Contralateral screw loosening of through-and-through fixation following unilateral sacroiliac joint fusion: illustrative case

Abstract

BACKGROUND

Sacroiliac (SI) joint dysfunction is a frequently underrecognized source of low back pain, implicated in 15%–30% of cases across select populations. Unilateral SI joint fusion remains the most commonly performed technique, utilizing either iliosacral screws or, more recently, through-and-through (TNT) screws.

OBSERVATIONS

A 65-year-old woman with a history of failed right-sided SI joint fusion underwent revision surgery with bilateral TNT screw fixation due to worsening symptoms. Follow-up imaging confirmed solid right-sided arthrodesis. However, progressive haloing and sclerosis were observed around the left iliac portion of the cephalad screw.

LESSONS

This case represents the first reported instance of persistent motion and screw haloing using TNT screw fixation when using two spanning screws. These findings highlight a potential limitation of TNT constructs due to load transfer, stress shielding, and asymmetric osseous integration, particularly in patients with a history of prior unilateral SI joint fusion.

https://thejns.org/doi/10.3171/CASE25668

Sacroiliac (SI) joint dysfunction is a frequently underrecognized contributor of low back pain, implicated in 15%–30% of cases across select populations.¹ In patients with a history of lumbar fusion, the prevalence may reach as high as 40%.^2–9 A 2023 systematic review by Manzetti et al. further identified a weighted average SI joint pain rate of 24% and degeneration rate of 38% following spinal arthrodesis.¹⁰ These findings underscore the substantial contribution of SI joint dysfunction to postoperative disability, particularly in the setting of spinal arthrodesis.

Unilateral SI joint fusion remains the most commonly performed technique and typically uses either iliosacral screws or, more recently, through-and-through (TNT) screws.^11,12 Iliosacral screws are inserted posteriorly from the ilium into the sacral ala or body, whereas TNT screws transverse the entire sacrum, spanning both SI joints.^13,14 Biomechanical studies have demonstrated that TNT screws provide enhanced resistance to rotational and translational forces, particularly when cortical bone purchase is maximized and multiple sacral levels are engaged.^11,12,15 In a recent systematic review and meta-analysis, Whang et al. evaluated safety and efficacy outcomes of minimally invasive SI joint fusion procedures. Their findings demonstrated significant improvements in patient-reported outcomes and high levels of radiographically confirmed fusion, particularly for the lateral transiliac approach.¹⁶

The trauma literature has extensively explored the advantages of TNT screw constructs, particularly in the fixation of vertically unstable sacral fractures and pelvic ring disruptions. In a cadaveric biomechanical study, Sagi et al. demonstrated that transsacral screws significantly improved rotational and vertical stability compared to iliosacral screws for unstable SI dislocations.¹⁷ Zhao et al. conducted finite element analysis showing that lengthened TNT screws spanning S1 and S2 offered superior vertical load resistance and minimized stress concentrations relative to SI fixation.¹³ Salášek et al. further confirmed that transiliac constructs provided more effective force distribution and reduced sacral micromotion than iliosacral screw fixation in transforaminal fracture models.¹⁸ These trauma-based findings highlight the enhanced biomechanical performance of TNT screws, although their long-term success relies on achieving bilateral arthrodesis. Trauma constructs typically assume symmetric fixation and bony fusion, which is not applicable to a clinical setting. For instance, rigid fixation across a single fused joint may divert mechanical loads toward the unfused side, amplifying micromotion and contributing to implant loosening.

Here we present a revision case of bilateral SI joint fusion using dual TNT screws following a failed unilateral fusion. Despite symmetric instrumentation, uneven load distribution and persistent micromotion led to radiographically confirmed haloing around the sacral aspect of the cephalad screw contralateral to the previous fusion. These findings are novel and raise important questions about the limitations of TNT fixation in the absence of bilateral fusion.

Illustrative Case

A 65-year-old woman with a history of chronic low back and pelvic pain initially underwent an open intra-articular distraction arthrodesis of the right SI joint in 2011. This procedure involved posterior distraction cage implantation and decortication of the articular surfaces. Postoperatively, the patient experienced persistent mechanical pain, functional limitations, and difficulty ambulating. Pain management included muscle relaxants, opioids, and topical analgesics, but she remained significantly impaired. A trial of radiofrequency ablation and medial branch blocks was performed with limited relief.

Because of her worsening symptoms and imaging evidence of right-sided facet impingement from hypertrophic bone overgrowth, she underwent a revision right SI joint fusion and index left-sided fusion using bilateral TNT screws on September 22, 2014 (Fig. 1). The surgery was performed via a posterior open approach. Intraoperatively, the previously placed distraction cage was removed using a trephine. The S1 superior articular facet overgrowth was excised, and the SI joint was decorticated. Bone graft from the exostectomy was packed into the joint. Bicortical fixation across both SI joints was achieved using 150- and 130-mm TNT screws at S1 and S2, respectively. She was kept non–weight bearing postoperatively, and imaging confirmed stable fixation without evidence of loosening (Fig. 2A).

