Set Screw Having Variable Pitch Thread for Use With Spinal Implant Systems

摘要

An improved set screw for use with spinal implants to attach an elongated member to a bone. The set screw has a body with a lower portion. An external thread is on the lower portion wherein at least a portion of the external thread has a variable pitch. The set screw will engage a receiver member by a predetermined longitudinal distance before encountering the variable pitch of the external thread on the set screw. A preselected torque value for seating the set screw is achieved at this predetermined longitudinal distance of thread engagement by the set screw into the receiver member. The variable pitch provides consistent clamping force even when excessive torque is applied to the set screw. An upper portion can be connected to the lower portion of the set screw through a frangible portion. The preselected torque value for seating the set screw is selected at a value less than the break-off or separation torque required to cause the frangible portion to separate the upper portion from the lower portion of the set screw.