AJR 2005; 185:756-762
© American Roentgen Ray Society
MRI of Anterior Cruciate Ligament Injuries and Associated Findings in the Pediatric Knee: Changes with Skeletal Maturation
Jeffrey S. Prince1,2,
Tal Laor1 and
Judy A. Bean3
1 Department of Radiology, Cincinnati Children's Hospital Medical Center and
University of Cincinnati College of Medicine, Cincinnati, OH.
3 Center for Epidemiology and Biostatistics, Cincinnati Children's Hospital
Medical Center and University of Cincinnati College of Medicine, Cincinnati,
OH.
Received August 25, 2004;
accepted after revision October 26, 2004.
Address correspondence to J. S. Prince.
2 Present address: Department of Medical Imaging, Primary Children's Medical
Center and University of Utah School of Medicine, 100 N Medical Dr., Salt Lake
City, UT 84113.
Abstract
OBJECTIVE. The objective of our study was to evaluate the MRI
characteristics of anterior cruciate ligament (ACL) injuries and associated
findings relative to skeletal maturity. We also contrast the frequency of
findings in this younger population to adult data.
MATERIALS AND METHODS. Eighty-two consecutive knees with an MRI
report diagnosis of ACL injury (partial tear, sprain, or complete tear) or
tibial spine avulsion fracture imaged over 4 years were retrospectively
reviewed. Patients were grouped by degree of skeletal maturity as determined
from the MR images. The examinations were reviewed for the type of ACL injury,
secondary imaging findings, and associated knee injuries. Findings were
correlated to skeletal maturity, and frequencies were compared with adult
data.
RESULTS. ACL injuries were more common in boys in the skeletally
immature group, but more common in girls in the skeletally mature group
(p = 0.03). Tibial spine avulsion fractures were most common in
skeletally immature patients (p < 0.01), whereas complete tears of
the ACL were most common in skeletally mature patients. Associated injuries
were less common in the skeletally immature group, but this trend did not
reach statistical significance. Most secondary signs of ACL injuries occurred
at similar rates in all groups with frequencies similar to those reported in
adults.
CONCLUSION. ACL injuries in skeletally immature patients are seen
more often in boys. Tibial avulsion fractures and partial tears are more
common in younger, less rigid skeletons that may absorb the forces of trauma.
As children mature, complete ACL tears and associated injuries occur in
frequencies approaching those patterns seen in adults. Similarly, skeletally
mature girls are affected more often than mature boys.
Introduction
MRI findings of anterior cruciate ligament (ACL) tears and
associated injuries are well documented in the adult knee
[1,
2]. In the past, injuries to
the ACL were thought to be uncommon in children. A recent increase in the
frequency of diagnosis [3,
4] raises concerns regarding
the management of ACL tears and associated injuries in skeletally immature
patients. Treatment decisions are influenced by the nature of the ACL injury,
the skeletal maturity of the patient, and concomitant internal derangements of
the knee [3,
5,
6].
Reports limited to skeletally immature patients have described the
diagnostic accuracy of MRI for ACL and associated injuries
[7,
8]. We undertook this study to
evaluate the MRI characteristics of ACL injuries and the associated findings
relative to skeletal maturity of the knee in all patients referred for imaging
to a pediatric institution. We also contrast the findings in this younger
patient population with those described in the literature in adults.
Materials and Methods
A retrospective review of all consecutive patients who had an MRI radiology
report diagnosis of an ACL injury (partial tear, sprain, or complete tear) or
tibial spine avulsion fracture from January 2000 through February 2004 was
performed. A database word search yielded 103 knees. Eighty-two knees in 79
patients met the following inclusion criteria: diagnostic MRI performed and
interpreted at our institution and investigators' consensus agreement in the
diagnosis of ACL or tibial spine injury. Twelve knees were excluded from the
study because the imaging examination was not performed at our institution and
often was incomplete, but these cases appeared in the database search because
they were reviewed at our institution in consultation. Approval from the human
subjects committee was obtained, and the need for patient consent was
waived.
