PRP injections treat a list of orthopedic conditions including knee osteoarthritis but may be used on other joints as well.

Platelet-rich plasma therapy, sometimes called PRP therapy or autologous conditioned plasma (ACP) therapy, attempts to take advantage of the blood’s natural healing properties to repair damaged cartilage, tendons, ligaments, muscles, or even bone.

Although not considered standard practice, a growing number of people are turning to PRP injections to treat an expanding list of orthopedic conditions, including osteoarthritis. It is most commonly used for knee osteoarthritis but may be used on other joints as well.

Treating Osteoarthritis with PRP

When treating osteoarthritis with platelet-rich plasma, a doctor injects PRP directly into the affected joint. The goal is to:

  • reduce pain;
  • improve joint function;
  • possibly slow, halt, or even repair damage to the cartilage.

Platelet-rich plasma is derived from a sample of the patient’s own blood. The therapeutic injections contain plasma with a higher concentration of platelets than is found in normal blood.

What is plasma? Plasma refers to the liquid component of blood; it is the medium for red and white blood cells and other material traveling in the bloodstream. Plasma is mostly water but also includes proteins, nutrients, glucose, and antibodies, among other components.

What are platelets?

Like red and white blood cells, platelets are a normal component of blood. Platelets alone do not have any restorative or healing properties; rather, they secrete substances called growth factors and other proteins that regulate cell division, stimulate tissue regeneration, and promote healing. Platelets also help the blood to clot; a person with defective platelets or too few platelets will bleed excessively from a cut.

Platelet-Rich Plasma Injection Procedure

There is no universally accepted medical definition for “platelet-rich plasma,” so a PRP injection that one patient receives can be very different from that of another. Variations occur for many reasons, including:

Patient characteristics. Blood composition (e.g. number of platelets) can differ from patient to patient.

Processing of blood. How a patient’s blood sample is processed (e.g. centrifuged and filtered) affects the concentrations of platelets and white blood cells in a PRP injection.

Additives. Doctors may augment platelet-rich plasma with substances that are thought to enhance the PRP’s healing properties.

What does platelet-rich plasma therapy do?

Doctors who use PRP therapy to treat osteoarthritis suppose that the platelet-rich plasma might:

  • inhibit inflammation and slow down the progression of osteoarthritis;
  • stimulate the formation of new cartilage;
  • increase the production of natural lubricating fluid in the joints, thereby easing painful joint friction;
  • contain proteins that alter a patient’s pain receptors and reduce pain sensation.

PRP solutions can vary because they are made from patients’ blood, and each patient’s blood is a little different. In addition, different physicians have different approaches for formulating and preparing a PRP solution for injection.

How is platelet-rich plasma made?

The most common way to prepare PRP involves centrifuging a patient’s blood sample. A vial of blood is placed in a centrifuge, where it is spun at intensely high speeds. The spinning causes the blood to separate into layers:

  • Red blood cells, approximately 45% of blood, are forced to the bottom of the vial.
  • White blood cells and platelets form a thin middle layer, called a buffy coat, which comprises less than 1% of the centrifuged blood.
  • “Platelet-poor” plasma, or plasma with a low concentration of platelets, makes up the remaining top layer, about 55% of the centrifuged blood sample.

Once the centrifuge process is complete the doctor or medical technician will remove the vial from the centrifuge and prepare the PRP solution for injection.

Centrifugation speed and time can vary. Differences in centrifugation speed and time affect the composition of PRP. There is no clear consensus on what the centrifugation process produces the best results for treating osteoarthritis.

What is in a platelet-rich plasma injection?

All PRP injections are not the same. The exact make-up of platelet-rich plasma depends on several variables, including the concentration of platelets, the concentration of white blood cells, and the use of additives.

Concentration of Platelets

Normal blood has 150,000 to 450,000 platelets per microliter (μL), and the concentration of platelets in platelet-rich plasma can vary from 2.5 to 9 times that. Concentration levels depend on the individual’s blood, how much blood was drawn, the centrifuge process (e.g., rotation speed and duration), and other clinical preparation methods.

While it may seem logical that plasma with the highest possible platelet concentration will get better results than plasma with a lower platelet concentration, that is not necessarily the case. One lab study suggested that plasma with concentrations 2.5 times that of normal blood was ideal, and higher concentrations might actually limit new cell growth.

White Blood Cell Count

The immune system depends on white blood cells to fight infection, so experts suspect that white blood cells inhibit tissues’ ability to heal, promoting inflammation, scar tissue, and damage to nearby tissues.

As with the concentration of platelets, the concentration of white blood cells is determined by an individual’s blood as well as clinical preparation methods.


