A Bone-knitter was a piece of medical technology used to help repair damage to bones.
Bone-knitters worked much faster than traditional means of repairing broken bones, which were healed by immobilizing bones in splints so that they could heal naturally. (TNG episode: "Contagion")
Such knitters were used to help heal individuals who suffered broken bones in the wake of the Tarsus IV Massacre and resulting civil unrest. (DSC novel: Drastic Measures)
When a miniaturized Hikaru Sulu broke his leg, normal bone-knitting lasers were much too large to be used, so Doctor Leonard McCoy used a microscope laser instead. (TAS episode: "The Terratin Incident")
In 2365 the bone-knitters on the USS Enterprise stopped working due to an Iconian computer program running rampant through the ship's computer systems. Dr. Katherine Pulaski advised one of her assistants to use a splint instead. (TNG episode: "Contagion")