Keywords
lateral patellar dislocation - medial patellofemoral ligament - osteochondral injury
- children - adolescents
Introduction
Lateral patellar dislocation (LPD) is one of the most common knee injuries, and it
corresponds to the main cause of hemarthrosis in adolescents.[1]
[2] A recent population-based study[3] found that the annual incidence of LPD is of 23.2 per 100 thousand people per year,
and that dislocations occur more frequently among patients aged 14 to 18 years. Most
of them occur during sports activities, after a valgus movement with axial load and
rotation with the knee extended.[3]
The risk of recurrence varies from 30% to 70%, and repeated episodes increase the
risk of persistent knee pain and degenerative changes in the joint.[2]
[4]
Patellofemoral joint stability depends on the complex interplay between active, passive,
and static stabilizers that limit patellar displacement through the entire range of
motion. The medial patellofemoral ligament (MPFL) has gained relevance in recent years,
being recognized as the main passive stabilizer of lateral translation of the patella
in the first 30° of flexion.[5] Injuries to the MPFL occur in 84% to 100% of the first episodes based on previous
magnetic resonance imaging (MRI) studies,[6]
[7]
[8] and they are recognized as a relevant risk factor for its recurrence.
The MRI is a valuable diagnostic tool in the context of acute LPD.[9]
[10] It can facilitate the diagnosis in pediatric patients who are often unaware that
they have suffered a dislocation and present an acute trauma to the knee that limits
clinical examination. The characteristic MRI findings in cases of LPD include joint
effusion, bone edema in the lateral femoral condyle and medial patella, and MPFL injury,
among others.[11]
[12] In addition, the MRI provides additional information on the associated pathologies
(capsular, ligamentous, chondral, and osteochondral lesions) and the underlying joint
morphology that may predispose to future instability.[6]
[12]
[13]
Osteochondral and MPFL injury patterns after LPD have not been well defined for the
pediatric population, since most studies[6]
[7]
[8] encompass mixed cohorts of children and adults. However, several studies[1]
[14]
[15] suggest the existence of differences in the patterns between both groups. The location
and extent of the MPFL lesion has significant clinical value regarding both treatment
and prognosis. Therefore, an adequate characterization of MPFL injury patterns is
essential.
The objective of the present study is to evaluate through MRI scans the patterns of
MPFL lesions and osteochondral lesions in children and adolescents after a first episode
of LPD.
Materials and methods
We conducted a retrospective cohort study of all children and adolescents who presented
a first-time acute LPD in a single pediatric emergency department between 2008 and
2012. The inclusion criteria were: a) patients aged 18 years or younger presenting
with a clinical history and physical examination consistent with a first-time acute
LPD; and b) MRI examination confirming an acute patellar dislocation (joint effusion
pattern and bone marrow edema secondary to osteochondral impaction of the lateral
femoral condyle against the medial aspect of the patella). Patients with previous
dislocations, previous knee surgeries, congenital dislocations, or clinical findings
incompatible with acute LPD were excluded.
The medical records were reviewed to obtain demographic data (age, gender, laterality),
the mechanism of injury, and data on participation in sports (preinjury Tegner activity
score).
MRI technique
Patients underwent imaging scans within the first two weeks of the dislocation, with
the knee in full extension. All MRI examinations were performed on 1.5-T or 3.0-T
scanners (1.5T Signa [GE Healthcare, Chicago, IL, US]with an 8-channel coil or a MAGNETOM
Skyra 3T [Siemens Healthineers AG, Erlangen, Germany] with a 15-channel coil).
Imaging evaluation
The images were retrospectively analyzed by two independent evaluators, a knee surgeon
and a radiologist, both with more than ten years of experience, and the final decision
was made through consensus. The MPFL was assessed according to a previously-reported
method using coronal, axial, and sagittal views of T2-weighted images.[7]
[8]
The MPFL was divided into three regions: femoral attachment, intrasubstance, and patellar
attachment. The fiber status of the MPFL was defined as normal, partial injury, or
complete injury. Complete injury was defined as complete fiber disruption and subsequent
extensive edema ([Fig. 1], [2]), while partial lesion was defined as an incomplete discontinuity with intraligamentous
edema, in which some MPFL fibers could be identified in continuity ([Fig. 3], [4]). If osteochondral lesions were observed, their location and size in millimeters
were described based on the largest cross-sectional measurement of the lesion.
Fig. 1 Axial proton-density weighted with fat saturation (PD fat sat) view showing (A) complete rupture of the MPFL at its patellar insertion (white arrow), with the classic
pattern of bone edema at the lateral femoral condyle (asterisk) and subchondral fracture
due to impaction in the inferomedial patella (curved arrow). Coronal PD fat sat view
showing (B) a full-thickness chondral lesion in the load-bearing area of the lateral femoral
condyle (clear arrow), a free osteochondral fragment in the suprapatellar recess (clear
arrowhead), and significant joint effusion. The intercondylar notch radiograph (C) confirms the presence of an osteochondral fragment in the suprapatellar recess (triangle).
