The introduction of stereotaxy and image guidance for spine surgery has been a major technological advancement. Image guidance systems provide real-time navigation of instruments and hardware.1 Image guidance has three main benefits compared with the freehand technique: 1) improved accuracy of implant placement (i.e., pedicle screws and interbody cages);2,3 2) reduced radiation exposure to the surgeons and surgical team;4,5 and 3) use in minimally invasive surgery, which can reduce exposure-related morbidity, operative time, and length of hospital stay.6–8
Image guidance frequently relies on intraoperative CT data obtained with the patient in the final surgical position and with a rigid dynamic reference frame (DRF) attached to a fixed landmark close to the operative field. DRFs can be clamped onto a variety of structures, including local spinous processes in open surgery, or an intraosseous pin can be placed with the freehand technique into the ilium in certain cases treated with minimally invasive surgery.1 In supine and some lateral operations, the pin can be placed in the anterior superior iliac spine or iliac crest.9 During prone or sometimes prone-lateral operations, the pin can be placed in the posterior superior iliac spine (PSIS).10,11 After the DRF is attached, images are captured with reference to the position of the DRF and a navigable 3D image is created and utilized intraoperatively.
Placement of intraosseous DRFs has potential to harm patients. Regardless of location within the ilium, these DRFs require a separate skin incision, are placed without clear visualization of gross superficial landmarks, and have the potential to lead to complications. There is a paucity of studies in the literature regarding the exact nature and prevalence of complications after intraosseous DRF placement in spine surgery. Much of what is known has been extrapolated from the orthopedic literature. Case reports and cohort studies of total hip and total knee arthroplasties report hematoma formation, infection, neurovascular injury, pin breakage, foreign object retention, pin site fracture, and pain.12–17 Complications and patient outcomes after iliac grafting are better described, but its applicability to iliac DRF placement is questionable because iliac grafting requires much more dissection and bony work and leads to more morbidity.18–21 The aim of this study was to determine the exact anatomical locations of DRFs in PSIS, determine the prevalence and nature of complications, and describe long-term outcomes after DRF placement in PSIS.
Methods
A retrospective review of patient medical records was performed at a single institution from August 2019 to February 2021. This study was approved by the IRB. The electronic medical record system was queried for all neurosurgical patients who underwent lumbosacral spine surgery during this time. Operative reports were reviewed, and patients were included if placement of a pin in PSIS for intraoperative navigation was reported. The medical records were then reviewed for demographic information, indications for surgery, and surgical outcomes. Intraoperative CT images were then reviewed by 2 independent researchers to determine the anatomical placement of the DRFs. DRFs were determined to be appropriately placed if they had entered the PSIS and remained within the dorsal ilium without breach into the sacrum, pelvis, or iliac crest. The images and patient records were reviewed for complications, such as sacral fracture, hematoma, infection, and pain at the pin site. Interrater reliability was calculated between the 2 independent reviewers by using 10 random patients from the cohort.
Results
Of 497 lumbar spine operations performed between August 2019 and February 2021 by 4 surgeons, 85 operations utilized intraoperative navigation with a PSIS pin. Thirteen operations were excluded due to an inability to visualize the entirety of the pin on intraoperative CT images that had been uploaded into the institution’s imaging system. Overall, 72 operations performed in 71 patients were included in this study. Interrater reliability between the 2 reviewers was 95.8%.
Patient Outcomes
Seventy-two operations were performed: 6 minimally invasive sacroiliac (SI) joint fusions; 8 anterior lumbar interbody fusions with percutaneous posterior instrumentation; 22 lateral lumbar interbody fusions with posterior instrumentation; and 36 posterior lumbar interbody fusions (Table 1). The most common indications for surgery were degenerative disc disease (n = 22), mobile spondylolisthesis (n = 17), spinal deformity (n = 12), neurogenic claudication (n = 12), SI joint pain (n = 6), and pseudarthrosis (n = 3). During the mean ± SD follow-up period of 9 ± 5.2 months, 66.7% of patients had resolution of their index symptoms, 23.6% had improvement of their symptoms without full resolution, and 9.7% had no improvement. A majority of patients (76.4%) had no adverse events after their operation; of those who had adverse events, there were 3 instances of adjacent-segment disease, 3 failed fusions (pseudarthrosis), 2 wound infections, 9 patients who developed new radicular or SI pain postoperatively, and 7 patients who required reoperation (Table 2). There were no instances of reoperation or evidence of inaccurate placement due to intraoperative fluoroscopy for navigated placement of spinal instrumentation in patients with and without properly placed DRFs.
