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Poster: 17A Novel Method to Determine the Lower Limb Mechanical Axis in theCoronal PlaneOBJECTIVEMichel Collette - Clinique Edith Cavell - Brussels, Belgium*Nicholas Hohl - Exactech - Gainesville, USA*Email: nicolas.hohl@exac.comCorrect implant positioning is widely recognized as a key factor in the long-term success oftotal knee arthroplasty. According to the literature, malalignment of 3° or less in the frontalplane was observed in 75-80% of cases performed using the classical method (intramedullaryrod allowing bone cut adjustment according to preoperative X-ray planning) and 90-95% ofcases using traditional computer assisted surgery (CAS). Despite improving alignment accuracy,CAS has not been accepted as widely as expected due to cost and time involved.Our goal is to describe a new method for determining lower-limb mechanical axis in the coronalplane which improves alignment accuracy and is easier to use and less expensive. This newmethod relies on mechanical instruments to align the frontal plane with an extramedullarysystem.METHODSAccording to basic Euclidean geometry, the perpendicular bisector of a chord circle passesthrough the center of a circle (Figure 1). When a knee is brought from abduction to adduction,it moves on a circle. The center of this circle is the hip’s center of rotation.The Exactech Mechanical Positioning System features a vertical bar firmly fixed to theoperating table rail and a horizontal arm overhanging the operated knee (Figure 2). The systemcan be used to mark any point on the femoral condyle and “remember” its spatial location. Thesituation in space of a selected point on the femur is determined both in abduction andadduction. A rod passing perpendicularly through the center of the segment joining these twopositions automatically crosses the center of the hip.The coronal plane femoral axis was determined using this method and alignment was verifiedusing an image intensifier to visualize the hip joint center in 20 total knee replacementsperformed between March and June 2007. The differences between the actual hip center and themechanical axis were documented in the results.RESULTSIn comparison with the ideal 0° reference axis:Mean error (in degrees) ± SD: 0.6 ± 0.6Error ≤1°: 80% of casesError ≤3°: all casesCONCLUSIONSThe Exactech Mechanical Positioning System determines the mechanical axis of the lower limbin the frontal plane without violating the femoral intramedullary canal. The results shown verifyfile:///E|/ISTA2010-Abstracts.htm[12/7/2011 3:15:47 PM]