Variation in the sacrum of phytosaurs: New evidence from a partial skeleton of Machaeroprosopus mccauleyi.

Phytosaurs are a group of Upper Triassic semi-aquatic archosauriform reptiles. Their variable skull morphology forms the foundation of our understanding of their relationships and paleoecology, while only a few studies have focused on demonstrating the existence of postcranial variation. The numbers of vertebrae in the sacrum are thought to vary from two, the plesiomorphic condition for archosauriforms, to three, with the addition of a sacralized dorsal (i.e., dorsosacral) vertebra. In this study, we demonstrate the presence of a sacralized first caudal (i.e., caudosacral) vertebra in a sacrum belonging to Machaeroprosopus mccauleyi. We rule out taphonomic distortion or pathology as explanations for the inclusion of this element in the sacrum, suggesting instead that it occurred through modifications of the same developmental processes that likely produced dorsosacral vertebrae in phytosaurs. Additionally, we show that a dorsosacral vertebra is common in phytosaur specimens from the Chinle Formation and Dockum Group of the southwestern United States and suggest that it may be widespread among phytosaurs. The addition of sacral vertebrae potentially aided adaptation to larger body sizes or more terrestrial lifestyles in certain taxa.

The influence of lumbosacral transitional vertebrae on lumbar lordosis and the angle of pelvic incidence.

Pelvic incidence and lumbar lordosis have only normative values for spines comprising five lumbar and five sacral vertebrae. However, it is unclear how pelvic incidence and lumbar lordosis are affected by the common segmentation anomalies at the lumbo-sacral border leading to lumbosacral transitional vertebrae, including lumbarisations and sacralisations. In lumbosacral transitional vertebrae it is not trivial to identify the correct vertebral endplates to measure pelvic incidence and lumbar lordosis because ontogenetically the first sacral vertebra represents the first non-mobile sacral segment in lumbarisations, but the second segment in sacralisations. We therefore assessed pelvic incidence and lumbar lordosis with respect to both of these vertebral endplates. The type of segmentation anomaly was differentiated using spinal counts, spatial relationship with the iliac crest and morphological features. We found significant differences in pelvic incidence and lumbar lordosis between lumbarisations, sacralisations and the control group. The pelvic incidence in the sacralised group was mostly below the range of the lubarisation group and the control group when measured the traditional way at the first non-mobile segment (30.2°). However, the ranges of the sacralisation and lubarisation groups were completely encompassed by the control group when measured at the ontogenetically true first sacral vertebra. The mean pelvic incidence of the sacraliation group thus increased from 30.2° to 58.6°, and the mean pelvic incidence of the total sample increased from 45.6° to 51.2°, making it statistically indistinguishable from the control sample, whose pelvic incidence was 50.2°. Our results demonstrate that it is crucial to differentiate sacralisations from lumbarisation in order to assess the reference vertebra for pelvic incidence measurement. Due to their significant impact on spino-pelvic parameters, lumbosacral transitional vertebrae should be evaluated separately when examining pelvic incidence and lumbar lordosis.

Auricular surface morphology and surface area does not influence subchondral bone density distribution in the dysfunctional sacroiliac joint.

The subchondral lamella of the sacroiliac auricular surface is morphologically inconsistent. Its morpho-mechanical relationship with dysfunction (SIJD) remains unstudied. Here, the iliac and sacral subchondral bone mineralization is compared between morphological subtypes and in large and small surfaces, in SIJD joints and controls. CT datasets from 29 patients with bilateral or unilateral SIJD were subjected to CT-osteoabsorptiometry. Surface areas and posterior angles were calculated and surfaces were classified by size: small (<15 cm3 ) and large (≥15 cm3 ), and morphological types: 1 (>160°), 2 (130°-160°), and 3 (<130°). Mineralization patterns were identified: two marginal (M1 and M2) and two non-marginal (N1 and N2). Each sacral and iliac surface was subsequently classified. Dysfunctional cohort area averaged 15.0 ± 2.4 cm2 (males 16.2 ± 2.5 cm2 , females 13.7 ± 1.6 cm2 ). No age correlations with surface area were found nor mean Hounsfield Unit differences when comparing sizes, sexes or morphology-type. Controls and dysfunctional cohort comparison revealed differences in female sacra (p = 0.02) and small sacra (p = 0.03). There was low-conformity in marginal and non-marginal patterns, 26% for contralateral non-dysfunctional joints, and 46% for dysfunctional joints. The majority of painful joints was of type 2 morphology (59%), equally distributed between small (49%) and large joints (51%). Larger joints had the highest frequency of dysfunctional joints (72%). Auricular surface morphology seems to have little impact on pain-related subchondral lamella adaptation in SIJD. Larger joints may be predisposed to the onset of pain due to the weakening of the extracapsular structures. Dysfunctional joints reflect common conformity patterns of sacral-apex mineralization with corresponding superior corner iliac mineralization.

