Christiaan N. Mamczak

Bony Anatomy

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.

Radiographic Anatomy

AP View

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).

Gardner1e-ch016-image001 — Figure 16-1 A screening AP pelvic radiograph demonstrates symphyseal widening and concern for a left hemipelvic rotational deformity. The backboard buckle obscures the posterior pelvic anatomy. Note that the right hip is posteriorly dislocated.

Gardner1e-ch016-image002 — Figure 16-2 This trauma AP radiograph depicts significant widening of the anterior and posterior pelvis (open book injury). This patient is at risk for hemodynamic shock and requires emergent reduction of the pelvic ring.

Gardner1e-ch016-image003 — Figure 16-3 This AP pelvic radiograph demonstrates a lateral compression injury to the right hemipelvis with considerable internal rotation deformity. Note the difference in obturator ring projections.

Gardner1e-ch016-image004 — Figure 16-4 This AP pelvic view shows a combined pelvic ring and acetabular injury pattern globally disrupting the normal anatomy: the left hemipelvis has a vertical translation with complete anterior and posterior instability. This patient requires reduction of the dislocated right hip, left lower extremity skeletal traction and a binder.

Inlet View

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).

Gardner1e-ch016-image005 — Figure 16-5 Clinical demonstration of a pelvic inlet view with the patient in a supine position.

Gardner1e-ch016-image006 — Figure 16-6 This pelvic inlet view depicts left hemipelvic posterior translation and anterior symphyseal widening. External fixation has dramatically improved the ring anatomy from the original deformity (Fig. 16-4).

Gardner1e-ch016-image007 — Figure 16-7 This inlet fluoro view demonstrates the anterior displacement of this left zone 2 sacral fracture relative to the right hemipelvis.

Gardner1e-ch016-image008 — Figure 16-8 This inlet fluoro view demonstrates improved reduction of the anterior pelvic widening. Bilateral pubic root fractures with symphyseal disruption represent a challenging injury pattern.

Gardner1e-ch016-image009 — Figure 16-9 This hyperinlet fluoro view confirms adequate reduction of the anterior symphysis in a ring injury where a pelvic INFIX was utilized.

Outlet View

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