The Introduction to a scientific paper- whether that paper is an original data report, literature review or case report- has a very specific function. Its primary purpose is to let the reader know why your paper was written. It therefore provides context for the work you present. Typically, the introduction will provide a short overview of the topic area for your paper. For example, if you are presenting a case report on the successful management of an unusual condition, the Introduction will usually give the reader information about the condition in question, as well as some information about the intervention that was so successfully used. This information will be broad and general in nature; the Discussion section is where you can delve into the details about the condition in question.
An Introduction need not be lengthy. In fact, it is likely that a lengthy Introduction contains information that more appropriately belongs in the Discussion section. And often the Introduction is written to demonstrate where a gap in the literature exists. That is, a certain measure of literature is cited, and the author then points out areas of confusion, disagreement or question and notes where his or her paper fits into that confusion, and how it will provide information that may help to resolve some or all of that confusion.
Let me offer an example. Say we were writing a case report discussing the chiropractic management of Condition AB. My Introduction might proceed along these lines: “Condition AB is a disease affecting X% of the population at any given time, with a lifetime incidence of Y%. The condition occurs when something goes wrong in your organ system, affecting the function of this organ [AUTHOR”S NOTE: I am making this all up so am keeping comments purposely vague]. As a result, this lack of function leads to problems in the musculoskeletal system. Medical treatment for Condition X includes this, that and the other thing. However, there is no information in the literature regarding chiropractic management for Condition AB. This paper presents a case which was successfully managed using high-velocity low-amplitude adjustment.” Now, each sentence here would normally be expanded (and each factual statement referenced correctly) upon so that this entry would run to approximately 3 paragraphs, but you can see all the necessary elements in place.
I highly recommend that to get a good sense of the elements I note above, simply read over a few papers from the Journal of Manipulative and Physiological Therapeutics or other chiropractic journal. This will give you a template from which you can begin to develop not only the Introduction to your paper, but the rest of the paper as well.