Keywords
knee dislocation - extensor mechanism - magnetic resonance imaging
Knee dislocations, referring to disruption of the tibiofemoral articulation, are uncommon
but devastating injuries. They most commonly occur following high-energy trauma; however,
low energy causes, such as sports, have been implicated as well.[1]
[2] Dislocations often occur in younger individuals and may reduce spontaneously prior
to clinical or radiological evaluation. The prevalence of tibiofemoral dislocations
is therefore likely underestimated owing to their propensity to reduce spontaneously.
Tibiofemoral dislocations frequently result in damage to the major stabilizers of
the knee, thereby increasing the risk of injury to the popliteal artery and peroneal
nerve.[3]
[4]
Knee dislocations can be categorized by the position of the tibia in respect to the
femur. The most common presentation is anterior dislocation usually due to a hyperextension
injury, followed by posterior dislocations occurring from an anterior directed force
to the flexed knee. Less commonly, the knee can dislocate medially, laterally, or
rotationally.[5] Multiple ligaments may be injured most commonly involving the anterior and posterior
cruciate ligaments with medial and lateral ligament tears less commonly identified.
These injuries can be classified by the Schenk system which is the preferred method
of grading knee dislocations [Table 1].
Table 1
Schenck's classification
|
Classification
|
Description
|
|
KD-I
|
ACL or PCL + collateral
|
|
KD-II
|
ACL + PCL
|
|
KD-III M
|
ACL + PCL + MCL
|
|
KD-III L
|
ACL + PCL + LCL
|
|
KD-IV
|
ACL + PCL + MCL + LCL
|
|
KD-V
|
Fracture dislocation
|
Abbreviations: ACL, anterior cruciate ligament; PCL, posterior cruciate ligament;
KD, knee dislocation; MCL, medial cruciate ligament; LCL, lateral cruciate ligament.
Extensor mechanism injuries are also possible; however, they are infrequently discussed
in the literature. The purpose of this article is to review the normal imaging findings
of the extensor mechanism, as well as abnormalities, following dislocation of the
knee joint.
Normal Imaging
Conventional radiography is often the initial imaging modality for the assessment
of the acutely injured knee. Following multiligamentous injuries, its main utility
is to exclude fractures and assess joint alignment. Abnormal positioning of the patella
may be a secondary indicator of extensor mechanism rupture[6]
[7] ([Fig. 1]). Magnetic resonance imaging (MRI), with its superior soft tissue contrast remains
the modality of choice for the assessment of internal derangements of the knee joint.
Fig. 1 Frontal (A) and lateral (B) radiographs of a 24-year-old male with a posterior knee dislocation following a
fall. Note the avulsion fracture of the inferior pole of the patella resulting in
patella alta (curved arrow) suggesting patellar tendon rupture. The patient was taken
directly to the operating room for patellar fracture fixation and patellar tendon
repair.
All ligaments and tendons are predominantly low in signal on all MRI pulse sequences.
A laminated appearance, however, is often present in the distal insertion of the normal
quadriceps tendon as a result of the convergence of individual tendon fibers. The
rectus femoris is located most anteriorly, the vastus medialis and lateralis comprise
the intermediate layer, and the vastus intermedius is the deepest portion of the tendon.
The vastus medialis can be subdivided into the vastus medialis oblique (VMO) and vastus
medialis longus. The normal VMO muscle should be nonedematous and flush against the
femur. The patellar tendon also is predominantly low in signal on all pulse sequences
except for two small triangular regions of the deep tendon proximally, at the origin
with the patella, and distally at its tibial insertion[7]
[8] ([Fig. 2]).
Fig. 2 Normal knee structures in a 24-year-old female. Coronal T1-weighted image (A) demonstrates the VMO muscle flush against the distal femur (arrow). Sagittal T1-weighted
(B) and T2-weighted (C) images demonstrate the normal laminar appearance of the quadriceps tendon (curved
arrow) and normal triangular regions of increased signal in the proximal and distal
patellar tendons (straight arrows). VMO, vastus medialis oblique.
The patella plays an important role in the protection of the distal femoral articular
cartilage and increases the mechanical leverage during extension by elevating the
extensor tendons from the axis of rotation of the knee.[8] The patella should be centrally located in the trochlear groove on axial MRIs. The
patellofemoral joint should be congruent without abnormal tilting of the patella.
On sagittal images, the patella can be easily identified between the junction of the
quadriceps and patellar tendons.
The medial patellofemoral ligament (MPFL) is an hourglass-shaped ligament that acts
as the major stabilizer of the patella. Its origin is a triangular space between the
adductor tubercle, medial femoral epicondyle, and gastrocnemius tubercle, and its
insertion is the superomedial aspect of the patella.[9] At MRI, the normal MPFL is a bilaminated, low signal structure intimately associated
with the VMO muscle, and is best visualized on axial imaging ([Fig. 3]).
