Hip Dysplasia Diagnostics: The 3 X-Ray Angles That Confirm a Diagnosis
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In our last article, we looked at what hip dysplasia is and why it can cause long-standing hip, knee and back pain. If you haven't read that one yet, start there, it covers what hip dysplasia is, common symptoms, and the risk factors worth knowing about.
This time, Laura Rutterford, Chartered Physiotherapist at Quay Kinetics Physio, goes further into the diagnostics: the three angles measured on X-ray that confirm a diagnosis and help determine how severe it is.
The three angles, at a glance
| Angle | X-ray view | What it measures | Normal | Flagged |
|---|---|---|---|---|
| Lateral centre edge angle | AP (front to back) | Sideways coverage of the femoral head | Over 25° | 18–25° borderline, under 18° severe |
| Tönnis angle | AP, whole pelvis | Slope of the acetabular roof | 0–10° | Over 10° |
| Anterior centre edge angle | False profile (side view) | Front-to-back coverage of the femoral head | 20–45° | Under 20° |
Lateral centre edge angle
This is measured on an AP X-ray, taken square on from front to back. A line is drawn from the centre of the femoral head straight upwards, then a second line to the edge of the acetabulum (the socket). The angle between those two lines is the lateral centre edge angle, and it tells us how much sideways coverage the socket gives the ball of the hip.
More coverage means more stability. On this measurement:
Over 25° is good coverage and a stable joint
18–25° is borderline. This is where we'd monitor the joint and see what your symptoms are telling us, rather than act immediately
Under 18° is regarded as severe, and usually where we'd start having earlier conversations about what other management and support might help
Tönnis angle
This is also taken from an AP X-ray, but of the whole pelvis rather than just the hip. It measures something different: not how much coverage you have, but the angle of the acetabular roof itself, essentially how much of a weight-bearing surface it provides.
Because pelvic position varies from person to person, this measurement is first calibrated against bony landmarks to establish a neutral baseline, then the roof angle is measured from there.
0–10° is the ideal range
Over 10° is flagged for further investigation. It indicates slightly less stability within the joint and usually means we want to understand more about what's driving it.
Anterior centre edge angle
The first two angles come from a front-to-back X-ray. This one is taken from a false profile view, essentially from the side, and it measures coverage in a different direction: how much the socket covers the femoral head from front to back, rather than side to side.
The method is similar to the lateral centre edge angle (a line from the centre of the femoral head, a second line to the front edge of the acetabulum, and the angle between them), just measured from that side-on image.
20–45° means good anterior coverage and no anterior instability
Under 20° raises questions about how much stability is present, what extra strength work might help, and can factor into decisions about surgical options
Why these three angles matter together
None of these measurements are taken in isolation. Used together, they help build a fuller picture of the joint, and that picture is what determines whether hip dysplasia is classified as mild, moderate or severe.
That classification, in turn, shapes the conversation about what comes next: whether conservative, non-surgical management is the right path, or whether it's appropriate to progress to a consultant for a more in-depth discussion about surgical or other invasive options.
Next steps
If hip dysplasia is something you suspect, based on your own history or after reading part one of this series, the starting point is a physiotherapy assessment. From there we can advise on:
whether imaging is appropriate, and what to ask for
the right referral wording for your GP
a treatment plan built around your specific findings
👉 You can book an appointment at Quay Kinetics Physio to explore this further.