Surgical Approaches for Sacral Fractures and Sacropelvic Injuries

Anatomy

Lumbosacral, lumbosacropelvic (LSP), and sacropelvic fractures are a group of fractures involving the sacrum, occurring at the transition zone between the lumbar spine and pelvis. The sacrum acts as the nucleus through which mechanical loads are transferred between the upper, axial skeleton and lower, appendicular skeleton. Unique anatomical features of the sacral spine enable it to perform this function. Descending from the thoracolumbar spine to the pelvis, several anatomical changes occur: the vertebral bodies become much larger to withstand higher axial compressive forces, and the facet joints transition from a coronal to an oblique to a sagittal orientation, creating more translational mobility. Moreover, the paraspinal musculature and ligaments become more robust to promote stability in the setting of higher mechanical forces. These paraxial reinforcements help to stabilize the mobile lumbar spine as it transitions to a rigid sacrum and pelvis. Furthermore, the musculoligamentous complex also must accommodate a significant change in the alignment of the spine. Over these few motion segments, the spine transitions from a mobile and lordotic morphology to a rigid and kyphotic sacrum. The sacrum is then fixed to the pelvis through the more lateral sacroiliac joints with dorsal and ventral ligamentous attachments, as well as via sacrotuberous and sacrospinous ligaments.

These sacroiliac ligaments connect the sacrum to the pelvis at a relatively fixed incline or angle. This angle is referred to as the pelvic incidence (PI), which is the angle formed between two lines: one extending between the bicoxofemoral axis (center point of the overlap of the bilateral femoral heads) and middle of S1 superior end plate and the other extending down, perpendicular to the middle of the S1 superior end plate. Under physiological conditions, the sacrum and pelvis rotate in tandem around the hip joints, compensating and adjusting for changes in mechanical loading of the axial skeleton above and/or appendicular skeleton below to maintain upright posture. These adjustments in sacropelvic positioning occur by changing the pelvic tilt (PT; angle formed between a line extending between the bicoxofemoral axis and middle of S1 superior end plate and another line extending vertically upward from the bicoxofemoral axis) and the sacral slope (SS; angle formed between a line extending parallel to the S1 superior end plate and another line extending horizontally from the posterosuperior corner of the S1 superior end plate). PI is thus a fixed, innate, patient-specific parameter, whereas PT and SS change to help maintain erect posture. The three values are related mathematically: PI = PT + SS. Awareness regarding the importance of these spinopelvic parameters has increased more recently with the increasing study of the biomechanical interplay between the spine and pelvis in pathological conditions such as lumbar spondylosis and its deformity correction, as well as in the setting of spinopelvic trauma and its management.

Together, the sacrum and its sacropelvic connections are among the strongest joints in the body and impart significant stability to the pelvis, making up its posterior or dorsal ring. ^, In fact, biomechanical studies have shown that removal of the sacrum distal to S1 results in a 50% decrease in pelvic stability; moreover, the sacrum plays no role in stability or ambulation distal to S2. As such, trauma resulting in sacropelvic fractures (or spinopelvic dissociation if severe enough to impart instability between the lumbosacral spine and pelvis) is relatively uncommon. Because of the inherent stability of the area, either significant force or intrinsic weakening of the bone (as seen in pathological/fragility fractures) is required to result in this pattern of injury. In this chapter, we discuss sacral fractures and spinopelvic dissociation, including their epidemiology, clinical presentation, and diagnosis, with an emphasis on fracture pattern classification schemes, surgical management, and outcomes.