All patients were imaged on a 1.5-T LX system (GE Healthcare), 1.5-T
Symphony system (Siemens Medical Solutions), or 3-T Trio system (Siemens
Medical Solutions). All examinations included at least the following
sequences: axial fast or turbo spin-echo proton density-weighted sequence;
coronal fast or turbo spin-echo proton density- and T2-weighted sequences; and
either sagittal conventional spin-echo proton density- and T2-weighted and
fast or turbo spin-echo T2-weighted sequences or sagittal fast or turbo
spin-echo proton density- and T2-weighted sequences. Additional sequences
included T1-weighted and T1- or T2-weighted gradient-recalled echo sequences.
All fast and turbo spin-echo sequences were fat suppressed.
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Fig. 1A —Coronal turbo or fast spin-echo proton density-weighted
images show examples of categories of skeletal maturity. Image of 9-year-old
boy from group 1 shows cartilage signal intensity across entire physis
(arrow).
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Fig. 1B —Coronal turbo or fast spin-echo proton density-weighted
images show examples of categories of skeletal maturity. Image of 14-year-old
boy from group 2 shows incomplete physeal signal (arrow), but physeal
plane is still visible.
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Fig. 1C —Coronal turbo or fast spin-echo proton density-weighted
images show examples of categories of skeletal maturity. Image of 18-year-old
girl from group 3 shows no physeal signal intensity can be seen.
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Examinations were reviewed by consensus of two pediatric radiologists who
were blinded to patient identification data and the radiology report.
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Fig. 2A —Types of anterior cruciate ligament (ACL) injury. Sagittal
fast spin-echo proton density-weighted image shows complete midsubstance tear
with both abnormal course and increased signal intensity of ACL in 17-year-old
boy (group 2).
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Fig. 2B —Types of anterior cruciate ligament (ACL) injury. Sagittal
turbo spin-echo proton density-weighted image shows normal course but abnormal
high internal signal consistent with partial tear of ACL in 15-year-old boy
(group 2).
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Fig. 2C —Types of anterior cruciate ligament (ACL) injury. Sagittal
turbo spin-echo T2-weighted image shows avulsion fracture of tibial spine
(arrow) and hemarthrosis (asterisk) in 12-year-old boy
(group 1). ACL has normal course, but abnormal internal signal from associated
partial tear.
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MRI Evaluation
Patients were categorized into one of three groups on the basis of their MR
images. The groups were defined as follows: group 1, immature (cartilage
signal intensity present throughout the physis on all sequences); group 2,
partially mature (incomplete or discontinuous cartilage signal intensity or
decreased signal intensity in the physis, but physeal plane still clearly
visible on all sequences); and group 3, mature (physeal scar nearly resolved)
(Figs. 1A,
1B, and
1C). Both the distal femoral
and proximal tibial physes were evaluated, with assignment to the less mature
group if there was a discrepancy in the degree of maturation between bones.
Patient age and sex were recorded.
MRI examinations were reviewed for the type of ACL injury (partial tear or
complete tear) or tibial spine avulsion fracture (Figs.
2A,
2B, and
2C). Images were also evaluated
for associated knee injuries including tear or contusion of the medial or
lateral meniscus (Fig. 3), tear
of the medial collateral ligament, and tear of the posterior cruciate ligament
(PCL) (Figs. 4A, and
4B). Meniscal tear was defined
as a linear increased signal abnormality that disrupts an articular surface on
more than one slice and is differentiated from the normal vascularity
frequently seen in children. Meniscal contusion was defined as amorphous
abnormal increased signal that made contact with an articular surface, after
acute trauma, but did not have a definite linear component
[9]. The following indirect
signs of ACL injury also were tabulated: bone contusion, joint fluid and type
(simple fluid, hemarthrosis, or lipohemarthrosis)
(Fig. 5), PCL buckling,
anterior tibial translation, uncovering of the posterior horn of the lateral
meniscus, deep lateral condylar sulcus sign, Segond fracture, and
semimembranosus muscle or tendon injury (Figs.
6A,
6B,
6C,
6D, and
6E).
