Extra-articular Hip Impingement: A New Discovery in Hip Preservation
There are two accepted forms of hip impingement currently documented in the literature. The two types are 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 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.
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
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.
Patients 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.
For health care professionals: To learn more about nonoperative and operative hip labral and FAI management, check out Ginger’s continuing education course on Extra-Articular Pelvic and Hip Labrum Injury: Differential Diagnosis and Integrative Management. The next opportunity to take the course is March of 2015 in Houston.