Imaging of the Achilles Tendon

14 Pages·2004·0.62 MB·English

4
Imaging of the Achilles Tendon
Robert R. Bleakney, Lawrence M. White, and Nicola Maffulli
Imaging plays a critical role in the diagnostic 4.2).2 These fi ndings may be subtle without the use
evaluation and assessment of patients with prob- of specifi c high-contrast (low-kilovolt) radio-
lems at and around the Achilles tendon, both in graphic technique.
the documentation and differential assessment of Morphologically, the normal Achilles tendon
disease as well as in the staging of the extent and should be no more than 8 mm thick in the antero-
severity of disease present. Imaging may addi- posterior (AP) dimension, being thickest proxi-
tionally provide important information regarding mally and tapering slightly along its distal third to
the status of the tendon and surrounding osseous its insertion on the calcaneal tubercle. The normal
and soft tissue structures following therapeutic retrocalcaneal bursa should produce a radiolu-
intervention, and in some instances may provide cency anterior to the distal insertional fi bers of the
prognostic information regarding ultimate tendon Achilles tendon that extends at least 2 mm below
function. In this chapter, we review the normal the superior surface of the calcaneus (Fig. 4.1).3
and pathologic imaging features of the Achilles Bursitis or thickening of the tendon at its calca-
tendon, highlighting the potential utility and limi- neal insertion may obliterate this normal radiolu-
tations of various imaging techniques in the non- cency on conventional radiography (Fig. 4.3). If
invasive assessment of the tendon and the adjacent erosions are seen to the posterior calca-
potential impact of imaging fi ndings on clinical neus in the region of the retrocalcaneal bursa,
patient care. then an underlying infl ammatory arthritis, such
Conventional radiography is currently the as rheumatoid arthritis or psoriatic arthritis, with
mainstay of bone and joint imaging, particularly infl ammatory bursitis and pannus formation
in trauma. As it lacks soft tissue contrast, radiog- should be considered.3 Conventional radiography
raphy provides limited information regarding the promptly reveals avulsive fractures, calcifi cation,
soft tissues. However, conventional radiography or ossifi cation of the tendon and adjacent soft
is fast, inexpensive, and readily available and may tissues. Ossifi cation of the Achilles tendon is rare,
still provide important information regarding the with an ossifi c mass contained within the sub-
Achilles tendon and adjacent structures.1 On stance of the tendon, usually seen in patients prior
lateral projection conventional radiographs, the to Achilles tendon rupture or chronic Achilles
normal margination of the Achilles tendon and tendinopathy (Fig. 4.4).4 In contrast, ethesopathic
adjacent pre-Achilles fat pad (Kager’s triangle) is ossifi cation (spur formation) at the calcaneal
seen as a sharp soft tissue interface along the ante- insertion of the Achilles tendon is a fairly common
rior (volar) margin of the tendon (Fig. 4.1).2 fi nding of little clinical signifi cance.
Rupture of the Achilles tendon, Achilles tendi- In contrast to conventional radiography, cross-
nopathy, or infl ammation/hemorrhage within the sectional imaging techniques such as ultrasound
pre-Achilles fat pad may obscure this sharp inter- and MRI have excellent soft tissue contrast
face between the tendon and adjacent fat (Fig. imaging capabilities, and have thus become the
25
26 R.R. Bleakney et al.
FIGURE 4.2. Lateral conventional radiograph of an ankle following
FIGURE 4.1. Lateral conventional radiograph of a normal ankle rupture of the Achilles tendon. There is marked thickening of the
demonstrating a well-defined anterior margin of the Achilles Achilles tendon, loss of the normal sharp anterior border (arrow-
tendon (arrowheads), the pre-Achilles/Kager’s fat pad (*), and the heads), and effacement of the pre-Achilles/Kager’s fat pad.
retrocalcaneal bursal recess (solid arrow).
modalities of choice for imaging assessment of
the Achilles tendon. Musculoskeletal ultrasound
(MSK US) is frequently utilized for assessment of
myotendinous disorders, particularly in Europe
where its use has been extremely popular and
widespread. Constant improvement in technol-
ogy, with higher frequency transducers (15 MHz),
smaller footprint probes, power Doppler (Fig.
