Key words
patellofemoral joint - knee - patella - patellofemoral instability - maltracking -
MRI
Introduction
Patellofemoral instability (PI) describes an elevated risk of dislocation/redislocation
of the patella. PI can occur after traumatic patellar dislocation caused by injury
to the patellofemoral ligaments and is associated with an increased risk of redislocation.
Alternatively, PI can be the result of unphysiological movement of the patella within
the trochlear groove (known as maltracking) resulting in recurrent patellar dislocation
or subluxation [1]. PI causes cartilage damage at the joint surfaces, which often ultimately results
in retropatellar arthrosis [2].
The incidence rate of PI is 7–49 per 100 000 inhabitants [3]. Young, athletic, active women are most commonly affected. Typical symptoms include
anterior knee pain and recurrent spontaneous patellar dislocation. Patients can remain
symptom-free for a long time. Initial clinical manifestation of PI is often preceded
by an acute injury to the knee [4]. An important clinical finding in PI is the “J sign”, which describes the sudden
lateralization of the patella in cranial extension [5].
Knowledge of the multiple, often connected, anatomical risk factors is important for
diagnosis and treatment planning in PI. Different treatment concepts are initiated
depending on the presence of maltracking and the presence or combination of various
risk factors [1]. In addition to clinical examination, imaging is a cornerstone of diagnosis and
treatment planning in PI. Methods include conventional radiography, magnetic resonance
imaging (MRI), and multidetector computed tomography (MDCT). Kinematic MRI and 4-dimensional
computed tomography (4D-CT) are additional innovative examination techniques that
make it possible to visualize movement of the patella in real time[6]
[7]. Finally, quantitative MRI is a promising method that allows early detection of
cartilage damage due to maltracking.
Anatomy and biomechanics
Anatomy
Articulation between the patella and the trochlea in the patellofemoral joint is a
complex sequence of movements with the bony structures, the quadriceps tendon, the
joint capsule, and the ligaments contributing greatly to stability. Deviations in
this physiological anatomy are risk factors for PI. The retropatellar joint surface
is comprised of a prominent lateral facet, a median ridge, and a medial facet. The
trochlea has a classic concave shape. With its upper pole attached to the quadriceps
tendon, the patella is surrounded by all four parts of the quadriceps femoris muscle.
Muscle fibers of the quadriceps tendon extend over the anterior surface of the patella
and connect as an aponeurosis to the patellar tendon which is attached to the tibial
tubercle [7]. One of the most important static stabilizers of the patella is the medial patellofemoral
ligament (MPFL) [8]. The MPFL runs almost horizontal to the vastus medialis oblique (VMO) between the
medial femoral epicondyle and the medial edge of the patella [9], The MPFL has a close anatomical location with fibers radiating into the joint capsule,
medial collateral ligament, and medial retinaculum. In addition, there are bundles
of fibers between the anterior portion of the MPFL and the tendon of the VMO, the
most important dynamic stabilizer against lateral patellar translation [10]
[11] ([Fig. 1]).
Fig. 1 a Medial view of the anatomical structures and the course of the MPFL. b, c MPFL (arrow) with almost horizontal orientation of the ligament fibers with respect
to the VMO (vastus medialis obliquus) originating at the medial femoral epicondyle
(*) and with insertion at the medial edge of the patella (tip of the arrow). LP = patella
ligament.
Biomechanics
During physiological flexion of the knee, the patella slides back and forth in the
trochlear groove accompanied by mediolateral translation. In full extension the patella
is still proximal to the trochlear groove. At the start of flexion (0–40°), only the
distal portion of the patella is in contact with the proximal part of the trochlear
groove. In this phase of movement, the MPFL plays a significant role in the stabilization
of the patella and prevents lateral dislocation [12]. In the case of flexion > 40°, the morphology of the trochlear groove becomes increasingly
important since the patella slides further into the groove.
Starting at a flexion of 60°, the muscular structures, primarily the VMO, have a stabilizing
function and center the patella in the trochlear groove during flexion.
