Posterior Sacroiliac Joint Fusion for Severe Sacroiliac Joint Disorders: A Technical Note on Surgery Using Transarticular Cylindrical Cages Combining Posterior Instrumentation and its Surgical Outcomes

Abstract

Introduction

Anterior sacroiliac joint (SIJ) fusion has been performed for severe SIJ disorders and has demonstrated stable long-term outcomes; however, its relatively high invasiveness remains a significant concern. To overcome this limitation, we developed a posterior fusion using transarticular cylindrical cages combined with posterior instrumentation. This study describes the posterior SIJ fusion technique and evaluates the surgical outcomes.

Technical Note

An S1 pedicle screw is inserted toward the sacral promontory, and two S2 alar-iliac screws are placed in the ilium. All three screws are connected to a rod. One or two cylindrical cages filled with cancellous bone are inserted into the SIJ via a trans-iliac bony channel lateral to the posterior superior iliac spine to promote fusion in the anterior-superior portion of the SIJ while preserving the posterior sacroiliac ligaments. Seventeen patients (20 joints) with SIJ disorders were followed for a mean period of 53±23 months. Bone union was obtained in 93.3% of joints. Postoperative SIJ pain evaluated by the visual analog scale significantly improved from 87±18 to 24±25 mm (p<0.0001), and activities of daily living, assessed by the Denver SIJ Questionnaire, improved from 32±11 to 18±16 (p=0.0002).

Conclusions

The posterior SIJ fusion technique using transarticular cylindrical cages via a trans-iliac bony channel combined with posterior instrumentation resulted in favorable postoperative outcomes in patients with SIJ disorders.

Introduction

The sacroiliac joint (SIJ) is located between the spine and hip joints and functions as a shock absorber and load transmitter^1-3). Repeated movement and/or accidental minor joint incongruency can cause a functional problem known as an SIJ disorder⁴⁾. A typical SIJ disorder presents as unilateral or bilateral pain localized around the posterior superior iliac spine (PSIS), sometimes accompanied by groin pain or lower extremity symptoms that do not correspond to a specific nerve root distribution⁵⁾. Diagnosis is based on a stepwise approach: identification of pain localized to the PSIS using the one-finger test⁶⁾, reproduction of pain with an SIJ shear test⁴⁾, and a diagnostic SIJ injection producing more than 70% pain relief⁷⁾. Differentiation from lumbar spine disorders is facilitated by the SIJ scoring system⁸⁾, based on characteristic clinical features, including PSIS-localized buttock pain identified by the one-finger test, groin pain, pain exacerbation while sitting on a chair, a positive SIJ shear test, and tenderness over the PSIS and sacrotuberous ligament. Patients with severe SIJ disorders may develop load transfer disorders, including difficulties in walking and sitting⁹⁾, which cause significant limitations in activities of daily living and work. Although most patients with SIJ disorders improve with conservative treatment, surgical intervention is considered for carefully selected patients with severe, refractory symptoms and load transfer disorders. These patients remain symptomatic despite more than six months of conservative management and exhibit significant functional impairment, including markedly reduced sitting tolerance (<15 minutes), gait disturbance requiring a cane, pain in the supine position, pain while lying on the painful side, non-dermatomal lower extremity numbness, and a history of trauma inducing SIJ pain¹⁰⁾.

Although anterior SIJ fusion has demonstrated stable long-term outcomes⁹⁾, its invasiveness remains a concern. Several posterior approaches placing implants and bone grafts in the posterior ligamentous region of the SIJ have been developed^11,12); however, access to the joint space is anatomically limited. We established a modified trans-iliac technique based on that of Wise and Dall¹³⁾, enabling reliable intra-articular cage placement. This study describes posterior SIJ fusion utilizing a cylindrical cage placed into the SIJ via a trans-iliac bony channel lateral to the PSIS combined with posterior instrumentation and evaluates its surgical outcomes.

Technical Note

A patient is placed in the prone position, and a paramedian skin incision is made from the L5/S1 facet joint to a point slightly distal to the S1 dorsal foramen. An S1 pedicle screw (PS) is inserted into the sacrum, targeting the sacral promontory, which provides the highest bone strength. Two S2 alar-iliac screws are inserted into the ilium to effectively suppress iliac rotation, and all three screws are connected to a rod. The heads of the three screws are aligned without gaps. To achieve this configuration, the S1 PS is inserted as cranially as possible, and the first S2 alar-iliac screw entry point is placed slightly cranial to the S1 dorsal foramen (Figure 1).

Figure 1.

Posterior sacroiliac joint fusion. An S1 pedicle screw was inserted toward the sacral promontory, and two S2 alar-iliac screws were placed in the ilium. All three screws were connected to a rod. One or two cylindrical cages filled with cancellous bone were inserted from the iliac side into the sacrum through a trans-iliac bony channel near the posterior superior iliac spine.

