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Measurement of the constraint of total knee components in a test machine provides an objective method of describing the laxity and stability characteristics of the implant itself, independent of the knee joint into which it would be implanted. A special fixture was designed and fitted to a Bionix multi-channel loading machine. The test consisted of applying a compressive load, applying a cyclic AP force or internal-external torque, and measuring all of the displacements and rotations. Three different commonly-used TKR’s showed widely different constraint characteristics. In the cyclic AP test, along with the cyclic AP displacement, displacements and rotations occurred in the other directions. This indicated that all degrees of freedom should be free to move, otherwise anomalous results would be obtained. The paper concludes with recommendations for standardized constraint tests.