Feasibility of ultrasound-guided sacroiliac joint injection considering sonoanatomic landmarks at two different levels in cadavers and patients




Sacroiliitis is often caused by rheumatic diseases, and besides other therapeutic options, treatment consists of intraarticular injection of corticosteroids. The purpose of this study was to assess the feasibility of ultrasound (US)-guided sacroiliac joint (SI joint) injection at 2 different puncture levels in cadavers and patients when defined sonoanatomic landmarks were considered.


After defining sonoanatomic landmarks, US-guided needle insertion was performed in 10 human cadavers (20 SI joints) at 2 different puncture sites. Upper level was defined at the level of the posterior sacral foramen 1 and lower level at the level of the posterior sacral foramen 2. In 10 patients with unilateral sacroiliitis, injection at the most feasible level was attempted.


Computed tomography confirmed correct intraarticular needle placement in cadavers by showing the tip of the needle in the joint and intraarticular diffusion of contrast media in 16 (80%) of 20 SI joints (upper level 7 [70%] of 10; lower level 9 [90%] of 10). In all 4 cases in which needle insertion failed, intraarticular SI joint injection at the other level was successful. In patients, 100% of US-guided injections were successful (8 lower level, 2 upper level), with a mean pain relief of 8.6 after 3 months.


US guidance of needle insertion into SI joints was feasible at both levels when defined sonoanatomic landmarks were used. If SI joint alterations do not allow for direct visualization of the dorsal joint space of the lower level, which is easier to access, the upper level might offer an appropriate alternative.


Inflammatory diseases, especially ankylosing spondylitis and psoriatic arthritis, are common causes of sacroiliitis (1). Besides physiotherapy and systemic medication, treatment consists of intraarticular injection of corticosteroids (2). However, sacroiliac joint (SI joint) injections can be a challenging procedure, due to the complex anatomy of the SI joint (3–5). A double-blind study by Rosenberg et al (6) has demonstrated an intraarticular success rate of 22% only among clinically guided SI joint injections. Therefore, image guidance seems to be a crucial issue to improve the intraarticular success rate of SI joint injections, and image-guided needle placement using fluoroscopy, computed tomography (CT), or magnetic resonance imaging (MRI) has been advocated in several studies (7–12).

Recently, an SI joint injection study assessed the value of ultrasound (US) guidance for intraarticular injection, reporting an overall intraarticular injection success rate of 76.7% when controlled by fluoroscopy. However, 23.3% of injections were performed extraarticularly (13). Nonetheless, US guidance is an economical and easily available imaging method without the use of ionizing radiation.

The purpose of this study was to assess the feasibility of US-guided needle injection of the SI joint at 2 different puncture levels in cadavers and to prove feasibility in patients.



We investigated 20 SI joints in 10 human, formalin-preserved, intact cadavers (5 men, 5 women; mean age 65.5 years, range 57–72 years). The cadavers were placed in a prone position for US and CT examinations. All cadavers were in legal custody of the local Department of Anatomy, Histology, and Embryology and dedicated to human studies according to the last will of the donors.

US technique.

US was performed using a Technos MPX scanner (Esaote Biomedica, Genoa, Italy) fitted with a curved array transducer (CA 430E 5-2, Esaote Biomedica) and operating at a gray-scale frequency between 2.5 and 6.0 MHz, adjusted to the frequency needed according to the penetration depth. US scanning and needle insertion were performed by a musculoskeletal radiologist with 5 years of experience in US-guided injections.

An axial US scan of the posterior area of the cadavers was used to identify landmarks of the 2 different levels (puncture sites) (Figure 1).

Figure 1.

Bony landmarks of the sacroiliac joint. The red line shows the upper level (at the level of the posterior sacral foramen 1) and the green line shows the lower level (at the level of the posterior sacral foramen 2). Arrowheads show the median sacral crest, and the star indicates the lateral sacral crest.

Upper level.

For primary orientation, the posterior superior iliac spine was visualized laterally, and the spinous process of the fifth lumbar vertebra medially. Then the transducer was moved caudally, depicting the dorsal surface of the sacrum with the median and lateral sacral crest, the gluteal surface of the ilium, and the posterior sacral foramen 1. The needle was inserted into the hypoechoic cleft located between the surface of the sacrum and the contour of the ilium (Figure 2).

