Minimally-invasive spinal surgery has been rapidly growing in popularity with the aim in improving short-term outcomes through the limitation of approach-related morbidity. In this chapter, the authors focus on complication avoidance in minimally-invasive spinal surgery, which begins with careful preoperative selection of all candidates in the clinic. Competency in MIS surgery requires an understanding of specific anatomic corridors utilized in MIS approaches. Several key concepts in complication avoidance are elaborated. Understanding the relevant patient-specific anatomy can avert major neurovascular injury. Uncomplicated access of the lateral disc space can still be complicated by subsidence in the long-run. Iatrogenic endplate injury due to aggressive disc space preparation can be viewed as a procedural complication as subsidence reverses indirect decompression. Tubular approaches have a higher learning curve and carry an increased risk for durotomy and nerve root injury. Careful preoperative consideration should go into the selection of these patients as severely spondylotic or morbidly obese patients can lead to increased operative durations, time spent in the prone position, increased anesthesia, airway edema, and all other associated morbidity. Lastly, the MIS posterior cervical foraminotomy minimizes muscular detachment of paraspinal neck musculature that is key in preserving normal cervical alignment. Utilization of decompression only procedures, even muscle splitting approaches in patients with preoperative cervical kyphosis should be done with careful consideration as there is still an elevated risk for worsening cervical deformity by loss of cervical lordosis and increased cervical sagittal malalignment.
Keywordsminimally invasive spinal surgery, complications, lateral lumbar interbody fusion, posterior cervical foraminotomy, MIS, tubular, MIS fusion
Complication avoidance in minimally invasive spinal surgery begins with careful preoperative selection of all candidates in the clinic.
Competency in minimally invasive spinal surgery requires an understanding of specific anatomic corridors utilized in minimally invasive spinal approaches. At the least, a full understanding of the relevant anatomy can avert major neurovascular injury.
Uncomplicated access of the lateral disc space can still be complicated by subsidence in the long run. Iatrogenic end plate injury due to aggressive disc space preparation can be viewed as a procedural complication because subsidence reverses indirect decompression.
Tubular approaches have a higher learning curve and carry an increased risk for durotomy and nerve root injury. Careful preoperative consideration should go into the selection of these patients because severely spondylotic or morbidly obese patients can lead to increased operative durations, time spent in the prone position, increased anesthesia, airway edema, and all other associated morbidity.
The minimally invasive spinal posterior cervical foraminotomy minimizes muscular detachment of paraspinal neck musculature that is key in preserving normal cervical alignment. Utilization of decompression-only procedures, even muscle splitting approaches in patients with preoperative cervical kyphosis, should be done with careful consideration because there is still an elevated risk for worsening cervical deformity by loss of cervical lordosis and increased cervical sagittal malalignment.
One of the fundamental tenets of minimally invasive spinal (MIS) surgery is the improvement of short-term outcomes through the limitation of approach-related morbidity. Tubular access for posterior cervical and lumbar pathology, lateral lumbar transpsoas surgery, minimally invasive lumbar interbody fusion, and percutaneous instrumentation have all grown in popularity with the common goal of lowering surgical morbidity, cost, and maximizing outcomes. In this chapter, the authors discuss select versatile MIS approaches such as the lateral lumbar interbody fusion (LLIF) and posterior paraspinal approaches to the lumbar and cervical spine. An emphasis on complication avoidance as well as intraoperative and postoperative complication management is made.
