Pelvic fixation in spine surgery may be used to correct pelvic obliquity and improve sitting balance in neuromuscular patients treated with posterior spinal fusion (PSF). Although many techniques exist, pelvic fixation continues to present challenges in the management of neuromuscular scoliosis.1 Patients frequently have multiple medical comorbidities, abnormal anatomy, and poor bone quality, which contribute to the high complication rate.2
At our institution, pelvic fixation in neuromuscular scoliosis has traditionally consisted of an iliac screw technique. The sacral alar iliac (SAI) screw technique as described in 2009 by Sponseller et al3 has been gaining popularity as it is low profile and often eliminates the need for a rod to iliac screw connector. Previous studies4–7 have described the advantages of each mode of fixation with regard to the technique and the biomechanical forces, but have not focused on comparing the clinical outcomes of these 2 modes of fixation in the neuromuscular population. In this study, we compared the outcomes for pelvic fixation using the SAI screw technique and the iliac screw technique with regard to the amount of spinal deformity and pelvic obliquity correction, relative complication rate, and the need for reoperation.
After obtaining institutional review board approval, we conducted a retrospective review of all consecutive patients who had a PSF with pelvic fixation, from January 2004 to January 2012, with a minimum of 2 years follow-up. These patients were grouped according to the type of pelvic fixation. Charts and radiographs were reviewed. The following parameters were measured on radiographs obtained preoperatively, postoperatively, and at most recent follow-up: thoracic and lumbar Cobb angle, pelvic obliquity, maximum thoracic kyphosis, and maximum lumbar lordosis. We defined failure as mechanical implant failure (ie, disengagement of rod to iliac screw connector, separation of screw tulip from screw shaft, set screw disengagement, and rod fracture).
A total of 109 patients were treated with PSF from January 2004 to January 2012, using SAI screws or iliac screws for pelvic fixation. The same surgeons were involved in both cohorts. Eight patients were excluded from the study for <2 years of follow-up. A total of 101 patients were included in this study, of whom 55 patients were treated with the iliac screw technique and 46 patients were treated with the SAI screw technique as described below. Connectors were used with the iliac screws but were not needed with any of the SAI screws.
Technique for Iliac Screws
An incision is made in the fascia just lateral to the posterior superior iliac spine (PSIS). The lateral aspect of the iliac wing is exposed subperiostally. Depending on surgeon preference the sciatic notch is palpated or the exposure is carried out so that it can be visualized. A retractor (Taylor or Sofield; Franklin Lakes, NJ) is used for this exposure and to allow visualization of the lateral iliac wing. A large rongeur is then used to remove the cartilage cap over the PSIS and expose the cortical channel. A pedicle probe is used to develop a tract into the lateral aspect of the iliac wing from the PSIS passing above the sciatic notch. A ball-tip probe is used to confirm that this channel does not have any cortical breeches and a screw is placed in this tract. C-arm fluoroscopy confirms the screw position. In the majority of cases a connector will be needed to facilitate securing this screw to the rod.
Technique for SAI Screws
The sacrum is dissected down to the S2 neural foramina. Neither the S1 nor S2 neural foramina need to be entered. If they are entered there is frequently bleeding, which may best be controlled with bipolar cautery. The entrance point is the S2 pedicle, which is generally midpoint between the S1 and S2 foramina. We recommend erring slightly medial of the midline as the trajectory tends to push lateral. We go through the cortex with a bur just a few millimeters deep. We use either a curved pedicle probe or a 2-mm-diameter drill bit, followed by a 3.2-mm drill bit to go across the sacro-iliac (SI) joint, aiming just superior to the sciatic notch.
It is often noted that at about 20 to 25 mm, when crossing the SI joint, more resistance is felt as we cross the 2 cortices rather than cancellous bone. This is crossed with either the pedicle probe or the drill bit. A C-arm image is obtained at that point to confirm the trajectory is just superior to the sciatic notch. We then transition to a specially made long-curved pedicle probe, with the length approximately 1.5× that of a standard pedicle probe that keeps the surgeon’s hands from bouncing or from being limited by the patient’s sides. The pedicle probe is continued across the SI joint between the inner and outer tables, just superior to the sciatic notch.