Topographic localization of the sacroiliac joint and superior gluteal artery branches on the posterolateral ilium.

Latrogenic vascular injuries at the posterior ilium during sacroiliac screw placements are not uncommon. Though intra-operative imaging reduces the risk of such injuries, anatomical localization of the sacral segments using discrete topographical landmarks is not currently available. This descriptive study proposes the use of an anatomical grid system to localize the sacroiliac articulation on the posterolateral ilium. It also investigates the positional variability of the branches of the superior gluteal artery (SGA) within areas defined by the grid. 48 dried adult hip bones were examined to determine the position of the sacral articular surface on the posterolateral surface of the ilium. A novel grid-system was defined and used to map the positions of the articulation of the first two sacral segments on the posterolateral ilium. Superficial and deep branches of the SGA were dissected in donor cadavers and their courses were virtually overlayed on the grid system. The grid system localized the sacral articular surfaces within a defined area on the posterior ilium. Arterial distributions indicated the presence of the superficial branch of SGA more frequently over the screw insertion area (at an intermuscular plane), while the deep branch ran closer to the ilium but antero-inferior to the screw placement areas. This study proposes a new topographical perspective of visualizing SGA branches with respect to the cranial sacral segments. Precise localization of vascular anatomy may help to reduce potential risk of injury during sacroiliac screw placements.

An anatomical study defining the safe range of angles in percutaneous iliosacral and transsacral screw fixation.

Percutaneous iliosacral screw fixation and transsacral fixation are challenging procedures requiring extensive knowledge of sacral anatomy to avoid damaging nearby neurovascular structures. Greater knowledge of anatomical screw trajectory and size allowances would be helpful to guide surgical placement. An anatomical study of 40 cadaveric sacra in specimens ages 18-65 was performed. Three-dimensional surface scans were obtained, and computer modeling software was used to simulate a 7.3 mm diameter screw with 1 mm buffer inserted orthogonal to the sacroiliac joint in the pelvic inlet and outlet views. Transsacral screws were also inserted into S1 and S2 vertebrae. For screws orthogonal to the sacroiliac joint, the overall mean screw insertion angle was 4.1° ± 7.5° (range, -18.3° to 22.0°) in the inlet view in the posterior to anterior direction, and 21.7° ± 5.1° (range, 8.2°-36.3°) in the outlet view in the caudal to cranial direction. Before breaching the sacrum, the range of sacral tunnel lengths was between 31.1 and 70.1 mm with a range of diameters between 9.3 and 13.3 mm. Transsacral screws inserted into either the S1 or S2 vertebrae did not breach the sacrum in 40% (16/40) at each level. 30% (12/40) of sacra could not safely accommodate both S1 and S2 transsacral screws. There is an initial screw insertion angle range of -4° to 12° in the inlet view and 16°-27° in the outlet view. There was always adequate size to accept a 7.3 mm or larger screw.

Sacral co-ossification in dinosaurs: The oldest record of fused sacral vertebrae in Dinosauria and the diversity of sacral co-ossification patterns in the group.

