Facet Wedge system feature s
Development of a new device requires novel ideas to approach the facet joint and apply the suggested principles of fusion. A minimal invasive access is preferred as minimal tissue damage will improve outcome and speed up recovery [22, 23]. This applies to different types of minimal invasive fusion techniques .
The primary step therefore might be a K-wire-guided system to access, prepare, and fuse the facet joint with the Facet Wedge system. However, considering the anatomy and the orientation of the lumbar facet joints, a true minimal invasive approach is difficult. In many cadaveric tests, it was found out that percutaneous fluoroscopically guided K-wire-guided placement of this particular device was not reliable on a routine basis. This is probably due to the angle of the facet joint in the transverse plane (Fig. 12.2) and to the difficulty to have a perfect fluoroscopic image of the degenerative facet joint. Direct visualization of the facet joint is eminent for perfect Facet Wedge positioning. The Wiltse approach used in a less invasive way is most logical to find the facet joint by blunt dilator dissection. Minimal invasive retractors can be used to have direct visualization facilitated by microscope or loupe glasses. Once direct vision is obtained, a K-wire can be introduced in the facet joint.
Facet joint angle of lumbar vertebra in transverse plane
An important tool is the so-called facet joint finder that is introduced over the K-wire. Once the finder is in the joint, the joint capsule can be identified and opened. If necessary osteophytes can be removed to create a flush surface of the facet joint that will allow for proper seating of the Facet Wedge and plate. The K-wire may be reoriented in the joint and then serve as a guide for the instruments to remove cartilage and prepare the joint for the fusion device (Fig. 12.3). In bilateral application of Facet Wedge, it makes sense to approach the contralateral side first before introducing the device. Unilateral wedge introduction did cause considerable difficulty to open the contralateral side in cadaveric testing. Therefore, it is recommended to always prepare both sides before implant introduction.
K-wire-guided instrument for facet joint preparation
Once the implant is introduced over the K-wire, the screw fixation will turn it into a stable construct (Fig. 12.4a–c). Further stability will be achieved with bone fusion through the holes in the wedge, which can be visualized on CT (Fig. 12.5).
Facet finder (a), facet rasp (b), Facet Wedge in situ with two screws (c)
CT of Facet Wedge system demonstrating bony facet fusion
12.4 Operation Technique
(Important steps—no complete instruction)
Positioning in prone position on a radiolucent table.
Attention: No direct reposition can be performed using the Facet Wedge
Mark the cranial (cr), the caudal (ca) border of the facet joint, and the midline (M). Also mark the midline offset (mo) to enter trajectory of the joint space (Fig. 12.6).
Incision (ca. 2 cm) and paraspinal approach to the joint using a retractor. After remaining soft tissue is removed, the facet joint capsule can be visualized (Fig. 12.7).
A cannulated rasp is then used to remove the superficial cartilaginous layers of the joint surface to expose bleeding bone. The rasp also works as a trial. There are three sizes of rasps/trials (S, M, L) corresponding to the implant size (Fig. 12.9).
After cartilage removal and insertion of an optimal sized trial/rasp, a reamer is pushed over the inserted trial/rasp to remove bone on the facet joint entry to create flat surface for optimal implant seating (Fig. 12.10). This procedure is to be performed on both sides.
The tools except the k-wire are then removed, and the correct sized Facet Wedge which is filled with bone (or bone substitute) can be inserted using the k-wire (Fig. 12.11).
The inserting tool also works as a screw guide. Before awling or screw insertion, the K-Wire needs be removed. Screw placement into both parts of the facet is then prepared by an awl. After awling in one part of the facet, a self-locking screw is inserted (Fig. 12.12). Then the screw guide can be turned, and the second screw into the other part of the facet can be prepared and placed (Fig. 12.13). Both screws are secured by tightening the screw to the recommended 1.2 Nm t orque.
X-ray planning of the correct incision and approach
Visualization of the joint capsule using a retractor
A k-wire is placed in the joint space
The joint is prepared using a cannulated rasp. The rasp also works as a probe. According to the used rasp (S, M or L), the correct implant size can be chosen
After the cartilage is removed, a reamer is used to flatten the surface of the facet joint
The Facet Wedge is filled with bone or bone substitute and is inserted in the prepared joint space
The inserter also works as a drill guide and guides the self-locking screws
After one screw is placed, the inserting tool can be turned and the other screw can be placed
12.5 Biomechanical Evaluation of the Facet Wedge
The only true articulation in the lumbar and lumbosacral spine is the facet joint. Therefore, it is consequent to fix this joint directly for segmental stabilization. This consideration is not new and its history is described above. Traditional translaminar facet screws are more comparable to a threaded bolt than to a “real” screw without any compressing across the facet joint. Several biomechanical studies found that pedicle screw fixation and facet fixation showed similar biomechanical characteristics with some limitations [25, 26].
In order to evaluate the kinematic properties of primary stability of the Facet Wedge, a biomechanical study was performed using a robotic based spine tester  (Fig. 12.14).
Experimental setup and X-ray from the evaluating study of the Facet Wedge
In the evaluation study, the Facet Wedge (FW), the translaminar screw fixation (TLS ), and the polyaxial pedicle screw system (PS) all produced a significant reduction in the ROM in all directions of motion of an intact motion segment.
Ultimately, all three posterior systems proved to be capable of effectively stabilizing an intact motion segment, which is consistent with other studies [25, 26].
In comparison of various posterior systems in order to stabilize an intact motion segment, the translaminar screw fixation was equivalent to trans-pedicular screw fixation in respect of primary stability for extension and flexion which was in line with the existing published findings for the translaminar screw . A similar behavior is described for screw fixation of the facet by the Boucher technique , although this was not investigated in the evaluation study. For extension and flexion, the Facet Wedge showed a trend—albeit without reaching statistical significance—toward superiority in ROM compared to pedicle screw instrumentation and translaminar screw fixation.
In the intact, idealized natural specimen, all implants (FW, PS, TLS) showed equivalent stabilities in respect of axial rotation. In lateral bending, translaminar screw fixation was significantly inferior to the pedicle screw system. This weakness of the facet screw in lateral bending has also been described by other study groups . Here, the Facet Wedge proved to be equivalent to pedicle screw instrumentation in terms of primary stability also in lateral bending in the intact motion segment.