Review of Current Minimally Invasive Spine Fusion Systems

screw over advancing wire
Fig. 1: Screw advancing over guide wire in pedicle (larger view)

C-shaped tower
Fig. 2: C shaped tower
(larger view)

Rod
Fig. 3: Rod passing through the towers into the screw heads
(larger view)

x-ray of construct
Fig. 4: X-ray of completed contruct (larger view)

Expandable port
Fig. 5: Expandable port opened under skin (larger view)

Endoscope
Fig. 6: Endoscope introduced through port into wound
(larger view)

screw rod
Fig. 7: Screw rod schematic contruct (larger view)

screw rod
Fig. 8: Screw rod construct-endoscopic photo (larger view)

wire advancing through pedicle
Fig. 9: Wire is advanced through the pedicle (larger view)

wire advanced over guide
Fig. 10: Wire is advanced over guide wire (larger view)

both screws placed
Fig. 11: Both screws placed,
rod being swung into
the screw heads (larger view)

screw driver tightens the set screw
Fig. 12: Screw driver tightens the set screw (larger view)

The following is a review of three types of minimally invasive systems currently used in the US for performing a spine fusion surgery with pedicle screw fixation.

1. System 1 - "Pathfinder" (Abbott Spine), "Viper" (DePuy Spine), "Paramount" (Innovative Spinal Technologies), "SphereX"'''''

The first minimally invasive spine surgery system necessitates an approximately 3 cm incision directly over the insertion site of the pedicle screws. The procedure involves no muscle retraction and minimal stripping of muscles. The pedicles are addressed by placing a guide wire followed by a series of specialized instruments. Then the pedicle screw is placed over the guide wire (see figure 1). The screws are attached to C-shaped towers, which extend above the skin (see figure 2).

It is through these towers that the rod is delivered into the screw head deep in the tissue and tightened in place with set screws. The towers are then removed (see figure 3 and figure 4).

This fusion surgery approach allows a posterolateral fusion to be performed with traditional instruments, and an interbody fusion is possible through tubes that use the same small incision. Multi-level fusions of more than two vertebra are possible with this technique.

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This minimally invasive spine fusion surgery system allows for decortication and fusion to be performed thru the same incision with minimal retraction of the muscles, using traditional instruments and a head light. An interbody fusion is possible through tubes, which are commercially available to be utilized through the same small incision. Multilevel fusions with a screw at each level are possible as well.

2. System 2 - "ATAVI" (Endius)

The next minimally invasive spine fusion surgery system is similar in its incision and muscle sparing approach. It employs a special expandable retractor (see figure 5) in the area of facet joints and transverse processes. An endoscope allows visualization of the involved tissue and operative site (see figure 6).

A Transforaminal Lumbar Interbody Fusion or TLIF may be performed through this portal. Pedicle screws are placed through the portal with fluoroscopic guidance and direct visualization. The rod is then inserted and attached to the screws with set screws (see figure 7 and figure 8). Placing the expandable retractor and adjusting the retractor during the surgery allows direct visualization.

This minimally invasive surgery system allows for decortication, fusion as well as interbody fusion through the same expandable lighted port which is used for placement of the pedicle screws. Multilevel fusions are possible as well.

3. System 3 - "Sextant System" (Medtronic)

The third minimally invasive spine surgery system is essentially a fixation system and does not allow for a posterolateral fusion. However, interbody fusions are possible with the addition of an access port (which requires a larger incision).

The spine fusion system provides pedicle screw fixation without fusion using three 1.5 cm incisions, with minimal soft tissue dissection. Each screw is placed through its own incision and a third incision is needed for placement of the rod into the screws. Screws are placed in a manner similar to the first system discussed (see figure 9 and figure 10).

The passage of the rod does create some muscle damage, and there are some nuisances associated with its passage (see figure 11). Once the rod is seated, placing and tightening the set screws is relatively straightforward (see figure 12).

This minimally invasive spine surgery system is essentially a fixation system and does not allow for a posterolateral fusion or interbody fusion through the regular incisions that are made for the placement of the pedicle screws. If the surgeon wishes to perform any of the latter procedures it would have to be done by enlarging the incisions and use of tubes as mentioned above. Multilevel fixation is possible with this system.

Minimally Invasive Interbody Fusions

Interbody fusions can be done with minimally invasive techniques. This article will discuss the posterior approaches to perform a minimally invasive interbody fusion, namely a TLIF.

These procedures are being performed through a posterolateral approach, which is known as the Wiltse approach. This technique approaches the spine from approximately 4 cm to the side of the midline, from both sides. The approach is not novel at all. However, the use of minimally invasive expandable retractors is. This same muscle splitting approach is now being done through a tubular retractor which has the potential of expanding once docked in place. These retractors are inserted into the body over a series of dilators which progressively enlarge the opening and split rather than cut the muscles of the back. This is the single most important advantage to this type of approach. Once the retractor is docked in position and expanded to the desired size, it is held in place via a mechanical arm. The retractor can be tilted and angulated to show the surgeon exactly the anatomy that needs to be visualized. Some of these retractors have their own lighting mechanism. They all essentially retract the muscle to allow the surgeon to perform the same operation as the traditional open surgery with less muscle and soft tissue trauma. Some are made of metal and cannot be X-rayed through, and others of more radiolucent material that can be X-rayed through.

The TLIF procedure is then performed as per routine by removing the bones that are covering the opening to the disc, protecting the surrounding nerves, cutting and removing the disc material, cleaning the bony walls within the disc (known as the endplate), inserting the cage thru the retractor into the disc space and packing bone graft around the cage. As mentioned before, the TLIF procedure is commonly performed at the same time with the posterior fusion. These two procedures can be performed through the same incisions.

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Written by Ali Araghi, DO