Platelet-Rich Plasma (PRP) therapy involves concentrating a patient’s own blood components to support tissue healing, such as in the knee joint. PRP is a volume of plasma with a platelet concentration significantly higher than that found in whole blood, typically aiming for three to five times the baseline concentration. These concentrated platelets are a natural source of numerous proteins and growth factors, like Platelet-Derived Growth Factor (PDGF) and Transforming Growth Factor-beta (TGF-β). Upon activation, these factors are released to help regulate tissue repair and regeneration. The precision of the preparation process directly influences the final concentration and is relevant for clinical success in treating knee conditions like osteoarthritis.
Initial Blood Collection Procedures
The preparation of Platelet-Rich Plasma begins with a sterile, peripheral venipuncture to collect a patient’s whole blood, similar to a standard blood test. The venipuncture site, often the antecubital vein, is meticulously cleaned to maintain an aseptic environment and prevent contamination. The collection volume is important because it dictates the potential yield of the final concentrate, with typical requirements for a knee injection ranging from 30 to 60 milliliters of blood.
The collected blood must be drawn into specialized tubes or syringes that contain an anticoagulant, most commonly Acid Citrate Dextrose Solution A (ACD-A) or sodium citrate. The anticoagulant prevents the blood from clotting and keeps the platelets in a non-activated state until the preparation process is complete. The volume of the final injectable PRP product can be as small as 3 to 8 milliliters, derived from the initial, larger volume of whole blood.
Centrifugation Protocols
After collection, the whole blood is placed into a clinical centrifuge, where the process of separation begins. Centrifugation works by applying centrifugal force, which separates the blood components based on their different densities. Red blood cells, being the heaviest, are pushed to the bottom of the tube, while the least dense plasma remains at the top. A thin, intermediate layer called the buffy coat forms between them.
Centrifugation protocols are highly variable and are generally categorized as either a single-spin or a double-spin method. The goal is to isolate the buffy coat layer where the concentrated platelets reside. The single-spin method typically involves one moderate-speed spin to separate the components, which may produce a lower platelet concentration, often two to four times the baseline.
Alternatively, the double-spin method uses an initial “soft spin” to separate the red blood cells and then a second, “harder spin” on the plasma layer. This further concentrates the platelets into a small pellet, potentially achieving a five to eight times concentration. Different commercial systems use proprietary, pre-set speeds, measured in revolutions per minute (RPMs) or relative centrifugal force (g-force), and specific time intervals to standardize this separation. The ultimate goal is to achieve a clinically effective concentration factor, usually considered three to five times the baseline platelet count, while minimizing the inclusion of red blood cells.
Isolating the Platelet Concentrate
Once the centrifugation process is complete, the blood is visibly stratified into three distinct layers, and the next step involves the manual and precise aspiration of the desired platelet-rich layer. The buffy coat, a thin, whitish layer situated just above the packed red blood cells, contains the highest concentration of platelets and most of the white blood cells. The layer above the buffy coat is Platelet-Poor Plasma (PPP), which is largely discarded.
A medical professional carefully draws the buffy coat and a specific volume of the adjacent plasma into a new, sterile syringe, avoiding the red blood cell layer to minimize contamination. The final volume of the concentrate is determined by the specific protocol and the intended therapeutic dose, which is often around 4 to 8 milliliters for a knee injection. Precision in this step is paramount because aspirating too much of the lower layer introduces unwanted red blood cells, which can cause increased post-injection pain, and removing too much of the upper plasma reduces the therapeutic platelet concentration.
Final Activation Before Injection
Before the prepared Platelet-Rich Plasma is injected into the knee, the platelets must be “activated” to initiate the release of stored growth factors from their alpha-granules. Activation causes the platelets to degranulate and initiates the clotting cascade, which is necessary for the therapeutic effect. There are two primary approaches to achieving this activation: in vitro (outside the body) or in vivo (inside the body).
The in vitro method involves mixing an exogenous activating agent, such as calcium chloride (CaCl2) or bovine thrombin, into the PRP concentrate immediately prior to injection. This provides a controlled and immediate release of growth factors before the product enters the joint space. Conversely, the in vivo method involves injecting the PRP in its resting, non-activated form, relying on contact with native collagen and other tissues within the knee joint to spontaneously trigger activation and growth factor release. The choice of activation method is often based on the physician’s preference and the target tissue.