Independent and additive stimulation of tendon repair by thrombin and platelets.

Background: Platelet concentrate application with added thrombin improves Achilles tendon repair in the rat. Upon tissue injury, platelets are activated by thrombin, which has many biological properties in common with growth factors. We wanted to differentiate the effect of platelets from that of thrombin.
Methods: The Achilles tendon was transected in 50 rats. Platelet gel was prepared from the blood of 10 other rats. The rats were given either platelet gel with active or neutralized thrombin implanted into the defect during the operation, or a local injection 6h postoperatively with 50 microL of either platelet concentrate, thrombin or saline. The rats were killed after 14 days and the tendons were mechanically tested.
Results: Compared to saline, platelet gel caused a 42% increase in force at failure, a 90% increase in energy, and a 61% increase in ultimate stress. Platelet gel with neutralized thrombin caused a 22% increase in force at failure, and energy and stress were less elevated. Injected platelet concentrate caused a 24% increase in force at failure, and thrombin caused a 10% increase. These effects and the differences between treatments were statistically significant.
Interpretation: Platelets and thrombin had independent and additive stimulatory effects on tendon repair. The clinical relevance is so far unknown.