Posterior Cervical Approach
The posterior approach to the cervical spine in an MIS fashion entails an understanding of the posterior cervical musculature. Normal cervical lordosis (CL) is a relatively modern research focus, consisting of studies of asymptomatic adults. In general, due to the lack of consensus on normal cervical alignment, there is a debate over the concept of cervical deformity. Yukawa et al. evaluated 1200 asymptomatic adults, finding the mean CL to be 13.9 ± 12.3 degrees and a range of motion of 55 degrees. Generally, a trend in the literature shows common cervical deformity criteria to include either CL less than 10 degrees, CL less than 0 degrees (kyphosis), or greater than 4 cm of cervical positive sagittal malalignment (CPSM), which is the difference in plumb lines between the centra of C2 and C7. Cervical deformity should not be an afterthought when planning even MIS posterior cervical surgery because the CL of the neck is maintained by the extensor musculature. Reduction in this tension band will result in gradual loss of lordosis. Detachment of the posterior muscular attachments from the spinous processes in the midline posterior cervical approach for laminectomy is widely performed with an instrumented fusion. The deep muscular attachments consist of the semispinalis cervices and multifidus muscles, and are avoidable in decompression surgery from a paramedian muscle-splitting approach. Even a single-level posterior foraminotomy, which entails the subperiosteal release of the muscle fibers from the lamina and drilling of less than 50% of a unilateral facet, is still observed to result in a loss of CL <10 degrees, an occurrence more common in patients with preoperative cervical deformity. This can be done with a tubular retractor. Shiraishi et al. describe their experience with 79 patients utilizing a curvilinear paramedian fascial incision to allow for access to the laminar facet junction by splitting the semispinalis cervices and interspinales muscles. This technique was not observed to result in cervical kyphosis; however, their follow-up was limited.
Lateral Lumbar Transpsoas Approach
Ozgur et al. first published results with the LLIF in 2006, describing an MIS surgical corridor to the lumbar disc and vertebral body. This lateral transpsoas, retroperitoneal approach has demonstrated in approximately 10 years to be versatile in addressing a variety of posterior degenerative indications with a low complication rate : degenerative scoliosis and global sagittal imbalance, neoplasms, osteodiscitis, and thoracolumbar trauma. The attempt to expand this approach illustrates the desire of surgeons and patients to overcome the morbidity attributed to dissection of the posterior musculature. Meticulous technique and surgeon awareness of critical neurovascular and retroperitoneal structures are key in avoidance of devastating injury.
The LLIF is unique in that every surgical step requires a broad anatomic knowledge to avoid injury. The muscular anatomy of the abdominal wall consists of three layers: the external oblique, internal oblique, and transversalis muscles, from superficial to deep. Injury to the subcostal nerve, arising from the T12 nerve root, can occur as the nerve runs between the transversus abdominus and internal oblique muscles and results in pseudohernia. The iliohypogastric (L1 supplied) runs anterior to the quadratus lumborum muscle, with an anterior branch running between the transversus abdominis and internal oblique layers susceptible to injury through approach or by heat transfer by monopolar cautery. The ilioinguinal nerve courses more posteriorly, but can travel ventral and inferior before piercing the transversus abdominis muscle to enter the inguinal canal. Injury to these sensory nerves can result in numbness or a painful neuroma. Deep to the muscular layer lies the peritoneal fascia and retroperitoneum. The abdominal fat and bowel are swept anteriorly with the psoas being the next anatomic structure of intended entry. The femoral nerve takes a more posterior to anterior position within the psoas muscle as one descends from cranial to caudal, with the greatest risk for injury at the L4–L5 disc space. Lastly, the genitofemoral arises from L1 and L2, traveling obliquely from a posterior to anterior direction on its eventual course to the abdominal wall.
The most feared complication of lateral surgery is catastrophic venous injury or arterial injury. A systematic approach to reviewing each individual patient’s anatomy is key to avoiding injury. Hu et al. demonstrated in an magnetic resonance imaging (MRI) study of the relevant vascular anatomy for the LLIF that the vena cava migrates laterally from L1 to L5 in most patients (up to 70%), and therefore an increased risk of venous injury on a right-sided approach is noted at the L4–L5 disc space. The importance to understanding where the vessels lie on each patient’s MRI is due to variations in individual anatomy. Another finding in the transitional lumbosacral zone is a lower sacral slope and a more triangular shape of the vertebral bodies, giving L5 more of an appearance seen in S1. Great care must be taken not to deviate medially and anteriorly along the side of the triangular vertebral body, but even more so here as the iliac vein has a more lateral course and runs at the greatest risk for injury.
