Minimally Disruptive Lateral Transpsoas Approach for Thoracolumbar Anterior Interbody Fusion

Abstract

It has been 15 years since the commercial introduction of the lateral transpsoas interbody fusion procedure, utilizing a specialized retractor system and dedicated intervertebral implants paired with, most importantly, real-time surgeon-directed neuromonitoring integrated into instrumentation to allow for identification and avoidance of the nerves of the lumbar plexus. Since then, hundreds of thousands of lateral interbody fusions have been performed globally for an increasingly complex set of pathologies, with incremental technological innovation being introduced regularly. This mini-open approach can be successfully used in the treatment of thoracic disc herniations and advanced coronal and sagittal plane deformities, in the treatment of spinal trauma and tumors, as well as in the more “routine” and common short-segment degenerative lumbar spinal disease. However, successful adoption and use of this detail-oriented technique requires knowledge and understanding of the regional anatomy, careful review of and incorporation of axial magnetic resonance imaging findings into surgical decision-making, adherence to the canonical surgical technique, and adherence to neuromonitoring feedback. This chapter will cover the background, surgical technique, and best practices of the lateral transpsoas approach as they are currently understood, based on a vast amount of preclinical and clinical evidence as well as collective surgical experience.

Appendices

Quiz Questions

• 1. What are the six key steps in making lateral access transpsoas procedures safe and reproducible?

• 2. Describe the absolute and relative contraindications to lateral access spinal surgery.

• 3. What anatomic structures should the operative surgeon specifically identify on the preoperative axial MRI or CT scan at L4–L5? L3–L4? L2–L3?

• 4. Do current neuromonitoring platforms detect the location of sensory nerves?

• 5. Describe the suggested technique for “mapping” the lumbar plexus utilizing preoperative radiographic studies, intraoperative tactile exploration, and visual and auditory responses from the neuromonitor.

• 6. What is the most important independent predictor of postoperative motor deficits in MIS lateral spinal surgery?

• 7. What is the key difference between the standard lateral transpsoas technique and the “shallow-docking” lateral transpsoas approach? (Mark one)

  • (a) A different approach trajectory

  • (b) The use of expandable implants

  • (c) The lack of requirement for neuromonitoring

  • (d) The need for an access surgeon

• 8. What techniques are used to avoid endplate violation and subsidence in lateral transpsoas interbody fusion?

  • (a) Properly orient the patient in a lateral position so the working corridor is perpendicular to the floor.

  • (b) Use slides when delivering the implant.

  • (c) Use angled instrumentation at L4–L5 with a high crest.

  • (d) Use physiologic, not supraphysiologic, sizing of implants, especially in the height (cranial-caudal) dimension.

  • (e) All of the above

• 9. When evaluating the presence of paralytics in the patient prior to performing a lateral transpsoas interbody fusion, a twitch test is performed. A successful twitch test involves:

  • (a) A train of four EMG stimulations that maintain at least 75% of the response of the first stimulation at the fourth.

  • (b) A train of four EMG stimulations that maintain complete (>99%) functional responses at all four stimulations.

  • (c) A positive twitch on peripheral stimulation by the anesthesiologist.

  • (d) A series of eight stimulations with at least 50% of the response maintained throughout.

• 10. Overbreaking the surgical table can lead to: (Mark all that apply)

  • (a) Tensioning of the psoas muscle and lumbar plexus

  • (b) A more difficult access due to the position of the pelvis and ribs

  • (c) Pressure points and the development of rhabdomyolysis

  • (d) Abnormal stimulated EMG findings in the surgical window

  • (e) Changes in TMAP findings

Answers

•  1. 

  • (a) Careful patient positioning

  • (b) Gentle retroperitoneal dissection

  • (c) Meticulous psoas passage using an integrated neuromonitoring platform

  • (d) Complete discectomy and fusion site preparation

  • (e) Proper interbody implant sizing and placement

  • (f) Constant awareness of impact of protracted surgical time on neural injury

• 2. Contraindications evolve with experience. Relative contraindications to the approach include instances where L5-S1 is indicated, where the approach is limited by the position of the iliac crest, and where a level cranial to approximately T4 is indicated, where vascular anatomy and the position of the scapula limit access for the approach. Other relative contraindications include patients with bilateral retroperitoneal scarring (e.g., prior kidney surgery), patients with anomalous vascular anatomy interfering with the lateral approach (as may occur in rotational deformities), and degenerative spondylolisthesis ≥ Grade II where exiting nerve roots are more anterior and limit access. In patients with lumbarized sacra where L5-6 is a functional L4-5 segment, the approach may not be possible due to the likelihood of a more anterior lumbar plexus limiting lateral disc space access. Many, if not all, of these anatomic considerations are easily determined on preoperative imaging studies. Absolute contraindications are best dictated by common sense, but it is the authors’ opinion that L5-S1 should not be addressed with this approach.

• 3. At all levels, a careful review of the vascular structures near to, or traversing, the operative window is mandatory. In like fashion, the lumbar plexus—if visible—should be evaluated. At higher levels, the location of the kidneys should be determined, while at lower levels anomalous vascular or visceral should be visible. By CT, lateral and ventral osteophytes can be assessed as well.

• 4. No

• 5. The key to any surgery is to “uncover” the anatomy, not to “discover” it. As in question 3, a thorough review of the preoperative imaging studies will prepare the surgeon for the location of the peritoneum, the retroperitoneal viscera, and the vascular structures. Upon incision and entry into the retroperitoneal space, the surgeon’s index finger can oftentimes tactilely locate the ilioinguinal, iliohypogastric, and genitofemoral nerves (which are not detected by IONM) and, when approaching from the left side, can palpate the arterial pulse as well. Once the initial dilator has traversed the retroperitoneal space and prior to dilation through the psoas muscle, stimulated EMG should be activated through the dilator and stimulation should be maintained throughout psoas dilation. To begin, use blunt dissection with the index finger and dilator through the fibers of the psoas muscle, slowly advancing the dilator towards the lateral disc and rotating throughout, paying attention to EMG responses and where the response was generated (direction) relative to the dilator. In addition to the directional stimulation of EMG using the dilator, discrete-threshold EMG responses are provided and indicate relative proximity to the dilator. The lower the threshold (in milliamps) required to evoke a response, the closer in proximity the motor nerve is to the stimulating field. Feedback is provided both visually and audibly, with thresholds below 5 mA indicating direct nerve contact, those between 5 mA and 10 mA reporting close proximity, and responses greater than 10 mA indicating a workable distance away from motor nerves. In the authors’ experience, such an approach represents a “mapping” of the individual patient’s local anatomy and mandates the integrated use of the surgeon’s visual, auditory, and tactile senses.

• 6. Time

• 7. c

• 8. e

• 9. a

• 10. a, c, e