Low back pain (LBP) is one of the most prevalent, disabling and costly medical conditions affecting modern society. LBP presents a significant challenge to effective treatment due to often multifactorial or unknown etiology. Since the 1980s, the sacroiliac (SI) joint has become increasingly recognized as a common source of LBP. In contrast to other sources of LBP such as internal disc disruption and even psychosocial factors, SI joint pain and degeneration are reliably identified with provocative manual tests and diagnostic injections. Fusion of the SI joint has been shown to provide enduring symptom relief, and minimally invasive techniques developed over the past decade have further reduced the operative risks associated with open fusion surgery. Minimally invasive SI joint fusion surgeries are typically performed by placing rigid implant components across the joint space. The implants provide mechanical fixation while bony fusion develops. Decortication of the SI joint space during the procedure produces a bleeding bone surface that allows for increased availability of autologous mesenchymal stem cells and growth factors at the fusion site. Coupled with the mechanical stability provided by the implant and autologous bone graft, decortication provides an optimal environment for bone growth and subsequent fusion of the joint. This report describes the background of SI joint disease, treatment, and the minimally invasive SImmetry? Sacroiliac Joint Fusion System (Zyga Technology, Inc., Minnetonka, MN, USA), with emphasis on the decortication instrumentation and procedure.
Low back pain is a highly prevalent and costly condition: it affects 70% - 85% of all people at some time in life, with prevalence between 75 and 100 per 1000 population among all age groups above 18 years [
The SI joint (
SI joint pain may result from several different sources such as direct or iatrogenic trauma, pelvic shear, childbirth, degenerative or inflammatory processes, or be idiopathic in nature [
lated degeneration, including intra and extra-articular osteophytosis and ankyloses, which is typically associated with increased motion of the joint [
The SI joints have complex and multi-planar morphology that challenges traditional radiography [
Conservative care for SI joint pain may include analgesics, physical therapy or spinal manipulation therapy. There is poor evidence for the effectiveness of therapeutic injections [
Over the past decade, minimally invasive SI joint fusion systems have been developed to achieve the pain relief and clinical benefit of a successful fusion, while lowering the surgical morbidity associated with open SI joint fusion procedures. Minimally invasive surgery (MIS) procedures have shown significantly improved operative results compared to open techniques [
Both mechanical and biological conditions are required for successful arthrodesis. Pluripotent mesenchymal stem cells originating in hemopoetic marrow differentiate into skeletal tissue such as cartilage, fibrous tissue or bone depending on these factors, as described by Carter [
external fixation (e.g. external fixatorsor casting). The trauma at the fracture site provides the initial bleeding, hematoma and callus formation that naturally create the biological environment conducive to healing and bone formation. In arthrodesis surgery these conditions must be created surgically, typically through cartilaginous debridement and decortication of the joint space.
As described above, the SI joint has been considered to be an amphiarthrosis. Continuing the fracture healing analogy, a painful, degenerative SI joint bears many similarities to an unhealed pseudarthrosis, where residual motion and the biological environment result in a chronically painful and unstable condition that requires true arthrodesis to resolve.
Decortication prior to placement of autologous bone graft in spinal fusion was first described by Hibbs in 1924 [
The SImmetry Sacroiliac Joint Fusion System implant is illustrated below in
comprises large 12.5 mm diameter cannulated implants at the fusion sites, as well as 8.5 mm diameter anti-rota- tion implants that provide additional mechanical stability during the development of arthrodesis. Unlike other MIS SI joint fusion implants that are impacted across or between the joint surfaces, the SImmetry implants are characterized by deep corticocancellous threads that provide mechanical fixation and stability without distraction of the articular surfaces. The threaded surfaces of the implants are prepared with a 2 - 4 μm surface roughness that has been shown to be optimal for osseointegration [
The four principal steps of the surgical procedure are minimally invasive lateral access, joint preparation, bone graft placement, and implant delivery. After a 1.5 - 2 cm incision is made, a dilator and guide pin are used to access the ilium and SI joint under fluoroscopic guidance. Acannulated drill is then advanced over the guide pin to create an osseous tunnel through the ilium. Ilium drillings are collected for use in the bone graft, avoiding the need for direct ICBG harvest. The joint is prepared as shown in
Fixation is then achieved with at least one 12.5 mm diameter cannulated implant, and one or more additional implants to ensure rotational stability. Typically an 8.5 mm implant is placed just superior to the 12.5 mm implant, but may also be placed inferiorly. Final fluoroscopic images are obtained to confirm correct placement, and the deep tissues and skin incision may be infiltrated with bupivacaine and epinephrine for postoperative pain control. Patients may be discharged the same day, or admitted to a 1 - 2 day hospital stay for pain control, with pain medication as needed. Fusion status is typically assessed with CT12 months after the procedure.
