Surgical alignment using references

1. A method for determining alignment of an instrument relative to a joint, comprising:

coupling a guide to the joint, the guide defining an axis and having an outer contour formed to substantially conform to a portion of the joint such that the guide couples to the joint in a single orientation and the guide defines an axis in a previously determined position relative to the joint;

attaching a first reference at a fixed position relative to the joint;

using a positioning system to determine a position of the axis relative to the first reference, the position of the axis being determined based upon the position of the guide while the guide is coupled to the joint;

removing the guide from the joint; and

after removing the guide from the joint, positioning an instrument relative to the axis based on a position of a second reference relative to the first reference.

2. The method of claim 1 , wherein the axis has a known inclination angle and a known anteversion angle relative to the joint when the guide is coupled to the joint.

3. The method of claim 1 , wherein using the positioning system to determine a position of the axis relative to the first reference comprises positioning a reference at a position having a known offset relative to the axis.

4. The method of claim 1 , wherein the outer contour of the guide is formed prior to use of the guide such that the outer contour substantially conforms to a receiving portion of the joint, and the guide mates with the receiving portion of the joint in a single orientation.

5. The method of claim 1 , wherein using the positioning system to determine a position of the axis relative to the first reference comprises aligning an identifier relative to the axis, the identifier comprising at least one of an electromagnetic field generator, a magnetic sensor, and a fiducial.

6. The method of claim 1 , wherein using the positioning system to determine a position of the axis relative to the first reference comprises engaging the instrument to the guide while the guide is coupled to the joint, and while the second reference is coupled to the instrument.

7. The method of claim 1 , wherein:

the second reference includes an infrared detector;

the first reference includes a fiducial; and

using the positioning system to determine a position of the axis relative to the first reference comprises using the positioning system such that the positioning system determines a relative position between the infrared detector and the fiducial.

8. The method of claim 1 , wherein:

the second reference includes an electromagnetic field generator;

the first reference includes an electromagnetic field sensor; and

using the positioning system to determine a position of the axis relative to the first reference comprises using the positioning system such that the positioning system determines a relative position between the electromagnetic field generator and the electromagnetic field sensor.

9. The method of claim 1 , wherein:

the outer contour of the guide is dimensioned to mate with an acetabulum of a particular patient in a single predetermined orientation;

coupling the guide to the joint comprises mating the guide to the acetabulum in the single predetermined orientation; and

attaching the first reference at a fixed position relative to the joint comprises affixing an electromagnetic field sensor or a fiducial to a pelvis that includes the acetabulum.

10. The method of claim 9 , wherein:

using the positioning system to determine the position of the axis relative to the first reference comprises engaging the instrument to the guide while the guide is mated to the acetabulum in the single predetermined orientation, the instrument being oriented in a first orientation relative to the acetabulum when in engagement with the guide; and

positioning the instrument relative to the axis comprises returning the instrument to the first orientation relative to the acetabulum after removing the guide from the joint.

11. The method of claim 1 , further comprising using the instrument to prepare the joint to receive an implant; and

implanting an implant at the joint.

12. The method of claim 11 , wherein using the instrument to prepare the joint to receive the implant comprises using a reamer that is positioned based on the position of the second reference relative to the first reference to ream an acetabulum; and

wherein implanting an implant at the joint comprises implanting an acetabular implant at the reamed acetabulum.

13. The method of claim 11 , wherein using the instrument to prepare the joint to receive the implant comprises reaming an acetabulum to a depth determined based at least in part on characteristics of an acetabular implant to be implanted at the acetabulum.

14. The method of claim 1 , further comprising implanting an implant at the joint based on the position of the instrument.

15. The method of claim 14 , wherein implanting the implant at the joint comprises impacting an acetabular implant along the axis.

16. The method of claim 1 , wherein positioning the instrument relative to the axis based on the position of the second reference relative to the first reference comprises:

obtaining, from the positioning system, output that indicates of an inclination angle or an anteversion angle of the instrument with respect to an acetabulum, the output being determined based on the position of the second reference relative to the first reference; and

adjusting the position of the instrument based on the output of the positioning system to achieve a desired inclination angle or a desired anteversion angle; and

wherein the method further comprises implanting an acetabular implant at the desired inclination angle or the desired anteversion angle with respect to the acetabulum.