J Orthop Sci DOI 10.1007/s00776-015-0733-5

ORIGINAL ARTICLE

Prevalence of radiological femoroacetabular impingement in Japanese hip joints: detailed investigation with computed tomography Tomohiro Mimura1 · Taku Kawasaki1 · Shin Itakura2 · Tomohiro Hirata1 · Hitomi Fuzikawa1 · Kanji Mori1 · Shinji Imai1 

Received: 6 October 2014 / Accepted: 21 April 2015 © The Japanese Orthopaedic Association 2015

Abstract  Background  Femoroacetabular impingement (FAI) has been highlighted as a new etiology for osteoarthritis of the hip, and its prevalence has been reported in the past decade. In the present study, we performed a detailed investigation of the anatomical parameters related to FAI and calculated the prevalence of FAI-related findings in asymptomatic Japanese hip joints using computed tomography. Methods We evaluated high-resolution reconstructed multislice computed tomography images in patients who had undergone computed tomography imaging in our institution for conditions unrelated to hip disorders. The examined parameters were as follows: center-edge (CE) angle; acetabular index; acetabular anteversion (five slices in the axial plane); and asphericity angle of the femoral head (AAFH) (six slices in multiple radial planes). The AAFH in the oblique axial slice through the center of the femoral neck is the so-called α-angle. We then examined the accurate prevalence of FAI-related findings in Japan. Results  We investigated a total of 103 hips. The mean age of the subjects was 59.4 years. The mean CE angle was 31.1° and the mean acetabular index was 7.0°. The mean acetabular anteversion was 20.3° at the level of the hip center, and decreased as the slice level neared the superior margin of the femoral head. The mean AAFH ranged from 40.6° to 49.2° in the radial planes. The AAFH was largest

* Tomohiro Mimura [email protected]‑med.ac.jp 1

Department of Orthopedic Surgery, Shiga University of Medical Science, Tsukinowa‑cho, Seta, Otsu, Shiga 520‑2192, Japan

2

Department of Orthopedic Surgery, Shiga Medical Center for Children, Shiga 524‑0022, Japan





at 60° rotated slice from the oblique axial slice through the center of the femoral neck. The prevalence of FAIrelated findings in these Japanese hip joints was assessed as follows. An AAFH of >50° in any slice was detected in 51.5 % of the hips, and acetabular anteversion was negative for all images in 16.5 % of the hips, meaning that a total of 56.3 % of the images met the criteria for radiological FAI. Conclusions With consideration of our results, we emphasize that “anatomical or radiological FAI” is not uncommon in Japanese hips. Therefore, the diagnosis of FAI should be performed with the clinical findings taken into account.

Introduction Femoroacetabular impingement (FAI) has recently been reported as a cause of osteoarthritis (OA) of the hip. It was first described by Myers et al. [1] and then in detail by Ganz et al. [2]. The etiology of FAI is abnormal contact between the acetabulum and femoral head-neck junction that leads to labral tearing and subsequent osteochondral disorders. FAI is divided into two subcategories: cam deformity and pincer deformity [2, 3]. A cam deformity is defined as an abnormal morphological finding of the femoral side of FAI, and is characterized by a bony bump of the anterior femoral head-neck junction or non-spherical femoral head. A pincer deformity is defined as a morphological abnormality of the acetabular side of FAI, and is characterized by excessive anterior coverage at the superior acetabulum or acetabular retroversion. Many studies have described these radiographic deformities and the prevalence of FAI [3–9]. However, many of the reports were roentgenogram image investigations, which run the risk of being affected by the patient’s position and posture [10, 11]. Therefore,

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several studies involving anatomical investigations related to FAI using computed tomography (CT) have recently been reported in Western countries [12–15]. In the present study, we aimed to perform a detailed investigation of the anatomical parameters related to FAI in asymptomatic hips using high-resolution reconstructed multislice CT images and to examine the accurate prevalence of FAI-related findings in Japan.