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Fig. 3 —Associated meniscal injuries. Sagittal conventional spin-echo
proton density-weighted image in 14-year-old girl (group 2) with complete
anterior cruciate ligament tear shows complex tear of posterior horn of medial
meniscus.
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Fig. 4A —Ligamentous injuries associated with anterior cruciate
ligament injuries. Coronal turbo spin-echo proton density-weighted image in
17-year-old boy (group 2) shows high signal adjacent to medial femoral condyle
and discontinuity of medial collateral ligament (arrow).
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Fig. 4B —Ligamentous injuries associated with anterior cruciate
ligament injuries. Sagittal conventional spin-echo proton density-weighted
image in 15-year-old boy (group 2) shows buckled, thickened posterior cruciate
ligament with abnormal increased signal intensity.
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Fig. 5 —Joint effusions. Axial fast spin-echo T2-weighted image shows
lipohemarthrosis in 11-year-old boy (group 1) with fluid-fluid levels of three
different signal intensities. Anterior or top layer (arrow) is of low
signal intensity on this fat-saturated image.
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Fig. 6A —Secondary imaging findings. Sagittal fast spin-echo
T2-weighted image through lateral condyle of 14-year-old boy (group 1) shows
MRI correlate of deep lateral condylar sulcus sign (arrow):
"lateral femoral condylar notch sign."
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Fig. 6B —Secondary imaging findings. Sagittal fast spin-echo
T2-weighted image in 16-year-old girl (group 3) shows bone marrow contusion as
high signal in marrow of lateral femoral condyle and posterior aspect of
lateral tibial plateau.
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Fig. 6C —Secondary imaging findings. Sagittal fast spin-echo proton
density-weighted image in 13-year-old girl (group 1) shows measurement of
anterior tibial translation. In this patient with complete tear of anterior
cruciate ligament, there is 17 mm of translation (between vertical
lines).
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Fig. 6D —Secondary imaging findings. Sagittal conventional spin-echo
proton density-weighted image in 16-year-old girl (group 3) with complete tear
of anterior cruciate ligament shows uncovering of posterior horn of lateral
meniscus (dashed line).
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PCL buckling was defined, as reported by McCauley et al.
[10], by measuring an angle
created by lines placed parallel to the femoral portion of the PCL and tibial
portion of the PCL. An angle of less than 105° was considered abnormal.
Anterior tibial translation was measured by placing vertical lines tangent to
the posterior aspect of the ossified lateral femoral condyle and the lateral
tibial plateau. A distance between the two lines of greater than 5 mm was
considered abnormal [11].
Uncovering of the posterior horn of the lateral meniscus was defined by a
vertical line drawn tangent to the posterior cortex of the ossified lateral
tibial plateau. If this line passed through any part of the posterior horn of
the lateral meniscus, this sign was present
[12].
Statistical Analysis
Fisher's exact test was used to compare the proportions of the findings
across the three groups of skeletal maturity as the count for the cells were
small. A p value of less than 0.05 was considered to be statistically
significant. The chi-square test was used to compare sex proportions across
the groups.
Results
Patient age and sex and the type of ACL injury grouped by skeletal maturity
are summarized in Table 1. Most
of the patients in the skeletally immature group were boys and those in the
skeletally mature group, girls. The different proportions of each sex in the
three groups reached statistical significance (p = 0.03).
The proportion of complete ACL tears in groups 2 and 3 was statistically
different from the proportion of complete tears in group 1 (p =
0.01). Compared with the more mature patients, the skeletally immature group
had fewer complete ACL tears and more partial ACL tears and tibial spine
avulsion fractures. Tibial spine avulsion fractures were most common in group
1 and became less common as the patients matured
(Table 1). This was
statistically significant (p < 0.01) among all groups.
The frequency of associated injuries in the knee grouped by skeletal
maturity are summarized in Table
2. Overall, medial meniscal injuries were present more frequently
than lateral meniscal injuries, neither showing statistical significance
affected by the degree of skeletal maturity. Medial collateral ligament
injuries and PCL injuries were seen in 22% and 10% of patients, respectively.
The skeletally immature children had a trend toward fewer associated injuries,
but this did not reach statistical significance.
