Disclaimer: I am not an MD and have not examined RDP. I have a PhD in Immunology and Pathology and this is an entirely remote opinion of publicly available information.
Take home message: Rick DiPietro has had chronic inflammation of knees (likely still does) and this leads to inevitable changes and degradation of joint function.
Throughout the evolution of humans one of the most prominent selective agents has been infectious disease. Until recent modern times the most prominent causes of mortality were infectious disease (e.g. In the USA in 1900, 7 of top 10 causes). With the advent of modern vaccines (to suppress viral pathogens) and antibiotics (to suppress pathogenic bacteria) infectious disease mortality is now rare. However the legacy of selective pressure from infectious disease is apparent (whether you believe in evolution or not).
Your immune system responses efficiently and destructively to perceived insults. As a result it does not discriminate between tissue destruction due to physical insults vs. infectious insults. If you twist your ankle and damage your ligaments your immune system will respond similar to a bacterial infection. Immune cells (T-cells, macrophages, neutrophils etc.) go in release destructive chemicals and try to neutralize perceived threats regardless of collateral damage.
This is why Ice, Ibuprofen and immune suppression is effective in injury healing. While doing nothing results in prolonged recovery time. Simply put your immune system is effective, very aggressive and in many cases not very specific.
Chronic inflammation: with survival being a top priority vs. infectious disease you can easily imagine that good joint biomechanics falls way down on the list of evolutionary pressures due to age.
Inflammation itself is a process where insulted tissue releases hormones (termed pro-inflammatory agents), which affect both the local tissue and the systemic immune system. Many of these hormones, including such hormones as Interleukins and prostaglandins, cause short term swelling, immune cell infiltration and some pain. However…. And this is the big However: If long-term inflammation occurs in the same area, the tissue/cells in that area respond as if a life threatening infection is occurring. This means "remodeling" of the tissue architecture to isolate off pathogens and change the local so that bad pathogens don’t escape to the rest of the organism……… this is generally beneficial, except in joints where architecture is the key to good biomechanics.
Now, unless anyone wants the specifics, joints are somewhat of a privileged area. Because they constantly receive physical insults from use, they don’t get a lot of immune system attention. The problem is some pathogens have evolved to exploit this (hide out in your joints). Your immune system knowns this – and hence the battle. Bad biomechanics and chronic inflammatory stimulation lead to immune attention. Problem is nobody has perfect biomechanics and if an athlete gets chronic joint injury...the result: arthritis and immune response/remodeling of the joints which lead to lost of function.
Imagine your kneecap moving in the groove is like driving a Porsche down the interstate. After inflammation: all those nice synovial cells have been infiltrated with bumpy fibroblasts, immune cells and others looking to isolate pathogens (*kind of like putting pot holes and rocks in the way of your Porsche/knee cap* ouch). Immune cells think an infection is in the joint, evolution has programmed this as a possible mortality event, your immune system will destroy your joint to preserve your longevity.
Furthermore, once an immune response builds, you acquire lots of cells ready to respond to any further incident (called memory cells). This is why vaccines work. In the case of joint disease where every weird bend of the knee could cause minor damage and an immune response (e.g. autoimmunity, Osteo-arthritis, Ankylosing spondylitis, RA etc). Once these start, rarely does the immune system completely regress, lots of immune cells programmed to respond to a specific insult. The problem becomes if this is inappropriate, a feedback mechanism gets set up and continual joint remodeling occurs. This is very common in older individuals (e.g. osteoarthritis). Its rare in younger people.
So think about this regarding DiPietro: 4+ years of inappropriate/severe knee joint inflammation. How could his knees be fully functional?
I'm really sorry to predict this but: There is no physiological way RDP can recover to his former physical abilities.