The fusion of the sacrum occurs in the major dinosaur lineages, i.e. ornithischians, theropods, and sauropodomorphs, but it is unclear if this trait is a common ancestral condition, or if it evolved independently in each lineage, or even how or if it is related to ontogeny. In addition, the order in which the different structures of the sacrum are fused, as well as the causes that lead to this co-ossification, are poorly understood. Herein, we described the oldest record of fused sacral vertebrae within dinosaurs, based on two primordial sacral vertebrae from the Late Triassic of Candelária Sequence, southern Brazil. We used computed microtomography (micro-CT) to analyze the extent of vertebral fusion, which revealed that it occurred only between the centra. We also assessed the occurrence of sacral fusion in Dinosauria and close relatives. The degree of fusion observed in representatives of the major dinosaur lineages suggested that there may be a sequential pattern of fusion of the elements of the sacrum, more clearly observed in Sauropodomorpha. Our analyses suggest that primordial sacral vertebrae fuse earlier in the lineage (as seen in Norian sauropodomorphs). Intervertebral fusion is observed to encompass progressively more vertebral units as sauropodomorphs evolve, reaching up to five or more fully fused sacrals in Neosauropoda. Furthermore, the new specimen described here indicates that the fusion of sacral elements occurred early in the evolution of dinosaurs. Factors such as ontogeny and the increase in body size, combined with the incorporation of vertebrae to the sacrum may have a significant role in the process and in the variation of sacral fusion observed.

Imaging anatomy of the vertebral canal for trans-sacral hiatus puncture of the lumbar cistern.

A standard lumbar puncture may be impossible for many anatomic or technical reasons. Previous accounts of caudal epidural anesthesia and other procedures via the sacral hiatus prompted us to test if image-guided percutaneous trans-sacral hiatus access to the lumbosacral subarachnoid cistern would be anatomically feasible. To study vertebral canal morphometry and curvature, we analyzed midsagittal computed tomography-myelogram images of 40 normal subjects and digitally measured sacral curvatures between S1 to S5 and S2 to S4 using two methods whereby a lower angle signifies a straighter sacrum. We measured midsagittal vertebral canal area, hiatus width, dural sac termination levels, and distance from sacral hiatus to the dural sac tip (needle distance). Subjects were F:M = 25:15, with a mean age of 44.9 years. The two S1-S5 full sacral curvature mean angles were 57.3° and 60.4°. Almost all sacral hiatuses were at S4, and dural sac terminations were at S1-S2. The mean S2-S4 sacral curvature was 25.1°, and the mean needle distance was 57.7 mm. Using two-way analysis of variance, there were significant sex differences for needle distances (p = .001), and full and limited sacral curvatures (p = .02, and p = .046, respectively). There were no significant linear regression correlations between age and sacral curvature, needle distance, canal area, or hiatus width. Therefore, despite a frequently prominent full sacral curvature, the combination of S1-S2 dural sac termination plus a relatively straight trajectory of the lower vertebral canal between S2 and S4 support the theoretical feasibility of percutaneous trans-sacral hiatus and vertebral canal access to the lumbosacral cistern using a standard spinal needle.

Relationship of aortic bifurcation with sacropelvic anatomy: Application to anterior lumbar interbody fusion.

INTRODUCTION: Various sacropelvic parameters such as the pelvic Incidence (PI) are used to predict ideal lumbar lordosis and aid surgical planning. The objective of this study was to establish the relationship between the location of the aortic bifurcation from the sacral promontory and sacropelvic measures including the PI. MATERIALS AND METHODS: One hundred sixty five computed tomography (CT) scans obtained for major trauma including the entire spine were identified. Sacropelvic parameters including PI, sacral anatomic orientation, pelvic thickness (PTH), and sacral table angle were measured. Aortic bifurcation was identified on sagittal and coronal imaging and the distance from the sacral promontory (bifurcation-promontory distance [BPD]) measured (mm). RESULTS: Mean age of the cohort was 44.3 years (SD 18.5; range 16-88 years); 61.8% male. The mean PI was 49.2° (SD 10.2°; range 30°-80°). The mean BPD was 66.4 mm (SD 13.1 mm; range 38.3-100 mm). In the majority, the bifurcation was at the level of the L4 vertebral body (72.7%). Only age (r = -.389; p < .0001) and PTH (r = .172; p = .027) correlated with the BPD to a significant degree. PI did not correlate with BPD (r = .061; p = .435). Linear regression analysis provided the following predictive equation: BPD = 34.3 mm + 0.30 × PTH. CONCLUSION: This study demonstrates a lack of any meaningful correlation between sagittal pelvic parameters and the distance of the aortic bifurcation from the sacral promontory. Surgical planning for fusion surgery in the lumbar spine should include assessment of spinopelvic parameters and if anterior access to the lumbar disc(s) necessary, vascular anatomy should be carefully assessed independent of these measures.