4.5), extended fi eld of view capabilities,5 3D
imaging, and tissue harmonics6 have all contrib-
uted in part to this popularity. In comparison with
MR imaging, MSK US has several advantages in
assessment of the Achilles tendon: it is readily
assessable, has a relatively quick scan time, and
has better patient tolerability. MSK US also allows
easy contralateral comparison. In addition, the
FIGURE 4.3. Lateral conventional radiograph of an ankle showing
personal interaction with the patient can produce a thickened distal Achilles tendon (arrowheads), loss of the normal
a more patient-directed examination, tailored to retrocalcaneal bursal recess, dystrophic insertional ossification
the investigation of specifi c clinical complaints (solid arrow), and a retrocalcaneal bursitis, replacing the normal
or symptoms. However, MSK US is operator low attenuation fat pad (*).
4. Imaging of the Achilles Tendon 27
dependent, with a long learning curve. Appropri-
ate training and experience are required for accu-
rate and effi cient use of this modality in clinical
practice. Higher frequency musculoskeletal ultra-
sound transducers provide better spatial resolu-
tion, and thus more detailed delineation of normal
and abnormal superfi cial soft tissues, but are of
limited value in the assessment of deeper
structures due to poor return of echoes. As a
result, MSK US has been increasingly used in the
evaluation of the superfi cial tendon, and in
particular the Achilles tendon, in the assessment
of tendinopathy and rupture to post-treatment
follow-up.7–18
For optimal ultrasonographic evaluation,
tendons should be interrogated/scanned along
both their long and short axes,9,19 orientating the
ultrasound probe so that the ultrasonic waves
reach the tendon perpendicularly. The highly
ordered pattern of parallel collagen tendon fi bers
shows the highest echogenicity when examined
perpendicular to the ultrasound beam (Fig. 4.6).
If this is not the case, the majority of the refl ected
ultrasonic waves will not be received by the trans-
ducer and tendons will appear hypoechoic or
anechoic (Fig. 4.7). This angle-dependent appear-
FIGURE 4.4. Lateral conventional radiograph showing extensive
ance of tissue structures is referred to as acoustic
ossification of the Achilles tendon.
fi ber anisotropy.20 Imaging artifacts related to
fi ber anisotropy can mimic the ultrasonographic
appearance of tendon pathology. Awareness of
the normal curvature of tendons and proper
FIGURE 4.5. Longitudinal ultrasound image of the Achilles tendon with Power Doppler, demonstrating increased vascularity at the
musculotendinous junction.
28 R.R. Bleakney et al.
ultrasonographic investigation, including dynamic
real-time imaging in more than one plane, are
A N I S O T R O P Y
essential to avoid this potential imaging pitfall.21
Except for its insertion on to the calcaneus, the
Achilles tendon has a relatively straight course,
compared with other ankle tendons, and is thus
Tendon less susceptible to anisotropy at US evaluation.
Nevertheless, careful examination of the Achilles,
particularly at the tendon’s calcaneal insertion,
with cranial and caudal angulation of the probe is
necessary to assess the inherent ultrastructural
FIGURE 4.6. Diagrammatic representation of anisotropy. When integrity and echogenicity of the tendon.
the ultrasound beam is perpendicular to the tendon, reflected The normal Achilles tendon has an echogenic
waves return to the transducer. If the tendon is off perpendicular, pattern of parallel fi brillar lines in the longitudi-
then the majority of the reflected waves will not be received by the nal plane and an echogenic round-to-ovoid shape
transducer. in the transverse plane (Fig. 4.8). The number of
echogenic lines visible in the tendon with ultra-
sound is simply a correlate of the ultrasound
A B
FIGURE 4.7. Transverse ultrasound images of a normal Achilles off perpendicular to the tendon demonstrating a hypoechoic
tendon. (A) Probe orientated at 90° to the tendon demonstrating (dark) tendon (arrowheads).
a normal oval echobright tendon (arrowheads). (B) Probe angled
cale
A B
FIGURE 4.8. (A) Longitudinal ultrasonographic image of a normal Achilles tendon. Note the echogenic, parallel fibrillar pattern (between
arrowheads). (B) Transverse ultrasonographic image of a normal Achilles tendon showing echogenic ovoid shape (arrowheads).
