Femoroacetabular Impingement (FAI)
Femoroacetabular impingement (FAI) occurs when anatomic variation of the hip causes impingement between the femoral head-neck junction and the acetabular rim during functional range of motion.
There are three types of Femoroacetabular Impingement (FAI):
- Cam impingement.
- Pincer impingement.
- Mixed impingement.
Cam impingement occurs due to femoral anatomic variations, typically there is an excessive bone at junction of femoral head and neck that can be caused by:
- Aspherical head.
- Decreased head-to-neck ratio.
- Decreased femoral offset
- Slipped capital femoral epiphysis SCFE deformity (femoral neck retroversion).
- Cam Impingement is more common in young athletic males.
- A typical injury pattern with cam impingement is a tear at the base of the labrum at the labral-chondral junction.
- The adjacent articular cartilage then becomes injured because of compression from the femoral head with its relatively larger radius of curvature rotating into the acetabulum.
- A “contrecoup” injury frequently is seen on the posterior femoral head and posteroinferior acetabulum owing to anterior cam impingement with subsequent increased pressure on the posterior hip cartilage.
See Also: Hip Joint Osteoarthritis
Pincer impingement occurs due to overcoverage of the acetabular rim causing impingement against the femoral neck with functional motion,
The area of overcoverage can be caused by:
- Acetabular retroversion (crossover sign).
- Acetabular protrusion (medial aspect of femoral head is medial to ilioischial line).
- Coxa profunda (acetabular fossa is medial to ilioischial line).
- Pincer morphology is more commonly seen in active middle-aged women.
- The injury pattern with pincer impingement is created by the femoral neck abutting the acetabular rim and labrum during the extremes of motion. The labrum is pinched between the bony surfaces and subsequently suffers more damage than the adjacent articular cartilage.
Combined impingement occurs when cam and pincer Impingement coexist in the same hip.
- Associated pincer deformities have been reported in 42% of patients being operated on for cam-type impingement.
- According to some authors, most hips treated for femoroacetabular impingement FAI have Combined Impingement.
Femoroacetabular Impingement Symptoms
- Hip / groin pain related to activities and exacerbated by hip flexion.
- Sitting on chair difficulty.
- Significant limitation of hip internal rotation tested at 90 degrees due to mechanical block.
- Impingement test or FADDIR test is positive (flexion, adduction, internal rotation)
See Also: Hip Special Tests
Recommended views include: AP view and false profile view.
Radiology Findings include:
- Pistol grip deformity (Femoral head and neck asphericity): indicates Cam impingement.
- Acetabular protrusion, retroversion, and coxa profunda.
- Crossover sign: indicates acetabular retroversion in Pincer impingement.
- Head-Neck Offset is measured on lateral radiographs.
- It is measured using lines parallel to the line defining the femoral head center and neck midline.
- A reduced head-neck offset increases the risk for neck impingement upon the acetabular rim.
- α-angle measure head-neck offset , it can be measured on frog-leg lateral radiograph.
- First line is drawn connecting the center of the femoral head and the center of the femoral neck.
- Second line is drawn from the center of the femoral head to the point on the anterolateral head-neck junction where prominence begins
- The intersection of these two lines forms the alpha angle.
- Normally 40 degrees or less.
CT scan: can be helpful to further assess for structural abnormalities.
MRI: used to assess articular cartilage and labral injuries.
Femoroacetabular Impingement Treatment
- Non-Operative Treatment is indicated in asymptomatic or mild symptomatic patients, or if there is no mechanical symptoms.
- Non-operative treatment of Femoroacetabular Impingement FAI include observation.
- Surgical treatment varies according to patient anatomy.
- Labrum should be repaired when possible; results appear to be superior compared to labral débridement.
Surgical treatment include:
- Hip arthroscopy:
- Trim femoral head/neck in Cam impingement.
- Repair / débridement of labral tear.
- Surgical hip dislocation (Ganz trochanteric osteotomy):
- Allows excellent exposure of proximal femur and acetabulum.
- Permits treatment of severe deformities.
- Preserves femoral head blood supply.
- Allows for repair of labrum and chondral flap tears.
- Complications are rare (<5%): trochanteric nonunion and heterotopic bone formation.
- Anterior Z-capsulotomy preserves posterior vessels to femoral neck and minimizes risk for osteonecrosis.
- Periacetabular osteotomy:
- Less common procedure for Femoroacetabular Impingement FAI.
- May be used to address a retroverted socket.
- Total hip arthroplasty:
- Patient age >60 years and end-stage Hip Joint Osteoarthritis.