Femoroacetabular Impingement (Hip Impingement)
Patient information — Dr Christopher Spelman, Orthopaedic Surgeon
What it is
Femoroacetabular impingement, or FAI, describes a hip in which the ball and socket make abnormal contact at the edges of the joint’s range of movement. Instead of moving smoothly, the bones collide — and over time this damages the labrum, the ring of cartilage that seals the socket, and the articular cartilage behind it.
FAI is a common cause of groin pain in young and active adults, and is thought to be one route by which some people develop hip arthritis relatively early.
Cam and pincer
Two shapes cause impingement, and many people have both (mixed).
Cam morphology — the femoral head is not perfectly round, with extra bone at the head–neck junction. As the hip flexes, this non-spherical portion is forced into the socket, shearing the cartilage. It is more common in men and is associated with high-impact sport during adolescence, while the growth plate is still open.
Pincer morphology — the socket covers too much of the ball, either generally or because it faces the wrong way (acetabular retroversion). The neck of the femur impinges against the rim, crushing the labrum. More common in women.
Symptoms
Groin pain is the usual complaint. Pain is typically worse with prolonged sitting, getting out of a car, squatting, or pivoting sports. Some patients notice clicking, catching or a sense of the hip locking, which usually indicates labral damage.
Symptoms build gradually. Many patients have modified their activity for years before seeking help.
Important: the shape alone is not the diagnosis. Cam and pincer morphology are common in people with no symptoms at all. FAI syndrome requires the combination of symptoms, clinical signs and imaging findings. Treating an X-ray rather than a patient is a well-recognised error in this field.
Diagnosis
Assessment includes history, examination — particularly hip flexion, adduction and internal rotation — and imaging. Plain X-rays show the bony shape. MRI, sometimes with contrast in the joint, shows labral and cartilage damage. A diagnostic injection of local anaesthetic into the joint can help confirm that the pain is coming from inside the hip.
Treatment
Physiotherapy comes first for most patients. A structured, supervised programme targeting hip and trunk strength, movement control and activity modification improves symptoms substantially in a large proportion of people.
Surgery — usually hip arthroscopy to reshape the bone and repair the labrum — is considered where a good physiotherapy programme has not delivered enough improvement.
What the evidence shows
The best evidence comes from UK FASHIoN, a multicentre randomised controlled trial across 23 NHS hospitals that randomised 348 patients with FAI syndrome to either hip arthroscopy or a personalised, physiotherapist-led programme (Griffin et al., 2018).
Both groups improved substantially. On the iHOT-33 hip quality-of-life score, the arthroscopy group improved from 39.2 to 58.8, and the physiotherapy group from 35.6 to 49.7.
Arthroscopy was better, but by a margin only just above the threshold of clinical importance. The adjusted mean difference was 6.8 points in favour of surgery, against a minimum clinically important difference of 6.1 points.
That result deserves to be read carefully in both directions. Surgery was genuinely superior, and the difference was statistically robust. But personalised physiotherapy produced a large improvement on its own, and the gap between them was modest. This is a meaningful argument for trying good conservative care first — not for abandoning surgery.
Adverse events were more common in the surgical group (72% of patients reported at least one, versus 60%), though most were minor, such as muscle soreness. Serious adverse events were uncommon; five of the six in the surgical group were treatment-related, including one post-operative joint infection (Griffin et al., 2018).
Notably, “good physiotherapy” here meant a specific, supervised, progressive, individualised programme — not a generic exercise sheet. The quality of the conservative arm is part of why it performed as well as it did.
Risks of hip arthroscopy
• Incomplete relief of symptoms — a proportion of patients do not improve, particularly where cartilage damage is already established
• Progression to arthritis. Arthroscopy does not reverse existing cartilage loss. Where arthritis is already present, outcomes are markedly less predictable
• Nerve injury — traction is applied to the leg during the procedure; numbness in the groin, foot or genital area can occur and is usually temporary
• Infection — uncommon but serious
• Blood clots
• Heterotopic ossification — bone forming in the soft tissues
• Further surgery, including revision arthroscopy or eventual joint replacement
Where periacetabular osteotomy fits
Some hips have both impingement and dysplasia, and some have acetabular retroversion — a socket facing the wrong direction — where the correct treatment is to reorientate the socket rather than trim it. Arthroscopy in an unrecognised dysplastic hip can worsen instability. This distinction is central to getting treatment right, and is one reason FAI is best assessed by a surgeon who also performs hip preservation surgery.
See [periacetabular osteotomy] and [adult hip dysplasia].
References
1. Griffin DR, Dickenson EJ, Wall PDH, et al. Hip arthroscopy versus best conservative care for the treatment of femoroacetabular impingement syndrome (UK FASHIoN): a multicentre randomised controlled trial. Lancet. 2018;391(10136):2225–2235. DOI
2. Grant TM, Diamond LE, Pizzolato C, et al. Comparison of walking biomechanics after physical therapist-led care or hip arthroscopy for femoroacetabular impingement syndrome: a secondary analysis from a randomized controlled trial. Am J Sports Med. 2022;50(12):3198–3209. DOI