4. Imaging of the Achilles Tendon 29
probe frequency.22 On transverse imaging, the tendon onto the calcaneus. Both are well delin-
normal Achilles tendon has a fl at to concave ante- eated at ultrasonography. The pre-Achilles bursa,
rior surface and measures 4–6 mm in anterior to also referred to as the retrocalcaneal bursa, lies
posterior (AP) diameter.8,12,23–25 While 6 mm is deep to the Achilles tendon between the Achilles
generally accepted as the upper limit of normal tendon and the subjacent calcaneus. This bursa is
for AP dimension, the measurement can be some- commonly seen in normal subjects, and may vary
what variable due to anatomical variation in the considerably in appearance and relative dimen-
shape of the Achilles tendon.25 Proximal to its cal- sions with fl exion and extension of the ankle.15
caneal insertion, the Achilles tendon lies immedi- The superfi cial tendo-Achilles bursa or retro-
ately superfi cial to the pre-Achilles fat pad, a Achilles bursa is an acquired bursa occurring in
triangular area of adipose tissue known as Kager’s the potential soft tissue interval superfi cial to the
triangle. At ultrasound imaging, the pre-Achilles distal tendon between it and the dorsal subcuta-
fat pad shows low mottled echogenicity relative to neous tissues. This bursa is not seen in normal
the normally echogenic tendon. Further anterior individuals, and its presence is typically post-
to this pre-Achilles fat pad is the deep fl exor com- traumatic or infl ammatory in etiology.23
partment of the calf, predominantly composed of Tendon thickening and hypoechogenicity are
the fl exor hallucis longus muscle, which overlies the most common abnormalities encountered in
the echobright acoustical interface of the poste- clinical ultrasonographic assessment of the
rior tibial and talar cortices (Fig. 4.9). Two bursae Achilles tendon. Focal or diffuse thickening of
can be present around the insertion of the Achilles the Achilles tendon is most commonly seen in
A
B
MTJ AT
Kager’s
FHL Calcaneus
fat pad
Tibia
C
FIGURE 4.9. (A) Longitudinal ultrasonographic image of the (C) Line diagram of the same field of view. Deep to the Achilles
Achilles tendon using extended field of view, from its soleal tendon (AT) are the flexor hallucis longis (FHL), the echogenic
musculotendinous junction (MTJ) to its insertion on to the (bright) tibial cortex, and Kager’s fat pad.
calcaneus. (B) Soft tissue interfaces marked by dotted line.
30 R.R. Bleakney et al.
association with tendinopathy (Fig. 4.10). Prior In contrast to potential diffi culties encountered
investigations have documented tendon thicken- in the differential evaluation of partial tears versus
ing ranging from 7 to 16 mm in patients with a tendinopathy, ultrasound is highly accurate in the
clinical diagnosis of tendinopathy.8 Similarly, in diagnosis of full thickness tears of the Achilles
athletes with a clinical diagnosis of Achilles tendi- tendon.11,28 Paavola et al. correctly diagnosed 25 of
nopathy, affected tendons were on average 78% 26 full thickness tears before surgery,28 and
thicker than the contralateral unaffected tendon.14 Hartgerink et al. showed that ultrasound can be
Focal hypoechoic areas within the normally echo- effective in the differentiation of full versus partial
bright tendon represent areas of tendinopathic thickness tears or tendinopathy, with a sensitivity
lesions.10,26 Many of the so-called spontaneous and specifi city of 100% and 83%, respectively, and
tendon ruptures are due to progressive degenera- an accuracy of 92%.11 Undectable tendon at the
tion of the tendon.27 Thickening of the tendon and site of injury, tendon retraction, and posterior
focal hypoechoic areas are seen with both tendin- acoustic shadowing at the site of a tendon tear
opathy and partial tearing, thus making the have been described as ultrasonographic signs of
differentiation between the two diffi cult.1,26,28 a full thickness tear (Fig. 4.11).11,12 Posterior acous-
However, Åström proposed that thickening of the tic shadowing deep to the torn tendon margins is
tendon >10 mm and severe intratendinous abnor- thought to occur secondary to sound beam refrac-
malities indirectly suggested partial rupture.26 tion by frayed/torn tendon ends.12 A potential
However, true partial tears are rare at surgery, and pitfall in the ultrasound evaluation of a complete
potential differentiating imaging features can at full thickness tear of the Achilles tendon is visu-
times be misleading. alization of an intact plantaris tendon medial to
A B
CALC
C
FIGURE 4.10. (A) Normal transverse ultrasonographic image of the and hypoechoic. (C) Longitudinal extended field of view ultraso-
Achilles tendon. (B) Transverse ultrasonographic image of an nographic image of Achilles tendinopathy demonstrating fusiform
Achilles tendinopathy. The tendon is thickened, heterogeneous, thickening of the Achilles tendon (arrowheads).
