What is platelet-rich plasma?
Platelet-rich plasma (PRP) is an autologous concentration of human platelets in a small volume of plasma. It is a multidisciplinary procedure, with a great amount of supporting literature for the field of cosmetic surgery, as well as oral implantology, ophthalmology, orthopaedics and sports medicine.
Plasma, which comprises 55% of blood fluid, is mostly water (90% by volume), and also contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), platelets, and blood cells themselves. As it is a concentration of platelets, it is also a concentration of the seven fundamental protein growth factors proved to be actively secreted by platelets to initiate all wound healing. These growth factors include the three isomers of platelet-derived growth factor. All of these growth factors have been documented to exist in platelets2, 3, but the platelets will need to be activated. On activation, they will release alpha granules, within which those growth factors are stored.
In addition, the activated thrombocytes have a multitude of signalisation molecules on their surface: CD9, CD-W17, CD41, CD42a-d, CD51, CD-W60, CD61, CD62P, CD63. As these concentrated platelets are suspended in a small volume of plasma, PRP is more than just a platelet concentrate; it also contains the three proteins in blood known to act as cell adhesion molecules for osteo-conduction and as a matrix for bone, connective tissue, and epithelial migration. These cell adhesion molecules are fibrin itself, fibronectin, and vitronectin.
Platelet origin, morphology and distribution
Platelets are cytoplasmic fragments of megakaryocytes (a type of white blood cell), which are formed in the marrow, are round or oval in shape, and approximately 2 μm in diameter4. They have a trilaminar cell membrane with a glycoprotein receptor surface overlying and partially interspersed with and penetrating a bilayer of phospholipids and cholesterol5.
Platelets lack nuclei but contain organelles and structures such as mitochondria, microtubules, and granules (α, δ, and λ)4–7. There are approximately 50–80 alpha granules per platelet, each bound by a unit membrane and formed during megakaryocyte maturation8. The granules are approximately 200–500 nm in diameter and contain over 30 bioactive proteins, many of which have a fundamental role in haemostasis and/or tissue healing8, 9.
The platelet cytoplasm contains an open, canalicular system that increases the effective surface area for intake of stimulatory agonists and the discharge of effector secretions. The sub-membrane region contains microfilaments of actin and myosin that mediate morphologic alterations5. These cells possess a tricarboxylic acid cycle and use glucose by means of the glycolytic and hexose monophosphate shunt pathways4. Their function is closely linked to their metabolic activity.
A small volume of blood (ideally 20 ml) is taken by venipuncture from the patient and inserted into the correct size of tube to be centrifuged. Platelet concentration (growth factors) will be dependant on:
The amount of whole-blood used
Platelet recovery efficiency
The final volume of plasma in which platelets are suspended.
For these reasons, to homogenate the value of the acceleration, which is of importance to the self-activation of the liquid, it is better to work with wide and short tubes. It is also important to choose a concentration factor of less than 4 to avoid a final concentration that is too high, which would be detrimental owing to a high risk of self-activation (premature). Self-activation in such cases is usually the result of an increase in concentration and platelet aggregation that follows their physical proximity.
It has been established that excessive acceleration will decrease the integrity of the platelet membrane, some of which are then activated10. Thus, for values of more than 400 G, 5% of platelets are activated11, and 40–70% of platelets will be activated at 3000 G12.
The results obtained with systems using a double centrifugation are similar to those that use a single centrifuge, but injections are more painful and inflammatory13, owing to the ratio between platelets, leukocytes and plasma, as well as the increased amount of manipulations. Furthermore, even if the desired concentrations are higher, the risk of self-activation will increase. PRP contains large numbers of cellular microparticles, including annexin V+ microparticles, which are lost to varying degrees when PRP is double centrifuged to remove platelets.
[pull_quote align=”right” ]It is well known that the wound healing process is complex and typically divided into three phases; inflammatory, regeneration, and remodelling[/pull_quote]
Platelet fragmentation during processing should be avoided, because it is the process of activation that results in the completion of the tertiary structure of some of the secretory proteins. As a result, such fragmentation during processing could result in the release of high levels of proteins with compromised bioactivity. The integrity of the platelet membrane can be preserved through the use of low gravity forces during the centrifugation process. Growth factors released from activated platelets initiate and modulate wound healing in both soft and hard tissue.
There are a number of different PRP harvesting kits available, but some contain thrombin—either from bovine or human origin—and therefore the end product cannot be categorised as autologous. Others use a chemical buffer to separate the plasma and red cells, and do not deliver pure PRP as a result. The separation should be physical, not chemical.