Osteoarthritis is a disorder of diverse etiologies, which commonly can affect the hip. 5,11,24 A Workshop on Etiopathogenesis of Osteoarthritis (OA) provided a definition and a classification system for OA that includes a category of patients with idiopathic osteoarthritis, for whom no etiologic factors can be determined. 6
The biomechanical principles for development of OA of the hip generally are based on the calculations of force transmission in that cartilage degeneration is thought to be initiated by concentric or eccentric overload. 1,22 A maloriented articular surface with decreased contact area in circumstances such as developmental dysplasia of the hip typically leads to excessive and eccentric loading of the anterosuperior portion and subsequently promotes the development of early OA of the hip. 18,19,27 This widely accepted theory implicating axial overload for the onset of OA of the hip fails to provide a satisfactory explanation for development of arthritis in groups of often young patients with apparently normal skeletal structures and intraarticular pressures.
Based on our clinical experience spanning more than a decade, we have evidence that in many cases of idiopathic arthritis, predisposing factors, in the form of femoroacetabular impingement, are present that are not appreciated readily using the traditional diagnostic modalities. 3,7,12,21 The theory implies that in certain aberrant morphologic features of the hip, abnormal contact between the proximal femur and the acetabular rim that occurs during terminal motion of the hip, leads to lesions of acetabular labrum and/or the adjacent acetabular cartilage. The phenomenon is more common in young and physically active adults. The early chondral and labral lesions continue to progress and result in degenerative disease of the joint if the underlying cause of impingement is not addressed. 7,13 A previous publication from this institution provided evidence for a type of femoroacetabular impingement to exist. 12
We describe the clinical presentation, radiographic findings, the mechanism, and the distinct types of femoroacetabular impingement. Furthermore, the nature of the surgical procedures used at our institution for the treatment of femoroacetabular impingement also will be outlined.
Femoroacetabular impingement usually presents in active young adults with slow onset of groin pain that often starts after a minor trauma. During the initial stages of the disease, the pain is intermittent and may be exacerbated by excessive demand on the hip such as athletic activities or prolonged walking. The pain often is present after sitting for a prolonged period. Based on the presence of normal radiographs of the hips, these patients sometimes are subjected to extensive diagnostic workup and inappropriate surgical therapeutic modalities including laparoscopy, laparotomy, knee arthroscopy, lumbar spine decompression, and inguinal hernia repair.
Examination of the hip often reveals limitation of motion particularly the internal rotation and adduction in flexion. The impingement test almost always is positive. This test is done with the patient supine, the hip is rotated internally as it is flexed passively to approximately 90° and adducted. Flexion and adduction leads to the approximation of the femoral neck and the acetabular rim. Forceful additional internal rotation induces shearing forces at the labrum and creating a sharp pain when there is a chondral lesion, a labral lesion, or both. 16 Occasionally, posteroinferior impingement also can occur. The provocative test to elicit the latter type of impingement is done by having the patient lie supine on the edge of the bed and have the legs hang free from the end of the bed, to create extension. External rotation in extension giving rise to severe deep seated groin pain is indicative of posteroinferior impingement. A positive impingement test has been correlated closely to acetabular rim lesions as seen on specific modern MRI arthrograms of the hip. 15
An orthograde AP radiograph taken with the patient standing and a lateral radiograph of the hip routinely are taken for all patients with the symptoms suggestive of femoroacetabular impingement. The optimal AP radiograph is taken when the coccyx points toward the symphysis pubis with a distance of 1 to 2 cm between them. The radiographs often appear normal at first. However, on detailed review some abnormalities may become apparent including the presence of a bony prominence usually in the anterolateral head and neck junction that is seen best on the lateral radiographs, reduced offset of the femoral neck and head junction, and changes on the acetabular rim such as os acetabuli or double-line that is seen with rim ossification. Close scrutiny of the femoral neck may reveal the presence of herniation pits,which we postulate, are indicative of impingement. Morphologic changes affecting the acetabulum and/or the proximal femur such as retroversion, relative anterior over coverage, coxa profunda, 9 protrusio acetabuli, coxa vara, extreme coxa valga, or occasionally subtle dysplasia only may become apparent on systematic examination of the plain radiographs. We routinely request radial sequence MRI arthrograms, done under well-established protocol and leading to very high quality images, to observe the labrum and the acetabular cartilage. 15,30 Magnetic resonance arthrograms also are capable of detecting or confirming nonsphericity of the femoral head, low offset of the neck, herniation pit, or ossification of the rim all resulting from impingement. Magnetic resonance arthrograms are very sensitive and specific for detecting labral and chondral lesions but have limitations for detecting undetached chondral separations. 30