Stability and instability
Stability and instability
Patellofemoral joint stability is described as “the patella being guided into the
trochlear groove and kept engaged within the trochlear groove” by constraint by passive
soft tissue tethers, bony geometry, and active muscle contraction as the knee flexes
and extends. PI is defined as the deficiency of passive constraint (patholaxity) when
the patella partially or completely leaves its physiological position under the influence
of a displacing force. Such forces can be generated by muscular tension, movement,
or external forces. An intact medial and lateral retinaculum, a physiological joint
formed by the patella and trochlear groove, and the height of the patella are factors
supporting patellofemoral stability. The quadriceps femoris provides important active
stability. In particular, the VMO counteracts patella lateralization during flexion
[9]. A conventional means of clinically evaluating the quadriceps femoris with regard
to the presence of maltracking is measurement of the Q-angle (quadriceps angle) at
25° flexion. The angle between the two intersecting lines (anterior superior iliac
spine to the center of the patella and center of the patella to the tibial tuberosity)
is measured. Values > 20–25° are considered a relevant pathological factor regarding
PI [13].
The MPFL is a main stabilizer in the patellofemoral joint. In the case of a knee with
an insufficient MPFL, the force needed for lateral translation of the patella is reduced
by 50 % in an extended position thereby greatly increasing the risk of lateral PI.
Further risk factors like patella alta, femorotibial torsion deformities, increased
TT-TG distance, or trochlear dysplasia also contribute to PI [3]
[14]. In light of the above factors that can all influence patellofemoral joint instability,
it is clear that PI usually has a multifactorial origin. The extent of the individual
factors in PI can vary greatly on an individual basis [15]. Therefore, it is important to identify these anatomical parameters with the help
of radiological imaging and to quantify them when possible since their presence and
extent influence the selection of the optimal treatment [16]. The most important risk factors as well as their classification and importance
are shown in the following ([Table 1]).
Table 1
Overview of risk factors for patellofemoral instability.
|
risk factors
|
|
trochlear dysplasia
|
|
|
increased TT-TG distance
|
|
|
structural defect of the MPFL
|
-
critical stabilizer of the patellofemoral joint
-
course in the deep portion of the medial retinaculum
-
most commonly injured ligament after patella luxation
|
|
patella alta
|
|
|
genu valgum
|
pathological: lateral deviation from the Mikulicz line > 10 mm or leg axis turned
outward by > 5°
|
|
pathological torsion angle of the axis of the leg
|
physiological torsion angle according to Strecker et al.:
|
Risk factors
Trochlear dysplasia
Trochlear dysplasia (TD) is considered the most important congenital risk factor for
PI [17]. A characteristic of TD is a flattened medialized trochlear groove, that however
does not affect the condyles at the anteroposterior level. As a result, the trochlear
groove does not ensure proper tracking of the patella. In addition, there is flattening
of the lateral slope of the lateral trochlear facet. The trochlear groove is then
often not only slightly concave but is often flat of even convex. According to Dejour
et al., there are four different types of trochlear dysplasia that are considered
predisposing risk factors for PI [18]. Type A is a mild form of dysplasia with only flattening of the trochlear angle
(> 145°). More severe forms of dysplasia are represented by types B-D. Type B dysplasia
is characterized by a flattened trochlea with a prominent supratrochlear spur or bump
on the joint surface. Type C indicates flattening of the trochlea with hypoplasia
of the medial joint facet and convexity of the lateral joint facet. Finally, type
D refers to complete flattening of the trochlea and an abrupt depression in the medial
facet (cliff sign), the most severe form of trochlear dysplasia according to Dejour
[18] ([Fig. 2]).
Fig. 2 Axial view of trochlear dysplasia according to Dejour. a Dysplasia type A with flattening of the trochlea (sulcus angle > 145°) but concavity
is preserved (arrow). b Dysplasia type B, lateral facet is flat to convex, possibly with supratrochlear spur
or bump (arrow). c Dysplasia type C, lateral facet is convex, and the medial facet is hypoplastic (arrow).
d Dysplasia type D, complete flattening of the trochlea with abrupt depression in the
medial facet (cliff sign; arrows).
In addition to the categorization according to Dejour, there are a number of geometric
measurement techniques that can be used to diagnose trochlear dysplasia. Determination
of the trochlear depth (or the groove angle), the trochlear facet asymmetry, and of
the lateral trochlear inclination are the most useful methods [19]. To perform MRI measurements, the selection of an imaging plane approximately 3 cm
above the joint line is recommended. However, this anatomical reference point varies
on an individual basis due to differences in the size of the patient's knee joint.