Figure 2.

Upper level of a sacroiliac joint (SI joint). Moving the probe caudally to the upper level of a right SI joint, the posterior sacral foramen 1 (white arrow) and hypoechoic cleft leading to the proximal SI joint (black arrow) can be seen. The black arrow indicates needle angulation. The arrowheads display the gluteal surface of the ilium.

Lower level.

For the lower level, the transducer was moved downward by delineation of the median and lateral sacral crest, at the dorsal surface of the sacrum and the gluteal surface of the ilium until the posterior sacral foramen 2 was visualized. As with the upper level, the needle was inserted into the hypoechoic cleft between the sacrum and ilium (Figure 3).

Figure 3.

Lower level of a left sacroiliac joint (SI joint), displayed by a paraaxial transducer position. The tip of the needle (black arrow) can be visualized under perpendicular US beam in the hypoechoic cleft of the SI joint (star). The black arrow indicates perpendicular needle positioning. The open arrow indicates the direction to the median sacral crest. The white arrow shows posterior sacral foramen 2, and the arrowheads present the gluteal surface of the ilium.

The SI joint consists of ear-shaped auricular surfaces of the ilium and sacrum, resulting in a mainly vertical and anterolateral orientation. The posterosuperior compartment is fibrous, whereas the anteroinferior compartment is synovial. The cartilage-lined portion extends more superiorly along the anterior aspect of the joint, so that the few inferior centimeters represent a chondral joint from front to back. The transition line between the cartilage and syndesmotic portions is inferiorly convex. The entire joint is superficially stabilized by strong anterior and posterior ligaments to the interosseous ligaments and the joint capsule (14, 15).

Needle placement.

Needle insertion using a 21-gauge needle (21 gauge × 3⅛ inches; 0.80 × 80 mm, BL/LB; Braun, Melsungen AG, Germany) was performed at both levels under US guidance by freehand needle placement. The tip of the needle was placed cranially to the puncture level by using a paraaxial transducer position first. Angulations of needle insertion were determined according to the orientation of the hypoechoic cleft. The hypoechoic cleft of the SI joint shows cranially a more medial to lateral orientation, which becomes slightly more caudally vertically oriented. Therefore, needle orientation is mainly vertical with only slight angulations from medial at the upper level and vertical at the lower level. After needle positioning under the skin by a paraaxial US scan, the needle was inserted toward the SI joint using a longitudinal transducer position, visualizing the needle parallel to the US beam. After reaching the entrance of the SI joint with the tip of the needle, a paraaxial transducer position allowed for further vertically oriented needle introduction under a perpendicular US beam, so that the tip of the needle could be visualized in the hypoechoic cleft. Care was taken to insert the needle directly toward the hypoechoic cleft, to avoid any bony spurs. Once the needle tip was depicted in the hypoechoic cleft, a further insertion of less than 1 cm was attempted by pushing the needle into the joint space (Figure 3). US-guided needle placement was performed in 10 upper levels and in 10 lower levels, and the procedure time was calculated from the beginning of the examination until the extraction of the needle.

Control CT.

After US-guided needle insertion, CT was performed in all cases by using a multislice CT (Somatom Volume Zoom 4; Siemens Medical Systems, Erlangen, Germany) with a detector collimation of 4 × 1 mm, gantry rotation time of 0.75 seconds, tube voltage of 120 kV, and tube current time-product of 200 mA seconds. Images were reconstructed with 2-mm effective slice width (2-mm increment), convolution kernel B60, and 512 × 512–pixel matrix. CT was performed to confirm the accuracy of intraarticular needle placement, which was considered placement of the tip of the needle precisely into the SI joint space and not into the fibrocartilaginous tissue or the retroperitoneal pelvic space. If intraarticular needle placement was found by CT, 0.5 ml non-ionic contrast material (Iomeron; Bracco, Milan, Italy) and 0.5 ml of saline was administered. CT was repeated to evaluate whether the contrast material was in an intraarticular location, which was defined as the outlining of the joint space by a contrast agent (Figure 4). If CT showed an extraarticular location of the tip of the needle, US-guided needle placement at the other level was performed. CT was further used to evaluate underlying structural morphology, which disallowed intraarticular needle insertion, such as ankylosis or bony spurs.

