Modular percutaneous spinal fusion

Abstract

A posterior spinal fusion system may include pedicle screws and one or more segments designed to be attached to the implanted pedicle screws via nuts to fuse spinal motion segments in a modular fashion. The pedicle screws may have semispherical receiving surfaces, and each segment may have mounting portions at both ends, with corresponding semispherical engagement surfaces. The nuts may also have semispherical surfaces. The semispherical surfaces permit polyaxial adjustment of the relative orientations of the segments and pedicle screws. Each mounting portion may have a passageway therethrough to receive the proximal end of the corresponding pedicle screw; each passageway may intersect the edge of the corresponding engagement surface to facilitate percutaneous placement. Such placement may be carried out through cannulae to provide a minimally invasive (MIS) implantation procedure.

1 . A system comprising:
 a first fixation member implantable in a first bone portion;   a superior segment comprising a caudal mounting portion, the caudal mounting portion comprising an engagement surface and a first passageway extending through the engagement surface of the caudal mounting portion, wherein the first passageway receives the first fixation member via relative motion between the first passageway and the first fixation member along a first direction generally perpendicular to an axis of the first fixation member; and   an inferior segment comprising a cephalad mounting portion, the cephalad mounting portion comprising an engagement surface and a second passageway extending through the engagement surface of the cephalad mounting portion, wherein the second passageway receives the first fixation member via relative motion between the second passageway and the first fixation member along a second direction generally perpendicular to the axis of the first fixation member;   wherein the engagement surfaces of the caudal and cephalad mounting portions are shaped to adjustably engage each other while the engagement surfaces are in engagement with one another.   
     
     
         2 . The system of  claim 1 , further comprising:
 a second fixation member implantable in a second bone portion superior to the first bone portion;   wherein the superior segment comprises a cephalad mounting portion attachable to the second fixation member such that, upon attachment to the first and second fixation members, the superior segment substantially prevents relative motion between the first and second bone portions.   
     
     
         3 . The system of  claim 2 , further comprising:
 a third fixation member implantable in a third bone portion inferior to the first bone portion;   wherein the inferior segment comprises a caudal mounting portion attachable to the third fixation member such that, upon attachment to the first and third fixation members, the inferior segment substantially prevents relative motion between the first and third bone portions.   
     
     
         4 . The system of  claim 3 , wherein the caudal mounting portions of the superior and inferior segments have substantially the same shape and the cephalad mounting portions of the superior and inferior segments have substantially the same shape. 
     
     
         5 . The system of  claim 2 , further comprising an additional segment comprising a caudal mounting portion attachable to the second fixation member, wherein the engagement surface of the cephalad mounting portion of the superior segment is shaped to adjustably engage an engagement surface of the caudal mounting portion of the additional segment. 
     
     
         6 . The system of  claim 1 , wherein the engagement surfaces of the caudal and cephalad mounting portions each comprise a generally semispherical shape, wherein each of the passageways receives a proximal end of the first fixation member. 
     
     
         7 . The system of  claim 6 , further comprising a nut shaped to engage the proximal end of the first fixation member to attach the caudal and cephalad mounting portions to the first fixation member. 
     
     
         8 . The system of  claim 7 , wherein the first fixation member comprises a receiving surface shaped to receive either of the cephalad mounting portion and the caudal mounting portion such that tightening of the nut presses the mounting portion against the receiving surface. 
     
     
         9 . The system of  claim 1 , wherein each of the superior segment and the inferior segment is percutaneously implantable through a cannula; and wherein at least one of the superior segment and the inferior segment is adjustable through a cannula to adjust the engagement surfaces. 
     
     
         10 . The system of  claim 1 , wherein at least one of the engagement surfaces comprises at least one surface irregularity shaped to restrict relative motion between the engagement surfaces in response to pressure urging the engagement surfaces together. 
     
     
         11 . The system of  claim 1 , wherein the first fixation member comprises a pedicle screw implantable in a pedicle of a first vertebra, wherein the superior and inferior segments are shaped to bridge intervertebral spaces between the pedicle of the first vertebra and pedicles of two vertebrae adjacent to the first vertebra. 
     
     
         12 . A system comprising:
 a first fixation member implantable in a first bone portion;   a second fixation member implantable in a second bone portion adjacent to the first bone portion;   a third fixation member implantable in a third bone portion adjacent to the first bone portion;   a first segment comprising a caudal mounting portion rigidly attached to the first fixation member and a cephalad mounting portion attached to the second fixation member; and   a second segment comprising a cephalad mounting portion rigidly attached to the first fixation member and a caudal mounting portion attached to the third fixation member;   wherein at least one of the cephalad and caudal mounting portions of at least one of the first and second fixation members comprises a passageway extending therethrough to permit entry of the corresponding fixation member into the passageway in response to motion of the passageway along a direction generally perpendicular to an axis of the corresponding fixation member;   
     
     
         13 . The system of  claim 12 , wherein the caudal mounting portions of the first and second segments have substantially the same shape and the cephalad mounting portions of the first and second segments have substantially the same shape. 
     
     
         14 . The system of  claim 12 , wherein each of the passageways is sized to receive a proximal end of the corresponding fixation member. 
     
     
         15 . The system of  claim 14 , further comprising first, second, and third nuts shaped to engage the proximal ends to attach the mounting portions to the first, second, and third fixation members, wherein each of the fixation members comprises a receiving surface shaped to receive at least one of the mounting portions such that tightening of the nut presses the mounting portion against the receiving surface. 
     
     
         16 . The system of  claim 15 , wherein each of the receiving surfaces comprises a generally semispherical shape. 
     
     
         17 . The system of  claim 12 , wherein at least one of the first and second segments is percutaneously implantable. 
     
     
         18 . The system of  claim 12 , wherein at least one of the mounting portions comprises an engagement surface comprising at least one surface irregularity shaped to restrict relative motion between the engagement surface and an engagement surface of an adjacent mounting portion. 
     
     
         19 . The system of  claim 12 , wherein the first, second, and third fixation members comprise pedicle screws implantable in pedicles of the first, second, and third bone portions, respectively, wherein the first and second segments are shaped to bridge intervertebral spaces between the pedicles of the first, second, and third bone portions. 
     
     
         20 . The system of  claim 12 , wherein the caudal mounting portion of the first segment comprises an engagement surface and the cephalad mounting portion of the second segment comprises an engagement surface, wherein the engagement surfaces of the caudal and cephalad mounting portions are shaped to adjustably engage each other.