AJR 2005; 184:1640-1646
© American Roentgen Ray Society
Congenital Dislocation of the Patella
Benjamin Z. Koplewitz1,2,
Paul S. Babyn1 and
William G. Cole3
1 Department of Diagnostic Imaging, Hospital for Sick Children and the
University of Toronto, Toronto, Canada.
2 Present address: Department of Radiology, Hadassah-Hebrew University Medical
Center, P.O. Box 12000, Jerusalem 91120, Israel.
3 Division of Orthopaedic Surgery, Hospital for Sick Children and the University
of Toronto, Toronto, Canada.
Received May 13, 2004;
accepted after revision August 24, 2004.
Address correspondence to B. Z. Koplewitz
(ben_kop{at}hadassah.org.il).
Abstract
OBJECTIVE. Our objective was to present the imaging findings for
congenital dislocation of the patella, an uncommon condition with variable
clinical manifestations in patients of different ages.
CONCLUSION. Sonography can clearly illustrate the presence and
location of the laterally displaced patella and the anatomy of adjacent joint
structures. The high resolution for soft-tissue, cartilaginous, and bony
structures of the immature skeleton makes sonography a valuable tool in the
management of congenital dislocation of the patella. Conventional radiography
is a simple method for diagnosis once ossification of the patella has
commenced and for postoperative follow-up. MRI allows visualization of fine
anatomic details and relationships between the involved structures of the
extensor mechanism and is the technique of choice for preoperative
planning.
Introduction
Congenital dislocation of the patella is an uncommon but well-recognized
orthopedic condition that can have different clinical presentations. The
patella develops normally as a sesamoid bone of the femur. Congenital patellar
dislocation (CPD) is considered to result from failure of internal rotation of
the myotome that forms the femur, the quadriceps muscle, and the extensor
mechanism. This failure normally occurs toward the eighth to tenth week of
embryonic development [1]. CPD
usually manifests immediately after birth with genu valgum, flexion
contracture, and external rotation of the tibia
[2–11].
In some cases, however, diagnosis may be delayed until early childhood. In
less severe cases, function may be impaired only minimally and the diagnosis
can be delayed further, until late childhood, adolescence, or even adulthood
[3,
4,
6]. This delay may lead to
premature degenerative changes and severe impairment of joint function
[2,
3,
6,
7,
10]. Early diagnosis is
important because it enables timely management, which permits improved
development of the knee joint, thus reducing or avoiding the onset of late
sequelae.
There is a relative paucity of literature focusing on the imaging findings
for congenital dislocation of the patella. To address this paucity and to
highlight this condition, we present the imaging findings and illustrate the
application of various imaging techniques in the preoperative evaluation and
postoperative follow-up of four children in whom CPD was diagnosed during
infancy and childhood.
Materials and Methods
Patients were retrospectively identified by a computerized search of the
databases of the diagnostic imaging department and the medical records of the
hospital using the keywords "patella" or "knee" and
"dislocation." Patients' charts were reviewed, and the data that
were collected included clinical presentation, imaging studies, operative
management, and postoperative follow-up. All available imaging studies were
reviewed, including conventional radiography, sonography, and MRI.
Radiographs in anteroposterior and lateral projections had been obtained
for two patients, and additional skyline views had been obtained for the other
two patients. Sonographic examinations were performed using an HDI 5000 unit
(Advanced Technology Laboratories) with a 7- to 10-MHz linear transducer or a
Sequoia unit (Acuson) with a 5- to 8-MHz or an 8- to 13-MHz linear transducer.
Sonograms of both the normal and the affected knees were obtained in the
longitudinal plane scanning from the medial through the anterior to the
lateral aspect of the knee, and in the transverse plane scanning from the
distal femur to the proximal tibia.
MRI was performed (under sedation or general anesthesia, as necessary)
using a 1.5-T unit (Signa, GE Healthcare). Sagittal and coronal T1-weighted
(TR range/TE range, 400–500/8–10) and fast spin-echo T2-weighted
(4,000/70) images with fat saturation were obtained. At least one
cartilage-specific sequence, proton density or multiple planar
gradient-recalled, in one or two planes was performed in each study, allowing
evaluation of the distal femoral epiphysis, patella, and proximal tibial
epiphysis.