MECHANISM OF FEMOROACETABULAR IMPINGEMENT
Femoroacetabular impingement is a condition of abnormal contact that may arise as a result of either abnormal morphologic features involving the proximal femur and/or the acetabulum or it may occur in patients, with otherwise normal or near-normal anatomic structure of the hip, who experience impingement as a result of subjecting the hip to excessive and supraphysiologic ROM. Often a combination of factors, including the presence of aforementioned causes may lead to femoroacetabular impingement. In patients with morphologic features of the acetabulum and/or the femoral neck, less motion is required before abutment around the hip occurs. Unlike a hip with a prosthesis, the native hip is under stricter constraint, and therefore cannot easily escape the detrimental effects of any contact or shearing forces.
TYPES OF FEMOROACETABULAR IMPINGEMENT
Based on the pattern and the various stages of chondral and labral injuries observed in situ during surgical dislocation of the hip, two distinct types of femoroacetabular impingement could be distinguished.
The first or cam impingement is caused by jamming of an abnormal femoral head with increasing radius into the acetabulum during forceful motion, especially flexion. 12,21 (Fig 1A). The resulting shear forces produce outside-in abrasion of the acetabular cartilage and/or its avulsion from the labrum and the subchondral bone in a rather constant anterosuperior rim area. Chondral avulsion in turn leads to tear or detachment of the principally uninvolved labrum.
The second or pincer impingement is the result of linear contact between the acetabular rim and the femoral head-neck junction (Fig 1B). The femoral head may have normal morphologic features and the abutment is the result of acetabular abnormality, often a general (coxa profunda) or local anterior over coverage (acetabular retroversion). The first structure to fail in this situation is the acetabular labrum. Continued impact of abutment results in degeneration of the labrum with intrasubstance ganglion formation, or ossification of the rim leading to additional deepening of the acetabulum and worsening of the over coverage. The persistent abutment, which often is anterior, with chronic leverage of the head in the acetabulum can result in chondral injury in the ‘contre-coup’ region of the posteroinferior acetabulum (Fig 1B).
Chondral lesions in pincer impingement often are limited to a small rim area and therefore are more benign. This is in contrast to deep chondral lesions and/or extensive labral tears that are seen with cam impingement (Fig 2). Pincer impingement is seen more frequently in middle-aged women with a desire for athletic activites(Fig 3). Cam impingement, however, is more common in young and athletic males.
The surgical approach for treatment of femoroacetabular impingement was described by Ganz et al. 7 Briefly, this involves placement of the patient in the lateral decubitus position. Using a trochanteric flip approach, the hip is exposed anteriorly and dislocated in the same direction, while respecting the integrity of the external rotator muscles including the piriformis. This allows full protection of the vascular supply of the femoral head. 8 After doing a Z-shaped capsulotomy, the hip is exposed and examined in detail. The surgical dislocation of the joint provides a full 360° view of the femoral head and the acetabulum for inspection. The site of impingement is identified and the labrum and acetabular cartilage are checked for the presence of any lesions. The surgical treatment comprises mainly of removing any nonspherical portion of the head, thereby improving the neck offset and subsequent clearance. Femoral neck osteoplasty particularly is an important part of alleviating cam impingement. During osteoplasty of the femoral neck, the retinacular vessels that are the main blood supply of the femoral head can be seen entering the superior portion of the neck and are carefully protected. For pincer impingement, this includes reducing the anterior over coverage by excising the bony prominence at the rim or poor mans periacetabular osteotomy. The articular cartilage covering the acetabular rim in the area of the over coverage often is soft and occasionally is detached. Using a nerve hook, the cartilage may be rolled like a slack carpet on the floor. The torn or degenerate area of the labrum also is excised and the remainder of the labrum, if substantial, is reattached to the rim using suture anchors. Excision osteoplasty involving removal of the bony prominence in the extraarticular portion of the femoral head also is done to increase the femoral neck offset by creating a better waist at the head and neck junction. After completion of the femoral and acetabular osteoplasty, the hip is reduced and checked for impingement-free ROM. Excision of the excessive bone is repeated if any residual impingement is detected. Trochanteric advancement, femoral neck or intertrochanteric osteotomy, whenever is deemed necessary, is done to further improve clearance and alleviate extraarticular impingement.
