The hip joint and its surrounding structures make up a very complex anatomic region of the body with deep to superficial layers. The ball and socket joint is made of the femoral head and the acetabulum.
Two types of cartilage are present in the hip:
- Articular: covers the surface of the femoral head
- Labrum: fibrocartilaginous ring that sits around the majority of the outer rim of the acetabulum
Within the joint capsule are three main ligaments that provide stability and 27 muscles that cross the hip joint which also contribute to stability and motion. More superficially, there are many vessels and nervous structures crossing the joint.
What is femoroacetabular impingement?
The femoral head is a spherical structure that articulates congruously with the hemispherical acetabulum. Femoroacetabular impingement (FAI) is a mechanical disorder that occurs when there is atypical abutment of the acetabulum with the femoral head-neck junction due to an abnormal shape of one or both bones. As this impingement occurs, the labrum and outer edges of cartilage are trapped between the bones and can become irritated or torn.
Three types of FAI
- Cam type: Aspherical femoral head creates mismatch between the shape of the ball and socket in which the femoral head exerts abnormal shearing forces across the labrum and cartilage as it enters the joint
- In pincer type: Excessive bone at the rim of the acetabulum causes an over-constrained joint and a crushing force on the labrum as the femoral head impinges upon the edge of the acetabulum
- Subspine impingement: Enlarged anterior inferior iliac spine, which sits just above the hip joint and is the attachment site for the rectus tendon, can impinge upon the femoral head with hip flexion
Providers do not know the cause of these bony deformities, but they could be associated with high levels of activity and participation in sports at an early age, especially hockey, basketball, lacrosse, soccer and football. Males typically have larger cam deformities, while pincer morphology is more common in females.
Femoroacetabular impingement symptoms
Patients who have experienced injury to these soft tissue structures typically feel pain in the groin during activities involving flexion or hip rotation. They may also experience a sensation of popping or catching. Due to joint pain, patients often develop abnormal compensatory muscle patterns that can cause pain in these muscles and tendons as well as exacerbate the joint pain from FAI.
While patients may not have a specific injury that triggered the pain, they may describe a subtle onset of pain with activities. Pain with sitting and pain with activities is common. Patients will feel true hip joint pain in the groin, but many describe a deep pain centered in an area wrapping from the groin to the lateral hip. This is called the "C" sign (patients often cup a hand around this area in a "C" to describe the location). Pain in the posterior hip, around the greater trochanter, or in the gluteal muscles is generally musculotendinous pain due to compensatory movement patterns.
Diagnosis of femoroacetabular impingement
In cases of FAI, doctors traditionally find groin pain with hip flexion and hip flexion along with internal rotation during examination. It's common that patients also experience limited hip internal rotation because the impingement limits the rotation of the femur into the acetabulum.
Providers initially perform a radiographic work-up with plain films if FAI is suspected. This includes an AP pelvis and lateral view of the hips. Instead of the more commonly obtained Frog lateral, a Dunn lateral view for FAI is the preferred approach. This is because the greater trochanter does not obscure the femoral neck in this position.
Doctors use these radiographs to help determine if there are bony deformities suggestive of cam, pincer or subspine impingement. An MRI is helpful to evaluate the labrum, cartilage and the surrounding muscular structures that may show signs of tendonitis or bursitis. MRI is also useful to rule out pathology that can mimic FAI pain such as stress fractures, neoplasms or avascular necrosis.
Treatment for femoroacetabular impingement
The initial treatment providers use for confirmed FAI with a labral and/or cartilage injury is activity modification, NSAIDs and physical therapy (PT). PT can't change the bony anatomy that caused FAI, and it can't heal the labral and cartilage injury. However, it can retrain the patient to use the appropriate muscles for their activity and improve pain to possibly prevent surgery.
Muscles cross the hip joint
Months of recovery before return to normal activities after surgery
Of patients return to sports at or above pre-op level
If pain is persistent and surgical intervention is warranted, doctors treat FAI with a minimally invasive hip arthroscopy. To relieve stress on the labrum and cartilage, providers perform a procedure to repair the torn labrum and to shave down the abnormal femoral and acetabular bone.
The goal of femoroacetabular impingement surgery is to relieve the bony deformity that caused the labral and cartilage injury, repair the torn labrum and prevent further degeneration of the hip joint so that the patient can return to unrestricted activities without pain.
After surgery, patients will be on crutches with partial weight bearing for several weeks and should begin PT within the first few days after surgery. Patients return to activities around six months after surgery.