Patient characteristics of the 72 included operations
| Characteristic | No. of Patients (n = 72) |
|---|---|
| Operations | |
| Posterior lumbar interbody fusion | 36 |
| Lateral lumbar interbody fusion w/ posterior instrumentation | 22 |
| Anterior lumbar interbody fusion w/ posterior instrumentation | 8 |
| Minimally invasive SI joint fusion | 6 |
| Indication for surgery | |
| Degenerative disc disease | 22 |
| Mobile spondylolisthesis | 17 |
| Spinal deformity | 12 |
| Neurogenic claudication | 12 |
| SI joint pain | 6 |
| Pseudarthrosis | 3 |
| Outcomes after surgery | |
| Resolution of symptoms | 48 |
| Improvement in symptoms | 17 |
| No improvement in symptoms | 7 |
Adverse events after surgery during the follow-up period for all 72 operations
| Adverse Event | No. of Patients (n = 17) |
|---|---|
| New pain | 9 |
| Pseudarthrosis | 3 |
| Adjacent-segment disease | 3 |
| Wound infection | 2 |
| Reoperation | 7 |
Anatomical Placement
Seventy-two intraoperative CT scans were analyzed. A majority of pins (77.8%) were placed correctly into the PSIS (n = 56). Of those placed into the PSIS, 15 had bicortical purchase within the PSIS. Of those not contained within the PSIS, 11 traversed the iliac wing, 11 had a component within the sacrum, 6 traversed the SI joint, and 2 had breached into the pelvis (Fig. 1).
Examples of intraoperative CT images showing malpositioned DRFs. A: A DRF placed with its tip within the left sacrum. B: A DRF placed within the left SI joint. C: A DRF positioned too deep within the pelvis, with its tip in the left psoas muscle.
Pin Site Complications
There were no instances of sacral or pelvic fracture due to pin placement, retroperitoneal hematoma, or pin site hematoma. There were no cases of pin site infection during follow-up. Four patients had persistent pin site pain at the initial 2-week follow-up examination. Of these 4 patients, 2 had malpositioned pins: one had the pin placed in the iliac wing, and the other had the pin placed in the sacrum. The other 2 patients had pins that were contained entirely in the PSIS. There were no complications associated with these pin placements, such as sacral fracture, pelvic fracture, or retroperitoneal hematoma. Three of these patients had resolution of pin site pain by the last follow-up examination (mean ± SD 10.1 ± 7.6 months), and 1 patient was lost to follow-up.
Discussion
In this retrospective study of 71 patients who underwent 72 lumbosacral spine operations, we have reported radiological and clinical outcomes after DRF placement within the PSIS. To our knowledge, this is the first published cohort study to describe radiological or clinical outcomes after DRF placement in the PSIS.
Placement of DRFs within the PSIS is valuable for certain types of spine operations. In minimally invasive surgical procedures of the lower lumbar spine or SI joint, placement of the DRF within the PSIS merely requires an additional stab incision. The DRF is then angled in such a way that it does not interfere with hardware placement.10,11,22 Additionally, accuracy is excellent, with some radiological studies reporting 96% accuracy for pedicle screw placement using an intraosseous DRF within the PSIS.23 Especially with the increased incidence of minimally invasive SI joint fusions24 and spine surgical procedures,25 surgeons must understand the potential for complications due to DRF placement.
