I remembered there is an article on this website, so this is the link for this:
https://www.mympnteam.com/resources/pain-and-mp...

I found a clear and simple explanation of why we experience bone pain in MPNs:
"In normal people bone marrow is confined to the central skeleton (cranium, ribs, sternum, vertebrae, and pelvis). The long bones contain mostly fat. In MPN, the bone marrow expands outward to the long bones and can be seen even out in the fingers and toes when bone marrow scanning is done.
The proliferation within the bone marrow cavity may cause pressure inside the bone that, in turn, causes inflammation of the covering of the bone (periostium).
The resulting periostitis may be painful. In some patients with bone and joint pain treatment with myelosuppressive therapy has decreased or eliminated these symptoms by reducing the amount of bone marrow in the cavity and reducing the pressure that it causes."
https://www.pvreporter.com/bone-joint-pain-mpn-...

And there are studies on abdominal and bone pain as symptoms present in MPNs:
"Chronic pain (abdominal, bone) is a prevalent and undermanaged feature of MPN disorders. Chronic non-MPN pain is further complicated by the development of the disorder. Many MPN patients lack understanding on its prevalence and furthermore feel ill-equipped to manage it. Patients have, however, found symptomatic benefit from various pharmacological and non-pharmacological treatment modalities that require further exploration. Results from this study suggest MPN patients should be assessed regularly for pain and offered early referral to specialized services for evaluation, source identification and treatment."
https://www.sciencedirect.com/science/article/p...

Also, a nice summary of ET symptoms and causes can be found here:
https://www.voicesofmpn.com/essential-thrombocy...

And more detailed and with chronic inflammation linked to MPN symptoms in this study:
"Prominent symptoms include fatigue (92.7%), early satiety (61.9%), abdominal pain (45.9%), abdominal discomfort (53.2%), inactivity (60.5%), headache (48.3%), concentration problems (61.7%), dizziness (55.2%), numbness (61.3%), insomnia (65.4%), sad mood (62.7%), sexuality problems (57.9%), cough (46.4%), night sweats (56.4%), itching (52.6%), bone pain (48.5%), fever (20.2%), weight loss (34.2%), and impaired quality of life (84.2%)"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC46...

The risk score we have at diagnosis is not always linked to how many symptoms we experience, meaning that low-risk ET patients can still experience unpleasant and sometimes disabling symptoms.

Pain and MPNs: Causes and Management | myMPNteam

https://www.mympnteam.com/resources/pain-and-mpns-causes-and-management

Essential Thrombocythemia Symptoms

https://www.voicesofmpn.com/essential-thrombocythemia-symptoms

Bone & Joint Pain In MPN Patients Explained | Proliferative | PV Reporter

https://www.pvreporter.com/bone-joint-pain-mpn-explained/

Impact of Inflammation on Myeloproliferative Neoplasm Symptom Development

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4619953/

One of the problems good people have is "we don't want to get hooked on pain meds." But we need to realize that medication is available to treat the correct symptoms. If physical activity can help relieve pain, is it wrong to take pain medication so you are able to do physical activity? If you know that at night when your body is tired, you get more pain, is it wrong to take pain medication before that pain increases so you can get some needed sleep? Even some physicians fear prescribing pain medication. Please be honest with your physicians and advocate for yourselves. There are different medications used for pain and not all of them are potent opioids. If your physicians ignore your request for help with pain, ask what you can do to relieve pain. yoga and deep breathing may help many people but if you still need help, ask for it. Advocate for yourselves.

Every day. It's so severe that I had to get on prescription pain medication. It took years of fighting to get it. I despise opiates, but the pain was taking over all of my thoughts. I could not focus on anything else or function much at all.

I have hip pain... keeps me up at night. No clue if it's arthritis, bursitis, or just joint pain from ET. I'm going to see the osteo doc soon.

Jakafi has reduced overall pain significantly, but not totally eliminated it. The use of Gabapentin at times has helped along with acetaminophen.