Secondary imaging findings of ACL injury are summarized in
Table 3. Bone contusions, joint
effusions, and anterior tibial translation were seen with high frequency in
all groups. A contusion pattern involving the lateral femoral condyle and
posterolateral tibial plateau was the pattern most often observed. Simple
effusions were common, present in 78% of all patients in our study, whereas
complex effusions were less common. Two of the three cases of hemarthrosis and
both cases of lipohemarthrosis occurred in skeletally immature patients with a
tibial spine avulsion fracture.
PCL buckling and a deep lateral condylar sulcus sign were seen in fewer
than half of patients. We also observed three cases (one in each group) of
thinning and deformity of only the articular cartilage overlying the lateral
condylopatellar sulcus, but no underlying bony deformity.
Uncovering of the posterior horn of the lateral meniscus was infrequent. A
Segond fracture and semimembranosus muscle or tendon injuries were very
uncommon (< 5%) across all groups. Both of the Segond fractures were seen
in skeletally immature patients. One Segond fracture was associated with a
complete ACL tear and the other, with a tibial spine avulsion fracture. Both
semimembranosus tendon injuries were partial tears at the insertion site and
were associated with other severe knee derangements. Neither knee was in the
mature group.
The group of seven skeletally immature patients with tibial spine avulsion
fractures infrequently had associated injuries; however, because of the small
number of patients in this group, no statistically significant differences
could be determined. One patient in this group had an injury caused by extreme
forces in which he also suffered a PCL tear, a medial collateral ligament
sprain, and a partial tear of the semimembranosus tendon. Another patient had
a lateral meniscal tear. The remaining patients had no associated injuries.
Secondary findings of ACL injury seen in this group with tibial spine avulsion
fractures included bone contusions (n = 6), anterior tibial
translation (n = 3), deep lateral condylar sulcus sign (n =
2), hemarthrosis (n = 2), lipohemarthrosis (n = 2), PCL
buckling (n = 1), uncovering of the posterior horn of the lateral
meniscus (n = 1), semimembranosus tendon injury (n = 1), and
Segond fracture (n =1).
Discussion
The MRI appearance of ACL injury and its associated findings is described
extensively in the adult population. However, to our knowledge, similar
in-depth evaluations have not been reported for children referred to a
pediatric institution for MRI. Historically, it has been suggested that most
ACL injuries in children result in avulsions at the insertion on the tibia.
The more recent studies in the orthopedics literature suggest that tears in
the ligament itself also are common
[13]. We performed this review
of all consecutive patients with ACL injury or tibial spine avulsion fracture
who were evaluated on MRI at our pediatric hospital with the intent to better
define the pattern of injury in a younger population and identify differences
when compared with those seen in adults, as documented in the literature. We
did not limit our study group to only the skeletally immature. Our study
revealed differences in injuries based on skeletal maturity, and we found
similarities and differences when comparing our findings with those published
reports of ACL injuries and associated findings in adults.
Patterns of Injuries as They Relate to Skeletal Maturity
The frequency of ACL injuries evaluated with MRI between the sexes showed a
statistically significant difference as children mature. ACL injuries were
more common in boys in skeletally immature children. With further maturation
(group 2), the sex difference diminished, and with skeletal maturity, we found
the same female dominance that has been reported in the literature in adults
[14,
15]. We did not have complete
information regarding the mechanism of injury in all patients to determine
whether a particular sport or group of sports was responsible for the trends
we report. However, we suspect that the maturity-related sex difference
between groups may be due to a greater number of boys participating in
high-velocity organized contact sports or sports with pivotal forces at an
earlier age [16]. As children
approach young adulthood, the skeleton undergoes changes in configuration that
result in a more valgus alignment of the knee in females. This difference in
architecture between the male and female mature skeletons is thought to
contribute to the higher rate of ACL injury in women
[15]. Various other risk
factors have been suggested as reasons for the relative increased incidence of
ACL injures in girls. These include the Q angle, the shape and size of the
femoral intercondylar notch, the thickness of the ACL, joint laxity, hormonal
influences, and training techniques
[14].