Estimation of the relationship between the sacral hiatus and other dorsal sacral parameters using principle component analysis.

PURPOSE: Correct localization of the sacral hiatus is essential for administering a successful caudal epidural block. The purpose of this study is to create a statistical model of sacral hiatus from dorsal sacral parameters to improve the location of the hiatus and thus, reduce the failure rate. The aim of this investigation was to examine the relationship of sacral hiatus morphology and dimension with sacral curvature. This study further examines the dorsal sacral parameters that could affect the sacral hiatus dimension. METHODS: Adult, human, dry sacra and three-dimensionally reconstructed sacra from computed tomography imaging of normal subjects were included in the study and measured using digital Vernier calipers of 0.01 mm accuracy and Geomagic freeform plus software, respectively. RESULT: The most frequent shape of the sacral hiatus was an inverted V (48%) followed by inverted U shape (32%), an irregular shape (12.3%), an M shape (4.7) and an A shape (2.8%). The data were represented by mean and standard deviation. Sacra with M-shaped hiatus had the lowest hiatal length (14.21 ± 5.44 mm), whereas sacra with an inverted V-shaped hiatus had the highest length (25.41 ± 11.3 mm). The anteroposterior diameter of the sacral hiatus at the base in males and females was found to be 3.46 ± 1.48 mm and 2.79 ± 0.83 mm, respectively (P < 0.001). The distance between the caudal end of the median sacral crest and the apex of the sacral hiatus (7.90 ± 6.74 mm, 4.4 ± 5.86 mm) also revealed sexual dimorphism (P < 0.001). CONCLUSION: The correlations between most of the dorsal sacral parameters and length of the sacral hiatus are significant. The intercornual distance is also moderately correlated with the distance between right and left lateral sacral crest S1 level. Dorsal sacral parameters predicts variance of the sacral hiatus dimension from 40 to 73% and this could be utilized for statistical model of the sacral hiatus.

[Three-dimensional MRI reconstruction research on the anatomical relationship among uterosacral ligament and ureter or rectum in pelvic organ prolapse patients].

Objective: To study the anatomical relationship among uterosacral ligament and ureter or rectum by using MRI three-dimensional reconstruction model in pelvic organ prolapse (POP) patients. Methods: According to the research standard, 67 POP patients were enrolled, who accepted pelvic MRI before surgery in Nanfang Hospital, Southern Medical University during May 2015 to March 2020. Three-dimensional model of uterosacral ligament was reconstructed. The intersection point of the fitting curve of uterosacral ligament and ischial spine level marked point P0, every 1 cm increasing from P0 towards the sacrum marked points P1, P2, and P3. Distances were measured between rectum or ureter to uterosacral ligament respectively at the P0-P3 horizontal levels. Results: (1) The distances between the left ureter and the left uterosacral ligament were (15.45±7.46) to (19.31±11.38) mm, and the distances between the right ureter and the right uterosacral ligament were (13.77±8.16) to (14.78±9.18) mm. At the P1 horizontal level ureters were the closest to uterosacral ligaments, and the right ureter was the closest to right uterosacral ligament [(13.45±9.34) mm] at P2 horizontal level in severe POP group. The farthest distance presented at the P3 horizontal level between bilateral ureters and uterosacral ligaments. (2) At the P0 horizontal level, the rectum was the closest to the bilateral uterosacral ligaments [left: (20.62±9.99) mm, right: (16.82±9.63) mm; P=0.026], and the rectum was closer to the right uterosacral ligament. There were no significant differences in the distance between rectum and bilateral uterosacral ligaments in mild POP group (P>0.05), and the results of severe POP group also showed the rectum was closer to the right uterosacral ligament [(15.64±10.31) mm at P0 horizontal level]. Conclusions: Right ureter and rectum are closer to the right uterosacral ligament. Gynecologists should pay more attention to avoid damaging the right ureter and rectum during the operation of the right uterosacral ligament in POP patients.

