Every payer has a definition of medical necessity and Medicare is no exception. Medicare Administrative Contractors (MACs) release a Local Coverage Determination (LCD) where they provide healthcare providers of all specialties their rules and definitions so they can determine if the care they are providing to their patients is payable. The following definitions pertaining to Chiropractic care come directly from Medicare:

Term Definition
Medical Necessity The patient must have a significant health problem in the form of a neuromusculoskeletal condition necessitating treatment, and the manipulative services rendered must have a direct therapeutic relationship to the patient's condition while providing a reasonable expectation of recovery or improvement of function
Acute Care When the patient is being treated for a new injury identified by X-ray or physical exam as specified above. The chiropractic manipulation is expected to result in an improvement in, or arrest the progression of, the patient's condition
Chronic When the patient's condition is not expected to significantly improve or be resolved with further treatment, but the continued therapy is expected to result in some functional improvement. Once the clinical status is stable for a given condition, and no additional objective clinical improvements are expected, further manipulative treatment is considered maintenance therapy and is not covered.
Maintenance A treatment plan that seeks to prevent disease, promotes health, and prolongs and enhances the quality of life; or therapy that is performed to maintain or prevent deterioration of a chronic condition. When further clinical improvement is not expected from continuous, ongoing care, and the chiropractic treatment is supportive rather than corrective in nature, the treatment is considered maintenance therapy. Chiropractic maintenance therapy is not considered medically reasonable or necessary under the Medicare program and is not payable.
Acute Exacerbation A temporary, but marked deterioration of a patient's condition that is causing significant interference with activities of daily living due to an acute flare-up of a previously treated condition. The patient's clinical record must specify the date of occurrence, nature of the onset, or other pertinent factors that support the medical necessity for treatment. As with an acute injury, treatment should result in an improvement of or an arrest of the deterioration in a reasonable period of time.
Chronic Exacerbation Represents an acute change that is a marked deterioration of the patient's condition and that is causing significant interference with activities of daily living. "Active treatment" can only occur as long as the patient is achieving significant clinical improvement.