Abnormal contact between the proximal femur (ball side of the joint) and acetabulum (cup side of the joint) may result from a variety of conditions. In some cases, it is genetically determined that you will have the problem. In some cases, you may have a previous fracture in the area of the hip ball. Chronic impingement may cause damage to the cartilage as a result of excessive contact with the abnormally shaped parts of the hip.
Pincer impingement occurs from direct contact on the acetabular rim at the femoral neck. Excessive anterior acetabular coverage, seen more often with a condition known as coxa profunda (a deep socket) or acetabular retroversion (where the cup is rotated backwards), are the primary culprits. The femoral head may be of normal appearance. Pincer impingement causes a compression or crushing of the labrum between the femoral neck and acetabulum.
The first structure to fail appears to be a small area of the labrum causing labral degeneration, ganglions around the joint or labral ossification (where the tissue becomes hardened). Continued abutment against an ossified labrum can lever the femoral head out of the socket creating head contact on the posteroinferior acetabulum with resultant chondral lesion or reactive osteophytes in that quadrant. Pincer impingement has been suggested to be more prevalent in women, mostly in those who engage in athletic activities.
In many instances, the pathologies overlap. Most patients will have a combination of pincer and cam impingement as an etiology for their clinical symptomatology. It is important to address both of these pathologies at the time of surgery or further degeneration of the labrum and joint may occur.
Standard xrays often illustrate the morphological relationships of the femoral head and neck. A true AP pelvis, AP and lateral of the affected hip highlight structural landmarks allowing assessment of head-neck offset and acetabular version. Either a frog or cross-table lateral view will confirm a CAM lesion. Several radiographic measures evaluate the femoral neck for dysplasia. The lateral center edge angle (CE angle of Wiberg) is measured on the AP view from the center of the femoral head vertically and an angle measured from the lateral rim. An angle less than 20 degrees is consistent with dysplasia. Greater than twenty-five degrees is considered normal with an angle between twenty and twenty-five degrees considered borderline. The femoral head extrusion index is the percentage of the femoral head outside of the acetabular roof. Greater than twenty-five percent is consistent with dysplasia
Measuring acetabular version requires a true AP pelvis radiograph. The film should align the coccyx and symphysis pubis with a separation distance of 1-2cm.( Lines are then traced from the anterolateral edge of the acetabulum along the anterior and posterior projections of the rim. With normal version, the lines should not cross. If the anterior wall is traced more laterally than the posterior wall, the acetabulum is retroverted. This creates a “figure-of-eight” tracing as the anterior wall crosses the posterior wall. In converse, if the posterior line is traced more laterally than the anterior wall, the acetabulum is anteverted.
If the tracings do not cross, then the acetabulum is felt to be in neutral version. In the early stages of FAI, the joint space is rarely affected.
Further diagnostic imaging can document intra-articular pathology and assist in preoperative planning. MRI arthrograms are capable of detecting labral and less reliably, chondral injuries. Marcaine should be combined with the gadolinium-DPTA for diagnostic measures. Asymmetric fluid presence surrounding the femoral neck on T2 images suggests inflammation in the peripheral compartment. The arthrogram will also define the three dimensional anatomy of the femoral head-neck offset, herniation pits, paralabral cysts, and ossification of the acetabular rim. While MRI arthrograms have very good sensitivity and specificity for detecting labral and chondral lesions, they are limited in their ability to reliably detect chondral delaminations. A CT scan with 3D image reconstructions is an excellent preoperative planning adjunct, especially if treating the lesion arthroscopically. The 3D CT scan offers a thorough analysis of a cam or pincer lesion, especially for those that are believed to extend posterior on plain radiographs.
Treating pincer lesions from acetabular retroversion is addressed arthroscopically with a pincer resection and a labral repair. Anterior overhang is often the culprit with an associated ossified antero-superior labrum secondary to chronic impingement. The procedure known as “rim trimming” often necessitates labral detachment and repair following resection of the overhang. Establishing the junction between the labrum and bone with an arthroscopic elevator or a fine tip radiofrequency device allows a clean detachment. The diseased labrum will often lend information regarding the location of the offending acetabular lesion. After the margins have been identified, a motorized shaver clears the soft tissue from the overhanging acetabulum followed by a motorized burr to resect the bony acetabular prominence (typically 5-7mm). Avoiding excessive acetabular resection is important to protect from causing hip instability. Reattachment of the labrum following bone resection is performed with suture anchors. This procedure is also performed under fluoroscopic guidance and with a true AP pelvis, the crossover sign should be routinely checked, stopping when neutral version as been achieved. Following resection of the pincer impingement, the arthroscope should be placed in the peripheral compartment to visualize the head-neck junction during range of motion. Mixed lesions are common and treatment of both pincer and cam lesions are often necessary to eliminate the impingement.