Theory 03/Rheumatology & Connective Tissue/Joint mechanics
Erosions Form Where Load Leaves
Erosions are blamed on pressure. They may form where the pressure stops.
Inflammatory joint erosions occur at sites of mechanical unloading rather than at sites of peak load, so the distribution of damage follows stress shielding rather than stress concentration.
01The evidence
Erosions cluster at the bare areas and margins of joints, which are sites where cartilage is thin and load transmission is interrupted rather than maximal.
02The tension
Damage models assume load and inflammation combine to destroy tissue, predicting erosion at high-load regions. The observed distribution is at the margins, which is not where peak load sits.
03The mechanism
Cartilage is maintained by load. Chondrocytes require cyclic mechanical signal to sustain matrix synthesis, and unloading triggers catabolic pathways. At the margins, load is transferred away from the cartilage, which removes the maintenance signal.
Inflammation lowers the threshold for this effect, so an unloaded region becomes catabolic faster in a diseased joint. That makes the pattern a mechanical one, with inflammation as an amplifier rather than as the cause.
04What this is not
Those models predict damage at high load. This predicts it at low load, which fits the marginal distribution.
05Predictions
- P1Erosion should map to stress-shielded zones.
- P2Loading the margin should reduce erosion progression.
- P3Peak-load regions should be spared early.
06Refutation
Erosions appearing first at peak-load regions would restore the load-damage model.
07What it changes
It would change joint protection and exercise advice in inflammatory arthritis.