .
Magnetic resonance neurography (MRN) is the direct imaging of nerves by optimizing selectivity for unique MRI water properties of nerves. It may be useful in delineating the anatomy of the lumbar plexus in complicated scoliosis cases or in those with unique anatomy like a rising psoas sign
Degenerative scoliosis typically develops over time. As the “major” curve worsens and the patient is no longer in coronal spinal balance, they compensate to keep their head centered over their pelvis. This compensation can result in the development of curves above and/or below the “major” curve. With age these curves can worsen and become stiff. The curve that develops below a major lumbar curve is called the fractional curve and is usually located at the lumbosacral junction on the opposite side of the major curve’s concavity. Obliquity at L5–S1, due to severe degeneration, congenital deformity, or leg-length inequality, can actually be the primary deformity and result in a major compensatory lumbar curve above it. Although the LLIF technique is not contraindicated in patients with fractional and stiff thoracic curves, these scenarios deserve particular attention. If the fractional and stiff thoracic curves are not identified and considered in the preoperative plan, worsening of the patient’s coronal balance can result (Fig. 20.6). Utilizing the LLIF technique in patients with stiff thoracic curves can “push” them further out of coronal balance. Additionally, if the L5–S1 interbody fusion is performed on the same side of the major curve’s concavity in patients with fractional curves, the coronal imbalance can worsen.
(a) Preoperative scoliosis AP x-ray of a patient who had adolescent idiopathic scoliosis that progressed into adulthood. Bending x-rays revealed a stiff thoracic dextroscoliosis (red arrow) and major thoracolumbar scoliosis (solid yellow line). The fractional curve is located at the lumbosacral junction (yellow dashed line). The patient is coronally imbalanced to the left. (b) Postoperative x-ray after a five-level LLIF (T12–L1, L1–L2, L2–L3, L3–L4, L4–L5) and an L5–S1 TLIF performed on the right side in the convexity of the fractional curve (opposite the yellow dashed line). The patient’s coronal imbalance is slightly worsened
Proper patient selection is the key to successfully treating adult patients with deformity using MIS techniques. Regardless of the utilization of open or MIS techniques, the goals of adult degenerative spinal deformity surgery are the same and include neural element decompression, establishing and maintaining sagittal and coronal global balance, and arthrodesis. Before the surgical application of MIS techniques is utilized to treat adult deformity, several qualifying questions need to be answered. First, can MIS techniques adequately decompress the neural elements? Second, can the spinal instrumentation be placed using MIS techniques? Third, can global coronal and sagittal balance be adequately restored? Lastly, can a solid arthrodesis be obtained?
Several classification schemes including treatment levels have previously been described for adult spinal deformity . In 2010, Silva and Lenke published a treatment-level guide detailing six treatment levels (degrees of severity) for the traditional open surgical management of spinal deformity, based on clinical and radiographic findings .
Of the six Lenke-Silva treatment levels, treatment levels I–IV could effectively be treated with current minimally invasive techniques based on published data [28, 29, 31].
Mummaneni and colleagues modified the Lenke-Silva scheme to create an algorithm for the minimally invasive treatment of spinal deformity, which is termed the MiSLAT (Mummaneni, Wang, Silva, Lenke, Amin, Tu) algorithm  (Fig. 20.7). The authors propose that adult deformity falling into MiSLAT treatment levels I through IV could be addressed utilizing MIS techniques. However, more severe and fixed deformities that fell in the MiSLAT treatment levels V and VI require more traditional open approaches to reliably correct the deformity.
