Doctors at the world’s leading medical universities and hospitals are showing that PRP can regenerate damaged knee cartilage and meniscus in patients suffering from knee osteoarthritis and PRP can also enhance healing after knee ligament reconstruction.
An October 2018 study in the journal Current Reviews in Musculoskeletal Medicine (2) says this:

“Recent research into the applications of PRP for knee osteoarthritis has further indicated both the efficacy and safety of PRP treatment. Although research has shown a tendency toward better efficacy at earlier stages of osteoarthritis, evidence exists to indicate positive effects at all stages of osteoarthritis. In summary, since knee osteoarthritis is an extremely prevalent condition that can be a challenge to treat, it is imperative that safe and effective nonoperative treatment methods be available to individuals that are suffering from this condition.”

In July 2018, medical university researchers in Ireland lead a multi-national European research team to conclude in their research:

“Platelet-rich plasma therapy is a simple, low-cost and minimally invasive intervention which is feasible to deliver in primary care to treat degenerative lesions of articular cartilage of the knee. This therapy appears to have minimal associated adverse events and may have beneficial effects in terms of pain, health utility, patient satisfaction, and goal-orientated outcomes.”(3)

What is interesting about this study is who the PRP helped:

The 12 participants in the study had an average age of 72.6 years and average Body Mass Index (SD) of 31.8 meaning that the average person in this study was obese and seven (58%) were males.
The most common goal of this group was to be pain-free, followed by walking normally without aid
Reduction of knee stiffness
prevention of knee replacement
and being able to dance and garden again.

In November 2017, researchers reported on the benefits of PRP compared to placebo injection in patients who had osteoarthritis in both knees. Published in the American journal of physical medicine & rehabilitation, the study showed PRP treatment significantly improves pain, stiffness, and disability in patients with knee osteoarthritis compared to normal saline (placebo) treatment.(4)
Also in November 2017, in the International journal of rheumatic diseases, researchers reported a summary of the most recent findings on the benefits of PRP for knee osteoarthritis.

This study performed an overview of the research to provide recommendations for PRP use in knee osteoarthritis patients through the best evidence. “They concluded that PRP is an effective intervention in treating knee osteoarthritis without increased risk of adverse events.”(5)
In the medical journal Arthroscopy, a journal devoted to obviously arthroscopy, surgeons are told that Platelet Rich Plasma injections (PRP), offers better symptomatic relief to patients with early knee degenerative changes (than hyaluronic acid or placebo), and its use should be considered in patients with knee osteoarthritis.(6)

This is a verification of early research from the Mayo Clinic which came to the same conclusion – PRP showed better improvement than hyaluronic acid injection and placebo in reducing symptoms and improving function and quality of life. Especially in in younger, active patients with low-grade osteoarthritis.(7)

This is from the Mayo Clinic research:

“Intraarticular platelet-rich plasma (PRP) injection has emerged as a promising treatment for knee osteoarthritis. Studies to date, including multiple randomized controlled trials, have shown that PRP is a safe and effective treatment option for knee osteoarthritis. Intraarticular PRP is similar in efficacy to hyaluronic acid and seems to be more effective than hyaluronic acid in younger, active patients with low-grade osteoarthritis. Treatment benefits seem to wane after 6-9 mos. There are numerous PRP treatment variables that may be of importance, and the optimal PRP protocol remains unclear.”