Bone screw

Abstract

A bone fixation system with variable z-axis translation is provided. The bone fixation system includes an outer tulip coupled to a bone fastener. An inner tulip, which provides a seat for an elongate fixation rod, is located inside the outer tulip. The inner tulip together with a seated rod is permitted to translate along the z-axis in an unlocked position. Also in the unlocked position, the bone fastener is free to angulate relative to the outer tulip. The bone fixation system includes a locked position in which the z-axis position of the inner tulip and rod relative to the outer tulip is fixed. Also in the locked position, the bone fastener is locked with respect to the outer tulip. The system may be adjusted between the locked and unlocked positions by way of a set screw or independently by rotation of the inner tulip relative to the outer tulip.

We claim: 
     
         1 . A bone fixation system, comprising:
 a bone fastener including:
 a bone engaging portion; and 
 a head integrally connected to the bone engaging portion; 
   an outer receiver having a proximal end, a distal end and a longitudinal axis; the outer receiver including:
 a sidewall extending between the proximal end and the distal end and having an inner surface and an outer surface; 
 an inner bore extending between a top opening at the proximal end and a bottom opening at the distal end; the bone fastener being connected to the outer receiver such that the bone engaging portion extends through the bottom opening; 
 two oppositely disposed arms defined by the sidewall and at least one rod channel defined between the two arms; the at least one rod channel being interconnected with the top opening and the inner bore; the inner surface of each arm having a longitudinally extending interlocking inner surface adjacent to a longitudinally extending smooth surface; 
 an interlocking inner surface at the distal end; 
   a first locking cap located inside the inner bore of the outer receiver; the first locking cap having an interlocking outer surface configured to interlock with the interlocking inner surface at the distal end; the outer surface of the first locking cap is interconnected by a top surface and a bottom surface; the first locking cap includes two tab-engaging surfaces;   an inner receiver sized to fit inside the outer receiver; the inner receiver including a base and two oppositely disposed arms extending upwardly from the base; the inner receiver includes at least one channel defined between the arms; each arm has an interlocking inner surface and an interlocking outer surface; the interlocking outer surface of each arm of the inner receiver being configured to engage the interlocking inner surfaces on the arms of the outer receiver; the inner receiver further including two oppositely disposed tabs extending downwardly from the base; the two tabs are configured for engagement with the two tab-engaging surfaces on the first locking cap; wherein the inner receiver is coupled to the first locking cap by the two tabs engaging the tab-engaging surfaces;   a second locking cap removably located between the arms of the inner receiver and having an interlocking outer surface configured to interlock with the interlocking inner surface of the arms of the inner receiver;   wherein the bone fixation system includes an unlocked position in which the bone fastener angulates with respect to the outer receiver and the inner receiver is free to translate longitudinally with respect to the outer receiver and a locked position in which the bone fastener and inner receiver are fixed with respect to the outer receiver.   
     
     
         2 . The bone fixation system of  claim 1  wherein the locked position is achieved by rotation of the inner receiver from being adjacent to the smooth surfaces on the outer receiver to being adjacent to the interlocking surfaces of the outer receiver to interlock the interlocking surfaces of the outer receiver with the interlocking outer surface of the inner receiver. 
     
     
         3 . The bone fixation system of  claim 1  wherein rotation of the inner receiver rotates the first locking cap into interlocking engagement with the interlocking surface of at the distal end and downwardly within the outer receiver against the head of the bone fastener to fix the position of the bone fastener relative to the outer receiver. 
     
     
         4 . The bone fixation system of  claim 1  further including an elongate fixation rod located between the two arms of the inner receiver and in the at least one channel of the inner receiver; wherein rotation of the second locking cap downwardly onto the rod fixes the position of the rod relative to the inner receiver and further rotation of the second locking cap rotates the inner receiver relative to the outer receiver into the locked position in which the longitudinal positions of the inner receiver and rod are fixed with respect to the outer receiver. 
     
     
         5 . The bone fixation system of  claim 1  further including at least one spring located between the inner receiver and the first locking cap. 
     
     
         6 . The bone fixation system of  claim 1  wherein one or more of the cooperating interlocking surface on the inner surface of each arm of the outer receiver, the interlocking surface in the bone fastener receiving location, the interlocking outer surface of the fastener locking cap, the interlocking inner surface of each arm of the inner receiver, the interlocking outer surface of each arm of the inner receiver, and the interlocking outer surface of the second locking cap is a threaded surface. 
     
     
         7 . The bone fixation system of  claim 1  the interlocking inner surface of one arm is opposite from the interlocking inner surface of the other arm and the smooth surface of one arm is opposite from the smooth surface of the other arm; 
     