This study was approved by our institutional review board. We conducted a patient-based study on patients who had undergone CT imaging from the chest to the abdomen. As other departments in our institution had ordered these scans, the CT scans were undertaken for conditions unrelated to hip disorders. The inclusion criteria for investigations were as follows: (1) whole pelvis and both hip joints were involved; (2) reconstructed axial slice thickness was ≤1 mm; (3) pelvic rotations and tilt were normal (described below in the section for standardization of CT images). The

exclusion criteria were as follows: (1) age under 20 years; (2) evident pathology of hip OA, such as joint space narrowing, osteophytes, or subchondral bone changes including cysts and sclerosis [16]. We retrospectively examined consecutive patients fulfilling the above criteria from 1 July 2013 to 31 July 2013. The parameters examined were as follows: age; sex; center-edge (CE) angle [17]; acetabular index [18]; and acetabular anteversion in the axial plane (five slices; first slice at the level of the superior margin of the femoral head, followed by four slices at 5, 10, and 15 mm below the superior margin of the femoral head, and at the level of the hip center) (Fig. 1). In addition, we measured the asphericity angle of the femoral head (AAFH) in multiple radial slices using previously described methods [19, 20]. For this, six radial slices were generated at 15° intervals, including slices with 0°, 15°, 30°, 45°, 60°, and 75° rotation from the oblique axial slice through the center of the femoral neck (hereafter referred to as S1, S2, S3, S4, S5, and S6, respectively) (Fig. 2). The AAFH of each slice was measured as previously described [21]: the S1 slice angle is the so-called α-angle [2, 3]. Negative acetabular anteversion in any slice was defined as pincer deformity, while an AAFH of >50° in any slice was defined as cam deformity.

Fig. 1  Measurement of the acetabular anteversion angles in five axial slices. a Reference plane for the acetabular anteversion angle measurements. Each line represents one of the five slice lines. The axial views of lines b, c, d, e, and f are shown in b, c, d, e, and f, respectively. b Axial view of the superior margin of the femoral head (line b). Angle ø is the acetabular anteversion angle at the superior margin of the femoral head. The angle is measured between the line joining the anterior and posterior edges of the acetabulum and the line per-

pendicular to the line joining the posterior edges at both acetabula. c Acetabular anteversion angle in the axial view at 5 mm below the superior margin of the femoral head (line c). d Acetabular anteversion angle in the axial view at 10 mm below the superior margin of the femoral head (line d). e Acetabular anteversion angle in the axial view at 15 mm below the superior margin of the femoral head (line e). f Acetabular anteversion angle in the axial view at the level of the center of the femoral head (line f)

Materials and methods Patients and parameters

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Prevalence of radiological femoroacetabular impingement in Japanese…

Fig. 2  Measurement of the asphericity angles of the femoral head (AAFHs) in multiple radial slices. a The dotted line is the axis through the center of the femoral neck, adjusted to be parallel to the femoral neck-shaft angle. The solid line is the reference plane for radial angle reconstruction. b The reconstructed plane demonstrates superimposed radial reference lines at 15° intervals. S1 is the oblique axial slice (dotted line in a), S2 is a plane rotated 15° from S1, S3 is a plane rotated 30° from S1, S4 is a plane rotated 45° from S1, S5 is a plane rotated 60° from S1, and S6 is a plane rotated 75° from S1. c

Reconstructed slice of the S1 plane. Angle ø is the AAFH; namely the α-angle. The angle is measured between the line joining the femoral head and the center of the neck and the line drawn from the center of the femoral head to the point where the spherical image of the femoral head is lost. d Reconstructed radial slice of the S2 plane. e Reconstructed radial slice of the S3 plane. f Reconstructed radial slice of the S4 plane. g Reconstructed radial slice of the S5 plane. h Reconstructed radial slice of the S6 plane

