Static overload occurs due to abnormal stress and asymmetrical loading between the femoral head and the acetabulum in the axially loaded position. This results in an incongruent joint with abnormal loading of the acetabular and femoral cartilage, with or without instability. Hip pain related to static overload does not require motion across the hip in order to be painful. The most common type of static overload is dysplasia with lateral or anterior undercoverage of the femoral head. It can also result from excessive femoral antetorsion or a proximal femoral valgus deformity. The key clinical component when evaluating patients with dysplasia is to identify whether the patients are experiencing pain from static overload, which may be best treated with osteotomies that modify the amount of acetabular coverage and/or the position of joint loading. Arthroscopy, which addresses pathology from dynamic components, is unlikely to be helpful for pathologies that are the result of static mechanisms. However, some patients with hip dysplasia also have components of dynamic impingement, which is modifiable with arthroscopic surgery.

Impingement occurs with dynamic movements across the hip joint, and therefore involves both fixed anatomical structures (the bones and joint) and soft tissues such as muscles (nerves, tendons, ligaments, and blood vessels). Dynamic intra-articular impingement occurs with cam impingement, pincer impingement, or a combination of both. Pain develops secondary to repeated functional movement patterns that exceed the anatomic and physiologic capacity of the joint [8]. As joint pathology develops, patients will frequently develop compensatory muscular dysfunction involving the adductor longus, proximal hamstrings, hip abductors, and hip flexors [9, 11, 13]. Muscle atrophy or weakness and motor control deficiencies should be addressed in physical therapy prior to possible surgical intervention. Even in patients with recalcitrant symptoms, it is important to address the muscle weakness postoperatively within pain-free limits in order to maximize their outcome.

Cam impingement is defined by a loss of femoral head–neck offset and an aspherical femoral head. The insufficient offset typically occurs at the 1–2 o’clock position of the femoral head (12 o’clock being the most superior aspect of the femoral head in a standing position). Cam impingement is the most common morphology found in young athletic males [14]. Pathologic changes are caused by repetitive entry of the aspherical portion of the femoral head into the hip joint during flexion and internal rotation. This results in a shear injury to the transition zone of the labrum and the adjacent acetabular articular cartilage [15, 16]. The clinical pathology from cam impingement is a detachment of the transition zone cartilage rather than an intrasubstance labral injury [6, 17]. The size of the cam correlates with the degree and location of cartilage injury [18]. Patients who present with a longer duration of symptoms typically have a higher severity of cartilage injury [17–19]. Transition zone tears that occur in cam deformities may have a better prognosis than intrasubstance labral tears because the transition zone preserves the vascular supply from the capsule [20].

The second common category of dynamic impingement is pincer or rim impingement, and results in compressive injury to the labrum. Rim impingement can result from acetabular retroversion or overcoverage. Acetabular retroversion can be seen on plain radiographs as a cross-over sign. However, the cross-over sign on plain radiographs can be manipulated by orientation of the acetabulum, so retroversion is best categorized on cross-sectional imaging [21–23]. Focal overcoverage at the anterior superior acetabulum can appear as a cross-over sign on plain radiographs even though the patient has normal acetabular version as seen on a CT scan. Focal resection of the overcovered area is most appropriate in this clinical scenario as opposed to an osteotomy to correct version. Both focal retroversion [12] and global overcoverage are more common in females [13, 24, 25]. Another mechanism for rim impingement occurs with retrotorsion of the proximal femur. Patients with retrotorsion have reduced functional internal rotation and increased external rotation [8, 26, 27]. Compressive injuries to the labrum cause an intrasubstance injury, which frequently results in heterotopic bone formation, making it less amenable to repair [21]. Rim impingement results in more limited chondral damage as compared to cam injury mechanisms [8]. With rim impingement, occasionally the femoral head will lever out of the joint from supraphysiologic motion against a relative point of rim overcoverage, resulting in a contrecoup chondral injury [12, 28]. Patients that fall on a flexed and adducted hip with a posteriorly directed force can experience hip instability. Hip subluxation or dislocation can result due to abnormal contact between the anterior femoral head against the acetabulum [28, 29]. Posterior hip instability, ranging from frank dislocation to subtle instability, has been documented in the setting of femoroacetabular instability due to posterior levering of the femoral head from decreased internal rotation [30]. While rim impingement can occur in isolation, the most common presentation is a mixed form of cam and rim impingement [6, 12].

