The hip joint is formed by the thigh bone — presenting as a narrow neck and a round ball (femoral head) — fitting into a socket on the side of the pelvis (acetabulum). The normal shape of the ball and socket allow close contact through an adequate range of motion. The endpoint for hip movement in any direction is usually the jamming of the head-neck junction against the rim of the socket.
As hip flexion is needed more than movement in any other direction, the normal socket and ball are turned forwards, allowing more motion before they jam. Due to congenital and developmental factors, the size, shape, or orientation of the ball and socket may not be ideal, causing premature jamming. Most active people do not initially feel the jamming and force their hips beyond the available range. Decreased hip motion without pain is a form of hip stiffness. Painful limitation of motion from jamming is impingement — femoroacetabular impingement (FAI).
Repetitive forceful jamming of the ball against the socket wall is the most common cause of hip labral tears. Continued jamming can damage joint cartilage and lead to arthritis at a young age. Repetitive jamming can also thicken the ball and socket, leading to jamming at progressively lesser ranges of motion. This condition may not cause significant pain in the early stages when treatment could prevent arthritis.
Modern Concept
The modern concepts of FAI were described by Professor Reinhold Ganz from Switzerland in the early 1990s. Hip impingement explains previously unknown causes of hip pain and premature arthritis in young adults.
Pubic symphysis irregularity is commonly associated with stiff hips. Even sacroiliac pain and lumbar pain can be associated with FAI. Athletic pubalgia, sports hernia, rectus abdominal strain, adductor muscle strain, avulsion fractures of the pelvis, and osteitis pubis are well-known associations.
Types of Impingement
Type 1 — Clinically obvious FAI — impingement occurs with less than the required movement in one or more directions from a bony block. The bony block signifies that the end range of FIR is from bones jamming anteriorly, not from posterior contracture, effusion, or guarding. Impingement in front is known to cause posterior damage from instability (contrecoup damage from FAI-induced instability), which can increase motion in some Type 1 hips making them appear like Type 2.
Type 2 — FAI with adequate range of motion — less obvious and less common.
Extra-Articular Impingement (Type 2A)
| Site | Mechanism | Type |
|---|---|---|
| AIIS | Impingement against the femoral neck in straight flexion | Type 1 |
| Greater trochanter | Against the ilium in flexion & abduction, and against the ischium in external rotation | Type 1 |
| Lesser trochanter | Against the ischium | Often Type 2 |
Intra-Articular Impingement (Type 2B)
| Subtype | Mechanism | Type |
|---|---|---|
| Cam | Always from asphericity of the head causing chondral debonding | Mostly Type 1 |
| Pincer | Usually acetabular, causes pinching of the labrum | All Type 1 |
| SCFE | Causes pincer-type and abrasion damage to the labrum and acetabular cartilage | Type 1 |
| Foveal | Pinching of ligamentum teres between the head and acetabulum | Mostly Type 2 |
| Mixed | Combination of the above subtypes | Varies |
Cam and pincer impingement can be confirmed only by the labral and acetabular cartilage damage patterns seen during surgery.
Clinical Features
Symptoms — pain is the most common complaint. Groin or lateral hip pain is typical, but referral to the thigh and knee is not uncommon. Flexion activities — sitting in low seats, getting in and out of a car, ice hockey goalkeeping — are expected causes, but pain only with running and sports is also seen. Clicking, popping, and locking are not uncommon. Patellofemoral pain, hip adductor tendinitis, hamstring strains, gluteal enthesopathy, sacroiliac strain, low back pain, and sports hernia are often associated and may be caused by hip impingement.
Signs — station and gait are usually unaffected unless there is severe deformity or joint irritation. Range of motion and pain-provoking tests are the most useful examination tools.
Examination Tests
| Test | What It Assesses |
|---|---|
| SLR | Tests for posterior instability in addition to sciatica and hamstring tightness |
| Straight flexion | Causes groin pain from AIIS impingement or inflamed labrum |
| FADDIR | Flexion-adduction-internal rotation — very sensitive for hip pathology but not specific; can also elicit posterior apprehension |
| FIR | Most useful test. Healthy FIR is 20–30°. Decreased FIR with reproducible pain and a bony block is very specific for anterior FAI. Increased FIR with FADDIR pain can suggest a containing cam or anterior dysplasia. |
| FABER | Stresses the SI joint; decreased range with lateral pain suggests anterolateral cam impingement |
| EADER | Extension-adduction-external rotation elicits posterior impingement and anterior apprehension |
Imaging
A good AP pelvis shows most acetabular and femoral morphological details. Lateral views at different angles show different parts of the femoral head-neck contour. CT is rarely required but shows femoral morphology best. MRI rules out other pathology, shows labral tears and advanced damage, and can differentiate borderline dysplasia from cam damage based on labral size. Circumferential alpha angle measurements are best performed on radial MRI sequences.
No imaging finding is diagnostic of FAI before damage patterns are established. The diagnosis should be made clinically and correlated with imaging to guide treatment. Image-guided intra-articular local anaesthetic injection can be diagnostic, therapeutic, and prognostic.
Treatment
Non-operative treatment is reasonable for a few months to avoid operating on one-time hip pain. It is limited to improving core and hip strength and decreasing lumbar lordosis. Physiotherapy should not attempt to improve hip movement range, as stretching usually aggravates impingement. Activity modification typically decreases pain.
Physiotherapy Caution
Stretching usually aggravates impingement. Physiotherapy should not attempt to improve hip movement range — it should focus on core and hip strength.
Surgical Treatment
The goal is correction of all potential causes of pain and provision of adequate impingement-free motion in a hip with good cartilage space. Correction of the bony cause should not unmask articular incongruity, instability, or abductor dysfunction. Labral tears caused by FAI should not be treated in isolation — correcting the causative morphology is essential. Labral tears can be debrided, repaired, or reconstructed based on tissue quality after bony correction.
| Impingement Type | Surgical Approach |
|---|---|
| Type 1 Cam & Pincer | Increased FIR and cam osteoplasty. Rim trimming always improves FIR (each mm removed increases FIR by 2–3°). Additional femoral and acetabular osteotomies if FIR cannot reach +20–30° safely with arthroscopy alone. |
| Type 2 Cam | Cam osteoplasty only, regardless of acetabular morphology, unless the labrum shows pincer damage. |
| AIIS impingement | Arthroscopic or open AIIS resection |
| GT impingement | Distal transfer of GT with relative neck lengthening |
| LT impingement | Arthroscopic or open LT resection, or distal transfer |
| Foveal impingement | Varus intertrochanteric osteotomy, often preceded by relative neck lengthening |
Today there is no role for prophylactic treatment — 70–90% of active children develop radiographic risk factors without symptoms and we do not know who will progress to FAI or arthritis.