Many people have injured their backs, knees, shoulders, etc. and are afraid to exercise for fear of doing more damage. The problem with avoidance means that you never create muscular stability for joints and tendons in that area.
By avoiding fitness, you could be more prone to future injuries due to lack of musculature. CrossFit teaches you how to move properly and exercise with correct mechanics, which in turn helps you regain functional capacity.
If you have permanent damage that is irreversible our trainers can always modify workouts to fit your needs.