MiSLAT algorithm for MIS treatment of adult degenerative deformity. MiSLAT I = decompression only; MiSLAT II = decompression and limited pedicle screw fixation of a portion of the coronal curve with posterolateral bone graft or TLIF; MiSLAT III = decompression and pedicle screw fixation of the apex of the lumbar curve with posterolateral bone graft or TLIF/extreme lateral interbody fusion (XLIF)/direct lateral interbody fusion (DLIF); MiSLAT IV = decompression and pedicle screw fixation of the lumbar spine with TLIF/XLIF/DLIF to include Cobb angles of the main curve; MiSLAT V = decompression and pedicle screw fixation and fusion extending into thoracic region for thoracic hyperkyphosis ± osteotomies; MiSLAT VI = correction of thoracolumbar scoliosis with three-column or multiple-facet osteotomies and multisegmental pedicle fixation and fusion. Iliac screw insertion is suggested for constructs extending longer than L2 to S1 (Adapted from Mummaneni et al. )
Mummaneni’s MiSLAT classification for the MIS treatment of spinal deformity is cumbersome and has a low interobserver and intraobserver reliability. His group subsequently created a less complex scheme, the minimally invasive spinal deformity surgery (MISDEF) algorithm to assist spine surgeons in selecting an appropriate surgical approach for spinal deformity ).
The MISDEF algorithm incorporates patient’s preoperative radiographic parameters and was simplified into three general surgical approaches, ranging from MIS direct or indirect decompression to open deformity surgery with osteotomies. This simplified approach resulted in substantial inter- and intraobserver agreement.
A Class I approach involves an MIS or mini-open muscle-sparing decompression alone or MIS fusion of a single listhetic level, regardless of curve apex. The Class I approach is accomplished either through small fixed tubular retractors (MIS) or via expandable tubular retractors placed through a muscle-sparing Wiltse or lateral approach (mini-open). Instrumentation may be placed through the expandable tubular retractor or via a percutaneous method. A Class II approach entails an MIS or mini-open decompression and interbody fusion of the curve apex or the entire coronal Cobb angle of the major curve. A Class III approach entails a traditional open surgical approach involving osteotomies and/or extension of the fusion into the thoracic spine. Navigation through the algorithm is based on established ideal sacropelvic parameters and global spinal balance. In general, progressively worse deformity requires higher-class approaches in the algorithm.
Not all deformity cases can be appropriately treated with MIS techniques. Due to the limitations of MIS in restoring significant sagittal plane imbalance, Class III deformities cannot be easily corrected using MIS techniques, as patients often require osteotomies, which can be extremely challenging using MIS techniques. The minimally invasive spinal deformity surgery (MISDEF) algorithm may provide a reliable and reproducible tool for surgeons to achieve their desired surgical goals when considering MIS versus open techniques in the treatment of adult spinal deformity.
20.3 Surgical Technique
The transpsoas lateral lumbar interbody fusion (LLIF) surgical approach can be more complicated when utilized for deformity correction in the scoliotic spine due to the associated coronal and rotational deformities. Not only can the bony anatomy be altered making radiographic identification challenging, but also other critical structures can rotate into the normal surgical path placing them at risk. The LLIF procedure has advantages over the traditional direct anterior lumbar interbody fusion (ALIF) technique. Open anterior thoracoabdominal approaches are associated with up to 40 % risk of complications including incisional pain, abdominal hernia, vascular injury, ileus, retrograde ejaculation, ureter and bladder injury, and ilioinguinal and iliohypogastric nerve injury [17, 18]. Proper surgical LLIF technique allows for an aggressive diskectomy and release of the contralateral annulus. The anterior longitudinal ligament (ALL) is preserved, which preserves stability but can also limit lordosis restoration. A more aggressive and thorough discectomy can be performed, which can prepare a larger graft bed for the LLIF implant resulting in a larger surface area for fusion. Proper implant sizing allows for spanning the vertebral annular ring apophysis, which is the strongest portion of the endplate . This outer rim of the dense cortical bone is stronger than the more central, weaker portions of the endplate where ALIF and TLIF cages rest (Fig. 20.8).
The ring apophysis is the strongest portion of the vertebral endplate composed of dense cortical bone located circumferentially at the periphery. The red rectangle illustrates the optimal position of the LLIF cage resting at the anterior 1/3 of the disk space spanning the ring apophysis
In addition to a greater area of endplate preparation, Tatsumi and colleagues found that the LLIF approach resulted in a significantly lower risk of endplate violation when compared to the TLIF approach (4 % vs 48 %) . If the endplate is not violated during disc preparation and the implant properly spans the ring apophysis, this can facilitate deformity correction, indirect decompression of the spinal canal and neuroforamen, and interbody fusion.