In addition to intra-articular impingement, trochanteric-pelvic, ischio-femoral, and subspine extra-articular impingement may occur. Trochanteric-pelvic impingement occurs when the greater trochanter abuts up to the pelvis during abduction and extension, and is typically caused by Perthes disease and the resultant short varus femoral neck [22, 31]. Ischio-femoral impingement is a result of abnormal contact between the ischium and the lesser trochanter. Clinical examination may identify this with posterior hip and buttock pain that are incited and exacerbated by extension, adduction, and external rotation of the hip [23, 24] Subspine impingement results in contact between the anterior inferior iliac spine and the inferior femoral neck in hyperflexion. Apophyseal avulsion injuries in adolescents can lead to an elongated anterior inferior iliac spine, and a narrowed subspine space. Extra-articular impingement can result in localized pain to the site of impingement, but it also could present atypically if nervous or vascular structures are caught in the sites of impingement.

The clinical evaluation and workup for patients with FAI can be challenging due to the various types of impingement and overlapping pathologies. A layered approach to diagnosis often will provide a comprehensive diagnosis [32]. The layers from deep to superficial are comprised of the osteochondral layer, capsule-ligamentous layer, muscular layer, and the neural layer. The diagnosis should take into account the effects of the underlying bony morphology upon these tissues. The history should guide the clinical examination, and should, in turn, direct the imaging studies conducted.

The patient’s history is the cornerstone of appropriate decision making in the setting of hip pain and FAI. Many patients with FAI by radiographic criteria alone are asymptomatic [33–37]. It is therefore essential to identify whether the patient’s symptoms are a result of FAI or a different etiology. Pain from FAI typically occurs with specific activities that create anatomical impingement. Internal impingement caused by a mixed cam and pincer mechanism frequently results in groin pain that is exacerbated by activities that place the hip into a position of impingement: flexion, adduction, and internal rotation (FADIR). Patients typically report difficulty sitting for long periods of time or pain with activities that require recurrent or constant hip flexion such as maintaining an athletic stance. Patients with a longer duration of symptoms will likely have more severe cartilage injury and may have a worse prognosis than patients with a shorter duration of symptoms. Traumatic injuries such as a fall onto a flexed knee with a flexed hip above it may signal a potential subluxation event. Snapping, popping, clicking, or catching may be the result of labral pathology, synovitis, loose bodies, a hypertrophic ligamentum teres, iliopsoas snapping over the anterior capsule-labral complex, or IT band snapping over the greater trochanter. The location of the pain also helps establish the diagnosis. Hip pain from mixed cam and pincer FAI frequently localizes to the groin, directly anterior to the hip joint. The senior author has noted a possible correlation between patients with posterior buttock pain that have synovitis of the ligamentum teres, with resolution of symptoms after ligamentum teres debridement. Pain frequency and temporal pattern are also important components of the history, since pain from FAI is less likely a constant phenomenon, and more likely to have a mechanical etiology. Constant pain may be unrelated, or it may reflect more severe joint damage. It is also important to understand the patient’s current activity levels and desired activity levels. The patient’s athletic participation should be assessed as well as the level at which they participate. The patient should be questioned about potential modifications that they have had to make to their normal performance. If surgical treatment is indeed the plan, expectations for return to sport should be assessed and counseled. Each of these questions will help the clinician better understand the patient’s problem and goals for treatment.

Effectiveness of prior treatments should be described during the initial assessment. Common modalities for hip pain include physical therapy, active release therapy, non-steroidal anti-inflammatory medications (NSAIDs), and steroid injections into the hip. The efficacy of the treatments can help the clinician understand the patient’s specific pathology and direct further treatment. It is also important to understand the types of treatment that occurred in physical therapy, passive therapy consisting solely of modalities is much less effective than a combination of deep tissue work and specific core and gluteal strengthening exercises. Also, the patient’s specific response to an intra-articular hip injection should be investigated, as often times the relief will be fleeting and therefore deemed to be unsuccessful by the patient. This could suggest that the local anesthetic identified the problem as being intra-articular, even though the steroid was not efficacious.

The first line of treatment for patients diagnosed with FAI is non-operative management. Activities that exacerbate the patient’s symptoms should be modified to alleviate pain. NSAIDs taken on a consistent basis may assist with pain and inflammation secondary to impingement. Physical therapy should be directed to address muscular weakness, motor patterning, posture, and soft tissue restrictions. If the patient has an increased pelvic tilt, physical therapy may be beneficial in restoring lumbopelvic control and aiding the patient in establishing greater arc of pain-free motion. No data currently demonstrates the efficacy of these interventions for symptomatic hips, and further research is needed in this area.

