An intimate relationship between form and function is. Anatomy of the Interosseous Region of the Sacroiliac Joint

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1 Anatomy of the Interosseous Region of the Sacroiliac Joint Alessandro L. Rosatelli, BSc, BSc(PT), MPhty (Manip), FCAMT 1 Anne M. Agur, BSc(OT), MSc, PhD 2 Sam Chhaya, MD 3 Study Design: Anatomical study of the region of the sacroiliac joint (SIJ) complex. Objectives: To document and quantify the surface topography of the region of the SIJ. Background: A review of the literature reveals that little consideration has been given to the region of the SIJ anatomically, biomechanically, and clinically. Methods and Measures: The region of 11 cadaveric specimens (9 formalin embalmed and 2 fresh frozen) were studied. Ten specimens were 55 years of age or older and 1 was 20 years old. To view the surfaces of the sacrum and ilium the specimens were either axially sectioned (1-cm slices) or disarticulated. One fresh-frozen and 6 embalmed specimens were disarticulated and the remainder axially sectioned. The topography (surface ridging and areas of ossification) of the region was documented in all specimens and in 2 specimens the surfaces were 3-dimensionally reconstructed using modeling and animation software (MAYA; Autodesk, Inc, San Rafael, CA). Results: Surface characteristics of the SIJ complex observed in specimens 55 years of age or older included moderate to extensive ridging of the region of the sacrum and ilium in 100% of specimens and ossification of the central region of the sacroiliac (SI) ligament in 60% of specimens. Conclusions: Central region ossification of the SI ligament and the presence of ridges and depressions over the opposing surfaces of the sacrum and ilium are features common to specimens that are in or beyond their sixth decade. These findings further support the contention that there is little to no movement available at this joint in older individuals. J Orthop Sports Phys Ther 2006;36(4): Key Words: ilium, joint morphology, pelvis, sacrum, surface topography An intimate relationship between form and function is commonplace with most biological systems. To better understand the medical enigma of low back pain (LBP) it is necessary to thoroughly understand all structures that could potentially refer pain to this region. Clinically it is important to identify pathology affecting the sacroiliac joint (SIJ), as this has been demonstrated to be a potential source of LBP. 12 Yet there remains a lack of a good clinical correlate between physical impairment(s) and disability. Therefore, the use of physical examination procedures, such as SIJ mobility tests, to assess potential pathoanatomic causes remains a 1 Master s candidate, Institute of Medical Sciences, Division of Anatomy, Department of Surgery, University of Toronto, Toronto, Canada. 2 Associate Professor, Division of Anatomy, Department of Surgery, University of Toronto, Toronto, Canada. 3 Assistant Professor in Musculoskeletal Radiology, University of Texas Health Science Centre at San Antonio, San Antonio, TX. Address correspondence to Alessandro Rosatelli, Division of Anatomy, Department of Surgery, Medical Sciences Building Room 1158, University of Toronto, Toronto, Ontario, Canada, M5S 1A8. a.rosatelli@utoronto.ca topic of much debate, particularly in light of their demonstrated poor intertherapist and intratherapist reliability. 14,15 The reliability of mobility tests depends on the examiners ability to feel movement in this joint. However, the available movement permissible in this joint depends not only on the morphology of the articular surfaces themselves but also on that of surrounding areas, including the region of the SIJ complex. It is important therefore to understand the anatomy of the region and how this may change with advancing age. Joint morphology inevitably influences not only the type of clinical tests that can be performed but also the types of treatment that are theoretically possible. The anatomy of the articular portion of the SIJ has been studied extensively. The sacral articular (auricular) area is c shaped and located on the lateral aspect of this bone. During the fetal and prepubescent years, the sacral surface is flat, smooth, and lined with hyaline cartilage. The articular surface of the ilium is also c shaped, but in contrast to the sacral articulating surface it is covered by fibrocartilage. 2 The smooth and planar articular surfaces of the SIJ, which are apparent at birth and early childhood, permit movement in all directions, 2 with the primary restraint to such motion provided 200 Journal of Orthopaedic & Sports Physical Therapy

