Femoroacetabular Impingement
S.M Hussain, D Paton, K Shah, S Patil
Department of Orthopaedics, Southern General Hospital, Glasgow
Correspondence to:
Mr SM Hussain, FRCS [T&O] Mr S Patil FRCS [T&O]
Adult Reconstructive Fellow Consultant Orthopaedic Surgeon
Dept of Orthopaedics Dept of Orthopaedics
Southern General Hospital, Southern General Hospital,
Glasgow Glasgow
Email: sheik.hussain@nhs.net shailasanjeev@gmail.com
Acknowledgements
The authors did not receive grants or outside funding in support for preparation of this manuscript. They did not receive payments or a commitment or agreement to or other benefits from a commercial entity.
Abstract:
Femoroacetabular impingement has been proposed as one of the causes leading to osteoarthritis of the hip. Features are mostly seen in young active individuals. Two morphological types, cam and pincer have been identified. Prompt clinical and radiological examination with appropriate surgical intervention can alleviate the symptoms and delay the degenerative changes associated with this condition.
Key words: Femoroacetabular impingemet, adult hip, hip pain, labrum
Introduction
Femoroacetabular impingement (FAI) has been recognised as one of the causes of hip pain in young adults.1-2 This abnormal hip morphology is presumed to be a causal factor in osteoarthritis.3-4 The condition was first described by Smith-Petersen in 1936.5 A major breakthrough in the field occurred in 2004, when Ganz and colleagues, having carefully investigated the vascular supply to the femoral head and neck, developed a new technique for surgical dislocation of the hip.6-7 This revived the concept of femoroacetabular impingement.
Morphological types
There are two morphological types of FAI. In cam-type, also known as pistol-grip impingement, the anterior-superior femoral head-neck junction is flattened or convex rather than concave. During terminal flexion, the ‘bump’ is forced beneath the acetabular rim. This causes a shearing stress on the labrum and adjacent cartilage, eventually causing complete labral detachment and chondral delamination. The aetiology of the cam morphology is not fully understood, but has been associated with subclinical SUFE, Perthes disease, malunited fractures of the femoral neck and avascualr necrosis8-9.
In pincer impingement there is an increase in the relative overall or anterior acetabular depth, caused by coxa profunda and acetabular retroversion, or true anterior acetabular over-coverage. “Contra coup” chondral lesions, believed to be caused by leverage of the femur against the protruding acetabular rim, are often found on the posteromedial femoral head. Pincer impingement is common in active middle-aged women. True pincer- and cam-type FAI is less common than mixed-type, which incorporates elements of both9.
Clinical features
History
Patients are usually physically active young and middle-aged adults. Common presenting complaint is groin pain, which may be constant and is exacerbated by athletic activities, walking long distances and prolonged sitting. Those activities, which involve repetitive hip flexion and internal rotation, are at increased risk. Symptoms are initially intermittent and insidious in onset but later become continuous when there are associated chondral damage and labral tears.
