Extra-articular impingement can occur from the area above (note the red area above the ball of the hip joint, that juts out from the pelvis. This is the AIIS, which can be a point of hip impingement that occurs outside the joint.
Types of Hip Impingement that Can Cause Hip Pain
by Dr. Ginger Garner PT, DPT, ATC-Ret
What types of hip impingement can cause pain and injury in the hip? First – there are two well known forms of hip impingement currently documented in the literature: 1) CAM type FAI (femoracetabular impingement) and 2) Pincer type FAI. These two types are found inside the joint, meaning they are considered intra-articular bony anomalies.
FAI is a common comorbidity found with hip labral injury (HLI); and in fact, FAI is a risk factor for HLI. Specifically, FAI is a bony impingement that arises in the femoral head-neck function and the rim of the acetabulum (see photo at right).
The edge of the acetabulum can be larger than needed, leading to a “pincer” type of impingement. The other type, “CAM” is when the femoral neck can be larger. The most common type is combined pincer and CAM.
The two types of FAI also generally occur together more than they do in isolation. However, it is possible that, combined with other issues like acetabular undercoverage or hip instability, CAM or Pincer-type FAI can be found a singular diagnosis.
Extra-articular Hip Impingement: A New Discovery in Hip Preservation
However, the arena of impingement in the hip is now evolving to consider other locations. In the past 5 years there has been buzz about other types of FAI. They aren’t classically considered FAI issues since this new type of identified impingement occurs outside (extra-articular) the joint. One type newly identified is known as anterior inferior iliac spine/subspinal hip impingement (AIIS). In a 2011 study of 3 case reports, AIIS was found and treated with arthroscopic AIIS decompression with positive results. A more recent 2012 study found excellent results at short-term follow up for surgical decompression of AIIS.
Identification & Diagnosis of AIIS Impingement
Both personal and professional experience in the area of AIIS has shown that AIIS is not always discovered on an AP (anterior-posterior) radiograph. However, it is possible to see a larger AIIS on an AP film. Another helpful (but not always definitive) diagnostic test is a CT scan with MRI 3D reconstruction (and no contrast). Bony contrast is more reliable with CT scan than the typically preferred MRA (which is better for soft tissue contrast).
In addition, the rectus femoris (RF) could be implicated in AIIS pathology because the same area receives the proximal attachment of the RF. The same 2011 study reported that the morphology and role of the RF in extra-articular impingement is “not well reported at this time.”
Likewise, the identification of AIIS as a primary driver of pathology in intra-articular hip injury (FAI and/or HLI) is rare. Some cases of AIIS are being found during hip arthroscopy to correct identified existing deficits such as FAI and/or HLI. This means that AIIS may be missed and should be included as a potential mechanism of injury, especially for anterosuperior labral tears in the 2 to 3 o’clock region.
People who have AIIS may present like a typical HLI patient, which means they may have a positive Thomas test, FADDIR test, or mechanical symptoms such as popping, clicking, grinding or giving way. It is important to note these signs and symptoms and work in a team approach with surgeons and physical therapists who specialize in hip preservation and reconstruction.
What We do At Garner Pelvic Health
Resources for Learning More
- Dr. Garner’s Hip Arthroscopy Blog
- Dr. Garner’s CEU Course on Hip Arthroscopy Rehab – Bridging the Gap between Sports Medicine and Pelvic Health
- Garner Pelvic Health – Free Consult with Dr. Garner
- Femoracetabular Impingement – American Academy of Orthopedic Surgeons