When the LLIF technique is utilized in the treatment of adult deformity, there are multiple critical factors to consider prior to the actual surgical procedure itself. After the patient’s preoperative imaging is critically reviewed and they have been determined an optimal candidate for LLIF deformity correction, proper patient positioning and intraoperative fluoroscopic x-rays are essential for a safe and successful outcome.
20.3.1 Patient and Bed Positioning
Patient positioning is critical, but often overlooked, when performing the LLIF procedure for deformity correction. Prior to placing the patient on the operating room (OR) table, an extension piece can be placed at the foot of the table. Once the extension piece is in place, the OR table can be reversed, or turned around, so that the patient’s head is placed on the extension located at the true end of the bed. The extension maximizes OR table length and gives the surgeon more room caudally. The bed can then be maximally translated away from the central metal bedpost (toward anesthesia). The placement of the extension piece, reversing and translating the OR bed, can result in more room under the table caudally and less restriction for the fluoroscopic C-arm relative to the OR table bedpost. This can result in better visualization of the caudal lumbar vertebra and pertinent bony anatomic landmarks (Fig. 20.9).
Reversed OR bed translated maximally toward anesthesia to allow room for proper visualization of the caudal lumbar vertebra with C-arm. The patient’s iliac crest is placed at the break in the OR table with the hips and knees flexed. Tape is placed across the chest and pelvis, as well as over the thighs and legs
Patients should be positioned in the lateral decubitus position with their iliac crest at the break of the table with all bony prominences well padded. A true lateral decubitus position is essential to this procedure. This position will allow for the abdominal contents to fall forward and away from the psoas more easily during peritoneal release from the retroperitoneal space, which can decrease the risk of injury to the peritoneum and its contents. Placing the patient’s iliac crest at the break of the OR table allows for flexing (breaking) the table, which can allow better visualization and access to the caudal lumbar levels, particularly L4–L5. It may also aid in the correction of the coronal deformity if the surgical approach is through the concavity of the lumbar curve. Most surgeons stand at the patient’s back when performing LLIF surgery, so the patient’s back should be positioned close to the posterior edge of OR table so that the surgeon does not have to lean considerably over the patient to visualize the operative field. Care has to be taken not to position the patient too far posterior so that the spine overlaps the metal bars on the side of the OR table. This can be particularly problematic when the bed has to be rotated toward the patient’s back due to the scoliosis in order to obtain neutral lateral x-rays. Rotating the patient posteriorly will rotate the metal bar on the side of the bed under the patient, possibly blocking C-arm visualization (Fig. 20.10).
(a) The OR bed is level and the metal bars on the side of the bed are not in the x-ray visual field. (b) Obtaining a true lateral of the planned operative level in a scoliotic spine often requires rotation of bed, which can bring the metal bars located on the side of the OR table under the spine interfering with x-ray visualization, (yellow dashed lines) identify location of the spine. (Red dashed lines) identify location of the metal bars located at the edges of the operating room bed.
The patient’s hips and knees should be flexed to approximately 60 and 90°, respectively, which takes tension off the psoas muscle and more importantly the lumbar plexus that lies within it. Femoral nerve strain at L4–L5 can increase with breaking the OR table by putting the psoas muscle and lumbar plexus on stretch.
O’Brien and colleagues demonstrated in a cadaveric model that table flexion results in preloading the femoral nerve when approaching L4–L5 . With 40° of table flexion at the pelvis, there was anterior displacement of the nerve by approximately 1.5 mm resulting in the highest nerve strain (average, 6–7 %) compared with 0°. Strain in the femoral nerve decreased with increasing hip flexion for both table flexion angles (40 and 0°). Flexing the hips and knees relaxes the psoas muscle and nerves minimizing the effects of breaking the table when required. Additionally, flexing the hips and knees may permit increased mobilization of the psoas and lumbar plexus allowing more displacement when the retractor is placed and opened, decreasing stretch and possibly neurological injury.