Fig. 2 Axial PD fat sat (A) and T2-weighted (B) views showing complete MPFL injury at its femoral attachment (white arrow) with
extensive bone edema at the lateral femoral condyle confirming lateral dislocation
(asterisk).
Fig. 3 Axial PD fat sat (A) and T2-weighted (B) views showing a partial injury to the MPFL, with bony avulsion of its patellar attachment
(white arrow), a subchondral impaction fracture of the inferomedial patella (curved
arrow), and a bone edema at the lateral femoral condyle (asterisk).
Fig. 4 Axial PD fat sat view showing a partial lesion of the MPFL at its femoral insertion
and bone edema due to impaction at the lateral femoral condyle (asterisk).
Results
The total sample was composed of 91 patients (60 men and 31 women) who met the aforementioned
inclusion criteria, and 50 right knees and 41 left knees were available for study.
The mean age was 14.5 years (range: 11 to 18 years), with a mean preinjury Tegner
activity score of 6 (range: 5 to 7). 64 patients practiced sports at the time of the
injury (70.3%, n = 64), and only 27 (29,7%) patients presented dislocation while performing
their daily-life activities (walking, sitting, squatting or going down a staircase).
Indirect trauma was the main cause of injury, affecting 80.2% (73) of the patients,
while direct injury was only observed in 19.7% (18) cases. Only 10 cases (11%) required
manually-assisted reduction by an emergency physician, as most lesions reduced spontaneously.
All MRI scans in the present series showed varying degrees of joint effusion, medial
retinacular edema, and bone hematoma pattern reflected on the medial patellar facet
and lateral femoral condyle, consistent with acute LPD. Lesions to the MPFL, either
total or partial, were found in 46 cases (50.5%). The other 49.5% of the cases presented
intra- and periligamentous edema, but there was no evidence of rupture of the MPFL
fibers. In total, 36 cases (39.5%) presented partial MPFL lesions, while the other
10 cases (10.9%) presented complete lesions.
The location and extent of MPFL lesions are summarized in [table 1]. The partial lesions were located mainly in the patellar insertion (26 cases), followed
by 7 cases in the femoral insertion, and 3 cases with multifocal partial tears. No
partial injury was exclusively intrasubstance.
Table 1
|
Lesion extent
|
Location
|
Number of cases (%)
|
|
Partial injury
36 cases (39.5%)
|
Patellar
|
26 (72.2%)
|
|
Femoral
|
7 (19.4%)
|
|
Multifocal
|
3 (8.3%)
|
|
Complete injury
10 cases (10.9%)
|
Patellar
|
7 (70%)
|
|
Femoral
|
2 (20%)
|
|
Multifocal
|
1 (10%)
|
|
No injury
(edema)
45 cases (49.5%)
|
Patellar
|
25 (55.6%)
|
|
Femoral
|
5 (11.1%)
|
|
Multifocal
|
15 (33.3%)
|
The complete lesions were also patellar in most (7) cases, followed by 2 cases of
lesion in its femoral insertion, 1 case of lesion in both areas, and no case of total
intrasubstance lesion. The 45 cases in which there was no evidence of fiber rupture
presented peri- and intraligamentous edema of the MPFL, and its location was mainly
in the patellar insertion (25 cases), followed by a multifocal lesion in 15 cases,
with only 5 (11.1%) cases of lesion femoral.
In general, considering all levels of MPFL injury described, from edema to complete
injury, these were located in the patellar insertion in 63.7% (58) of cases.
Osteochondral lesions were observed in 35 patients (38%) after the first episode of
LPD. These were unifocal in all cases, except for 1 patient who presented it simultaneously
with a patellar lesion and a lesion to the lateral femoral condyle. A total of 22
(62.8%) osteochondral lesions were found in the patella, and 12 (34.2%), in the lateral
femoral condyle. The mean maximum transverse diameter of the osteochondral lesions
was of 11.3 mm (range: 2 mm to 23 mm).
Discussion
The main findings of the present study are that the MPFL lesion was present in 50.5%
(46) of the cases of LPD in children evaluated after a first episode, a figure lower
than that reported in the literature.[11]
[15] In this pediatric cohort, the most frequent location was the patellar region, both
for partial and total tears.
A frequent injury in children and adolescents, LPD has a high rate of recurrence,
which may generate long-term disability. Several risk factors contribute to redislocation
after the first episode, including trochlear dysplasia, patella alta, extensor apparatus
misalignment, genu valgum, rotational deformities, and MPFL injuries, among others.
