Abstract: The disclosure relates to a dental treatment chair assembly wherein the assembly is detachably positioned inside the backrest of the chair and comprises a number of cartridge boxes; each of the boxes contains therein an instrument and an extensible connecting pipe and means for assisting manual extension and reeling-in of the pipes from the top of the shoulder part of the backrest, whereby a dentist can arrange the type, number and position of dental instruments in compliance with his convenience and desire by arranging the cartridge boxes. Any instrument can be removed and repaired by withdrawing the cartridge box that contains the defective instrument.