Fri., 10/17/08 Pelvis/Injury Prevention, Paper #29, 11:50 am OTA-2008
Computed Tomography Assessment of Hip Instability in Posterior Wall Acetabular Fractures
Berton R. Moed, MD (n); David Ajibade, MD (n); Heidi Israel, PhD (n);
Saint Louis University School of Medicine, Department of Orthopaedic Surgery,
St. Louis, Missouri, USA
Purpose: Studies using two-dimensional CT-derived criteria indicate that, in general, posterior wall fractures involving less than 20% of the posterior wall are stable and able to withstand physiologic loads, whereas those involving greater than 40% to 50% are unstable, leaving a wide range of posterior wall fractures classified as indeterminate. The purpose of this study was to assess the ability of static measurement of posterior acetabular wall fragment size using CT to predict to predict dynamic hip stability status, as determined by dynamic stress examination under anesthesia.
Methods: Three methods were used in a blinded fashion to calculate posterior wall fracture fragment size using two-dimensional computed tomograms in 33 patients with isolated posterior wall acetabular fractures. These methods include those previously described by Calkins et al, which measures the smallest amount of intact acetabular arc and Keith et al, which measures fragment size at the level of the fovea, as well as the senior author’s technique (a modification of Keith et al) using the level of largest posterior wall deficit. Each method classifies hip instability into three groups: group I, stable; group II, indeterminate; and group III, unstable. All patients underwent dynamic fluoroscopic stress testing under general anesthesia (examination under anesthesia [EUA]) to determine actual hip stability status, which served as the gold standard.
Results: EUA showed 18 stable hips and 15 unstable. The analyses showed that the methods of Calkins et al and Keith et al had a substantial percentage of incorrect predictions, especially in the critical group I patients (those predicted to be stable, but actually unstable). The percent incorrectly predicted for these group I patients was 33% (positive predictive value 66.7%) for the Calkins et al derived data and 14.3% (positive predictive value 85.7%) for the Keith et al derived data. In contradistinction, for the senior author’s method, specificity, sensitivity, and positive predictive value were all 100% with 0% incorrectly predicted; all fractures predicted by this method to be stable (group I) or unstable (group III) were found by EUA to have been appropriately described. Each method had a large number of group II (indeterminate) fractures. However, with the senior author’s method, there was an increase in the number of group II fractures (23), as compared to the group II numbers for Calkins et al (7) and Keith et al (18). Reanalysis of the data for better potential cut-points revealed that none of the methods could be improved in this way.
Conclusion and Significance: The senior author’s method is the only reliable technique that is predictive of stability for small fracture fragments, while also being predictive of instability for large fracture fragments. However, these findings are based on small patient numbers and there remain a substantial number of fractures involving 20% or more of the posterior wall that are both stable and unstable by EUA. Therefore, we recommend dynamic fluoroscopic stress testing of the hip under general anesthesia to help guide the management of isolated fractures of the posterior wall of the acetabulum.
If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
• The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an “off label” use). ◆FDA information not available at time of printing. Δ OTA Grant.