4. Imaging of the Achilles Tendon 31
FHL
A
B
FIGURE 4.11. (A) Longitudinal extended field of view ultrasono- (B) Longitudinal extended field of view ultrasonographic image of
graphic image of a complete tendon tear showing the gap in the a complete tendon tear. Note more retraction, compared to case
tendon (*) and the torn tendon ends (solid arrows). The muscle A, with a larger gap in the tendon, torn tendon ends (solid arrows),
belly of flexor hallucis longus is well seen deep to the tear. and echogenic (bright) fat herniating into the tendon gap (*).
the torn fi bers of the Achilles (Fig. 4.12). The sound evaluation.8,16,30–32 Rupp et al. tried to cor-
normal plantaris tendon may be mistaken for relate long-term clinical outcome after surgery for
residual intact Achilles tendon fi bers, and can lead Achilles tendon rupture with ultrasound mor-
to a false diagnosis of a high-grade partial tear phology of the tendon, but found that, while ultra-
rather than a complete tendon tear.11,29 Dynamic sound is able to reveal long-lasting changes of the
ultrasound assessment of a complete Achilles morphology of the tendon, it was of only limited
tendon rupture can further reveal whether the value in evaluation of the functional result.16,32
retracted torn tendon ends can be approximated Calcifi cations may also occur at the site of a prior
to one another on plantar fl exion. This may be of tear: they are seen as hyperechoic areas casting
use when deciding between conservative versus acoustic shadowing (Fig. 4.13).8,30,31 Despite the
surgical treatment. ability of ultrasound to accurately depict struc-
Following successful management of Achilles tural abnormalities of the Achilles tendon, only
tendon rupture and tendinopathy, tendon abnor- moderate correlation exists between ultrasound
malities can persist on ultrasound despite resolu- appearance and clinical assessment of chronic
tion of patient symptoms. Following conservatively Achilles tendinopathy.33 In addition, Khan and
or surgically treated Achilles tendon ruptures, the co-workers showed that the baseline ultrasound
tendon can continue to appear thickened and appearance of the tendon, in the setting of chronic
irregular with focal hypoechoic areas at ultra- tendinopathy, was a poor predictor of subsequent
32 R.R. Bleakney et al.
A B
FIGURE 4.12. Transverse (A) and longitudinal (B) ultrasonographic images of a full thickness tear of the Achilles tendon, demonstrating
an intact echogenic plantaris tendon (solid arrows).
clinical outcome.33 Conversely, other investiga- abnormal tendon, suggesting neovasculariza-
tors have shown that tendon inhomogeneity can tion.37 Zanetti et al. demonstrated that the pres-
be used to predict clinical outcome in painful ence of neovascularization is a relatively specifi c
Achilles tendons.34,35 Nehrer et al. additionally sign for a clinically painful tendon. However, the
reported that patients with a clinical diagnosis of presence of neovascularization did not affect the
Achilles tendinopathy, with a normal ultrasound patient’s outcome adversely.35
appearance of the Achilles tendon, had a signifi - The multiplanar imaging capabilities of MRI
cantly better clinical outcome, compared to indi- combined with its excellent soft tissue contrast
viduals with abnormal fi ndings at ultrasound.36 characteristics make it ideally suited for imaging
They also documented that patients with tendon of the Achilles tendon. Sagittal and axial planes
thickening and focal hypoechoic areas had are most useful in the evaluation of the Achilles
higher rates of subsequent spontaneous tendon tendon commonly using a combination of T1 and
rupture.36 T2 weighted imaging sequences.38,39 In general, T1
The recent addition of color and power Doppler or intermediate weighted sequences provide
imaging to ultrasound has allowed for the nonin- optimal delineation of anatomic detail, and T2
vasive study of blood fl ow and vascularity within weighted sequences are most sensitive to the
and surrounding the Achilles tendon (Fig. 4.5). In abnormal increase in fl uid signal that accompa-
patellar tendinopathy, color Doppler has demon- nies most pathological conditions of the tendon.40
strated increased vascularity in the setting of an Short tau inversion recovery (STIR) and fat satu-
A B
FIGURE 4.13. (A) Longitudinal extended field of view ultrasono- torn Achilles tendon. There are focal echogenic (bright) areas of
graphic image of an insertional Achilles tendinosis with echogenic calcification (solid arrow) with posterior acoustic shadowing
(bright) calcification (solid arrows) in the thickened distal Achilles (arrowheads). The Achilles tendon is thickened, hypoechoic, and
tendon. (B) Transverse ultrasonographic image of a previously heterogeneous.
4. Imaging of the Achilles Tendon 33
ration T2 weighted sequences may additionally
serve to increase signal contrast between free
water and the surrounding fat and adjacent
tendon.