Clinical examination
On examination there may be evidence of reduced range of hip movement in flexion and internal rotation.10-11 Positive impingement test is a consistent physical examination in patients with FAI.12 In the supine position, the hip is flexed to 900, adducted and internally rotated [Figure 1]. As the hip is flexed and adducted the femoral neck abuts against the acetabular rim. Further passive internal rotation then results in shearing forces at the labrum reproducing the symptoms experienced by the patient. This positive clinical sign is suggestive of anterior superior impingement. 10
Figure 1 The anterior impingement test
Similarly, posterior impingement can be elicited by extending the patient’s leg over the side of the examining table and rotating externally. This reproduces the pain suggestive of posterior inferior impingement. 11
Investigation
Plain Radiograph:
A plain anteroposterior (AP) view of the pelvis and cross table lateral view is taken for all patients suspected of FAI. The AP view is taken in a supine position with hips in 150 of internal rotation to compensate for femoral neck anteversion and also for visualization of the lateral femoral/head neck junction.13 In a standardized AP radiograph, the coccyx points toward the symphysis pubis with the distance less than 2 cm between them. The tear drops, iliac wings and obturator foramina appear symmetrical.12 [Figure 2]
Figure 2 Standardized pelvic anteroposterior radiograph
The features on the femoral side in an anteroposterior view suggestive of impingement are: loss of normal concavity at the femoral head neck junction resulting in flattening of the junction, pistol-grip deformity [Figure 3], presence of herniation pits (radiolucent fibrocystic changes) along the anterior superior aspect of the femoral neck indicative of impingement14 and features suggestive of asymptomatic SUFE.15
Figure 3 Normal head neck Offset [Fig 3A]; Pistol grip [Cam type lesion] deformity of femoral head as a result of loss of head neck Offset [Fig 3B]
On the acetabular side there may be features suggestive of acetabular retroversion. In a normal anteverted acetabulum the anterior wall covers less of the femoral head and lies medial to the posterior wall meeting at the superolateral margin of the acetabulum. In a retroverted acetabulum the anterior wall is lateral to the posterior wall and hence the walls tend to cross each other distally giving rise to a figure of eight ‘cross over sign’.16 [Figure 4]
Figure 4 The crossover sign. Normal acetabular version [Figure 4A] Crossover sign suggestive of retroversion [Figure 4B]
The posterior wall sign aids in selecting a particular surgical intervention.16 Normally in an anteroposterior radiograph the posterior wall should lie at the level or lateral to the centre of femoral head. A positive posterior wall sign suggests that the posterior wall lies medial to centre of femoral head.
The presence or absence of osteoarthritic changes should be documented and graded based on the criteria described by Tonnis.17-18 [Table 1]
Table 1 Grading of OA using the criteria described by Tonnis
Osteoarthritis Grading by Tonnis
Grade 0 No arthritic changes
Grade 1 Sclerosis, mild joint space narrowing, minimal osteophyte formation
Grade 2 Small cystic changes, moderate joint space loss
Grade 3 Large Cysts, Complete joint space loss
MR Arthrography:
MR arthrography has been used increasingly to diagnose FAI.19 The femoral head asphericity is quantified using an alpha angle, which is measured on axial MRI views taken parallel to femoral neck.19-20 The angle is subtended by a line drawn along the centre of the femoral neck and head with a line drawn along the centre of the femoral head to a point at which the femoral head neck contour changes with regards to the femoral head sphericity. An angle above 550 has been considered abnormal.19 MRI arthography also helps to diagnose and precisely localize the labral and chondral damage.21
Retrospective analysis of MRI arthrography carried out at our centre revealed that MRI arthrography was quite accurate in diagnosing labral pathology but was not so accurate in identifying cartilage lesions. Two experienced musculoskeletal radiologists analyzed 20 MRI arthrograms of patients clinically diagnosed of FAI. The sensitivity and specificity of labral tear for observer 1 was 86.7% and 100% respectively, and for observer 2 was 73% and 100% respectively. The sensitivity and specificity of all cartilage lesions combined for observer 1 was 39.25% and 92.8% respectively, and for observer 2 was 21.5% and 96.6% respectively. Inter-observer correlation was good for labral tear lesions and poor for articular cartilage lesions. This study emphasises the limitations of MRA in the diagnosis of cartilage lesions.
Management:
Recent literature evidence has pointed to FAI being a leading cause of primary osteoarthritis (OA).7,12,19 Despite the pathoanatomy of hip impingement being mechanical, genetic predisposition also plays a role in causing OA.22 An initial trial of non-operative treatment with anti-inflammatory medications and activity modifications has been proposed.
Surgery
The surgical management of FAI consists of non-arthroplasty and arthroplasty options. The non-arthroplasty option focuses on improving the head and neck offset by clearing the abutment between femur and acetabulum. This can be achieved by open surgical hip dislocation, hip arthroscopy or combined arthroscopy and minimal open procedure.