The entire trochlear facet should always be covered with cartilage. To determine the
lateral trochlear inclination, the angle between the subchondral bone of the lateral
trochlear facet and a tangent along the posterior edge of the femoral condyles is
measured. An angle < 11° is considered pathological here. Trochlear facet asymmetry
is calculated from the ratio of the width of the medial lateral facet to the lateral
trochlear facet (normal value > 40 %). The trochlear depth is defined as the distance
from the cartilage surface to the deepest point of the groove (normal value > 3 mm)
([Fig. 3], [4]). The shape of the patella (Wiberg's classification A-C), which is based on the
configuration of the lateral and medial joint facet, is considered a further cause
of TD. Type A (medial and lateral facet equal length and concave) as well as type
B (flattened medial slightly shortened facet) are considered non-pathological patella
shapes, while a convex, shortened medial joint facet (Wiberg Type C) is considered
a risk factor for the development of patellofemoral instability [20]. A pathologically increased patella tilt (angle between the posterior edge of the
femoral condyles and the axis of the patella on the axial plane) is a further “patellar”
risk factor for PI and a consequence of TD ([Table 2]) [51].
Fig. 3 Measurement parameter regarding trochlear morphology. (I) Lateral trochlear inclination:
Angle (°) measured between a line along the subchondral bone of the lateral trochlea
a and the dorsal femoral condyle plane b. Limit value < 11°. (II) Trochlear facet asymmetry and trochlear depth. Trochlear
facet asymmetry: Length of the medial trochlear facet d divided by the length of the lateral trochlear facet c in percentage = d/c – 100 %. Normal value > 40 %. Trochlear depth: The distance of
the trochlear groove f is subtracted from the mean of the distances of the lateral e and medial g trochlear facet to the dorsal femoral condyle plane = (e + g)/2–f. Normal value > 3 mm.
Fig. 4 16-year-old female with patella instability and recurrent bilateral patellar dislocation.
a MRI after spontaneous patellar dislocation shows lateral subluxation of the patella
with postcontusional edema in the lateral femoral condyle (*). In addition, rupture
of the MPFL (arrow) and the lateral retinaculum (dashed arrow). Signs of trochlear
dysplasia (type B according to Dejour) on radiography b and MRI d, e with flattened lateral trochlear inclination (7°) and decreased trochlear depth (2 mm).
Decision to perform trochleoplasty with medialization of the tibial tuberosity and
MPFL augmentation using autologous gracilis tendon (postoperative radiological follow-up
c, f).
Table 2
Patella Instability Severity Score (PISS) according to Balcarek et al. 2014.
|
risk factors
|
points
|
|
age
|
|
|
|
0
|
|
|
1
|
|
bilateral instability
|
|
|
|
0
|
|
|
1
|
|
trochlear dysplasia
|
|
|
|
0
|
|
|
1
|
|
|
2
|
|
height of the patella
|
|
|
|
0
|
|
|
1
|
|
TT-TG distance (mm)
|
|
|
|
0
|
|
|
1
|
|
patella tilt (°)
|
|
|
|
0
|
|
|
1
|
|
maximum number of points
|
7
|
Patella-Nail Syndrome is a rare form of congenital osseous dysplasia that is typically
associated with severe patellofemoral instability. Pathognomonic changes caused by
this disease include dysplasia of the patella, the fingernails, and the head of the
radius, and typical iliac horns. Characteristic patellofemoral joint findings include
a dysplastic and lateralized patella as well as dysplasia of the femoral trochlea,
with the lateral femoral condyle often representing the patellar groove. Cartilage
damage can often already be detected on MRI at the time of diagnosis ([Fig. 5]).
Fig. 5 19-year-old patient with confirmed Patella-Nail Syndrome and chronic bilateral patella
instability. Transverse CT images a, b and 3D reconstruction c show left-sided trochlear dysplasia (Dejour C). The trochlear groove shows marked
flattening, and the articulation of the patella is highly lateralized with the lateral
femoral condyle. The fat-saturated proton-weighted MRI sequence shows a loss of substance
and signal changes in the cartilage at the lateral retropatellar joint facet (arrow)
indicating the onset of chondropathy d.
Medial patellofemoral ligament (MPFL)
Injury to the medial retinaculum is the most common pathomorphology after patellar
dislocation. In almost all cases, the MPFL is also affected [15]. Current studies show the patellar insertion as the most common location (50–90 %)
[21]. The MPFL is the most important passive stabilizer against lateral translation when
the knee is almost fully extended so that a structural defect of the MPFL often results
in hypermobility of the patella. As a result, MPFL injuries should be treated surgically
depending on the risk of recurrent dislocation to prevent future PI [8]
[12] ([Fig. 6]).