Figure 4.

Control computed tomography (CT). CT scan of a left sacroiliac joint (SI joint) after needle placement and contrast agent injection. Correct intraarticular placement of the needle (white arrow) and correct outlining of the SI joint space by contrast agent (black arrow) in the joint of a cadaver.

We analyzed the success rate of US-guided needle insertion in comparison with CT findings. To assess the differences between the success rates for the US-guided needle insertions at the 2 levels, Fisher's exact test was performed using Java-supported biometry of the University Münster. A P value less than 0.05 was considered statistically significant.


Ten consecutive patients (4 men, 6 women; mean age 25.6 years, range 18–35 years) with established spondylarthritis according to the European Spondylarthropathy Study Group criteria (16) and modified New York criteria (17) (ankylosing spondylitis: n = 2; psoriatic arthritis: n = 3; undifferentiated spondylarthritis: n = 3; reactive arthritis: n = 2) reporting unilateral symptoms in the SI joints refractory to systemic therapy were referred for SI joint injection. A subjective rating of the patient using a 0–10 dolorimetry scale (0 = no pain, 10 = most severe pain) was recorded, and oral and written consent for injection was obtained from all patients. A US-guided injection of 40 mg crystalline triamcinolone acetonide (Volon A 40 mg/ml; Dermapharm, Vienna, Austria) was performed in all SI joints by the same physician who performed the experimental study according to the established sonoanatomic landmarks evaluated in the cadaver section. A 21-gauge needle was used and 0.5 ml mepivacaine hydrochloride (Mepinaest purum 2%; Gebro Pharma, Fieberbrunn, Austria) was added to the corticosteroids; thus, a total volume of 1.5 ml was injected.

Procedure time was calculated, but no control CT scan was performed. Furthermore, the distance from the skin to the tip of the needle was measured by obtaining a measurement from the skin toward the hyperechoic cortical line of the bony landmark at the entrance to the dorsal hypoechoic cleft of the SI joint of each level. Patients were followed up after 3 months by telephone and were asked for a pain rating and if they would undergo the procedure again.


Cadaver results.

In all cases, US was appropriate to visualize defined landmarks, such as the spinous process of the fifth lumbar vertebra, the median and lateral sacral crest, the dorsal surface of the sacrum, the contour of the iliac crest, posterior sacral foramen 1 and 2, the posterior superior iliac spine, and the gluteal surface of the ilium. By using US, the hypoechoic cleft between the surface of the sacrum and the ilium toward the SI joint, which was the target used to guide needle insertion, was delineated at the upper level in 7 (70%) of 10 cases, and at the lower level in 9 (90%) of 10.

The success rate for US-guided intraarticular needle placement was confirmed by CT at the upper level in 7 (70%) of 10 cases and at the lower level in 9 (90%) of 10. After contrast agent administration, all of these cases showed a correct outlining of the joint space. The overall success rate for US-guided intraarticular needle placement for both levels was 80% (16 of 20 SI joints) (Table 1).

Table 1. Results in cadavers
CadaverAge, yearsSideLevelTime (minutes)SuccessfulOther level
Mean65.5  3.09  

In 4 (20%) of 20 cases, US-guided needle insertion failed due to extensive SI joint alteration in terms of degenerative joint narrowing or bony spurs, which were found by CT. In all 4 cases, the tip of the needle was localized at a bony spur at the entrance of the SI joint. However, US-guided needle positioning at the other level was performed successfully in these 4 cases, when controlled by CT with and without a contrast agent.

Overall, SI joint infiltration was performed 11 times at the upper level and 13 times at the lower level. Eight (72.7%) of 11 SI joint infiltrations at the upper level and 12 (92.3%) of 13 at the lower level were successful, and altogether, 10 (100%) of 10 SI joints were injected at the upper or lower level. Mean procedure time was 3.09 minutes (range 2–5 minutes) and was longer in the first cadavers (>4 minutes) than in the last cadavers (<3 minutes).

Patient results.