In addition, a modified technique based on that described by Wise and Dall¹³⁾ is used to achieve bony fusion of the anterior-superior SIJ region. This method involves inserting one or two cylindrical cages (INTER FIX™, Medtronic Sofamor Danek, Memphis, TN, USA, Inc.) with diameters of 12 mm or 10 mm and a length of 25 mm, filled with cancellous bone, into the anterior-superior SIJ region while largely preserving the posterior sacroiliac ligaments. One or two trans-iliac bony channels are created lateral to the PSIS, and cylindrical cages are placed in the SIJ from the iliac side toward the sacrum. First, the entry point on the ilium is determined under fluoroscopic or navigation guidance (Figure 2). An entry hole is created in the ilium using an 8-mm crown reamer, which is subsequently enlarged to match the diameter of the intended cylindrical cage using a Kerrison rongeur and curettes. During this procedure, cancellous bone is harvested from the ilium and later packed into the cylindrical cages. The channel is then advanced by sequential reaming from 6 mm to 10 mm to create a pathway across the articular region of the SIJ. Subsequently, the cylindrical cage filled with cancellous bone is inserted (Figure 3A). The articular surface of the SIJ is characterized by a complex undulating morphology. Therefore, preoperative planning that aligns the trajectory of the cylindrical cages with the S1 PS angle makes cage insertion into the joint space easier. Previous attempts to place cages strictly along the articular surface have proven difficult, and the current practice favors a trajectory that crosses the joint line rather than one along the joint line (Figure 3B and C). Importantly, radiographs show that the tip of the cylindrical cage should not extend anteriorly beyond the iliac cortical density line (Figure 4). Patients are allowed to walk from the first postoperative day without weight-bearing restrictions. Stair climbing is avoided for six weeks postoperatively to minimize peak loading on the SIJ.

Figure 2.

Localization and navigation-guided planning of the cylindrical cage insertion pathway.

(A) Gray circles indicate the entry points of bone channels created lateral to the PSIS.

(B) Intraoperative photograph showing determination of the entry point using a navigation system.

(C) Real-time navigation images demonstrate accurate confirmation of the cylindrical cage insertion trajectory relative to the sacroiliac joint.

PSIS: posterior superior iliac spine

Figure 3.

Transarticular insertion of the cylindrical cage across the sacroiliac joint.

(A) Intraoperative photograph showing insertion of the cylindrical cage filled with cancellous bone along the prepared transarticular pathway.

(B) Schematic illustration comparing two insertion trajectories: previous attempts to place the cylindrical cage strictly along the articular surface, which are technically demanding, and the currently favored transarticular trajectory that crosses the joint line (*) rather than along the articular surface.

(C) Postoperative CT image confirming transarticular placement of the cylindrical cage across the sacroiliac joint.

CT: computed tomography

Figure 4.

Confirmation of insertion depth and position of the cylindrical cages.

(A) Lateral fluoroscopic view showing the insertion depth of the cylindrical cages. The three arrows indicate the radiographic landmark, namely the anterior iliac cortical density line.

(B) Schematic illustration showing the anatomical relationship between the cylindrical cages (*) and the sacroiliac joint articular region. The dotted area represents the sacroiliac joint articular region, which is not directly visualized on fluoroscopy but is schematically indicated based on anatomical landmarks. Both cages are placed in the upper anterior portion of this articular region. The bold line represents the anterior iliac cortical density line.

Surgical Outcomes

A total of 17 patients (6 men and 11 women: mean age, 44±7 years) with 20 joints, including three bilateral SIJ fusions, were investigated. This study was approved by an institutional ethics committee. The mean follow-up duration was 53±23 months. All patients had a preoperative SIJ score of ≥5 points⁸⁾ and ≥70% pain relief following a diagnostic SIJ injection, confirming the diagnosis of an SIJ disorder. Three patients had comorbid lumbar spine disorders for which lumbar fusion was performed in conjunction with SIJ fusion (Figure 5).

Figure 5.

Representative images of combined lumbar and sacroiliac joint fusion. Postoperative radiograph (A) demonstrating posterior lumbar fusion combined with unilateral posterior sacroiliac joint fusion. Coronal (B) and axial (C) CT images demonstrating posterior lumbar instrumentation and transarticular sacroiliac joint fusion using a cylindrical cage.

Combined fusion of the lumbar spine and SIJ is feasible in patients with concomitant lumbar spine disorders.

CT: computed tomography

Pain in the SIJ region, assessed using the visual analog scale (VAS), significantly improved from a preoperative score of 87±18 mm to a postoperative score of 24±25 mm (p<0.0001). Functional impairment, assessed using the Denver SIJ Questionnaire¹⁴⁾ (DSIJQ; range: 0-50 points, with higher scores indicating greater disability), also improved significantly from a preoperative score of 32±11 points to a postoperative score of 18±16 points (p=0.0002) (Table 1). Pre- and postoperative comparisons of the VAS and DSIJQ scores were analyzed using the Wilcoxon signed-rank test, with a significance level set at p=0.05. All statistical analyses were conducted using JMP^Ⓡ 14.2 software (SAS Institute Inc., Cary, NC, USA).