It is widely recognized in the literature that MPFL injury contributes to patellar
instability, this being the main restrictor for lateral translation of the patella
during the first 30° of knee flexion.[5] Therefore, an accurate assessment of the patterns of MPFL injuries after a first
episode of LPD is key to a successful treatment approach.
Some studies[11]
[15] suggest that the patterns of MPFL injuries differs between children and adults;
however, the findings are still contradictory due to the difficulty in comparing the
various studies, which include mixed cohorts of children and adults, as well as new
and chronic dislocations. In addition, not all studies detail whether edema was found
as a sign of an acute episode on MRI scans.
The literature[8]
[16]
[17] traditionally mentions that MPFL injuries occur in the femoral region. However,
the publication of case series with mixed or pediatric cohorts showed that MPFL patellar
lesions were not as uncommon as previously thought.[8]
[15]
Using MRI Scans, Elias et al.[7] reported injury patterns of the MPFL in a mixed cohort of children and adults. They
observed lesions in the patellar insertion in 76% of the cases, while in 49% of the
cases in which the femoral origin of the ligament was observed, it was injured, highlighting
that 48% presented multifocal lesions.
Balcarek et al.,[6] in a mixed cohort, found that multiple-site injuries to the MPFL were not as common,
only being reported in 22% of the cases of LPD. These results are consistent with
those of previous reports, and are similar to the findings of the present study.
Regarding the site of MPFL injury, Kepler et al.,[15] in an exclusively pediatric cohort of 43 patients, observed and reported a predominance
of isolated injury in the patellar insertion (61%), finding only 12% of isolated injuries
at the femoral insertion, and 12% of multifocal lesions.[15]
Confirming this hypothesis, Felus and Kowalczyk,[14] in a cohort of 50 pediatric patients who underwent surgery after a first-time LPD,
reported a patellar MPFL lesion in 66% of the cases. After an analysis by age, the
results showed a rate of 79% of lesions at this site in skeletally-immature patients
versus 54% in skeletally-mature patients, reaffirming what has been evidenced in the
present work.
The MPFL attachment to the medial border of the patella remains cartilaginous until
the ages of 16 to 18 years, while the femoral attachment to the distal femur ossifies
at around 13 to 15 years. The hypothesis is then raised that lesions at the border
between cartilage and bone could explain the susceptibility of younger patients to
suffer these lesions in the patellar region.[14]
Askenberger et al.,[2] in a cohort of patients under 14 years of age with acute LPD, reported lesions at
the patellar attachment in isolation or as part of a multifocal lesion in 95% of the
cases, while only 4% had isolated femoral injuries.[2]
Seeley et al,[1] published one of the largest pediatric cohorts to date, reporting patterns of MPFL
injury in 111 adolescents after an acute episode. In line with the present study,
they[1] showed that not all LPDs present with MPFL ruptures in this age group, finding injuries
to this ligament in 78.4% of the cases (87/111), that is, a rate 27.9% higher than
that of the present study. They[1] describe the location in the femoral insertion zone in isolation in 14%, and in
the patellar insertion zone in 31% (34), While in 33% (37) of the cases there was
evidence of a multifocal MPFL lesion.
However, there are also reports of an incidence of lesions in the patellar region
in fewer than a third of patients.[18]
[19] As aforementioned, the results should be viewed with caution based on sample sizes,
inclusion criteria, injury acuity, and age of the cohorts in question.
Osteochondral injuries are a common consequence of LPD that can lead to knee pain
and long-term complications. Previous studies[20]
[21] have reported an incidence of these injuries after an acute episode ranging from
21% to 54%.
It has been suggested that the mechanism of injury is an impaction of the medial facet
of the patella against the lateral femoral condyle.[22] In line with our findings, Kepler et al.,[15] in a pediatric cohort of 44 patients, reported an incidence of 23% of osteochondral
lesions, with a mean maximum diameter of 12.5 mm.
Treating osteochondral injuries can be very challenging, so starting with an accurate
diagnosis is crucial. The high incidence of these injuries clearly supports the use
of MRI scans after the first episode of LPD.
The main limitation of the present study is its retrospective design; while its main
strengths are the large number of patients included, which turned out to be one of
the largest cohorts published to date dedicated exclusively to pediatric patients,
and we were able to confirm the acute temporality of the lesion in all cases through
MRI scans. For future studies, linking the analysis of the findings with skeletal
maturity may help improve the recognition of the patterns of MPFL injuries.
Conclusion
The MRI has been shown to be a valuable diagnostic tool for patellar instability in
children and adolescents. In the present study, MPFL injuries were only present in
half of the cases of LPD in children, with partial MPFL injury at the patellar insertion
being the most common pattern in skeletally-immature patients during the first episode
in our sample. More than a third of children with LPD have a concomitant osteochondral
injury, mainly in the patella, so we consider that the use of MRI scans is essential
for the accurate recognition of the injury pattern and its associated injuries in
this age group.
