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1 Department of Pediatric Radiology, Massachusetts General Hospital, 32 Fruit
St., Boston, MA 02114.
2 Present address: Department of Radiology, Southampton General Hospital,
Southampton, Hants SO16 6YD, England.
3 Department of Radiology, Children's Hospital, 300 Longwood Ave., Boston, MA
02115.
4 Department of Radiology, Huntington Hospital, Huntington, NY 11743.
Received May 19, 2003;
accepted after revision July 28, 2003.
Address correspondence to D. Jaramillo.
Abstract
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MATERIALS AND METHODS. We reviewed knee MRIs of 126 young children and adolescents suspected to have internal knee derangement, including 82 with open physes and a control group of 44 who were skeletally mature. High-resolution proton density and T2-weighted pulse sequences were used in all patients. The prevalence of common injuries in the two groups was compared using chi-square analysis. Levels of interobserver agreement for evaluation of chondral lesions in the skeletally immature group were determined using the kappa statistic.
RESULTS. In the skeletally immature group, chondral lesions were the
most prevalent injuries (prevalence = 0.34, p = 0.009) followed by
meniscal and anterior cruciate ligament injuries (prevalence = 0.23 and 0.24,
respectively). No significant difference in the prevalence of chondral injury
before and after physeal closure was seen (p = 0.45). There was no
significant difference in the prevalence of anterior cruciate ligament
injuries between the two groups, but meniscal injuries were more prevalent in
the skeletally mature patients (prevalence = 0.41, p = 0.037).
Interobserver agreement for chondral injuries in the group with open physes
was good (weighted 
= 0.45–0.51).
CONCLUSION. The most common injuries occurring as a result of acute trauma to the immature knee were chondral. In patients with open physes, chondral injuries were significantly more prevalent than anterior cruciate ligament and meniscal injuries.
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At our institution, a high-resolution MRI protocol has been used routinely for the past 4 years. We performed a retrospective study to assess the prevalence of chondral injuries and other lesions after acute knee trauma in children and adolescents and to determine whether there are differences in prevalence of injury related to skeletal maturity.
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Patients
Patients younger than 21 years old examined at Massachusetts General
Hospital from January 2000 to December 2001 for internal knee derangement were
included in the study (142 individual examinations). Two patients with
congenital deformities, two with joint disorders (juvenile rheumatoid
arthritis and Lyme disease), three with tumors (chondroblastoma,
osteochondroma, and lymphoma), one with a prior burn, one with a history of
knee surgery, and one with osteomyelitis were excluded. Six patients were
excluded because the examination was technically inadequate, which was defined
as one that did not include the entire proximal tibial physis or that had
motion artifacts. The remaining 126 examinations were considered both
technically adequate and without significant underlying disease.
Patients were classified as skeletally immature if at least one of the two major physes of the knee was open on MRIs (i.e., cartilaginous signal evident throughout the entire growth plate of the distal femoral or proximal tibial physes).
MRI
The studies were performed on 1.5-T systems (Signa, General Electric
Medical Systems, Milwaukee, WI). Each study included the following sequences:
intermediate-weighted fast spin-echo localizing images (TR/TE, 1,000/10);
axial fast spin-echo proton density–weighted images (3,500/35;
echo-train length, 8); sagittal fast spin-echo proton density–weighted
images (3,500/11; echo-train length, 4); fast spin-echo T2-weighted images
(4,000/100; echo-train length, 8); coronal fat-suppressed fast spin-echo
proton density–weighted images (3,500/11; echo-train length, 4); and
usually conventional T1-weighted spin-echo images (600/10). Imaging parameters
for proton density–weighted sequences were as follows: field of view, 16
cm; section thickness, 3.0 mm; interslice gap, 1 mm; matrix size,
256–320 x 512; and number of signals acquired, 2. The examinations
were performed using an extremity linear receive–transmit coil. Fat
suppression was achieved through chemical shift selective saturation.
Image Analysis
Each study was reviewed by one of three radiologists experienced in
musculoskeletal radiology without knowledge of a patient's specific history or
mechanism of injury. All three radiologists evaluated the skeletally immature
group for evidence of chondral injuries, which allowed assessment of
interobserver agreement for the detection of these lesions. Disagreements were
settled by consensus.
The tibial plateau, femoral condyles, and patella were examined for evidence of fracture, bone bruising, and chondral injury. The cruciate and collateral ligaments and the quadriceps, patellar, and popliteus tendons were inspected, and any sprain, tear, or avulsion injury was noted. The menisci were evaluated for the presence of abnormal intrasubstance signal and frank tears. Effusions and injuries to the patellar retinaculum, physes, and apophyses were also noted.
Chondral injuries were assessed using a modified version of the Outerbridge classification [10, 11] (Fig. 1A, 1B, 1C, 1D, 1E). We arbitrarily defined a full-thickness cartilaginous injury (grade 3) as an injury in which the entire signal intensity of the articular surface was lost. We called subchondral bone involvement (grade 4) only when clear signal intensity changes were present in the adjacent subchondral bone. Fifteen cartilaginous zones were assessed: three running from anterior to posterior on the medial and lateral femoral condyles and each tibial plateau; the medial and lateral facets; and apical surface of the patella.
