Setting up a bicycle for someone with restricted knee articulation My father, now in his 70's, had a left knee replacement several years ago. He has limited movement in that knee - probably about 85 degrees on a good day. His knee can extend fully, but cannot bend as much as it should. As a result, he cannot pedal a bike he cannot get his left leg over the top of the pedal stroke. 

He is desperate to get back onto a bike, he knows he won't be ripping up the road and winning races like the old days, but wants the freedom to go for a slow easy rides.  

Does anyone have suggestions as to a solution to the problem. We don't mind spending money if we know the solution will work, but he can't afford to throw big money at wild ideas that may or may not work. If we find a solution, a new bike is likely. 

We have access several mountain bikes in a range of sizes to trail ideas, and we have all the tools and skills to build bikes up from parts and rearrange things as needed.  

EDIT:
Thanks everyone - all great suggestions. What am am going to do is got tto the LBS and get hold of a cheap set of short cranks for around $30- probably from a kids bike, in the 120-150 size range, and see if he can ride that. Depending how he goes, I'll try a standard right crank and a short left crank. If all else fails, we will go for the Power Cranks 

UPDATE: In case anyone else has similar problems.
Got some 152mm cranks for $30. Set an old  mountain bike up with the seat as far back as possible. The old man is able to ride reasonably comfortably, now has a smile from ear to ear - any suggestions how to remove it - doesn't fit with the grumpy old man image :) 

Has got me wondering how many people with minor knee problems would benefit from short(er) cranks.......
Look at Power Cranks.  They are a crank set with the ability to freewheel independently of each other.

It allows a slow build to fully articulated movement if additional rehab is still a goal, and using each crank independently means using a bike is still possible, even if doing so uses a pedaling motion which would be awkward for someone who had an option for full movement.

They also have adjustable crank lengths on each independent arm, which would allow you to use a shorter crank on the leg with the limited movement.

They are often used as a rehab tool, are a real, readily available, and not ridiculously priced product, and are install-able with normal bicycle tools. 

One caveat: They are shifting to a new poly carbonate fixing cap. If you order, I recommend requesting the old alloy fixing caps. The last set of poly caps I installed broke easily.

FTR, I have installed and used this product, and have no relationship with the manufacturer, nor will I be compensated in any way for this post.