(E-191) Analysis of Fracture Mechanics of the Sacrum in Cadaveric Pelvises

Introduction:: Sacral injuries are rare and often difficult to diagnose in the clinical setting due to heterogeneous presentations. Fractures of the sacrum are not only dangerous, their treatment poses higher levels of difficulty for fixation and postoperative management. Little information has been gathered to date regarding its mechanism of injury, except for the fact that these injuries rarely occur in isolation, with only 5% of sacral fracture incidents occurring as isolated incidences. As the largest axial joint in the body, the sacroiliac joint is formed by the outer wings of the sacrum and the ilium and is held together by a large network of ligaments. Like other joints in the lower body, the SI joint absorbs shock for the spine as the sacrum transmits axial load from the spine to the lower limbs, and as such, it can be damaged from injuries that involve both the lower extremities and spine. Currently, minimal research is available to detail the mechanisms of injury for the sacrum compared to the SI joint, and this study aims to analyze the fracture mechanics of the sacrum using cadaveric sacra and pelvises. Data obtained from this study would be useful not only in the fields of fracture and injury mechanics but also the medical field as it would provide insight on fixation methods for native biomechanical stabilization of the pelvis in event of traumatic injury.


Materials and Methods:: Cadaver specimens were provided through the United Tissue Network. Five cadaver sacra and two pelvises were obtained, and the distance between the anterior superior iliac spines, anterior inferior iliac spines, iliac crests, longitudinal diameter of the pelvic inlet from posterior to anterior borders, and transverse diameter of the pelvic inlet from lateral border to lateral border as well as the pelvis height for the whole pelvises and the width of the sacral ala, distance between superior articular processes to the coccygeal apex, and the average interforaminal distance, all in centimeters, were manually measured. Soft tissue dissection was performed to ensure visibility of bony structures, and specimens were set in molds so the medial sacral crest at S3-S4 laid at 30 degrees from the vertical. Epoxy-hardener mixture was used to immobilize the specimens, and they were secured to the MTS Bionix Servohydraulic Testing System using C-clamps after curing for a minimum of 1 hour. A tile was attached to the vertical impact mechanism in order to simulate a fall from a height onto a hard tiled surface. The vertical impact mechanism was set to lie 10 cm above the surface of the epoxy, and loading rates were 1 mm/s for all samples. Data collected from the MTS system were analyzed using MATLAB and JMP Pro 16, and the fractures generated were characterized according to the AO, Denis, and Isler classification systems as well as the morphologic injury patterns should the fracture pertain to zone three.


Results, Conclusions, and Discussions:: All samples sustained fractures during simulation using a loading rate of 1 mm/s. Fractures were all incomplete with varying degrees of displacement for both the whole pelvises and sacra. Though all were incomplete fractures, these ranged from unilateral to bilateral and displaced to nondisplaced. Paired t-test results determined that the difference between the force required to fracture the sacrum in isolation and in conjunction with the SI joint was statistically significant. Analysis of variance test results conducted on all specimens determined that sexual dimorphism did not play a role in the average or maximum axial load sustained; however, sacrum size was found to play a role in maximum force prior to failure when disregarding sex. Maximum axial load sustained was found to be significantly reduced when comparing specimens with osteopenia in isolation with those that had osteopenia in conjunction with cardiovascular diseases or additional health concerns. Additionally, it was determined that samples with normal bone density and cardiovascular concerns were able to endure more axial force than those with solely osteopenia. Specimens in which the sacrum remained anatomically intact with the pelvis sustained significantly less force prior to failure whereas those that were no longer intact endured larger axial force. This is similar to known principles in injury mechanics that ligaments and other supporting structures fail prior to bone fracture. Regarding fractures present, the presence of the pelvis elicited fracture patterns that were less severe than those present in isolated sacra, and singular fracture types were present in the pelvises whereas isolated sacra presented with multiple fractures. The study concluded that from a vertical impact on the sacrum, coupled bending moments were generated, resulting in multiple fracture patterns. In future research, the accuracy in measuring angle orientation for each specimen prior to testing should be reevaluated. Deviations in angle orientation could cause inconsistency in fracture patterns and data analysis, as the axial loading would be applied in different areas of the sacrum or coccyx. Additionally, an increase in sample size and the consideration of bone mineral density would allow for more analysis and thus allow for accurate conclusions.

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