Kelly Turner

While at work, you usually don't have much equipment to use. Add to that the fact you have a knee injury, and your options for a lower body workout are even more limited.
I would suggest investing in resistance band. You can do lower body exercises like leg lifts, donkey kicks, calf raises and hamstring curls against the resistance band without too much pressure on your knees and the band can easily fit into a drawer or be stashed under your desk.
If you can't bend your knees at all, you are probably better off avoiding lower body exercises until your doctor tells you it is OK. A physical therapist can also suggest some lower body strengthening exercises that won't worsen, and could possibly even improve, your specific injury.