Adult spinal deformity (ASD) is a problem of growing clinical importance and concern. Interest in ASD has steadily risen, as evidenced by a continuous increase in PubMed-listed manuscripts, from 57 in 1980 to 416 in 2014. Surgical techniques have changed considerably over that period, including more commonly used segmental pedicle screw fixation with or without posterior-based osteotomies. Within ASD, there is a special emphasis on sagittal plane deformity due to its correlation with health-related quality-of-life (HRQOL) outcomes.1 Sagittal plane deformity is increasingly prevalent in an aging population. Traditionally, it was thought that open procedures were required to treat adult sagittal deformity such as flat back syndrome. In open procedures, posterior-based osteotomies have become the most common means of correcting sagittal plane deformity. Posterior column osteotomies, including Smith Peterson osteotomy, have been indicated for milder deformities. Three column osteotomies (3CO), including pedicle subtraction osteotomy (PSO) and vertebral column resection, have been the mainstays of correcting more severe rigid deformities. With the advent of less invasive surgical (MIS) techniques in recent years, new procedures have also emerged for treating sagittal plane deformity.
Anterior column realignment (ACR) is a relatively new, minimally invasive technique for treating sagittal imbalance.2 The ACR procedure involves a lateral trans-psoas approach to perform a complete discectomy and deliberate release of the anterior longitudinal ligament (ALL) and annulus. Hyperlordotic interbody cages, either 20° or 30° of lordosis, are then placed into the disc space and fixed to the vertebral body with one or two screws. ACR is thus an anterior column lengthening procedure, in contrast to a posterior shortening 3CO procedure. The procedure is completed by placing posterior instrumentation, and reducing the sagittal plane deformity using either MIS or open techniques depending on the desired posterior release, surgeon preference, and experience.
Although emerging as a viable option for ASD, traditional posterior osteotomy techniques have relevant advantages and disadvantages. 3CO allows significant correction of rigid deformities through a single segment. PSO yields approximately 20 to 35° lordosis, in comparison to approximately 8 to 10° per level with posterior column osteotomies.3 3CO procedures, however, also carry high morbidity, including over 20% risk of new motor weakness and nearly 100% risk of any complication, especially in older patients.4–10 Additionally, while patient-reported HRQOL measures generally improve, the number of patients reaching clinically important difference may be modest, from 24.4% reaching ODI substantial clinical benefit to 57.6% reaching SRS-22 pain minimum clinically important difference at 2-year post-op.11 From a HRQOL perspective, there is considerable room for improvement.
Published data suggests open approaches remain advantageous for more severe deformities. In a multicenter minimally invasive database, patients with marked sagittal deformities (SRS-Schwab “++”) saw improvement in back pain (VAS), but did not significantly improve in leg pain (VAS), pelvic parameters (pelvic incidence–lumbar lordosis mismatch, pelvic tilt, or sagittal vertical axis), or disability (ODI).12 Mummaneni et al13 have proposed an algorithmic approach to MIS techniques for ASD correction, and suggested that MIS approaches will be ineffective for more severe sagittal plane deformity. The purpose of this paper was to describe the ACR technique and present a review of the history and current ACR literature. We hope that by identifying proper indications for this procedure, better outcomes will be achieved for patients with ASD.
The ACR technique has evolved to include both anterior ALL and annulus release, hyperlordotic cage placement and screw fixation and posterior-based release and fixation/fusion. Diligent preoperative planning helps to increase the probability of a safe surgery and postoperative alignment goals. Preoperative MRI and/or CT myelography should be studied to determine the optimal side for approach, with the safest trajectory and working plane. The surgeon should scrutinize the relation of the vessels relative to the spine, including looking for a safe plane for dissection anterior to the spine. Contraindications to the procedure include a fused disc space at the level of interest and prior retroperitoneal vascular injury. L4-5 level is often a more challenging level and should be scrutinized more closely for anterior location of the psoas. Finding a safe docking location for retractors is more challenging when the psoas (with the traversing nerves of the lumbar plexus) is displaced anteriorly. Given the risks involved, ACR should only be performed by surgeons who are experienced and facile with lateral, retroperitoneal approaches.
