Objective: To determine the effects of exercise on the distribution and pharmacokinetic parameters of 99mtechnetium-methylene diphosphonate (99mTc-MDP) following intra-articular (IA) injection in horses.
Animals: 5 horses.
Procedures: In this randomized, controlled, crossover study, one antebrachiocarpal joint (ACJ) per horse was assigned to the Exercised group (n=5), and the contralateral ACJ was evaluated in the Non-Exercised group (n=5) after a minimum of 7 days. Following IA injection of 99mTc-MDP (148 MBq), blood and scintigraphic images of the carpus were obtained at 5, 10, 15, 20, 25, 30, 45, 60, 90, 120, 240, 360, 480, 600, 720, and 1440 minutes. Plasma and scintigraphic radioactivity were determined over time, and pharmacokinetic parameters were generated via non-compartmental and compartmental analyses. Each horse was monitored by physical and lameness examination and ACJ synovial fluid analysis before injection and at days 1, 2, 3, and 7.
Results: Lameness was not observed. Mean synovial fluid WBC count increased at day 1 (Exercised: 721 ± 234 cells/uL, p < 0.05; Non-Exercised: 948 ± 223 cells/uL, p < 0.01), but returned to baseline at days 3 and 7. Mean time to maximum plasma radioactivity was earlier in the Exercised group (16.00 ± 2.35 min) than the Non-Exercised group (43.75 ± 3.64 min) (p < 0.001). Linear regression of the scintigraphic radioactivity-time curves showed a greater negative slope in the Exercised group within the first 25 minutes (p = 0.03). There was no difference in absorption or elimination rate constants in a 2-compartment model.
Conclusions and Clinical Relevance: Intra-articular 99mTc-MDP was safe and effective for evaluating synovial solute distribution. Exercise significantly increased early transfer of 99mTc-MDP from the ACJ into plasma, although absorption and elimination rate constants were not affected. Exercise may affect synovial clearance and withdrawal times of IA medications.