Thank you for presenting your interesting technique. You conclude that "it is possible to obtain sufficient information concerning the extent of necrotic involvement with a single anteroposterior plain radiograph of the affected femoral head." The question this raises is how much information is really "sufficient". In many situations, examining only a single AP x-ray is indeed sufficient for clinical purposes. Often no measurements are needed as a careful estimate of lesion size may tell us what we need to know to determine treatment.

However, it is well established that important information may not be available from only one view, for example a small degree of femoral head flattening, seen only in the lateral view. If present, this could well change treatment and prognosis.

For clinical research it is important to obtain as much information as possible regarding the extent of necrosis as several studies have confirmed that the size of the lesion is quite important in establishing a prognosis, determining treatment, following progression or resolution, and evaluating various forms of management. This can not be done with a single radiograph. The femoral head and the necrotic lesion are three dimensional and one must therefore use a different technique to accurately determine the volume of necrosis and the percent of head involvement. Serial CT scans or MRI sections are the most accurate; however quantitative digital image analysis of both AP and Lateral radiographs gives a close approximation, as described in our publication.(1). This technique is more accurate than using angular measurements which cannot effectively determine the size of an irregular, three dimensional lesion which can vary in location within the head.

Thus, although lesion size is important, other factors must be considered when evaluating a patient with osteonecrosis. These include lesion size and extent of collapse, if present; location; proximity to the articular suface and amount of surface involved; joint line narrowing and acetabular involvement; as well relevant clinical factors.

Reference:

1. Steinberg DR, Steinberg ME, Garino JP, Murray D, Udupa JK. Determining lesion size in osteonecrosis of the femoral head. J Bone Joint Surg Am. 2006;88(Supp 3)27-34.

NOTE: Revised Fig. 3 was uploaded on July 18, 2007.

To The Editor:

We read with great interest the article, "Determining Lesion Size in Osteonecrosis of the Femoral Head"(1) which describes different methods of measuring osteonecrotic lesion size, such as the Combined Necrotic Angle based upon X-rays described by Marcel Kerboull et al.(2), the Index of Necrotic Extent based upon MRI described by Kyung-Hoi Koo et al.(3) and the Modified Index of Necrotic Extent described by Sebastien Cherian et al.(4). All three methods make use of a trigonometrical function, such as the measurement of an angle or an area and require two images.

In our opinion, it is possible to obtain sufficient information concerning the extent of osteonecrotic involvement with a single anteroposterior plain radiograph of the affected femoral head. For this purpose it is advantageous to use the negative image of the anteroposterior plain radiograph, to make it easier to define the osteonecrotic area (Figure 1 and Figure 2). As with the previous methods, the technique we propose uses a trigonometrical function consisting of resolving the osteonecrotic area, and subdividing it into triangles. The area of each of these triangles can be calculated simply by measuring the base and the height; therefore the desired area is half the base times the height (Figure 3). The sum of the different areas is the full osteonecrotic area of the femoral head. We have named the proposed method as “S-G (Salvi-Grappiolo) osteonecrosis triangling method”.

Figure 1 – The radiograph (on the left) shows an osteonecrotic area located in the head of the femur (arrow). The negative image of the same radiograph (on the right) permits an easy evaluation of the osteonecrotic area, visible as a darker area (arrow).

Figure 2 – The same femoral head after its removal and sagittal cut. The osteonecrotic lesions are visible (arrows).

Figure 3 – The proposed method permits to subdivide the negative image of the osteonecrotic area (Figure a) into some triangles (Figure b). Each area of these triangles can be easily obtained through a simple trigonometrical function (Figure c). The full osteonecrotic area is the sum of the different areas of the triangles.

The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

References:

1. Steinberg DR, Steinberg ME, Garino JP, Dalinka M, Udupa JK Determining lesion size in osteonecrosis of the femoral head J Bone Joint Surg Am. 2006 Nov;88 Suppl 3:27-34.

2. Kerboul M, Thomine J, Postel M, Merle d'Aubigne R The conservative surgical treatment of idiopathic aseptic necrosis of the femoral head J Bone Joint Surg Br. 1974 May;56(2):291-6.

3. Koo KH, Kim R Quantifying the extent of osteonecrosis of the femoral head. A new method using MRI J Bone Joint Surg Br 1995 Nov;77(6):875-80.

4. Cherian SF, Laorr A, Saleh KJ, Kuskowski MA, Bailey RF, Cheng EY Quantifying the extent of femoral head involvement in osteonecrosis J BoneJoint Surg Am. 2003 Feb;85-A(2):309-15.