**3. Aspen® MIS spinous process fusion system**

The Aspen® Minimally Invasive Fusion System is a collection of spinous process fixation devices that are designed for rigid posterior fixation from T1 to S1 levels (see **Figure 1**). Each device consists of spinous process plates that come in three configurations (standard, medium, "Flared 5-1"), a "post plate" (a cylindrical device that is threaded in between the interspinous ligament and eventually joins

**Figure 1.** *Aspen® MIS fusion system standard size spinous process plate [19].*

### **Figure 2.** *Aspen® minimally invasive fusion system fully assembled [19].*

the two spinous process plates) and a set screw that locks the system together. The cylindrical barrel in between the two plates can also hold approximately 0.5 cc to 3 cc of bone graft material. The system also has its own set of surgical tools to facilitate the insertion.

The final assembled Aspen® Minimally Invasive Fusion System interspinous fixation device is shown in **Figure 2**. The system is FDA approved and indicated for use in the United States as an adjunct to interbody and/or posterior fusion or as a standalone fixation device from T1 – S1 levels [8] in degenerative, traumatic, and deformity pathologies.

## **4. Surgical technique**

The Aspen MIS system is placed with a patient in prone position through a 3–5 cm incision, enough to expose the length of the spinous process. Subperiosteal dissection is used to elevate the paraspinal muscles of the spinous process and lamina. The fusion site should be clear of connective and soft tissue then decorticated. The supraspinous ligament (SSL) can be removed or kept intact. Keeping the SSL intact helps preserves the natural anatomy and can prevent over distraction. The interspinous ligament is pierced as anterior as possible with a dilator (**Figure 3**).

A fluoroscopy image can be taken at this point to confirm anterior placement and appropriate level of dilator. The interspinous space is opened with a lamina spreader and measured to determine implant size. The interspinous space is decorticated with a rasp (**Figure 4**).

The barrel diameter is selected based on the fit of the rasp or spreader. The barrel length comes in a standard 21 mm size, appropriate for thick spinous processes or medium 18 mm when there are hypertrophied facets. The post plate implant is attached first to the left of the spinous process, then the barrel which is packed with graft material through the interspinous space, and finally the locking plate to the right of the spinous process (**Figure 5**).

Autograft and/or allograft can be placed posterior to the graft between the spinous process and across the lamina. The device should sit in the proper anterior *The Aspen MIS Spinous Process Fusion System DOI: http://dx.doi.org/10.5772/intechopen.95941*

**Figure 3.** *Dilator is used to create space between the interspinous ligament [19].*

**Figure 4.** *Rasp for decortication [19].*

**Figure 5.** *Attachment of the post plate [18].*

**Figure 6.** *A/P and lateral images of Aspen MIS fusion system at L4-L5.*

placement and not protrude above the lumbodorsal fascia before compressing the plates and tightening the set screw. If the implant is placed too far posterior there is an increased risk of spinous process fracture. The spikes should be fully seated into the bone, but care should be taken to not over-compress and weaken the cortex.

The Aspen® MIS Fusion System should be removed in the case of nonunion or if any components loosen or break. The provided set-screwdriver or a T10 Torque driver can be used to loosen the locking set screw. The plates can then be lifted with a Cobb elevator and removed from the spinous process. **Figure 6** shows lateral and antero-posterior radiographs of a full assembled Aspen® MIS Fusion System.