Subjective symptoms were graded as 8 or higher (mean 9.1, range 8–10) in all patients before US-guided SI joint injection (Table 2). Because our experimental findings demonstrated better results at the lower level, US-guided needle placement was aimed primarily at the lower level. In 8 (80%) of 10 painful SI joints, the hypoechoic cleft between the sacrum and ilium was clearly visible at the lower level, and US-guided injection was performed at this level. In 2 cases (20%) the hypoechoic cleft was not distinctly depicted, therefore a more proximal approach at the upper level was attempted. In both cases, the hypoechoic cleft was well displayed at this puncture site and infiltration could be performed without problem. The mean procedure time was 3 minutes (range 2–5 minutes) for injection at the lower level. At the upper level, 4.25 minutes and 4.5 minutes, respectively, were needed when calculating from the beginning of the examination until the procedure was finished. The mean distance from the skin to the tip of the needle was 4.3 cm (range 3.8–5.3 cm). After 3 months, all patients reported distinct pain relief (mean 0.5, range 0–2). Mean improvement was 8.6 after US-guided SI joint injection. All patients declared that they would undergo the procedure again if necessary.

Table 2. Results in patients
PatientAge, yearsLevelTime (minutes)Dolorimetry scale*
Before3 months afterDifference
  • *

    Dolorimetry scale representing subjective values from 0 (no pain) to 10 (maximal pain), before injection and 3 months after injection.

Mean25.6 3.3259.10.58.6


Several studies have demonstrated the therapeutic efficacy and good success rate of image-guided intraarticular SI joint injections performed by CT, fluoroscopy, or MRI (7–12, 18). Intraarticular injection seems to be the key for long-term pain relief, but accurate intraarticular needle placement is known to be difficult without image guidance, due to the complex SI joint anatomy with division from a true synovial lower part toward a fibrous upper part. Joint space narrowing and bony spurs occurring during the disease can be challenging when entering the joint with a needle. Even in the knee joint, which is relatively easy to access, clinically guided intraarticular injection was difficult when no effusion was present (19). Accordingly, clinically guided SI joint injections showed an intraarticular success rate of only 22% when controlled by CT (6). Therefore, image-guided injections seem to be preferable. CT guidance is already used for SI joint injections in routine clinical settings (7, 10, 20). Moreover, Dussault et al reported an intraarticular success rate of 97% in a study of 31 fluoroscopy-guided SI joint injections in 24 patients (8). Günaydin et al performed MRI-guided injections in 16 SI joints and demonstrated good results (21), which is in line with 2 other studies using MRI guidance (12, 22). However, CT- and MRI-guided SI joint injections are more time consuming and cost intensive than US.

US, a readily available imaging technique for guiding injections, has been shown to enable detection of inflammatory activity in the dorsal SI joint (23) and to be suitable for image-guided SI joint injection, as Pekkafahli et al demonstrated in 60 SI joints of 34 consecutive patients (13). The first 30 injections showed a successful intraarticular injection rate of 60%, the subsequent 30 injections reached an intraarticular injection rate of 93.5% when controlled by fluoroscopy (13). However, 14 (23.3%) of 60 injections were placed extraarticularly, where the contrast media was detected too cranially in the fibrocartilaginous tissue or too caudally reaching the retroperitoneal pelvic space (13). Reference to sonoanatomic landmarks, which were evaluated in the present study, proved to be of value in guiding intraarticular needle insertion into the SI joint.

Our findings in cadavers demonstrate an overall success rate of 100% (10 of 10) for US-guided SI injections; however, the success rates at the 2 levels were slightly different. A higher success rate (92.3%) was found using the most caudal approach (lower level), but, alternatively, the upper level may also be an appropriate approach (72.7%) when the lower level appears to be unsuitable (e.g., due to bony spurs). Comparison between the lower and upper levels showed no statistically significant difference (P = 0.3). However, some advantages of the lower level have to be mentioned. The lower level of the SI joint is relatively superficially located in nonobese patients, is vertically oriented, and can therefore be reached by perpendicular needle direction, whereas a slight angulation of the needle is necessary at the upper level; therefore, SI joint injection might be easier to perform at the lower level. If the hypoechoic cleft at the lower level can be clearly seen, it is advisable to inject directly into the synovial portion with a vertically oriented needle positioning. The danger of injecting into the retropelvic space can be avoided by strict relation to the sonoanatomic landmarks as described in our study.