The anterior stage involves a sequence of steps to safely release the ALL and annulus. The patient is positioned lateral, as for a standard lateral lumbar interbody fusion (LLIF) and a retroperitoneal approach is made to the disc space. Retractors should be docked posteriorly so that three quarters of the disc is exposed anterior to the posterior blade. A wide annulotomy is made, initially keeping the ALL intact. This enables a more complete and safe discectomy. A complete contralateral annulotomy is performed after full ipsilateral annulus release. The contralateral annulotomy can be performed with a wide Cobb elevator. Gentle anterior dissection is completed anterior to the ALL with curved Penfield dissectors. Some surgeons prefer to do the anterior exposure before the annulotomy and disc removal when the anatomy is intact but the release is done after the discectomy and annulus release. The disc posterior to the ALL must be removed thoroughly to facilitate the ALL release.
Full and safe visualization of the anterior aspect of the spine (ALL and annulus) is critically important prior to proceeding with the release. To prevent injury to the anterior vascular structures, it is imperative to develop the adventitial plane directly anterior to the ALL and annulus. The surgical plane is developed with a blunt instrument, such as a Penfield #4 or endoscopic Kittner to make sure that the retractor is between the ALL and vascular structures and the vein is not folded between the ALL and the retractor. Prior to the case start, the patient should be typed and crossed for packed red blood cells should the need for transfusion arise.
The anesthesiologist should be alerted to closely monitor vital signs during the time of greatest risk of vascular injury. The surgeon should always be prepared for vascular bleeding with hemostatic agents and packing materials. If vascular injury occurs, the first step is to slow or control bleeding. For a large vessel tear, a vascular surgeon may need to be emergently consulted for primary repair. With adequate preparation and planning, such injuries are extremely rare.
Fluoroscopy is used to confirm that the anterior dissection reaches to the depth of the contralateral pedicle. Once the plane has been developed, a larger retractor is used to protect structures anterior to the ALL. The width of the final retractor should be wider so that it is in contact of at least one of the endplates and does not fall into the disc space once the ALL is released. Once the discectomy is complete, the ALL has been isolated, and the anterior structures are protected, the ALL can be released with a long handle scalpel or any method of choice. Anterior osteophytes are rarely encountered. There is usually enough opening to enter the disc space but an osteotome can be used if needed. Once the release is completed a spreader may be used gently to test the completeness of the release and facilitate further release of the ALL and annulus if needed. Care should be taken not to overpower distraction and cause end plate fracture. Implant trialing and finalization of the procedure is performed using standard techniques, with emphasis on intraoperative alignment confirmation. Implants for LLIF ACR include 20° and 30° lordotic implants with tabs for internal fixation. Internal fixation to the vertebral bodies is important to decrease the likelihood of implant migration. ALL release otherwise leaves the segment highly mobile. The tabs can be fixed with lateral screws to one or both vertebrae. The choice of one or two lateral fixation screws has not been adequately studied and is left to surgeon preference. Fixation with one screw allows more flexibility for subsequent posterior correction; however, two screws protect from translation of the vertebra if a full posterior release is performed. Placement of an ACR cage with two lateral screws requires a broader opening, and is thus more challenging at L4-5.
Case Example 1—Historical First Case of ALL Release and Realignment
A 64-year-old woman with prior L3-S1 posterior spinal fusion presented with positive sagittal alignment. She had focal kyphosis of 39° at L3-4. In 2005, she underwent the first ever ACR, to our knowledge, at L3-4, by senior author (BAA) with 49° correction and resulting L3-4 segmental lordosis of 10°. Figure 1 shows relevant images from the case. No hyperlordotic cage nor lateral screws are seen in the image because it was the first case performed and interbody cages with tabbed inserts were not yet available.
Case Example 2—LLIF/ACR
A 66-year-old male patient with sagittal deformity and flatback syndrome, status post prior L1-L3 PSF and L5-S1 ALIF and posterior spinal fusion, presented with disabling back pain. Preoperative SVA was 8.3 cm, PI 69°, LL 41°, PT 27°. He underwent posterior removal of prior instrumentation with L2-3 posterior column osteotomies. Two days later he underwent stage 2 LLIF at L1-2 and L3-4 with L2-3 ACR, and was then turned prone for planned stage 3 L1-S1 PSF. Postoperative SVA improved to 4.4 cm, LL 69°, and PT to 18° (Figure 2). He did not require any ICU stay and was discharged home on postoperative day 3 from the definitive procedures (stages 2 and 3).