Adult Degenerative Scoliosis
Adult degenerative scoliosis and adult spinal deformity (ASD) represent challenges in identifying safe working corridors and pose increased risk for vascular injury, bowel injury, or retroperitoneal injury. Blizzard et al. reported an arterial avulsion from an unidentified source injury retractor placement on the convexity of a multilevel degenerative scoliosis correction. After retractor expansion, episodic arterial bleeding was encountered; however, it was quickly controlled after immediate placement of vascular clips on what was thought to be a segmental artery. Because no further blood loss was encountered, the surgical course continued, and the scheduled T12 to L5 LLIF proceeded. The patient presented with delayed back pain symptoms several days later, with a computed tomography (CT) of the abdomen demonstrating a delayed renal infarction, indicating a left renal artery occlusion by the vascular clips. The renal pole will often appear within the trajectory or dorsal to the approach in thoracolumbar junction treatment of a degenerative scoliosis curve. In adult idiopathic scoliosis, or adolescent idiopathic scoliosis in the adult, significant rotation can occur in tandem with the typical lumbar degenerative curve. Careful review of the preoperative anatomy can mean life and death because the prevertebral vessels may be in a different location adjacent to the vertebral body. Another case report of vascular injury resulted from lumbar two-segmental artery injury and coagulation, which presented with hemodynamic compromise 48 hours after the uneventful L2–3 LLIF. This pseudoaneurysm was successfully managed with endovascular coil and glue embolization.
Posterior Paraspinal Lumbar Approach
The posterior lumbar paraspinal muscle-splitting approach is similar to the cervical paraspinal muscular approach in terms of surgical goals, which are to decrease postoperative axial pain, facilitate faster recovery, and to limit deformity due to surgical destabilization of the posterior spinal attachments. Watkins described a paraspinal muscular approach between the sacrospinalis muscles medially and the quadratus lumborum laterally. All paraspinal muscular approaches are commonly mistaken for and often referred to as the Wiltse approach. The Wiltse approach is more medial and describes an intermuscular approach between the multifidus and longissimus muscles. Due to the lack of clearly identifiable landmarks, this approach can be difficult. The use of an MIS retractor will obviate the direct visualization that is afforded by a large incision. Vialle et al. evaluated 50 cadavers in an attempt to clarify the approach. They found the appropriate distance from midline to be a mean of 4.04 cm at which the surgeon would most easily encounter the natural cleavage plane between the multifidus and longissimus muscles. Most techniques utilizing MIS tubular retractors in the posterior lumbar spine use fluoroscopy for guidance and therefore relegate the nuances of intraoperative anatomic identification of paraspinal muscles to an academic exercise. Instead, the safe use of tubular retractors with paraspinal approaches is through safe technique with K-wire and dilator placement under fluoroscopic guidance.
Lateral Retroperitoneal Transpsoas Approach (LLIF)
Complication avoidance starts with patient selection. Although the indications for management with a lateral approach are expanding, there are numerous pathologies that are relatively contraindicated, or are only recommended after considerable experience with the LLIF is gained. For example, the use of the LLIF for anterior column release and placement of a hyperlordotic cage for the restoration of lumbar lordosis in flatback deformity may be a less invasive alternative than a PSO in an elderly patient with progressive decline in function; this is not a typical procedure for a surgeon with limited experience.
The dissection of the psoas muscle should not continue without an understanding of the relationship of the dilator position in the psoas muscle with respect to the femoral nerve. There are numerous reports regarding the location of the nerve within the psoas muscle. In one anatomic study, Uribe et al. divide the discs into four equal zones, numbered anterior to posterior, as a way to create a helpful understanding of the position of the lumbosacral plexus and femoral nerve in relation to anteroposterior position along the disc space. For example, in their study, at L2–3, all of the nerves were in the posterior zone (IV), except with the genitofemoral nerve crossing fairly reliably in the midpoint of the L2–3 disc (between zones 2 and 3)(II). The genitofemoral nerve is a sensory nerve, and its location cannot be determined by stimulus-evoked electromyography (EMG). Injury could result in perineal numbness, or a neuroma can potentially result from transection. Thigh flexion weakness can occur, and most often, this weakness improves. One additional consideration is the use of neuromuscular blockade during intubation, which in a team with inconsistent anesthesia providers, this administration could occur without communication to the team. The rationale for neuromuscular blockade with posterior surgery is that the half-life is short enough to allow stimulus-evoked or continuous EMG, or motor-evoked potential use by the appropriate needed time point. However, with a lateral approach, the time to access and stimulate the psoas muscle occurs much earlier, most often before the neuromuscular blockade would wear off.
