Distraction arthrodesis of the sacroiliac joint: 2-year results of a descriptive prospective multi-center cohort study in 171 patients

Study design

Patients with chronic SIJ pain were included from 20 German hospitals in this descriptive prospective multi-center cohort study, with pain persisting for 6 months or more. Only patients with exhausted options of conservative treatment (≥ 6 months) were included, such as physiotherapy, manual therapy, therapeutic ultrasound, SIJ orthoses, peri- and intra-articular infiltration and SIJ denervation. The inclusion period was from January 2011 to June 2012. The study design was questionnaire based, surveying on preoperative and postoperative data at 6 weeks and 3, 6, 12 and 24 months. The patients themselves completed all postoperative questionnaires free from external influence at home. Besides magnetic resonance imaging (MRI) and X-rays of the lumbar spine preoperative imaging diagnostics included X-ray and computed tomography (CT) scans of the pelvis. A corresponding imaging of the pelvis was scheduled to be obtained immediately following the surgery and in a 6-month follow-up. The exclusion criteria included the presence of a tumor or bacterial infection, multiple prior surgical procedures to the SIJ, sacral insufficiency fractures and bony defects in the area of the recess of the ilium and sacrum following bone graft harvesting. Patients with ongoing pension claims or on disability leave were also excluded. Preoperative RF neurotomy was no precondition for the patients to be included in our study. No age-related exclusion criteria were defined. All relevant ethics committees approved the study. All patients gave their informed and written consent.

Indication for surgery

The patients had to present their maximum pain below the lumbosacral level and around the posterior superior iliac spine (PSIS). A combination of provocation tests of the SIJ (distraction test, compression test, thigh thrust test, FABER test, Gaenslen test and sacral thrust test) was used [5, 13, 14] for physical examination of the patients. Either peri- or intra-articular SIJ injections were performed additionally, considered positive with a pain reduction of 50% or more. X-rays of the pelvis together with CT scans of both SIJs were indicated to diagnose secondary osteoarthritis consistent with adjacent segment degeneration after lumbar/lumbosacral fusion, accessory joints (false joints of unknown etiology located in the recess of the sacroiliac joint either between the S2 or more rarely between the S1 transverse process of the sacrum against the ilium on one or both sides; Fig. 1) [15, 16], dysplasia, post-partum arthritis, post-traumatic arthritis, axial spondyloarthritis and primary arthritis of the SIJ.

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However, it was not mandatory for all of the above-mentioned criteria to be fulfilled completely to enroll eligible patients in this study. The decision was made by the surgeon on basis of a conclusive combination of medical history, clinical tests, SIJ injections, X-ray and CT scan.

Rationale and surgical technique

Several studies demonstrated that applying a pelvic belt or a Hoffmann–Slätis frame over the wing of the ilium might increase the force closure of the SIJ [17, 18, 19, 20]. A distraction arthrodesis over the recess is hypothesized to have similar effects, but is placed on the short lever side of the ilium via a posterior approach to the SIJ [21]. Inserting an interference screw in the joint recess between the sacrum and ilium at the level of S2 brings about distraction near the joint and as a result of the accompanying ligamentotaxis, causing a repositioning of the joint surfaces. The distraction enables primary stabilization of the SIJ culminating in fusion (Fig. 2a, b).

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The surgery was performed under general anesthesia. A dorsal 4–6-cm-long midline approach was used, with the center of the skin incision corresponding to the upper border of the sacrum. The incision was then extended to the thoracolumbar fascia, continuing laterally to the posterior superior iliac spine (PSIS). A lengthwise fascia incision was made 1.5 cm medial to the PSIS and somewhat further caudally than the skin incision but at the same length. After blunt retraction of the spinal erectors towards medial, the posterior sacroiliac ligaments were exposed and resected over the recess of the SIJ between the S1 and S2 transverse processes of the sacrum. In a next step, the interosseous ligaments were removed and the cortical surfaces of the ilium and sacrum exposed, along with the posterior joint space opening at the floor of the recess. A drill was used to open the iliac and sacral cortical bone. A guidewire was positioned along the weight-bearing axis of the ilium using an image intensifier in three predefined planes. The individual width of the recess together with additional possible distraction between the ilium and the sacrum were acquired with a distraction instrument used to determine the size of the required implant (hollow, tapered titanium screw with large fenestrations; implant diameter 13 mm/15 mm/17 mm or 19 mm; length 30 mm). While maintaining distraction between ilium and sacrum the interference screw was inserted in the recess. A large quantity of allograft or bone substitute material (30–45 cc) according to the size of the recess was deposited in and around the implant until the recess and the implant was filled completely (Figs. 3, 4a–c, 5a, b).

