Osteoarthritis (OA): Given the advanced tricompartmental arthritic changes, high-grade degenerative tear of the medial meniscus, and chronically torn ACL, OA is the most likely diagnosis. The combination of these findings is consistent with the wear and tear associated with OA, especially in the context of a chronically unstable knee joint due to the ACL injury.

Post-Traumatic Arthritis: The presence of a chronically torn ACL suggests a history of trauma, which could have initiated the arthritic process, making post-traumatic arthritis a plausible diagnosis.
Meniscal Tear with Secondary Osteoarthritis: The high-grade degenerative tear of the medial meniscus could be both a cause and a result of the arthritic changes, as meniscal injuries can lead to altered joint mechanics and accelerate OA.

Septic Arthritis: Although less likely given the chronic nature of the findings, septic arthritis is a potentially deadly condition if missed. Any signs of infection (e.g., fever, increased white blood cell count, redness, warmth) would necessitate immediate attention.
Osteonecrosis: Particularly of the femoral condyles, could be a complication of chronic steroid use, trauma, or other factors, and might present with similar pain and degenerative changes.

Rheumatoid Arthritis (RA): While RA can cause tricompartmental arthritis, it typically presents with systemic symptoms and a more symmetric joint involvement pattern. However, it's a rare consideration in this context without additional systemic or laboratory findings suggestive of RA.
Pigmented Villonodular Synovitis (PVNS): A rare condition characterized by synovial proliferation and pigmentation, which can cause joint degeneration and meniscal tears, but is much less common than OA or post-traumatic arthritis.
Hemochromatosis: A genetic disorder leading to iron overload, which can cause arthropathy similar to OA, but is rare and would require specific laboratory findings to support the diagnosis.