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There is a higher risk that the tendon of extensor pollicis longus will rupture in undisplaced Colles' type of fracture than in those which are displaced. This difference in incidence is due to the integrity of the extensor retinaculum in undisplaced Colles' type of fracture causing the tendon to be held tight against the fracture callus in the floor of the tunnel which may result in an attrition rupture. In the displaced Colles' fracture the extensor retinaculum is torn from the bone and thus permits the tendon to escape from contact with the fracture site.
Helal et al. (1982) studied this question.