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Follow-up treatment

Post-surgery the patients were mobilized for 6–8 weeks with a partial weight bearing (20 kg) on the operated side not to lose the achieved intraoperative distraction and ligamentotaxis. Vitamin D supplement therapy was carried out for 3 months postoperatively to promote bone metabolism [22, 23]. Patients undergoing surgery of both SIJs underwent two separate procedures. However, the second side was not operated on until full bearing was achieved on the side operated first.

Outcome measures

Preoperative data collection took place at the respective hospital of surgery. The postoperative surveys were mailed to the participants and completed by the patients independently. The questionnaires included: Million Visual Analogue Scale (MVAS), Oswestry Disability Index (ODI), Short-form McGill Pain Questionnaire (SF-MPQ), 12-Item Short-Form Health Survey (SF-12), and a pain drawing (PD). The postoperative questionnaires also contained questions about patient satisfaction.

A preoperative pain drawing was assessed as ‘typical’ (Fig. 6a) if the PD covered a region corresponding to the ‘Fortin area’ [24, 25]. If the patient additionally indicated referred pain in the leg on the same side or the inguinal region of the affected SIJ, the PD was also classified as ‘typical’ [26, 27]. Drawings that primarily showed pain over the midline at the level of L5 and further cranial or in the lower abdomen or hypoesthesia of the lower extremities or multiple pain locations in the entire body were classified as ‘atypical’ (Fig. 6b).

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Two radiologists familiar with the surgical procedure evaluated all X-ray and CT datasets independently and blinded to each other’s ratings. This evaluation included the position of the implant, the ossification between the sacrum and the ilium in the area of the recess and the interference screw, as well as the vacuum phenomenon in the SIJ (Fig. 7a, b). If the results of the two radiologists diverged, the principal investigator additionally evaluated the case.

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Statistical analysis

Group comparisons were performed using the Mann–Whitney U test, Wilcoxon signed-ranks test, Chi-square test and McNemar test. For multiple testing, confidence levels were adjusted using the Holm–Bonferroni-method. All calculations and visualizations were implemented in the Python programming language using community managed open source libraries of scientific tools (NumPy, SciPy, StatsModels, pandas, Matplotlib). For the scoring of the 12-Item Short-Form Survey (SF12), physical and mental component summaries (PCS/MCS) were calculated by reversing items 1, 8, 9, and 10. Scores were then calculated as a percentage ranging from the minimal (0%) to the maximal (100%) possible sum of all items (PCS: items 1–5 and 8; MCS: remaining items). Patients treated on both sides were considered as independent data sets regarding each operation.

Patient numbers and operations

Preoperative questionnaires were received from a total of 171 patients. An operation of the contralateral SIJ was performed in 7 patients (4.1%) in the 24-month surveillance period. At the study endpoint (24 months post-surgery), 143 12-month datasets from 137 patients and 137 24-month datasets from 132 patients were available for evaluation (22.8% loss to follow-up at 24 months). Revisions due to misplacements of the implant or persistent pain were reported in six patients (4.3%) in the postoperative course. All of these patients were followed-up after revision surgery for a period of 12–24 months and were included in the evaluation.