Often times we will use image in 2 planes (perfectly perpendicular to the iliac wing so the sciatic notch is best seen and in the plane of the ilium). Adjustments may be made based upon these biplanar images. Typically, the screws should be between 90 and 110 cm long; in smaller children we certainly use screws of smaller size. While we have not had any screws break at the SI joint, other busy centers have reported this; therefore, we try to use screws that are 8.5 cm and wider. It is optional to use cannulated screws and a wire. A ball-tip probe confirms we are wholly within bone and the screw is inserted under power. Orthogonal views again confirm correct position of the screw. A technical pitfall is that sometimes the screw tulip would push against the sacrum and direct the screw through the medial wall. At times, a bit of the sacrum must be removed with a bur to allow the screw head to go in straight.
An alternative method, which we recommend during the learning curve for this procedure, is a small exposure of the outer table of the ilium. Approximately a 4-cm incision is made just along the outer corner of the PSIS. A finger is placed along the outer table, either extraperiosteal or subperiosteal to palpate the depression of the sciatic notch. With the fingertip at the sciatic notch, placement of the pedicle probe is greatly facilitated.
The great majority of the time the screw head is deeper than the PSIS and a connector from the screw to the rod is not needed.
The data were analyzed with Stata12 (2011; StataCorp., College Station, TX) to determine statistically significant differences (measured at P-value <0.05) between outcome measures by 2-sided t test or χ2 test where appropriate. The Fisher exact test was used under those circumstances with fewer subjects in groups of interest. Pearson correlation coefficient was also calculated when indicated. Multivariate analysis using logistic regression was performed where univariate analysis showed statistically significant variables of interest.
The mean age at surgery was 14.1 years (9 to 20 y) in the iliac screw group and 13.8 years (4 to 20 y) in the SAI group. The mean preoperative Cobb angles were 79 degrees (22 to 136) and 78 degrees (22 to 129) for the iliac screw group and the SAI screw group, respectively. In this series, none of the SAI screws required a connector and none of the iliac screws were connected to the vertical rod without a rod-screw connector.
There were no significant differences preoperatively between the 2 groups with regard to age, sex, weight, preoperative Cobb angle, pelvic obliquity, thoracic kyphosis, and lumbar lordosis. The preoperative parameters are summarized in Table 1.
The most common diagnosis was cerebral palsy (55%, 55/101), followed by Duchenne muscular dystrophy (14%, 14/101). There were no significant differences between the groups with regard to the diagnosis except for spina bifida, which was higher in the iliac screw group (9 patients in the iliac screw group compared with 1 patient in the SAI group, P=0.01) (Table 2). The duration of follow-up was similar between the 2 groups, with a mean of 39.8 months (range, 22 to 80 mo) in the iliac screw group and 36 months (range, 22 to 65 mo) in the SAI screw group (P=0.18).
There were no differences in the number of vertebral levels fused, correction of the major curve achieved, or correction of pelvic obliquity in either group (Table 3). An average number of 15.8 levels (range, 13 to 18 levels) were fused in the iliac screw group and 15.6 levels (range, 14 to 18 levels) in the SAI screw group (P=0.41). The mean postoperative primary Cobb angle was 38 degrees (range, 1 to 67 degrees) in the iliac screw group compared with a mean postoperative primary curve Cobb angle of 37 degrees (range, 1 to 75 degrees) in the SAI screw group (P=0.77). The postoperative average pelvic obliquity was 11 degrees (range, 0 to 35 degrees) in the patients with iliac screws and 12 degrees (range, 0 to 31 degrees) in the patients with SAI screws, respectively (P=0.48). There was a small but statistically significant difference between the postoperative kyphosis in the iliac screw group (mean=53 degrees; range, 20 to 75 degrees) compared with the SAI group (mean=46 degrees; range, 12 to 76 degrees) (P=0.002).
A summary of postoperative complications is shown in Table 4. There were significantly more implant failures, implant failures requiring additional surgery, and infections requiring additional surgery in the iliac screw group.
