Segmental Interbody, Muscle-Preserving, Ligamentotaxis-Enabled Reduction: “SIMPLER” Technique for cMIS Correction of ASD

“Midline muscle sparing” approach. (a) Standard midline skin incision. (b) Elevation of lipocutaneous flaps, staying superficial to muscular fascial layers. (c) Completed midline muscle-sparing approach with bilateral lipocutaneous flap elevation to the Wiltse paraspinal interval.

Robotic pin placement. (a) Palpating the posterior superior iliac spine through the midline muscle sparing incision allows for safe pin placement and prevents errant sacral placement. (b) Completed pin placement through stab incision. This reference pin is the site for robotic arm attachment.

Computed tomography (CT) to fluoroscopic registration. (a) Anteroposterior fluoroscopic registration with robotic arm reference markers. (b) Fluoroscopic image obtained from anteroposterior. (c) Lateral/oblique fluoroscopic registration with robotic arm reference markers. (d) Fluoroscopic image obtained at lateral/oblique. The spinal cortical densities in combination with the navigation markers are matched to a corresponding preoperative CT image. This is the basis for navigation registration.

“Trust-but-verify” robotic instrumentation safety protocol. (a) Long-handled scalpel blade is introduced to create a fascial opening for working channel insertion. (b) Working channel insertion is introduced down to bone, dilating through the muscle to provide a channel for tool passage. (c) The high-speed burr is introduced to open the initial pedicle screw trajectory. (d) A pedicle tap 1 mm less than the pedicle screw diameter is introduced to the pedicle channel. (e) A long ball-tipped probe is introduced to “feel” the walls of the pedicle for breaches. (f) A Kirschner wire (K-wire) introduction sleeve (“straw”) is placed. (g) K-wire is placed through the “straw” and gently impacted into bone to prevent dislodgement from the prepared pedicle channel. (h) Perifascial incision can be packed with gauze to limit bleeding upon removal of working channel. (i) K-wires are managed superficially and later inspected with fluoroscopy for safe trajectory. If there is ever concern for pedicle breach, a ball-tipped probe can be inserted at any step to inspect the pedicle walls.

Placement of pelvic instrumentation. A small fascial opening is made medial to the posterior superior iliac spine (PSIS). A small cuff of fascia tissue is left attached to the PSIS for lateral repair. The screw head is left prominent and visible above the fascia. Later, a screw extender tower can be attached before the screw is driven below the fascia. Inset radiograph demonstrates the “subcrestal” iliac screw trajectories that allow the screw head to eventually be driven below the fascia.

Passing the rod. (a) With instrumentation completed, an overcontoured rod is harmoniously bent, ensuring there are no sharp angular bends that notch the rod. (b) The rod is slid under the fascia into all the screw extenders. The midline muscle-sparing incision makes this aspect of minimally invasive surgery much easier than percutaneous techniques.

Reduction to the apex of lordosis. Starting at the apex of lordosis, a rod reducer is inserted into the screw extender. This reduction tower has numerical readouts that are displayed in a window on the side of the extender. These numeric values correspond to the distance needed for complete rod reduction into the screw head. Once the rod reduction tower is inserted into the extender, it is advanced until resistance is encountered. The display window should read 8 mm or less. If a greater value is observed, remove the rod and bend more lordosis into the rod.

Serial rod reduction to the apex of lordosis. This diagram demonstrates equal force distribution across multiple screw reduction towers. The goal is for reduction to the apex of lordosis first. With an initial display of 8 mm at the apex of lordosis, adjacent towers will likely read slightly higher in an appropriately overcontoured rod. By reducing the adjacent screw towers, this will lessen the forces on the rod at the apex of lordosis. Once the adjacent screw towers are reduced to 8 mm, the surgeon can reduce the apex to 4 mm. Switching back to the adjacent towers, the surgeon reduces L3 and L5 down to 4 mm. The apex should now easily reduce (RD). Once the apex of lordosis is reduced, the adjacent towers can be reduced as well. Reduction of the adjacent levels after the apex will hopefully impart a force onto the spine and create lordosis.

