Mechanostat Disease: Osteoporosis and Osteopetrosis as the Mirror of Wolff's Law

Osteoporosis is the exact MIRROR of Wolff remodelling (§4). Where supra-threshold load flips bone to the dense basin, sustained sub-threshold (disuse) load lowers the dense-basin escape barrier, so density drains back toward the resorbed basin. The relative loss rate rises monotonically with disuse (1.00× maintained → 2.12× at spaceflight → 2.59× at complete unloading) — barrier-limited escape, FORCED by the same bistable substrate [F].

Below the Frost mechanostat setpoint (MES_resorption ~75 µε, cited [L]) the loss rate climbs from baseline through mild disuse (1.28×), bedrest and spaceflight (2.12×) to complete unloading (2.59×), with settled density falling from 1.00 to 0.840. Past the spinodal drive the dense basin vanishes outright (catastrophic resorption). Mirror of T3 [F]; setpoint [L]; absolute BMD and %/month [O].

The other arm of the mechanostat

T3 proved that loading flips bone to the dense basin with hysteresis. Bone disease is the same double well read the other way: disuse and post-menopausal osteoporosis is sustained sub-threshold load. The unloading drive lowers the escape barrier OUT of the dense basin, so the Kramers escape rate — the resorption rate — rises as the load drops further below the Frost minimum effective strain. Nothing new is added: it is the T3 settle run below threshold.

Loss accelerates as load falls (cited setpoint)

The rate is normalised to 1.00× in the maintenance ‘lazy zone’. It rises smoothly with deeper unloading, matching the clinical ordering bedrest < spaceflight < complete immobilisation. The cited weight-bearing BMD loss of ~1.0–1.5 %/month in spaceflight and bedrest sets the SCALE [L]; the substrate supplies the rising SHAPE, not the absolute percentage [O].

Osteopetrosis: the loop loses its lower branch

The opposite disease is osteopetrosis (CLCN7 / TCIRG1 osteoclast failure). Here the resorption pathway is disabled — the DOWN-branch of the T3 loop is removed. On unloading, normal bone returns to baseline density (0.00) but osteopetrotic bone stays locked high (1.00): the hysteresis loop can no longer descend. The MECHANISM (removed down-branch → density cannot fall) is shown [V]; the resulting brittleness is a material property and stays [O].

Honest boundary

Paget's disease (disorganised runaway cross-basin cycling) and renal osteodystrophy / osteomalacia (a mineralization-supply defect, not a switch-threshold defect) have no clean single-switch signature and are recorded as [O] in the out-of-class register (§20).