An axillary roll should be placed to prevent brachial plexus injury. Padding should be placed between the OR table and down leg to protect the peroneal nerve and the skin from breakdown over the bony prominences. A pillow should be placed between the patient’s legs. Additionally, an arm board should be placed to support the patient’s down arm. An arm holder or a pillow can be placed to support the patient’s up arm. Once the patient is provisionally positioned, tape should be applied across the patient’s upper chest and lower hips to secure them to the OR table outside of the planned operative field. This prevents the patient from shifting or rotating intraoperatively. Tape should also be applied to the patient’s thighs and legs to secure them in flexion (Fig. 20.9). Tape can be applied multiple times, even circumferentially around the patient and bed to prevent patient migration during the procedure.
Once the patient is secured to the OR table, the table can be flexed at the iliac crest for optimal visualization of L4–L5. The bed should be extended at the patient’s feet to prevent the hips and knees from extending. The bed can be placed in reverse Trendelenburg to compensate for the breaking in the table to level the patient’s torso.
20.3.2 Fluoroscopic Imaging
Given the segmental deformities often seen in scoliosis, the OR table and fluoroscopy often need to be adjusted at each level to ensure optimal radiographic imaging (Fig. 20.11). It is the authors’ recommendation that the bed, not the C-arm, should be adjusted to obtain true AP and lateral x-rays of each individual operative level. With the C-arm locked at 0°, the bed can be rotated until a true AP image is obtained, and the spinous process of each level is in perfect midline position between the two pedicles and the endplates are parallel (Fig. 20.12). With the C-arm locked at 90°, the bed can be inclined or declined (Trendelenburg vs. reverse Trendelenburg) until a true lateral image is obtained with parallel endplates and overlapping pedicles and facet joints (Fig. 20.13). Moving the table and not the C-arm allows the surgeon an easier reference point of a true AP (surgeon’s hand perfectly horizontal and parallel to the floor) and lateral (surgeon’s hand perfectly vertical and perpendicular to the floor). True lateral x-rays can help prevent endplate violation during disk prep and implant placement, which can eliminate the advantage of deformity correction and indirect decompression of the LLIF technique. More worrisome, without true AP x-rays, vertebral rotation can lead to implant encroachment posteriorly in the neuroforamen and nerve injury, or anteriorly resulting in catastrophic vascular injury.
The scoliotic spine often has rotational deformity. With the bed level, there is significant rotational deformity. Rotating the patient posteriorly results in an AP view of the spine with the C-arm in the lateral position at 90°. Rotating the patient anteriorly, keeping the C-arm at 90°, results in a true lateral view of the spine
(a) Rotation of the spine. (b) True AP x-ray with the spinous process of L4 equally bisecting the pedicles with parallel endplates
(a) Parallax resulting in endplate double densities. (b) True lateral x-ray with parallel and overlapping endplates, pedicles, and facet joints
After draping the operative field with 10 × 10 drapes, the skin can be prepped with alcohol. Prior to the incision, each planned LLIF surgical level should be visualized with a true AP and lateral x-ray to ensure all planned operative levels can be visualized and accessed. The bony ribs can be palpated and marked on the skin. A flexible radiopaque guide wire can be used to identify the disks of each planned operative level, along with outlining the iliac crest on the skin to identify any bony restrictions, particularly the iliac crest restricting access to L4–L5 (Fig. 20.14).
Skin markings identifying operative levels and possible limitations to access. Using fluoroscopic C-arm, four operative levels, the ribs and iliac crest are marked on the skin. A flexible guide wire is placed along the iliac crest, and fluoroscopy is used to identify access of L4–L5
Tape can be used to mark the floor relative to the position of the C-arm, which allows the C-arm machine to be moved in and out of the operative field with precision (Fig. 20.15). Additionally, the position of the C-arm gantry identifying true AP and lateral x-rays for each planned operative level can be marked with tape and the levels labeled (Fig. 20.16). This preoperative marking of the skin can minimize the skin incision, and labeling the floor and C-arm gantry with tape can expedite the intraoperative localization process of each planned surgical level and provide a reproducible guide for other radiology technologists who may not have been present at the beginning of the case.
