Tearing the anterior cruciate ligament that traverses the knee joint is one of the most common sports injuries. The ligament doesn't heal by itself, and most people who damage it must undergo reconstructive surgery and six months of rehabilitation. To decrease both the pain and the duration of the recovery, Tissue Regeneration, a Medford, MA-based startup, is creating a replacement ligament based on technology developed at Tufts University. To make the replacement, which also helps the body form new ligament tissue, the company modifies the structure of natural silk. The modifications allow cells in the knee to infiltrate and adhere to the material, and they encourage the growth of new tissue. The material has also been treated so that the silk scaffold slowly degrades, gradually transferring weight-bearing responsibilities to the newly formed natural ligament. The material could also be used in the shoulder, elbow, and wrist. The company hopes to start human trials in 2006 and have the silk-based device on the market by 2008.