The concept of abnormal morphologic features around the hip leading to OA of that joint in not novel. Stulberg et al 28 reported on an abnormal head-neck relationship on AP radiographs of their patients presenting with idiopathic arthritis. They coined the term pistol grip deformity to describe the radiologic appearance of this morphologic abnormality but did not elucidate the mechanism responsible for the deformity. Other authors also have alluded to abnormal anatomic relationship between the femoral head and neck as a possible cause for OA. 5,19,23,27,29 The majority of patients with femoroacetabular anomalies lack a clear history of hip disease. Therefore, as suggested by some authors, the morphologic abnormalities around the hip in these patients are most likely the result of a subtle developmental abnormality such as coxa profunda, 9 retroversion, 25 or subclinical epiphyseal slip or displacement. 10
Several investigators have implicated subclinical displacement of the femoral epiphysis as a risk factor for OA and have proposed the term head-tilt or post-slip to describe the deformity resulting from mild slips. 10,12,19,28 In one study using a three-dimensional volume computer model, Rab 23 showed that considerable anterior metaphyseal impingement occurred during walking and sitting in patients with slipped capital femoral epiphysis (SCFE). Abutment of the metaphysis against the acetabular rim was thought to be responsible for an increase in the intraarticular pressures, and ultimately lead to degenerative changes in the joint. 23
The association between SCFE and femoroacetabular impingement has been confirmed in a clinical study from our institution. 14 Anatomic derangement lead to reduced clearance of the joint and detectable abutment of the metaphysis against the acetabular rim in patients with SCFE. 14 Depending on the stage of presentation, chondral and labral injuries also were observed in patients with SCFE. A similar situation also is present after femoral neck fracture, when the femoral head is fixed in retrotilt. 4
Other, although not all inclusive, possible disease processes resulting in morphologic derangement around the hip includes residual childhood diseases such as Perthes, and prior surgical interventions such as femoral osteotomies. In our series of patients having surgical treatment for femoroacetabular impingement, we have been able to identify a subgroup of patients with prior periacetabular or intertrochanteric osteotomy in whom impingement was a direct result of reduced joint clearance. 20 Although detailed analysis of the outcome of surgical intervention still is ongoing, the preliminary results indicate that surgical dislocation of the hip and improvement of the head and neck offset was successful in addressing these patients’ symptoms arising from the underlying impingement. It therefore is imperative that the goal of improving joint congruity and coverage, by doing osteotomies around the hip, is not achieved at the expense of reducing clearance and creating subsequent femoroacetabular impingement.
