Introduction 2 4 5 6 The purpose of our prospective study was to determine the value of radiographs in young patients with non-acute knee symptoms and suspected intra-articular pathology who are scheduled for MRI based on the results of a physical exam. We therefore determined the prevalence of osseous abnormalities visible on radiographs and MRI and the yield of MRI compared with that of radiographs. In addition, we measured the radiation dose of standard radiographs of the knee. Material and methods Over 3 years, 961 patients referred from general practitioners to the outpatient surgery or orthopedic surgery clinics of three hospitals (two general hospitals and one university hospital) because of non-acute knee symptoms agreed to participate in our prospective study. The symptoms had to have been present for at least 4 weeks. The institutional review board of each hospital approved the study. We obtained written informed consent from all patients. At study entry a standardized history, including inventory of traumatic events and other knee-related events such as morbidity and medical procedures was taken. Physical exam was performed by one of 15 (orthopedic) surgeons or by residents under their supervision. All the patients included underwent MRI and radiographs of the knee. Recent radiographs taken for the same complaints that were obtained prior to study inclusion were not repeated. 1 Table 1 Excluded patients. Of the 961 patients referred, 163 (17.0%) were excluded Exclusion criteria n Symptoms of less than 4 weeks’ duration 65 Aged under 16 years 1 Aged over 45 years 6 Previous surgery 8 Retropatellar chondromalacia 14 Previous MRI 1 Radiographs unavailable or not obtained 65 MRI or arthroscopy prohibited 2 Immediate arthroscopy required 30 Criteria are not mutually exclusive. One out of six musculoskeletal radiologists (all with at least 5 years’ experience) evaluated the radiographs without knowledge of the MR images and recorded their findings on a Case Record Form (CRF). The radiation dose of standard antero-posterior and lateral radiographs was determined in two of the three hospitals using flat ionization chambers. We used PCXMC software (STUK, Finland) to convert the measured radiation doses into effective doses. 7 1 8 10 11 12 We performed the MR studies in all three hospitals on an identical 0.5-T system (Gyroscan T5, Philips Medical Systems, Best, the Netherlands), with the same software release using a dedicated transmit–receive knee coil. The standardized scanning protocol consisted of three sequences: a sagittal and a coronal dual spin-echo (SE) sequence and a sagittal T1-weighted 3D gradient-echo (GE) sequence with frequency selective fat suppression. The following parameters were identical for both SE sequences: field of view 140–160 mm, echo time (TE) 20/80 ms. The coronal dual SE had a repetition time (TR) of 2,100 ms, a 256 × 205 matrix and a slice thickness of 5 mm with a 0.5-mm interslice gap. The sagittal dual SE had a TR of 2,350 ms, a 256 × 179 matrix and a slice thickness of 4 mm with a 0.4-mm interslice gap. The parameters for the sagittal frequency selective fat-suppressed T1-weighted 3D GE were: TR 70 ms, TE 13 ms, 45° flip angle, field of view 160 mm, 256 × 205 matrix, 4 mm slice thickness with a 2-mm overlap. The total imaging time of the standard protocol (including the initial survey sequence) was 26 min. 8 12 13 7 14 1 We divided the completed CRFs over two groups: group A had no history of trauma, and group B had a history of trauma of more than 4 weeks before presentation at the hospital. We used the Chi-squared test to identify significant differences between the yield of radiographic and MR diagnoses, and between the number of findings in group A and that in group B. To look for statistically significant differences between the yield of radiographs and MRI within groups A, B and the study population as a whole, we used McNemar’s Chi-squared test. Results The median age of the 798 patients included was 31 years (range 16–45 years), and 261 patients (32.7%) were female. Minimal and median duration of symptoms were 4 and 20 weeks respectively (range 4–1,490 weeks). Three hundred and thirty-two patients reported no history of trauma (group A), and 466 patients reported an old (more than 4 weeks prior to consultation) history of trauma (group B). Magnetic resonance imaging depicted arthroscopically treatable intra-articular pathology in 341 patients (42.7%). Medial and lateral meniscal tears were found in 225 (28.2%) and 111 patients (13.9%) respectively. The average radiation dose of supine antero-posterior and lateral radiographs was 0.2 and 0.3 mSv respectively. The median duration of the interval between radiographs and MRI was 8 days (95 percentile, 85 days). In 196 patients (24.6%) radiographs were obtained prior to inclusion in the study. These radiographs were not repeated since the knee complaints had not changed between the moment they were obtained and inclusion in this study. 