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Arthroscopy
Arthroscopy reports were available for 11 patients, five of whom were
skeletally immature. The arthroscopy reports were correlated with MRIs with
respect to the presence and location of articular injuries.
Statistical Analysis
Interobserver reliability for chondral lesions in the skeletally immature
group was assessed using the kappa statistic. The ranges for kappa with
respect to the level of agreement according to Landis and Koch
[12] are shown in
Table 1. The significance of
differences in prevalence of injuries between the groups was established using
two-sample tests of proportion. Associations between injuries and skeletal
maturity were evaluated using the chi-square test.
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Articular chondral injuries were present in 18 of the 44 skeletally mature patients (prevalence = 0.41). The difference in the prevalence of chondral injury between the two groups was not statistically significant (p = 0.45). However, the distribution of chondral lesions in the two groups was significantly different. Thirty-one of the 43 chondral injuries in the skeletally immature group were femoral, compared with only 22 of 41 chondral lesions in the mature group (Table 2, p = 0.04). In contrast, only 10 of 43 lesions in the skeletally immature were patellar (Fig. 6A, 6B) compared with 16 of 41 chondral lesions in the mature group (p = 0.059).
Thirty-three osteochondral injuries involving the subchondral bone (grades 4 and 5) were seen in 24 skeletally immature patients (prevalence = 0.29). Six loose intraarticular bodies were present in four patients (three patellar [Fig. 6B], two femoral, and one tibial in origin). Fifteen osteochondral injuries were identified in 10 skeletally mature patients (prevalence = 0.23), and no loose bodies were seen in this group.
Interobserver agreement for chondral injuries in the skeletally immature group was good for both grade and location of the lesions with weighted kappa values for all observations ranging from 0.45 to 0.51. Table 3 shows the variation in weighted kappa values by location; agreement was better for the lateral femoral cartilage than for the patella. Kappa values for the few tibial lesions (Fig. 8) showed great variation.
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Arthroscopy
Arthroscopy was performed in five of the patients in the skeletally
immature group. Chondral injuries were seen at arthroscopy in two of the five,
and the cartilage was intact in the remaining three. On MRIs, only these two
of the five patients had cartilaginous injuries.
In the six of the patients in the skeletally mature group, arthroscopy was performed. Chondral injuries were reported in the arthroscopy reports of three. All had MRI evidence of cartilaginous injuries that agreed in location and number with those seen arthroscopically. In the remaining three, the status of the cartilage was not mentioned. MRIs showed a grade 1 lesion in the patellar apex in one and a grade 2 lesion in the medial femoral condyle in another.
Meniscal and Ligamentous Injuries
Twenty-four meniscal injuries were present in 19 of 82 skeletally immature
patients (prevalence = 0.23), eight involving the medial meniscus and 16
involving the lateral meniscus. Significantly more meniscal
injuries—eight medial and 13 lateral in 18 of the 44 patients
(prevalence = 0.41)—were detected in the skeletally mature group
(p = 0.037). Twenty (12 boys, eight girls) of 82 skeletally immature
patients had anterior cruciate ligament injuries (prevalence = 0.24), with a
similar prevalence (11/44; prevalence = 0.25) noted in the skeletally mature
patients. The difference in the prevalence of anterior cruciate ligament
injuries in the two groups was not statistically significant (p =
0.94).
Therefore, a significant excess of chondral injuries compared with meniscal and anterior cruciate ligament injuries was seen in the skeletally immature group (p = 0.009), but chondral and meniscal injuries were equal in prevalence in the skeletally mature patients.
Other Injuries
Four patients had Salter-Harris fractures that were not detected
radiographically. Eight other patients had subtle physeal injuries (minor
widening and alteration in physeal signal or paraphyseal bruising). No
fractures were present in the skeletally mature group.
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A marked increase in injuries, particularly those related to sports, has been noted in older children [13] and adolescents. A recent series documented a 54% increase in injuries from 1983 to 1999 [14]. Chondral injuries are significant because they may predispose a patient to premature osteoarthritis [15]. This tendency is especially true in patients with full-thickness cartilage injuries, which may be amenable to autologous chondrocyte transplantation [16]. Loose intraarticular bodies can cause locking and pain. Prior MRI reports have emphasized injuries to the ligaments and menisci. In fact, several of the largest series of MRI examinations of the knee in children did not comment on chondral injuries [2–5]. This omission may be because of the lack of sensitivity of conventional MRI sequences, particularly T1-weighted images, to injuries involving the articular cartilage.
