Acute onset of severe right hind limb lameness following paddock turnout
13 year old Westphalian gelding used for dressage referred to the University of California after an acute onset of lameness following a paddock turnout that occurred 5 days before admission. Prior to this acute onset of lameness the horse had a history of mild desmitis at the origin of the proximal suspensory ligament in the same limb. On admission the patient was 4/5 lame on the right hind limb. Flexion of the distal extremity was negative and flexion of the proximal extremity was graded as positive. Radiographs of the distal extremity, fetlock, hock, and stifle performed by the referring veterinarian were unremarkable. Diagnostic analgesia was attempted by the referring veterinarian and at the referral institution, but the horse was deemed dangerous. A bone phase scintigraphy exam of the pelvis and hind limbs was performed 60 minutes after intravenous injection of technetium 99m-MDP. A radiographic study was performed the following day.
Radiographs were repeated 4 weeks later and similar findings were reported. Considering the absence of clinical improvement it was elected to performed positron emission tomography (PET) with 18F-NaF and computed tomography (CT) of the region using a previously reported protocol1.
Transverse PET, fused PET/CT and CT video clip (from left to right) through the right distal tibia. Lateral is to the left. There is marked focal NaF uptake (SUV max 37) of the subchondral bone of the medial groove of the distal tibia. Regional sclerosis and mild periosteal reaction are also identified in the same area showing moderate NaF uptake.
The reported changes are consistent with subchondral bone damage (necrosis versus edema) and secondary peri-lesional sclerosis and periosteal reaction.
Outcome:
The horse was reevaluated 3 months after the diagnosis and the right hind lameness had mildly improved and was graded as 3/5. After eleven months the horse remained 3/5 lame on the affected leg. A bone phase scintigraphy exam was repeated and revealed persistent focal increased radiopharmaceutical uptake in the caudomedial aspect of the distal tibia. The owners elected humane euthanasia and the postmortem exam revealed osteonecrosis and fibrosis with cartilage degenerations at the articular surface of the medial trochlear groove.
Discussion:
In 2004, Garcia-Lopez et al2. reported similar subchondral cystic lesions using CT. In the same publication in different cases scintigraphy showed diffused radiopharmaceutical uptake distally and medially in the tibia and the authors identified the medial malleolus of the tibia as the affected portion. We suspect that a similar pathology might be the origin of the uptake seen on scintigraphy, but the lack of resolution and the uniplanar images of the scintigraphy prevented proper identification of the affected region. In our particular case, PET/CT showed precisely that the IRU was located at the distal tibia and the medial cochlea at the level of the subchondral bone plate.
The exact pathophysiology of the reported changes is unknown. Considering the acute onset of clinical signs, traumatic contusion and secondary osteonecrosis of the subchondral bone plate remains the most likely explanation. Additionally, the reported changes in the articular cartilage at the site of the lesion suggests a primary subchondral injury versus a non-subchondral lesion that at a later stage extends to the subchondral bone and articular cartilage. In humans, traumatic subchondral lesions of the knee are typically seen as reversible oedema-like lesion that resolves within 2-4 months 3. In this case minimal clinical improvement was observed over a period of approximatively 15 months suggesting the presence of additional bone necrosis that was later confirmed in the postmortem exam.
This report highlights the importance of functional cross-sectional imaging in the tarsal region of horses over traditional uniplanar imaging modalities. In this case, CT clearly showed regions of sclerosis and periosteal reaction that correlated well with regions of 18F-NaF uptake. Recent studies suggest that 18F-NaF PET can identify regions of active bone remodeling in the canine elbow and equine distal tarsal joint that are silent on CT4,5.