T he study by Mahfouz and colleagues provides a number of interesting arguments regarding distal femoral preparation in TKA. Traditional instrumentation systems provide surgeons with flexibility in component placement; however, these systems can introduce potential errors in alignment and bone resection planes. Different knee systems have variable implant thicknesses and progression between sizes, making instrumentation comparisons even more difficult. The use of computer navigation or patient-specific guides may eliminate some of these errors but can potentially create new ones. With computer navigation, inaccuracies in registration may create erroneous alignment and resection planes. Any movement of fixed referencing guides can also lead to incorrect resection planes. For patient-specific guides, incorrect templating for surgical planning, or incorrect placement of the cutting guide may lead to incorrect resection planes. Furthermore, to balance the ligaments with patientspecific guides, additional bone resection is often required. In addition, the distal femoral anatomy shows significant variation between individuals and current total knee implants can only approximate that anatomy. While alignment is important, the success of a TKA may be better defined by the kinematics of the reconstructed whole. This paper demonstrates some of the variability that can occur based on the use of specific landmarks for distal femoral bone preparation. The greatest variation detected was in the amount of posterior condylar bone resected with the various techniques. This error is inherent in systems that use anterior cortical referencing and is clearly shown by the data. The use of a PCA+3 posterior condylar resection may be correct in many knees, but when using a ligament balancing technique could be shown to be incorrect as much as 40% of the time. The distal femoral flexion discrepancy between the mechanical axe (MA)-1 and MA-2 technique raises an important point with regard to positioning of the femoral implant. Relative extension of the distal femoral cut will alter This CORR Insights is a commentary on the article ‘‘A New Method for Calculating Femoral Anterior Cortex Point Location and Its Effect on Component Sizing and Placement’’ by Mahfouz and colleagues available at: DOI: 10.1007/s11999-0143930-1. The author certifies that he, or any member of his immediate family, has no funding or commercial associations (eg, consultancies, stock ownership, equity interest, patent/ licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article. All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request. The opinions expressed are those of the writers, and do not reflect the opinion or policy of CORR or the Association of Bone and Joint Surgeons. This CORR Insights comment refers to the article available at DOI: 10.1007/s11999-0143930-1. D. A. Fisher MD (&) Center for Joint Replacement, Indiana Orthopedic Hospital, 8450 Northwest Blvd, Indianapolis, IN 46278, USA e-mail: dafisher@orthoindy.com; dafishermd@aol.com CORR Insights Published online: 7 October 2014 The Association of Bone and Joint Surgeons1 2014