Some doctors mix additives into the platelet-rich plasma. These additives, called thrombin and calcium chloride, artificially activate the platelets, stimulate clotting, and may enhance platelet-rich plasma’s regenerative properties.

Platelet-Rich Plasma Injection Procedure

Researchers studying PRP and osteoarthritis often work with patients who have knee osteoarthritis, a condition that experts estimate will affect nearly half of all Americans at some point during their lives. Two clinical studies that examine PRP to treat knee arthritis are described below.

One study, published in 2013, involved 78 patients with osteoarthritis in both knees (156 knees). Each knee received one of three treatments: 1 PRP injection, 2 PRP injections, or 1 placebo saline injection. Researchers evaluated the subjects’ knees 6 weeks, 3 months, and 6 months after injection. Researchers found:

Knees treated with 1 or 2 PRP injections saw a reduction in pain and stiffness as well as improvement in knee function at 6 weeks and 3 months.

At the 6-month mark, positive results declined, though pain and function were still better than before PRP treatment.

The group that received placebo injections saw a small increase in pain and stiffness and a decrease in knee function.

The platelet-rich plasma used in this clinical study had 3 times the platelet concentration of normal blood and had been filtered to remove white blood cells.

A second, smaller study examined patients who had experienced mild knee pain for an average of 14 months. Each arthritic knee underwent an MRI to evaluate joint damage and then received a single PRP injection. Patients’ knees were assessed at the 1 week, 3 months, 6 months, and 1-year marks. In addition, each knee underwent a second MRI after one year. Researchers found:

One year after receiving a PRP injection, most patients had less pain than they did the year before (though the pain had not necessarily disappeared).

MRIs showed that the degenerative process had not progressed in the majority of knees.

While knee cartilage did not seem to regenerate for patients, the fact that arthritis did not worsen may be significant. Evidence suggests that an average of 4 to 6% of cartilage disappears each year in arthritic joints.

Platelet-rich plasma injections are outpatient procedures. Because the patient’s blood must be drawn and prepared for injection, a typical procedure may take anywhere from 45 to 90 minutes.

Knee Injection

Whether the patient has a one-time injection or a series of injections spaced over weeks or months is up to the individual patient and doctor. If a series of injections is planned, a doctor may recommend single blood draw during the first visit and use of fresh PRP in the first injection, and freezing and thaw the remaining PRP as needed for future injections. However, some experts believe freezing and thawing PRP negatively affects its usefulness and prefer to do a separate blood draw for each PRP injection.

Pre-Injection Precautions

The American Academy of Orthopaedic Surgeons recommends patients adhere to the following pre-injection guidelines:

  • avoid corticosteroid medications for 2 to 3 weeks prior to the procedure;
  • stop taking non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen, or arthritis medications such as Celebrex, a week prior to the procedure;
  • do not take anticoagulation medication for 5 days before the procedure;
  • drink plenty of fluids the day before the procedure;
  • some patients may require anti-anxiety medication immediately before the procedure.

Platelet-Rich Plasma Injections, Step-by-Step

This is an in-office procedure that involves a blood draw, preparation of the PRP, and the injection:

Blood is drawn from a vein in the patient’s arm into a vial (typically 15 to 50 mL or less than 2 ounces of blood is needed).

The blood is processed using a centrifuge machine.

A doctor or technician prepares the centrifuged platelet-rich plasma for injection.

The affected joint area is cleansed with disinfectants such as alcohol or iodine.

If ultrasound is being used, a special gel will be applied to an area of skin near the injection site. An ultrasound probe will be pressed against the gel-covered skin. A live image of the joint will be projected on-screen for the doctor to see.

The patient is asked to relax; this will facilitate the injection and also can make the injection less painful.

Using a syringe and needle, the doctor injects a small amount (often just 3 to 6 mL) of platelet-rich plasma into the joint capsule.

The injection area is cleaned and bandaged.

The platelet-rich plasma typically stimulates a series of biological responses, and the injection site may be swollen and painful for about 3 days.

After the PRP Injection: Immediate Follow-up Care

Patients are advised to take it easy for a few days and avoid putting a strain on the affected joint.
Doctors may require or suggest that a patient:

  • does not take anti-inflammatory pain medication; another pain medication may be prescribed by the doctor;
  • wears a brace or sling to protect and immobilize the affected joint; a patient who receives an injection at the ankle, knee, or hip may be advised to use crutches;
  • uses a cold compress a few times a day for 10 to 20 minutes at a time to help decrease post-injection pain and swelling.

See When and Why to Apply Cold to an Arthritic Joint

Patients who do not have physically demanding jobs can usually go back to work the next day. Patients can resume normal activities when swelling and pain decrease, typically a few days after the injections. Patients should not begin taking anti-inflammatory medications until approved by the doctor.

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