Low gearing and knee health
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.