Of note, the retroperitoneal approach to ALL release can also be accomplished anteriorly. This is routinely done as part of anterior lumbar interbody fusion (ALIF) and will be referred to as an ALIF-ACR when hyperlordotic implants are used through an anterior retroperitoneal approach. Once a thorough release is accomplished, trialing is performed. Trials with a built-in amount of lordosis are used including both 20° and 30° options. When 30° implants are used, the anterior dimensions needed for placement of the interbody include a 2-cm anterior lengthening. A significant advantage to this approach is the ability to treat L5-S1, which is not approachable from a direct lateral position due to the iliac crest and lumbar plexus. The disadvantages include a more technically challenging retroperitoneal dissection, usually requiring an access surgeon, and the need to open the anterior abdominal musculature.
Case Example 3—ALIF/ACR
A 75-year-old woman with a history of multilevel thoracolumbar degenerative disk disease, thoracolumbar kyphosis, and prior L3-4 posterior spinal fusion presented with significant back pain and right-sided leg pain. Imaging revealed PT 27°, PI 61°, LL 29°, and SVA 15 cm. She underwent a first stage ALIF-ACR at L5-S1. She was then turned lateral for a planned second stage lateral interbody fusion at T12-L1, L1-2, and L2-3. At L1-2, a lateral ACR was performed. She was taken back to the operating room for a planned third stage posterior spinal fusion and instrumentation from T12-S1 on postoperative day 2. This was performed with percutaneous screw and rod placement. She progressed appropriately in the postoperative period. Post-op radiographic spinopelvic parameters improved to: PT 21°, PI 59°, LL 52°, and SVA 6.9 cm. She was stable and ready for discharge to acute rehab on postoperative day 5 after the second stage surgery. Case images are included in Figure 3.
TECHNICAL CONSIDERATIONS/COMPLICATION AVOIDANCE
Once the lateral retractor is docked, surgery must proceed as efficiently as possible, as prolonged trans-psoas retraction has been correlated with increased rates of neurologic injury.14 Up to 62.7% of patients may have thigh symptoms postoperatively from a lateral trans-psoas approach.15 The lumbar plexus passes through the psoas, which adds anatomic risk of neurological injury from lateral MIS approaches. This risk can be mitigated with intraoperative neuromonitoring, either testing stimulation thresholds for triggered, motor evoked potentials, somatosensory evoked potentials or a combination of the above. MIS retractor systems can allow for direct stimulation of the dilators and the posterior blade as they are positioned, allowing the surgeon to make adjustments if low stimulation threshold indicates proximity to a motor nerve. Transabdominal muscle action potentials are a possible application for cauda equina neuromonitoring.
Because of the hyperlordotic nature of ACR implants, it is possible to have reduction of the neuroforaminal area after cage placement. An adequate anterior and posterior implant height must be selected to allow for adequate neuroforaminal space after implant placement. The anterior stage of ACR is a highly destabilizing procedure. Posterior fixation is thus considered necessary to stabilize the spine. Posterior osteotomies combined with compression of the vertebrae above and below can be used to effect greater degrees of lordosis correction and to ensure foraminal patency.
We conducted a literature review of all published MIS ACR reports. PubMed was searched using the key words “anterior column realignment,” “anterior column release,” “anterior longitudinal ligament release,” or “anterior elongation AND sagittal imbalance.” Any preclinical or clinical article describing MIS ACR was included. Manuscripts not directly related to ACR for sagittal balance correction were excluded.
There is a small but growing body of literature describing anterior column realignment. Twelve ACR papers met inclusion criteria (see Table 1). Three biomechanics studies have been completed. Uribe et al16 completed a cadaveric study to examine effects of ALL release and varying degrees of lordotic implant insertion. ALL release combined with 30° cage placement led to mean 11.6° segmental lordosis correction, compared with 9.5° with 20°, 4.1° with 10°, and 0.9° with 10° cage and no ALL release. Foraminal height increased by 10.4% with ALL release and 30° cage compared with 6.3% for 10° and no ALL release. In a separate study, Uribe et al17 used finite element analysis to demonstrate that release of the ALL can increase lumbar lordosis. Deukmedjian et al18 completed an early feasibility study of ACR. In four clinical cases, ACR led to mean lordosis increase of 10° per segmental level and 25° overall per patient. All cases had posterior pedicle screw fixation in addition to the anterior stage. Three of four patients had PI-LL mismatch of 10 or less after correction. All ended with SVA less than 5 cm.