Results
Four patients were identified. Table
1 summarizes their clinical presentation.
Two infants were referred for imaging studies as part of an evaluation of
multiple anomalies detected at birth: One had genu valgum with fixed bilateral
flexion knee contractures in addition to other multiple anomalies. The other
had recurrent dislocations of the elbows, hips, and both knees as a part of
Larsen's syndrome. The remaining two children had a mild deformity of the knee
with only minimal functional impairment, which had been noted by their parents
to worsen with time.
Preoperative imaging studies included radiography for all four patients,
sonography for two, and MRI for three. Radiography and sonography were
performed after initial clinical diagnosis and during follow-up; MRI was
performed before tentative operative repair. Conventional radiography was
performed in the anteroposterior and lateral projections for two patients as
part of a skeletal survey. Additional skyline views were obtained for the
remaining two patients. Radiographs showed mild to moderate genu valgum in all
patients and lateral dislocation of the patella in the two patients in whom
the patella had ossified (Fig.
1). In the anteroposterior projection, the patella on the affected
side could not be visualized in its normal location in these two patients,
whereas the contralateral patella could be seen situated normally. The lateral
femoral condyle on the affected side was flattened and hypoplastic
(Fig. 1, left side). The
lateral view showed a decrease in both the size and the degree of ossification
of the dislocated patellae (Fig.
2A,
2B,
2C,
2D) and varying degrees of
superior patellar dislocation. In the two patients for whom skyline views were
obtained, these clearly showed lateral dislocation of the underdeveloped
patella, a shallow intercondylar sulcus, and a dysplastic lateral femoral
condyle (Fig. 2A,
2B,
2C,
2D). In one patient (patient
4), an incidental finding was noted of an additional lytic lesion in the
anteromedial aspect of the distal femoral metaphysis, not involving the growth
plate. A sclerotic margin later appeared around this lesion, which was
considered a fibrous cortical defect.
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Fig. 1. —Congenital dislocation of patella in 22-month-old boy who had
recurrent dislocation of both knees as part of Larsen's syndrome.
Anteroposterior radiographs of both knees show bilateral genu valgum and
decreased height of lateral femoral condyle and lateral tibial epiphysis. On
left side, partially ossified patella (arrowheads) can be seen
overlying hypoplastic lateral femoral condyle.
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Fig. 2A. —3-year-old girl with left leg deformity and limp due to
congenital dislocation of patella. Shown are lateral views (A and
B) and skyline views (C and D). Radiographs of left knee
(A and C) show small, indistinct patella (arrowheads),
seen as additional curvilinear soft-tissue opacity overlying lateral aspect of
knee, with small, fragmented ossification center (arrow). In right
knee (B and D), patella (arrowheads) is normal in size,
configuration, and location, with well-developed ossification center.
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Fig. 2B. —3-year-old girl with left leg deformity and limp due to
congenital dislocation of patella. Shown are lateral views (A and
B) and skyline views (C and D). Radiographs of left knee
(A and C) show small, indistinct patella (arrowheads),
seen as additional curvilinear soft-tissue opacity overlying lateral aspect of
knee, with small, fragmented ossification center (arrow). In right
knee (B and D), patella (arrowheads) is normal in size,
configuration, and location, with well-developed ossification center.
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Fig. 2C. —3-year-old girl with left leg deformity and limp due to
congenital dislocation of patella. Shown are lateral views (A and
B) and skyline views (C and D). Radiographs of left knee
(A and C) show small, indistinct patella (arrowheads),
seen as additional curvilinear soft-tissue opacity overlying lateral aspect of
knee, with small, fragmented ossification center (arrow). In right
knee (B and D), patella (arrowheads) is normal in size,
configuration, and location, with well-developed ossification center.
