Paget disease of bone
How overactive giant osteoclasts set off fast, chaotic remodelling in a few bones, why the result is a mosaic of enlarged but weaker bone, and how that causes bowing, fractures, deafness, nerve compression, high-output failure and, rarely, osteosarcoma.
By the end, you can
- Describe coupled remodelling, cement lines and lamellar bone, and the markers that track bone turnover.
- Explain the role of abnormal osteoclasts, SQSTM1 and other genes and possible environmental triggers, and why particular bones are affected.
- Relate the osteolytic, mixed and sclerotic phases to the mosaic pattern, the gross and X-ray appearances and the rise in alkaline phosphatase.
- Explain how enlarged, weak, vascular bone causes bowing, fractures, deafness, nerve compression, high-output heart failure and osteosarcoma.
- Recognise the classic tests and differentials of Paget disease, and explain how bisphosphonates and other treatments act on its mechanism.
What it is
Paget disease of bone (osteitis deformans) is a chronic focal disorder of the adult skeleton in which overactive, abnormal osteoclasts drive excessive bone resorption, followed by fast, disorganised formation. The affected bones become enlarged, deformed and vascular, yet weaker than normal. It may involve one bone (monostotic) or several (polyostotic), and each site passes through osteolytic, mixed and sclerotic phases.
3 types to tell apart
- Monostotic Paget disease
- One bone affected; now the commoner pattern as the disease has become milder.
- Polyostotic Paget disease
- Several bones, almost always asymmetrically; more likely to cause a very high alkaline phosphatase and complications.
- Juvenile Paget disease and Paget-like syndromes
- Rare inherited disorders: osteoprotegerin deficiency in childhood, and VCP variants combining Paget disease with muscle disease and dementia.
Normal function
Coupled remodelling leaves an orderly record
Bone is remodelled by small teams of cells. Osteoclasts, large cells formed by the fusion of monocyte–macrophage precursors and usually carrying a handful of nuclei, dissolve a pit or tunnel. Their formation is driven by RANKL from osteoblast-lineage cells and osteocytes, and held back by osteoprotegerin, a decoy receptor. Osteoblasts then refill exactly the space removed. This coupling keeps the amount and place of formation matched to resorption.
Where resorption stopped and formation began, a thin, wavy cement line remains. Each is bounded by one, and fragments of older osteons survive between them as . New bone is lamellar: collagen laid in orderly sheets, each turned relative to the next like plywood and aligned with the loads the bone carries, with spaced regularly between them. Fast growth and fracture repair make woven bone, with randomly arranged collagen, which is weaker and later replaced.
Osteoblasts release bone-specific alkaline phosphatase as they make and mineralise matrix, and collagen fragments enter the blood as osteoblasts form it (PINP) and as osteoclasts break it down (CTX). Blood levels of these markers therefore track how fast the skeleton is turning over: low in a healthy adult, high in a growing child.
Underlying change · Step 1 of 4
Overactive giant osteoclasts: genes, triggers and the bones affected
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 2 of 4
Three phases: osteolytic, mixed and sclerotic
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 4
Bigger but weaker bone, and why alkaline phosphatase rises
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 4 of 4
Compressed nerves, a working heart and sarcoma
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 4 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .