MRI findings of cyclops lesions of the knee

Abstract

Cyclops lesions develop in the anterior aspect of the intercondylar notch typically after anterior cruciate ligament (ACL) reconstruction or injury. It is a lesion consisting of fibrous tissue with or without cartilage and bony components. A cyclops lesion is one of the causes for reduced extension and, in the cases reported here, also knee pain or discomfort after ACL reconstruction. We present the MRI features, particularly the features on proton density weighted turbo spin echo (PDW TSE) and proton density weighted turbo spin echo fat saturation (PDW TSE FS) sequences of four cases of cyclops lesions, and distinguish between the MRI findings of large and small lesions. We also describe a cyclops lesion after a posterior cruciate ligament reconstruction, not described in literature before.

Full text

MRI findings of
cyclops lesions of the knee
C Minné MBChB (Pret), FC Rad Diag (SA), MMed Rad D (UL)
University of Pretoria, Department of Radiology
MD Velleman MBChB (Pret), FC Rad Diag (SA), MMed Rad D (Pret)
Radiologist, Little Company of Mary Hospital
FE Suleman MBChB (Natal), FC Rad Diag (SA), MMed Rad D (UL)
University of Pretoria, Department of Radiology
Reprint requests:
Dr Cornelia Minné
Email: riaminne@gmail.com
Cell: 073 228 8031
CASE RE P O RT A N D
RE V I E W O F TH E LITERATURE
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Abstract
Cyclops lesions develop in the anterior aspect of the intercondylar notch typically after anterior cruciate ligament
(ACL) reconstruction or injury. It is a lesion consisting of fibrous tissue with or without cartilage and bony com-
ponents. A cyclops lesion is one of the causes for reduced extension and, in the cases reported here, also knee pain
or discomfort after ACL reconstruction. We present the MRI features, particularly the features on proton density
weighted turbo spin echo (PDW TSE) and proton density weighted turbo spin echo fat saturation (PDW TSE FS)
sequences of four cases of cyclops lesions, and distinguish between the MRI findings of large and small lesions. We
also describe a cyclops lesion after a posterior cruciate ligament reconstruction, not described in literature before.
Key words: Cyclops lesion, arthrofibrosis, ACL repair complication, PCL repair complication, motion loss, knee
stiffness, post-operative knee pain
Introduction
Cyclops lesions or focal anterior arthrofibroses are lesions
that develop in the anterior aspect of the intercondylar
notch, typically after anterior cruciate ligament (ACL)
reconstruction or injury. They are lesions consisting of
fibrous tissue with or without cartilage and bony compo-
nents. Cyclops lesions are one of the causes for reduced
extension after ACL reconstruction; however, in three out
of four cases reported here, knee pain and discomfort was
the main complaint. We present cyclops lesions in four
patients, including a patient with a previous posterior cru-
ciate ligament (PCL) repair, which has not been described
in literature.
Case 1
A 23-year-old male patient had a single bundle bone-
patellar tendon-bone ACL reconstruction three weeks
after a rugby injury. Four months after ACL reconstruc-
tion he presented with complaints of intermittent knee
locking and posterior knee pain. An MRI of the right knee
was done to investigate the cause.
A cyclops lesion is one of the causes for reduced
extension and, in the cases reported here, also knee pain
or discomfort after ACL reconstruction
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The MRI (Figure 1 a and b) demonstrated an intact ACL
repair, patellar tendon thickening, small supra-patellar
effusion, synovitis in the region of Hoffa’s fat pad and a
cyclops lesion (2.8 × 2.8 × 1.5 cm) anterior to the ACL in
the intercondylar notch of the femur. The lesion demon-
strated a mild heterogeneous intermediate signal that was
higher than muscle on proton density weighted turbo spin
echo (PDW TSE) sequences (Figure 1a).
It was heterogeneous and hyperintense with a low signal
intensity rim on proton density weighted turbo spin echo fat
saturation (PDW TSE FS) sequences (Figure 1b). The later-
al collateral ligament was thickened and oedematous due to
the previous repair. No impingement was demonstrated to
account for any of the clinical features. Arthroscopy
revealed a cyclops lesion.
Case 2
A 36-year-old male presented with knee pain and a histo-
ry of previous posterior cruciate ligament (PCL) repair.
