(Orthopedics Online) In the past decade, minimally invasive spine surgery techniques have been rapidly developed. Minimally invasive spine surgery reduces postoperative pain and recovery time because it involves less stretching and stripping of soft tissues. With the advent of microscopic techniques, tissue retractors and some special surgical instruments, surgeons can perform previous surgical operations through a small incision. As with open surgery, the goal of minimally invasive surgery is also to provide adequate decompression of neural structures, stabilization of spinal motion segments, and correction of spinal deformities. This article outlines the current state of development of minimally invasive spine surgery and discusses the key biological concepts of posterior lumbar decompression and posterior posterolateral fusion techniques.
I. Problems faced by traditional lumbar spine surgery
Minimally invasive posterior lumbar spine surgery is based on several important concepts.
(1) Avoiding muscle crushing injuries from automatic spacers,
(2) Avoid stripping and cutting the tendon stops of important muscles, especially those of the multifidus on the spinous process,
(3) Use important neurovascular and muscular anatomic gaps for access as much as possible,
(4) Reduce the damage to the surrounding soft tissues by reducing the width of the working channel.
An important goal of minimally invasive posterior spine surgery is to reduce injury to two groups of posterior paravertebral muscles.
(1) One group is the deep paramedian transverse spinous muscle group, which includes the multifidus, interspinous, intertransverse and short gyrus muscles.
(2) The other group is the more superficial group of lateral erector spinae muscles, including the longest muscle and the iliopsoas muscle. These muscles extend from the beginning of the thoracolumbar segment of the spine to the caudal end. Of these, the multifidus muscles play a particularly important role in the maintenance of dynamic spinal stability.
Traditional trans-posterior lumbar decompression and fusion surgery has caused varying degrees of damage to the paravertebral soft tissues. Kim et al. compared the strength of trunk muscles in patients undergoing traditional open fixation with those undergoing percutaneous fixation and found a 50% improvement in lumbar extensor strength in patients undergoing percutaneous fixation. However, there was no improvement in open surgery patients.
Selective type II muscle fiber atrophy, extensive myofiber reorganization (a manifestation of reinnervation), and myofiber worm-like changes were found in muscle biopsies from patients undergoing revision spine surgery. Kawaguchi et al. suggested that the mechanism of extrusion injury to the paravertebral muscles by an automatic spreader is similar to that of injury to the extremity muscles by a tourniquet. During the use of an automatic spreader, the pressure in the paravertebral muscle increases, which in turn leads to a decrease in intramuscular blood perfusion.
The extent of muscle damage was positively correlated with intramuscular pressure and duration of traction. stevens used magnetic resonance to assess changes in the multifidus muscle after surgery. Tsutsumimoto et al. also used MRI to evaluate the postoperative multilocular muscles in patients undergoing PLIF with a conventional posterior median incision and PLIF with a small incision Wiltse approach. changes. Compared with the posterior median incision, the small-incision Wiltse approach significantly reduced the postoperative atrophy of the multifidus muscle and increased intramuscular T2 signal intensity.
Another mechanism that causes muscle degeneration and atrophy after conventional surgery is muscle denervation. The innervation of the multifidus muscle is unisegmental, a feature that makes the muscle susceptible to denervation. Damage to the neuromuscular junction due to prolonged stretching of the muscle can also lead to muscle denervation. A study of muscle biopsies in patients with failed lumbar spine surgery syndrome also found significant chronic muscle denervation.
Kim et al. compared circulating levels of tissue damage markers in patients undergoing minimally invasive spinal fusion surgery with those in patients undergoing traditional open surgery and found that creatine kinase, fructokinase diphosphate, pro-inflammatory cytokines (IL-6 and IL-8), and anti-inflammatory cytokines (IL-6 and IL-8) were significantly lower in patients undergoing open surgery compared with those undergoing minimally invasive surgery. inflammatory cytokines (IL-10 and IL-1 receptor antagonist) levels appeared to be exponentially altered in open surgery patients compared to minimally invasive surgery patients.