The most feared complication of lateral surgery is catastrophic venous injury or arterial injury. This cannot be overstressed, that successful implementation of MIS spinal surgery requires the proper selection of patients. Therefore always review the preoperative vascular anatomy on the axial MRI imaging, noting in particular the course of the iliac bifurcation and level. Always evaluate the position of the nerves within the psoas, which is posteriorly located, and any anterior vasculature. This gives you an estimation of your working corridor.
Anatomic radiographic studies have shown a consistent relationship with a high iliac bifurcation, resulting in a frequent lateral course of the iliac vein across the L4–5 disc space. As mentioned above, a lumbosacral transitional vertebra can occur, and the patient can have six lumbar vertebrae, such that the iliac bifurcation occurs a level higher than in most patients. Vascular or neurologic injury is a possibility ( Fig. 56.2 ). In a retrospective review of 351 patients that underwent LLIF at L4–5, 2.8% (n = 6) patients were noted to have a lumbarized sacrum, meaning that they had six lumbar vertebrae and a mobile L6–S1 disc space. Sharp injury to retroperitoneal viscera and vascular structures is a life-threatening procedural complication, as illustrated in a case report where anterior migration of a retractor blade resulted in catastrophic common iliac vein injury and subsequent death. Understanding the relevant vascular anatomy is key to preventing life-threatening complication. The vena cava has been shown to migrate laterally from L1 to L5 and in 70% of patients is at considerable risk for injury from a right-sided approach at the L4–L5 level. Another iliac vein injury has been reported at the L4–5 level from a right-sided approach due to an early iliac vein bifurcation, and a relatively narrow working corridor on preoperative MRI. Adult degenerative scoliosis represents another challenge in identifying safe working corridors and poses a risk for vascular injury. One example of this was reported by Blizzard et al., who reported an arterial injury during adult scoliosis correction. During retractor expansion, episodic arterial bleeding was identified, requiring intraoperative vascular surgery assistance. The bleeding was promptly controlled with vascular clip application, allowing the T12 to L5 LLIF to proceed due to low blood loss. However, delayed renal infarction resulted, due to injury to the left renal artery. In another patient, a delayed left L2 lumbar artery pseudoaneurysm was diagnosed after hemodynamic compromise, which occurred 48 hours after an uneventful L2–3 LLIF. This was successfully managed with endovascular coil and glue embolization.
In most instances, brisk bleeding should be tamponaded immediately with pressure held for several minutes. Most injuries along this approach are venous and will be controlled when substantial pressure is held. Use of immediate cautery at the location of a small injury to the lumen of a large vein will convert this injury to a large tear, and a less controllable source of bleeding. In the event of uncontrolled bleeding, an immediate decision should be to call for blood, notify the anesthesiologist, call for the assistance of a general surgeon, and extend the incision to allow for a wider exposure and superior vessel control.
