Letter to the Editor: Computer-assisted Navigation Increases Precision of Component Placement in Total Knee Arthroplasty

Abstract

To the Editor: We read with interest the article “Computer-assisted Navigation Increases Precision of Component Placement in Total Knee Arthroplasty” by Haaker et al in the April 2005 edition of Clinical Orthopaedics and Related Research.3 We appreciate the commendable effort made by the authors for reporting an elaborately conducted study. Comparisons of navigation-assisted total knee arthroplasty (TKA) and conventional TKA rarely mention all the angles of prosthesis implantation and limb alignment and then provide the statistical analysis for each angle. However, the authors used some angles that did not match the references mentioned; moreover, they commented on results as better for a navigation system without providing adequate references for the values that they considered normal or exact. In the Materials and Methods section, the authors stated the sagittal femoral component angle was the angle between the ventral cortex axis of the femur and a line drawn perpendicular to the distal part of the femoral component. There is disparity between the figure quoted for this angle and the text description. The angle shown in the figure is measured between the tangent to the ventral femoral cortex and tangent at the prosthesis. No perpendicular line to the distal part of the femoral component is drawn. Moreover, the three references mentioned in the text for this angle do not match with the description in the text or the figure. The first reference, an article by Ewald,2 reported the Knee Society roentgenographic evaluation system, used a sagittal femoral angle measured between a line perpendicular to the distal metal-cement interface of the femoral component and a line parallel to the femoral shaft axis rather than the lines used by the authors. The second reference, an article by Mahaluxmivala et al,4 also mentioned the Knee Society roentgenographic evaluation system. The third reference was a paper regarding mechanism of failure in TKA which does not mention a sagittal femoral component angle.5 While reporting the sagittal femoral alignment in the Results section, the authors used the term exact alignment for 90° sagittal femoral component angle. There is no report in the literature regarding the exact or ideal sagittal femoral component angle, as this angle varies with anatomic anterior bowing of the femoral shaft. Even if we measure this angle using the method described by the authors in the figure, the central femoral cortex axis is not likely to be a straight line perpendicular to the tangent at the distal prosthetic-bone interface. In the Results section, the authors mentioned that the coronal femoral component was better aligned in the navigation group compared with the conventional TKA group, even though the mean angle in both groups was within 1° of exact perpendicular and the 95% confidence interval also was within 1° of exact perpendicular. The angle measured by the authors is the femoral half of the mechanical axis as they measured it between a line drawn from the center of the hip to the center of the knee and tangent at the prosthesis. The mechanical axis within 3° valgus or varus is associated with a better outcome,1,6 however, there is no report in the literature about a better outcome with varus alignment compared with valgus alignment within 1° of perpendicular for the concerned angle. We understand that the difference between two groups can be significant, but there is no evidence to determine which group has better alignment, although from the wide range of angles in the conventional group it can be interpreted that outliers might be more frequent in this group. The other angles also are measured along lines different from those mentioned in the Knee Society roentgenographic evaluation system. However, these can be accepted as the authors' personal method for measuring the angles because they did not mention that these angles were measured in accordance with the Knee Society roentgenographic evaluation system. D. S. Pandher, MS, K. J. Oh, MD, PhD Department of Orthopedics, Konkuk University Hospital, Konkuk University School of Medicine, Seoul, South Korea R. S. Boaparai, MS, G. S. Josan, MS Department of Orthopedics, Govt. Medical College, Amritsar, India