KNEE NAVIGATION; ACCURACY OF REGISTRATION AND POTENTIAL FOR MALALIGNMENT

Abstract

Introduction: Several studies have shown that malalignment of primary knee replacement of more that three degrees result in rapid failure and less than optimal functional outcome. We investigated whether inaccuracies in registration resulted in malalignment and also what degree of registration error would result in an unacceptable result. The Stryker image free navigation system was used.

Method: Using a dry bone leg with restrained joints, we varied the points where registration was set and measured the resulting errors in alignment. Registration points used were 1: centre of the knee, 2: AP axis, 3: centre of the ankle and 4: medial malleolus. The true registration point was translated medially or laterally up to 10 mm or inclined up to ten degrees to simulate inaccurate placement.

Results: 1: less than 1° varus for 10mm of medial translation, 1.5° valgus for 10mm lateral translation on distal femoral cut. 2a: 0.5° valgus for 10mm medial translation and 0.5° valgus for 10° medial angulation on distal femoral cut. 2b: 0.5° varus for 10mm medial translation and no effect for 10° angulation on tibial cut. 3a: Software error with 10mm translation laterally and 1.5° varus for 10mm of lateral translation. 3b: 1.5° posterior slope and valgus angle for 30 degrees of lateral angulation. 4: 1° valgus for 10mm lateral translation and no effect on tibial slope.

Conclusion: There is not only no significant intra-observer error, but also that even combining registration errors may result in a cut error of only 1.5 degrees. The software itself also protected against any significant errors.