The pelvis is a complex, three-dimensional ring structure with many irregular topographic surfaces that are not clearly defined by tangential imaging.
The bony pelvis is composed of the right and left ilium, which articulate posteriorly with the sacrum (sacroiliac/SI joint) and become confluent with the right and left pubic bones for the anterior symphyseal joint.
Pelvic ring fracture patterns and/or symphyseal and SI joint dislocations are dependent upon the mechanism of injury, the imparted force, the direction of that force, and the degree of bony resiliency (with a spectrum from pediatric to geriatric patients).
The anterior symphyseal joint between left and right pubic bodies is fairly linear and easily assessed for abnormal widening (>2 cm) on standard AP pelvic imaging.
The posterior sacroiliac joints have greater anatomic variability with an oblique orientation to the coronal, sagittal, and axial planes. Standard AP imaging offers some diagnostic clues to widening or displacement, but specialized views (i.e., inlet, outlet, and obturator-inlet) better delineate this joint.
Sacral dysmorphism of the superior sacral segment may be present in up to 40% of cases with the following radiographic markers: (1) upper sacral segment collinear with the level of the iliac crests (AP and outlet views), (2) irregular, noncircular superior sacral segment foramina (outlet view), (3) underdeveloped transverse processes (a.k.a. mammillary bodies) (AP and outlet views), and (4) residual upper sacral segment disks (AP and outlet views) and tongue-in-groove SI joint contour as seen on CT imaging.1
The ilium is most commonly used for external fixation and pelvic INFIX frames. Traditionally, iliac crest fixation occurs 3 to 4 cm posterior to the ASIS at the gluteus medius pillar with pins aimed between the iliac inner and outer tables. Supra-acetabular frame or INFIX fixation utilizes a large corridor of bone beginning anterolaterally at the AIIS and traversing posteromedially toward the PSIS. Due to its location near the equator of the bony pelvis, supra-acetabular fixation constructs yield the greatest power and control of hemipelvic displacements.
The AP pelvic view is an excellent initial diagnostic tool for the assessment of acute trauma patients with a suspected pelvic ring injury. Note that static images with pelvic sheeting or binders may misrepresent the actual degree of pelvic ring instability (Fig. 16-1).
Although an excellent initial diagnostic tool, the AP pelvic view alone is insufficient to accurately assess the complex anatomy. A refined knowledge of specific pelvic radiographic views is imperative to assure that adequate reduction and safe instrumentation have been achieved.
“Open book” (APC) injuries will display various degrees of symphyseal widening (with or without rami fractures), as well as posterior pelvic sacroiliac widening/dislocation or sacral fractures (Fig. 16-2).
Lateral compression injuries may show laterality of ramus and sacral fractures with degrees of internal rotation and occasionally contralateral hemipelvic windswept deformities (Fig. 16-3).
Vertical shear injuries display affected hemipelvic vertical instability of the anterior and posterior pelvis structures (Fig. 16-4).
Although traditional techniques describe a 45-degree inlet view, posterior pelvic screening assessment in the average patient is generally best illustrated with a 20-degree cranial-to-caudal tilt.2 The actual degree of necessary intraoperative fluoroscopy angle is dependent upon patient positioning, individual natural pelvic obliquity, or posterior pelvic displacement from injury but can be estimated from the sacral sagittal CT (Fig. 16-5).3
Posterior pelvic sacroiliac dislocation/widening and crescent fractures are well visualized with this view, including the degree of anterior or posterior displacement of the SI joint. Anterior pelvic translation and rotational (internal or external) deformities are also well visualized (Figs. 16-6 and 16-7).
A hyperinlet (>45 degree) of the anterior pelvis can display the degree of symphyseal or ramus fracture displacement, widening, and malrotation (internal or external). Increasing inlet tilt is performed until the superior and inferior rami overlap. Postreduction inlet images should demonstrate an acceptable posterior and anterior reduction (Figs. 16-8 and 16-9).
Although traditional radiographic techniques describe a 45-degree outlet view, screening radiographs for posterior pelvic injury are generally best illustrated with a 55-degree caudal-to-cranial tilt in the average patient (Fig. 16-10).2
Craniocaudal hemipelvic displacement and flexion deformities of the anterior and posterior pelvis are well visualized on the outlet view. Again, the actual degree of necessary intraoperative fluoroscopy angle is dependent upon patient positioning, individual natural pelvic obliquity, or posterior pelvic displacement from injury but can be estimated from the lateral sacral CT (Figs. 16-11 and 16-12).3