Morphological evolution of the carnivoran sacrum.

The sacrum is a key piece of the vertebrate skeleton, since it connects the caudal region with the presacral region of the vertebral column and the hind limbs through the pelvis. Therefore, understanding its form and function is of great relevance in vertebrate ecomorphology. However, it is striking that morphometric studies that quantify its morphological evolution in relation to function are scarce. The main goal of this study is to investigate the morphological evolution of the sacrum in relation to its function in the mammalian order Carnivora, using three-dimensional (3D) geometric morphometrics. Principal component analysis under a phylogenetic background indicated that changes in sacrum morphology are mainly focused on the joint areas where it articulates with other parts of the skeleton allowing resistance to stress at these joints caused by increasing muscle loadings. In addition, we demonstrated that sacrum morphology is related to both the length of the tail relativised to the length of the body, and the length of the body relativised to body mass. We conclude that the sacrum in carnivores has evolved in response to the locomotor requirements of the species analysed, but in locomotion, each family has followed alternative morphological solutions to address the same functional demands.

Histology and pneumaticity of Aoniraptor libertatem (Dinosauria, Theropoda), an enigmatic mid-sized megaraptoran from Patagonia.

Aoniraptor libertatem is a mid-sized megaraptoran that comes from the Late Cretaceous (Turonian) Huincul Formation at Río Negro province, Patagonia, Argentina. In this study, we conducted a detailed analysis of pneumaticity of the sacrum and tail of Aoniraptor. This shows a complex structure within these vertebrae, being composed by small diverticulae surrounding large pneumatic canals and a central chamber that opens outside through pleurocoels or pneumatic canals. Further, we carried out a histologic analysis which confirms the pneumatic nature of these anatomical features. Both analyses found that chevrons in Aoniraptor were invaded by pneumaticity, a feature that appears to be unique to this taxon. In addition, a comparative analysis between Aoniraptor and other theropods (e.g. Gualicho and other megaraptorans) was carried out. This resulted in the modification of previous schemes about the evolution of pneumaticity through Theropoda, the finding of some evolutionary pneumatic traits through Megaraptora, and the usefulness of pneumatic traits as a taxonomic tool.

The iliosacral joint in lizards: an osteological and histological analysis.

The development of the iliosacral joint (ISJ) in tetrapods represented a crucial step in the evolution of terrestrial locomotion. This structure is responsible for transferring forces between the vertebral column and appendicular skeleton, thus supporting the bodyweight on land. However, most research dealing with the water-to-land transition and biomechanical studies in general has focused exclusively on the articulation between the pelvic girdle and femur. Our knowledge about the contact between the pelvic girdle and vertebral column (i.e. the ISJ) at a tissue level is restricted so far to human anatomy, with little to no information available on other tetrapods. This lack of data limits our understanding of the development and evolution of such a key structure, and thus on the pattern and processes of the evolution of terrestrial locomotion. Therefore, we investigated the macro- and microanatomy of the ISJ in limb-bearing squamates that, similar to most non-mammalian, non-avian tetrapods, possess only two sacral ribs articulating with the posterior process of the ilium. Using a combination of osteology, micro-computed tomography and histology, we collected data on the ISJ apparatus of numerous specimens, sampling different taxa and different ontogenetic stages. Osteologically, we recorded consistent variability in all three processes of the ilium (preacetabular, supracetabular and posterior) and sacral ribs that correlate with posture and locomotion. The presence of a cavity between the ilium and sacral ribs, abundant articular cartilage and fibrocartilage, and a surrounding membrane of dense fibrous connective tissue allowed us to define this contact as a synovial joint. By comparison, the two sacral ribs are connected to each other mostly by dense fibrous tissue, with some cartilage found more distally along the margins of the two ribs, defining this joint as a combination of a syndesmosis and synchondrosis. Considering the intermediary position of the ISJ between the axial and appendicular skeletons, the shape of the articular surfaces of the sacral ribs and ilium, and the characteristics of the muscles associated with this structure, we argue that the mobility of the ISJ is primarily driven by the movements of the hindlimb during locomotion. We hypothesize that limited torsion of the ilium at the ISJ happens when the hip is abducted, and the joint is likely able to absorb the compressional and extensional forces related to the protraction and retraction of the femur. The mix of fibres and cartilage between the two sacral ribs instead serves primarily as a shock absorber, with the potential for limited vertical translation during locomotion.