2 by the strong sacroiliac (SI) ligament. However, the surface topography of the articular region of the joint changes with advancing age. As early as the third decade of life ridges and depressions begin to form, making the joint surfaces nonplanar. The presence of ridges and depressions on the articular surfaces of the SIJ likely increases the frictional resistance to motion and imparts greater stability to the joint. 19 In addition, the surface texture of the SIJ becomes more irregular, taking on a coarser quality (ie, microscopic fibrillations of the articular cartilage) with advancing age. 18,19 This further limits movement by increasing the coefficient of frictional resistance between opposing articular surfaces. A review of previous literature reveals that attention has been focused on the microscopic and macroscopic structure of the articular (auricular) surfaces of the joint, with little consideration given to the region of the SIJ complex. Anatomical descriptions of the region of the SIJ are generally limited to the descriptions of the ligaments described below and illustrated in Figure 1. 1 The Interosseous SI Ligament This extensive ligament bridges the irregular space between the sacrum and ilium posterosuperior to the articular portion of the joint. 22 It consists of numerous short, strong ligamentous bands which pass from one bone to the other in different directions. 10 The ligament is divided into a deep part located posterior to the joint cavity, and a superficial part that blends with the posterior SI ligament. The Posterior SI Ligament The ligament is situated posterior to the SI ligament and is sep- arated from it by the posterior rami of the sacral spinal nerves and vessels. The superior part of this ligament originates from the intermediate and lateral sacral crests to attach laterally onto the posterior superior iliac spine (PSIS) and the internal lip of the iliac crest. Inferior fibers, from S3 and S4, ascend to the PSIS and internal lip of the iliac crest as well. These latter fibers are often continuous laterally with part of the sacrotuberous ligament and medially with the thoracolumbar fascia. 22 The Anterior SI Ligament This is an anteroinferior capsular thickening which is well developed near the arcuate line and the posterior inferior iliac spine. It is generally thin elsewhere. 22 The Sacrotuberous Ligament The ligament runs obliquely from the PSISs, the inferior sacrum, and the superior coccyx inferiorly to the medial margin of the ischial tuberosity. The inferior fibers of gluteus maximus and the tendinous portion of the long head of biceps femoris blend with this ligament. 22 The Sacrospinous Ligament The ligament is thin and triangular. It attaches laterally to the ischial spine and medially, by its broad base, to the lateral margins of the sacrum and coccyx, anterior to the sacrotuberous ligament, with which its fibers are intermingled. 22 The degree of movement permissible at any joint is dependant not only on the surface morphology and orientation of the articulating surfaces, but also in part on extra-articular components which lie external to the joint. Previous studies have addressed only the articular portion of the SIJ complex. Hence, the purpose of the study is to document and quantify the surface topography of the region of the SIJ complex. RESEARCH REPORT METHODS Sample Eleven cadaveric specimens (9 formalin embalmed, 2 fresh frozen) were studied. Ten (5 female, 5 male) of 11 specimens had an average age of 67 years (range, years) and 1 male specimen was 20 years of age. The specimens were obtained from, and the dissection was carried out in, the Division of Anatomy, Department of Surgery, University of Toronto, following the guidelines for use of cadaveric material in research as set forth by the Chief Coroner of the Province of Ontario. FIGURE 1. Ligaments joining the sacrum (S) and ilium (IL). Anterior sacroiliac (SI) ligament (A), SI ligament (X), posterior SI ligament (P), sacrotuberous ligament (ST), sacrospinous ligament (SP), coccyx (C) (from Grant s Atlas of Anatomy, 11th edition, with permission). Procedures Three different techniques were used to visualize the SIJ complex: (1) disarticulation, (2) axial sectioning, and (3) 3-D computer modeling. J Orthop Sports Phys Ther Volume 36 Number 4 April

