Prone transpsoas direct lateral lumbar fusion may improve operative efficiency
The desire for a minimally invasive technique to restore intervertebral disc height for indirect decompression and sagittal and coronal alignment while preserving posterior structures resulted in modern lateral lumbar interbody fusion.
Building on this experience, recent innovation has led to the development of prone transpsoas lateral lumbar interbody fusion (LLIF). Prone positioning allows for simultaneous access to the anterior and posterior elements of the spine, allows abdominal contents to move away from the surgical field and alleviates the need for intraoperative positioning change. This technique can thus improve operative efficiency and versatility without compromising safety.
Source: Bryan Heard, MD, PhD, FRCSC; Scott Mallozzi, MD, MA; Isaac L. Moss, MD, MASc, FRCSC; and Hardeep Singh, MD
Patient case
A 62-year-old woman presented for evaluation of decreased walking tolerance in the context of chronic back and leg pain that was worsening for several months. Her pain was localized to the low back, with radiation down the front and back of both thighs, right worse than left. Pain impeded walking for more than one-quarter of a mile and was improved with rest and change of posture. She reported significant and worsening disability.
Imaging showed a grade 1 spondylolisthesis at L3/4, with dynamic instability on flexion extension radiographs. MRI confirmed lateral recess and central stenosis at the L3/4 level (Figure 1). Her symptoms persisted despite physical therapy and epidural injections. She elected to undergo prone transpsoas interbody fusion at L3/4 with percutaneous posterior instrumentation.
Technique
Following the application of neuromonitoring leads for somatosensory evoked potentials and electromyography (EMG), the patient is positioned on a standard Jackson frame with a specialized pad and bolster system (ATEC Spine). Bolsters are positioned at the level of the greater trochanters and the chest to both stabilize the patient and provide coronal plane bending to increase the space between the pelvis and the ribs to improve access on the desired side of approach.
The choice of side of approach is dictated by the spinal, muscular and vascular anatomy. If scoliosis is present, access through the concavity of the curve is recommended to improve access to the cranial and caudal extents of the lumbar spine and allow for access to a greater number of levels through fewer incisions. The height of the iliac crest and shape of the psoas are important considerations, especially at L4/5, as a high crest may impede access to the disc space and an anteriorly positioned “Mickey Mouse” psoas (Figure 2) may put the lumbar plexus at risk while accessing the disc space. Position of the great vessels should also be considered, as these may be at greater risk on one side of approach. If there is no anatomic feature-guiding side of approach, approach through the symptomatic side is often recommended.
Procedure
Using anteroposterior fluoroscopy, the table is rotated to ensure the patient is in the true prone position. Rotating the table as opposed to the C-arm is recommended to help the surgeon with orientation throughout the procedure. The level of interest is then localized with lateral fluoroscopy and incision is planned from the dorsal aspect of the foramen to approximately the anterior two-thirds of the disc space.
A 4-cm skin incision is made and carried through the subcutaneous tissue and abdominal wall fascial. Blunt finger dissection is then used to create a path through the obliques and transversus abdominus muscles. Electrocautery is avoided through the abdominal wall musculature to avoid nerve injury and development of a pseudohernia. Once the finger is through the abdominal wall, the quadratus lumborum, transverse process, iliacus or ribs (depending on level) can be palpated to confirm that the retroperitoneum was successfully accessed. In a controlled fashion, the finger is swept caudally to cranial to develop a clear path to the lateral aspect of the psoas. Palpation of a clear path to the lateral aspect of the psoas is a crucial safety to avoid injury to abdominal contents.
Using lateral fluoroscopy, the first of three dilators is advanced through the retroperitoneal space, through the psoas and onto the middle portion of the disc space. The dilator is guided to the psoas by a ventrally positioned finger to protect abdominal contents. Directional triggered EMG through the dilator is used to localize the motor nerves of the lumbar plexus dorsal to the docking point by ensuring EMG stimulation thresholds are higher ventrally compared with dorsally. The difference between ventral and dorsal stimulation threshold is more important than the absolute value. A K-wire is then used to stabilize the initial dilator, followed by subsequent dilation with two additional dilators, repeating triggered EMG stimulation with each to confirm position ventral to the lumbar plexus.
