Although the sacroiliac (SI) joint has long been assumed to be a nearly immobile joint and has been treated nonsurgically,1 newer evidence shows that it may be responsible for up to 30% of low back pain.2 Treatments for the SI joint have been well documented and range from therapy, medications, and injections to surgical management.3–5 Prior studies have documented surgical and anatomic considerations for arthrodesis of the painful SI joint6–9; however, surgical management has been limited due to anatomic and biomechanical considerations. The SI joint is difficult to approach anteriorly because it lies deep within the pelvis behind the viscera or posteriorly deep under the gluteal muscles.10,11 Thus, conventional fusion techniques require large, morbid exposures for placement of plates and screws and there is significant risk associated with the surgical approach.12,13
Techniques typically used in the setting of trauma have been developed for the percutaneous placement of screws across the SI joint14 and have developed over time for use in arthrodesis. In recent years, implants have been developed that are more specific for SI joint arthrodesis and can be placed with similar methods. Various implants have been evaluated, including cannulated screws, hollow modular anchorage screws, and bone plugs.6–9,15 More recently, the iFuse implant (SI-Bone, San Jose, California) has been introduced and has been used for arthrodesis of the pathologic SI joint.16,17 However, the surgeries still require advanced intraoperative imaging techniques and knowledge of pelvic anatomy, as there can be significant risk to neural and vascular structures if the safe zones are violated. Additionally, anatomic research and statistical shape modeling has shown that there is tremendous variability in the shape of the SI joint, making it more difficult to visualize the safe zones during instrumentation (Figure 1).
Statistical shape model of the sacroiliac joint surface showing the variation in shape. A crescent-shaped sacroiliac joint (A) and a scone-shaped sacroiliac joint (B) are shown.
Using 3-dimensional (3D) imaging and computer navigation to treat SI joint and sacral pathology allows for customized implant placement relative to patient-specific pelvic anatomy. Two additional advantages include more confident placement of implants and decreased exposure to radiation for the surgeon and operating room staff. There have been reports documenting the difficulty with visualization of the pelvis and complexities of sacral anatomy that can lead to complications with percutaneous screw placement.18,19 Further, neurologic complications have been documented with screw breeches into the sacral neural foramen, or anterior cortical disruption with irritation of the L5 nerve.18 These risks have led surgeons to identify other means for placement of hardware. Studies have shown that implants can be safely and accurately placed with the use of 3D imaging in various settings, including trauma, tumor, and degenerative disease.9,11,20–24 Secondarily, radiation exposure tends to be higher in these cases, which require radiation through the pelvis and multiple fluoroscopic images. At least one study has included percutaneous transsacral screw placement, comparatively evaluating radiation doses between conventional fluoroscopy and computer navigation. In that study, when Brainlab (Brainlab, Munich, Germany) was used for 3D imaging and navigation, reductions occurred in the radiation dosage.25 To the current authors' knowledge, no similar studies have been reported with O-arm and StealthStation (Medtronic, Minneapolis, Minnesota) navigation. In this article, the use of the O-arm in combination with StealthStation navigation and the iFuse implant is described for SI joint arthrodesis. The technique allows the surgeon and staff to step out of the room during image capture, minimizing their exposure to radiation, and leads to more confident placement of implants.
Materials and Methods
Prior to data collection, Colorado Multiple Institutional Review Board approval was obtained. Surgery was considered for patients with positive provocative physical examination maneuvers, with greater than 80% pain relief from a computed tomography-guided SI joint injection, and failing at least 6 months of nonoperative management. Patients 18 to 85 years old who had an arthrodesis of the SI joint performed using a minimally invasive technique with O-arm and StealthStation navigation were evaluated retrospectively. All patients were treated at the University of Colorado Hospital by 1 of 3 attending surgeons. The iFuse implant was used for arthrodesis in all patients. Basic patient demographic data, including age, gender, diagnosis, surgical details, surgical complications, pre- and postoperative Oswestry Disability Index and visual analog scale pain scores, and Denver Sacroiliac Joint Questionnaire (DSIJQ) responses, were collected and evaluated. Complications were tracked both intraoperatively and postoperatively.
After induction with general anesthesia, the patient is positioned prone on a Jackson Spine Frame. The typical operating room setup is shown in Figure 2. The operative side is bolstered with additional padding as needed to allow greater access to the lateral aspect of the flank/hip (Figure 3). Once the area has been prepped and draped, the posterior superior iliac spine is identified. A point approximately 10 mm proximal and 10 mm anterior to the posterior superior iliac spine is identified and marked. Using a scalpel, a small knick incision measuring approximately 5 mm is made in the skin. Blunt dissection is used to create a track from the skin incision to the lateral aspect of the ilium. A terminally threaded pin with a small attachment sleeve for the navigation system frame is introduced into the incision and advanced into the ilium, perpendicular to the bone surface, until the navigation frame holder is well fixed (Figure 4). The navigation array is attached and checked to ensure that it is rotationally stable and well attached in the ilium. Images are obtained using the O-arm.