In the minimally invasive surgery group, most of the markers returned to baseline levels by 3 days postoperatively. In contrast, in the open surgery group, it took 7 days. Glycerophospholipids are the basic structure of cell membranes, and glycerol is an important component of glycerophospholipids. When the integrity of the cell membrane is disrupted, glycerol is released into the tissue fluid. the Ren study found that for patients who underwent postero-lateral lumbar fusion, the concentration of glycerol in their paravertebral muscles was significantly higher than the concentration in their deltoid muscles.
Another important goal of minimally invasive spine surgery is to provide limited resection of bony structures, thereby reducing the potential for postoperative spinal instability. In conventional total laminectomy, the disruption of synovial joint integrity combined with the loss of the midline interspinous ligament-tendon complex can lead to spinal flexion instability. To overcome the resulting potential for spinal instability, hemilaminectomy was created, subsequently preserving the spinous structures, the corresponding tendon stops of the multifidus on the spinous process, and the supraspinous and interspinous ligaments. Finite element analysis has shown that minimizing the removal of bony structures and ligamentous tissues can maximize the preservation of normal motion of the lumbar spine.
Second, minimally invasive lumbar decompression
1.Microscopic disc nucleus pulposus removal under minimally invasive channel
Microdiscectomy for disc herniation through minimally invasive access is currently the most commonly used minimally invasive spine surgery technique in the United States. This system, invented by Foley and Smith, consists of a series of concentric dilating cannulas and thin-walled tubular working channels of various lengths. The classic working channel is 18 mm in diameter and the procedure is usually done under the working channel using a microscope.
Several recent studies comparing minimally invasive disc nucleus pulposus removal with traditional open surgery have shown less tissue damage, less nerve interference, less blood loss, less postoperative pain symptoms, shorter hospital stay, and faster recovery and return to work in minimally invasive surgery. A randomized controlled study of traditional open microdisk nucleus pulposus removal and microdisk nucleus pulposus removal under minimally invasive access showed that surgery under minimally invasive access was safe and effective.
Site-specific pathologic changes determine the placement of the working channel. Minimally invasive lumbar decompression allows adequate decompression of the central spinal canal, lateral saphenous fossa, and intervertebral foramen regions. In addition, the disc tissue outside the foramen can be removed. The surgical approach needs to be planned prior to decompression of the different areas. For extraforaminal nerve decompression, the working channel can be placed on the transverse intervertebral membrane between the transverse processes, which is first identified and opened to reveal the travel roots, and once the travel roots are identified, the herniated disc tissue can be located deep within the nerve roots.
2.Minimally invasive lumbar hemilaminectomy
An important principle of minimally invasive lumbar decompression is to preserve the tendon stop of the multifidus on the spinous process. In a conventional total laminectomy, the spinous process is removed and the multifidus muscle is drawn to the sides. It is not possible to repair the start of the multifidus muscle on the spinous process when closing the wound. However, with the hemi-laminectomy technique, a complete decompression of the spinal canal can be performed unilaterally through the working channel. Tilting the working channel dorsally allows visualization of the underside of the spinous process and the contralateral lamina, and gentle downward pressure is applied to the dural sac to remove the ligamentum flavum and the contralateral superior articular eminence, thus completing bilateral decompression.
The anatomy of the superior lumbar spine is different from that of the inferior lumbar spine. At the level of L3 and above, the lamina between the spinous process and the articular process is very narrow, and if a unilateral approach is used, in order to perform decompression of the ipsilateral lateral saphenous fossa, more resection of the ipsilateral superior articular process must be performed. Another option is to use a bilateral approach technique, in which decompression of the right lateral saphenous fossa is accomplished by hemilaminectomy on the left side, and vice versa. A preliminary study used this bilateral approach technique to decompress 7 segments in 4 patients, with an overall mean operative time of 32 minutes per segment, a mean blood loss of 75 ml, and a mean postoperative hospital stay of 1.2 days. Preoperative neurogenic claudication disappeared in all patients and no complications occurred.
