Effects of Time to Start Training After Acute Patellar Tendon Enthesis Injuries on Healing of the Injury in a Rabbit Model

Background: A patellar tendon injury is a common injury in sports. The optimal time to start training after an acute, proximal patellar enthesis injury is still unclear. Hypothesis: The time to start training after an acute, proximal patellar enthesis injury significantly affects healing of the patellar tendon 4 weeks after the injury. Study Design: Controlled laboratory study. Methods: The left hindlimbs of 35 mature female rabbits were randomly assigned to 5 injury groups including a 4-week natural healing group (NH4W) and 4 training groups that started low-intensity training at 24 hours (POST24), 48 hours (POST48), 72 hours (POST72), and 96 hours (POST96) after an acute patellar tendon injury, with 7 limbs in each group. The right hindlimbs of the NH4W group were used as a control group (CON). An acute, proximal patellar enthesis injury was created in all injury groups. The training groups underwent low-intensity quadriceps training for 2 hours per day and 3 days per week for 4 weeks. Histological and radiographic data were collected and analyzed. Results: The cell densities of the training groups were significantly lower than those of the NH4W and CON groups (P = .01). The fibrocartilage zone was significantly thicker in the POST24, POST48, and POST72 groups compared with the CON and NH4W groups and was the thickest in the POST24 group (P = .01). The bone surface to bone volume ratio was significantly higher in all the injury groups compared with the CON group and in the POST24 group compared with the other groups (P = .01). Trabecular thickness was significantly lower in all the injury groups compared with the CON group and in the POST24 group compared with the other groups (P = .01). Conclusion: Resting without training in the first 96 hours after an acute patellar tendon enthesis injury resulted in the best recovery of cell density in the tendon enthesis 4 weeks after the injury. Starting training 96 hours after the injury resulted in the best recovery of fibrocartilage zone thickness. Starting training 48 to 96 hours after the injury resulted in the best healing of the bone component of the attachment site 4 weeks after the injury. The optimal time to start training may be longer than 96 hours after an acute patellar tendon enthesis injury for the best overall healing of the tendon enthesis 4 weeks after the injury.