A notch on the femoral head that tracks instability - and should change nothing yet

Design

Retrospective single-centre MR arthrography study, level of evidence IV

Population

392 hips in patients with non-traumatic hip pain considered for hip preservation surgery, mean age 32

Primary outcome

Prevalence of gluteus minimus indentation and its association with acetabular and femoral parameters

Effect

Present in 7% overall, 22% of dysplastic vs 4% of non-dysplastic hips; OR 1.5 (95% CI 1.2-1.9) per 5 degree fall in lateral centre-edge angle; OR 1.3 (1.1-1.6) per 5 degree rise in femoral torsion

Three hundred and ninety-two hips, from 761 imaged by MR arthrography for non-traumatic hip pain that had not settled with three months of conservative treatment, were reviewed for gluteus minimus indentation - a lateral femoral head morphological finding previously described in cerebral palsy and severe dysplasia. Mean age was 32; 18% had dysplasia with a lateral centre-edge angle below 22 degrees and 59% had femoral torsion above 25 degrees. None had cerebral palsy.

The indentation was present in 7% of hips overall, but in 22% of dysplastic hips against 4% of the rest. After controlling for acetabular version, retroversion index, caput-collum-diaphyseal angle, age and sex, each 5 degree decrease in lateral centre-edge angle raised the odds 1.5-fold (95% CI 1.2-1.9) and each 5 degree increase in femoral torsion raised them 1.3-fold (95% CI 1.1-1.6). In dysplastic and high-torsion hips, those with the indentation had a smaller relative cartilage surface area and a greater labral volume and labral contribution to the joint surface - the pattern of a hip where the labrum is doing work the acetabulum should be doing.

The authors are unusually direct about the limits, and they are worth repeating: the frequency and significance of this finding in asymptomatic hips are unknown, and its presence should not influence treatment until its relationship to prognosis or treatment response is established. Read it as a marker to note and study, not to act on.

• Note the finding if you see it on radial MR sequences, but do not let it change the operative plan
• It was present in only 7% of symptomatic hips, and its rate in asymptomatic hips is unknown
• Where you see it, check lateral centre-edge angle and femoral torsion - both track with it
• The associated cartilage and labral pattern suggests instability, but this is level IV evidence
• Do not report it to a patient as a sign of joint damage; its prognostic meaning is undetermined

Why it matters

A morphological sign tied to cerebral palsy turns out to appear in ordinary dysplastic and high-torsion hips.

Don't overread it

The authors state directly that this should not influence treatment until its prognostic relevance is established.

The statistics, in plain English

The odds ratios here are per 5 degrees, which is how a continuous measurement should be reported, but it means the effect compounds: a hip 15 degrees below the dysplasia threshold carries the 1.5-fold increase three times over. The more important limitation is the denominator. Every hip in this study was symptomatic enough to reach MR arthrography after three months of failed conservative treatment, so the 7% prevalence describes that population and no other. Without an asymptomatic comparison group, a finding that is more common in dysplastic hips cannot be separated from a finding that is simply common in hips shaped that way.

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