A medical professional may have been negligent in providing care to a patient, but sometimes that negligence is not the cause of the injury suffered by a patient. Because the law requires a connection between fault and injury, not all instances of medical malpractice allow for an award of damages. Determining causation in medical malpractice cases often is very complicated and usually requires the assistance of expert witnesses.

Expert witnesses are usually required in medical malpractice cases to establish the standard of medical care in the geographical area or in the area of medical specialty at issue. In addition, expert testimony is required to establish that the malpractice caused the patient's injuries, unless the cause is obvious to a layperson, such as where a wrong arm is amputated or similar error.

In many cases, the cause of injury is less clear, and can be spread among many health care providers. For example, a patient may be treated by a number of doctors, nurses, and medical technicians in the course of a hospital stay. Determining which of these practitioners may have been negligent, and how that negligence may have caused a patient's injury, can be extremely complex. The first doctor may have incorrectly diagnosed a patient, but a subsequent doctor may have been negligent in failing to correct the diagnosis. A subsequent series of mishaps in the operating room, each by a different technician, may require naming each technician as a defendant because each mishap contributed to the injury. Additional injury may have been caused by the use of a defective medical device or drug, or the negligence of an operating room doctor. In such cases, experts are needed to determine the cause of injury in light of the unfortunate sequence of events.