The Use of Scintigraphy in the Diagnosis of Bone Spavin

Bone Spavin (Degenerative Joint Disease of the distal tarsal joints) is a well-known cause of hindlimb lameness in the horse. The diagnosis of this condition is usually achieved by clinical examination, selective analgesia of the hindlimb (including intra-articular anaesthesia of the intertarsal and tarsometararsal joints), and radiography. While the diagnosis of the condition poses no problem in advanced cases, the early stages can escape diagnosis by conventional means. In some cases clinical symptoms can be present for a long time before radiographic changes develop; in other cases, a mild bilateral spavin can present disguised as a back problem. Gamma-Scintigraphy has been successfully used to investigate osteoarthritis both in Human and Veterinary Medicine. The technique has proven to be an extremely sensitive indicator of joint abnormalities, and because it relays on the metabolical activity and vascularity of bone rather than gross physical changes, it has advantages over radiography in the early detection of bone remodelling. In the present study, the clinical, radiographic and scintigraphic findings of 17 horses admitted for lameness examination to the Equine Hospital of the University of Glasgow Veterinary School, and in which a diagnosis of Bone Spavin was made are reported. Post-mortem and pathology findings are reported in one case. Bone and Soft Tissue Scintigraphy are compared with conventional methods in its ability to diagnose Bone Spavin, and the correlation between scintigraphy and clinical signs, and scintigraphy and radiography is calculated. Scintigraphy was carried out using a portable scintillation counter connected to a hand held probe. The isotope used was 99m Tc- MDP at a dose rate of 2 Mbq/Kg. injected intravenously. Soft tissue scans were performed 25 min. and bone scans 3 hrs. post-injection. A computer soft-ware program base on a commercial worksheet was used to manipulate the numerical data and display the results graphically for easier interpretation. Cases of bone spavin showed marked increases in isotope uptake in the tarsus, usually with peak increases in the DIT and TMT joints. A value of 40% increase over the contralateral limb was considered significant. The sensitivity of scintigraphy in the detection of Bone Spavin was of 100% in cases of confirmed bone spavin, which compared favourably with the 81,2% of radiography. The correlation between the degree of isotope uptake detected by scintigraphy and the degree of lameness was poor (r = 0.17 for bone phase and r = -0.39 for soft tissue scintigraphy). There was moderate correlation between scintigraphy and radiography. In only 65% of the cases positive radiographic and positive scintigraphic findings were matched. The correlation between the values of the bone scans and the radiological scores of the tarsal joints was good for the left DIT joint (r = 0.74), poor for the right PIT joint (r = 0.99) and moderate for the rest of the joints (r ranging from 0.45 to 0.59). The correlation coefficients for soft tissue phase scintigraphy were similar. In three horses, an increase of activity in the tarsal joints was detected by scintigraphy in the absence of radiographic signs of osteoarthritis, and with a clinical picture suggestive of lower back injury. Follow up examinations of these cases revealed that the positive scintigraphic findings had been an indicator of incipient joint degeneration, as they became clinically symptomatic. This findings illustrate the value of scintigraphy in the early detection or "prediction" of bone spavin, and osteoarthritis in general. They also suggest that subchondral bone changes occur very early in the process of osteoarthritis, and add weight to the hypothesis that they may be instrumental in its development