Prolotherapy is a newer technology. Dextrose (a type of sugar) is injected into ligaments or tendons. This acts as an irritant and triggers wound healing. New collagen is formed, which is the material of tendons a ligaments. This new collagen shrinks as it develops resulting in a tighter tendon or ligament.
Prolotherapy is a treatment that has been around for a long time, but until recently was not used for tendons, and musculoskeletal disorders. Since this use is newer, there are studies being done to evaluate prolotherapy effects.
Beware of clinics promoting stems cells or other injections without evidence based research, or that make claims that seem too good to be true.