Koga graduated from Tokyo Medical and Dental University (TMDU) in 1999, and then joined its Department of Orthopaedic Surgery. After completing a clinical fellowship, he then obtained a PhD at TMDU on cartilage regeneration using mesenchymal stem cells. Following that, he went to the Oslo Sports Trauma Research Centre, Norway, to research knee anterior cruciate ligament (ACL) injury prevention. He was able to clarify the mechanisms involved in ACL injury for the first time in the world using a model-based image-matching technique. He is still working on injury prevention as a member of the scientific committee of the International Olympic Committee (IOC) World Conference on Prevention of Injury and Illness in Sport and is a member of the Academic Advisory Board of the IOC Diploma in Sports Medicine.

Koga has performed many double-bundle ACL reconstructions, as well as reconstructions of multiligament knee injuries and meniscus surgeries. He has carried out clinical and biomechanical research on knee ligament surgeries, especially focusing on double-bundle ACL reconstruction. Another current interest of his is meniscus preservation, and he has developed a new surgical procedure called arthroscopic centralization to reduce or prevent meniscus extrusion.Koga has published more than 200 papers and has given lectures or presentations at many international congresses. He is also working as a member of the International Society of Arthroscopy, Knee Surgery, and Orthopaedic Sports Medicine (ISAKOS) and is a member of the programme committee of the ACL Study Group.

Koga became a professor of the Department of Joint Surgery and Sports Medicine at TMDU in 2020. He also serves on the editorial board of the Journal of Orthopaedic Science, the Annals of the Joint and Asia-Pacific Journal of Sports Medicine, Arthroscopy, Rehabilitation, and Technology.