Gross and Histopathologic Correlation of Low‐field Magnetic Resonance Imaging Findings in the Stifle of Asymptomatic Horses

Abstract

With the recent introduction of a 0.25T rotating MRI system, clinical evaluation of the equine stifle joint is now possible in the average equine athlete. A recent publication described common abnormalities of horses with stifle lameness detected with a low-field MRI system; however, postmortem corroboration of the lesions detected was not possible. Therefore, our objective was to compare postmortem findings with low-field MRI findings in equine cadaver stifle joints. Ten fresh cadaver stifle joints from horses without clinical signs of stifle disease were evaluated using low-field MRI, gross dissection, and histopathology. In eight stifles, either the lateral or medial cranial meniscotibial ligament had an irregular shape, fiber separation, or moderate abnormal signal intensity (SI) on all sequences. In five stifles, the medial femoral condyle had articular cartilage fibrillation with or without an osteochondral defect over the weight bearing surface of the medial femoral condyle. All stifles had abnormal SI on all sequences within the patellar ligaments that corresponded with adipose tissue infiltrating between the collagen bundles. Other abnormalities identified included articular cartilage fibrillation of the tibial condyles in three stifles, and articular cartilage fibrillation with chondral defects in the patella in three stifles. All abnormalities detected with low-field MRI were corroborated by gross dissection. Findings from the current study supported the use of low-field MRI for detection of stifle joint lesions in horses and demonstrated that some stifle joint pathologies may be subclinical in horses.

High field magnetic resonance imaging is comparable with gross anatomy for description of the normal appearance of soft tissues in the equine stifle

“…This increase in signal intensity was also made at the junction between the horns and the respective meniscotibial or meniscofemoral ligaments in all stifles. No gross abnormalities were identified to correlate with this change in signal intensity, indicating that it is likely attributable to the effects of magic angle, as have been reported in the human knee and elsewhere in the veterinary literature . Subtle fibrillation of the lateral margin of the medial meniscus was observed on gross evaluation in three of nine stifles but not predicted by MRI evaluation.…”

“…On magnetic resonance images, the curving fibers of the lateral and medial margins of the medial meniscus are subject to magic angle artifact that is observed as intermediate weighted signal compared to the low signal intensity of the mid body of the structure. Magic angle at this location is confirmed by comparing the region of increased signal on proton density FSE sequences to the homogenous, low signal of the meniscus on T2 FSE sequences, which are less susceptible to magic angle artifact (Figure ) . These are clearly visualized on dorsal and sagittal plane sequences at the cranial and caudal margins of both menisci, lateral margin of the lateral meniscus and medial margin of the medial meniscus.…”

Intermediate patellar ligament desmopathy often occurs in conjunction with other stifle abnormalities

“…It is important to note that the normal IPL has a mildly heterogeneous, and often well‐defined striated appearance at the distal portion of the ligament, which should not be confused with desmopathy . Histopathologically, in normal IPLs, adipose tissue is interspersed between collagen bundles creating the striated appearance seen on ultrasound . On MRI, this histologic configuration is characterized by well‐marginated increased signal intensity .…”

Fracture of the medial intercondylar eminence of the tibia in horses treated by arthroscopic fragment removal (21 horses)

“…Ultrasonography is very useful to identify soft tissue damage and especially meniscal and patellar ligament injuries, which may not be visualised arthroscopically [18][19][20]. More advanced imaging modalities, such as computed tomography or magnetic resonance, provide a more complete examination of the different structures of this complicated joint including soft tissue, articular cartilage and subchondral bone that cannot be accurately assessed by more conventional diagnostic imaging modalities [21][22][23]. This occurred with Case 4 in our study, whereby severe subchondral bone degeneration was not diagnosed on routine plain radiographs but was only visible on post-mortem computed tomographic examination.…”