Chondral injuries are difficult to detect on MRI. Two techniques are used most commonly for this purpose. Fat-suppressed spoiled gradient-recalled echo imaging initially emerged as the best technique to show injuries to the surface of the joints [17]. Unfortunately, this technique does not depict the menisci and ligaments well. More recently, high-resolution fast spin-echo imaging with a longer TE has been shown to correlate well with arthroscopy for the depiction of chondral injuries [10]. This technique is, in turn, very sensitive to other lesions in the knee; therefore, it has become an integral part of many imaging protocols. With this sequence, the articular cartilage is of lower signal intensity than the adjacent joint fluid. An abnormality in the cartilage is seen as a discontinuity in the low-signal-intensity cartilage. Detection of subtle injuries (grades 1 and 2 in this series [Figs. 2 and 3A, 3B]) continues to be challenging, and further improvements in MRI techniques are necessary to refine the diagnosis of these lesions.
Subchondral bone marrow edema, best noted on fat-suppressed proton density– and T2-weighted sequences, frequently accompanied cartilaginous injuries. Increased signal intensity of bone alerts the radiologist to the presence of a subchondral injury. Interobserver agreement was good overall and in the specific locations. Our data suggest that agreement is slightly better in the femur than the patella, but the overall range of agreement is the same for both areas.
Differentiation of the deep layers of articular cartilage and subchondral bone using routine MRI is difficult [18–20]. The transition between cartilage and bone will appear to vary depending on factors such as TE and the orientation of the cartilage. For this reason, we decided to arbitrarily define a subchondral injury as being present only when a clear focus of abnormal signal intensity on the bone was present. Some of the lesions classified as grade 3 may indeed have had minimal involvement of the subchondral bone.
The prevalence of anterior cruciate ligament injury in both skeletally immature and mature patient groups in our study (0.24 and 0.25, respectively) was similar to that reported previously [2–4]. The overall prevalence of meniscal injury was 0.23 and 0.41 before and after physeal closure, respectively. In previous studies, researchers have found prevalence of meniscal injury to be approximately 0.25 in skeletally immature patients [2] and as high as 0.59 in skeletally mature patients [4]; the findings from these prior reports agree with our findings. However, the medial meniscus was injured significantly more frequently than the lateral meniscus in these studies, whereas we found a significant excess of lateral meniscal tears both before and after physeal closure. We do not have an explanation for the high prevalence of lateral meniscal injuries in our group of patients. Only three lateral discoid menisci were noted in the immature group, and none was noted in the mature group; this difference is unlikely to account for the predominance of lateral meniscal injuries. The prevalence of physeal injuries and of other occult injuries was low in comparison with a previous report [21].
Whether immaturity protects against chondral injuries is uncertain. Biomechanical differences related to the presence of residual epiphyseal cartilage under the articular cartilage would be expected. We compared patients before and after physeal closure to evaluate this difference. Although the prevalence in younger patients was slightly lower, this difference was not significant. Most of the patients in the immature group were older than 10 years; at this stage of development, most of the epiphyseal cartilage has ossified. Beyond biologic differences, it is possible that the patterns of injury differ before and after skeletal maturity because activities change between late childhood and puberty, early adolescence, and late adolescence [13, 22].
Our study does not include a systematic validation with arthroscopy. Arthroscopy is performed less frequently in children than in adults [23]. Patients were referred from a number of sources: orthopedic surgeons, sports medicine physicians, and pediatricians in the community. Information about outcome or eventual arthroscopy and surgery could not be obtained systematically. In the five skeletally immature patients who underwent arthroscopy, there was complete agreement between MRI and arthroscopy. In the six skeletally mature patients who underwent arthroscopy, two lesions seen on MRIs were not reported arthroscopically. High-resolution MRI, however, has already been validated with arthroscopy; therefore, we believe that it is valid to discuss these injuries in the absence of arthroscopic correlation [10]. Our goal was to evaluate differences in distribution and prevalence of injuries, rather than to validate the technique.
Another potential limitation of our study is its retrospective nature, given that our population reflects the local patterns of referral to MRI. The clinical presentations encompassed by "internal derangement of the knee" vary considerably. Inclusion of only patients presenting with specific clinical indications to a well-defined group of clinicians who had uniform criteria for referral to MRI would have been ideal. We have no reason to suspect, however, that there was a bias toward specific injuries. We arbitrarily selected the upper age limit as 21 years. We evaluated interobserver variability in the immature population only because this was the population of interest. One could argue, however, that the MRI studies of the immature patients underwent closer scrutiny than those of their mature counterparts because studies of the former were reviewed three times, whereas those of the latter were analyzed only once.
In summary, our study shows that chondral injuries are the most common lesions after acute knee trauma in the skeletally immature population. The prevalence of these lesions does not change significantly with skeletal maturity. Chondral lesions are subtle but can be significant clinically and may easily remain undetected. We recommend that in skeletally immature patients MRI evaluation of internal knee derangement include sequences that are sensitive to articular cartilage injury.
Acknowledgments
We thank Elkan F. Halpern for help with the statistical analysis; Linda
Banzi for help with illustrations; and Sherry Brec, Noemi Chavez, and
Elizabeth Olear for help with the preparation of this manuscript.
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