Corticosteroid injections with local anesthetic may be a useful modality to localize pain and treat inflammation. Patients that have significant pain relief following a local anesthetic injection have a 90 % likelihood of an intra-articular abnormality [42]. While it is expected that symptoms from intra-articular impingement will resolve with an injection, symptoms from extra-articular impingement should not be affected [43]. Lower volume anesthetic injections (<5 mL) are less likely to cause discomfort from capsule over-distension, which may result in a persistent pain from the injection even though the patient’s underlying symptoms are completely masked. If the injection is diagnostic, it is important that a clinical examination is performed prior to the injection, and again shortly after it is completed. This allows the patient and the surgeon to compare how effective the injection was in alleviating impingement symptoms. It is unknown whether corticosteroid injections provide any benefits or detriment to the hip joint in the treatment of FAI.

Many studies have demonstrated that open and arthroscopic surgical approaches can be effective for the correction of mechanical abnormalities due to symptomatic FAI [11, 44–51]. The operative intervention that the surgeon is most comfortable with should be used to correct the pathoanatomy identified during the workup. In the setting of FAI, the surgeon should be able to perform an acetabuloplasty, a femoral head osteoplasty, chondroplasty, and labral debridement or refixation. There are unique challenges to both open and arthroscopic techniques to treat these pathologies. Some pathologies are better suited for open procedures, such as FAI with underlying femoral torsion or acetabular version abnormalities.

The indication for surgery in patients that have a diagnosis of FAI is recalcitrant pain despite conservative modalities. Ideally, the patient exhibits clear evidence of a treatable structural pathoanatomy that correlates with soft tissue injury patterns and pain on clinical examination. The ideal candidate should present with no evidence of articular cartilage wear.

Surgical contraindications include cartilage loss and asymptomatic patients with incidentally found labrum tears or FAI. Cartilage loss is evaluated on both plain radiographs and MRI, and correlates with a poor prognosis for FAI surgery. Factors that may be indicative of a poor outcome include loss of >50 % of the cartilage thickness in a weight bearing zone, less than 2 mm of joint space in the weight bearing zone, or the presence of full thickness cartilage loss with exposed subchondral bone [47]. Many patients may be diagnosed with FAI or a labral tear incidentally in the process of a workup for a different pathology. Asymptomatic patients with FAI or a labral tear should not undergo surgery based upon the current best available evidence [33–37, 52].

For patients who undergo surgical intervention for FAI, numerous complications should be considered and discussed with the patient prior to the procedure. The most likely complication is continued pain from insufficient bone resection and persistent postoperative impingement [53]. Inadequate resections are related to surgeon inexperience and inaccurate portal placement, and is the most commonly cited cause for failure in the literature [53, 54]. Many other intraoperative complications such as iatrogenic cartilage injuries, labral injury due to traumatic access into the central compartment, or instrument breakage, are likely underreported. Transient or permanent nerve palsies can also result from prolonged traction times and inexperience [54]. Injuries to the sciatic, femoral, lateral femoral cutaneous, and the pudendal nerves occur at a rate of 10 % of the cases performed [55, 56]. Placement of the perineal post for traction results in the potential for pressure necrosis of the skin in the perineum secondary to prolonged traction times. Abdominal compartment syndrome has been reported in the literature as a result of fluid extravasation from the hip into the abdomen [57, 58]. Avascular necrosis is a complication which likely results from disruption of the blood supply to the femoral head from traction or surgical techniques [59]. Instability and dislocations have been reported to occur after capsulotomy in patients undergoing hip arthroscopy. Patients with dysplasia are at a higher risk for postoperative instability [27]. Heterotopic ossification occurs in as many as 8 % if cases of FAI surgery [60]. Heterotopic ossification likely arises secondary to increased bone debris from the osteoplasty as well as increased bleeding associated with capsular incisions. It can be reduced to <1 % of patients with NSAID prophylaxis with Indomethacin 75 mg daily for 4 days followed by Naprosyn 500 mg twice daily for 4 weeks [60]. Most cases of heterotopic ossification are asymptomatic; however, there are rare cases of symptomatic Brooker III or IV heterotopic ossification after hip arthroscopy. Heterotopic ossification restricts motion and causes pain that may require revision surgery. The incidence of deep venous thrombosis is reported to be as high as 3.7 %, but the senior author’s experience is that the rate is substantially lower [53, 61]. Femoral neck fracture is another rare complication that can occur from over-resection of the cam deformity [62].