MRI of the right knee, done to find the cause for the knee
pain (Figure 2a and b), revealed patellar tendon thicken-
ing and traction changes, a small effusion and synovial
thickening in the supra-patellar bursa. The patellar carti-
lage was thinned and degenerative. Mild thinning of the
femoral condyle cartilage with early osteophyte formation
was observed. The posterior horn of the medial meniscus
had signal disturbance and morphology indicating previ-
ous surgery or injury. The lateral meniscus had a small
vertical tear in the peripheral zone with an associated
para-meniscal cyst. Both ACL and PCL had signal
changes but were intact. The PCL was thickened. A
cyclops lesion (2.8 × 2.7 × 1.6 cm) was noted anterior to
the ACL in the intercondylar notch and mild synovitis was
present in the region of Hoffa’s fat pad. The cyclops lesion
was heterogeneous on all sequences with a hypo-intense
rim. There was mainly hyperintense signal on PDW TSE
FS (Figure 2a) and a mildly heterogeneous intermediate
signal higher than muscle on PDW TSE sequences (Figure
2b). A very low signal intensity nodule was seen within
the lesion along the antero-superior border. A cyclops
lesion was confirmed and excised at arthroscopy.
Figure 1a. A follow-up sagittal PDW TSE image of the
right knee of the patient described in case 1, with an
ACL repair demonstrates a cyclops lesion (arrow) of
heterogeneous intermediate signal intensity which is
higher than the signal of the surrounding muscles
Figure 1b. An axial PDW TSE FS image of the right
knee of case 1 revealed a cyclops lesion (arrow) anteri-
or to the reconstructed ACL. The lesion had heteroge-
neous but mainly high signal intensity and a low signal
intensity rim. A small joint effusion persisted.
Figure 2a. Sagittal PDW TSE image of the right knee of
case 2 with a mild heterogeneous intermediate signal
intensity cyclops lesion (black arrow) and an area of
calcification (white arrow) seen anteriorly in the lesion
as a low signal intensity structure
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Case 3
A 27-year-old male presented with anterior knee pain
without knee lock or extension block seven months after
an ACL repair. MRI of the right knee (Figure 3) showed a
thickened patellar tendon, supra-patellar effusion, bone
contusion and oedema in the anterior aspect of the tibial
plateau as well as anterior and superior to the bony tract
of the ACL repair. The repaired ACL was intact. A cyclops
lesion (2.2 × 1.4 × 2.4 cm) was seen anterior to the ACL in
the intercondylar notch. The lesion demonstrated a mild
heterogeneous intermediate signal that was higher than
muscle on PDW TSE sequences. It was heterogeneous
hyperintense on PDW TSE FS sequences. A subsequent
arthroscopy revealed a cyclops lesion correlating with the
MRI findings.
Case 4
A 22-year-old male athlete with a previous ACL recon-
struction and medial meniscus repair presented after a re-
injury. He complained only of lateral knee discomfort. An
MRI of the left knee demonstrated thickening of the patel-
lar tendon, a small supra-patellar effusion, bone oedema
and contusion in the postero-lateral tibia. There was thin-
ning of the cartilage over the medial and lateral femoral
condyles and signal disturbance in the posterior horn of
the medial meniscus due to previous meniscus repair. A
small cyclops lesion (0.9 × 0.9 × 1.1 cm) was seen at the
apex of Hoffa’s fat pad anterior to the ACL in the inter-
condylar notch. It was heterogeneous and isointense to
muscle on the PDW TSE sequences (Figure 4), isointense
to muscle on the PDW TSE FS sequences and hyperin-
tense on short tau inversion recovery (STIR) sequences.
Figure 2b. Coronal PDW TSE FS image of the right
knee of the patient in case 2 demonstrated degenerative
changes of the knee. The heterogeneous high signal
intensity cyclops lesion (black arrow) with a low signal
intensity rim was seen in the intercondylar notch. The
very low signal intensity nodule seen in the supero-lat-
eral border of the lesion represented a calcification in
the lesion (white arrow).
Figure 3. Sagittal PDW TSE FS image of the right knee
of the patient described in case 3 demonstrated a het-
erogeneous high signal intensity cyclops lesion (arrow)
anterior to the repaired ACL.
Figure 4. Sagittal PDW TSE image of the left knee of
the patient in case 4 shows an intermediate signal
intensity cyclops lesion (arrow) in the apex of Hoffa’s
fat pad. The lesion is isointense to muscle with a central
bony component. The bony component is seen as a
high signal intensity structure with a very low signal
intensity rim.
The pathogenesis of a cyclops nodule is not certain
but is most likely multi-factorial
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The lesion contained a bony fragment seen as a high sig-
nal intensity structure with a very low signal intensity rim
within it. The repaired ACL was intact. The cyclops lesion
demonstrated on MRI was confirmed and excised at
arthroscopy.