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Fig. 2D. —3-year-old girl with left leg deformity and limp due to
congenital dislocation of patella. Shown are lateral views (A and
B) and skyline views (C and D). Radiographs of left knee
(A and C) show small, indistinct patella (arrowheads),
seen as additional curvilinear soft-tissue opacity overlying lateral aspect of
knee, with small, fragmented ossification center (arrow). In right
knee (B and D), patella (arrowheads) is normal in size,
configuration, and location, with well-developed ossification center.
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Sonography was performed on the two patients who had been diagnosed in
infancy. In one infant, sonography initially was performed at the age of 4
months and showed a normal left knee but failed to show a patella on the right
side. Follow-up sonography when the patient was 22 months old showed a
hypoplastic, elongated patella on the lateral aspect of the right knee (Fig.
3A,
3B). The small, dislocated
patella showed homogeneous, low echogenicity, similar to that of the
contralateral normal cartilaginous patella. In the second infant, sonography
at the ages of 1 and 5 months showed marked dislocation of the small patellae
bilaterally and hypoplastic quadriceps and patellar tendons.
MRI was performed on three of the patients. In patient 1, MRI performed at
the age of 19 months showed a hypoplastic quadriceps muscle and attenuated
posterior cruciate ligament. The patella, the lateral meniscus, and the
anterior cruciate ligament could not be identified
(Fig. 4). In patient 2, the
cartilaginous patella was markedly dislocated laterally and superiorly. In the
last patient, MRI showed marked lateral displacement of the left patella,
which had a small ossification center and normal cartilage. Signal intensity
was increased adjacent to the lateral patellar retinaculum on the T2-weighted
sequence (Fig. 5A,
5B,
5C,
5D).
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Fig. 4. —8-month-old girl with congenital dislocation of patella. On
sagittal T1-weighted MR image of flexed knee, hypoplastic cartilaginous
patella (arrowheads) is hardly visible, showing marked lateral and
superior displacement.
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Fig. 5A. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (arrowheads). Skyline view radiograph shows
lateral displacement of patella (arrowheads), which is still mostly
cartilaginous.
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Fig. 5B. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (arrowheads). On sagittal high-resolution
proton-density (B), coronal fast spin-echo T1-weighted (C), and
axial fast spin-echo fat-suppressed T2 (D) sequences, anatomic
relations between femoral epiphysis (E), tibial epiphysis (T), and fibular
head (F) illustrate marked lateral and superior displacement of hypoplastic
patella (arrowheads). Arrow indicates quadriceps tendon.
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Fig. 5C. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (arrowheads). On sagittal high-resolution
proton-density (B), coronal fast spin-echo T1-weighted (C), and
axial fast spin-echo fat-suppressed T2 (D) sequences, anatomic
relations between femoral epiphysis (E), tibial epiphysis (T), and fibular
head (F) illustrate marked lateral and superior displacement of hypoplastic
patella (arrowheads). Arrow indicates quadriceps tendon.
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Fig. 5D. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (arrowheads). On sagittal high-resolution
proton-density (B), coronal fast spin-echo T1-weighted (C), and
axial fast spin-echo fat-suppressed T2 (D) sequences, anatomic
relations between femoral epiphysis (E), tibial epiphysis (T), and fibular
head (F) illustrate marked lateral and superior displacement of hypoplastic
patella (arrowheads). Arrow indicates quadriceps tendon.
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Patient 1 is being treated with physiotherapy, with interval increases in
the knee range of motion. Patient 2 had bilateral casting followed by surgical
repair, with improved function. The last two patients had surgical repair,
which included lateral release and patellar realignment, with good function at
2–6 months' follow-up.
Postoperative follow-up radiographs showed an increase in the size and
ossification of the patella, which gained an almost normal configuration (Fig.
6A,
6B,
6C,
6D). Although the position of
the patella was corrected, 6 months after operative repair the intercondylar
groove still remained shallow (Fig.
6D).
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Fig. 6A. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (same patient as in Fig.
5A,
5B,
5C,
5D). Lateral radiographs
(A and B) and skyline views (C and D) before
(A and C) and after (B and D) surgical repair.