Table 1.

Changes in Pre- and Postoperative Sacroiliac Joint Region Pain and Activities of Daily Living.

	Pre	Post	P-value
VAS (mm)	88±18	24±25	<0.0001
DSIJQ (points)	32±11	18±16	0.002

Values express mean±standard deviation.

DSIJQ: Denver Sacroiliac Joint Questionnaire; VAS: visual analog scale

Patient-reported outcome measures were evaluated in five domains: satisfaction with surgery, symptom reduction, improvement in quality of life, willingness to undergo the same procedure again, and willingness to recommend the surgery to others. Response options included completely agree, agree, neutral, disagree, and completely disagree. For surgical satisfaction, responses were: completely agree (n=12), agree (n=6), neutral (n=1), disagree (n=0), and completely disagree (n=1), indicating that 90% (18 out of 20 joints) of patients reported satisfaction (“agree” or higher). Similarly, high satisfaction rates were observed for the other domains (Figure 6).

Figure 6.

Patient-reported outcome measures. The majority of patients responded with “Agree” or higher regarding satisfaction with surgical outcomes, symptom relief (reduction of complaints), improvement in QOL, and willingness to undergo the same surgery again (“Would you choose surgery again?”). In contrast, responses to the item “Would you recommend the surgery?” were more neutral than those to the other categories.

QOL: quality of life

A computed tomography (CT) scan within one week after surgery revealed malposition of the cylindrical cage in one joint. Follow-up CT scans were performed in 14 cases (15 joints) at a mean of 25 months (range, 5-59 months) after surgery to evaluate screw loosening and bone union. No screw loosening was observed in any case. Bone union was observed in 13 cases (14 joints, 93.3%), and nonunion was noted in one joint.

Discussion

Whether the posterior SIJ fusion technique could serve as a viable alternative to the anterior approach was initially uncertain. However, based on the present postoperative results, posterior fusion resulted in favorable clinical outcomes, including improvements in activities of daily living. Although it remains uncertain whether the use of cylindrical cages significantly affects surgical outcomes, a previous finite element analysis suggested that the addition of cylindrical cages in the posterior technique provides stability almost similar to that achieved with the anterior technique¹⁵⁾. Therefore, we routinely insert one or two cylindrical cages during surgery via a trans-iliac bony channel created lateral to the PSIS, advancing them across the articular portion of the SIJ.

Wise and Dall¹³⁾ first demonstrated that a minimally invasive posterior SIJ fusion technique using transarticular cages could achieve a high fusion rate and significant pain relief, reporting an 89% fusion rate and a mean improvement of 4.9 points in low back pain VAS scores. Their technique aimed to stabilize the joint through intra-articular bony fusion using cages alone and relied on bone morphogenetic protein, without supplemental posterior instrumentation such as screws or rods, which may raise concerns regarding initial mechanical stability.

Since 2012, minimally invasive SIJ fusion via the lateral approach has been widely used, and favorable postoperative outcomes have been reported^16,17). However, cases of implant loosening on the sacral side have also been documented using this method^18,19) due to the relatively weak bone quality of the sacral ala, and careful postoperative management, such as weight-bearing restrictions, may be required.

In contrast to these two types of techniques, our posterior approach allows reliable initial fixation by firmly holding the sacrum with an S1 PS and controlling iliac rotation with two S2 alar-iliac screws. Surgical indications for SIJ fusion are primarily based on load transfer disorders that cause gait disturbances and difficulty in sitting^9,20). In our technique, S2 alar-iliac screws are placed in the anatomical region presumed to represent the physiological load transfer line^21,22). This construct is therefore intended to restore physiological load transmission across the SIJ and directly address the underlying biomechanical pathology. Notably, in our clinical series, no cases of postoperative pubic symphysis pain or subsequent pubic fixation were observed following bilateral SIJ fusion procedures using this posterior approach, complications that have been reported after anterior SIJ fusion⁹⁾. This finding suggests that posterior stabilization along the load transfer line may avoid excessive stress concentration at the pubic symphysis. Taken together, these biomechanical considerations and clinical observations indicate that this posterior SIJ fusion technique may represent a promising surgical option for patients with severe SIJ disorders characterized by load transfer disorders. However, surgical indications were limited to carefully selected cases based on thorough clinical evaluation and diagnostic SIJ injection. Further validation is required before generalizing the diagnostic and surgical approach to a broader patient population.

Conclusions

The posterior SIJ fusion technique using transarticular cylindrical cages via a trans-iliac bony channel combined with posterior instrumentation resulted in favorable postoperative outcomes in patients with SIJ disorders.

Author Contributions: D.K.: Conceptualization, methodology, investigation, and writing of the original draft. E.M. and H.O.: Writing/review and editing, and supervision. T.S.: Data analysis.

Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.

Ethical Approval: The use of clinical data was approved by the institutional ethics committee of JCHO Sendai Hospital (Approval No. 2024-23).

Informed Consent: Informed consent was obtained from the patient for the use of anonymized data and publication.