Fig. 6 30-year-old female with traumatic patellar dislocation. a Conventional radiography of the patella shows lateral subluxation of the patella.
In addition, there is a small bony avulsion medial to the patella (arrow). b MRI (PD weighting with FS transverse) confirmed the suspicion of MPFL rupture with
bony avulsion (arrow). c, d Normal postoperative finding in radiological follow-up after arthroscopy with removal
of the loose body and MPFL reconstruction via gracilis transplant.
Patella alta
A high-riding patella is specified as a relative anatomical risk factor for the development
of PI [17]
[22]. In the physiological state, the patella slides into the trochlear groove as flexion
increases and is stabilized by the groove. In the case of a pathologically large distance,
contact of the joint facets is delayed so that the patella has less bony stability.
Lateral X-ray of the knee at approx. 30° flexion is a simple means of determining
the height of the patella. The Insall-Salvati Index (ISI) and the Caton-Deschamps
Index (CDI) are two established measurement methods. The ISI is the ratio of the length
of the patellar tendon to the longest sagittal diameter of the patella [23]. The CDI is based on the ratio between the length of the retropatellar joint surface
and the distance between the caudal pole of patella and the anterior tibial joint
surface. In both measurement methods, patella alta is defined with a ratio > 1.2.
TT-TG distance
If the tibial tuberosity (TT) as the attachment point of the patellar tendon is lateralized
compared to the trochlear groove (TG), the force vector is directed outward resulting
in a predisposition to PI. The TT-TG distance is a simple and reproducible method
for determining the valgus stress (lateralization) on the patella. The distance from
the lowest point of the trochlear groove to the center of the tibial tuberosity (TT)
on axial views of cross-sectional images is measured. Both anatomical measurement
points are projected onto a 90° tangent with respect to the posterior edge of the
femoral condyles. The distance between both lines represents the TT-TG distance. Physiological
TT-TG values are < 15 mm. Values of ≥ 16 mm in connection with maltracking and distances
> 20 mm are considered pathological and there is an indication for surgical tuberosity
transfer [17]
[24] ([Fig. 7]). For several years, the TT-PCL (tibial tuberosity – posterior cruciate ligament)
distance has been increasingly used in the clinical routine since it can be measured
in a flexion-independent manner in contrast to the TT-TG distance [25].
Fig. 7 17-year-old patient after recurrent patellar dislocation while playing soccer. a Radiograph shows bony avulsions of the medial patella (arrow). b–d Detection of a rupture of the MPFL in MRI examination (PD weighting with FS, transverse)
(*). The calculated TT-TG distance (24 mm) is abnormally high c, d. Decision to perform open surgical MPFL reconstruction (autogenous gracilis tendon)
and osteotomy of the tibial tuberosity with medialization according to Elmslie (8 mm)
with normal finding in the postoperative follow-up e, f.
Genu valgum
Genu valgum is considered a risk factor for patellofemoral instability [26]. Valgus alignment of the axis of the leg results in lateralization of the patella
and a modified patellar tilt. Biomechanical studies were also able to show that a
valgus deformity of the axis of the leg has a significant effect on patellar tracking.
A leg axis that is turned outward by > 5° or lateralization of the leg axis > 10 mm
from the Mikulicz line in combination with corresponding clinical symptoms is discussed
here as a valgus deformity requiring surgery [27].
Torsion deformities
Torsion deformity is described as a further risk fact for PI. The term “inwardly pointing
knee” was first described by Cooke et al. in 1990 [28]. In approx. 12 % of all patients with maltracking, a torsion deviation was identified
as the cause. An internal torsion of 24.1° (± 17.4°) is described in the literature
as the standard value for femoral torsion and an external torsion of 34.9° (± 15.9°)
for the tibia [29]. Exact threshold values for surgical correction of torsional deviations have not
yet been defined in the literature. Torsional deviations of 10° or more with corresponding
clinical symptoms are being discussed [29] ([Fig. 8]).