FIG. 1.

A and B:Standing preoperative posteroanterior (A) and lateral (B) lumbar spine radiographs (May 29, 2014) showing a right-sided distraction cage from prior SI joint fusion. C and D: Postoperative posteroanterior (C) and lateral (D) pelvis radiographs (March 12, 2015) demonstrating bilateral TNT screw placement.

FIG. 2.

Lucency progression of TNT screws. Each panel displays an axial CT slice and the corresponding Ferguson view radiograph. A: Immediate postoperative images (CT scan: December 23, 2014; radiograph: November 6, 2014) showing stable TNT screw fixation without evidence of loosening. B: Early lucency observed at the left iliac portion of the cephalad screw (CT scan: September 23, 2015; radiograph: June 11, 2015), consistent with left-sided SI joint nonunion. C: Progression of peri-implant haloing and sclerotic remodeling of the left cephalad screw (CT scan: June 27, 2016; radiograph: June 22, 2016). D: Advanced loosening with persistent lucency of the left cephalad screw and lack of arthrodesis at the left SI joint (CT scan: February 9, 2023; radiograph: August 3, 2023). Circles indicate the area of lucency.

Despite radiographic evidence of right-sided fusion 1 year postoperatively, early radiolucency and haloing were observed at the left iliac portion of the cephalad TNT screw on a pelvic radiograph dated June 11, 2015 (Fig. 2B). This is suggestive of a left-sided SI joint motion. A follow-up CT study on September 23, 2015, confirmed peri-implant lucency, consistent with early loosening (Fig. 2B). Repeat CT and radiographic imaging in June 2016 and later in 2023 demonstrated progressive haloing and sclerotic remodeling (Fig. 2C and D).

Her clinical course was further complicated by a motorcycle accident in May 2021, which resulted in thoracic compression fractures, bilateral femur fractures, and right-sided soft tissue injuries. Notably, the patient had previously developed complex regional pain syndrome (CRPS) in the right lower extremity following her 2014 revision SI joint fusion. She later underwent a posterior spinal fusion from L2 to L4 with Smith-Petersen osteotomies and transforaminal lumbar interbody fusions (TLIFs) in November 2022. Although her sagittal alignment was improved, new adjacent segment degeneration at L4–5 and residual pelvic asymmetry placed increased stress on the left SI joint (Fig. 3). Clinical examinations consistently revealed tenderness along the left ilium, and Ferguson views demonstrated progressive screw lucency (Fig. 2).

FIG. 3.

Full-body anteroposterior (left) and lateral (right) radiographs obtained after the patient’s November 2022 posterior spinal fusion from L2 to L4 with Smith-Petersen osteotomies and TLIFs.

The patient returned to the clinic most recently at 11 years after her bilateral SI joint fusion with TNT screws. Her spinopelvic pathology and related symptoms were largely unchanged from her previous visit. This was confirmed with radiographic imaging (Fig. 4). Despite persistent radiographic lucency at the left SI joint, the patient was managed conservatively based on a comprehensive clinical and radiographic assessment. Follow-up imaging demonstrated stable TNT screw positioning without progression of mechanical failure, solid posterior fusion from L2 to L4, and preserved overall spinal alignment. Her predominant pain was localized to the lumbosacral junction and was attributed to adjacent segment degeneration at L4–5 and L5–S1, facet arthropathy, and spinal stenosis related to a prior traumatic compression fracture. Importantly, she experienced significant symptomatic improvement following diagnostic bilateral medial branch blocks, suggesting a facet-mediated pain generator and predicting favorable response to radiofrequency ablation. Given the absence of progressive pelvic instability, the presence of alternative pain generators, and the elevated risk associated with extending fusion to the pelvis in a patient with CRPS and multiple prior injuries, nonoperative management was favored. This approach prioritized symptom control while avoiding a large revision procedure, with surgical intervention reserved only if conservative measures fail.

FIG. 4.

Most recent Ferguson view (February 27, 2025), demonstrating persistent haloing at the left iliac tip of the cephalad TNT screw, consistent with progressive loosening and lack of arthrodesis. Postoperative changes include posterior spinal fusion from L2 to L4 with instrumentation and interbody cages following a 2022 revision for adjacent segment degeneration. This image highlights the utility of Ferguson views in detecting implant haloing at the iliac entry point, which may be subtle or missed on standard posteroanterior or lateral radiographs.

