This is a complicated question, so I'll try to break it down into two issues.
First, let's talk about the ACL tear diagnosis. The anterior cruciate ligament (ACL) is one of the four main stabilizers of the knee. It provides support for the knee when twisting and turning maneuvers are performed. Injuries to this ligament are quite common, particularly in the young, athletic population. The classic symptoms of an acute ACL tear include significant pain, swelling, and often a "pop" or "tearing" sensation as the ligament ruptures. Most people can't walk immediately, and the injury is often severe enough that a doctor's visit or trip to the ER ensues. Chronic, or long-standing, ACL tears cause symptoms of instability or giving way, mainly with cutting, pivoting or jumping activities. Actions like normal walking, sitting, or standing often are painless and are easily performed. Treatment is tailored to the individual patient - surgery to reconstruct the ACL is very common in the athletic population, while therapy and bracing are very reasonable for patients with a more sedentary lifestyle.
Arthritis, on the other hand, involves the wearing down of the cartilage cap on the end of the bones in joints. This process can occur because of an injury or a disease, but it is most commonly seen as part of aging. Osteoarthritis, or degenerative joint disease, often manifests as pain, stiffness, and limitations in motion. The pain is often described as an "ache", but it can be sharp and deep as well. It normally progresses slowly, but it definitely can progress over time. Treatment is aimed at decreasing the symptoms and maintaining a normal lifestyle. Medications for pain, injections, physical therapy, braces, cane usage, and sometimes surgery all have a role in the treatment of arthritis, but the treatment is definitely dependent on the how severe the pain, stiffness, and swelling are.
So, this question mentions both a ligament tear and arthritis. Both can coexist in the same knee, but the only way to know for sure is to have an experienced doctor or surgeon perform a thorough exam.