Before surgical repair, dislocated patella (arrowheads) is small, and
its ossification center is fragmented and irregular. Postoperative follow-up
radiographs show well-developed, regular ossification center within centrally
located patella (arrowheads). Incidental finding of fibrous cortical
defect in distal femur can also be seen.
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Fig. 6B. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (same patient as in Fig.
5A,
5B,
5C,
5D). Lateral radiographs
(A and B) and skyline views (C and D) before
(A and C) and after (B and D) surgical repair.
Before surgical repair, dislocated patella (arrowheads) is small, and
its ossification center is fragmented and irregular. Postoperative follow-up
radiographs show well-developed, regular ossification center within centrally
located patella (arrowheads). Incidental finding of fibrous cortical
defect in distal femur can also be seen.
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Fig. 6C. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (same patient as in Fig.
5A,
5B,
5C,
5D). Lateral radiographs
(A and B) and skyline views (C and D) before
(A and C) and after (B and D) surgical repair.
Before surgical repair, dislocated patella (arrowheads) is small, and
its ossification center is fragmented and irregular. Postoperative follow-up
radiographs show well-developed, regular ossification center within centrally
located patella (arrowheads). Incidental finding of fibrous cortical
defect in distal femur can also be seen.
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Fig. 6D. —6-year-old boy with left knee deformity due to congenital
dislocation of patella (same patient as in Fig.
5A,
5B,
5C,
5D). Lateral radiographs
(A and B) and skyline views (C and D) before
(A and C) and after (B and D) surgical repair.
Before surgical repair, dislocated patella (arrowheads) is small, and
its ossification center is fragmented and irregular. Postoperative follow-up
radiographs show well-developed, regular ossification center within centrally
located patella (arrowheads). Incidental finding of fibrous cortical
defect in distal femur can also be seen.
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Discussion
Congenital anomalies of the patella include absence, hypoplasia, or
dislocation. Absence or hypoplasia of the patella occurs as part of several
syndromes such as nail-patella syndrome
[12–14],
genitopatellar syndrome
[15–19],
Meier-Gorlin syndrome [20,
21], or small patella syndrome
[22,
23]. Dislocation of the
patella can also present as a part of several syndromes having increased
tissue laxity, such as Down syndrome and Larsen's syndrome
[5–8,
11,
24], as in our two younger
patients, but it can also appear as an isolated deformity, as in our two older
patients.
Patellar dislocation can manifest with a spectrum of clinical
presentations. It should be suspected when genu valgum is noticed at any age
or when recurrent dislocations occur. It can also be the cause for a limp or
unstable walking, as in two of our patients. Some authors distinguish between
two separate entities, that is, congenital versus developmental dislocation,
according to patient age and severity of symptoms at presentation
[2,4–7,10].
Congenital or persistent dislocation of the patella is present at birth, is
permanent and irreducible, and is associated with a flexion contracture. This
type is often associated with genetic syndromes with increased joint and
connective tissue laxity [6,
10]. Syndromes that show an
increased incidence of CPD include Down syndrome
[4,
5], Larsen's syndrome,
arthrogryposis [4],
nail-patella syndrome [6,
12–14],
Rubinstein-Taybi syndrome [5],
Ellis-van Creveld syndrome, and diastrophic dysplasia
[24]. Most of these cases are
noticed soon after birth, as happened in our first two patients, who had a
valgus deformity and limited range of motion, and require early surgical
repair. Another form of patellar dislocation in flexion that results from
quadriceps shortening was described in children treated by multiple injections
into the vastus lateralis for neonatal infection
[25,
26]. In these children,
progressive quadriceps contracture and loss of the full range of knee flexion
later developed, and when the knee was flexed, the patella would gradually
sublux and dislocate laterally. None of the children included in this series,
however, had received multiple quadriceps injections as neonates.