After dilation, the prone transpsoas retractor is advanced over the dilators and attached to the table via an articulating arm using fluoroscopy to confirm that retractor is orthogonal to the lateral aspect of the spine. The dilators are removed, and the retractor is opened ventrally to the desired location at the ventral aspect of the disc space. The operative field is examined visually and with an EMG probe to confirm absence of motor nerves within the field. A shim is then placed through the anterior blade of the retractor into the disc space to stabilize the retractor under anteroposterior fluoroscopic guidance (Figure 3). The retractor is then opened dorsally until the desired exposure is achieved. An EMG probe is once again used to confirm absence of motors nerve in the field, and a second shim is placed to stabilize the posterior retractor blade.
At the cranial and caudal levels of the lumbar spine, access can be impeded by the 11th or 12th rib or the iliac crest. Depending on the patient-specific anatomy, access can be below or between the ribs. In some cases, the retractor can be docked at an oblique angle to the spine and, once shims are in place, it can be used to lever the retractor to an orthogonal orientation relative to the spine. At L1/2, it may be necessary to dilate between the ribs to gain access to the spine. If orthogonal docking is not possible, disc space preparation and interbody placement can still be achieved using angled instruments. If lateral listhesis is present, it is beneficial to dock at the level of the vertebral body that is closer to the skin.
With the retractor in place, an annulotomy is made across the length of the exposed operative field. The contralateral annulus is then released with a cobb using anteroposterior fluoroscopic guidance. A variety of instruments can be used to perform a thorough discectomy and prepare the cartilaginous endplate for fusion. It is important to preserve the integrity of the bony endplate to prevent implant subsidence and loss of disc height restoration.
After disc preparation is complete, trial implants are used to determine final implant height and medial-lateral length (Figure 3). Anteroposterior width is determined based on exposure achieved when opening the retractor. Position of the final trial should be confirmed with anteroposterior and lateral fluoroscopy. The appropriately sized interbody is packed with the desired bone graft material and then impacted into place under anteroposterior fluoroscopic guidance (Figure 4). If the anterior longitudinal ligament ruptures either due to anterior placement of the implant or attenuation and failure during disc height restoration, a lateral plate is recommended to stabilize the interbody device within the disc space.
After final interbody placement, hemostasis is achieved and the retractor is slowly withdrawn while visually confirming that there is no bleeding along the dilated tract. The abdominal wall fascia must be securely closed to avoid development of an incisional hernia. If working with an assistant, one can then begin any posterior work as lateral closure is undertaken.
At the early time point of 6 weeks after surgery, the patient was satisfied and significantly improved in all patient-reported outcome measures.
Discussion
A recent multicenter retrospective study (365 patients with 545 levels fused, followed for 7.3 ± 5 months) examining the outcomes of prone transpsoas surgery revealed that approximately 8.2% of patients developed transient postoperative thigh pain and 5.8% developed transient lower extremity weakness. The reoperation rate was approximately 3%. A persistent femoral nerve palsy was present in two patients (0.5%). These findings are on the low end of previously reported complication rates for lateral decubitus LLIF surgery. The length of stay and readmission rates favored prone transpsoas compared with other LLIF procedures. The authors concluded that prone transpsoas is a safe and efficacious approach to LLIF.
- References:
- Singh H, et al. Int J Spine Surg. 2024;doi:10.1444/8625.
- Soliman MAR, et al. Neurosurgery. 2023;doi:10.1227/neu.0000000000002555.
- For more information:
- Bryan Heard, MD, PhD, FRCSC; Scott Mallozzi, MD, MA; Isaac L. Moss, MD, MASc, FRCSC; and Hardeep Singh, MD, can be reached at the department of orthopaedic surgery at UConn Health in Farmington, Connecticut. Heard’s email: bheard@uchc.edu. Mallozzi’s email: mallozzi@uchc.edu. Moss’s email: imoss@uchc.edu. Singh’s email: hasingh@uchc.edu.
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