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
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.