The proportion of children with complete ACL tears changed significantly
among the groups in our cohort. The skeletally immature patients had a greater
percentage of partial tears of the ACL than the skeletally mature group, in
which complete tears were more common. The increased frequency of partial
tears in skeletally immature children also has been described in the
orthopedics literature [13].
In the skeletally immature child, the tibial eminence is not fully ossified or
is recently ossified. This makes the underlying cancellous bone more prone to
fail than the strong anterior cruciate ligament
[17]. Before physeal fusion,
the more pliable intercondylar tibial eminence offers less resistance to
traction forces than the ACL
[18], and as such, the ACL
usually fails at the chondroosseous transition near the tibial spine,
resulting in an avulsion [19,
20]. In our study, tibial
spine avulsion fractures were more common in the skeletally immature group,
comprising 26% of ACL injuries in this group. This proportion of tibial spine
injury was significantly more common in the skeletally immature group than in
the nearly mature group (4%) and the skeletally mature group (4%), which had
rates of tibial spine avulsion fracture similar to those reported in the adult
population [2]. Because the
physis is the weakest part of the musculoskeleton during adolescence
[21], a completely ossified
proximal tibial epiphysis strengthened by fusion with the tibial metaphysis is
more resistant to avulsion, thus making complete ACL tear more likely.
Internal derangements of the knee are commonly associated with ACL
injuries. Meniscal tears are described as the most common injury associated
with ACL tears in adults with rates of 40-70%
[1,
2]. The medial meniscus is a
secondary restrictor of anterior tibial translation and can be injured by the
same mechanisms as the ACL. However, we saw a trend toward fewer associated
meniscal injuries among the skeletally immature population when compared with
the more mature patients. In adults, medial meniscal tears are present in 48%
of ACL injuries [1], whereas
lateral meniscal injuries are generally less common (21%)
[1,
2]. In our study population,
the frequency of meniscal injuries was less than the published rates in
adults, but in our study population, like in adult populations, the medial
meniscal injuries were more common.
Injuries to the medial collateral ligament observed are associated with ACL
injuries. Medial collateral ligament injuries in our study group occurred at a
rate similar to that published in the adult literature (18%)
[1]. PCL injuries associated
with tears of the ACL are uncommon in adults
[15] and were the least common
associated injury in our study population.
A less rigid immature skeleton may absorb some of the traumatic forces and
thus be protective, resulting in fewer associated derangements in children
with ACL injuries. Only one lateral meniscal tear and no medial meniscal tears
occurred in the skeletally immature patients with tibial spine avulsion
fracture, suggesting that the tibial spine avulsion fracture may dissipate the
force of injury in the immature knee.
Secondary Signs of ACL Injury
Joint effusions were common in all groups of patients. Hemarthrosis was
uncommon with only three cases, two of which were in patients with avulsion
fractures (group 1). The two cases of lipohemarthrosis were also in patients
with a tibial avulsion fracture (group 1). These complex effusions were likely
more common in this subgroup because of a bone injury. However, in other
pediatric studies, hemarthrosis has been reported in 47% of patients with ACL
injury [13]. The reason for
the difference between our study group and published data on the frequency of
complex effusions is not clear but may be related to time between injury and
MRI.
The other secondary signs of ACL injury showed no significant difference
between groups and occurred at rates similar to those published in the adult
literature. Contusion in the lateral femoral condyle and posterior aspect of
the tibial plateau is a pattern often described with pivot shift injuries, a
common mechanism of ACL injury
[22]. This was the most common
pattern of contusion in all groups in our study. It occurred at a rate (79%)
within the range that has been reported in the literature for adults (68-97%)
[1].
The deep lateral condylar sulcus sign is a depression in the lateral
femoral condyle at the lateral condylopatellar sulcus secondary to impaction
injury of the lateral condyle on the posterior aspect of the tibial plateau at
the time of the ACL injury
[23,
24]. This sign is reportedly a
reliable sign of ACL tear, but there can be condylar depression as a normal
anatomic variant [23]. A
depressed lateral femoral condyle can be seen in approximately 50% of adult
patients with ACL tear [23].
