Anthropometric Measurements of the Human Knee: Correlation to the Sizing of Current Knee Arthroplasty Systems

Background: There is a paucity of data concerning the morphological dimensions of the distal part of the femur, the proximal part of the tibia, and the patella. The objective of this study was to analyze the exact anatomic data collected from a large group of patients undergoing total knee arthroplasty and to correlate the measurements to the dimensions of current prosthetic systems. Methods: Eight different centers collected morphologic data from the distal part of the femur, the proximal part of the tibia, and the patella from 337 knees during total knee arthroplasty. Microcaliper measurements from templates and measuring guides were used to decrease intraobserver variation. The study included 188 women (209 knees) and 107 men (128 knees) who had a mean age of sixty-nine years. A characterization of the aspect ratio (the medial-lateral to anterior-posterior dimensions) was made for the proximal aspect of the tibia and the distal part of the femur. Known dimensions from six prosthetic knee systems were compared with the morphologic data. Results: A wide variation in the aspect ratio for the femoral component was seen among the six different prosthetic systems. For women, there was a significant association between the component size and the amount of medial-lateral overhang, with larger sizes having more overhang (p < 0.0001). Although the femoral aspect ratio for the morphologic data showed higher ratios for smaller knees and proportionally lower ratios for larger knees, the designs showed little change in the aspect ratio. The tibial aspect ratio from the morphologic data showed a higher ratio for smaller knees and a proportionally lower ratio for larger knees. The Duracon component tracked the decline in aspect ratio fairly well, whereas the other brands either did not change with anterior-posterior dimension or actually increased (NexGen). Gender differences in the morphologic data were shown by the variable tibial aspect ratios. A comparison of the bone dimensions from the study data and the dimensions of the implants indicated that the smaller sizes were too small while the larger sizes tended to be too large. The average overall unresected patellar thickness was 23.7 mm. Conclusion: The results of this study will allow manufacturers to make more appropriate determinations of the sizes and aspect ratios of components for use in total knee arthroplasty.