| Basic and Clinical Science Advances in Understanding FAI- Index

III. Disease Epidemiology and Prevalence

Michael Leunig PD Dr. med.

Disease epidemiology is the study if health-event, health-characteristic, or health-determinant patterns in a population.

Etiology of hip OA: OA has long been considered secondary (eg, to congenital or developmental deformities) or primary (presuming some underlying abnormality of articular cartilage). Recent information supports a hypothesis that so-called primary osteoarthritis is also secondary to subtle developmental abnormalities and the mechanism in these cases is femoroacetabular impingement rather than excessive contact stress.

History: Four decades ago, Murray suggested a relationship between a rather subtle deformity of the proximal femur, which he called the ‘‘tilt deformity,’’ to the subsequent development of osteoarthritis (OA) of the hip. Stimulated by Murray’s observations, Solomon and colleagues in South Africa and Harris and colleagues in the United States extended Murray’s original suggestion.

Confirmation of hypothesis: The two key developments are (1) the recognition of the mechanism for the development of the hip OA caused by mild deformities and (2) the possibility that correction of that mechanism could retard the development of the OA. First pioneered and stimulated by Ganz and coworkers, and now confirmed by a variety of observations from many centers worldwide, a mechanism has been defined that explains how these subtle developmental abnormalities adversely affect the joint and lead to OA in many cases. The dominant mechanism is FAI-induced by motion of the well-constrained hip. In those cases of severe deformity of the hip secondary to developmental abnormalities, the mechanism of OA is excessive contact stress history on the cartilage secondary to reduced contact area. This is not so for the more subtle deformities that produce FAI since the contact areas are not reduced.

Prevalence in males

Background. Clinically, cam impingement is often seen in young active male individuals referred to an orthopedic surgeon because of groin pain, and internal rotation is usually found to be diminished. However, it remains unclear how often a cam-type deformity with a non- spherical femoral head and decreased head-neck offset can be found in the general population of young males. We therefore aimed to examine the prevalence of cam-type deformities in a population-based inception cohort study of young males.

Objective. To determine the prevalence of cam-type deformities on hip magnetic resonance imaging (MRI) in young males.

Methods. This was a population-based cross-sectional study in young asymptomatic male individuals who underwent clinical examination and completed a self-report questionnaire. A random sample of participants was invited for MRI of the hip. We graded the maximal offset at the femoral head-neck junction on radial sequences using grades from 0 to 3, where 0 = normal, 1 = possible, 2 = definite, and 3 = severe deformity. The prespecified main analyses were based on definite cam-type deformity grades 2 or 3. We estimated the prevalence of the cam-type deformity adjusted for the sampling process overall and according to the extent of internal rotation. Then we determined the location of the deformity on radial MRI sequences.

Results. A total of 1,080 subjects were included in the study and 244 asymptomatic males with a mean age of 19.9 years attended MRI. Sixty-seven definite cam-type deformities were detected. The adjusted overall prevalence was 24% (95% confidence interval [95% CI] 19– 30%). The prevalence increased with decreasing internal rotation (P < 0.001 for trend). Among those with a clinically decreased internal rotation of < 30°, the estimated prevalence was 48% (95% CI 37–59%). Sixty-one of 67 cam-type deformities were located in an anterosuperior position.

Summary. Cam-type deformities can be found on MRI in every fourth young asymptomatic male individual and in every second male with decreased internal rotation. The majority of deformities are located in an anterosuperior position.

Prevalence in females

Background. We already reported on the prevalence of femoral deformities in a young asymptomatic male population. The aim of this a second study was to determine the prevalences of both femoral and acetabular types of impingement as potential risk factors for hip OA in a quasi-population-based cross-sectional cohort study of young females.

Methods. Study subjects were young females aged 18 to 19 attending grammar school or selected vocational schools for manual or non-manual occupations. Participants completed a set of questionnaires pertaining to pain, stiffness, and physical function, and internal rotation was measured reliably using a validated examination chair. A random sample of the examined participants was subsequently invited to obtain magnetic resonance images (MRI) of the hip. Cam-type deformities were assessed semiquantitatively using scores from grades 0 to 3: 0 = normal, 1 = mild, 2 = moderate, 3 = severe. The depth of the acetabulum was defined as the distance (in mm) between the center of the femoral head and the line connecting the anterior and posterior acetabular rims. The value was positive if the center of the femoral head was lateral to the line connecting the acetabular rim. Values ≤3 mm were considered to represent increased acetabular depth. Overall prevalence estimates with 95% confidence intervals (95% CI) accounted for the oversampling of participants with decreased (<40°) and increased (≥50°) internal rotation using post-stratification weights. Prevalences were calculated separately for participants with decreased, normal, and increased internal rotation.

Results. Subjects who underwent imaging included 80 asymptomatic participants with a mean age of 19.3 years and a mean body mass index of 21.2 kg/m2. Grade 1 femoral (cam- type) deformities were found in 22% (95% CI 13 to 34). No MRI showed evidence of a definite cam-type deformity (grade ≥2). The prevalence of increased acetabular depth was 10% (95% CI 5 to 19). No correlations were found between increased acetabular depth and decreased internal hip rotation.

Summary. In this cohort of young patients, definite cam-type deformities in women were rare compared to men, whereas the prevalence of increased acetabular depth was higher, suggesting that FAI has different gender-related biomechanical mechanisms. An increased acetabular depth was not associated with signs of labral damage.

Conclusion on prevalences. Definite cam-type deformities in women are rare compared to men, whereas the prevalence of increased acetabular depth is higher, suggesting that FAI has different gender-related biomechanical mechanisms. Finally, increased acetabular depth was not associated with signs of labral damage.


References

1. Ganz, R., et al., The etiology of osteoarthritis of the hip : an integrated mechanical concept. Clin Orthop Relat Res, 2008. 466(2): p. 264-72.

2. Reichenbach, S., et al., Association between cam-type deformities and MRI-detected structural damage of the hip: A cross-sectional study in young males. Arthritis and rheumatism, 2011.

3. Reichenbach, S., et al., Prevalence of cam-type deformity on hip magnetic resonance imaging in young males: a cross-sectional study. Arthritis Care Res (Hoboken), 2010. 62(9): p. 1319-27.