FRANK J. MILNE STATE-OF-THE-ART LECTURE


Fig. 11. A, Sagittal section through the distal aspect of a collateral ligament of the distal interphalangeal (DIP) joint with generalised chondroid metaplasia and widespread fissure development. There are chondrocyte clusters (black arrows). The fissures (white spaces) have smooth margins, whereas processing artefacts are usually irregular. There are areas of hyalinisation representing alteration to an amorphous-type acellular structure (black arrow heads). Pale areas (e.g., surrounding the chondrocyte clusters) represent proteoglycan deposition. (Haematoxylin & Eosin,  40). B, Sagittal section through the distal aspect of a collateral ligament of the distal interphalangeal joint. The ligament is hyalinised with no normal ligamentous architecture. There is chondroid metaplasia. There is a naturally occurring cleft between the ligament and the distal phalanx within which are clumps of fissuring degenerate ligamentous tissue (arrow heads). The bone margin (arrows) is irregular reflecting resorption. (Haematoxylin & Eosin,  10). C, Sagittal section through the solar aspect of a distal phalanx. The solar aspect faces distally (arrows) and has an irregular margin. Proximally there is normal lamellar cortical bone. There is an abrupt and irregular transition from bone to abnormal fibrovascular tissue containing osteoclasts. This section was obtained directly distal to the site of insertion of the injured ipsilateral collateral ligament of the DIP joint. It is proposed that injury may result in altered stability of the DIP joint, predisposing to abnormal loading of the ipsilateral aspect of the distal phalanx, and secondary osseous changes. (Haematoxylin & Eosin,  40).


Diagnostic Imaging


Lesions may be identified ultrasonographically, but only a limited length of the ligament can be as- sessed. Ultrasonographic assessment revealed le- sions consistent with desmitis characterized by enlargement of the ligament, and areas of reduced echogenicity were identified in 85 of 313 horses (27%), with lesions confirmed with the use of MRI.39 On MRI, CL injury was defined as increased signal intensity in T1 and T2*-weighted (W) gradient echo (GRE) or fast spin echo (FSE) images in part or all of the ligament, with enlargement or change in shape, or altered definition of the margins, with or without alteration in signal intensity in the perligamentar tissues. Some horses also had increased signal in- tensity in fat-suppressed images.33–38 In all stud- ies, medial lesions were identified more frequently than lateral injuries; lesions were sometimes bi- axial. Interpretation of MR images is potentially confounded by the magic angle effect (MAE) in im- ages acquired in both low-field48 and high-field mag- nets49 related to the orientation of the fibers, especially at the origin; the lateral CL is especially susceptible in images acquired standing because of its more sloping orientation. Use of sequences with long echo times is useful but does not abolish the MAE. Concurrent osseous pathology is discussed below.


Verification of MRI by Comparison With Histology


To verify the interpretation of MR images, a com- parative high-field MRI and histological study was performed in horses both with and without sus-


106 2013  Vol. 59  AAEP PROCEEDINGS


pected CL injury.41 It was hypothesized that ab- normal signal intensity and tissue contour would represent change in tissue structure detected through histological examination. The aims were to compare results in horses free from lameness and those with chronic lameness and to describe possible progression of lesions. One or both feet from 12 horses free from lameness (Group N) and 25 horses with foot-related lameness (Group L) were examined with the use of MRI and by gross postmortem exam- ination. The MR images were graded (0–3) by one analyst. Sagittal histological sections from the proximal and distal aspect of each CL were exam- ined histologically and were assigned a histological grade: 0  normal or the presence of transitional fibrocartilaginous metaplasia, considered to be within normal limits, near the origin and/or inser- tion; 1  localized pallor; 2  diffuse, extensive, or multifocal fibrocartilaginous metaplasia; 3  exten- sive fissuring degeneration and any associated osse- ous pathology (Fig. 11A,B). The overall score assigned to each CL was the sum of the proximal and distal grades. Scintigraphic images from lame horses were also evaluated. In horses from Group N, 25 CLs were graded


normal on both MR images and histology. The ma- jority of CLs had a histology score of 0. Two CLs were graded 1 on MR images but were histologically normal. Two CLs had MR abnormalities verified histologically. However, two CLs appeared normal on MR images but were histologically abnormal. In Group L, in 72 of 89 CLs the results of MRI and histopathology concurred. Eighteen CLs were


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