Method and apparatus for determining acetabular component positioning

An instrument for establishing an ipsilateral hemipelvic reference plane for a pelvis, the pelvis including an acetabulum, an anterior superior iliac spine, an ischium, and an ilium, the instrument comprising:

• a hub having a vertical axis;
• first and second arms emanating from the hub, the first and second arms
• angularly separated from each other,
• having ends opposite the hub, and
• defining a nominal plane;
• first, second, and third legs, the first leg attached at or proximate to the end of the first arm, the second leg attached at or proximate to the end of the second arm, and the third leg attached at or proximate to the hub, the first, second, and third legs
• extending from the nominal plane defined by the first and second arms, and
• terminating in tips;
• and
• an alignment indicator attached to the hub and extending out from the hub, the alignment indicator rotatable in the nominal plane defined by the first and second arms to have an azimuth orientation, and the alignment indicator pivotable between the vertical axis of the hub and the nominal plane to have an elevation,
• wherein when the instrument is docked to the pelvis:
• the tip of the first leg contacts a region of the ischium where it joins a posterior wall of the acetabulum,
• the tip of the second leg contacts the anterior superior iliac spine, and
• the tip of the third leg contacts the ilium at a point free of identifiable landmarks,
• to establish the ipsilateral hemipelvic reference plane at the pelvis.