Page 458 - Adams and Stashak's Lameness in Horses, 7th Edition

424 Chapter 3

lameness may be abolished by anesthesia of the deep The incidence of MRI lesions of stifles examined in
branch of the lateral plantar nerve. an 0.25‐T low‐field open magnet was recently reviewed
VetBooks.ir eral digital flexor) have been reported at the level of the lameness, meniscotibial ligament degeneration, fibrilla­
In one study of 61 horses with stifle
Injuries of the tarsal sheath portion of the DDFT (lat­
in two studies.
111,187
tion, and tearing were the most common abnormality
distal tarsus and at the level of the sustentaculum
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123
tali. Chronic calcaneal bursitis may be accompanied (87%), closely followed by osteoarthritis (74%).
by focal cortical bone loss from the tuber calcis with Degeneration or tearing of a meniscus (56%) or cruciate
generalized reactive osseous fluid throughout the proxi­ ligament (59%) were also common. Less frequently
mal portion of the calcaneus. observed were bone marrow lesions or bone contusions
(30%), tibial enthesopathy of meniscotibial ligament
insertions with cystic bone degeneration (18%), sub­
MAGNETIC RESONANCE IMAGING OF THE chondral bone defects or osseous cyst‐like lesions of the
STIFLE REGION tibial or femoral condyles (21%), and collateral
Introduction desmopathies (10%). Another study, including 70 horses
with stifle lameness without conventional imaging
MRI examination of the stifle has enormous poten­ abnormalities, identified meniscal lesions as the most
tial for improved diagnosis of injuries of this complex common abnormality (95%). Cartilage fibrillation or
187
joint as many of the soft tissue structures of the joint defects (66%), bone contusions (50%), and cruciate
cannot be evaluated comprehensively with any other ligament abnormalities (43%) were also commonly
imaging modality. MRI provides a more complete evalu­ encountered. Although these observations suggest that
ation of the stifle than exploratory arthrosocopy. 87 many of these lesions were not only very common but
MRI of the stifle has rarely been possible in live also present concurrently, the clinical significance of
horses. However, recent equipment improvements both some low‐field MRI abnormalities in the stifle has been
in low‐ and high‐field magnet design have made the sti­ questioned previously. 153
fle accessible to routine MRI examinations under gen­
eral anesthesia. Clinical MRI of the stifle in selected
horses has been possible in ultrashort to short, wide MRI Abnormalities in the Stifle Region
bore (70 cm) high‐field magnets (1.5‐T Siemens Meniscal and Meniscal Ligament Injuries
Magnetom Espree ; 3.0‐T Siemens Verio Open‐Bore
TM
TM
system, Siemens Medical Solutions Inc., Malvern, PA, The obvious advantage of MRI is that it can evaluate
USA). Moreover, technical innovation allowing for the entire meniscus including areas not visible arthro­
84
rotation of open magnets has now made stifle MRI scopically or ultrasonographically, as well as internal
accessible to all horses under general anesthesia with a lesions that do not reach the articular surfaces. An
0.25‐T low‐field MRI system (Figure 3.208; Rotating increase in internal signal or anatomical disruption of
Vet MR Grande XL, Esaote, SpA, Genova, Italy), with the normally hypointense triangular structure of the
the exception of particularly short‐legged horses or meniscus is indicative of damage (Figure 3.250). Intra‐
ponies. 111 substance signal increase without signs of synovitis or
Different stifle MRI protocols have been suggested cartilage damage is considered a degenerative change.
111
for high‐field 38,77,87 and low‐field magnets. 153,186 Contrast However, it should be remembered that menisci are also
enhancement with gadopentetate dimeglumine adminis­ prone to magic angle effects as a cause of signal
tered intravenously has also been useful to clarify fur­ increase. Meniscal injuries are best recognized on PD
38
ther lesions in the stifle. Various imaging planes with or T2 sagittal or frontal images. Damage to the menis­
87
87
the joint in flexion and extension have been cotibial or meniscofemoral ligaments can occur in isola­
evaluated. 38 tion or with concurrent meniscal damage. MRI changes
The MRI anatomy of the stifle has been described in include intra‐substance signal increase or loss of archi­
111
cadaver limbs using both high‐field and low‐field mag­ tecture, loss of margin definition or fraying, and tears.
nets. 38,77,153 Both systems allowed for detailed evaluation Osseous resorption and degenerative cyst formation can
of all soft tissue structures of the stifle. In addition, MR occur as a manifestation of enthesopathy at the tibial
images provided excellent resolution of articular carti­ insertions of the meniscotibial ligaments cranial to and
lage and subchondral bone. 38,77 The unique shape and on either side of the medial intercondylar eminence of
fiber orientation of the menisci predisposed to the effects the tibia.
of magic angle, specifically between the deep external
and shallow internal margins and at the junction
between the horns and the respective meniscotibial or Cruciate Ligament Injuries
meniscofemoral ligaments. In another study using a It has been stated that both cruciate ligaments should
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0.25‐T low‐field open magnet, MR imaging findings in be uniformly hypointense on PD and T2 images and
10 fresh cadaver stifles of asymptomatic horses corre­ that the caudal cruciate ligament can be used as a refer­
lated well with gross and histopathological findings. ence structure to assess signal change in the cranial cru­
Irregular areas of signal increase in all patellar ligaments ciate. However, others have claimed that the cranial
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corresponded to adipose tissue infiltrating between the cruciate ligament has low to intermediate signal inten­
collagen bundles. The authors concluded that the study sity in its proximal half, while its distal half has interme­
supported the use of low‐field MRI for detection of sti­ diate to high signal corresponding to twisting of the
fle joint lesions in horses, even though the pathologies ligament fibers from proximal to distal and the presence
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observed had been subclinical. 153 of shallow striations and fanning of the ligament.