Discussion
Loss of knee extension is a post-operative complication of
ACL reconstruction. The most common cause for
impaired knee extension is graft impingement, followed
by cyclops lesions, also known as localised arthrofibro-
sis.1,2 Other causes include excessive graft tension, non-
anatomical graft position, inadequate rehabilitation,
fibrosis of the fat pad, supra-patellar and intercondylar
adhesions, entrapment of the patella and capsular con-
tracture.1,3
A cyclops lesion is an intra-articular fibrous nodule at
the anterior edge of the intercondylar notch anterior to
the ACL.1,4 It is most frequently seen after ACL recon-
structive surgery but has also been reported after injury.
The reported frequency of cyclops lesions after ACL graft
reconstruction is 1%–10%.2Arthroscopically the cyclops
lesion is a nodule with a focal area of reddish-blue discol-
oration due to vascular channels.1
The pathogenesis of a cyclops nodule is not certain but
is most likely multi-factorial. Jackson and Schaefer postu-
lated that debris raised by drilling of the tibial tunnel, or
broken graft fibres, gives rise to a cyclops lesion. However,
Marzo et al suggested micro-trauma from graft impinge-
ment.1,5 Histologically the cyclops lesion has a centre of
granulation tissue surrounded by dense fibrous tissue.1,6 As
the lesion matures it develops fibrocartilaginous tissue
and may also contain bone, synovium, fat and fibrous tis-
sue.1,2,4 These lesions may originate from one of the fol-
lowing: the ACL graft, a residual ACL stump, a tibial tun-
nel trap door, intercondylar fibrosis or metaplasia in
Hoffa’s fat pad.2,4
Cyclops syndrome is diagnosed when a patient has a
cyclops nodule demonstrated by MRI and presents with
loss of knee extension. Cyclopoid scars on the other hand
contain only fibroproliferative tissue and do not prevent
full extension.5Only the first case presented here had
intermittent knee locking and would thus be regarded as
cyclops syndrome; the other cases only presented with
knee pain or discomfort without knee lock or flexion
deformity and thus only qualify as cyclopoid scars.
The second case in the described case series developed a
cyclops lesion after a PCL repair. This is most unusual as
no other case report could be found in the literature of a
cyclops lesion following a PCL repair.
MRI sensitivity to diagnose a cyclops lesion in general
is reported as 85%, specificity as 84.6% and accuracy of
84.8 %.4Larger lesions (more than 1cm in at least one
dimension) increases specificity to 100% and accuracy
to 91% but sensitivity remains 85%. The content of a
cyclops nodule will determine its MRI appearance and
therefore there is a variety of possible signal intensities.
The most commonly reported appearances are: low-to-
intermediate signal intensity on T1-weighted sequences,
heterogeneous or intermediate signal intensity on T2-
weighted sequences and intermediate-to-high signal
intensity on PDW sequences.1,2,4,5,7,8 In the case series
reported here, the large cyclops lesions (cases 1, 2 and 3)
have a similar MRI appearance to each other while the
small lesion in case 2 differs from the rest. Large cyclops
lesions are reported in the literature to have heteroge-
neous high signal intensity on PDW TSE FS as in our
patients with large lesions. They have slightly heteroge-
neous intermediate signal intensity on PDW TSE but
with higher signal intensity than muscle. The small
cyclops lesion in case 4 had a homogeneous signal
intensity isodense to muscle on PDW TSE FS and PDW
TSE. This lesion also had a central bony component
seen as a high signal intensity structure with a very low
signal intensity rim. No link could be found in the liter-
ature between the aetiology, the size of the cyclops
lesion and the presence of a bony component.
The type of reconstruction must also be taken into
account when evaluating the MRI of a patient who had
an ACL reconstruction as a remnant bundle reconstruc-
tion can mimic a cyclops lesion.9None of the patients
presented here, however, had a remnant bundle recon-
struction.
Conclusion
Cyclops lesions are the second most common reported
cause for loss of knee extension after ACL repairs,
although not a frequent finding. In this case series knee
pain was a more prominent feature than loss of knee
extension. A cyclops lesion can thus be a cause for knee
pain and discomfort after cruciate ligament reconstruc-
tion and should therefore be excluded in patients pre-
senting with persistent knee pain post-operatively.
Furthermore this is the first description of a cyclops
lesion after a PCL repair that we are aware of and indi-
cates that cyclops lesions are not exclusively found after
ACL reconstructions.
Cyclops syndrome is diagnosed when a patient
has a cyclops nodule demonstrated by MRI and presents
with loss of knee extension
MRI sensitivity to diagnose a cyclops lesion in general
is reported as 85%, specificity as 84.6% and accuracy of 84.8 %
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