Facet Joints. Image credit: en.wikipedia.org/wiki/Facet...
The facet joints connect the vertebrae together at the rear of the spine, as shown in the image above. They help to stabilize the spine in conjunction with muscles and ligaments.
Several people here have encountered facet joint problems so this may be common in PWP.
The tendency of people with Parkinson's to lean forward puts excess stress on the lumbar spine, both the intervertebral discs and the facet joints. When the facet joints get unhappy they get stiff. The key symptom that confirms facet joint impairment is difficulty straightening up upon standing, especially after sitting for an extended time. This can add big time to Parkinson's motor impairment - for me personally it turned a situation that was under control into OMG I cannot walk without dangerously and desperately staggering.
Reference: centenoschultz.com/symptom/...
Lumbar facet joint impairment can also impact any of the muscles in the lower torso. This can look like a wandering dystonia which can be interpreted as Parkinson's progression but is not.
Map of lumbar facet joint pain referral patterns: europepmc.org/articles/PMC4...
What to do about it
There are numerous videos on YouTube of exercises for facet joints. I found that they helped a little bit, maybe. Traditional medical interventions are focused on symptomatic pain relief, which would do nothing for impairment, which I found to be the main problem. There is a new medical treatment in the form of infusions of platelet-rich plasma, a blood fraction obtained from the patient's own blood. There is some evidence that this helps, but you have to find a clinic that offers this, and it is a long process involving several infusions.
What I have found that seems to help
Wearing loose clothing, I lie on my back on a bed with my feet and lower legs dangling. This really hurts but it feels like a good hurt as it straightens out my lumbar spine. I maintain this position for as long as I can stand it or until it feels like it is causing undue strain. I then elevate my knees to my chest and hold them there for a while. I then return to the legs dangling position, and keep repeating this process for a while. I do this several times a day. Also taking frequent breaks from sitting is helpful.
Additional detail by me on this subject and on sciatica: tinyurl.com/ktv33aye