Various of conditions affecting the acetabulum also can lead to morphologic changes predisposing the hip to impingement. Some of the conditions observed to cause femoroacetabular impingement include acetabular retroversion, coxa profunda, protrusio acetabuli, and posttraumatic deformities. Retroversion of the acetabulum has been described as a posteriorly orientated acetabular opening with reference to the sagittal plane. 25 A retroverted acetabulum may occur as part of more complex acetabular developmental deformities, 19 or it may be seen as an isolated entity. 2 Retroversion results in a prominent anterolateral acetabular edge, creating an obstacle for flexion and internal rotation and in turn predisposes the hip to femoroacetabular impingement. This situation is made worse if the prominent acetabular edge impinges against a proximal femur with a low head and neck offset such as seen in hips with a pistol grip deformity. 11,28 Symptomatic impingement resulting from underlying acetabular retroversion has been treated successfully with reverse periacetabular osteotomy in a group of 26 patients from our institution. 26 Femoroacetabular impingement that was alleviated by removing the relative anterior over coverage could be confirmed in all patients. Labral and chondral lesions in the anterosuperior region of the acetabulum resulting from the repetitive trauma of impingement were observed in more than 1/2 of the patients in the aforementioned series, which may explain the association of acetabular retroversion with development of OA. 29
Coxa profunda and protrusio acetabuli, by increasing the relative depth of the acetabulum also can result in femoroacetabular impingement. 9,13 One unpublished study from our institution detected a high incidence of femoroacetabular impingement in patients with coxa profunda when compared with a group of patients with normal and asymptomatic hips. This finding is intuitive in that deepening of the socket results in a relative decrease of the femoral neck, a decrease in the neck to head ratio, a decrease in femoral neck offset, or an increase in relative circumferential over coverage of the femoral head. All the latter conditions affecting the acetabulum with a relatively normal proximal femur lead to pincer abutment of the rim against the femoral neck and lesions limited to the rim area with deep chondral lesions being rarely encountered. Pincer impingement is more common in middle-aged women with morphologic abnormalities of the acetabulum. This is in contrast to cam impingement, which is more common in young males with morphologic abnormalities involving the femoral head. A previous study from our institution, using an MRI-based quantative anatomic study of the femoral head and neck, confirmed that anatomic variations in the proximal femur were responsible for the cam femoroacetabular impingement. 12 A significant reduction in the mean femoral anteversion and mean head-neck offset in the anterior aspect of the femoral neck was seen in patients presenting with impingement when compared with a group of age-matched and gender-matched control subjects. 12 Subsequent studies, using standardized MRI, have confirmed that hips of patients with symptomatic impingement have significantly less concavity at the femoral head-neck junction, when compared with normal hips. 21
Cam impingement results in deep chondral injuries and subsequent labral detachment. Two sets of observations made during surgical dislocation of the hip have furnished the evidence in support of chondral injury leading to labral tear and not the reverse as suggested by some surgeons doing arthroscopic examination of the hip. 17 First, all labral tears or detachments occur at the articular margin and not the capsular margin. The other observation is that chondral injuries without labral tears frequently are seen at the early stages of the impingement process. We think that solitary labral tears arising from an acute traumatic event is rare. Labral tears not associated with chondral injuries only are observed in patients with early pincer impingement. Labral tears, particularly in the anterosuperior region of the acetabulum, seen during arthroscopic examination of the hip, 17 most likely represent femoroacetabular impingement. Although some of these patients may have a history and clinical examination suggestive of traumatic etiology and confirmatory of labral tear, it is the underlying femoroacetabular impingement, however subtle, that leads to a labral lesion as part of a more extensive injury. This premise is supported by the observation that the majority of labral tears, seen during hip arthroscopy, also are associated with chondral injury. 17 McCarthy et al, 17 reporting more than 400 hip arthroscopies, noted a highly significant association between the presence of labral lesions and degeneration of the articular surface.The labral and articular lesions almost always were located in the same region of the acetabulum in their series and the relative risk of significant chondral lesion approximately doubled in the presence of labral lesions. 17 Approxiamtely 2/3 of their patients with fraying or a tear of the labrum had evidence of chondral damage. It is plausible that some chondral lesions may go undetected during hip arthroscopy that in turn accounts for the higher than expected incidence of isolated labral tears.
Through extensive clinical observations with more than 600 surgical dislocations of the hip, we have observed that femoroacetabular impingent results in lesions of the joint and acts as a precursor for early degenerative disease of the hip in patients with idiopathic arthritis. Previous and ongoing studies have helped us gain insight into the pathomechanism of conditions resulting in femoroacetabular impingement and paved the road for additional investigations to unravel the mechanism of this entity. Understanding the exact mechanism of impingement, as a possible cause of arthritis of the hip, would enable therapeutic opportunities to be developed and explored. Although long-term results are awaited, the surgical treatment of patients with femoroacetabular impingement has been encouraging so far. It is proposed that recognition of this entity and early intervention before the degenerative process is advanced, is likely to have a considerable impact on the natural history of the disease, delaying the onset of end-stage arthritis in this young group of patients.
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