Open surgical hip dislocation
With open surgical dislocation it is possible to have a 360 degree view of both acetabulum and femur.6 The results of open procedure are reliable and reproducible.6,23
The preoperative factors to consider prior to committing for joint preservation procedures include the physiological age of the patient and the extent of chondral/labral damage.24 In addition patients with progressive arthritis beyond Tonnis grade II have less satisfactory outcomes.18 The novel approach of surgical hip dislocation developed by Ganz and colleagues arose out of detailed study of the vascular anatomy of the hip6. Using open surgical dislocation, anatomical abnormalities on both femoral and acetabular side can be addressed. 26
Open surgical dislocation for FAI has shown good to excellent outcomes in 75- 80% of the patients.18, 28 Reasons for unfavourable outcomes include: progression of joint degeneration, insufficient correction of femoral and acetabular pathology,18, 29 and intraarticular adhesions.30 The adhesions can be released by performing hip arthroscopy.
Periacetabular Osteotomy
This reorientation osteotomy of the acetabulum31 is indicated when there is acetabular retroversion resulting in pincer impingement with associated positive posterior wall sign.1,23,31, Siebenrock reports 90% good to excellent results with this procedure at an average of 30 months follow up.32 Three dimensional CT scans of the pelvis are obtained for assessment of the acetabular orientation and percentage of femoral head cover prior to this procedure.33
Combined Arthroscopy and Mini open anterior Hueter approach.
This approach is feasible when the femoral head lesion is localised over the anterior aspect. Hip arthroscopy is performed initially to address intra-articular pathology followed by mini open approach using anterolateral incision of approximately 2 to 4cm as described by Hueter to address the femoral side.34
Arthroscopy
Recently, with the increasing understanding of FAI, arthroscopic techniques to access the hip have been described. The principles involved in arthroscopic treatment are similar to those with the open surgical approach.35 However, it is associated with a shorter rehabilitation period and reduced post-operative morbidity. The overall complication rate is less than 1.5%.36 The main lasting complications are insufficient osseous reshaping.
Arthroplasty
With advanced chondral damage arthroplasty may provide more predictable outcome.
Evidence that treatment alleviates symptoms
Three studies of 19–30 patients reported substantial improvement in Merle d’Aubigne scores from means of 12–14.1 preoperatively to 15–16.9 following surgical dislocation with a follow-up of a mean of 2.7 to 4.7 years.18 Between 13-31% required total hip replacement during the follow-up period. In each of these cases the patients had been reported clinically, radiographically and morphologically to have advanced OA. Bizzini 37 reported on five professional ice hockey players who underwent surgery for FAI and returned to competitive activity after a mean of 9.6 months. A study published in 2009 documenting outcomes at a mean of 3.2 years follow-up after open dislocation of 48 hips showed similar results to previous studies, with Merle d’Aubigne scores increasing from a mean of 13 to 16.8.38
Recently fewer studies have shown the benefits of arthroscopic surgery. Philippon et al. reported the results of 112 patients undergoing arthroscopic surgery.35 At a follow-up of 2.3 years there was a mean increase in the modified Harris hip score (HSS) from 58 to 84. 86% had returned to work within eight weeks. Ten patients underwent total hip replacement within 16 months, but each had pre-operative evidence of osteoarthritis. Stähelin found that arthroscopic osteoplasty for cam-impingement increased the Nonarthritic Hip Score in 22 patients from a mean of 49 to 74 after six months.39
Conclusion
The anatomical studies and development of the new surgical approach by Ganz and colleagues has allowed three major advances namely: the option of surgical treatment, with excellent functional results and low complication rates; accurate diagnosis of minor bony, cartilaginous and labral defects in symptomatic patients; lastly, it has increased the level of understanding of the process by which FAI causes damage and may facilitate the prevention of a very high proportion of cases of osteoarthritis of the hip.
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