http://www.greenfacts.org/glossary/pqrs/skeletal-fluorosis.htm

Skeletal fluorosis
Definition:

Skeletal fluorosis is a health effect of excessive accumulation of
fluoride in bones leading to changes in bone structure and making them
extremely weak and brittle.

The early stages of skeletal fluorosis are characterized by increased
bone mass, detectable by x-ray. If very high fluoride intake persists
over many years, joint pain and stiffness may result from the skeletal
changes.

The most severe form of skeletal fluorosis is known as "crippling
skeletal fluorosis," which may result in calcification of ligaments,
immobility, muscle wasting, and neurological problems related to
spinal cord compression.

Source: GreenFacts
Related words:

Bone - Dental fluorosis - Fluoride