The bone marrow’s three blood-cell families
Bone provides structure, but the marrow inside many bones is also a living blood-cell factory. It produces red blood cells, white blood cells and megakaryocytes—the large cells that release platelets into circulation.
Red blood cells carry oxygen from the lungs to tissues through haemoglobin. White blood cells comprise several immune-cell families that identify threats, coordinate inflammation and help clear damaged material. Platelets are small cell fragments rather than complete cells. The older phrase “yellow blood cells” has sometimes been used in patient education, but it is not standard modern medical terminology.
These components work together after injury. Oxygen delivery, immune activity, clot formation, removal of damaged tissue and repair signalling overlap rather than happening as three isolated jobs.
What platelets do after an injury
When a blood vessel is disrupted, platelets adhere to the injured area, change shape and aggregate to help form a clot. This creates early stability and a temporary matrix in which later repair processes can occur.
Platelet alpha granules release signalling proteins that include platelet-derived growth factor, transforming growth factor beta, vascular endothelial growth factor, epidermal growth factor and fibroblast growth factor. These signals can influence cell migration, blood-vessel formation, collagen production and extracellular-matrix remodelling.
Growth factors do not simply “regrow” a tissue on command. Their effects depend on concentration, timing, the surrounding cells, mechanical load, inflammation and the condition of the tissue receiving the signal. More platelets or more growth factor is therefore not automatically better.
Why tendons and ligaments may recover slowly
Tendons transfer force from muscle to bone; ligaments help stabilize joints by connecting bone to bone. Many regions within these tissues have a more limited blood supply than highly vascular organs, which can influence the pace and quality of repair.
Blood supply is only part of the story. A chronic tendon problem is not identical to a fresh tear, and persistent pain does not always mean that tissue remains actively damaged. Age, smoking, diabetes, medications, previous injury, joint mechanics, sleep, nutrition and repeated loading can all change recovery.
A useful assessment therefore identifies the structure involved and asks why it is being irritated. An injection cannot correct every movement, alignment or loading problem by itself.
How platelet-rich plasma is prepared
PRP is autologous: the starting material comes from the patient. Blood is drawn into collection tubes and processed in a centrifuge so that components separate according to density. The platelet-containing plasma is then collected and, depending on the system, concentrated further.
The final product can vary considerably. Systems differ in the volume of blood drawn, number and speed of centrifuge cycles, platelet concentration, presence of white cells, residual red cells, activation method and injected volume. These differences may alter how the preparation behaves.
That variability is one reason PRP research can be difficult to compare. A result obtained with leukocyte-poor PRP for knee osteoarthritis cannot automatically be applied to leukocyte-rich PRP used for a tendon condition.
Assessment, placement and the treatment visit
The process should begin with a diagnosis and a review of alternatives, not simply a decision to inject. The practitioner considers the history, physical examination, relevant imaging, medications, bleeding risk, infection risk and the patient’s goals.
When treatment is appropriate, the target tissue is identified and the prepared PRP is placed into or around the selected area using a sterile technique. Ultrasound guidance may be used when it improves visualization and placement. Local discomfort and a short-lived increase in pain or swelling can occur afterward.
The clinic’s original article emphasized tissue alignment before healing. The broader principle remains useful: the biological signal and the mechanical environment must support one another. The exact positioning, bracing or rehabilitation plan must be individualized rather than applied as one fixed protocol.
What clinical research currently shows
PRP has been studied for lateral elbow tendinopathy, plantar fasciitis, patellar and Achilles tendinopathy, rotator-cuff conditions, muscle injury and osteoarthritis, among other uses. Some systematic reviews report improvements in pain or function for selected conditions, particularly at medium-term follow-up.
The evidence is not uniformly positive. In knee osteoarthritis, some reviews favour PRP over comparators, while rigorous saline-controlled trials have found no significant advantage in pain, function or cartilage volume. Differences in participant selection, PRP composition, dose, injection number and outcome measurement may contribute to conflicting results.
The most responsible conclusion is condition-specific: PRP may be reasonable for selected patients after assessment, but it is not a universal repair treatment, it has not been proven to regenerate every damaged structure, and results cannot be guaranteed.
Recovery is part of the treatment
Early recovery commonly emphasizes relative protection of the treated area and avoidance of activity that sharply aggravates symptoms. Complete immobilization is not automatically desirable; the amount and timing of movement depend on the tissue, procedure and clinical findings.
A brace may be appropriate when instability or repeated stress threatens the target tissue, but it is not required for every PRP treatment. Progressive rehabilitation usually matters because tendons, ligaments and muscles adapt to carefully dosed load.
The original clinic guide described an eight-to-twelve-week biological window for ligament and tendon repair. That can be a useful expectation for some cases, but actual timelines vary. Follow-up should guide the return to mobility, strength work, sport and occupational demands.
Risks, limitations and useful questions
Because PRP is prepared from the patient’s own blood, allergy to the platelet product is not the usual concern. Risks still include pain flare, bruising, bleeding, infection, injury to nearby structures and lack of improvement. Blood-draw reactions and procedure-specific risks should also be discussed.
Patients should ask what diagnosis is being treated, why PRP is being considered, what preparation is used, whether guidance is recommended, what alternatives exist, how recovery will be managed and what outcome would count as success.
Suitability may be affected by active infection, blood or platelet disorders, anticoagulant or antiplatelet medication, significant anaemia, pregnancy-related considerations and other health factors. These decisions belong in an individualized clinical assessment.