Several studies have evaluated the safety and efficacy of minimally invasive posterior lumbar decompression. The learning curve of minimally invasive spine surgery has received attention with a high complication rate at the beginning of some studies.Ikuta reported their experience with bilateral lumbar decompression for lumbar spinal stenosis using a unilateral approach with good short-term outcomes in 38 of 44 patients.The mean improvement in JOA scores was 72%. There was a lower rate of postoperative complications, less intraoperative blood loss, a lower need for postoperative pain medication, and a shorter hospital stay compared to the open surgery control group.
There was a 25% complication rate, which included four dural tears, three access lateral inferior articular eminence fractures, one postoperative cauda equina syndrome requiring reoperation, and one postoperative epidural hematoma requiring reoperation.
In a prospective study by Yagi, 41 patients with lumbar spinal stenosis were randomly divided into two groups: one group (20 patients) underwent minimally invasive microendoscopic decompression, and the other group (21 patients) underwent conventional laminectomy decompression with a mean follow-up of 18 months. Compared with the traditional laminectomy decompression group, the minimally invasive decompression group had a shorter mean hospital stay, less blood loss, lower muscle isoenzyme levels of creatine phosphokinase in the blood, lower VAS scores for low back pain 1 year after surgery, and faster recovery.
Satisfactory neurological decompression and symptom relief were achieved in 90% of the patients in this group. No postoperative spinal instability occurred in 1 case. Castro performed microendoscopic spinal decompression using an 18-mm working canal in 55 patients with lumbar spinal stenosis. With a mean follow-up of 4 years, 72% of the patients had excellent or excellent results and 68% had excellent subjective satisfaction. the mean reduction in ODI scores was 30.23 and the mean reduction in VAS scores for leg pain was 6.02.
Asgarzadie and Khoo reported 48 cases of lumbar spinal stenosis treated by minimally invasive lumbar decompression. 28 patients underwent single segment decompression and the other 20 received two segment decompression. The minimally invasive group had less mean intraoperative bleeding (25 vs 193 ml) and a shorter hospital stay (36 vs 94 hours) compared with the control group, i.e., conventional open laminectomy. 32 of the 48 patients were followed up at 4 years postoperatively.
Walking tolerance improved in all patients at 6 months postoperatively and was maintained in 80% of patients until a mean of 38 months postoperatively. Improvements in ODI scores and SF-36 scores were maintained throughout the follow-up period. No complications of nerve damage occurred in 1 case in this group.
Minimally invasive spinal surgery techniques may be more appropriate in older, frail or obese cases. In a retrospective case study, Tomasino performed single-segment microdiscectomy or laminectomy using dilated access in a group of obese and normal patients, respectively, and compared the results between the two groups and found no significant differences in incision length, operative time, blood loss, or complication rates.
Pao performed only minimally invasive lumbar decompression in 13 cases of lumbar stenosis combined with first-degree lumbar spondylolisthesis. Postoperatively, all cases had good clinical results and did not show any increase in the degree of slippage.
Sasai treated 23 cases of degenerative lumbar spondylolisthesis and 25 cases of degenerative lumbar spinal stenosis using a unilateral approach with bilateral decompression. After two years of follow-up, the neurogenic intermittent claudication score and the ODI score were slightly worse in the degenerative lumbar spondylolisthesis group, although overall the two scores were similar in both groups. Kleeman treated 15 patients with lumbar spinal stenosis combined with degenerative lumbar spondylolisthesis with a mean slip of 6.7 mm using a decompression technique that preserved the spinous process and interspinous ligaments. after an average follow-up of 4 years, two patients had worsened slippage and symptoms, and 12 patients had good or excellent clinical outcomes.
3.Transvertebral foraminal lumbar intervertebral fusion
Transforaminal lumbar interbody fusion (TLIF) was first proposed by Blume and Rojas and popularized by Harms and Jeszensky. This technique evolved from the posterior lumbar interbody fusion (PLIF) first proposed by Cloward, which required extensive canal decompression and bilateral nerve root retraction to expose the spinal space, whereas the TLIF procedure exposes the spinal space unilaterally through the intervertebral foramen. A major advantage of the TLIF procedure is that posterior lumbar canal decompression and anterior interbody fusion can be accomplished simultaneously through a single posterior incision.