3 FIGURE 2. Superior (S), central (C), and inferior (I) areas of the region of the sacroiliac joint (SIJ) complex. The joint has been disarticulated to show a medial view of the ilium and lateral view of the sacrum (from Grant s Atlas of Anatomy, 11th edition, with permission). Lines delineate approximate axial plane used for cross-sectional analysis of the SIJ complex. examined using a dissection microscope. All specimens (sectioned and disarticulated) were assessed by region using the following criteria: Extent of ridging (ie, the degree of unevenness or flatness) associated with the region of each SIJ complex was assessed. Based on this initial assessment, a 3-point scale was devised to quantify the degree of ridging. Slight ridging was defined as having little or no evidence of surface irregularity (Figure 3A). Moderate ridging was defined as having an region that demonstrated some degree of irregularity (Figure 3B), while extensive ridging was defined as sacral and/or iliac surfaces that were heavily invested with surface Disarticulation One fresh-frozen and 6 embalmed specimens were disarticulated. First, the soft tissues (ie, skin, subcutaneous tissue, muscles, vessels, and nerves) were removed to expose the osseoligamentous pelvic girdle. Next, the joint margins of each SIJ were identified and the joints carefully disarticulated by cutting through the anterior, posterior, and SI ligaments using a scalpel. During this process, great care was taken not to interrupt the bony topography of the sacral and iliac surfaces of the articular and regions of the joint. In cases where the ligaments or articular surfaces were ossified, a fine chisel was used to assist in the disarticulation process. The SIJ complex was then carefully separated to expose the articular and regions. Joint surfaces of the disarticulated specimens were photographed. Axial Sectioning Seven specimens (1 fresh frozen, 6 formalin embalmed) were sectioned in the axial (or transverse) plane using a band saw from cranial to caudal with a slice thickness of approximately 1 cm (Figure 2). Axial sections were chosen, as these represent the imaging plane typically used when conducting CT scans of the pelvis. To facilitate sectioning the formalin-embalmed specimens were also frozen. To localize changes in joint morphology in both the disarticulated and axially sectioned specimens the tuberosity of the ilium and sacrum of each SIJ complex was subdivided into approximately equal thirds: superior, central, and inferior from cranial to caudal (Figure 2). 1 The superior, central, and inferior regions of each surface was FIGURE 3. Classification of the surface topography of the region of the sacroiliac joint (SIJ). Black lines superimposed on a 3-D reconstruction of the portion of the ilium demonstrates: (A) slight ridging pattern, (B) moderate ridging pattern, and (C) extensive ridging pattern. 202 J Orthop Sports Phys Ther Volume 36 Number 4 April 2006

4 irregularities. Two independent observers assessed the extent of ridging in the first 4 specimens using this 3-point rating scale for degree of ridging. Each observer rated the extent of ridging identically Presence of calcification or ossification within the SI ligament was identified and the extent mapped Location and distribution of any bony irregularities (eg, osteophytes) were recorded 3-D Modeling Two specimens were selected for modeling. A 56-year-old female specimen with extensive changes in SIJ topography was compared with the region of the 20-year-old male specimen. Using a Microscribe 3DX Digitizer (Immersion Corporation, San Jose, CA) and MAYA (Autodesk, Inc, San Rafael, CA) modeling and animation software, the surface topography of the region of the SIJ complex was reconstructed in 3-D. Prior to digitization, each specimen was firmly secured to prevent movement during the digitization process. To facilitate removal of the ligament, specimens were submerged in a mild solution of 10% potassium hydroxide for a period of 1 to 2 hours. Residual traces of the ligament were removed with the aid of a dissecting microscope. Once the surface of the sacrum and ilium of each specimen was denuded of all ligament and soft tissues, the specimens were then ready for surface reconstruction. Surface reconstruction entailed placing small dots (ie, formulating