Typical operating room setup.
Patient on a Jackson Spine Frame with a bolster under the operative side (A). Operative side draped (B). Instruments used: A, threaded pin and navigation frame; B, ball-tipped navigation probe; C, navigation array; D, navigated guidewire soft tissue protector; E, drill/broach/implant soft tissue protector; F, drill; G, implant impactor; H, impactor/slotted mallet; I, broach; J, reverse-thread guidewire (C).
Advancing the threaded pin perpendicular to the iliac crest (A). Hand-tightening the pin and frame until stable (B). Attaching the navigation array (C).
Images are transferred to the StealthStation for navigation. The ball-tipped navigation tool is calibrated. On the StealthStation, a projection is added to the tip of the ball-tipped navigation device (Figure 5). This typically measures 1 × 100 mm and is used for deep visualization of the bone while the probe is on the skin. The SI joint is initially evaluated to identify the margins of the joint surface and to review the patient's specific 3D anatomy. Three points corresponding to approximate entry sites for the implants on the ilium are then selected and marked on the skin. A typical implant pattern includes placement of implants in the S1 body, one anterior and another posterior, and placement of a third implant into the S2. However, the placement of implants is largely dictated by patient anatomy. The 3 marked points often create a triangle on the lateral aspect of the buttock/thigh. A linear incision is made in such a way that all 3 points can be easily accessed with minimal interference of the soft tissues. The incision is made sharply and carried through the subcutaneous soft tissue down to the fascia.
Identifying approximate implant entry points using the ball-tipped navigation probe on the skin with a StealthStation-generated projection (Medtronic, Minneapolis, Minnesota) (A, B). Marking locations on the skin and mapping the incision (C, D) with approximate implant entry points labeled 1, 2, and 3.
A calibrated, navigated soft tissue protector is preloaded with a guide pin that is held with a kocher clamp such that only a small portion of the pin (1 mm) is present outside the distal aspect of the guide (Figure 6). With a virtual projection having a diameter of 1 mm and a length of 100 mm added onto the tip of the guide, the guide and pin are advanced through the wound to the level of the fascia once the appropriate entry point and trajectory are confirmed. The projection is used to visualize the ideal trajectory using the StealthStation and the first implant entry site is again identified. The authors' preference is to begin with the S1 anterior implant. When the soft tissue guide and guide pin appear to be in an adequate position, the fascia is pierced and the guide is advanced to the lateral ilium. With the guide and the pin docked on the wall of the ilium, the virtual projection is changed to project a vir tual implant of optimal length, typically 8.5 mm in diameter and 25 to 70 mm in length. The guide pin is then advanced the length of the implant into the ilium using the marks on the back end of the pin to monitor depth.
Placing the navigated guidewire soft tissue protector (A) and using StealthStation projections (Medtronic, Minneapolis, Minnesota) to determine implant size and location (B). Advancing the guidewire to the depth marked on the wire prior to surgery (A inset, C). Arrows indicate mark made on the guidewire.
The navigated guide is carefully removed, leaving the guide pin in place. The following steps are performed without the assistance of navigation; thus, great care is taken to control the guide pin at all times. The implant soft tissue protector is placed over the guide pin and advanced through the fascia down to the lateral ilium. It is sometimes necessary to expand the fascial rent sharply to allow for passage of the soft tissue protector, and finger palpation is often used down to the lateral ilium to push away soft tissue and potential neurovascular structures. Using the appropriate size of cannulated drill, 3 cortices are drilled on power while holding the guide pin with a kocher clamp (Figure 7). To avoid advancement or notching of the guide pin, the drill is used at full speed and with light pressure applied while the soft tissue protector is held only loosely. The implant is then placed over the guide pin and advanced until it is well seated. The implant position can then be evaluated using the navigated guide pin sleeve (Figure 8). This is advanced over the pin to the lateral aspect of the implant, and approximate position can be evaluated with the Stealth-Station projections. A virtual projection of the implant is then saved for reference to avoid overlap during placement of the additional implants.
Drilling over the guidewire (A). Placing the implant over the guidewire (B). Seating the implant with the impactor (C).
Planning the third implant position using the navigated guidewire soft tissue protector with a projection. The green and blue projections mark the placement of the first 2 implants. (A). Final projected placement of all 3 implants (B). Final O-arm (Medtronic, Minneapolis, Minnesota) spin showing the position of the first (C) and the second and third (D) implants.
The guide pin is removed and these steps are repeated for the other implants. Once the appropriate number and position of implants is reached, a final O-arm spin is taken to verify implant positioning. The navigation array and setup is removed. All wounds are irrigated and closed. Fascial closure is generally not necessary, as the defects are small. Patients are instructed to be touch-down weight bearing on the operative side for a total of 6 weeks.
Forty-seven patients (53 SI joints) underwent arthrodesis of the SI joint using the technique described here. Patients' ages ranged from 25 to 82 years (average, 51 years). There were 33 women and 14 men. The average follow-up was 35.6 weeks; at the writing of this article, follow-up ranged from 3 to 77 weeks. Forty-one patients had unilateral SI joint arthrodesis, with 6 undergoing either staged (n=4) or simultaneous bilateral (n=2) SI joint arthrodesis.
