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INTRODUCTION
In long-level fusion surgery that extends to the sacrum, fixation failure and pseudarthrosis in the lumbosacral junction continue to be a challenge for spine surgeons7). A 24% rate of pseudarthrosis at the lumbosacral junction in adult scoliosis surgery was reported, ant it has been shown to be associated with adverse clinical outcomes8). These adverse outcomes could be attributed to poor bone quality of the sacrum, complex regional anatomy, and the substantial biomechanical forces1,3).
To address these issues, various techniques of sacropelvic fixation including trans-iliac bars, iliac wing screws, and iliosacral screws have been proposed. Among these, Galveston method and iliac wing screw fixation techniques have most commonly been used, with high fusion rates2). Compared to Galveston technique, iliac wing screw technique is technically less demanding, and the pull-out strength of the iliac wing screw is three times that of the Galveston rod10,16). However, there are several concerns regarding the conventional iliac wing screw technique, which have such as increased risk of infection owing to wide dissection of soft tissue, implant prominence, and painful hardware2,3,18).
S2 alar-iliac (S2AI) screws, an alternative for the traditional iliac wing screws, have recently been introduced and have become popular. Several studies have reported successful results with the S2AI screws in adult and pediatric populations13,17,19). The major advantages of S2AI screws over traditional iliac wing screws are that they have a lower profile, they are in line with the cephalad instrumentation without the need for offset connectors, and they do not require a separate fascial incision2,14,17). Further, previous researchers have demonstrated that S2AI screws can be inserted by free-hand technique or by percutaneous methods12,15).
However, we experienced several cortical violations due to misplacement of the screw from the original pilot hole, which we had confirmed as intraosseous. To address this issue, we developed a novel S2AI screw insertion technique using a K-wire and cannulated screw, which could ensure accurate placement of S2AI screw.
MATERIALS AND METHODS
Study design and patients
This was a single-center, retrospective case series involving adult patients with spinal deformity who required sacropelvic fixation as part of a long-level thoraco-lumbar fusion.
Surgical procedure
S2AI screw placement was performed by free hand technique, using anatomical landmarks of the dorsal aspect of the sacrum. The starting point of the S2AI screw was located at the midpoint between the lateral aspect of the S1 and S2 foramina and 2 mm medial to the lateral sacral crest. After flipping a patient to the prone position on the operating table, a portable lateral plain radiograph was acquired and imaginary lines were drawn for the ideal pedicle screw trajectory and S2AI screw trajectory. A 3-mm cutting burr was used to create a posterior cortical breach, approximately 5 mm in depth. The gearshift was advanced to make a pilot hole passing through the sacroiliac joint and directing the anterior inferior iliac spine, as previously described15). The pilot hole could be verified by a ball-tip sounder to palpate the five bony borders (medial, lateral, superior, inferior, and bottom).
A K-wire was then inserted through the pilot hole created by the gearshift. The screw length could be determined by using a ball-tip sounder on which the length was previously indicated. Subsequently, the pathway was tapped with a 6.5-mm cannulated tap along with the K-wire. Finally, a cannulated screw was introduced over the guide wire into the pathway. Plain posterior anterior and lateral radiographs were taken to confirm the adequate placement of the screws (Fig. 1).
RESULTS
Sixteen patients (thirteen females and three males), with a mean age of 69.4±9.5 years, underwent long level instrumented fusion surgery using S2AI cannulated screws. The preoperative diagnoses were degenerative flatback (nine patients), degenerative lumbar scoliosis (two patients), postoperative flatback (two patient), post-traumatic flatback (two patients), and pseudarthrosis (one patient). The mean number of fused levels was 8.8 (range, 4–16). The demographic data of the patients is summarized in Table 1.
A total of 33 S2AI screws were inserted in sixteen patients (Table 2). One of these was a conventional non-cannulated screw, which was inserted with the traditional iliac wing screw at the same side. The diameter of two screws was 7.5 mm and that of 31 screws was 8.5 mm. The length of most screws was 80 mm, except seven screws; three of 70 mm, four of 90 mm. The average caudal angle in the sagittal plane was 28.3±3.9°, while the average horizontal angle in the coronal plane was 40.2±3.8°.
Post-operative plain radiographs and computed tomography scans confirmed the screw placement. No violations of the cortical wall, sciatic notch, acetabulum, or pelvis were observed in 30 out of 32 (93.8%) cannulated S2AI screws (Fig. 2). Among total of 33 S2AI screws, there was one conventional non-cannulated screw, in which a violation of the lateral cortex of the ilium was noted (Fig. 3).
DISCUSSION
S2AI screw fixation technique affords more stable biomechanical durability against torsion force owing to the direction and length of the screws, resulting in more reliable stability compared to that provided by traditional iliac wing screw fixation20). Moreover, S2AI screw is technically easy to place because it requires minimal additional dissection, and is aligned in line without the need of the offset connector2,6). Nevertheless, the placement of S2AI screw still remains a challenge for spine surgeons, and its misplacement may result in frequent cortical violations and serious neurovascular complications7). Therefore, we introduced a novel technique of S2AI screw placement using a K-wire and cannulated tapper and screw.
In the present case series, 30 S2AI screws out of 32 (93.8%) were accurately placed in sixteen consecutive patients. Thus, using a K-wire and cannulated screw, S2AI screws could be inserted with acceptable success rate.
Cortical violation during screw placement can be minimized by careful use of gearshift probing9). Although the pilot hole violates the cortical wall, it can be identified using the ball-tip sounding device and salvaged thereafter11). However, the pilot hole alone does not ensure that the screw will follow the pilot hole trajectory4), particularly in S2AI screwing as in case of patient 3. We attribute the misplacement during tapping and screwing to failure to cross the sacroiliac joint adequately, poor bone quality of the sacrum with weak cancellous bone, and the narrow isthmus portion of the ilium. We recognized that most of the misplacement occurred during tapping through the pilot hole. Introduction of a cannulated tapper over a K-wire followed by a cannulated screw enabled us to accurately place the S2AI screw.
Tapping is an essential procedure that ensures proper placement of pedicle screws9). Pedicle screw placement without tapping would result in screw malpositioning, even if the pilot hole was created accurately4). However, tapping itself through a long pilot hole also carries a risk of misplacement, which may trap the neurovascular structure by the sharp grooves of the tapping device. Use of a K-wire together with a cannulated tapper and screw could help reduce the risk of screw malpositioning and the resultant potential complications.
In most cases, S2AI screw placement is performed under intra-operative C-arm fluoroscopy. However, screw placement under fluoroscopic guidance requires more surgical time and carries a risk of radiation exposure for both surgeons and patients, despite the use of protective equipment and devices. Moreover, a recent study demonstrated that lead aprons were less protective than expected5). As previously reported in the cadaveric study15), free hand insertion technique of S2AI screw could be an alternative method to address these concerns. Further, free hand S2AI screw insertion, using a K-wire, and cannulated tapper and screw may be useful for more accurate placement of the S2AI screw. Further studies with larger number of patients are warranted to verify the effectiveness and safety of this novel technique.
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