Review Question - QID 219209

Despite involving less than 20% of the weightbearing surface of the acetabulum, this patient's posterior wall acetabular fracture exits superiorly and should be examined with a hip stability exam under anesthesia.

Posterior wall acetabular fractures represent the most commonly encountered elementary pattern of acetabular fracture. Much literature has been published regarding the surgical treatment of isolated posterior wall fractures, with early studies initially reporting that the size of the posterior wall acetabular fracture correlated with the stability, deeming those involving >40% of the weightbearing surface automatically unstable, those involving between 20 and 40% as indeterminate, and those involving <20% being thought of as generally stable and able to be treated with protected weightbearing. Several recent studies and case reports, however, have shown that fractures sized <20% can still be unstable, and contemporary practice includes a formal examination under anesthesia to determine whether instability is present, particularly when the cranial exit point of the fracture is seen on CT as exiting near the acetabular dome.

Firoozabadi et al. reviewed 185 consecutive patients with isolated unilateral posterior wall acetabular fractures to determine whether or not the radiographic parameters of femoral head coverage by the residual intact portion of the posterior wall, acetabular version, and location of the fracture or an associated history of dislocation were determinates of hip stability in patients with posterior wall acetabular fractures. The authors found that an examination under anesthesia (EUA) determined 116 hips to be stable and 22 hips as unstable, with 23% of the unstable hips having wall sizes less than 20% and an average cranial exit point of the fracture from the dome of 5.0 mm, compared to 9.5 mm in the stable group. They concluded that using wall sizes less than 20% is not a reliable indicator of stability, whereas the location of the exit point of the fracture in relation to the dome of the acetabulum is a radiographic marker that can be used to aid physicians in determining stability, with those extending into the dome demonstrating a statistically significant increase in hip instability.

Patel et al. reviewed the factors associated with instability of the hip joint after posterior wall acetabular fractures. The authors reviewed 68 patients and found that the proximity of the superior exit point of the fracture to the acetabular dome had a significant association with hip instability; however, subsequent multivariate logistic regression modeling revealed that none of these factors were significant independent risk factors. They concluded that EUA should remain the main clinical determinant of hip stability status, even in the presence of these radiographic predictors.

Figure A is an axial cut of a CT scan showing a small, isolated posterior wall acetabular fracture.

Incorrect Answers:
Answer 1: A bedside stability exam would not be an appropriate way to assess the stability of this posterior wall acetabular fracture, and should be performed with fluoroscopy under anesthesia.
Answer 2: Though the patient may need a definitive ORIF if unstable, this would be premature in this particular setting.
Answer 3: Though the superior gluteal neurovascular bundle can be commonly injured in the setting of posterior wall fractures, without signs of hemodynamic instability, an interventional radiology consultation would not be indicated.
Answer 5: A trial of protected weightbearing for 6-8 weeks can be done as definitive treatment for small, minimally displaced posterior wall fractures that are stable. This patient's fracture exits through the dome, portending instability, and should be tested with an EUA prior to initiating a trial of protected weightbearing if deemed stable in the OR.