Tape (yellow arrows) marking the OR floor where true AP and lateral x-rays are obtained to allow efficient and reproducible access into and out of the operative field
Tape marking the C-arm gantry with each planned operative lumbar level labeled. This provides reproducible guide for visualization of each planned surgical level throughout the operation, even with different radiology technologists
20.3.3 Access to the Psoas
Access to the lateral lumbar spine can be performed using a one- or two-incision technique. Regardless of the technique, the skin incision over the planned operative level is made first, exposing the subcutaneous fat, which is dissected and retracted out of the operative field. The external oblique (EO) muscle is the first muscle encountered, and its muscle fibers run obliquely toward the umbilicus. This muscle layer can be bluntly dissected to expose the internal oblique (IO) muscle, whose muscle fibers run perpendicular to the EO muscle. The IO can be bluntly dissected over each individual lumbar disk level to reveal the transverse abdominal muscle or it can be cut. Care must be taken to bluntly dissect the IO muscle first to avoid injuring the subcostal, iliohypogastric, and ilioinguinal nerves, which originate from the T12 and L1 nerve roots and course anteriorly and inferiorly and pierce and innervate the abdominal wall musculature. Injury to any of these nerves can result in abdominal wall paresis causing an abdominal bulge or droop, which can be permanent or temporary depending on the degree of nerve injury (Fig. 20.17). Cahill and colleagues reported a 4.2 % incidence of postoperative abdominal wall bulge, which they related to permanent injury to the motor nervous supply to the lateral abdominal wall . Once the IO muscle is dissected, the transverse abdominal (TA) muscle is visualized and dissected. As its name contends, the TA fibers run transversely or horizontally. Immediately deep to the TA muscle is the transversalis fascia. This aponeurotic membrane can be distinct and lies between the inner surface of the TA muscle and the parietal peritoneum. Once this is dissected, the retroperitoneal fat can be visualized.
(a) The abdominal wall and its innervation. The subcostal, iliohypogastric, and ilioinguinal nerves are labeled. (b) Patient with right abdominal paresis after a right-sided LLIF approach that resolved a 6-month post-op
Access can also be obtained through a two-incision technique. After the first incision located directly over the planned surgical disk space is performed, dissection is carried to the transversalis fascia as described above. A second small incision can then be placed a finger-length posterior to the first incision and can be used as a direct access point to the retroperitoneal space. Once the second incision is made, blunt muscle dissection is performed to the transversalis fascia posteriorly, which can be penetrated with a pointed clamp into the retroperitoneal space (Fig. 20.18). Through the second more posterior incision, blunt finger dissection can sweep the peritoneum, fat, and any adhesions off the transversalis fascia and psoas muscle. The transverse process is a bony landmark that can be palpable for localization confirming the position in the retroperitoneal space. Once blunt dissection is performed, the same finger can be directed to the first incision and used as a visual marker for dissection of the transversalis fascia ensuring entry into the retroperitoneal space. The initial dilator can then be placed through the first incision using the finger in the second incision to guide the dilator to the retroperitoneal space and psoas muscle.
