Femoroacetabular impingement

In a healthy hip, the femoral head turns inside the acetabulum without touching its edges. In femoroacetabular impingement there is a mechanical conflict: on flexing and rotating, bone strikes the rim of the acetabulum. That contact, repeated thousands of times, progressively damages the labrum and the neighbouring cartilage.

There are two forms, and they frequently coexist. In the cam type the junction between the femoral head and neck is not concave enough and that prominence is forced into the joint. In the pincer type the acetabulum over-covers the head and the rim acts as a stop.

One point must be clear: having cam or pincer morphology on a radiograph is not a disease. It is very commonly found in people with no symptoms at all. The syndrome exists when there is the triad of symptoms, examination signs and imaging findings — all three, not just the last.

How it feels

This affects young, active people. The classic symptom is groin pain on flexing the hip: after long periods sitting, when driving, putting on shoes, crossing the legs, or after sport. Many patients describe the pain by making a C shape over the hip.

It may come with clicking, a sense of locking or of the hip catching. Stiffness is usually noticed on standing up after sitting. Patients have often been treated for months as an adductor strain or a lumbar problem before reaching the right diagnosis.

How it is assessed

Examination is central: the flexion-adduction-internal rotation manoeuvre reproduces the pain, and internal rotation is usually limited. Well-taken radiographs allow measurement of femoral neck morphology and acetabular coverage; MRI — sometimes with intra-articular contrast — shows the state of the labrum and the cartilage.

There is one step that cannot be skipped: measuring acetabular coverage to rule out dysplasia. A dysplastic hip can look like impingement, and treating it as such — resecting bony rim — worsens the instability. It is one of the costliest errors in hip preservation surgery.

What the evidence says

The UK FASHIoN trial randomised 348 patients with impingement syndrome to hip arthroscopy or to a personalised, supervised physiotherapy programme. At 12 months both groups had improved, and the difference in favour of arthroscopy exceeded the pre-specified threshold for clinical relevance.

The correct reading of that result is not “everyone should be operated on”. It is that surgery has a real, measurable benefit in well-selected patients, and that a serious physiotherapy programme also improves a great many people. Patient selection is the part that decides the outcome.

Treatment, in order

We start by modifying what provokes the conflict: avoiding repeated deep flexion, adjusting the squat and the sporting movements that trigger the pain, and working on pelvic control and gluteal and abdominal strength. In selected cases an intra-articular injection helps control the pain and also confirms that the problem is inside the joint.

Surgery is considered when symptoms persist despite properly delivered conservative treatment, in a hip without established arthritis. Arthroscopically, the bony prominence is reshaped to eliminate the conflict and the labrum is repaired when it is repairable. That last point matters: a systematic review comparing labral repair with debridement found better patient-reported outcomes with repair.

In someone who already has established arthritis, arthroscopy does not solve the problem. There the conversation is a different one.