The SAI group had a lower implant failure rate (7%, 3/46) compared with the iliac screw group (24%, 13/55) (P=0.031). In the iliac screw group 24% (13/55) of the patients had implant failures. The modes of failure in the iliac screw group were as follows: 18% (10/55) had failures of the rod to iliac screw connector (6 failed at the screw and 4 failed at the rod, Figs. 1, 2); 5% (3/55) had separation of screw tulip from screw shaft. In the SAI screw group there were 7% (3/46) of implant failures. The modes of failure were as follows in the SAI screw group: 1 separation of screw tulip from screw shaft and 2 patients where the set screws disengaged (Figs. 3, 4).
Of the 13 patients in the iliac screw group with implant failures, 31% (4/13) had clinical sequela and underwent reoperation (2 patients for pain, 1 patient for loss of correction, and 1 patient for both pain and loss of correction). Of the 4 symptomatic patients who underwent revision surgery, the mode of failure was rod to iliac screw connector in 75% (3/4) of the reoperated cases and separation of screw tulip from screw shaft in 1 case. In the SAI screw group, 2 of the 3 patients with implant failures were symptomatic and underwent revision (1 for pain and 1 for loss of correction).
Two main implant systems were used in both the iliac screw group and the SAI screw group. In 74 patients we used system A and in 18 cases we used system B. There was no difference in the failure rates observed: 11/74 (15%) implant failures with system A and 3/18 (16%) implant failures with system B (P=0.8).
In the iliac bolt group 22% (12/55) of the patients developed wound infections, of whom 67% (8/12) required intravenous antibiotic treatment and required irrigation and debridement, whereas 33% (4/12) required removal of implants after multiple debridements failed to resolve the infection. In the SAI screw group 15% (7/48) of the patients developed wound infections, of whom 71% (5/7) required intravenous antibiotic treatment combined with irrigation and debridement, whereas 29% (2/7) were treated with removal of implants after multiple debridement failed to resolve the infection. The SAI group had a slightly lower wound infection rate compared with the iliac bolt group but this was not statistically significant (P=0.25).
Excluding implant failures and wound infections, 18% (10/55) of the patients in the iliac screw group had other complications that required additional surgery: 5% (3/55) had decubitus ulcers, 2% (1/55) had a neurological change, and 11% (6/55) had a prominent screw. In the SAI screw group, 11% (5/46) of the patients had additional operations for reasons other than implant failure or infection: 4% (2/46) for decubitus ulcers, 2% (1/46) for neurological changes, 2% (1/46) for SAI screw prominence, and 2% (1/46) for prominent implants in the thoracic spine.
Patients with spina bifida had more soft tissue complications compared with the other etiologies. Five out of 10 patients (50%) with spina bifida had soft tissue complications. Three patients had a wound infection, 1 patient had decubitus ulcer, and 1 had prominent screw with skin breakdown. All 5 patients were part of the iliac bolt group.
At final follow-up, there was 1 asymptomatic patient in the iliac screw group with a radiolucency surrounding the screw of >3 mm without other implant failure. No association was found in the subgroup analysis between the patient’s underlying diagnosis and implant-related complications, though this was limited by the small sample size in each of the subgroups.
We also investigated implant density at L5, S1, and ilium as prior series have identified this as correlating with the rate of hardware failure.8 We had 1 implant failure in a patient who had all 6 screws (2 at L5, S1, and ilium), whereas all other patients with failure had a lower implant density (mean=3 screws). The 1 failure in the setting of 6 screws was in an obese child with Bardet-Biedle syndrome and severe developmental delay, whose sibling with the same syndrome also had a pseudoarthrosis and several surgical procedures for implant failure. Although this is a very rare syndrome, we suspect that these patients are at particularly high risk of pseudarthrosis and implant failure. This may have impacted our results, but in contrast to the previous study implant density at L5, S1, and the ilium was not significantly associated with the rate of implant failure (P=0.13) in this series.
Pelvic fixation in the treatment of neuromuscular scoliosis continues to be a challenge. The SAI technique described by Sponseller et al3,7 offers theoretical advantages of less dissection, less prominence, less need for a rod to iliac screw connector, and a trajectory that in many cases allows for a longer screw length for pelvic fixation. In addition, the SAI screws are associated with less postoperative sacroiliac pain9 in reported series when compared with a heterogenous group of 27 patients with various other modes of pelvic fixation. To our knowledge, our series is the largest series to date directly comparing the complication rate of the SAI screw technique with the more traditional iliac screw technique in neuromuscular scoliosis.