Two-incision technique. The first incision allows access to the LLIF level. The second incision allows access to the retroperitoneal space and guidance from the first incision to the psoas muscle
The retroperitoneal fat can be swept anteriorly to visualize the fascia enveloping the psoas muscle. The genitofemoral (GF) nerve lies directly on top of the psoas fascia. The GF nerve originates from the upper L1 to L2 segments of the lumbar plexus and passes caudally and emerges from the anterior surface of the psoas muscle. It can be encountered in the center on the LLIF operative field commonly at L3–L4. The nerve continues downward and divides into two branches, the genital branch and the femoral branch. The genital branch passes through the deep inguinal ring and enters the inguinal canal. The genital branch continues down and supplies the scrotal skin in men and accompanies the round ligament of the uterus terminating in the skin of the mons pubis and labia majora in women. The femoral branch passes underneath the inguinal ligament, traveling adjacent to the external iliac artery, supplying the skin of the upper anterior thigh and groin. Whether each operative level is approached with individual small dissections through the muscle layers and fascia or a mini-open incision is performed for better visualization, the psoas fascia should be released to expose the muscle fibers beneath it. Using a “no-look” approach, moving the initial dilator 2–3 mm cranially and caudally, in-line with the muscle fibers, can release the fascia as the dilator is advanced into the psoas muscle and docked (Fig. 20.19). This can decrease the risk of dragging the fascia and GF nerve into the psoas causing stretch with dilator advancement or compression with retractor opening, both of which can cause injury and anterior thigh symptoms. Wanding the initial dilator more than 2–3 mm cranially and caudally could result in nerve injury.
The genitofemoral (GF) nerve crossing the psoas muscle at L34. The psoas fascia has been released, exposing the psoas muscle fibers and freeing the GF nerve prior to dilator insertion
20.3.4 Transpsoas Approach and Retractor Docking
Sequential dilation of the psoas muscle can then be performed for each planned operative level using biplanar fluoroscopy and directional electromyography (EMG) or mechanomyography (MMG) neuromonitoring. The initial dilator is placed through the psoas and docked at the anterior 1/3 of the disk space where it is secured with a guide wire and confirmed with lateral x-ray. Sequential dilation of the psoas muscle is performed followed by placement of the retractor, which is fixed to the operating table. Each dilator should be monitored with directional EMG or MMG. Neuromonitoring alerts using triggered EMG or MMG can allow repositioning of the dilators and retractor to avoid nerve injury. Most nerve injuries are associated with instrumentation at the L4–L5 level [48–52].
Surgeons can start at increasing thresholds of 15–20 milliamps (mA), which may provide a safer transpsoas working corridor. If a response is obtained, the mA can be lowered until a response is not elicited. Feedback can be provided visually from the patient with muscle jerk of the extremity and from the neuromonitoring technologist, with thresholds below 5 mA usually indicating direct nerve contact . Threshold responses between 5 and 10 mA indicate close proximity, and responses greater than 10 mA usually are indicative of a safe working distance away from the motor nerves [54, 55].
Prior to placing the retractor, the finger of a sterile latex glove can be cut off and placed over the retractor, which can allow retractor opening while preventing retroperitoneal fat and psoas muscle from entering the operative field. Additionally, angling the retractor fixation arm vertically, instead of horizontally, can apply additional downward pressure and stability and may eliminate the need for a bone fixation pin or shim in the disk (Fig. 20.20). This can prevent retractor migration and creep of the psoas muscle under the retractor blades. Vertebral body blade fixation pins can result in unrecognized bony bleeding or segmental artery injury, and disk shims fix the posterior retractor blade, which is in close proximity to the lumbar plexus risking injury. AP and lateral images are then used to confirm position of the working corridor.
(a) A finger can be cut off of a sterile latex glove and placed over the retractor blades. This can prevent retroperitoneal fat and muscle from entering the operative field while still allowing the retractor to open. (b) Angling the retractor fixation arm vertically can apply additional downward pressure preventing retractor migration and may eliminate the need for bone fixation pins or disk shims
Once the retractor is placed and a safe working corridor is established and confirmed with x-ray, the discectomy can be performed. Minimize retractor opening to prevent stretch of the lumbar plexus or injury to the vertebral segmental vessels located at the midportion of the vertebral body (Fig. 20.21).
