Osteolytic Bone Disease (Myeloma, Giant-Cell Tumour) as Osteoclast/Osteoblast Uncoupling

Osteolytic bone disease is osteoclast/osteoblast UNCOUPLING — the exact mirror of osteopetrosis. Osteopetrosis (§9) disables resorption and locks density HIGH; multiple myeloma disables FORMATION (osteoblast suppression), so loading cannot re-form bone and density stays lytic (0.15 vs a normal 1.00 under the same load). Giant-cell tumour adds a strong RANKL resorptive drive that resorbs even dense bone [V].

On the T3/T6 mechanostat, normal bone couples resorption (down) and formation (up): from a resorbed state, load re-forms bone (density 0→1.00). Multiple myeloma suppresses osteoblasts (DKK1) while RANKL drives resorption, disabling the formation arm — loading cannot lift density past a low cap (0→0.15), a lytic lesion that does not heal. This is the precise MIRROR of osteopetrosis (resorption disabled → locked high). Giant-cell tumour drives RANKL osteoclast recruitment, resorbing even dense bone (→0.00). Mechanism [V]; absolute lesion size/incidence [O].

The mechanostat has two arms; disease disables one

Bone remodeling couples two opposing processes: osteoclast RESORPTION (the down arm) and osteoblast FORMATION (the up arm). The T3 hysteresis loop (§4) and osteopetrosis (§9) already exercised this: osteopetrosis disables the resorption arm, so density can only go up and locks HIGH. Osteolytic bone disease is the opposite lesion — disable the FORMATION arm.

Myeloma: formation disabled, locked lytic

In multiple myeloma, tumour-secreted DKK1 and related factors suppress osteoblasts while RANKL drives osteoclast resorption. In mechanostat terms the up arm is disabled: starting from a resorbed state, increasing load re-forms bone in normal tissue (density 0.00 → 1.00) but in myeloma the density cannot climb past a low cap (0.00 → 0.15) — the punched-out lytic lesion that, unlike a fracture (§16), does NOT heal under loading because the bone-forming response itself is gone. This is the exact mirror of osteopetrosis: one disease removes the down arm and locks density high, the other removes the up arm and locks it low.

Giant-cell tumour: a strong resorptive drive

Giant-cell tumour of bone recruits osteoclasts through massive RANKL signalling. That is a strong resorptive (negative) drive: applied to dense bone it pushes the switch past the resorption spinodal and density collapses (→ 0.00) — aggressive local osteolysis. The same RANKL axis is why denosumab (anti-RANKL) treats both myeloma bone disease and giant-cell tumour, which the shared mechanism makes natural.

Why this is the mechanostat, not the carcinogen kernel

These are tumour-associated, but the BONE lesion is not a carcinogen dose-response (§7) — it is osteoclast/osteoblast uncoupling on the remodeling switch. Modelling them here, as perturbations of the T3/T6 mechanostat, places the mechanism where it actually acts. The absolute lesion size and incidence are not fixed by the substrate and stay [O].

Analgesic lever map (cross-reference)

Osteolytic / myeloma bone pain is the clean resorptive-drive L2 case: the antiresorptive (denosumab / zoledronate) that relieves the RANKL drive is simultaneously the mirror treatment and the analgesic. Cancer bone pain is clinically opioid-first-line, and the opioid lever is honestly outside the non-opioid scope. See §25 the three-lever threshold logic for the inherited technique (concept DOI 10.5281/zenodo.20733420), and §27 osteolytic L2 detail for the worked lever sweeps. The analgesic axis adds no new physics: pain is a threshold-crossing rate on the same R19 barrier this chapter already uses.