In the developmental form, in contrast to the congenital form, the
dislocation is not permanent and is reducible. The patella is located stably
in the femoral groove when the knee is flexed but tends to drift laterally as
the knee extends. This drifting usually happens in full extension, and the
patella relocates during flexion. This type usually manifests when the child
begins to walk, because of knee instability that is worse if the condition is
bilateral [4,
7]. At times, this type may be
well tolerated and is diagnosed only in late childhood when a knee deformity
is noticed in an otherwise asymptomatic child
[25], as was the case in two
of our patients (patients 3 and 4).
Other authors consider both types as one entity of congenital dislocation
of the patella [3,
6]. The first and second types
have been reported to coexist in the same patient or within the same family
[3,
6,
10]. A milder form of patellar
dysplasia and malalignment with lateral deviation of the quadriceps mechanism
is more common in adolescence and manifests as recurrent subluxation or
dislocation (as happened in two other older patients who were referred for
preoperative imaging but were not included in this series)
[3]. This type is often
referred to as habitual subluxation and dislocation in extension. Congenital
dislocation of the patella can thus be considered part of a spectrum of
developmental dysplasia of the patella and of the extensor mechanism
[6].
The natural history of untreated CPD is the gradual development of
dysplastic knee abnormalities, as seen in our two older patients
[6,
7]. These include flattening of
the lateral femoral condyle, with an increasing degree of genu valgum and
external tibial rotation. The dislocated patella remains hypoplastic and can
show thinning of the cartilage along its lateral border, and the intercondylar
sulcus remains shallow (Fig.
5A,
5B,
5C,
5D). Continuous unequal knee
loading can lead to narrowing of the lateral knee joint space and subchondral
cyst formation. Recognition and surgical repair of this uncommon condition
during early childhood is therefore important.
Traditionally, patellar dislocation has been diagnosed through radiography
[4–6].
In older children and in adolescents, anteroposterior radiographs can show the
size and the position of the patella, although these are better illustrated in
the lateral and skyline projections (Figs.
1,
2A,
2B,
2C,
2D, and
6A,
6B,
6C,
6D). In infants and toddlers,
absence of the normal, unossified, or partly ossified patella and associated
soft-tissue changes can pass unnoticed if the diagnosis of CPD is not
considered. Anteroposterior views can show the degree of the lateral femoral
condyle hypoplasia, the severity of joint space narrowing, and the relative
position of the tibia in relation to the femur
(Fig. 1). The size and position
of the patella, however, are better assessed in the lateral projection. On
lateral views of the knees, one has to search carefully for the unossified
patellar cartilage anterior to the knee. Absence of the soft tissues composing
the extensor mechanism and of the patella from its normal location can lead to
the diagnosis in young children in whom the patella is not yet completely
ossified [2,
6] (Fig.
2A,
2B,
2C,
2D). Skyline views show the
location, shape, and size of the patella (Fig.
2A,
2B,
2C,
2D); the condition of the
intercondylar sulcus; and the degree of lateral femoral condyle dysplasia;
these views are therefore important in postoperative follow-up (Fig.
6A,
6B,
6C,
6D). However, in most cases of
CPD, ossification is delayed, making radiographic diagnosis more difficult.
Radiography also does not enable assessment of patellar cartilage
development.
With the development of high-frequency transducers, sonography is
increasingly being used in the assessment of bone and joint disorders. In
infants and young children, sonography can show the bony and cartilaginous
parts of the patella and of the femur and tibia
[24,
27–31].
The patella has an elongated, somewhat rounded rectangular shape in its long
axis, as viewed in the sagittal or coronal plane (Fig.
3A,
3B), and a rounded triangular
shape in the transverse plane
[27,
31]. At birth, the
cartilaginous patella shows a homogeneous, low echogenicity; an echogenic
ossification center can be noted several months later. In our patients with
patellar dislocation, the patella remained small and was of homogeneous, low
echogenicity (Fig. 3A).
Sonography can also show the knee ligaments and their bony insertions, thus
providing important information on the overall anatomy of the knee joint.