Great apes and humans evolved from a long-backed ancestor.

There is current debate whether the Homo/Pan last common ancestor (LCA) had a short, stiff lumbar column like great apes or a longer, flexible column observed in generalized Miocene hominoids. Beyond having only four segments, three additional features contribute to lumbar stiffening: the position of the transitional vertebra (TV), orientation of the lumbar spinous processes, and entrapment of lumbar vertebrae between the iliac blades. For great apes, these features would be homologous if inherited from a short-backed LCA but likely functionally convergent through dissimilar phenotypes if evolved from a long-backed LCA. We quantitatively and qualitatively analyzed human, ape, and monkey thoracic and lumbar vertebrae using 3D surface scanning and osteological measurements to compare spinous process morphology and sacral depth. We also used a large sample of hominoid vertebral counts to assess variation in the position of the TV and lumbosacral boundary. All extant hominoids modally place the TV at the ultimate thoracic. However, humans and orangutans place the TV at the 19th postcranial vertebral segment, whereas other apes place the TV at the 20th. Furthermore, chimpanzees, gorillas, and orangutans each have distinct patterns of spinous process angulation and morphology associated with lumbar stiffening, while human spinous process morphology is similar to that of longer backed gibbons, monkeys, and Miocene hominoids Morotopithecus and Pierolapithecus. Finally, chimpanzees are unique compared with other hominoids with a greater sacral depth facilitating lumbar entrapment, and there are differences among African apes with respect to the mechanisms governing variation in the lumbosacral boundary. These differences suggest that lumbar stiffening is convergent among great apes and that human bipedalism evolved from a more generalized long-backed ancestor. Such a model is more consistent with evidence of TV placement in Australopithecus.

The association between sacral morphology and sacroiliac joint conformity demonstrated on CT-based bone models.

INTRODUCTION: Knowledge of the stress distribution on structures around the sacroiliac joint (SIJ) is required to treat or prevent SIJ disorders. The purpose of this study was to reveal the association between sacral morphology and SIJ conformity. MATERIALS AND METHODS: This cross-sectional study included 11 adult patients with unilateral SIJ pain who underwent computed tomography (CT) imaging of the pelvis. Bony coordinate systems for the sacrum and innominates were embedded using anatomical landmarks. Local coordinate systems for the auricular surfaces of the sacrum and innominate were also defined. Conformity of the SIJ was quantified by the offset of the coordinate systems between the auricular surfaces of the sacrum and innominate. Repeated measure ANOVA and multiple regression analysis were used for statistical analyses. RESULTS: There were large variations across subjects in sacral morphology and SIJ conformity. There were no differences in any of the six degrees-of-freedom positions or orientations of the bilateral sacral and innominate auricular surfaces between the symptomatic and asymptomatic sides. The SIJ downward rotation on the asymptomatic and symptomatic sides were 0.0 [-1.0, 1.1]° and 2.1 [1.2, 3.0]°, respectively. Smaller downward rotation of the sacral auricular surface based on the sacral bony coordinate system had significant association with the greater SIJ downward rotation (standard partial regression coefficient: -.44, p = .043). CONCLUSIONS: The results indicate that the morphology of the sacrum is associated with poor SIJ conformity and that separation of the superior portion of the SIJ can be a risk factor for SIJ pain.

