A LEOFF Plan 2 member has asymptomatic congenital osteoarthritis, a degenerative joint disease. While off duty, the member suffered a knee injury. Absent the osteoarthritis, the knee injury may not have been disabling. However, the preexisting degenerative joint disease prevented full recovery from the injury, leaving the member unable to resume LEOFF duties. In this case, the member could qualify for a LEOFF Plan 2 nonduty disability benefit.