Hope,

While it might be impossible for you to now retrieve some of those records, which could influence the date of disability that the SSA would agree to/your backpay award, your current docs should still be able to document disability based on when they started seeing you. They can document that you had past treatments with x,y,and z, etc. The exact findings of each past MRI, test, etc isn't critical to your claim in most cases. With few exceptions (blindness, ALS, terminal cancer, etc) it's NOT about the diagnosis, it's about how your symptoms of that disease/condition impact your ability to work. Your current docs can document that.