a grid) using a fine marker over the entire surface of the sacrum and ilium of each specimen. Points on the surfaces were separated by 2-mm intervals. Each point was digitized using the Microscribe 3DX Digitizer to obtain x, y, and z coordinates. The data were then compiled in 3-D using a customized program developed in our laboratory in conjunction with MAYA. A 3-D, fully manipulatable topographical reconstruction of the surfaces of each SIJ complex was created. The modeled surfaces were then articulated as in situ. RESULTS The region of the ilium is convex, while the sacral surface is concave. Furthermore, it was observed that the SI ligament consisted of numerous short, dense bands that crossed the intervening space between the ilium and sacrum (Figure 4A). Moderate or extensive ridging of the surfaces of the ilium and sacrum was identified in all 10 specimens with average age of 69 years. Ridging was extensive in 6 specimens (age range, years), while moderate ridging was found in 4 specimens (age range, years). The severity of ridging was not found to be gender related, with extensive ridging present in 3 male and 3 female specimens and moderate ridging present in 2 male and 2 female specimens. In contrast, in the 20-year-old specimen only a slight ridging and depression pattern was observed on both the iliac and sacral surfaces. These results are summarized in the Table. While the degree of ridging did not appear to be related to gender, there did appear to be a relationship with respect to age. Slight ridging was found in the 20-year-old specimen, while the median age of specimens with moderate ridging and extensive ridging was 58 years and 75 years, respectively. Moderate and severe ridging was most prevalent in the central region of the SIJ complex (Figure 4B). Of the 10 specimens investigated that were in or beyond their sixth decade, 6 were observed to have distinct regions within the SIJ complex where the SI ligaments had become ossified (Table). In all cases, ossification was most pronounced within the central portion of the region of the SIJ complex (Figure 4C and 4D). Ossification of the SI ligament effectively fused the posterior aspect of the sacrum and ilium. One specimen demonstrated extensive bridging osteophytes over the anterosuperior aspect of the anterior SI ligament, which in turn encroached onto the cartilaginous component of the joint. Modeling of the Topography of the Interosseous Region of the SIJ Complex The 56-year-old female specimen and 20-year-old male specimen were modeled. When examined microscopically, the region of the 56-yearold specimen was found to have extensive ridging and a 1- to 2-cm area of ossification in its central portion. The 20-year-old specimen was markedly different in that it showed slight ridging only and no ossified area within the SI ligament. Computer modeling of the digitized points to reconstruct the surface topography of the region of sacrum and ilium permitted detailed 3-D visualization (Figure 5B and 5D) of surface features previously documented by visual inspection in 2-D (Figure 5A and 5C). Modeling further highlighted the magnitude of ridging and the presence of a distinct central area of ossification of the SI ligament in the 56-year-old specimen. In contrast, modeling of the 20-year-old specimen clearly demonstrated that the surfaces were relatively smooth and entirely free of ossification. The features of the region of the SIJ complex are clearly visible once they are digitized RESEARCH REPORT J Orthop Sports Phys Ther Volume 36 Number 4 April

5 FIGURE 4. (A) Axial section showing surfaces of ilium and sacrum (solid black lines). Hatched area between black lines (arrow) indicates the sacroiliac (SI) ligament. Ilium (I), sacrum (S). (B) Axial section (83-year-old male) showing extensive ridging, particularly within the central region. Solid black and white lines represent the surface boundaries of the sacrum (S) and ilium (I), respectively. Superior region (Sup), central region (Cent), inferior region (Inf). (C) Axial section (84-year-old female) showing where a bony bridge (circled area) has formed within the SI ligament. Anterior and posterior boundaries of the articular part of the joint are indicated with Xs. Ilium (I), sacrum (S). (D) Medial view of a disarticulated ilium (84-year-old female). The roughly circular area indicates the site of ossification of the SI ligament. and their surface