Peng compared the clinical and imaging outcomes of minimally invasive TLIF surgery with those of conventional open TLIF surgery. The results were similar at two-year follow-up, but the minimally invasive group had less initial postoperative pain, faster recovery, shorter hospital stays, and lower complication rates. dhall retrospectively compared 21 patients each with minimally invasive TLIF surgery and traditional open TLIF surgery and found no difference in clinical outcomes between the two groups after two years of follow-up, but the open group had significantly more bleeding and a significantly longer hospital stay.
Selznick concluded that minimally invasive TLIF surgery for revision cases was technically feasible and did not result in increased bleeding and complication rates of nerve damage as initially reported. However, the high incidence of dural tears in revision cases makes minimally invasive TLIF surgery to manage revision cases challenging and should be performed by surgeons experienced in minimally invasive surgery.
A prospective study by Kasis found that PLIF surgery with limited exposure resulted in better clinical outcomes and shorter hospital stays compared to traditional open surgery. He identified the following 5 points
(1) Preservation of posterior spinal structures;
(2) Avoidance of lateral stripping to the transverse process;
(3) Complete bilateral resection of the synovial joints;
(4) Less complications of nerve damage;
(5) avoidance of the use of autologous iliac bone graft, all of which are closely related to the improvement of clinical outcome.
4.Lateral lumbar intervertebral fusion
Lumbar interbody fusion is a very common technique with the following 3 advantages.
(1) Removal of the disc tissue that is the source of pain;
(2) High fusion rate;
(3) restoration of intervertebral space height and lumbar lordosis. Lumbar interbody fusion includes trans-anterior interbody fusion, trans-posterior interbody fusion, transforaminal interbody fusion, or endoscopic lateral interbody fusion via an extraperitoneal approach. Minimally invasive retroperitoneal lateral interbody fusion via the lumbaris major route has been reported in the literature. This technique is accomplished in the retroperitoneal space via the psoas major muscle under neurophysiological monitoring and fluoroscopic guidance.
Below the level of L4/L5, the iliac wing blocks the revealing of the vertebral space from the lateral side. Since the lumbar plexus is located within the posterior half of the psoas major muscle, limited dissection of the anterior 1/3 to anterior 1/2 of the psoas major muscle reduces the risk of nerve damage. In addition, the use of intraoperative electromyographic monitoring may also reduce the risk of nerve damage. Disruption of the bone endplates should be avoided when dealing with the intervertebral space and implanting an intervertebral fusion, and the orientation of the intervertebral fusion should be determined by positive and lateral fluoroscopy.
Interbody fusion can achieve indirect decompression of the foramina by restoring neural foraminal height and spinal dislocation alignment. The decision to also perform a posterior fusion and decompression is made on a body-by-body basis.Knight reported early complications in 43 female and 15 male patients who underwent minimally invasive lateral lumbar interbody fusion: abnormal postoperative sensory femoral pain in 6 cases and L4 nerve root injury in 2 cases.
Ozgur reported 13 cases who underwent single- or multisegmental lateral lumbar interbody fusion. Anand reported 12 cases that underwent both lateral interbody fusion and L5/S1 transforaminal fusion. The average fusion was 3.6 segments, and the Cobb angle was corrected from a preoperative 18.9 degrees to a postoperative 6.2 degrees. 39 patients were treated by Pimenta using the lateral fusion technique, with an average fusion of 2 stages.
The lateral bending angle improved from an average of 18 degrees preoperatively to an average of 8 degrees postoperatively, and the anterior lumbar lordosis angle increased from an average of 34 degrees preoperatively to an average of 41 degrees postoperatively. All cases were able to walk on the ground and take ordinary meals on the day of surgery. The average blood loss was less than 100 ml, the average operative time was 200 minutes, and the average hospital stay was 2.2 days. Both pain scores and functional scores improved postoperatively. In a larger case series, Wright reported 145 patients from multiple study sites undergoing lateral lumbar interbody fusion for degenerative lumbar spine disease.