Analysis of S1 DRG Programming to Determine Location of the DRG and Ideal Anatomic Positioning of the Electrode.

INTRODUCTION: Dorsal root ganglion (DRG) stimulation has been established as a therapy in the treatment of chronic pain. Ideal electrode placement is guided by proper identification of the location of the DRG. The location of the S1 DRG is not well delineated and can be variable making ideal location of the electrode placement difficult based on fluoroscopic imaging. METHODS: This is a retrospective analysis of postoperative programming of S1 DRG leading across two centers. There were 34 lead placements in 24 patients included in this study. Programming parameters and contacts used were evaluated based on the position of the electrode in reference to the sacral border. RESULTS: The majority of the patient programming parameters were recorded at six weeks following the implant. Most commonly, the programming used a simple continuous bipole configuration. Of the 34 leads programmed, 17 (50%) had programming on the sacral border, 14 (41%) were considered posterior, and 3 (9%) were anterior to the sacral border. CONCLUSION: This analysis of S1 DRG programming demonstrates that ideal positioning of the majority of the contacts for the electrode should be posterior and along the sacral border on fluoroscopic imaging. These findings also suggest that the S1 DRG may be located most reproducibly at the border of the intraforaminal and intracanalicular region.

The sacral screw placement depending on morphological and anatomical peculiarities.

PURPOSE: Various pathologies of the lumbosacral junction require fusion of the L5/S1 segment. However, pseudarthroses, which often come along with sacral screw loosening, are problematic. The aim of the present investigation was to elaborate the morphological features of the L5/S1 segment to define a so-called "safe zone" for bi- or tricortical screw placement without risking a damage of the iliac vessels. METHODS: A total of one hundred computed tomographies of the pelvis were included in this investigation. On axial and sagittal slices, pedicle morphologies, the prevertebral position of the iliac vessels, the spinal canal and the area with the largest bone density were analyzed. RESULTS: Beginning from the entry point of S1-srews iliac vessels were located at an average angle of 7° convergence, the spinal canal at 38°. Bone density was significantly higher centrally with a mean value of 276 Hounsfield Units compared to the area of the Ala ossis sacri. The largest intraosseous screw length could be achieved at an angle of 25°. The average pedicle width was 20 mm and the pedicle height 13 mm. CONCLUSIONS: A "safe-zone" for bicortical screw placement at S1 with regard to the course of the iliac vessels could be defined between 7° and 38° convergence. Regarding the area offering the largest bone density and the maximal possible screw length, a convergence of 25° is recommended at S1 to reduce the incidence of screw loosening. Screw diameter, as a further influence factor on screw holding, is limited by pedicle height not pedicle width.

Physiological in vitro sacroiliac joint motion: a study on three-dimensional posterior pelvic ring kinematics.

The sacroiliac joint (SIJ) is a well-known source of low back and pelvic pain, of increasing interest for both conservative and surgical treatment. Alterations in the kinematics of the pelvis have been hypothesized as a major cause of SIJ-related pain. However, definitions of both the range and the extent of physiological movement are controversial, and there are no clear baseline data for pathological alterations. The present study combined a novel biomechanical setup allowing for physiological motion of the lumbosacral transition and pelvis without restricting the SIJ movement in vitro, combined with optical image correlation. Six fresh human pelvises (81 ± 10 years, three females, three males) were tested, with bodyweight-adapted loading applied to the fifth lumbar vertebra and both acetabula. Deformation at the lumbopelvises was determined computationally from three-dimensional image correlation data. Sacroiliac joint motion under the loading of 100% bodyweight primarily consisted of a z-axis rotation (0.16°) and an inferior translation of the sacrum relative to the ilium (0.32 mm). Sacroiliac joint flexion-extension rotations were minute (< 0.02°). Corresponding movements of the SIJ were found at the lumbosacral transition, with an anterior translation of L5 relative to the sacrum of -0.97 mm and an inferior translation of 0.11 mm, respectively. Moreover, a flexion of 1.82° was observed at the lumbosacral transition. Within the innominate bone and at the pubic symphysis, small complementary rotations were seen around a vertical axis, accounting for -0.10° and 0.11°, respectively. Other motions were minute and accompanied by large interindividual variation. The present study provides evidence of different SIJ motions than reported previously when exerted by physiological loading. Sacroiliac joint kinematics were in the sub-degree and sub-millimeter range, in line with previous in vivo and in vitro findings, largely limited to the sagittal rotation and an inferior translation of the sacrum relative to the ilium. This given physiological loading scenario underlines the relevance of the lumbosacral transition when considering the overall motion of the lumbopelvis, and how relatively little the other segments contribute to overall motion.