topography rendered in 3-D. The surface of the ilium is convex, while that of the sacrum is concave. Furthermore, the surface of the iliac bone is wedge shaped, being wider posteriorly and narrower anteriorly (Figure 6). A narrow furrow separating the articular surface from the surface was observed extending the length of both the posterior margin of the articular surface and the anterior margin of the surface (Figure 6). DISCUSSION The SIJ complex consists of 2 parts: (1) an anteroinferior synovial joint between the c-shaped articular (auricular) surfaces of the sacrum and ilium, and (2) a posterosuperior syndesmosis situated between the surfaces (tuberosities) of the ilium and sacrum. Enclosing these 2 parts of the joint are the anterior and posterior SI ligaments. 204 J Orthop Sports Phys Ther Volume 36 Number 4 April 2006

6 TABLE. Summary of findings of sacroiliac joint (SIJ) region topography (n = 11). Specimen Type Age (y) Sex Specimen Disarticulated Specimen Sectioned Extent of Ridging Interosseous Region Ossification Embalmed 55 F Y Extensive Central Embalmed 56 F Y Extensive Fibrotic ligaments Embalmed 67 M Y Y Extensive Central Embalmed 83 M Y Y Extensive Superior anterior bridging osteophytes with dense ligaments Fresh frozen 84 F Y Extensive Central Embalmed 91 M Y Y Extensive Central Embalmed 58 M Y Moderate Central Embalmed 58 F Y Moderate N/O Embalmed 58 F Y Moderate Central Embalmed 80 M Y Moderate N/O Fresh frozen 20 M Y Slight N/O Abbreviations: F, female; M, male; N/O, no ossification within the region; Y, yes. * Presence of ossification within the region. Specimen examined bilaterally. In this study we found that 60% of the specimens in or beyond the sixth decade of life had partial ossification of the SI ligament. These ossification centers were found in the central area of the region of the SIJ complex. Ossification of the region fused the ilium and sacrum posteriorly, which can be extrapolated to suggest that no movement through the SIJ complex was possible in these specimens. With advancing age, the onetime syndesmosis had become a partial synostosis. In addition to finding frequent areas of ossification, we observed an extensive to moderate presence of ridges within the region of the joint in all 10 specimens (100%) aged 55 years and over. These changes were again more numerous centrally. Specimens demonstrating more extensive ridging of the region will likely present with reduced joint mobility by virtue of an increase of the coefficient of friction between opposing bony surfaces. This structural change to the region was well established in most of our specimens by the sixth decade of life, but likely begins much earlier. The smooth surface of the ilium and sacrum becomes more irregular with advancing age and can occur in conjunction with ossification of the SI ligament as demonstrated in this study. These structural changes occur gradually and may be part of a natural aging process. Interestingly, most of the degenerative changes seen in the region of the SIJ complex appeared to occur centrally. This may represent the region wherein there is the greatest concentration of compressive and/or shear forces through the SIJ complex. This degenerative enthesopathic process would in turn contribute to both osseous bridging and limited potential for mobility. Structural changes to the SIJ region that occur with aging have important implications for clinical practice and in particular the rationale for using various mobility tests in detecting joint dysfunction. The underlying assumption behind mobility testing for the SIJ is that this joint is capable of moving in vivo and that an alteration in normal mobility can be felt by the experienced clinician. Abnormal SIJ mobility may therefore manifest itself in the form of RESEARCH REPORT J Orthop Sports Phys Ther Volume 36 Number 4 April