Fig. 8 29-year-old female with severe left-sided knee pain with known patellofemoral instability
and recurrent patellar dislocation. a MRI examination of the knee joint shows proper position of the patella with unremarkable
cartilage finding. b Axial projection of the patella shows a torsion deformity with overlapping of the
tibia and the femoral condyles. c CT for determination of the torsion angle shows abnormally high external torsion
of the tibia, bilateral, with an angle of 56° on the right and 51°on the left. Due
to the presence of symptoms exclusively on the left side, a decision was made to perform
unilateral corrective surgery. An internal torsional osteotomy of the tibia (proximal
to the tuberosity of the tibia) with a fixed-angle plate and MPFL reconstruction using
ipsilateral autologous gracilis tendon were performed. Normal postoperative finding
on radiological follow-up d.
Patellar instability severity score (PISS)
Patellar instability severity score (PISS)
Depending on the patient's anatomic and demographic risk factors, the risk of recurrence
after initial patellar dislocation can vary significantly. Accordingly, the expected
success of conservative or surgical treatment approaches depends greatly on the combination
of various factors [52]. Predictive scoring systems like the patellar instability severity (PISS) score
are helpful clinical tools that allow assessment of the risk of recurrent dislocation
after initial patellar dislocation based on risk factors. Patient age, bilateral instabilities,
trochlear dysplasia, TT-TG, patellar tilt, and patellar height are taken into consideration
in the PIS score and are added up in a point system as a function of their extent.
A point value of ≥ 4 points is associated with an up to 5 times greater risk of recurrent
dislocation ([Table 2]) [53].
Treatment options
The classification of PI and/or maltracking in individual subtypes according to Frosch
et al. can be used to support a comprehensive, structured treatment approach [30]. It must be taken into consideration that PI and maltracking are different pathologies
and this difference must also be taken into consideration in treatment. After initial
patellar dislocation, surgical treatment of the underlying pathology is indicated
due to the high risk of recurrent dislocation and the often associated subsequent
chondral damage. In addition to the fixation or removal of cartilage or cartilage-bone
fragments, reconstruction of the MPFL is a commonly used treatment approach. PI (but
not maltracking) can be treated this way and a recurrence of patellar dislocation
can be prevented.
In contrast, maltracking is usually caused by bone and requires careful analysis.
In the case of a pathological TT-TG distance, tuberosity osteotomy can be used as
a therapeutic option to change the TT-TG distance and the patellar height [31]. Normalization of the TT-TG is critical here. Overcorrection must be avoided. In
the case of incomplete growth and open epiphyseal plates, a transfer of the patellar
tendon insertion can be performed [31]. In patients with chronic PI, trochlear dysplasia is present in up to 96 % of cases
[32]. While MPFL reconstruction is usually sufficient in patients with a mild form of
dysplasia (Dejour type A), trochleoplasty is often additionally necessary in the case
of severe forms of dysplasia (types B-D) [31]
[33].
In the case of an axial deformity in the form of genu valgum or a torsional deformity
as the cause of PI, adjustment osteotomy is the method of choice [31]. Distal femoral osteotomy is a validated method in the treatment of symptomatic
genu valgum in fully grown patients [27] ([Fig. 9]). However, it must be taken into consideration that the bone deformity is not always
in the femur but is located in the tibia in up to 20 % of cases. This would then result
in a correction of the tibial axis. Therefore, an exact preoperative analysis of the
leg geometry is essential for planning purposes.
Fig. 9 24-year-old patient after MPFL augmentation, external, in the case of habitual patellar
dislocation approx. 8 years ago. Presented with persistent symptoms in the right knee.
The full leg X-ray shows genu valgum with the Mikulicz line running approx. 25 mm
lateral to the center of the knee a. MRI detected subluxation of the patella with chondropathy of the retropatellar articular
cartilage (arrow) b. Sagittal T1-weighted MRI shows a high-riding patella with a Caton-Deschamps Index
(CDI) of 1.3 c. In addition, excessive TT-TG distance (23 mm). MPFL reconstruction with lateral
open wedge femoral osteotomy due to pathological leg length difference (right < left)
and osteotomy of the tibial tuberosity with medialization were then performed. Postoperative
conventional imaging showing complete correction of the prior genu valgum d, however detection of loosening of the osteotomy of the tibial tuberosity (arrow
tips) due to a lack of compliance (premature use of the knee) e. Follow-up after revision showing correct position of the osteosynthesis plate on
the tuberosity f and correct axial position of the patella in the groove g. Note: Due to the advanced cartilage damage, distalization of the tuberosity was
not performed.
