The following points require consideration in the biomechanics of pelvic fractures (401-406):

• The pelvis is a rigid ring structure. Instability implies fracture-dislocation at more than one site.

• The pelvis is subject to vertical shear and compressive forces. Compression is either anterior-posterior or lateral. Forces are inflicted directly on the pelvic wall or indirectly (e.g., loading the femur by knee contact resulting in hip dislocation).

• The stability of the pelvis is dependent on the integrity of the posterior weight-bearing sacroiliac complex and major ligaments. Classification of pelvic fractures is based on the stability of the posterior pelvis.

Three types of pelvic fractures are recognized:

Type A: A fracture that either does not involve the ring or is minimally displaced.

Type B: A fracture that is rotationally unstable but vertically stable (e.g., symphysis pubis separation, pubic ramus fracture). Sacral compression fractures are included if posterior stability is partly preserved.

Fig. 45. Industrial accident: arm pinned in machinery. Victim dead at scene; no other injuries. (A) Crushed arm. (B) Postmortem radiograph shows fracture of humerus. (Courtesy of Dr. M. Moussa, London Health Sciences Centre, London, Ontario, Canada.)

Type C: Rotational and vertical instability because of posterior pelvic instability (e.g., vertical fracture of the ilium, sacroiliac joint dislocation, vertical sacral fractures). Posterior pelvic ring trauma is almost always associated with anterior ring injury.

Motor vehicle impact is the most common cause of pelvic fracture (403,407). The incidence of pelvic fracture increases in near-side collisions because of contact with an intruding door (402,408). About one-fourth of automobile fatalities have pelvic fractures (401,409). The majority are type C fractures consistent with severe crashes (401,409). The consequent retroperitoneal hemorrhage contributes to the cause of death (403,407,410,411). Other injuries are frequent (401,403,406,407, 411-413).

Externally, there may be visible distension of the abdomen and thigh (404). Deformity of the pelvis may be seen or palpated. During examination of the pelvic cavity, the pathologist can palpate the pelvic ring, taking care to avoid glove puncture. If necessary, a postmortem radiograph is more accurate (401,402,404,409,414). Instead of the usual anterior-posterior projection, angling the X-ray beams 45° toward the feet and head allows better visualization of the pelvic inlet and sacrum, respectively (404).

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