The effect of a medially based anterior capsular shift on translational and rotational range of motion and posterior capsular strain was investigated in an in vitro model. Six cadaveric shoulders were tested in a robot-assisted shoulder simulator. Translational and rotational range of motion were reduced by the capsular shift, particularly with the shoulder at higher elevation angles. At 60 degrees of elevation, anterior translation was decreased 1.9 +/- 2.9 mm, and posterior translation was decreased 2.3 +/- 6.2 mm. External rotation was decreased 11.5 degrees +/- 10.2 degrees, and internal rotation was decreased 8.9 degrees + 5.7 degrees. Posterior capsular strain change was dependent on elevation angle. At 30 degrees of elevation, strain tended to increase 5.0% +/- 7.3% in the inferior aspect and 2.9% +/- 2.6% in the superior aspect, with no change detected in the medial aspect. At 60 degrees of elevation, strain increased 6.6% +/- 8.0%, 3.2% +/- 2.6%, and 4.4% +/- 3.5% in the inferior, middle, and superior aspects, respectively. Our results support the use of the medially based anterior shift for shoulders with anterior-inferior instability or multidirectional instability with posterior involvement.