7 movement impairment in the lumbosacral region of the spine, which in turn has been postulated as a potential substrate in LBP. Studies using intraarticular SIJ blocks have estimated the occurrence of SIJ pain in patients with chronic back pain below L5-S1 to be about 15% to 30%. 5,12 As diagnostic joint blocks are not readily available to most clinicians, the importance of having valid clinical tests to narrow the number of possible structures (eg, SIJ, lumbar spine, hip, etc) that may be responsible for a patient s LBP is imperative. This is particularly important when the results of such tests influence and in some cases determine the plan of care. For example, manipulation or mobilizations of the SIJ are commonly used when a painful joint is classified as hypomobile, while stabilization/motor control retraining is used when it is classified as hypermobile. Mobility tests for the SIJ have been found to be unreliable 14,15 and their regular use as diagnostic tools is questionable. 6,7,9,15,17 Several factors may help to account for their poor diagnostic utility in clinical practice. The SIJ has been shown to have very little mobility, 13,14 hence the 1 to 2 mm of movement that may occur, if at all, is likely to be difficult, if not impossible, for most clinicians to perceive. The current study demonstrated that the region of the SIJ complex was ossified in the majority (60%) of cases in subjects over 60 years of age. The presence of an bridge between the ilium and sacrum effectively fused these 2 bony regions together. Furthermore, ossification of the region can occur without the concomitant ossification of the articular portion of the SIJ complex. Hence, even if the articular portion of the SIJ was capable of demonstrating some small degree of movement, this might be nullified by the presence of ossification within the central region of the same joint complex. The presence of moderate to extensive ridging in the region of the SIJ complex in 100% of the specimens (aged 55 years FIGURE 5. (A and B) The articular and regions of the sacrum (S) and ilium (I) of an axially sectioned 56-year-old specimen in A is modeled in B. The region where the ligament has become ossified is circled. The articular part of the sacroiliac joint (SIJ) is located anteriorly (Ant) and is indicated with a bracket. The region lies posterior (Post) to the articular area. (C and D) The articular and regions of a 20-year-old male specimen. The axially sectioned specimen shown in C is modeled in D. Note the smooth surfaces with only slight ridging and no ossification. All abbreviations are as previously listed in A and B. 206 J Orthop Sports Phys Ther Volume 36 Number 4 April 2006

8 FIGURE 6. Disarticulated ilium of a 91-year-old male specimen. (A) Three-dimensional surface topography of the ilium showing both the and articular surfaces of the sacroiliac joint complex. Note the surface of the ilium is wedged shaped and is wider posteriorly and narrower anteriorly. The narrow furrow between the articular and regions is demonstrated by horizontal black lines. (B) Photograph of specimen modeled in 6A. or older) may be a natural occurrence of the aging process, helping to strengthen the union between the sacrum and ilium. These findings lend further evidence to support the contention that the SIJ has minimal or, perhaps in many cases, no capability for movement, especially in patients in or beyond their sixth decade. 20 The use of mobility tests in this age group should be viewed as dubious at best. The use of mobility tests alone to classify the SIJ as being either hypermobile or hypomobile should not be considered pathognomonic of SIJ dysfunction and thus necessitate any particular form of intervention. Hence, the process of differentiating between symptomatic and asymptomatic SIJs is best ascertained through the use of valid and reliable tests (for a specific population of patients) in conjunction with a structured examination and clinical-reasoning process. 3 Age-related changes to the SIJ articulation have been described by Sashin 12 and confirmed by others. 2,4,11,20 However, the changes described by these authors focus on the cartilaginous component, not the component. In the articular portion of the SIJ, changes seen are irregularity and thinning of the articular cartilage. This is followed by fibrillation, fibrous replacement, and fibrous ankylosis. 