The segments fused ranged from one to four (72% single segment; 22% two segment; 5% three segment; 1% four segment). The intervertebral support (86% PEEK; 8% allograft; 6% intervertebral fusion device) was used in combination with bone forming protein (52%), demineralized bone matrix (39%), and autologous bone (9%), respectively. 20% of the procedures were performed with intervertebral fusion alone, 23% with a lateral nail bar system, and 58% with a posterior pedicle screw system. The mean operative time was 74 minutes and the mean blood loss was 88 ml. transient genitofemoral nerve injury occurred in two cases and temporary reduction in hip flexion strength occurred in five cases. Most patients were ambulatory the day after surgery and were discharged on the first postoperative day.
Akbarnia reported 13 patients treated with multisegmental lateral fusion for lumbar scoliosis greater than 30 degrees. An average of 3 segments were fused, and posterior fusion and fixation were performed simultaneously in all cases. At an average follow-up of 9 months, substantial improvement in both lumbar scoliosis and anterior lordosis was achieved. 1 case required revision surgery for displacement of the intervertebral implant and 1 case developed herniation at the incision site where the lateral fusion was performed. In all cases, weakness of the psoas major muscle or numbness of the thighs and weakness disappeared within 6 months after surgery. Short-term postoperative VAS scores, SRS-22 scores, and ODI scores were improved compared with preoperative scores.
Anand obtained similar results in his study of a group of 12 patients. With 2 to 8 fused segments (mean 3.64), the mean bleeding was 163.89 ml (standard deviation 105.41 ml) for the anterior operation and 93.33 ml (standard deviation 101.43 ml) for the posterior percutaneous pedicle screw fixation. The mean operative time was 4.01 h (standard deviation 1.88 h) for the anterior approach and 3.99 h (standard deviation 1.19 h) for the posterior approach. The Cobb angle improved from a mean of 18.93 degrees (standard deviation of 10.48 degrees) preoperatively to 6.19 degrees (standard deviation of 7.20 degrees) at a mean of 75 days postoperatively.
5. Minimally invasive posterior fixation technique
Minimally invasive pedicle screw implantation can be achieved using either a percutaneous or a small paramedian incision, both of which aim to preserve the function of the multifidus muscle as much as possible. The percutaneous pedicle screw placement technique is fluoroscopically guided using a Jamshidi trocar needle, which is inserted into the pedicle and then withdrawn and a guide wire is inserted along the trocar. A serial expansion catheter is placed along the guidewire to expand the soft tissue, and then tapping and hollow pedicle screw placement is performed under guidance of the guidewire. The connecting rod can be placed in a percutaneous fashion to minimize soft tissue damage.
The minimally invasive small incision pedicle screw placement technique involves a longitudinal incision slightly lateral to the lateral edge of the pedicle, followed by separation between the multifidus and longest muscles. After grade-by-grade expansion of the soft tissue, the working channel is placed, the isthmus and cephalad and caudal papillae are exposed, and a high-speed grinding drill is used to open the opening, followed by tapping into the pedicle with an arch probe. Hollow or non-hollow pedicle screws can be used. The isthmus, synovial joint, and transverse process can be decorticated for implant fusion under the working channel.
The minimally invasive small incision technique has several advantages over percutaneous pedicle screw placement: first, the anatomy can be identified under direct vision, using either hollow or non-hollow pedicle screws. Second, the technique reveals a larger area for posterior implant fusion. However, there is a risk of injury to the medial branch of the posterior spinal nerve, which travels down to the transverse process of the caudal segment and branches posteriorly to innervate the multifidus, intertransverse and intertransverse ligaments, and the articular eminence of the cephalic segment, using a minimally invasive small incision technique.
