The Ottawa Knee Rules are highly sensitive for identifying knee fractures and should be used to determine which patients with acute knee injury require radiography.51,57 X-rays are not indicated for soft tissue injury assessment of the knee.
Ultrasound is not recommended for evaluation of menisci or cruciate ligament injuries.49
MRI is the imaging of choice for internal knee derangement.51 However, its use should be confined to more doubtful, difficult and complex knee injuries.56
Imaging issues, benefits and risks
Clinical examination, when combined with MRI, provides the most accurate non-invasive source of information currently available for pathological findings in the menisci and the ACL.58<
When comparing MRI and diagnostic arthroscopy for ACL and meniscal tears, MRI is superior and offers the health benefit of avoiding invasive surgery.50 In one study, almost half of patients presenting with an acutely locked knee had their management changed from surgical to conservative based on MRI findings.51
However, careful evaluation by an experienced examiner identifies patients with surgically treatable meniscus and ACL tears with equal (or better) reliability than MRI.50,53,54
The ACL can regain continuity after partial or complete rupture.59 Hence, the relative efficacy of surgical reconstruction and rehabilitation for the short-term and long-term outcomes after ACL rupture is debated. Results of a 2013 randomised controlled trial encourage doctors to consider rehabilitation as a primary treatment option following an acute ACL tear in young adults.60
MRI studies have higher false positive than false negative results.54,58 MRI shows lesions in the knee joint in most (almost 90%) middle-aged and elderly people in whom knee radiographs do not show any features of osteoarthritis, regardless of pain.61
MRI has been shown to have a false positive rate of 65% for identifying medial meniscal tears and 43% for lateral meniscus tears when compared with surgical findings.62
MRI of the knee is often performed in cases where a diagnosis is uncertain, and abnormal findings, such as meniscal tears, could be suspected to be the cause of the symptoms. However, incidental meniscal findings are common on MRI within the general population. For example in one study, 19% of women aged 50–59 years and 56% of men aged 70–90 years had asymptomatic meniscal injuries on MRI. Meniscal findings have been shown to increase with age.63 Additionally, no conclusion can be drawn about the natural course of meniscal injury seen at MRI imaging.59 A period of conservative management and re-evaluation is warranted in most cases.
MRI is not effective at detecting all forms of injury of the knee. For example:
- radial meniscal injuries are difficult to visualise on MRI and account for a large number of tears missed by MRI58
- MRI is not the most reliable tool for diagnosing recurrent meniscal tears, detecting only 66% compared with 88% with arthrography58
- the accuracy of MRI decreases in patients with multiple injuries.64
MRI is the method of choice for the non-invasive diagnosis of meniscal and ACL tears. Despite the high performance of this method, some cases are challenging and the criteria described in the literature are not sufficient to reach a diagnosis.65