Demographic and surgery-related data

Clinical outcomes

There was a highly significant decrease in the MVAS (Fig. 8), from 63% preoperatively to 36% after 24 months (p < 0.001). The ODI (Fig. 9) decreased from a preoperative score of 51% to 33% after 24 months (p < 0.001). The SF-MPQ (see Table 2) of all patients improved from an average of 50% preoperatively to 31% after 24 months (p < 0.001). The physical component summary (PCS) increased from 22% preoperatively to 41% 24 months postoperatively (p < 0.001). The mental component summary (MCS) increased from 40% preoperatively to 55% 24 months postoperatively (p < 0.001). With regard to the development of pain symptoms in the course of the study, a decrease in pain was observed from 74 points on the VAS (Fig. 10) preoperatively to 37 points after 24 months (p < 0.001). When the patients were specifically asked about the decrease in leg pain, their responses yielded a decrease from 60 points on the VAS preoperatively to 32 points after 24 months (p < 0.001).

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Table 2

Outcomes at baseline and after surgery (12 and 24 months); difference = 24 months − baseline value

	Baseline	12 months	24 months	Difference	p value
	Mean (SD)	Mean (SD)	Mean (SD)
MVAS	63 (26)	38 (29)	36 (30)	− 27	p < 0.001
ODI	51 (26)	35 (27)	33 (28)	− 18	p < 0.001
SF-MPQ	50 (16)	33 (23)	31 (25)	− 19	p < 0.001
VAS total	74 (20)	39 (29)	37 (30)	− 37	p < 0.001
VAS leg	60 (27)	33 (29)	32 (30)	− 28	p < 0.001
SF-12 (PCS)	22 (15)	40 (26)	41 (27)	19	p < 0.001
SF-12 (MCS)	40 (20)	54 (24)	55 (25)	15	p < 0.001

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Patient satisfaction

When asked about their current level of satisfaction, 73.3% of all patients reported that they felt better or much better 24 months postoperatively than prior to surgery. In contrast, 13.7 and 13.0% of the patients reported that they felt the same or worse. After 12 months, 79.0% of the participants responded ‘yes’ to the question as to whether they would recommend the surgical method to a good friend and 78.5% responded with ‘yes’ after 24 months.

Pain drawing

Analgesic use

At the time of surgery, 90.9% of the patients were taking analgesics. Before the SIJ fusion, 49.3% of the patients used weak or strong opioids and after 24 months this was decreased to 30.3%. After 24 months, a total of 49.3% of the participants reduced their pain medication (Fig. 11). Although 15.1% of the patients reported to use more analgesics 24 months after the SIJ fusion, only three of these patients stated that they were feeling worse than before surgery. All three of them had a non-union in the follow-up CT scan because of a bad preparation and bone grafting of the recess. None of them had a poor implant positioning. Two of them had an atypical pain drawing preoperatively.

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Imaging

A complete series of CT scans (pre-/postoperatively/6 months or more) was available for 115 operations. The radiologists’ assessments on fusion and vacuum phenomena diverged in four of the cases (3.5%). 36 operations (31.3%) were assessed as unequivocally fused, with a considerably higher fusion rate of 67% when BMP-2 was used compared to a fusion rate of 19% when no BMP-2 was used (p < 0.001). In terms of operations with no observed vacuum phenomenon (preoperative vacuum phenomenon in SIJ space, no longer present in the last follow-up CT), the fusion rate amounted to 83% in contrast to a fusion rate of 13% for operations showing a vacuum phenomenon (p < 0.001). In 33% of the cases a poor implant position (implant inserted too far into the ilium or the sacrum) was observed. Overdistraction of the SIJ as an indirect sign of a rupture of the surrounding sacroiliac ligaments was not found in any of the cases. Statistics showed no significant correlation regarding fusion against all scores (ODI, MVAS, SF-MPQ, VAS) or the reported improvements of back or leg pain by the patient.

Complications

Intraoperative and implant-associated postoperative complications occurred in one case, where a sensory L5-radiculitis was caused by too liberally applied bone substitute. After surgical revision and removal of the bone substitute, symptom relief was achieved in 5 days.