2 1 3 2 2 2 Table 2 Osseous abnormalities detected on radiographs only, on radiographs and MRI, and on MRI only in 332 patients with non-acute non-traumatic knee complaints Osseous abnormality R (%) R and MR (%) MRI (%) Total (%) p OA 13 (3.9) 18 (5.4) 39 (11.7) 70 (21.1) 0.00 ASL 0 (0) 3 (0.9) 7 (2.1) 10 (3.0) 0.02 LB 0 (0) 1 (0.3) 6 (1.8) 7 (2.1) 0.03 Fractures 0 (0) 0 (0) 0 (0) 0 (0) NA Incidental finding 0 (0) 1 (0.3) 0 (0.0) 1 (0.3) 1.00 Bone marrow edema 0 (0) 0 (0) 20 (6.0) 20 (6.0) NA  Total 13 (3.9) 23 (6.9) 72 (21.7) 108 (32.5) 0.00 1 14 3 1 1 4 5 6 3 7 8 2 3 3 Table 3 Osseous abnormalities detected on radiographs only, on radiographs and MRI, and on MRI only in 466 patients with non-acute traumatic knee complaints Osseous abnormality R (%) R and MR (%) MRI (%) Total (%)  p OA 15 (3.2) 13 (2.8) 65 (13.9) 93 (20.0) 0.00 ASL 0 (0) 3 (0.6) 22 (4.7) 25 (5.4) 0.00 LB 0 (0) 1 (0.2) 4 (0.9) 5 (1.1) 0.13 Fractures 0 (0) 4 (0.9) 3 (0.6) 7 (1.5) 0.25 Incidental finding 0 (0) 4 (0.9) 2 (0.4) 6 (1.3) 0.5 Bone marrow edema 0 (0) 0 (0) 96 (20.6) 96 (20.6) NA  Total 15 (3.2) 25 (5.4) 192 (41.2) 232 (49.8) 0.00 Categories are not mutually exclusive. Fig. 1 a, b c–e e  Fig. 2 a, b c–d  p p p 4 3 Table 4 Osseous abnormalities detected on radiographs only, on radiographs and MRI, and on MRI only in 798 patients with non-acute knee complaints Osseous abnormality R (%) R and MRI (%) MRI (%) Total (%) p OA 28 (3.5) 31 (3.9) 104 (13.0) 163 (20.4) 0.00 ASL 0 (0) 6 (0.8) 29 (3.6) 35 (4.4) 0.00 LB 0 (0) 2 0.3) 10 (1.3) 12 (1.5) 0.00 Fractures 0 (0) 4 (0.5 3 (0.4) 7 (0.9) 0.25 Incidental finding 0 (0) 5(0.6) 2 (0.3) 7 (0.9) 0.5 Bone marrow edema 0 (0) 0 (0) 96 (12.0) 96 (12.0) NA Total 28 (3.5) 48 (6.0) 264 (33.1) 340 (42.6) 0.00 Categories are not mutually exclusive. In 6 (30%) of the 20 group A patients with bone marrow edema, it was either associated with intra-articular damage of non-osseous origin or it was an isolated finding. The associated osseous abnormalities in the other 14 patients were as follows: bone marrow edema was found in 4 of the 10 patients with articular surface lesions, in 1 of the 7 patients with a loose body, and in 9 of the 70 patients with osteoarthritis. 2 In our population we did not find any other osseous lesions like osteopenia or femoropatellar disease. Also, we did not find chondrocalcinosis on radiographs. 1 1 2 3 4 5 6 7 8 Fig. 3 a b–d  Fig. 4 a b–c  Fig. 5 a b  Fig. 6 a b, c  Fig. 7 a, b c, d  Fig. 8 a b, c  Discussion Key characteristics of our population such as age, gender, clinical suspicion of intra-articular abnormalities, intra-articular abnormalities detected with MRI, and history suggest that our conclusions can be applied to typical populations scheduled for MRI to analyze non-acute knee problems. 1 1 6 6 The concordance between radiographs and MRI is not high because of the large number of diagnoses (77.6%) made only with MRI. In groups A and B osteoarthritis, articular surface lesions, and bone marrow edema were significantly more often diagnosed with MRI than with radiographs. Loose bodies were significantly more frequently diagnosed with MRI in group A only. This higher yield of MRI is no surprise, since abnormalities such as articular surface lesions and bone marrow edema are known to be better, or even exclusively appreciated on MRI. These findings have, in addition to the intra-articular abnormalities detected with MRI, potential clinical significance in view of the patients’ presenting symptoms. The advantage and comprehensiveness of MRI is illustrated by its ability to demonstrate bone marrow edema that is associated with other abnormalities. In the patients with a history of old trauma, bone marrow edema was mainly (92.7%) associated with intra-articular damage. In the group with no history of trauma, however, bone marrow edema was mainly (70%) seen in association with osseous abnormalities, thus facilitating the diagnosis of articular surface lesions and osteoarthritis. This prospective study has several limitations that are mainly related to the decision to follow usual care. This means that radiographs were available at the time of MRI. To overcome this limitation, one radiologist (JLB) re-evaluated MR studies of patients with abnormalities on conventional radiographs without knowledge of the radiographs, as described. 1 1 We tried to mimic usual care as much as possible in this study. However, in our study MRI was performed within 2 weeks of inclusion in the study. Due to the limited availability of MRI compared with conventional radiographs in most hospitals, MRI often cannot be performed at such short notice. This can lead to a considerable waiting time for MR examinations and this may direct physicians to prior conventional radiographs to rule out gross pathology and reassure the patient. To prevent these unnecessary radiographs, the waiting time for MRI must be as short as possible. In the Netherlands waiting times for MRI in most institutions are down from several months a couple of years ago to several weeks nowadays. 1