Regev conducted a cadaveric comparison of two minimally invasive pedicle screw insertion techniques and found that the minimally invasive small incision insertion technique was more likely to cause injury to the medial branch of the posterior spinal nerve. He suggested that percutaneous implantation techniques are preferable in adjacent cephalic segments if one wants to reduce the loss of innervation of the multifidus muscle in the adjacent cephalic segment.
The safety and accuracy of minimally invasive pedicle screw placement techniques have been reported, and Ringel evaluated 488 pedicle screws implanted percutaneously in 103 patients. Only 3% of the screws were found to be in an unacceptable position, and 9 screws required revision and replacement. All of these studies reflected the safety and effectiveness of minimally invasive posterior spinal fixation. In a meta-analysis of 130 studies with 37,337 pedicle screws implanted, the overall screw placement accuracy was 91.3%.
6. Limitations and disadvantages
① Radiation exposure
Among all minimally invasive pedicle screw placement techniques, the percutaneous placement technique causes the least damage to soft tissues and is therefore often used for single- or multisegment fusion procedures. However, the use of this technique is dependent on intraoperative multidimensional fluoroscopic monitoring. With advanced multidimensional fluoroscopic equipment, it takes ten minutes or more to implant two pedicle screws in a single segment, yet a segment can be completed in less than five minutes with a lateral fluoroscopic approach. With the increased implantation time associated with the application of advanced multidimensional fluoroscopic equipment, there is a corresponding increase in cumulative exposure to radiation.
Some studies have reported that fluoroscopically guided pedicle screw implantation exposes the surgeon to 10 to 12 times the amount of radiation received in non-spinal orthopedic procedures. Nevertheless, the convenience and high degree of accuracy afforded by the C-arm have made intraoperative fluoroscopic monitoring a necessary component of advanced minimally invasive spine surgery.
A prospective study analyzing the radiation exposure of 24 patients undergoing minimally invasive TLIF surgery suggested a mean exposure time of 1.69 minutes per patient (0.82 to 3.73 minutes), which the authors concluded was low compared to interventions with general fluoroscopic monitoring.Kim’s study demonstrated that the use of navigation assistance during minimally invasive TLIF surgery allows the surgeon to image acquisition away from the surgical area, thereby significantly reducing the amount of radiation exposure. In addition to reducing radiation exposure, the use of navigation also eliminates the need for cumbersome lead gown protection and eliminates the need for postoperative fluoroscopy.
② Learning curve for minimally invasive spine surgery
A Webb survey showed that most spine surgeons believe minimally invasive spine surgery techniques are effective and wish they could perform more minimally invasive spine procedures. However, because of these two limitations, most surgeons do not continue to perform minimally invasive spine surgery. complication rate during surgery.
Dhall found a higher rate of device-related complications, and Peng reported a longer operative time for minimally invasive TLIF compared with open TLIF. In order to improve the learning curve of minimally invasive spine surgery, a better understanding of the most challenging parts of the procedure is needed to develop more appropriate instruments, and it is also important to improve training techniques.
Overview
The posterior spine is dynamically stabilized by groups of muscles that are adjacent to the spinal structures and are connected to the spine by tendon stops. Stability and motion of human spinal segments are controlled in both active and passive ways. The multifidus is a powerful spinal “stabilizer” with short and strong muscle fibers that generate a lot of force through contraction over a short distance. Traditional open posterior median incision surgery severely compromises the function of the multifidus muscle due to stripping of the tendon stops, blockage of the blood supply to the muscle, and compression of the muscle tissue.
The initial goal of developing minimally invasive spinal surgery techniques was to preserve normal muscle function by reducing surgical damage. The rationale for this technique is to maintain normal function with minimal disruption of normal anatomy while safely exposing the surgical area. The traditional automatic spreader has been replaced by a tubular retractor fixed to the operating table to reduce pressure on the muscles, vessels, and nerves due to the crushing injury to the muscles. As minimally invasive spine surgery techniques continue to evolve, it is important to conduct prospective long-term clinical studies to properly evaluate the risks and benefits of different minimally invasive techniques.