If US cannot visualize the hypoechoic cleft, US-guided needle insertion will be difficult or impossible due to bony spurs, joint space narrowing, or ankylosis. When only a small bony spur compromises the US beam penetration by extensive posterior acoustic shadowing, US transducer tilting can be helpful. At the upper level, a slight angulation of the needle from medial to lateral is necessary to insert the needle parallel to the SI joint course. Provided landmarks are essential to avoid complications due to incorrect extraarticular puncture, resulting in subperiosteal, fibrocartilaginous, and intrapelvic spread of injected material. We did not encounter any of these extraarticular needle locations in our series.

CT confirmed the presence of joint space narrowing or bony spurs in 4 of 4 cases at both levels, where the posterior portion of the SI joint could not be visualized by US and no intraarticular needle placement could be achieved. However, in all of these joints, US correctly guided needle infiltration at the other level. Furthermore, the mean age of the cadavers used in this study was relatively high (65.5 years), and was an age at which the rate of joint space narrowing and bony spur formation is known to increase (24); however, in all SI joints, US-guided injection was feasible. Considering sonoanatomic landmarks in 10 of 10 (100%) patients, SI joint injections by US guidance were feasible at either the lower or upper level, and after 3 months distinct pain relief was observed in all cases, which further confirms the feasibility of US-guided SI joint injections.

Advantages of US-guided SI joint injection include not only the widespread availability of US and cost effectiveness, but also the lack of radiation. Radiation exposure under fluoroscopy-guided SI joint injection ranges from 12–30 mGy/minute for the skin and 0.1–0.6 mGy/minute for the gonads (18). For CT-guided injections, values up to 10 mGy/minute were reported (7). Absence of radiation is especially important, as mainly younger patients or even pediatric patients undergo therapeutic SI joint injections, in whom radiation is a more serious issue of concern, e.g., in terms of lifetime risk of cancer. Without the risk of radiation, US could also be helpful for repeated procedures in chronic sacroiliitis and for contralateral side injection. Lack of radiation is also an advantage of MRI, but this imaging method is a more time- and cost-consuming procedure with contraindications such as a pacemaker and claustrophobia. Additionally, US-guided injections can be easily performed in real time, whereas other modalities such as MRI and CT have to use an additional fluoroscopic imaging device. Further studies should compare the value of all different imaging methods for SI joint injection.

We have to acknowledge several limitations of this study. Our patient population was small, but US guidance was feasible in all cadavers and patients. By virtue of ethical concerns, no control CT scan for assessing correct intraarticular needle positioning, was conducted in patients. However, considerable pain relief was found in all patients, suggesting intraarticular steroid placement. Furthermore, US-guided needle insertions were performed by only a single investigator, therefore inter- and intraobserver variability were not calculated. However, a learning curve in terms of a decrease in time for the procedure was found, suggesting that technical improvement was possible. No learning curve was found for intraarticular needle placement itself; therefore, it can be assumed that after appropriate training by considering provided landmarks, beginners can also achieve equally high success rates; however, no data on this issue were evaluated in this study. By using mostly axial transducer positioning, direct visualization of the needle in a longitudinal plane was not possible, although the tip of the needle can be seen well in real time, when moving the needle into the hypoechoic cleft of the posterior SI joint. This study presents sonoanatomic landmarks essential to facilitate SI joint injection, but further improvement of US guidance might be obtained by image fusion and transducer needle guidance kits. Additional limitations regarding the use of established landmarks are anatomic variants such as the lumbosacral fusion, but we did not encounter such a case. Detailed US examination of such variants should be performed in further studies.

US-guided needle insertion has been shown to be feasible for SI joint injection when taking into account defined sonoanatomic landmarks. We suggest performing US-guided SI injections at the lower level, which is easier to access, and in cases where the lower approach appears to be unsuitable, the upper level represents a valuable alternative.


Dr. Klauser had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study design. Klauser, De Zordo, Feuchtner, Sögner, Schirmer, Gruber, Sepp, Moriggl.

Acquisition of data. Klauser.

Analysis and interpretation of data. Klauser.

Manuscript preparation. Klauser, De Zordo, Feuchtner, Sögner, Schirmer, Gruber, Sepp, Moriggl.

Statistical analysis. Klauser, De Zordo.