Diagnostic imaging
Conventional radiography
Traditionally, conventional X-ray of the knee joint on two planes (a. p. and lateral)
and axial projection of the patella (in 30–45 % flexion) is the standard imaging method
for ruling out fracture. However, it was able to be shown in multiple studies that
the evaluation of anatomical risk factors is limited due to the use of inexact imaging
settings [34]. Therefore, torsional deformities can mask or simulate trochlear dysplasia if a
true lateral view is not used or in the case of flexion > 45° on the axial projection
[34]. Therefore, conventional imaging is being increasingly supplemented by cross-sectional
imaging methods like CT and MRI, particularly for preoperative planning.
Anterior-posterior native full leg standing radiographs are used to diagnose genu
valgum. The most common indication for full leg imaging is determination of the axis
of the lower extremity on the coronal plane prior to surgical correction of the leg
axis [35], A deviation of the axis of the leg from the physiological mechanical axis of the
knee joint (Mikulicz line; connecting line between the center of the femoral head
and the center of the upper ankle joint with a course 4 + 2 mm medial to the center
of the knee joint) is registered for this purpose ([Fig. 10]). Additional joint angles (e. g., mechanical lateral distal femoral angle, etc.)
can be determined for further analysis of deformity caused by axial malalignment.
Following surgery, the position of the osteosynthesis material can be monitored with
respect to material defects or loosening ([Fig. 9]). An essential quality feature when acquiring a full leg image is centering of the
patella between the femoral condyles and strict extension of the knee joint, which
is usually associated with an outward rotation of the feet of 8–10° [54].
Fig. 10 Full leg images (right) to determine the axis of the leg: a Patient with normal mechanical leg axis (Mikulicz line runs through the center of
the knee), b young patient with pathological outward angulation of the leg axis (lateralization
of the Mikulicz line 25 mm from the center of the knee) and c patient with advanced gonarthrosis and varus angulation of the leg axis (medialization
of the Mikulicz line 38 mm from the center of the knee).
MDCT
Multidetector CT (MDCT) is an established examination modality that primarily allows
characterization of the bone structure and a detailed evaluation of bone injury patterns.
MDCT is valuable with respect to diagnosis and treatment planning in rotational deformities
and bony torsional deformities. Acquisition of selective axial CT images of the hip,
knee, and ankle is a relatively simple and dose-reduced method for measuring a torsional
deformity. To determine the femoral anteversion angle, the angle between the center
of the hip joint and the center of an ellipse around the greater trochanter is measured
[36]. The tibial torsion angle is the angle between a line drawn dorsal to the proximal
tibial plateau and the axial transverse axis of the distal mortise ([Fig. 8]). The measurement can also be performed with MRI, which should be used as a radiation-free
alternative in young patients [37].
MRI
Today, MRI is considered a validated method for the diagnosis and characterization
of PI [21]. It allows precise evaluation of the scope of injury after patellofemoral dislocation.
A clear advantage is the ability to visualize associated injuries, primarily of the
MPFL and articular cartilage [21]
[38]. Patellofemoral maltracking can also result in fluid accumulation in the surrounding
soft tissue due to constriction or abnormal mechanical stress. Therefore, edema in
the superolateral Hoffa's fat pad can be an indirect indication of maltracking [39].
When evaluating the severity of trochlear dysplasia, MRI yields higher interobserver
agreement compared to conventional radiography [40]
[41]. For the differentiation between mild dysplasia (type A according to Dejour) and
more severe dysplasia (types B–D) that often represent an indication for surgery,
an interobserver agreement of over 90 % can be achieved with MRI [38].. Additional quantitative measurements (sulcus angle, trochlear depth, trochlear
inclination, and trochlear facet asymmetry) can help the examiner to determine severity
[38].
MRI is currently considered the method of choice for determining the TT-TG distance.
Compared to CT, similarly high reproducibility of TT-TG distance measurements can
be shown for MRI [37]. However, it must be taken into consideration in an intermodality comparison that
deviations in the TT-TG distance occur as a function of the degree of flexion when
positioning the knee joint in the MRI coil. If possible, maximum extension of the
knee joint should be ensured since current studies describe a reduction of TT-TG values
in the case of a greater degree of flexion [15]
[24].
