Systems and devices for connecting a longitudinal member to a bone portion

1. A spinal fixation device, comprising:

a fastener having:

a shank; and

a head comprising:

a first spherical surface extending from a proximal end of the shank;

a proximal surface;

a second spherical surface extending from the proximal surface of the head; and

a radially extending shoulder extending around a periphery of the head between the first and second spherical surfaces of the head, wherein the shoulder is disposed closer to the proximal surface of the head than the proximal end of the shank, along a longitudinal axis of the shank;

a housing having a passage formed along a longitudinal axis thereof and adapted to seat the spherical surface of the head of the fastener;

a spacer received in said passage of said housing and engageable with said head; and

a compressible member disposed within the housing and positioned between the spacer and the housing, the compressible member generating a frictional force between the head and the housing to enable the shank to be maintained in a desired angular orientation relative to the housing in the absence of the spinal fixation element.

2. The device of claim 1 , wherein the compressible member is positioned above the shoulder of the head.

3. The device of claim 1 , wherein the compressible member is substantially C-shaped.

4. The device of claim 1 , wherein the spacer has at least two slots aligned along the longitudinal axis of the housing, the slots extending distally from a proximal edge of the spacer.

5. The device of claim 1 , wherein the compressible member exerts the frictional force on the head through the spacer.

6. The device of claim 5 , wherein:

the housing has a channel in an inner wall thereof configured to receive at least a portion of the compressible member; and

the spacer has a circumferential groove in an outer wall thereof configured to receive at least a portion of the compressible member.

7. The device of claim 6 , wherein the entire channel in the inner wall of the housing and the circumferential groove in the spacer are disposed below the bottom-most portion of a channel in the proximal surface of the spacer adapted to receive a spinal fixation element.

8. The device of claim 1 , wherein the passage of the housing and the spacer mate at a cylindrical interface when the spacer is engaged with the head such that the spacer is rotatable within the passage.

9. The device of claim 1 , wherein the first spherical surface is configured to engage the passage of the housing over a surface area that is greater than that of an engagement between the second spherical surface and the spacer.

10. A spinal fixation device, comprising:

a fastener having:

a shank extending along a longitudinal axis; and

a head comprising:

a proximal surface having a tool recess;

a first spherical portion extending from the proximal surface in a distal direction, the first spherical portion having a first radius;

a distal neck connected to the shank; and

a second spherical portion extending from the distal neck in a proximal direction, the second spherical portion having a second radius larger than the first radius;

wherein the first and second spherical portions are connected to each other along a radially extending surface that is closer to the proximal surface of the head than the distal neck of the head, along the longitudinal axis of the shank;

a housing comprising:

a transverse slot configured to receive a rod;

a longitudinal passage configured to receive the head; and

a first part spherical surface sized to engage the second spherical portion; and

a spacer comprising:

an upper portion configured to engage the longitudinal passage; and

a second part spherical portion configured to engage the first spherical portion.

11. The spinal fixation device of claim 10 , further comprising a compressible member disposed between the longitudinal passage of the housing and the upper portion of the spacer, the compressible member configured to exert a longitudinal force against the head through the spacer.

12. The spinal fixation device of claim 11 , wherein:

the housing has a channel on an inner surface thereof configured to receive at least a portion of the compressible member; and

the spacer has a circumferential groove in an outer surface thereof configured to receive at least a portion of the compressible member.

13. The spinal fixation device of claim 11 , wherein the compressible member is adapted to expand or contract in a radial direction.

14. The spinal fixation device of claim 10 , wherein the spacer has at least two slots aligned along a longitudinal axis of the housing, the slots extending distally from a proximal edge of the spacer.

15. The spinal fixation device of claim 10 , wherein the fastener further comprises a radially extending shoulder extending around a periphery of the head between the first and second spherical portions of the head.

16. The spinal fixation device of claim 10 , wherein:

the second part spherical portion engages the first spherical portion along a first interface extending along a first arc length oriented in a longitudinal cross-section taken along the longitudinal axis; and

the first part spherical portion engages the second spherical portion along a second interface extending along a second arc length oriented in the longitudinal cross-section;

wherein the second arc length is greater in length than the first arc length when the housing and the spacer are disposed concentrically about the longitudinal axis on the head.

17. The spinal fixation device of claim 10 , wherein the upper portion of the spacer is rotatable against the longitudinal passage of the housing when the second part spherical portion is engaged with the first spherical portion.