In our study, we saw a lateral condylar notch on MRI in 33 (40%) of 82
patients, a similar rate.
In the absence of a competent ACL, the tibia may sublux anteriorly relative
to the femur, thereby allowing laxity and buckling of the PCL. In the
literature, this is reported in approximately 50-70% of adult patients
[10,
12]. This sign was present
less commonly in our study. In our experience, the PCL has a somewhat
straighter, more horizontal course in children when compared with skeletally
mature patients. We postulate that as a consequence of this more horizontal
course, more subluxation of the tibia relative to the femur is required to
result in buckling of the PCL.
Anterior tibial translation occurs in the setting of an incompetent ACL in
58% of adult patients and has a 69% accuracy
[11]. In our study, this sign
was seen in 72% of patients. Uncovering the posterior horn of the lateral
meniscus resulting from anterior subluxation of the tibia
[25] has been reported in
18-22% of adult patients with ACL tears
[10,
12]. Likewise, a similar rate
was seen among patients in our study, although it was somewhat less common
among skeletally immature patients. PCL buckling, anterior tibial translation,
and uncovering of the posterior horn of the lateral meniscus in pediatric
patients may be difficult to evaluate accurately with the current definitions,
because the criteria do not take the unossified epiphyseal cartilage into
account when measuring. Further study into the validity of these measurements
in immature bones must be undertaken.
Of note, we saw only two Segond fractures in our study group, both in group
1. This was not a statistically significant difference among the groups
because of the small number of cases, but this trend again might reflect the
relative weakness of the immature epiphysis compared with the strength of the
highly organized, resistant ligaments of the knee.
Avulsion of the semimembranosus tendon insertion site has been described as
an uncommon injury in adults, with a high association with ACL injury
[26]. In our series, we saw no
avulsion fractures of the semimembranosus tendon attachment site; however, we
did see two partial tears of the tendon near its insertion sight. Both of
these patients had severe knee injuries with extensive internal derangement
(groups 1 and 2).
Our study is limited by the lack of surgical or arthroscopic correlation to
confirm the MRI findings. Partial tears of the ACL are more difficult to
diagnosis than complete tears
[1], possibly underestimating
their frequency. In addition, patients with suspected complete ACL tears may
be referred more frequently for MRI than those with more stable partial tears,
thus biasing the imaged population. The MRI examinations were reviewed by
consensus, so interobserver variability and intraobserver reliability testing
was not performed. This study is also limited by any inherent bias introduced
through patient referral patterns to a pediatric institution. Our hospital,
however, does serve the general population of a large tristate area, receiving
referrals from both community-based and hospital-based pediatricians and
orthopedists.
In summary, in our cohort, ACL injuries in younger patients are seen more
often on MRI in boys, but as the skeleton matures, girls are affected more
frequently. The pattern of types of ACL injury also changed with the degree of
skeletal maturity. Tibial spine avulsion fractures were significantly more
common in the skeletally immature group, and complete ACL tears were more
common in the partially and fully mature groups. Skeletally immature patients
tended to have fewer associated injuries on MRI, but this difference did not
reach statistical significance. The lack of associated injuries is most
notable in the immature patients with tibial spine avulsion fractures. This
trend may be due to a combination of physiologic laxity of the immature joint
and dissipation of the force of injury by a less rigid growing skeletal
system.
When compared with the frequencies of most associated injuries in adults,
those in children were similar except meniscal injuries. These were less
common in the immature patients than reported in adults. Secondary signs of
ACL injury showed little difference across the groups and occurred with
similar frequencies as those reported in the adult literature. Signs, such as
uncovering of the posterior horn of the lateral meniscus and anterior tibial
translation, that are dependent on bony landmarks within the knee require
further investigation regarding how these may be modified as the cartilaginous
and bony proportions change.
Recognizing the relationship of skeletal maturity and patient sex to the
frequency of ACL injuries and type seen with MRI and the frequency of
associated injuries and secondary findings may be helpful when interpreting
examinations in children and young adults.
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Abstract
Introduction