A further advantage of MRI compared to conventional radiography and CT is the ability
to evaluate the articular cartilage and thus the “true” geometric configuration of
the joint[6]. Cartilage damage in the patellofemoral joint is one of the most common complications
of PI [15]. In addition to structural defects, increases in the T2 signal intensity at the
lateral joint facet can be detected as possible early degenerative cartilage changes
in patients with PI [42]. In addition to morphological sequences, quantitative MRI examination techniques
in cartilage tissue of the patellofemoral joint have been examined in recent years
[43]
[55]. Connections between changes in T2 / T1rho relaxation times and patellofemoral maltracking
were able to be shown [43]
[44]. The goal of this type of examination is to detect initial degenerative cartilage
changes as a result of patellofemoral maltracking ([Fig. 11]). Future studies must show the value of quantitative MRI for monitoring cartilage
changes in the preoperative and postoperative course (e. g. after MPFL reconstruction)
in patients with PI.
Fig. 11 MRI images of a 20-year-old patient with patellofemoral instability in the presence
of trochlear dysplasia (type C according to Dejour) and a high TT-TG distance (23 mm).
Proton-weighted MRI a shows morphologically intact articular cartilage without substance defects. The quantitative
T1rho b and T2* measurements c show a pathological increase in relaxation times, particularly at the trochlear joint
surface, indicating possible early degenerative changes in the cartilage.
Kinematic imaging
Both kinematic MRI and 4 D CT allow evaluation of the sequence of movements of peripheral
joints [45]
[46]. Image data is acquired during flexion-extension with high spatial resolution. The
first kinematic MRI for dynamic visualization of the patellofemoral joint was acquired
by Shellock et al. in 1988 [45]. By acquiring sequential axial images during passive knee flexion, MRI visualization
of maltracking was able to be achieved. In 2000, McNally et al. described a dynamic
real-time examination of the patellofemoral joint on MRI using an inflatable plastic
balloon that is continuously deflated during active knee extension [47]. In current feasibility studies regarding kinematic MRI during physiological knee
flexion and extension, the clinical benefit of dynamic imaging for the evaluation
of patellofemoral maltracking was able to be shown [48]
[49]. Therefore, kinematic MRI was able to show significant differences in the mediolateral
translation of the patella between healthy subjects and patients with maltracking.
Furthermore, using the examination technique it was possible to show the effects of
anatomical risk factors on maltracking during physiological movement and muscle contraction
[45]
[56].
Moreover, kinematic MRI is a robust and objective examination method for evaluating
surgical success ([Video 1, 2]). Dynamic imaging is currently the most sensitive examination technique even with
respect to recurrent maltracking, which is usually initially clinically asymptomatic
[49]
[50].
However, it must be taken into consideration that even though the majority of cases
of maltracking can be diagnosed based solely on kinematic imaging, conventional MRI
should always be performed as a basic diagnostic method to determine objective radiological
measurements (e. g. TT-TG distance, torsion angle, TD) [49]. There are also currently (not yet) any standardized examination conditions and
established evaluation criteria for a kinematic imaging examination of the knee joint
in routine radiological/clinical diagnosis.
Summary
MRI is currently the method of choice for diagnosing patellofemoral instability and
predisposing anatomical causes. In addition to the exact visualization and quantification
of anatomical risk factors, MRI makes it possible to detect associated structural
injuries and subsequent damage caused by maltracking, e. g., to the articular cartilage.
MDCT is used as a complementary method to MRI in the evaluation of bone structures.
With the help of modern kinematic MRI examination techniques and CT, the sequence
of movements of the patella can be visualized in real time. Dynamic examinations have
added diagnostic value because they visualize patellofemoral maltracking precisely
and in a time-resolved manner and influence factors like quadriceps contraction during
movement can be taken into consideration.
Particularly when multiple risk factors are present, radiological findings help to
determine the dominant, causative pathology of patellofemoral instability and thus
represent an important component of treatment planning.
Video 1 Multi-slice GRE sequence with high time resolution in a 20-year-old patient with
patellofemoral maltracking with a pathologically high TT-TG distance (21 mm), presence
of trochlear dysplasia type C and insufficiency of the medial ligaments. Decision
to perform surgical treatment with MPFL reconstruction and transfer of the tibial
tuberosity.
Video 2 Postoperative follow-up with successful correction of the previously pathological
lateral translation of the patella in the trochlear groove.
