<prompt> In some old fractures, cailaginous tissue forms over the fractured bone ends with a cavity in between containing clear fluid. This condition is known as:Delayed unionSlow unionNon unionPseudahrosis  <response>: Pseudahrosis is a false joint that may develop after a fracture that has not united properly due to inadequate immobilization. Inadequate immobilization permits constant movements at the fracture site so that the normal constituents of callus do not form consequently the callus may be composed mainly of fibrous tissue cailage, perpetuating the instability and resulting in delayed union or nonunion. If a non union allows for too much motion along the fracture gap, the central poion of the callus undergoes cystic degeneration and luminal surface can actually become lined by synol like cells, creating a false joint filled with clear fluid. This is known as pseudahrosis. Ref: Robbins Pathologic basis of Disease 7e, Page 1289; Apleys system of Ohopedics fractures 8e, Page 156,164,544 ; Natarajans ohopedics , Page 27; Murcers Ohopedic surgery 9e, Page 433, 811, 1095, 672,736. 