Arthroscopic surgery for FAI is the treatment of choice by the senior author. The surgery is divided into eight separate steps in order to systematically approach each component of pathoanatomy. The steps are: (1) patient positioning, (2) portal access, (3) joint visualization, (4) rim preparation, (5) labral refixation, (6) establishing access to the peripheral compartment, (7) femoroplasty, and (8) capsule closure. The procedure is divided into these steps in order to delineate clear goals with each step that will maximize patient safety and good outcomes.

The first step of surgery is patient positioning. The goal is to position the patient so that safe, reliable access can be obtained for the procedure while minimizing risk to the patient. Consistency in positioning techniques with the surgeon and the ancillary staff will minimize errors. Hip arthroscopy can be accomplished in both the supine and lateral positions. The ipsilateral arm should be positioned securely with padding so that it remains outside of the operative field throughout the procedure. A large, padded perineal post is used to decrease the risk of pressure necrosis or pudendal nerve palsy during traction. The feet should be securely attached with appropriate padding so that traction can be maintained throughout the procedure. A small amount of counter-traction should be applied to the contralateral hip during setup to provide a fulcrum through which the hip post can maintain traction.

Traction is initially applied by placing longitudinal manual traction to the ipsilateral hip while it is positioned in about 30° of abduction. The hip is then adducted to a neutral position while visualizing the amount of distraction with fluoroscopy. Approximately 8–10 mm of joint space widening confirms adequate distraction. It also is helpful for the surgeon to be attentive to the suction seal on the hip and the ease with which the hip is dislocated. Hips that easily dislocate with minimal traction may be less stable, which may indicate how robust a capsule repair is required to prevent postoperative instability. If the traction seal is not broken with distraction, then a spinal needle can be carefully placed into the joint at the beginning of the procedure. Internal or external rotation of the hip may also facilitate hip joint access for patients that have proximal femoral torsion deformities (attempt to mitigate the deformity by rotating the foot in the opposite direction of the deformity). It typically requires about 50 pounds of force to ensure adequate distraction during the procedure. Traction time should be minimized to reduce the risk of pressure necrosis or neurologic traction injury.

The second step in hip arthroscopy is to gain access to the joint through portal placement. Byrd described three portals to access the hip joint: the anterolateral peritrochanteric portal, the posterolateral peritrochanteric portal, and the anterior portal [63, 64]. Since then, many different portals have been described, but the two most commonly used portals are the anterolateral peritrochanteric portal and an anterior portal. Additional portals include the distal anterolateral accessory portal, the posterolateral peritrochanteric portal, the proximal anterolateral accessory portal, and various distal entry points that optimize suture anchor placement [65].

The first portal established is the anterolateral peritrochanteric portal. A spinal needle is placed approximately 1–2 cm superior and 1–2 cm anterior to the anterosuperior edge of the greater trochanter. While the hip is in traction, a palpable band (the iliotibial band under tension with the attachment of the gluteus maximus) can be felt just superior to and along the anterior edge of the greater trochanter. The portal should be placed just posterior to this band. A spinal needle is then advanced parallel to the floor into the hip joint. Ideally, the spinal needle plane on AP fluoroscopy is parallel to the sourcil of the acetabulum, and positioned as closely to the femoral head as possible without hitting the articular cartilage. This placement decreases the likelihood of passing the needle and subsequent cannula through the labrum. As the suction seal is released by way of an air arthrogram, there may be an increase in the amount of distraction. If the spinal needle maintains proximity to the femoral head with increased distraction, then it is less likely that the labrum was penetrated. Lastly, the joint should be insufflated with about 20 mL of saline, which should then produce a fluid flashback to confirm intra-articular placement without labral or soft tissue blockage. After this is verified, a guide wire is then placed through the spinal needle until it rests in the central fossa of the acetabulum. A small diameter cannula (4.5–5.0 mm) is then advanced into the joint over the guidewire. A 70° arthroscope is then advanced into the joint through the cannula. The scope is aimed in the anterior direction in order to visualize the junction between the femoral head and the anterior acetabulum and labrum. The fluid pump is left off until an outflow is established.