Low gearing and knee health Background

I have been riding a single speed bikes over the last few (8) years in both flat (Perth, Australia) and hilly terrain (Brisbane, Australia). I love riding single speed. It has become part of my identity.  I used to joke that I'll get gears when I'm 40. I am 39 now.

I estimate that I ride 5000km/year now.

I am considering switching to a geared bike in big part because I'm concern that it affects my knees and the symptoms will appear in few years.

I don't have knee issues of any kind at the moment and I can get to top 10% of riders on Strava on many segments. Top 20% is often easy. What I am trying to say here is that I like to push and ride as hard as my abilities allow me and that I am fit. 

My daily commute involves few >10% gradient short hills (&lt;60s effort) and I like riding these hills up. I go longer way to work to ride more hills.

Ocassionaly I ride a 7% gradient hill with about 9 minutes of effort. It's hard and rewarding. 

I often take pleasure (after) riding hills seated with low to sometimes very low cadence.

There is quite a lot of opinion out there about knees and the effect of low gearing. 

Question

What is the truth about knees and using low gears (i.e. low cadence, high pressure)? Is there a consensus about its negative effect on knee health especially long term?
Knee issues are mostly a symptom of bad pedalling mechanic (over extension, misalignment, imbalance, that sort of thing). Low cadence "grinding" does not fundamentaly change that unless you start altering your pedal stroke as your cadence goes down.

Note that for fixed gear riding, you also need to consider how very high cadence (descending) may also alter your stroke. In terms of knee problems, fixed gear skids are another potential knee pain inducer as most people will bend one leg inward to get leverage against the frame.

So in short, if your pedal stroke is sound, riding at low cadence should not represent a higher knee injury risk.