Arthritis.org says that osteoarthritis (OA), also called degenerative joint disease, is the most common, chronic condition of the joints - precipitated when the cartilage between the joint spaces degrades, leading to pain, stiffness, and swelling.
Causes of OA may include: certain genetic traits increasing susceptibility, excess body weight, especially visceral abdominal body fat that sends out inflammatory signals, which can damage joints, injury and overuse to joints, and bone and joint disorders.
Let me also add a history of concussions and lower extremity injury - such as what you may find with retired NFL players.
Reporting in the June 2017 issue of the Journal of Athletic Training (JAT) - Osteoarthritis Prevalence in Retired National Football League Players with a History of Concussions and Lower Extremity Injury - researchers from the University of North Carolina, University of Georgia, and Walter Reed National Military Medical Center, Bethesda, Maryland, concluded that, "Concussion may be associated with OA in retired NFL players.