There were 2 intraoperative complications. Both involved a guide pin breaking in situ. In one case, the residual pin was removed through a small secondary incision posteriorly over the SI joint. In this patient, the small incision was made to burr the joint and pack graft, and the pin was subsequently identified and removed. In the other case, the pin was deep in the sacral bone and did not interfere with final implant placement. The retained pin was not thought to be a threat to vascular, neurologic, or other soft tissue and was believed to be well seated. The pin was left in place and this was discussed with the patient postoperatively. Neither case was noted to have postoperative complications related to the pin breakage.
The average preoperative Oswestry Disability Index value was 23.5 (32 patients), with average values at 6 weeks, 6 months, and 1 year postoperatively of 27.8 (18 patients), 25.8 (16 patients), and 16.2 (14 patients), respectively. Patients followed for more than 1 year were found to have an average Oswestry Disability Index value of 14.7 (9 patients). Similarly, the average preoperative visual analog scale score was 6.3 points (47 patients). At 6 weeks, 6 months, and 1 year postoperatively, average visual analog scale scores were 4.4 points (17 patients), 4.6 points (31 patients), and 2.8 points (8 patients), respectively. Patients more than 1 year beyond surgery had an average visual analog scale score of 2 points (3 patients).
A new questionnaire specific to the evaluation of SI joint pain was also introduced. Results of the DSIJQ were recorded for 15 patients preoperatively, yielding an average score of 28.7. On this scale, higher scores denote greater SI joint-related disability (range, 0–50). The average DSIJQ scores at 6 weeks, 6 months, and 1 year postoperatively were 28.5 (2 patients), 12.3 (3 patients), and 6.3 (4 patients), respectively. No patient had a DSIJQ score recorded beyond 1 year.
All patients more than 1 year beyond the original surgical date were thought to have stable implants and a solid fusion based on radiographic and clinical evaluation.
The SI joint is a source of low back pain and even radicular leg pain.26 However, treatment of SI joint pain has been limited for several reasons. Historically, the diagnosis of SI joint pain has been difficult to confirm and has been relegated to a diagnosis of exclusion.27–31 Further, surgical treatment of the SI joint has been difficult at best, requiring complex exposures and having significant perioperative morbidity.10–13
Recently, renewed interest in the SI joint as a source of low back pain has led to the development of newer techniques, including minimally invasive fusion procedures. Although fluoroscopy can be used, and is often familiar to orthopedic surgeons, the pelvic anatomy is complex and not always easily visualized.18,19 Further, the anatomy of the SI joint is not completely understood and is still being studied. Techniques have been described using 3D imaging and navigation for the placement of various SI implants.9,20–24,32 However, fusion rates have varied in those studies reporting radiologic outcome, and research has been ongoing as to the best implant to achieve a stable SI arthrodesis. For these reasons, the current authors developed the technique described here to place implants for the purposes of SI joint fusion.
In this study, the authors were able to safely place implants using the O-arm and StealthStation navigation. There were no neurologic complications. There were no significant intraoperative complications, and in the 2 patients with broken guide pins, no harm was caused. Advantages of the technique included the ability to more selectively place implants and to intraoperatively evaluate implant positioning with O-arm 3D imaging. Based on the intraoperative computed tomography imaging, implant trajectory, bone placement, and configuration could be optimized. Implants could be placed in areas of improved bone purchase, such as along the anterior sacrum, with reduced concern for cortical breakthrough and L5 injury. Further, although no implant required repositioning, a second computed tomography image was obtained and allowed visualization of implant position while the patient was in the operating room.
Another benefit of the O-arm is limited exposure to radiation for the surgeon and the operating room staff. The operating room personnel were able to stand outside the room during the O-arm spins, minimizing their exposure to radiation. Although the computed tomography imaging may increase the patient's exposure to radiation, this has not been clearly evaluated. In studies evaluating similar techniques for placement of pedicle screws, the radiation dosage was found to be comparable to that for patients requiring a pre- or postoperative computed tomography scan and intraoperative fluoroscopy.33 As previously described, the use of Brainlab imaging for 3D navigation produced doses of radiation for patients and staff that were comparable to or decreased from those with techniques with fluoroscopy.25 To the current authors' knowledge, no similar studies have been conducted specifically evaluating O-arm radiation dosing in the setting of SI joint procedures. This would be an interesting avenue for further study.
The use of the O-arm and StealthStation navigation for the placement of SI joint implants is a safe and effective method for fusion of the SI joint. This technique reduces surgeon and staff exposure to radiation. Further, the navigated technique allows the surgeon to customize trajectories for patient-specific pelvic anatomy intraoperatively and to more selectively place implants based on 3D imaging. As the authors employ this technology, they are also learning more about the variations in SI joint anatomy. Such knowledge allows the fixation to be specifically placed, triangulating it for better stability.
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