Magnetic resonance diagnosis of subchondral bone trauma in the equine tarsus

Introduction/Purpose: Subchondral bone damage has been diagnosed using Magnetic Resonance (MR) in the equine distal limb as areas of reduced signal intensity in T1 weighted sequences and increased signal intensity in STIR sequences1. A recent study showed no correlation between degree of lameness and radiographic changes in the tarsus2. The purpose of the present study is to describe MR findings in horses with acute onset of severe lameness localized in the tarsal region. Materials and Methods: Horses were selected for inclusion that had acute onset of severe lameness localized to the hock by local analgesia and the results of radiographic and ultrasonographic examination did not conclusively explain the degree of lameness. MR images were acquired by use of a 0.2T imaging system, with the horse under general anesthesia; 3D T1 gradient echo, T2 fast spin echo and STIR sequences were acquired on sagittal, dorsal and transverse planes. Results: We included 3 horses (used for show jumping, age range 6–12 years) showing 3/5 unilateral pelvic limb lameness localized to the hock. Radiographic examination showed enthesiophyte formation on the dorsal aspect of the central and third tarsal bones and dorso-proximal aspect of the third metatarsal bone in case 1; case 2 showed mild degenerative joint disease of the distal intertarsal joint and no radiographic changes were observed in case 3. MR images showed marked irregular thickening of the subchondral bone plate and abnormalities of the adjacent spongiosa of the third and central tarsal bones in all cases. Increased signal on STIR sequences involved the dorso-medial aspect (Case 2 and 3) or axial portion (case 1) of the third and central tarsal bones. In addition, case 3 showed a linear area of hyperintense signal in each sequence on the dorso-medial aspect of the central tarsal bone, interpreted as an incomplete slab fracture. Therapeutic plan included distal intertarsal joint medication (case 1) and extracorporeal shockwaves (case 2). Case 1 and 2 were rested for 1 and 8 months respectively before return to the previous competition level. Case 3 is still in the rehabilitation process. Discussion/Conclusion: Increase signal intensity in STIR sequence could be related to different pathologic situation such as hemorrhage, edema, bone necrosis or fibrosis1. In the present report, horses showed acute onset of lameness suggestive of bone trauma, microfracture of the subchondral bone and fluid accumulation within the bone as the more likely cause of the abnormal signal observed. Subchondral bone trauma can involve the small tarsal bones and MR is crucial for lesion identification and therapeutic plan when radiographic and/or ultrasonographic findings are equivocal.