Apparatus and method for measuring a length between implanted bone anchors

1. An apparatus for measuring a distance between first and second implanted bone anchors, the apparatus comprising:

a caliper body having a first longitudinal axis;

a first caliper arm slidably coupled to the caliper body and having a second longitudinal axis, wherein the first caliper arm is disposed in a fixed angular relationship to the caliper body such that the second longitudinal axis of the first caliper arm is always substantially perpendicular to the first longitudinal axis of the caliper body; and

a second caliper arm rotatably coupled to the caliper body and having a third longitudinal axis, wherein the first longitudinal axis, the second longitudinal axis, and the third longitudinal axis define a plane, and the second caliper arm is rotatable within the plane between a first position and a second position.

2. The apparatus of claim 1 , wherein the second caliper arm is rotatable relative to the caliper body between a first configuration and a second configuration.

3. The apparatus of claim 2 , wherein the third longitudinal axis of the second caliper arm and the first longitudinal axis of the caliper body define an acute angle in the first configuration and an obtuse angle in the second configuration.

4. The apparatus of claim 1 , wherein the caliper body includes a first metering portion and a second metering portion, the first caliper arm operable to display a position of the first caliper arm relative to the caliper body, and the second metering portion operable to display a position of the second caliper arm relative to the caliper body.

5. The apparatus of claim 1 , wherein the caliper body comprises an elongate rod portion having a cross-section including a longitudinal groove, the longitudinal groove being parallel to the first longitudinal axis and located at a corner of the rectilinear cross-section.

6. The apparatus of claim 5 , wherein the first caliper arm includes a rectilinear central bore having a flange portion, the rectilinear central bore being slidably disposed on the elongate rod portion such that the flange portion is slidably disposed in the longitudinal groove.

7. The apparatus of claim 1 , wherein the caliper body includes a plurality of dimples extending between a first end of the caliper body and a second end of the caliper body.

8. The apparatus of claim 7 , wherein the first caliper arm includes a nub operable to engage at least one of the plurality of dimples to positively locate the first caliper arm relative to the caliper body.

9. The apparatus of claim 1 , wherein:

the second caliper arm includes a second support member rotatably coupled to the caliper body, and a second tip member removably coupled to the second support member, and

the first caliper arm includes a slide element slidably coupled to the caliper body, a first support member coupled to the slide element, and a first tip member removably coupled to the first support member.

10. The apparatus of claim 9 , wherein at least one of the first tip member and the second tip member are disposable.

11. The apparatus of claim 9 , further comprising:

gear teeth disposed at an end of the second support member within the caliper body; and

a metering arm having a free end providing an indicator and a second end having gear teeth engaged with the gear teeth of the second support member.

12. The apparatus of claim 1 , wherein the caliper body includes a head element having a first end and a second end, and the second caliper arm includes a first end and a second end, and wherein the second caliper arm is rotatably coupled to the first end of the head element between the first end and the second end of the second caliper arm.

13. The apparatus of claim 12 , wherein the second end of the second caliper arm is adjacent the second end of the head element.

14. The apparatus of claim 12 , wherein the second end of the head element includes an arcuate slot having a first end and a second end, and wherein the second end of the second caliper arm is adjacent the first end of the arcuate slot in a first configuration, and adjacent the second end of the arcuate slot in a second configuration.

15. The apparatus of claim 1 , wherein the second caliper arm is attached to the caliper body at a rotation axis that is located in a fixed position relative to an elongate rod portion of the caliper body.

16. An apparatus for measuring a distance between first and second implanted bone anchors, the apparatus comprising:

a caliper body having a first longitudinal axis;

a first caliper arm having a slide element slidably coupled to the caliper body and having a second longitudinal axis, wherein the first caliper arm is rotationally fixed relative to the slide element; and

a second caliper arm rotatably coupled to the caliper body, the second caliper arm rotatable about an axis perpendicular to a plane defined by the first longitudinal axis and the second longitudinal axis.

17. The apparatus of claim 16 , wherein the first caliper arm is substantially perpendicular to the caliper body, and wherein the second caliper arm and the caliper body define an acute angle in a first configuration and an obtuse angle in a second configuration.

18. The apparatus of claim 16 , wherein the caliper body includes a first metering portion and a second metering portion, the first caliper arm operable to display a position of the first caliper arm relative to the caliper body, and the second metering portion operable to display a position of the second caliper arm relative to the caliper body.

19. A method for measuring a distance between first and second implanted bone anchors with a caliper including a caliper body, a first caliper arm and a second caliper arm configured to operate in a single plane defined by the caliper body, first caliper arm and second caliper arm, the method comprising:

linearly translating the first caliper arm along the caliper body;

generally aligning a longitudinal axis of the first caliper arm with a longitudinal axis of the first bone anchor;

generally aligning a longitudinal axis of the second caliper arm with a longitudinal axis of the second bone anchor by rotating the second caliper arm relative to the caliper body; and

measuring a distance between distal ends of the first and second caliper arms as a function of linear translation of the first caliper arm along the caliper body by viewing a first graduated scale through an aperture in the caliper body in the single plane, and rotation of the second caliper arm relative to the caliper body by viewing a second graduated scale through a slot in the caliper body in the single plane.

20. The method of claim 19 , further comprising removing a first portion of the first caliper arm from the caliper and removing a second portion of the second caliper arm from the caliper.