Prior studies have demonstrated that compared with the Galveston technique, both techniques of pelvic fixation with screws have the advantage of improved stability with a lower rate of pseudoarthrosis.2,10 There was no difference regarding the correction of the Cobb angle and the pelvic obliquity between the 2 groups in our study. Sponseller et al3 reported significantly better pelvic obliquity correction with SAI screws when compared with pelvic fixation with sacral or iliac screws but similar outcomes in Cobb angle correction. Their series had similar complication rates between the SAI group and the control group but had a smaller number of patients (26 patients in the SAI group compared with 27 patients in the control group). It is notable that 5/16 (31%) failures in this series occurred >2 years after surgery, so the absolute number of failures may be even higher with longer follow-up.
In this series, the SAI group had a lower implant failure rate compared with the iliac screw group. Most of the implant failures were due to disengagement of the rod to iliac screw connector. This emphasizes the advantage of the SAI screw technique, which eliminates the need for a connector. In this study 18% (10/55) of the iliac screw group had implant failure due to disengagement of rod to iliac screw connector. If we exclude the rod to screw connector failures, the failure rate of SAI screws of 6.5% (3/46) is similar to that of iliac screws 5.5% (3/55), so the real advantage of the SAI technique may be obviating the need for a screw to rod connector.
There was 1 patient in each group who had a separation between the tulip and screw shaft. This mode of failure was the same in both groups. We did not observe any failures at the point where the screw crosses the SI joint.
Although, the average surgery date for the iliac screw group was 1.4 years earlier than the average date of the SAI screw group, the average follow-up was similar for both groups (39.8 vs. 36 mo; P=0.18). Therefore, we do not feel the higher rate of implant failure in the iliac screw group is attributable to the earlier date of surgery. It is, however, entirely possible that with longer term follow-up more failures may be observed in 1 or both groups.
Other studies have demonstrated that rod disengagement and radiolucency around the screws are common when using the iliac screw technique.11,12 Phillips et al11 compared the outcome of pelvic fixation using 1 or 2 iliac screws for each iliac wing. They reported 5/50 (10%) cases of rod disengagement and 5/50 (10%) cases of radiolucency around the screws. In addition, they found that the use of 2 screws on each side was associated with fewer implant-related complications: 23.3% (7/30) compared with 15% (3/20) when 2 screws on each side were used.
We observed only 1 case of radiolucency around the screw edge >3 mm without any associated implant failure and without symptoms. Prior studies of Galveston rods were associated with a high rate of radiolucency surrounding the implants13,14 (35% to 65%).
Wound infection after scoliosis surgery including pelvic fixation in neuromuscular patients continues to be a problem regardless of the mode of fixation. The rate of wound infection (superficial and deep) after pelvic fixation in neuromuscular patients varies between 4.2% and 20%.15–17 In our study, the SAI group had a slightly lower wound infection rate compared with the iliac screw but this was not statistically significant (P=0.25). Ramo et al16 recently investigated the rate and risk factors for infection after PSF in 302 neuromuscular patients and reported a rate of infection of 12.3%. Pelvic fixation was found to be a statistically significant risk factor for infection regardless of the mode of fixation. Other potential risk factors for wound infection included: severity of the deformity and complexity of the procedure, surgeon experience, and health status of the patients.18
Limitations of this study include its retrospective study design and lack of randomization of the groups. Although the duration of follow-up is similar, the average surgery date for the iliac screw group was 1.4 years earlier than the average date of the SAI screw group. Hence, the trend of fewer wound infections in this group may be related to other changes made during this period, such as use of vancomycin in the wound, using neuromuscular checklists, and irrigating with soap solution.
In conclusion, the SAI screw technique had a lower rate of implant failure, fewer revisions for implant failure, fewer revisions for pelvic screw prominence, and fewer complications than iliac screws. A major advantage of the SAI technique over the iliac screw technique is obviating the need for a screw to rod connector.
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Keywords:Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.
sacral alar iliac screw; iliac screw; implant failure; neuromuscular scoliosis; connector; pelvic fixation; pseudoarthrosis