>Bulletproof Silver 110 BCD Cranks only $36.95

Bulletproof BMX cranks can be used as a single or double chainring setup.

These use the highly versatile 110 mm bolt circle, and fit standard tapered-square bottom bracket axles.

Chainline w/107 mm bottom bracket:

* 43.4 mm as double (middle of spider)
* 47.1 mm outer ring
* 39.7 mm inner ring
* 158 mm tread ("Q factor")

530 grams/pair (165 mm)

Arms only, no bolts nor chainrings included.

Lengths available (not all in stock): 140 mm, 150 mm, 155 mm, 160 mm, 165 mm, 170 mm <

Harris Cyclery

If your legs are the same length I would really hesitate to have different length crank arms. It seems to my non-expert self that that would just create imbalances that would exacerbate the effects of your knee issue. Have you asked a PT about it?