Unique among minimally invasive SI joint fusion systems, the SImmetry Decorticator provides a simple means to decorticate the fusion bed, creating the optimal biological conditions for solid arthrodesisvia a minimally invasive, lateral approach. The decorticators, illustrated in
orientation of the instrument; cutting element with integral guide ribbon; knob for incremental deployment of the cutting element; and depth stop for optional control of decorticator depth. Three sequentially larger decorticators are utilized to allow for a large graft volume of 2.1 cm3. The intuitive design of the decorticators allows joint preparation to be achieved inapproximately 5 minutes via a simple, lateral approach that is perpendicular to the joint surface. The lateral approach preserves the peri-articular structures, thus avoiding further iatrogenic destabilization of the joint.
The flexible distal end of the decorticator includes a guide ribbon and cutting element that extend beyond the margins of the tunnel. The integral guide ribbon is able to follow the undulating surface of the SI joint with the cutting element denuding cartilage and decorticating the surface of the fusion site. This enables the decorticator to access and prepare approximately 5 cm2 of the ilial and sacral surfaces beyond the implant, representing up to 50% of the area of the auricular joint surface [
When the prepared joint area is packed with bone graft and ultimately secured with the threaded implants, optimal conditions for intra-articular fusion are achieved. The decorticated subchondral bone and hemopoietic marrow provide the initial blood, mesenchymal stem cells, and ultimate vascularization that lead to strong, bony
fusion [
Chronic low back pain is a highly prevalent, disabling condition with enormous cost to society. Of the multiple possible underlying etiologies responsible for low back pain, the SI joint is recognized as a significant source. Chronic SI joint pain is a disabling condition with significant impact on patient quality of life. The symptoms often resemble axial low back pain from other sources as well as radiculopathy, however a combination of manual provocative tests and controlled diagnostic injections have led to a reliable algorithm for differential diagnosis of pain originating in the SI joint.
The anatomy of the SI joint is complex, and has been described as an amphiarthrosis due to having both diarthrodial and synarthrodial features. The joint is subject to classic age-related degeneration as well as trauma from childbirth and iatrogenic injury following lumbar fusion. Studies of degenerative SI joints reveal increased articular motion of the pathological joint. The result is a chronic, painful condition analogous to a pseudarthrosis, where the combination of poor vascularity and instability create biological and biomechanical conditions that perpetuate the situation. Local pain may originate from the chronic, degenerative inflammatory response in this highly innervated, complex muscular region; sciatic irritation may result in pain referred to the buttock, calf and foot.
Fusion of the SI joint has long been established as the ultimate solution for patients who have failed conservative therapy. Arthrodesis accompanied by decortication, bone grafting and mechanical fixation creates the biological and biomechanical conditions for intra-articular bone growth, restabilizing the joint to break the pattern of chronic instability and pain. Open surgical SI joint fusion as performed throughout the 20th century provides symptom resolution when successful. However, open fusion is a highly invasive procedure, with significant operative morbidity associated with surgical access, graft harvesting and placement.
Minimally invasive SI joint fusion has become increasingly popular over the past decade. These procedures significantly reduce the surgical time and complications that plagued open fusion. Minimally invasive procedures typically compromise with minimal access to the joint area, limiting the amount of decortication and graft material able to be deposited at the intended fusion site. If insufficient joint area is biologically and biomechanically available for proper incorporation of the graft and fusion mass development, the construct will largely rely upon the mechanical fixation provided solely by the minimally invasive implants. Over time, these may loosen, with a return to the chronically unstable, painful condition that the fusion was intended to address.
The SImmetry Sacroiliac Joint Fusion System addresses these issues both with the design of the implants, and particularly with the unique decortication technique utilizing the SImmetry Decorticator. The decorticator enables the surgeon to remove articular cartilage and decorticate 5 cm2 of the SI joint surrounding the 12.5 mm diameter cannulated implant. This prepared area, representing up to 50% of the auricular joint surface, ensures ample bioavailability of pluripotent mesenchymal stem cells, growth factors and vascularity from the subchondral bone and marrow. With the prepared area packed with bone graft and stabilized with the threaded implants, optimal conditions for a solid fusion are established. Autologous blood containing stem cells and growth factors proliferates into the osteoconductive graft material. The mechanical fixation maintains the proper stress environment, and vascular supply from the subchondral bone and marrow allow for the stem cells to differentiate into woven bone rather than the fibrocartilagenous tissue that would result without these necessary conditions.
Pain originating in the SI joint is a highly prevalent and disabling condition. When SI joint disease is positively diagnosed and refractory to conservative care, fusion of the joint is indicated. Open surgical fusion may be effective, yet has major operative risks and morbidities. While minimally invasive SI joint fusion techniques greatly improve upon the disadvantages of open fusion, this typically comes at a cost of minimal joint preparation, resulting in a much smaller fusion mass and increased risk of nonunion. The SImmetry Sacroiliac Joint Fusion System builds upon the operative advantages of a minimally invasive approach, with the addition of aninnovative decortication technique that allows preparation of up to 50% of the auricular surface of the SI joint. The decorticated subchondral bone and marrow provide a vascular environment rich in growth factors and mesenchymal stem cells that, when combined with the rigid fixation provided by the threaded implants, create an optimal environment for development of a solid fusion.
Brian Beaubien is an employee of Zyga Technology, Inc. (Minnetonka, MN, USA). Jon Bock Group Authors received financial support from Zyga Technology.