Each year in the United Kingdom (UK) 15% of patients who consult general practitioners (GPs) do so for musculoskeletal disorders. The annual consulting rate for internal derangement of the knee is 32 per 1000 patient years, similar to rheumatoid arthritis . Imaging of the knee is a common musculoskeletal application of Magnetic Resonance Imaging (MRI) . Although there is evidence for the technical  and diagnostic [4–6] performance of MRI for knee problems, there is uncertainty about the appropriate use of MRI, in particular when it should enter the diagnostic pathway for patients with suspected internal derangement of the knee [7,8]. This question is crucial to patient diagnosis, management and outcome, and thus to cost-effectiveness.
Systematic reviews have consistently demonstrated that MRI is an accurate diagnostic test for detecting meniscal and cruciate lesions [7–12]. For example, at least 85% of meniscal lesions and 90% of healthy menisci found at arthroscopy are detected with MRI . A trial that included 209 patients with negative MRI results who were randomised for arthroscopic or conservative treatment also concluded that MRI is accurate for the diagnosis of knee injuries . Therefore the evidence supports the use of MRI for diagnosing these common problems. This has led some to suggest that it is valuable to GPs in making appropriate and informed decisions [14,15]. Negative MRI findings could allow GPs to reassure patients, treat them conservatively in primary care, avoid unnecessary orthopaedic referrals and hence reduce waiting times [2,3] and save costs . Alternatively positive MRI findings could confirm GPs' clinical diagnoses and decision to refer to an orthopaedic specialist who would decide whether arthroscopy or other interventions were required without the need for a follow-up appointment. The radiologist's report could assist hospital specialists in prioritising outpatient appointments . Finally the estimated cost of accidents is £15 billion to the nation and approximately £1.2 billion per annum to the National Health Service (NHS) . Early access to MRI through referral from primary care could contribute to the diagnosis and management of these patients and potentially prevent the onset of chronic problems and the psychological and economic consequences of loss of physical fitness.
In contrast, some take the view that patients benefit more by being referred quickly and directly by their GP to see an orthopaedic specialist . This would allow hospital specialists to use MRI much more selectively, limiting it to those patients for whom a decision to operate has already been made and thus reduce resources spent on MRI. They argue that imaging may confuse the clinical picture if it detects asymptomatic abnormalities, possibly leading to unnecessary referrals and interventions . Not all GPs understand the implications of MRI findings as reported by radiologists, and this could result in false reassurance and delays in appropriate treatment .
A survey of the availability of MRI discovered that of 121 public sector departments with MRI who responded, 74 (61%) provided direct access to GPs for imaging of the knee . These findings imply wide variation in GPs' access to MRI and provide further evidence of the uncertainty where MRI should enter the diagnostic pathway. To the extent that the distribution of scanners reflects past demands of fundholding GPs, this variation may be more politically driven than evidence based. Furthermore referral behaviour varies among GPs with direct access to MRI. How such access affects the case mix of patients referred to orthopaedic clinics is not known. Investment in primary care is increasing, not least to prevent unnecessary referral to secondary care; Primary Care Trusts will control over 80% of the NHS budget by 2007/8 . In addition, the UK Department of Health has announced plans to reduce waiting times for diagnostic tests . MRI provision is expected to rise by around 12% a year ; nearly one million MRI examinations are now performed in England each year . There is a real danger that GP access to MRI will become standard policy without rigorous evaluation. In contrast, timely access to a reliable diagnostic tool in primary care has potential for better care and reduced costs.
So our multi-centre, pragmatic randomised trial with two parallel groups and concomitant economic evaluation addresses the question whether patients presenting to GPs with suspected internal derangement of the knee should be referred for MRI or directly to an orthopaedic specialist? The principal objectives are to evaluate: whether the use of MRI in primary or secondary care affects subsequent diagnosis and management; whether it improves patient outcomes; and whether it reduces net costs to the NHS, patients and society. In summary, our trial will inform policy whether to allocate resources to permit MRI for suspected internal derangement of the knee to enter the diagnostic pathway in primary care through early access for GPs, or to restrict it to secondary care at the request of orthopaedic specialists.