Radiological examination and standardization of CT images

Cary, NC). One-way analysis of variance (ANOVA) with a Scheffé post hoc test was used to determine the significance of differences among five or six groups. Student’s t-test was used to determine the significance of differences between two groups. Values of P 39° and 19.4 % showed an acetabular index of ≤0°, meaning that 71.8 % (74/103) of the total images met the criteria for radiological FAI. The prevalence in our results is higher than those in previous investigations in Japan. The differences between the present study and the previous studies may arise from whether the investigations were performed using CT or radiography. Our study involved multislice CT scans, with cam deformity investigated in six slices and pincer deformity investigated in five slices. Lepage-Saucier et al. [25] reported normal α-angles in asymptomatic Belgian populations using CT. They showed that the upper limit of the 95 % reference interval limit for the α-angle was 83° at the 1:30 position in radial slices (same slice as S4 in our study), which was far beyond the abnormal thresholds found in the previous literature (α-angle of >50° [3, 6, 8] or >60° [24, 26]). They concluded that the current morphological parameters needed to be redefined for the diagnosis of FAI. We also believe that the previously reported prevalences of radiological FAI in Japan may change to larger numbers if the parameters were investigated in detail using multislice imaging. This study has several limitations. The first is that the study was patient-based. The CT scans were ordered by other departments in our institution, such as cardiology and endocrinology, and therefore we could not know whether the hips were really asymptomatic; there is the possibility that some patients had been receiving treatment for hip problems at other institutions. The second limitation is that the only available information was the patient’s age, sex, and CT images. Because this study was permitted by our institutional review board under the stipulation that we could not check the clinical records or personal information, we could not know the patients’ symptoms, roentgenogram, past history, underlying disease, body mass index, and activity or occupation. The third limitation is that the sample size was relatively small. Some authors have shown that FAI is one of the major causes of hip OA in young patients and active patients [2, 27, 28], and that cam deformity in particular is a special risk for OA [29, 30]. Meanwhile, other authors have reported that radiological FAI findings do not lead to the development of significant OA [31–33]. Paliobeis et al. [34] emphasized that the presence of coexisting developmental dysplasia of the hip should be taken into account for the treatment of patients with FAI. The issue of whether FAI is a risk factor for the development of hip OA remains controversial. Moreover, accurate criteria for FAI are still unknown. For example, various cutoff values for the α-angle are used for the diagnosis of FAI,

Prevalence of radiological femoroacetabular impingement in Japanese…

with some authors reporting that angles of >50° are pathological [3, 6, 8] and others reporting that angles of >55° [13– 15] or >60° [24, 26] are pathological. In fact, if an α-angle of >55° was employed as the criterion for cam deformity, 34.0 % (35/103) of all images showed cam deformity, meaning that 40.7 % (42/103) of all images met the criteria for radiological FAI. If an α-angle of >60° was employed as the criterion for cam deformity, 18.4 % (19/103) of all images showed cam deformity, meaning that 29.1 % (30/103) of all images met the criteria for radiological FAI. We conclude that FAI-related findings were common in Japanese hips, similar to the case for Western countries, upon detailed investigation with CT. However, we also recognize that it may be incorrect to generalize our results to regions throughout Japan. Nevertheless, our results show the important fact that findings of “anatomical or radiological FAI” are not uncommon in Japanese asymptomatic hips. The diagnosis of FAI should be performed with the clinical findings taken into account. As the next step, our concern is to investigate these parameters in FAI patients diagnosed by their clinical symptoms. Investigations of these parameters in hips with “clinical FAI” and comparisons with the data generated from the present study are warranted. Conflict of interest  The authors declare that they have no conflicts of interest.

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Prevalence of radiological femoroacetabular impingement in Japanese hip joints: detailed investigation with computed tomography.

Femoroacetabular impingement (FAI) has been highlighted as a new etiology for osteoarthritis of the hip, and its prevalence has been reported in the p...
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