Case reports have corroborated feasibility of the technique for achieving radiographic and clinical improvement in selected patients. Berjano et al19 published a video case report showing L1-5 lateral lumbar interbody fusion with L2-3 ACR combined with open posterior column osteotomies and T10-pelvis fixation (ODI 53, VAS back 5/10) with 28° PI-LL mismatch, PT 27°, and 17 cm SVA. Postoperative SVA improved to 2.4 cm with PI-LL to −9°, PT to 13°. ODI and VAS back pain improved to 28 and 2, respectively. Smith and Berjano20 published a separate case report of a 72-year-old man with severe back pain with a 15 cm SVA, PI 31°, LL 0°, and T10-L5 Coronal Cobb 30°. He was treated with T10-L5 lateral interbody fusion, L3-4 ACR, and lateral vertebral body fixation alone (i.e., no posterior supplementation) for deformity correction, with correction of the deformity to an SVA of 5.5 cm, PI-LL 4°, T10-L5 Cobb 3°. In a brief review and accompanying supplementary video, Berjano21 reviews concepts on lateral MIS approaches for deformity correction and reports several cases, including a sagittal imbalance case with insufficient correction in a 73-year-old woman after L4 PSO. She was treated with subsequent L3-4 ACR and L4-5 LLIF, and achieved improvement in radiographic parameters: SVA from 12 cm to 2.8 cm, PT from 27° to 18°, and LL from 36 to 62°.
Larger case series have provided additional information about benefits and risks of the ACR procedure. Akbarnia et al2 retrospectively reviewed 17 patients treated with ACR from 2005 to 2011 including the first patient treated in 2005 (Figure 1). PT improved from mean 34° to 24°, thoracic kyphosis increased from 23° to 38° (intraop) to 51 (final), LL from 16 to 38.° EBL for the anterior stage ranged from less than 10 mL to 800 mL. Forty-seven percent of patients suffered any complication, major or minor. 17.6% (3/17) of patients developed post-op transient quadriceps weakness, 5.9% (1/17) experiencing persistent motor deficit at last follow-up. There was one common iliac artery laceration while the vascular surgeon approached the spine for a previously placed anterior plate removal; the laceration was repaired primarily. Berjano et al22 retrospectively reviewed 12 ACR patients treated from 2014 to 2015. Mean LL improved from 20° to 51° with mean 27° single-level lordosis from ACR. All patients achieved SVA less than 5 cm and 82% had postoperative PT less than 20°. They reported an 18% (2/12 patients) major complication rate with one patient suffering a bowel perforation and one posterior wound infection.
Murray et al23 retrospectively reviewed 31 patients treated with ACR (47 total ACRs) from 2011 to 2014 at a single institution. Mean LL improved from 32.3° to 49.9°, PI-LL mismatch improved from 26.5° to 11°, and SVA from 10 to 6.2 cm. They reported complications in 29% of patients (9/31) with 8 iliopsoas weakness and one patient with retrograde ejaculation. No vascular or bowel complications were noted. Turner et al24 completed a multicenter retrospective radiographic study of 34 patients treated with ACR (58 ACR levels) from 2005 to 2013. In the multicenter database, mean LL improved from 26.7 to 50.8 (last follow-up), PI-LL from 29.4° to 6.6°, and PT from 28.3° to 22.1°. ACR with posterior osteotomy led to 18.7° increase in mean disc angle, compared with 12.8° increase for ACR without posterior osteotomy and 5.7° increase for LLIF without release of the ALL.
Deukmedjian et al25 published a retrospective case series of seven ACR patients treated from 2010 to 2012. ACR led to 24° mean improvement in LL (24° pre-op to 48° post-op) with 17° improvement per ACR level. Mean PT improved from 32° to 25° and mean SVA from 9 cm to 4.1 cm. EBL averaged 125cc for the anterior stage. Mean length of stay was 8.3 days, but patients were observed at least 5 days between the anterior and posterior stages. They reported no blood transfusions, no vascular, and no visceral complications. They reported one superficial infection, yielding an overall complication rate of 14%. Marchi et al26 completed a short-term retrospective case series on eight patients treated with ACR from 2009 to 2010. At 6 months, ACR led to mean SVA improvement from 11.7 to 6.2 cm, PT from 35.2° to 23.8°, and LL from 2.3° to 27.1°. VAS improved from 88 to 51 at 1 week and ODI from 82 to 44 at 6 weeks.
