Saline (also saline solution) is a sterile solution of sodium chloride (NaCl, more commonly known as salt) in water. In the correct concentration, saline is considered to be a safe liquid medium for human tissue to survive and function. Saline solutions are available in various formulations for different purposes. Concentrations vary from low to normal to high. Normal saline (NS) is the commonly used term for a solution of 0.91% of NaCl, about 300 mOsm/L. This concentration is often called physiological or isotonic saline. NS is used frequently in intravenous drips (IV) to support blood flow. Hypertonic saline is a higher concentration of salt solution, sometimes used in an injection procedure known as prolotherapy discussed in the next section.

The use of isotonic saline solution in the knee has been used for decades, pre-dating more current injection therapies and may be very effective, though short-lived. The mechanism of action is simply that of irrigation and dilution of intra-articular debris, neurotransmitters and metabolic waste products within the joint.

The procedure involves numbing the skin and injecting and withdrawing quantities of saline within the joint. These quantities may vary from small amounts to liters. A similar procedure is performed surgically with high volumes of saline, known as an arthroscopic irrigation and debridement or lavage.

• Arthritis
• Pain reduction
• Swelling reduction
• Inflammation reduction
• Treatment of infection
• Diagnostic purposes

Studies generally show improvement with joint injection and lavage, though this change may be short-lived. The degree of improvement from lavage generally coincides with the degree of severity of knee degeneration. Avouac, et. al., demonstration modest reductions of pain and improved function in arthritic knees, which largely diminished by 3 months. A report from Johns Hopkins did not show improvement over placebo.
http://www.hopkins-arthritis.org/arthritis-news/2002/irrigation.html.

These are uncommon. Saline injections are often used as the placebo in case-controlled studies.