Knee mobility / limited squat depth on 61yo male

[Heavy Drop]

Rip,

My dad is 61 years old and a farmer. As such he is very physically active but I am trying to get him to do more training with a barbell. The problem is that his left knee does not bend more than about 95-100 degrees due to a full knee reconstruction he had in the early 70's, before they did things arthroscopically. The knee is not painful at the end of ROM, it just won't move any farther.

Given this limitation, he cannot squat to full depth with good form. We have a garage gym setup at the farm that includes a squat cage so the issue is not the availability of equipment. Right now my dad's workouts include some barbell exercises such as deadlifts and push presses but they do not include squats. I think he should be squatting for obvious reasons that do not need to be explained on this board.

Questions for you as follows:
1) Would it be worthwhile for him to incorporate squats into training with good form down to the maximum depth his knee allows? In his case, I estimate this to be about 3" above parallel. I'm concerned that the costs of him doing non-full-depth squats may outweigh the benefits - but also really think he should be squatting.
2) I was also thinking that Zerchers might make sense as he can get a little more depth on those given the more upright back position (his upper body isn't flexible enough for him to front squat properly). He can't Zercher to full depth either, but he can get lower than back squat, maybe 1" above parallel.

Thank you.

[Mark Rippetoe]

What is the condition of the meniscus in the bad knee?

[Heavy Drop]

About half of the cartilage was taken out in the surgery ~40 years ago. The remaining half has certainly seen wear and tear over the years, but the knee appears to be in pretty good shape. My dad lifts/carries heavy shit on a daily basis and occasionally goes on a 2-3 mile jog without issues. I want him to squat to maintain his strength through his later years, but am concerned that a non-full-depth squat will fuck up his knee. The Occum's Razor solution would be to have him incorporate above-parallel squats (or Zerchers) into his training and stop if this causes pain. I just wanted to sanity check this to see if you think that this is a stupid idea that we shouldn't even try.

Also, to clarify a point in the original post (not sure if really matters), 95-100 degrees of flexion is 95-100 degrees from zero. So at max range flexion his tibia makes an 80-85 angle with the femur.

Thanks Rip.

[Mark Rippetoe]

Above parallel squats will do far more harm to his knee than good. I'd just have him deadlift and not do any partial squats.

[Heavy Drop]

Thanks, Rip.