     
         8 . A bone fixation system, comprising:
 a bone fastener including:
 a bone engaging portion; and 
 a head integrally connected to the bone engaging portion; 
   an outer receiver having a proximal end, a distal end and a longitudinal axis; the outer receiver including:
 a sidewall extending between the proximal end and the distal end and having an inner surface and an outer surface; 
 an inner bore extending between a top opening at the proximal end and a bottom opening at the distal end; the bone fastener being connected to the outer receiver such that the bone engaging portion extends through the bottom opening; 
 two oppositely disposed arms defined by the sidewall and at least one rod channel defined between the two arms; the at least one rod channel being interconnected with the top opening and the inner bore; the inner surface of each arm having a longitudinally extending interlocking inner surface adjacent to a longitudinally extending smooth surface; 
 an interlocking inner surface at the distal end; 
   a first locking cap located inside the inner bore of the outer receiver; the first locking cap having an interlocking outer surface configured to interlock with the interlocking inner surface at the distal end of the outer receiver;   an inner receiver sized to fit inside the outer receiver; the inner receiver including a base and two oppositely disposed arms extending upwardly from the base; the inner receiver includes at least one channel between the arms; each arm has an interlocking inner surface; the inner receiver is located inside the inner bore of outer receiver such that the oppositely disposed arms of the inner receiver are disposed in the location of the smooth surfaces and the interlocking inner surface of each arm is flush with the adjacent interlocking surfaces of the outer receiver;   a second locking cap located between the arms of the inner receiver and having an interlocking outer surface configured to interlock with both the interlocking inner surface of the arms of the inner receiver and the adjacent interlocking inner surface of the arms of the outer receiver;   wherein the inner receiver includes an unlocked position in which the inner receiver is free to translate longitudinally with respect to the outer receiver and a locked position in which the inner receiver is fixed with respect to the outer receiver; and   wherein the locked position is achieved by the interlocking outer surface of the second locking cap engaging both the interlocking inner surface of the outer receiver and the interlocking inner surface of the inner receiver.   
     
     
         9 . The bone fixation system of  claim 8  wherein the bone fastener includes an unlocked position in which the bone fastener angulates with respect to the outer receiver and a locked position in which the bone fastener is fixed with respect to the outer receiver; the locked position is achieved by rotation of the first locking cap into interlocking engagement with the interlocking inner surface at the distal end and downwardly within the bore of the outer receiver against the head of the bone fastener to fix the position of the bone fastener relative to the outer receiver. 
     
     
         10 . The bone fixation system of  claim 8  further including an elongate fixation rod located between the two arms of the inner receiver and in the at least one channel of the inner receiver; wherein rotation of the second locking cap downwardly onto the rod fixes the position of the rod relative to the inner receiver. 
     
     
         11 . The bone fixation system of  claim 8  wherein the first locking cap is accessible from the at least one rod channel for being moved relative to the outer receiver. 
     
     
         12 . The bone fixation system of  claim 8  further including at least one spring located between the first locking cap and the inner receiver. 
     
     
         13 . The bone fixation system of  claim 8  the interlocking inner surface of one arm is opposite from the interlocking inner surface of the other arm and the smooth surface of one arm is opposite from the smooth surface of the other arm; 
     
     
         14 . A bone fixation system, comprising:
 an outer receiver including a sidewall that defines an inner bore and an open proximal end; the sidewall has an inner surface and an outer surface that defines at least one channel opening to the inner bore and the open proximal end; the outer receiver includes an interlocking outer surface formed on the outer surface;   an inner receiver including a sidewall that defines an interior, an open proximal end, and an inner base at a distal end; the sidewall having an inner surface and an outer surface that defines at least one channel opening to the interior and the open proximal end; the inner receiver includes an interlocking inner surface formed on the inner surface; the inner receiver being located inside the outer receiver such that the at least one channel of the inner receiver is substantially aligned with the at least one channel of the outer receiver; the inner receiver being movable along a longitudinal axis relative to the outer receiver in an unlocked position; and   a locking cap having an interlocking outwardly-facing surface and an interlocking inwardly-facing surface; the interlocking outwardly-facing surface being movable independently and relative to the interlocking inwardly-facing surface; the interlocking outwardly-facing surface being located in the inner receiver and interlocked with the interlocking inner surface of the inner receiver in a first locked position and the interlocking inwardly-facing surface being interlocked with the interlocking outer surface of the outer receiver in a second locked position; wherein the second locked position prevents the longitudinal translation of the inner receiver relative to the outer receiver.   
     
     
         15 . The bone fixation system of  claim 14  further including a rod disposed inside the at least one channel of the inner receiver; wherein the first locked position fixes the rod to the inner receiver between the base and the locking cap. 
     
     
         16 . A bone fixation system, comprising:
 an outer receiver having an inner bore and an open proximal end;   an inner receiver having an inner bore and an open proximal end; the inner receiver being located inside an outer receiver; the inner receiver being movable along a longitudinal axis relative to the outer receiver in an unlocked position and the inner receiver being longitudinally fixed relative to the outer receiver in a locked position;   a set screw located inside the inner receiver and movable rotationally about and along the longitudinal axis relative to the inner receiver;   wherein converting between the locked and unlocked positions is achieved by rotation of the inner receiver relative to the outer receiver;   wherein the locked position is achieved by rotation of the set screw downwardly relative to the inner receiver.   
     
     
         17 . The bone fixation system of  claim 16  further including a bone engaging portion configured to anchor in bone coupled to the outer receiver. 
     
     
         18 . The bone fixation system of  claim 16  further including an elongate fixation rod located inside the inner receiver and under the set screw; the elongate fixation rod being movable longitudinally with the inner receiver. 
     
     
         19 . The bone fixation system of  claim 16  wherein in the unlocked position the bone engaging portion is movable with respect to the outer receiver and in the locked position the bone engaging portion is fixed with respect to the outer receiver. 
     
     
         20 . The bone fixation system of  claim 16  wherein the set screw engages both the inner receiver and the outer receiver directly to effect the locked position. 
     
     
         21 . The bone fixation system of  claim 16  wherein the set screw engages the inner receiver which in turn engages the outer receiver to effect the locked position.