3D statistical model of the pelvic ring - a CT-based statistical evaluation of anatomical variation.

The pelvic ring is a highly complex construct with a central role for human stability and mobility. The observable interindividual differences in skeletal anatomy are caused by anatomical variation in the innominate bones as well as the sacrum, further to differences in the spatial arrangement of these bones to each other. The aim of this study was to generate a 3D statistical model of the entire pelvic ring in order to analyse the observed interindividual differences and anatomical variation. A series of 50 anonymized pelvic CT scans of uninjured Japanese adults [30 males, 20 females, average age of 74.9 years, standard deviation (SD) 16.9 years] were processed and analysed, resulting in a 3D statistical overall mean model and separate male and female mean models. Principal component analysis (PCA) of the overall statistical model predominantly showed size variation (20.39%) followed by shape variation (14.13%), and a variation of the spatial arrangement of the sacrum to the innominate bones in different anatomical peculiarities (11.39 and 8.85%). In addition, selected internal and external pelvic parameters were manually measured with the objective of further evaluating and quantifying the observed interindividual as well as the known sex-specific differences. A separate statistical model of the grey value distribution based on the given Hounsfield unit (HU) values was calculated for assessing bone mass distribution, thus an indication of bone quality utilizing grey values as a quantitative description of radiodensity was obtained. A consistent pattern of grey value distribution was shown, with the highest grey values observed between the sacro-iliac joint and the acetabulum along the pelvic brim. Low values were present in the sacral ala, in the area of the iliac fossa as well as in the pubic rami next to the symphysis. The present model allows a differentiated analysis of the observed interindividual variation of the pelvic ring and an evaluation of the grey value distribution therein. Besides providing a better understanding of anatomical variation, this model could be also used as a helpful tool for educational purposes, preoperative planning and implant design.

Variation in bony landmarks and predictors of success with sacral neuromodulation.

INTRODUCTION AND HYPOTHESIS: We assessed variations in sacral anatomy and lead placement as predictors of sacral neuromodulation (SNM) success. Based solely on bony landmarks, we also assessed the accuracy of the 9 and 2 protocol for locating S3. METHODS: This is a retrospective cohort study performed from October 2008 to December 2016 at the University of North Carolina at Chapel Hill. Fluoroscopic images were used to assess sacral anatomy and lead location. Success was defined as >50% symptom improvement after stage I and clinical response at most recent follow-up. RESULTS: Of 249 procedures, 209 were primary implants and 40 were revisions among 187 (89.5%) women and 22 (10.5%) men. Success rate was 83.3% for primary implants and 89.4% for revisions. Success was associated with shorter implant duration (21.3 ± 22.2 vs 33.6 ± 25.8 months), higher body mass index (30.3 ± 7.8 vs 27.6 ± 6.1 kg/m2), and straight vs curved lead (90.5% vs 80.5%) (all p = .05), but not with sacral anatomy or lead placement. In assessing the 9 and 2 protocol, mean distance from coccyx to S3 did not equal 9 cm: 7.4 ± 1.0 vs 7.2 ± 0.8 cm (p = .26), while mean distance from midline to S3 did equal 2 cm: 1.9 ± 0.4 vs 2.0 ± 0.7 cm (p = .37). CONCLUSIONS: Variations in sacral anatomy and lead placement did not predict SNM success. The 2-cm protocol was verified while the 9-cm protocol was not, although neither was predictive of success, which may obviate the need to mark bony landmarks prior to fluoroscopy.