Determining lesion size in osteonecrosis of the femoral head: A practical approach

Abstract

Background: To provide the best possible evaluation and treatment for patients with osteonecrosis of the femoral head (ONFH) it is necessary to determine the size of the necrotic lesion and extent of joint involvement as well as the stage. There has been a steady trend towards the use of comprehensive classifications, but there remains a difference of opinion as how best to determine lesion size. Purpose: To determine the accuracy and ease of use of various angular measurements and other techniques for measuring lesion size in ONFH. Methods: Eighteen hips with Stage I, II ONFH were evaluated using three-dimensional MRI measurements of lesion volume as the standard against which to compare simple visual estimates and area and angle measurements on plain radiographs and MRI. Results: The necrotic region occupied between 2.2% and 59% of the femoral head volume on MRI measurements. Many plain films in this specific series did not depict the lesion margins well, which made delineation, estimation and measurements of lesion size less accurate than in previous studies. There was a rough correlation between angle measurements and lesion size. This correlation was closer with the Index of Necrosis than with the Combined Necrotic Angle, which routinely overestimated size. The accuracy of the Index was improved when adjusted to reflect a femoral head angle of 250 degrees rather than 180 degrees. Numerical limits were derived with which angular measurements could be correlated for 70% of hips with Grades 〞A〞, 〞B〞 and 〞C〞 lesion size, as described in the University of Pennsylvania and ARCO classifications. Simple visual estimates from MRI studies were as accurate as angular measurements. Conclusions: For routine clinical use, visual estimates from good quality radiographs or MRI are generally satisfactory. Angular measurements do give an indication of prognosis and treatment, although they are estimates rather than true measurements of lesion size. 3-D MRI volume measurements are the most accurate method of measuring lesion size. They are now easier to use and should be considered for research and publications.