On sagittal MR images, the anterior and poste-
rior aspects of the normal Achilles tendon should
be parallel to one another distal to the soleus
insertion (see reference 43). The normal average
AP dimension of the Achilles tendon on MRI is
6 mm.25 At axial imaging, the anterior aspect of the
tendon should be fl at to concave (Fig. 4.14). The
length of the Achilles tendon is variable, ranging
from 20 to 120 mm between the musculotendi-
nous origin and the calcaneal insertion of the
tendon.41 The presence of an accessory soleus
muscle produces an apparently shorter Achilles
tendon, as the accessory soleus may have an inser-
tion directly onto the anterior margin of the
Achilles tendon, mimicking a more distal muscu-
lotendinous origin (Fig. 4.15).42 The normal
Achilles tendon is of low signal (black) on all MR
imaging sequences, refl ecting its ultrastructural
composition of compact parallel arrangements of
collagen fi bers and its low intrinsic water content
(Fig. 4.16).25,40,43 However, the magic angle phe-
nomenon can produce areas of increased signal
within normal tendons, observed as tendon fi bers
approach an orientation angle of 55° relative to
the main magnetic fi eld.44,45 While the Achilles has
FIGURE 4.14. Axial T2 weighted sequence of a normal ankle, dem-
a relatively straight course, this effect can occur as
onstrating a normal Achilles tendon (arrowheads), adjacent plan-
taris tendon (solid arrow), and Kager’s fat pad (*).
A B
FIGURE 4.15. Sagittal T1 weighted
image (A) and axial T2 weighted image
(B) of the ankle showing an accessory
soleus (*).
34 R.R. Bleakney et al.
A B
FIGURE 4.16. Sagittal T1 (A) and T2 with fat saturation (B) images, of a normal Achilles tendon. Note the parallel anterior and posterior
tendon surfaces (arrowheads) and its low signal (black) appearance on both sequences.
the Achilles fi bers spiral internally.43 The magic fusiform tendon shape, AP tendon thickening,
angle effect is usually seen with echo times of less convex bulging of the anterior tendon margin,
than 20 msec (e.g., T1 weighted and proton and areas of increased intratendinous signal on
density/intermediate acquisitions), but the effect T1 and T2 weighted sequences (Fig. 4.17).43,50
should not be present on T2 weighted (long echo Areas of increased signal within the tendon on T2
time) acquisitions.45 Recently MR magic angle weighted sequences are thought to represent more
imaging has been used to advantage in imaging of severe areas of collagen disruption50,51 and partial
the Achilles tendon. With this technique the tearing.26 AP thickening of the tendon greater
Achilles tendon is imaged at 55° relative to the than 10 mm correlates with pathological fi ndings
main magnetic fi eld rather than at the usual 0°. In of partial tendon tearing.26
this way, signal related to the magic angle Full thickness tearing of the Achilles tendon is
phenomenon becomes detectable within the manifest on MR imaging as complete disruption
tendon.46–48 Using this method, intratendinous of the tendon fi bers with tendon discontinuity
STIR signal change was more apparent, and con- and high signal intensity within the tendon gap
trast enhancement was much more evident within acutely on T2 weighted images (Fig. 4.18).52
the tendon.47 MR imaging with plantar fl exion, when feasible,
As with ultrasonographic assessment, a pre- may provide additional information regarding
Achilles/retrocalcaneal bursa can be seen in separation of the torn tendon ends and potential
asymptomatic individuals at MR evaluation. The apposition of the tear margins. Potential diagnos-
bursa normally measures up to 6 mm craniocau- tic diffi culties may arise in the MR imaging dif-
dally, 3 mm medial to lateral, and 2 mm anterior ferentiation of chronic tendinopathic changes
to posterior.49 Achilles paratendinopathy mani- with a complete non-retracted tendon tear
fests as linear or reticular areas of increased signal from cases of chronic tendinopathy with partial
on T2 weighted images, paralleling the deep tendon tearing. As with ultrasound examination,
margin of the Achilles tendon, representing areas MRI following Achilles tendon repair typically
of edema or increased vascularity. However, both illustrates tendon thickening and intrasubstance
ultrasound and MRI evaluation are unreliable in imaging heterogeneity (Fig. 4.19).32,51,52 A decrease
the assessment of the paratenon.26 in intrasubstance signal, together with an
Classic MR imaging features of Achilles increased tendon size, may gradually progress
tendinopathy include morphologic fi ndings of a over one to two years after surgery, possibly