Uniplanar bone screw
A uniplanar bone anchor including a housing and a bone screw is provided. The lower region of the housing defines mating elements that mate with engaging elements on the bone screw to limit movement of the housing relative to the bone screw to a single plane.
1. A bone anchor comprising:
a housing comprising a bore extending therethrough defining an inner surface and a distal end comprising two opposing arms defining a channel extending transverse to the bore, the housing further comprising a first set of diametrically opposed openings disposed therein, each opening extending through the housing transverse to the bore;
a bone screw comprising a head and a shank extending from the head defining a bone screw longitudinal axis, the bone screw positionable within the housing such that the head resides in the housing and the shank extends outside the housing, the head comprising a spherical surface and a second set of diametrically opposed openings disposed therein that extend transverse to the bone screw longitudinal axis and correspond to the first set of diametrically opposed openings extending through each side of the housing;
a pair of pins, each pin receivable through a respective one of the first set of diametrically opposed openings extending through each side of the housing and a respective one of the second set of diametrically opposed openings disposed in the head to enable uniplanar movement of the housing relative to the bone screw; and
a multi-part, biasable retainer assembly positionable inside the bore of the housing above the head of the bone screw, the retainer assembly comprising a retainer comprising an inner annular surface for engaging the head of the bone screw and an annular biasing member for biasing the retainer towards the head of the bone screw so that the housing is adjustable to a desired position relative to the bone screw prior to receiving a rod in the channel.
2. The bone anchor of claim 1 , wherein each of the pair of pins comprises an outer comprising a first diameter and an inner portion comprising a second diameter that is less than the diameter of the outer portion.
3. The bone anchor of claim 2 , wherein the first set of diametrically opposed openings disposed in the housing receive the outer portion of the pins with a friction fit.
4. The bone anchor of claim 2 , wherein the second set of diametrically opposed openings disposed in the head comprise a diameter that is greater than the diameter of the inner portion of each pin to allow for swinging movement of the head along the bone screw longitudinal axis relative to the pair of pins.
5. The bone anchor of claim 1 , wherein the second set of diametrically opposed openings disposed in the head are formed from a single throughbore in the head.
6. The bone anchor of claim 5 , wherein the pair of pins comprises a single pin extendable through the first set of diametrically opposed openings disposed in the housing and the single throughbore in the head.
7. The bone anchor of claim 1 , wherein the bone screw is received into the bore from the distal end of the housing.
8. The bone anchor of claim 1 , wherein the inner annular surface further comprises a concave inner annular surface comprising a radius of curvature matched to a radius of curvature of the spherical surface of the head.
9. The bone anchor of claim 8 , wherein the the annular biasing member further comprises a wave washer.
10. The bone anchor of claim 8 , wherein the retainer assembly further comprises an annular retaining ring.
11. The bone anchor of claim 1 , wherein a lower portion of the bore extending through the housing has a thread disposed therein to allow threads disposed on the shank to pass therethrough.
12. A bone anchor comprising:
a housing comprising a bore extending therethrough defining an inner surface and two opposing arms defining a channel extending transverse to the bore, the housing further comprising a first pair of diametrically opposed openings disposed in a lower portion of the housing, each opening of the first pair of openings extending through opposing sides of the housing transverse to the bore;
a bone screw comprising a head and a shank extending from the head defining a bone screw longitudinal axis, the bone screw positionable within the housing such that the head resides in the housing and the shank extends outside the housing, the head comprising a spherical surface and a second pair of diametrically opposed openings disposed therein that extend transverse to the bone screw longitudinal axis and correspond to the first pair of diametrically opposed openings extending through the sides of the housing;
a pair of pins, each pin receivable through a respective one of the first pair of diametrically opposed openings extending through the sides of the housing and a respective one of the second pair of diametrically opposed openings disposed in the head to enable uniplanar movement of the housing relative to the bone screw; and
a multi-part, biasable retainer assembly positionable inside the bore of the housing above and in contact with the head of the bone screw, the retainer assembly comprising an annular retainer for engaging the head of the bone screw and an annular biasing member for biasing the retainer towards the head of the bone screw so that the housing is adjustable to a desired position relative to the bone screw;
wherein the bone screw and the retainer assembly are configured for top loading into the housing.
13. The bone anchor of claim 12 , wherein each of the pair of pins comprises an outer portion comprising a first diameter and an inner portion comprising a second diameter that is less than the diameter of the outer portion.
14. The bone anchor of claim 13 , wherein the first set of diametrically opposed openings disposed in the housing receive the outer portion of the pins with a friction fit.
15. The bone anchor of claim 13 , wherein the second set of diametrically opposed openings disposed in the head comprise a diameter that is greater than the diameter of the inner portion of each pin to allow for longitudinal movement of the head along the bone screw longitudinal axis relative to the pair of pins.
16. The bone anchor of claim 12 , wherein the second set of diametrically opposed openings disposed in the head are formed from a single throughbore in the head.
17. The bone anchor of claim 16 , wherein the pair of pins comprises a single pin extendable through the first set of diametrically opposed openings disposed in the housing and the single throughbore in the head.
18. The bone anchor of claim 12 , wherein the annular retainer comprises a concave inner annular surface comprising a radius of curvature matched to a radius of curvature of the spherical surface of the head.
19. The bone anchor of claim 18 , wherein the annular biasing member further comprises a wave washer.
20. The bone anchor of claim 18 , wherein the retainer assembly further comprises an annular retaining ring.
21. The bone anchor of claim 1 , wherein a lower portion of the bore extending through the housing is constricted in order to retain the head of the bone screw in the housing.
22. The bone anchor of claim 1 , wherein the retainer assembly comprises a pair of diametrically opposed concave part cylindrical surfaces defined on an upper surface thereof for receiving a rod when the surfaces are aligned with the channel extending transverse to the bore of the housing.