Amount of acetabular rim resection for FAI correlates to change in center-edge angle

A recent study has pinpointed the formula for determining center-edge angle change.

SAN DIEGO — When resecting the acetabular rim in the hips of patients undergoing arthroscopic trimming for femoroacetabular impingement, 1 mm of acetabular rim trimming equals 2.4° of change in the center-edge angle and 5 mm of bony resection equals 3.2° of change, according to investigators here.

The findings were presented by Andrew B. Wolff, MD, at the 28th Annual Meeting of the Arthroscopy Association of North America.

“There is a possibility of over-resection of the acetabular rim with consequent progression of arthritis or instability,” Wolff said. “It is difficult intraoperatively to ascertain what the limits of this resection are. In order to inimize this risk, it is important to be able to correlate the amount of resection performed with pre- and postoperative radiographic imaging.”

A formula to determine change

Preoperative and postoperative CE angle and millimeters of rim reduction were prospectively collected in 58 hips that underwent arthroscopic rim reduction by Marc J. Philippon, MD. Measurements of the CE angle on pre- and postoperative AP pelvis radiographs were made by two orthopedic surgeons. To determine the amount of rim reduction intraoperatively, the lunate surface was measured using an arthroscopic rule at the 12 o’clock-position.

Average rim reduction was 3.2 mm, and the average change in CE angle was 3.9°. Wolff’s group found that the change in the CE angle could be determined by the following formula: Change in CE angle = 2.2 + (0.2 x [rim reduction in millimeters]).

According to the study, the interobserver intraclass correlation coefficient for radiographic measurement of the CE angle was 0.92, indicating excellent interobserver reliability.

Things to keep in mind

Moderator J.W. Thomas Byrd, MD, agreed that the study provided a “nice rule of thumb,” but added there are some factors to consider.

“I think you need to keep in mind — and they reflected on it — that the maximum pincer lesion is usually anterior to the 12-o’clock position, so you need to be careful about calculating the CE angle at 12 o’clock,” he said.

For more information:

• J.W. Thomas Byrd, MD, can be reached at Nashville Sports Medicine and Orthopaedic Center, Baptist Medical Plaza I, 2011 Church St., Suite 100, Nashville, TN 37203; 615-284-5800; e-mail: byrd@nsmoc.com. He receives research funding and royalties from Smith & Nephew.
• Andrew B. Wolff, MD, can be reached at Nirschl Orthopaedic Center for Sports Medicine and Joint Reconstruction, 1715 North George Mason Drive, Ste. 504, Arlington, VA 22205; 703-525-2200; e-mail: wolff_andy@yahoo.com. He has no direct financial interest with any product or company mentioned in this article.

Reference:

• Wolff AB. Rim reduction for the treatment of pincer-type FAI correlates with pre & postoperative center edge angles. Paper #SS-32. Presented at the 28th Annual Meeting of the Arthroscopy Association of North America. April 30-May 3, 2009. San Diego.