Fig. 3 Normal knee structures in a 24-year-old female. Axial T2-weighted image demonstrate
the patella centered in the trochlea. Note the normal VMO muscle (curved arrow) and
the normal bilaminar appearance of the MPFL (straight arrow). MPFL, medial patellofemoral
ligament; VMO, vastus medialis oblique.
Patella Injuries
Patella Fractures
Fractures of the patella are uncommon accounting for less than 2% of all skeletal
injuries.[10] The most common mechanism is direct impact to the knee such as in a fall. The patella
is a sesamoid bone embedded in the quadriceps tendon. This subcutaneous position of
the patella increases the potential of an open patella fracture or injury from a direct
blow.[11] The diagnosis is uncomplicated as patients will often present with swelling, pain,
and reduced active extension in the affected knee.[12] In most cases, radiographs are sufficient, however, nondisplaced fractures may require
MRI for proper diagnosis.
Patellar Instability
Knee dislocations may produce MRI finding similar to isolated patellar dislocation.
This includes VMO elevation and edema, MPFL tears, and abnormal patellar alignment.[13] MPFL tears are common and are present in the majority of knee dislocators. Similar
to isolated patellar dislocations, the most common site of injury is the femoral origin
of the ligament. However, unlike patellar dislocations, there is no evidence of transient
patellar dislocation with impaction of the medial patella with the anterior lateral
femoral condyle. The proposed mechanism of MPFL tears is stretching and elongation
of the ligament from tibiofemoral malalignment[13] ([Figs. 4] and [5]).
Fig. 4 A 34-year-old male with posterior knee dislocation. Lateral knee radiograph demonstrates
elongation of the MPFL dashed line (A). Axial T2-weighted images (B, C) demonstrate elevation of the VMO (curved arrow) and elongation of the MPFL (arrows)
with a hematoma at the femoral origin of the ligament (*). Note the lateral subluxation
of the patella. MPFL, medial patellofemoral ligament; VMO, vastus medialis oblique.
Fig. 5 Axial T2-weithted image demonstrated complete tear of the femoral attachment of the
MPFL (arrow). MPFL, medial patellofemoral ligament.
Tendinous Injury
Patella and quadricep tendon tears are relatively uncommon injuries. Direct trauma
may cause an isolated tear of the quadriceps or patellar tendons. Such injury typically
will not completely disable the extensor hood, since the medial and lateral retinacula
remain intact; therefore, some extensor function will persist.[14]
[15]
Indirect trauma, on the other hand, can result in complete transection of the extensor
mechanism, resulting in complete loss of active or passive extension. Normal tendons
usually do not rupture under stress.[14] In healthy patients, tendons are able to tolerate up to 17.5 times the normal body
weight.[16] Patients with systemic illnesses that result in tendon decline are at higher risk
for tendinous injuries.[16] These include chronic renal failure, rheumatoid arthritis, diabetes, systemic steroid
use, hyperparathyroidism, gout, fluoroquinolone use, and connective tissue disorder.
The literature is sparse regarding tendon injuries of the extensor mechanism in the
setting of knee dislocations.[13]
[17] Tendinous injuries do occur, are less common than MPFL injuries, and are usually
partial tendon tears instead of complete ruptures. In one study, approximately one-third
of individuals with knee dislocations had evidence of partial tears of the patellar
tendon at MRI.[13] These injuries manifested as diffuse increased signal throughout the course of the
tendon [Fig. 6]. The clinical significance of these tendon changes, however, is unknown and may
be well tolerated by individuals. Complete patellar tendon tears requiring surgical
repair however, do occur but are uncommon. In one study, complete patellar tendon
tears were identified in 2 of 14 patients with knee dislocations. One was as an avulsion
of the distal tibial insertion while the other was associated with a comminuted patellar
fracture.[13] Other authors have published similar results of proximal or distal patellar tendon
injuries including distal patellar tendon avulsions and patellar fractures.[17] Neither of these studies reported injuries to the quadriceps tendon and to the authors
knowledge, quadriceps tendon injures following knee dislocation have not been reported
in the literature. If quadriceps tendon tears occur following knee dislocations, they
are likely to be a very rare events.
Fig. 6 Axial (A) and sagittal (B) T2-weighted images demonstrate diffuse high signal throughout the patellar tendon
(arrows).
Conclusion
Knee dislocations are uncommon but devastating injuries most often affecting the major
stabilizers of the knee. Extensor mechanism injuries following knee dislocation include
patellar fractures, patellar tendon, and MPFL tears. Tears of the MPFL are common
and may result in patellar instability. Physicians should therefore be aware of this
potential complication. Most patellar tendon injuries are partial tears; however,
complete tendon tears may occur usually at the proximal or distal attachment sites.
MRI is considered the gold standard for evaluating these injuries due to its superior
contrast resolution.