MIS approaches to treat more complex ASD have been historically limited in their ability to improve sagittal alignment. ACR, while an advanced technique, allows for true sagittal deformity correction and it should be viewed as one procedure in an armamentarium of surgical options for treating sagittal deformity. ACR may be used alone or in conjunction with additional lateral interbody fusion levels (with or without preservation of the ALL). The choice of number of lateral interbody levels, cage size, whether or not to section the ALL, and any additional posterior-based corrective techniques should be guided by preoperative planning to make sure spinopelvic parameters are properly optimized. Surgical lordosis correction in a lower segmental level leads to more overall sagittal correction for ACR, just as in posterior-based three column osteotomy techniques. An additional goal should be to achieve as close to a physiologic curve as possible.
When combined with a posterior osteotomy, ACR leads to approximately 19° increase in mean disc angle, 6° more than without osteotomy and 13° more than LLIF alone.24 With the advent of hyperlordotic cages, 10 to 27° of mean segmental lordosis correction have been reported with ACR. Complication rates range from 18 to 47%. Comparison between different institutions is challenging due to varying definition of complications for both lateral approach-related versus overall surgical complications. Lateral approach-related complications include visceral injury, quadriceps palsy, other neurologic deficit (transient, permanent). Transient hip flexion weakness is expected from a lateral, transpsoas approach; other approach-related complications occur far less frequently and are not expected. Table 2 lists possible major and minor surgical complications.
To incorporate this technique into a surgeon's armamentarium, one should have significant experience with LLIF as well as caring for patients with ASD. Inadequate knowledge of anatomy or incorrect position of retractors could lead to catastrophic injury to the great vessels, kidneys or abdominal contents. Despite concerns about the spine-lengthening aspects of ACR, especially in the setting of calcified aorta, there have been only anecdotal reports of vascular injuries. Knowledge and experience with advanced adult deformity cases is necessary to judiciously select patients.
The MISDEF algorithm was developed to help guide surgeons in choosing between MIS and open procedures. The authors define three categories of deformity severity: I with SVA < 6 cm, II with SVA > 6 but PT < 25° and PI-LL < 30°, and III with SVA > 6 and PT > 25° and PI-LL > 30°.13 They suggest that MIS techniques may be adequate for type I and II cases, but that open deformity surgery is recommended for type III cases. This MIS versus open guidance remains a work in progress, as surgical techniques and technologies continue to evolve. Furthermore, the increased use of ACR may require a modification to the algorithm further expanding the utility of MIS techniques.
There is a paucity of published data on ALIF-ACR with hyperlordotic cage placement. As in the lateral approach, ALIF-ACR appears to be a viable alternative to posterior 3CO. Due to the need to transgress the anterior abdominal musculature and to retract the common iliac vessels laterally, ALIF-ACR approach will carry its own challenges compared with the lateral ACR approach. However, this can be accomplished safely and efficiently by having adequate experience with anterior approach or assistance of an anterior approach surgeon. One benefit is the ability to avoid psoas retraction and retraction of the lumbar plexus; however, significant retraction is required of the major vessels, the ureter and commonly manipulation of the sympathetic plexus during lateral exposure of the disc space. Further study is needed to compare and contrast morbidity from each approach.
ACR has emerged as a minimally invasive technique for sagittal deformity correction that may replace 3CO in carefully selected cases. Diligent attention to anatomy, neuromonitoring, and technique details is required to safely perform ACR procedure. Multiple case reports and retrospective case series now provide evidence to validate its use in achieving 19° to 27° of segmental lordosis correction. Complication rates vary from 18 to 47% but are likely to decrease as techniques are refined and standardized, patient selection is optimized, and more posterior-based MIS techniques are utilized.
- ACR is an emerging, minimally invasive technique able to treat sagittal deformity with the potential for lower morbidity compared with posterior osteotomies.
- This work represents the first comprehensive literature review (12 papers) on complications, radiographic, and clinical outcomes after ACR, using 20° to 30° lordotic intervertebral implants, in the treatment of adult sagittal deformity.
- Between 10° and 27° of segmental lordosis correction was reported after ACR, with complication rates ranging from 18% to 47%, depending on the extent of use of adjuvant posterior column osteotomies.
- The most common adverse events reported were hip flexion weakness (9.3%) and transient paresthesia or dysesthesia (12%), with major complications, such as bowel perforation or vascular injury, being rare (one of each reported).
- As an emerging, less invasive technique, ACR has similar restorative capacity to other corrective techniques with similar or lower complications rates.
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