Incorrect rod reduction sequence. The above-noted diagram demonstrates the improper technique for the reduction of the rod. When the adjacent screw towers around the apex of lordosis are fully reduced (RD) before the apex, this creates a scenario that will only result in screw pullout. The 5.5 mm titanium alloy rod has some flexibility but will not accommodate an acute deformation into the screw head with the adjacent levels fully reduced. Surgeons should take care to reduce to the apex of lordosis first.

Serial rod reduction toward the upper instrumented vertebra (UIV). Reducing the rod toward the UIV will eventually force the rod into the screw heads with very little effort. In an appropriately overcontoured rod, rod reducers inserted near the UIV should require almost no reduction.

Rod reduction into the pelvic screw head: With a modified iliac screw trajectory, polyaxial screw head can still rotate to accommodate variation in rod position. (a) Pictured is the iliac fixation driven under the fascia with an attached screw extender. The rod has not yet been fully reduced. The rod was aggressively overcontoured to ensure that the apex of lordosis is within 8 mm, but this results in a prominent rod tip. Further reduction will eventually drive this rod below the fascial level (b) With serial reduction around the apex of lordosis, the iliac screw tower is visualized by itself with an intact midline supraspinous ligament. (c) With the reduction of adjacent screws and eventually the iliac screw, the rod disappears under the fascia to prevent prominence and ensure that a full fascia closure is achievable. The dotted lines represent cuffs to fascia tissue that can be closed later.

Spinal translation during rod reduction. With the rod held in fixed coronal position, the dedicated rod reduction towers can impart significant forces on the residual spinal deformity. Additionally, distributing the forces across multiple adjacent screw towers helps prevent screw pullout during this process.

Spinal derotation during rod reduction. Throughout the reduction process, forces are being applied in multiple planes. Derotation can occur with pressure applied to the reduction towers. It should be noted that linked segmental derotation is not performed in this technique.

Fusion technique: the segments of spinal instrumentation that do not contain anterior interbody fusion grafts must still undergo a formal fusion procedure. (a) With spinal instrumentation and rod reduction fully completed, the areas in yellow boxes require a formal posterior fusion. The fascial openings from the spinal instrumentation are connected. (b) The paraspinal interval is dissected to expose the pars interarticularis, facet joints, and the lateral laminae of the levels to be fused. (c) The fusion bed containing decorticated bone surfaces medial to the instrumentation, packed with bone-morphogenic protein 2 and allograft. Note that the midline is intact without detachment of muscular origins or insertions. Radiographic insert demonstrates a healed fusion with this type of technique. Note the flowing bone along the pars-facet-pars complex.

Closure of midline muscle sparing approach. (a) Running barbed sutures are performed bilaterally to completely close the fascial openings. (b) Bilateral fascia has been closed in a watertight fashion. Note that the midline has not been violated. There is no need for a deep drain. (c) The lipocutaneous flaps are reattached to the midline with a suturing technique that contains bilateral deep dermal layers and the midline ligamentous complex of the spine. The suture is thrown superficial-to-deep in the deep dermal layer, then through the ligamentous midline spine, and then deep-to-superficial through the contralateral deep dermal layer. The outlined circles demonstrate the appropriate layers for this stitch. This is repeated until the lipocutaneous flaps are completely reattached, and dead-space has been eliminated. Suprafascial drains are not necessary.

Case example using Segmental Interbody, Muscle-Preserving, Ligamentotaxis-Enabled Reduction protocol: preoperative radiographs of a 62-year-old man with adult spinal deformity. Spinopelvic parameters include 53° coronal Cobb angle for the thoracolumbar structural curve (L1−L4), pelvic incidence of 42°, lumbar lordosis of 4°, mismatch of 38°, pelvic tilt of 31°, and C7 SVA of 11.5 cm. Computed tomography images also demonstrate severe facet and intradiscal spondylosis without ankylosis. The spinal magnetic resonance imaging demonstrates significant rotational deformity but also shows the various anterior interbody corridors (white arrows) based on the patient’s vascular anatomy.