Sonography has the advantage of being a multiplanar examination, with
adjustment of the examined planes according to the specific bone or joint
anatomy, as demonstrated in our two patients. Sonography has been used as a
dynamic examination in different knee angles and can illustrate the anatomic
location of the patella in relation to the femur at different flexion angles.
The intercondylar sulcus angle can also be measured by sonography
[6,
24,
27–31].
In our patients, we were not able to assess the knees dynamically because of
marked flexion contracture in one patient and knee dislocation in the
other.
CT provides information on fine bone detail and has been used for the
assessment of knee disorders in children and adolescents. Kinematic scanning
(in different flexion angles) can illustrate the patellar location in relation
to the femoral diaphysis [32,
33].
Accurate definition of articular cartilage and separation of cartilaginous
structures from soft tissues can be difficult without the use of contrast
medium. However, CT uses ionizing radiation, and young children may require
sedation. For these reasons, CT currently is not in widespread use for knee
joint assessment in infants and young children.
MRI can discriminate cartilaginous from adjacent joint structures
accurately
[32–36].
The ability to image cartilage makes MRI invaluable in the preoperative
assessment of children and infants with congenital dislocation of the patella.
In this series, we used T1-weighted proton density, fast spin-echo T2-weighted
images with fat saturation, and multiple planar gradient-recalled sequences in
different patients for visualization of the displaced patella (Figs. 4 and
5A,
5B,
5C,
5D). Appropriate technique
should include thin sections to avoid the overlooking of a hypoplastic patella
and the false diagnosis of patellar absence. The course of the quadriceps
tendon should be followed carefully and a hypoplastic, displaced patella
sought. The quadriceps muscle may appear thin and fibrotic. The size, shape,
and orientation of the developing patella can thus be defined. Before a
tentative surgical correction, the size and position of the quadriceps muscle
and the quadriceps tendon insertion need to be defined, as do the patellar
tendon and the medial and lateral patellar retinacula. Multiplanar assessment
of the bones, menisci, and ligaments of the knee complete the evaluation and
enable assessment of which operative procedure is most beneficial to each
individual. MRI demands a high degree of cooperation, and sedation may be
required in many of the younger patients. Nevertheless, the contribution of
MRI to preoperative planning outweighs the need for sedation in these
patients.
Treatment aims at realigning the maldeveloped, laterally displaced extensor
mechanism by using early casting followed by surgical correction
[6,
7,
10,
25]. In planning the operative
repair, the orthopedic surgeon needs to use the detailed anatomic information
that the various imaging studies can provide. These details should include the
degree of quadriceps muscle hypoplasia, the course of the quadriceps and
patellar tendons, the presence of any boney changes in the distal femur and
proximal tibia, the size and location of the patella, and possible changes in
the medial and lateral patellar retinaculum. In young infants and children,
most of this information can be obtained using high-resolution sonography.
MRI, on the other hand, provides an overall detailed anatomic perspective and
permits a better understanding of the mutual anatomic relationships of the
involved structures. Our impression, therefore, is that MRI should be an
integral part of the preoperative evaluation of any patient with suspected
CPD.
CPD can be assessed using several complementary imaging techniques.
Awareness of this uncommon condition with its variable manifestations, and
knowledge of the findings that can be seen with the different imaging
techniques, can lead to a timely diagnosis and prevent or reduce the
development of chronic degenerative joint changes. When suspected, congenital
dislocation of the patella can be diagnosed easily by sonography or by
radiography of both knees. The high resolution for soft-tissue, cartilaginous,
and bony structures of the immature skeleton makes sonography the technique of
choice for initial evaluation of neonates or infants with genu valgum and
flexion contracture. In older children, once ossification of the patella has
commenced, radiography becomes more useful, including the use of skyline views
both for initial diagnosis and for postoperative follow-up. CT can illustrate
bone details accurately, when required. MRI is valuable for visualizing bone,
soft-tissue, and muscular changes and for assessing the developing patella and
articular cartilage. MRI best illustrates the overall anatomic relationships
of the involved structures of the extensor mechanism and therefore should be
performed as a part of any preoperative assessment and planning.
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Abstract
Introduction
