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HipFurther Opinion

Predictors of progression of osteoarthritis in femoroacetabular impingement

NV Bardakos, RN Villar

J Bone Joint Surg [Br] 2009;91-B:162-9.

Given the predictions of increased demand for hip arthroplasty as we move further into the 21st  century, our objectives must be to improve the performance of arthroplasty in terms of its function and longevity, and to limit the numbers of patients progressing to end-stage disease by early  intervention. Recent appreciation of the process of femoroacetabular impingement has led to increased interest in surgical treatments which address morphological abnormalities. Currently such treatment is justified on the basis of symptomatic improvement, largely supported by uncontrolled case series with the other recent study by Bardakos and Villar being a rare exception.1 However, one cannot yet advise patients that such surgery will prevent progression to end-stage osteoarthritis.

In the light of current and historical evidence, one may hypothesise that the ‘idiopathic’ degeneration of the hip is a factor of its morphology, as are the durability of its articular cartilage and the activity level of the patient. Accurate classification of these variables in order to phenotype an individual, combined with observation of progression, will enable us to work out what the most important predictors of progression are and therefore which should be modified. Prospective clinical trials of new treatments, in carefully selected and characterised cohorts, will then hopefully confirm the validity of these observations. In time, advances in understanding of cartilage biology may enable us to address this aspect but we are not currently in a position to do so. Activity is hard to control and measure but there is a need for new validated scores appropriate for young patients, to replace less sensitive methods currently in use such as the UCLA activity score. The morphology of the hip is more easily assessed and more readily tangible for an orthopaedic surgeon, and has consequently been the subject of many recent publications as cited by Bardakos and Villar. Numerous ‘abnormal’ characteristics are described, largely derived from symptomatic study populations. One suspects that many of these features occur in ‘normal’ individuals who never  present for treatment, and there is a paucity of information regarding what is ‘normal’ in the general population for most of these parameters. A universal definition and classification of these variables is desirable but difficult to achieve, as the imaging protocols vary between studies, in terms of both modality employed and technique within that modality. The alpha angle is a case in point. The original description was from an MRI study, with a non-gender specific cut off of 50° suggested.2 This threshold may be extrapolated to lateral radiographs,3 but this assumption has not been validated. Gosvig et al4 have suggested gender-specific thresholds when assessed on an AP radiograph, but the AP radiograph is not ideal for assessing the deformity of interest.5 The situation is even more confused when one considers the acetabulum, because of the sensitivity to pelvic positioning and tilt. Does it matter if the patient is standing or supine and should that individual’s pelvic tilt be standardised to comply with perceived normality,6 thereby affecting the projection of their  acetabulum? How does one rationalise coxa profunda as an important indicator of global over-coverage when the floor of the acetabular fossa often touches the ilioischial line, particularly in females?7 Or perhaps this is the reason why they develop osteoarthritis more frequently? It could be argued that it is simply not appropriate to assess morphology from a two-dimensional radiograph. However, radiography remains cheap and available, and although three-dimensional imaging techniques may be superior the definition and classification of abnormality is not necessarily easier.

Bardakos and Villar have highlighted the complexity of the disease process and our assessment of it. They have recognised the importance of yet another variable to consider, the medial proximal femoral angle, which has received little attention in the recent literature. A cam lesion on its own will not necessarily lead to progressive disease. This observation is likely to be in keeping with those of many practising surgeons. The addition of localised acetabular over-coverage increases the probability of progression which is reassuring as it follows the accepted pathomechanism of impingement. The authors should be congratulated for achieving a study of such follow-up. They acknowledge the deficiencies of the study design, and through the appreciation of these deficiencies the obstacles and challenges we face in understanding the degeneration of the hip and how to advance our treatments are emphasised:

1) Consensus is required on how to best image morphology.

2) Validated and functional definitions of normal and abnormal are needed to classify individuals.

3) A reliable method to assess disease progression in the short-term is necessary. Radiographic studies like this are rare because such long-term follow-up is difficult, and a clinical trial would need a rapid assessment of efficacy to yield a result. There is an urgent need for validation of alternative markers of progression for introduction into trials.

4) How to identify cohorts? Patients presenting with pain may be too late in the disease process for intervention to alter the natural history, and their morphology may have been altered by the disease process itself. There is still uncertainty as to whether the cam lesion is the chicken or the egg, and therefore, whether its removal will change the natural history. We need to target young adults, either through population screening studies, or identifying a potentially predisposed group such as family members of the same or subsequent generations.

References

1. Bardakos NV, Vasconcelos JC, Villar RN. Early outcome of hip arthroscopy for femoroacetabular impingement: the role of femoral osteoplasty in symptomatic improvement. J Bone Joint Surg [Br] 2008;90-B:1570-5.
2. Notzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg [Br] 2002;84-B:556-60.
3. Clohisy JC, Carlisle JC, Beaule PE, et al. A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg [Am] 2008;90-A(Suppl4):47-66.
4. Gosvig KK, Jacobsen S, Palm H, Sonne-Holm S, Magnusson E. A new radiological index for assessing asphericity of the femoral head in cam impingement. J Bone Joint Surg [Br] 2007;89-B:1309-16.
5. Meyer DC, Beck M, Ellis T, Ganz R, Leunig M. Comparison of six radiographic projections to assess femoral head/neck asphericity. Clin Orthop 2006;445:181-5.
6. Tannast M, Zheng G, Anderegg C, et al. Tilt and rotation correction of acetabular version on pelvic radiographs. Clin Orthop 2005;438:182-90.
7. Armbuster TG, Guerra J, Jr., Resnick D, et al. The adult hip: an anatomic study. Part I: the bony landmarks. Radiology 1978;128-1:1-10.

 

Pollard T, Specialist Registrar

Nuffield Department of Orthopaedic Surgery,University of Oxford, Nuffield Orthopaedic Centre, Oxford, United Kingdom

E-mail: Tom.Pollard@ndos.ox.ac.uk