Bone anchor with locking cap and method of spinal fixation

A bone anchor for attaching a fixation element to a bone, the bone anchor comprising:

• an anchor member having a head portion and a bone engaging portion for attachment to the bone;
• an anchor head receiving the head portion of the anchor member into a hollow interior bore so that the anchor member is polyaxially rotatable with respect to the anchor head, the anchor head includes a generally U-shaped opening for receiving the fixation element;
• and
• a locking cap comprising a main body and a set screw, the main body engageable with a top portion of the anchor head, the set screw including external threads and a flared portion at a bottom end, the main body including internal threads that mate with the external threads of the set screw, the set screw being friction fit to the main body in a preload condition such that initial rotation of the set screw in a first direction causes the main body to rotate with the set screw until a predetermined torque is applied to the set screw relative to the main body, the flared portion engaging a bottom end of the main body in the preload condition to prevent rotation of the set screw relative to the main body in a second direction, opposite the first direction, in the preload condition.

A bone anchor for attaching a fixation element to a vertebrae, the bone anchor comprising:

• an anchor member for attachment to the vertebra;
• an anchor head having a top portion and a bottom portion, the anchor head having a generally tubular-shape bore, an interior bore, a top opening in the top portion communicating with the interior bore and a bottom opening in the bottom portion communicating with the interior bore, the top portion having an end wall, at least one groove located proximate the top opening, a U-shaped opening formed in the anchor head to receive the fixation element, the end wall including at least one dimple formed thereon;
• and
• a locking cap comprising a main body and a set screw, the main body being releasably enqaqeable with the top portion of the anchor head, the set screw being friction fit and rotatably mounted to the main body in a preload condition, a predetermined torque being applied in a first direction to the set screw relative to the main body to rotate the set screw relative to the main body from the preload condition to a free condition, wherein the set screw further comprises a flared portion at a bottom end that engages a bottom end of the internal threads of the main body in the preload condition thus preventing rotation of the set screw relative to the main body in a second direction, opposite the first direction, in the preload condition,
• wherein the main body further comprises at least one deflectable tab and at least one flange, the at least one deflectable tab interacting with the at least one dimple so that the at least one tab is deflected as the main body is rotated in a first direction with respect to the anchor head from an open position into a closed position, the at least one flange positioned in the at least one groove in the closed position to provisionally retain the fixation element inside the U-shaped opening, the dimple resisting rotation of the main body in a second direction opposite the first direction in the closed position.

A bone anchor for attaching a fixation element to a bone, the bone anchor comprising:

• an anchor member having a head portion and a bone engaging portion for attachment to the bone;
• an anchor head having a top portion, a bottom portion and a hollow interior bore extending from the top portion to the bottom portion, the head portion of the anchor member receivable within the hollow interior bore of the anchor head, the top portion of the anchor head including a generally U-shaped opening for receiving the fixation element, a longitudinal axis and a retaining surface facing the hollow interior bore proximate the top portion, the retaining surface being located a first distance from the longitudinal axis;
• and
• a locking cap removably engageable to the top portion of the anchor head, the locking cap including a retaining surface located a second distance from the longitudinal axis in an open position, the second distance being greater than the first distance, the retaining surfaces of the anchor head and the locking cap facilitating a snap lock of the locking cap relative to the anchor head as the locking cap is engaged with the anchor head in the open position,
• wherein the locking cap further comprises a main body and a set screw, the main body being releasably enqageable with the top portion of the anchor head, the set screw being friction fit and rotatably mounted to the main body in a preload condition, a predetermined torque being applied in a first direction to the set screw relative to the main body to rotate the set screw relative to the main body from the preload condition to a free condition, wherein the set screw further comprises a flared portion at a bottom end that enqaqes a bottom end of the internal threads of the main body in the preload condition thus preventing rotation of the set screw relative to the main body in a second direction, opposite the first direction, in the preload condition.

A combination bone anchor and fixation element for securing adjacent vertebra to each other, the combination comprising:

• an anchor member for mounting to the vertebra, the anchor member having a head portion and a bone engaging portion;
• an anchor head having a bottom portion, a top portion and a longitudinal axis, the top portion including an end wall, the anchor head coupled to the anchor member proximate the bottom portion, the anchor head including a hollow interior bore and a generally U-shaped opening for receiving the fixation element;
• a sleeve positioned within the hollow interior bore, the sleeve including opposing stanchions that define at least a portion of a U-shaped channel, the stanchions being flexible and defining a width therebetween at a top opening, the width being smaller than the outer diameter of the spinal rod so that the spinal rod is snap-fitted into the sleeve during insertion of the rod into the U -shaped channel and the U-shaped opening;
• and
• a locking cap removably mountable to the top portion of the anchor head, the locking cap blocking movement of the fixation element out of the anchor head along the longitudinal axis when the locking cap is in a closed position,
• wherein the locking cap further comprises a main body and a set screw, the main body being releasably engageable with the top portion of the anchor head, the set screw being friction fit and rotatably mounted to the main body in a preload condition, a predetermined torque being applied in a first direction to the set screw relative to the main body to rotate the set screw relative to the main body from the preload condition to a free condition, wherein the set screw further comprises a flared portion at a bottom end that engages a bottom end of the internal threads of the main body in the preload condition thus preventing rotation of the set screw relative to the main body in a second direction, opposite the first direction, in the preload condition.