Informed Consent

The necessary informed consent was obtained in this study.

Discussion

This case illustrates a real-world failure pattern predicted by biomechanical models but not previously documented clinically: unilateral SI joint motion with screw haloing following bilateral TNT fixation. Although the construct spanned both SI joints, only the previously fused (right) side achieved solid arthrodesis. The contralateral side, with index instrumentation, demonstrated progressive haloing and radiolucency. These findings suggest persistent motion, reinforcing the concept that symmetric fixation does not guarantee elimination of motion.

Observations

Although bilateral TNT screw constructs are increasingly used in patients with altered spinopelvic biomechanics or prior fusion, many biomechanical and trauma studies assume symmetric load distribution. TNT screws have demonstrated resistance to vertical shear and rotational forces in cadaveric and finite element models, particularly when long, fully threaded screws are used to maximize bicortical purchase and fixation.^12,13,17

Biomechanical studies have repeatedly shown that asymmetric fixation can amplify contralateral joint stress and increase load transfer to the unfused side, particularly in patients with low bone density or prior lumbar instrumentation.^15,19 In this patient, early haloing at the iliac tip of the cephalad screw, a site of high stress concentration, progressed over time despite no infection or hardware breakage, indicating motion-induced loosening.

Implant-specific factors including thread pitch, diameter, and length contribute to mechanical stability.²⁰ Wegmann et al. demonstrated that periscrew radiolucency may signal nonunion, especially under asymmetric loading conditions.²¹ The progressive nature of haloing in this case supports a chronic micromotion etiology, not an acute mechanical failure. This patient’s postoperative course was complicated by a posterior L2–4 fusion. Although this corrected her sagittal alignment, it increased spinal rigidity and shifted compressive loads distally. Previous studies have shown that lumbar fusions elevate stress across the SI joints and may potentiate implant fatigue or loosening when arthrodesis is incomplete.^16,22,23

Lessons

The presented case confirms long-standing biomechanical concerns with unilateral healing after bilateral TNT screw placement. One potential radiographic tool for assessing implant-related changes is the Shimizu pedicle screw loosening scale, originally developed to evaluate fusion status and instrumentation failure in adult spinal deformity surgery.²⁴ This system grades loosening on a scale from 0 to 3, based on the presence and severity of radiolucent zones around the screw. Higher scores indicate more advanced loosening. However, its application in the SI joint context has not been validated, and the binary nature of radiolucency assessment may limit its sensitivity in detecting subtle yet clinically significant mechanical changes.²⁴

To improve its utility for the TNT screw construct, particularly in patients with asymmetric SI joint healing, we propose a minor adaptation: the addition of a modifier that distinguishes radiolucency without a sclerotic rind (a) and with a sclerotic rind (b), which may indicate chronic micromotion.²⁴ For instance, in Fig. 2D the sacral aspect of the cephalad screw would be grade 1b under this modified scheme. This distinction may provide additional prognostic information regarding chronicity and potential irreversibility of loosening.

Preoperative surgical planning that incorporates construct symmetry, bone quality, and postoperative imaging assessment may help anticipate and prevent mechanical complications. To our knowledge, this is the first published case demonstrating unilateral SI joint fusion failure and progressive screw haloing following bilateral TNT fixation. Despite symmetric instrumentation, asymmetric biological healing resulted in micromotion and radiographic loosening localized at the iliac tip of the cephalad screw. This case validates biomechanical predictions regarding load transfer and stress shielding in the setting of prior unilateral fusion, particularly in patients with altered spinopelvic mechanics.

Importantly, this case highlights that successful hardware placement does not ensure symmetric arthrodesis. TNT constructs may lack the multiplanar force shielding required to resist asymmetric forces in patients with prior unilateral SI joint fusion. Long-term radiographic surveillance remains critical, especially in high-risk populations. Enhanced surgical planning, including consideration of load distribution, fusion symmetry, adjunctive grafting, and bone quality, may help prevent construct failure as TNT screw constructs become more widely adopted in elective SI joint fusion procedures.

Disclosures

Dr. Polly reported personal fees from SI-BONE, Medtronic, Springer, and Globus during the conduct of the study; grants from SI-BONE, Medtronic, and MizuhoOSI; patents with royalties paid for TORQ, and for Granite from SI-BONE; and being an investor in See All AI.

Author Contributions

Conception and design: Garcia, Polly. Acquisition of data: Garcia, Polly. Analysis and interpretation of data: all authors. Drafting the article: Garcia. Critically revising the article: Hendricks, Polly. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Garcia. Administrative/technical/material support: Hendricks. Study supervision: Polly.

Correspondence

Rafael Garcia: University of Minnesota, Minneapolis, MN. rgarciaandujar0419@gmail.com.