2,4,20 The results of this study cannot be compared with previous studies on SIJ morphology, as the region of the SIJ complex has not been previously examined. Furthermore, as with previous studies that have looked at the SIJ, this study s findings are limited by the small sample size and lack of specimens in the third, fourth, and fifth decades. The paucity of information available on the region of the SIJ, coupled with the novel preliminary findings of this paper, may be impetus for future studies in this area. Future studies should consist of a larger sample size and incorporate specimens over a much broader age range with equal gender distribution. CONCLUSION This study demonstrated that central region ossification of the SI ligament and the presence of ridges and depressions over the opposing surfaces of the sacrum and ilium are features that were common in specimens in or beyond their sixth decade. These findings are clinically relevant as they represent morphological features that further limit mobility through the SIJ complex. New insights into the complex surface architecture of the SIJ will encourage clinicians to more rigorously assess the result of mobility tests of the SIJ. Assessing mobility in the SIJ in the older population is not likely to yield any meaningful information. Clinicians should therefore exercise caution when interpreting the results of such tests. In addition, they might consider whether there is any merit in using them and furthermore question the legitimacy of employing commonly used therapeutic treatment techniques such as joint manipulation for the SIJ. Future studies should be undertaken to visualize and investigate the region of the SIJ in other age groups and with larger sample sizes. RESEARCH REPORT J Orthop Sports Phys Ther Volume 36 Number 4 April

9 ACKNOWLEDGMENTS The authors would like to thank William Wood, Terry Irvine, and Jerry Topham (Division of Anatomy, Department of Surgery, University of Toronto) for their technical assistance and Autodesk, Inc for the use of MAYA. REFERENCES 1. Agur A, Dalley A. Grant s Atlas of Anatomy. 11th ed. Baltimore, MD: Lippincott Williams & Wilkins; Bowen V, Cassidy JD. Macroscopic and microscopic anatomy of the sacroiliac joint from embryonic life until the eighth decade. Spine. 1981;6: Laslett M, Young SB, Aprill CN, McDonald B. Diagnosing painful sacroiliac joints: A validity study of a McKenzie evaluation and sacroiliac provocation tests. Aust J Physiother. 2003;49: Macdonald GR, Hunt TE. Sacroiliac joints; observations on the gross and histological changes in the various age groups. Can Med Assoc J. 1952;66: Maigne JY, Aivaliklis A, Pfefer F. Results of sacroiliac joint double block and value of sacroiliac pain provocation tests in 54 patients with low back pain. Spine. 1996;21: Meijne W, van Neerbos K, Aufdemkampe G, van der Wurff P. Intraexaminer and interexaminer reliability of the Gillet test. J Manipulative Physiol Ther. 1999;22: Potter NA, Rothstein JM. Intertester reliability for selected clinical tests of the sacroiliac joint. Phys Ther. 1985;65: Resnick D, Niwayama G, Goergen TG. Degenerative disease of the sacroiliac joint. Invest Radiol. 1975;10: Riddle DL, Freburger JK. Evaluation of the presence of sacroiliac joint region dysfunction using a combination of tests: a multicenter intertester reliability study. Phys Ther. 2002;82: Sashin D. A critical analysis of the anatomy and the pathological changes of the sacroiliac joints. J Bone Joint Surg. 1930;12: Schunke GB. The anatomy and development of the sacroiliac joint in man. Anat Rec. 1938;72: Schwarzer AC, Aprill CN, Bogduk N. The sacroiliac joint in chronic low back pain. Spine. 1995;20: Sturesson B, Selvik G, Uden A. Movements of the sacroiliac joints. A roentgen stereophotogrammetric analysis. Spine. 1989;14: Sturesson B, Uden A, Vleeming A. A radiostereometric analysis of movements of the sacroiliac joints during the standing hip flexion test. Spine. 2000;25: van der Wurff P, Hagmeijer RH, Meyne W. Clinical tests of the sacroiliac joint. A systemic methodological review. Part 1: reliability. Man Ther. 2000;5: van der Wurff P, Hagmeijer RH, Meyne W. Clinical tests of the sacroiliac joint. A systemic methodological review. Part 2: validity. Man Ther. 2000;5: Vincent-Smith B, Gibbons P. Inter-examiner and intraexaminer reliability of the standing flexion test. Man Ther. 1999;4: Vleeming A, Stoeckart R, Volkers AC, Snijders CJ. Relation between form and function in the sacroiliac joint. Part I: Clinical anatomical aspects. Spine. 1990;15: Vleeming A, Volkers AC, Snijders CJ, Stoeckart R. Relation between form and function in the sacroiliac joint. Part II: Biomechanical aspects. Spine. 1990;15: Walker JM. Age-related differences in the human sacroiliac joint: a histological study; implications for therapy. J Orthop Sports Phys Ther. 1986;7: Walker JM. The sacroiliac joint: a critical review. Phys Ther. 1992;72: Williams P. Gray s Anatomy. 38th ed. Edinburgh, UK: Churchill Livingstone; J Orthop Sports Phys Ther Volume 36 Number 4 April 2006