Studies in the literature regarding the complications and clinical outcomes after intraosseous DRF placement are sparse. The first study to examine clinical outcomes after intraosseous DRF placement was performed by Lambers and colleagues,26 who used intraosseous DRF in the iliac crest for total hip arthroplasty. In their prospectively studied cohort of 43 patients, they reported pin site pain in 30% of patients 3 weeks after surgery, which decreased to 9% at 6 weeks and 2% at 12 weeks. These rates are much higher than those of our cohort, in which pin site pain was reported by only 4 of 72 patients (5.5%) at the 2-week follow-up and resolution of pain was reported by almost all these patients at the last follow-up (75%). One explanation for this may be the location on the ilium. Lambers et al.26 placed DRFs on the iliac crest anteriorly to complete their surgical procedures, and our DRFs were placed in the PSIS. Additionally, they reported no instances of nerve injury, pin site infection, fracture, or screw breakage,26 which is similar to our findings. Half of the patients with pin site pain had malpositioned DRFs. Given the small sample size, it is difficult to say whether malpositioned DRFs are more likely to cause persistent pin site pain. Prior to this study, there were no reports of clinical outcomes after intraosseous placement of DRF in the PSIS.
Previous studies have described the use of DRF within the PSIS, but complications due to pin site placement were not the primary outcomes and therefore were not reliably reported. For example, 2 retrospective cohort studies on the use of navigated iliac bolts and guidewireless pedicle screw placement reportedly placed DRF within the PSIS in 5 and 36 patients, respectively.11,27 No complications due to DRF placement were described. A prospective study compared the accuracy of pedicle screws that were randomly assigned to be placed using DRF within the PSIS or DRF secured to only the skin. In the 101 patients who had a DRF within the PSIS, no complications due to DRF placement were explicitly reported.23 Another larger cohort study examined operative times and pedicle screw placement using DRF within the PSIS. Best et al.10 reported clinical outcomes after 289 posterior lumbar operations, of which the only complications reported were spinal fluid leak, lumbar wound infection, and postoperative urinary retention. There were no instances of pedicle screw misplacement. None of the complications were attributed to DRF placement within the PSIS, and complications due to DRF placement were not explicitly reported.10 The accuracy of navigation is likely due to the stability of the DRF instead of the actual location of placement. There are many ways to ensure stable pin placement. At our institution, the tip of a monopolar cautery device is used to dissect through the soft tissue and directly palpate the PSIS. The pin is then seated on the bone and malleated in place with a distinctive high-pitched ring as it traverses the cortical bone. Finally, once the pin is in place, pin stability is tested with gentle traction to make sure it has adequate fixation prior to intraoperative CT. Overall, studies of the use of DRFs within the PSIS have not reported complications, possibly because complications were not the primary outcome of interest.
To our knowledge, this is the first paper to critically assess the potential consequences of DRF placement into the PSIS. Our findings suggest that although placement is not always accurate, the current approach to DRF placement is unlikely to lead to long-term harmful complications for patients. The most notable limitation of this study was its sample size. More incidences of complications from DRF placement may be detected with a larger cohort. We did not ultimately include all patients who had a DRF placed within the PSIS; it is possible that the 13 patients who were excluded due to inability to visualize the entirety of the pin also had malpositioned DRFs. This study was also limited by potential bias because patients may have been more hesitant to report certain outcomes to physicians than neutral research staff, and patient recall is required to report subjective outcomes.
Conclusions
This was the first study to examine radiological and clinical outcomes after DRF placement within the PSIS. In this study, most DRFs were placed correctly within the PSIS. There were no serious complications due to DRF placement, such as pin site infection, fracture, retroperitoneal hematoma, or neurological injury. Although 22% of DRFs were malpositioned, this did not result in inaccurate navigation because the pin was stably placed. The stability of the DRF, not its exact location, is what matters for accurate navigation. Only 5.5% of patients reported persistent pin site pain, but this pain resolved by the last follow-up in a majority of patients.
Disclosures
Dr. Park is a consultant for Globus, NuVasive, and DuPuy Synthes; receives royalties from Globus; and receives non–study-related clinical or research support from Cerapedics, ISSG, SI Bone, and DePuy Synthes.
Author Contributions
Conception and design: Holste, Saadeh. Acquisition of data: Holste, Zaki. Analysis and interpretation of data: Holste, Zaki, Saadeh. Drafting the article: Holste, Wieland. Critically revising the article: Park, Holste, Zaki, Saadeh. Reviewed submitted version of manuscript: Park, Holste, Saadeh. Approved the final version of the manuscript on behalf of all authors: Park.
Supplemental Information
Previous Presentations
Portions of this study were presented at the 2022 AANS/CNS Joint Section on Disorders of the Spine and Peripheral Nerves, Las Vegas, NV, February 23–26, 2022.
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