Excessive retractor opening can result in stretch and compression of the lumbar plexus and/or injury to the segmental vessels
The retractor is opened just enough for adequate visualization and cage placement, the limits of which are usually just cranial and caudal to the endplates. Additionally, be conscious that excessive posterior blade retraction can compress the nerve between the retractor blade and the transverse process. Work quickly to decrease the risk of compressive or ischemic injury to the nerve due to prolonged psoas retraction. In a prospective multicenter trial, Uribe and colleagues evaluated whether triggered EMG monitoring could predict postoperative symptomatic neuropraxia (SN) throughout the retraction process during LLIF procedures. Postoperatively, 13 of 323 (4.03 %) patients had a new motor weakness that was consistent with SN of the lumbar plexus on the approach side. Retraction time was significantly longer in those patients with SN versus those without (32.3 vs. 22.6 min, p = 0.031) . Chaudhary et al. observed diminished motor evoked potentials (MEPs), not at the time of initial retractor placement, but after prolonged retractor opening in patients with postoperative motor nerve injuries. Prolonged mechanical compression and stretch were felt to be the mechanisms of injury . This proposition is supported by studies that have reported a higher likelihood of nerve injury with increasing surgical times .
Even with meticulous conscientious dilator and retractor placement in relation to the lumbar plexus, nerve and psoas muscle irritation is a potential side effect even in single-level LLIF procedures. Suggestions to reduce the risk of nerve injury include preoperative administration of gabapentin or Lyrica, along with 10 mg IV dexamethasone may prophylactically combat the inflammatory cascade in nervous and muscular tissue limiting the extent of injury [58, 59]. In cases where extended psoas retraction is required, such as in multilevel deformity cases, releasing the retractor and allowing for the muscle and soft tissue to relax may decrease the likelihood of such injury. Lastly, shallow docking above the psoas muscle allows dissection of the psoas with a Penfield dissector allowing direct visualization of lumbar plexus prior to dilator placement .
As with most minimally invasive techniques, there is a learning curve to overcome. We have found, as have others, that the risk of nerve injury declines steadily with greater experience . Le et al. reported a significant reduction in the incidence of postoperative numbness of nearly 60 % (26.1–10.7 %), with their refined technique over a 3-year period . Experience and evolution of a surgeon’s technique can minimize the risk of iatrogenic nerve injury resulting in dramatic changes in patient outcome.
20.3.5 Preparing the Disk Space
With the retractor in place and opened, the dilators are removed leaving the guide wire in place. The guide wire can provide a reference point for the annulotomy. Using an EMG or MMG probe, the annulus and remaining strands of psoas muscle around the guide wire and edges of the retractor blades are stimulated to ensure there are no nerves crossing the operative field. A bipolar cautery and Penfiled dissector can be used to facilitate removing the remaining strands of muscle providing clear identification of the annulus. The annulus can then be incised vertically with a scalpel both dorsal and ventral to the guide wire delineating the AP working space for cage insertion (Fig. 20.22).
Leaving the guide wire in place until the ventral and dorsal annulotomies are performed provides a constant reference point for localization
Annulotomy size is based on the selection of the width of the LLIF cage. Cage widths range from 18 to 27 mm, but more common widths include 18, 21, and 22 mm. Marchi et al. reviewed the incidence and effect of subsidence in patients with stand-alone short-segment 1- or 2-level lateral lumbar interbody fusions with two different width cages, 22 and 18 mm . Patients who had wider 22 mm cages had greater lordosis correction, along with lower rates and grades of subsidence. At 12 months, 70 % in the standard group (18 mm) and 89 % in the wide group (22 mm) had Grade 0 or I subsidence, and 30 % in the standard group and 11 % in wide group had Grade II or III subsidence (Fig. 20.23). Subsidence was detected early at 6 weeks postoperatively and correlated with transient clinical worsening in VAS scores. Progression of subsidence was not observed after the 6-week time point. Additionally, subsidence occurred predominantly (68 %) at the inferior endplate. Although fusion rate was not affected by cage dimension (p > 0.999) or by the incidence of subsidence (p = 0.383), most patients requiring secondary revision spinal procedures experienced Grade II and III subsidence (six of ten patients). This illustrates the importance of not violating the endplates and limiting subsidence.
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