'Must be that old football injury / timeI fell off my bike / years of hard work / etc.'
Is this the case? What exactly is the role past injury in determining how much you suffer from arthritis?
As usual, the answer isn't simple.
The cause of osteoarthritis is multifactorial. Genetics, fitness, weight, work, injury all have their say. Each if these variables has its own spectrum as well. There is a little overweight and there is a lot. There are some whos work is mildly aggravating to the knees and others that have a strong negative effect.
Injury is the same. While no history of injury is obviously the best, some injuries have a much bigger say than others in causing arthritis.
Fractures are one such example. Breaking your leg is never good for it but if you break it in the middle of the shaft, away from the ends, its impact on your future risk of arthritis is usually minimal. Breaking through the joint surface itself - called an intra-articular fracture - is very bad news. Our joints are made super-smooth to bear weight and provide movement for thousands of steps every day. If you break through this surface, the bone will heal but the surface will never be completely smooth again. Even the smallest of rough surfaces quickly increases the rate of wear in a joint. A severe fracture of the upper shin bone - called a tibial plateau fracture - have such a negative effect that I would consider early arthritis inevitable in most cases. A high percentage of patients suffering this injury eventually require a knee replacement.
A torn meniscus in the knee - a common injury often called a torn cartilage - also increases the risk of osteoarthritis but not to the same extent. The menisci act as shock absorbers so partially removing them via key-hole surgery can lead to an increased rate of wear. At least the former practice of removing the entire meniscus has been replaced by trimming of the affected piece only.
So to answer that question: Was my old knee injury responsible for my arthritis?
If you had an intra-articular fracture: Definately
If you had arthroscopic surgery (key-hole) for a torn meniscus: Probably
Lots of lesser injuries: Maybe
In most cases you will never know. Sure, you will look back hoping to find a reason, an incident to blame. But more often than not, there is no identifiable cause.
Such is life