Subsidence is a serious complication of LLIF or any procedure that relies on indirect decompression of the pathology, which is often from foraminal stenosis, subarticular recess stenosis, or ligamentous hypertrophy and central canal stenosis. Many intraoperative decisions can impact the development of subsidence, which varies in the literature from 0.3% to 22%. Multiple patient-specific risk factors for subsidence can include an elderly patient, T score less than −2.5 on femoral neck bone densitometry, recombinant human bone morphogenetic protein-2 use (rhBMP-2), and increased surgical complexity. As a percentage of troublesome levels, the L4–5 level is also at a relatively elevated risk for subsidence due to a high iliac crest, which can force the angle of entry from orthogonal to tangential, resulting in elevated forces directed into end plates. Moreover, as with any MIS procedure, poor fluoroscopy visualization commonly facilitated by patient body-habitus resulting in poor penetration of x-rays can affect the clarity of end plate margins and increase the change of a nonorthogonal trajectory. Overall, subsidence rates have been estimated to range from 0.3% to 22%, largely the work of retrospective radiographic case-control studies. Several studies have implicated surgeon decision-making as a risk factor for subsidence, namely by overly aggressive graft height in a collapsed disc space. Tohmeh et al. found that increased cage height and decreased length and width correlate with increased subsidence at the 1-year mark. Le et al. found in an analysis of subsidence risk factors that overdistraction of the end plates by improperly large grafts may be the most important of the three-dimensional parameters in terms of subsidence risk. In their study, they found the majority of subsidence cases to occur at 12 mm or greater, ultimately recommending a 10-mm cage maximum in either parallel or lordotic geometries. Satake et al. noted similar findings, with the incidence in end plate injury ranging from 8.3% to 21.4% when increasing from 10 to 12 mm in craniocaudal graft height (+13.1%), respectively.
This was followed by overly narrow cage widths as the second most common predictor of subsidence in the postoperative period. However, the risk of neurovascular injury by exceeding the anteroposterior diameter of the working channel between the nerve and vascular structures outweighs the risk of subsidence. Graft length is also important, but the issue of undersized grafts and lack of appropriate contact with the denser cortical apophyseal ring is relatively less common than oversizing a graft. An undersized graft has been biomechanically shown to have a greater peak load to failure. One additional caveat to the appropriate graft length is that in rotational deformities and an oversized graft, exceeding the depth of the contralateral annular ring with any instrument could result in injury to the vena cava, aorta, or bowel. It is recommended to note the depth of each instrument with fluoroscopy and to have the surgeon use his or her nondominant hand to maintain this position along the instrument; the hand becomes a positive stop on the patient’s body, preventing bottoming out. There are many systems for preventing this sort of complication because this complication can occur with any form of lumbar interbody access. One last topic is the use of rhBMP-2 in LLIF, which is quite commonplace. High dosages of rhBMP-2 (>2 mg) have not been shown to raise the fusion rates beyond lower doses, such as 1 to 2 mg. RhBMP-2 is thought to accelerate the cortical uptake of graft and in turn to result in an earlier risk of developing subsidence. This earlier subsidence, when studied, did not carry out significantly. Subsidence is an area that warrants further study with an attention to standardized definitions of the term, or a focus on linear representations of subsidence measurements.
A dural tear is one of the most common complications of spinal surgery, particularly in revision surgery, deformity, and MIS spinal surgery. In most cases, the dura can be repaired primarily and made watertight. Adjuncts include fibrin sealant, a dural patch, flat bed rest, and lumbar cerebrospinal fluid (CSF) diversion. It is rarely encountered where the wound needs to be extended to allow for a definitive closure.
Lateral extracavity, retropleural approaches have been popularized for the treatment of mid- to low-thoracic central and paracentral disc herniations, with and without calcifications. In many instances these approaches can remain extrapleural, and a plane of dissection along the excised rib provides a clear working channel to resect the calcified disc herniation. In the even of a frank CSF leak, seen in trauma or with a calcified, adherent disc herniation, the CSF leak may or may not be in the field of view. If the repair is not visualized, an alloderm patch can be sutured over the potential area followed by fibrin glue. This should be followed by intraoperative lumbar drain placement and CSF diversion, possibly with a chest tube, as well by monitoring fluid collection at the wound bed and in the pleural cavity. The concern is that a large CSF collection can develop within the thoracic cavity due to the large cavity and due to the negative pleural pressure. Flat bed rest and steady CSF drainage at 10 cc/h for a minimum of 2 days will help facilitate closure; in addition, there should be careful observation for the development of severe headaches. It is important to be aware of the development of hygromas or subdural hemorrhage, which is a potential risk from CSF fistula formation.