Ortho Expert has been developed to provide a specialist with the necessary information and medical knowledge on all levels of practicing, education and self-education. Ortho Expert includes special and general medical knowledge, encyclopedic and reference data, generalized experimental knowledge and classical knowledge. All this knowledge is presented in a convenient and accessible form in a systematized and structured manner. Ortho Expert provides the doctor/practitioner with lists of possible diagnosis with validity for each diagnose, based on the input the doctor/practitioner has given. Especially for this, complete new Knowledge algorithms have been developed, together with a full and transparent explanation system in order to justify the reasoning of the system.