Background
Accurate identification of native cruciate ligament insertion sites is required to settle the correct tunnel position during reconstruction, or appraise tunnel positioning after reconstruction. The ability to determine these insertion sites might be challenged by considerable variation in reported location and size of the ligament footprints. The three-dimensional osseous morphology varies per person and affects the position of the cruciate ligament insertions.
Purpose
The aim of this study is to determine the localization of the femoral and tibial native insertion sites of the cruciate ligaments while adjusting for variations in three-dimensional bone geometry.
Methods
Statistical shape models (SSMs) of both the distal femur and proximal tibia were created based on segmentations of 103 MR images of knees without signs of osteoarthritis (Kellgren-Lawrence grade 0). The compactness, specificity and generalization of these models were determined.
CT scans of ten lower leg specimens with marked cruciate ligament insertion sites were performed. The SSMs were fitted on segmentations of the CT images. The difference in the segmented bones and SSM produced surface models was calculated using the root mean squared (RMS) Hausdorff distance. The segmented cruciate ligament attachment sites of each specimen were addressed to project the insertions on the fitted SSM shape. The correspondence points constantly indicating the same insertion were determined, and defined as the ligament attachments on the SSMs.
Results
The SSMs of both the distal femur and proximal tibia each consisted of 4296 correspondence points placed in space as to represent the three-dimensional bony knee shape. The quantitative evaluation showed both SSMs were compact, as seven modes for the femur and nine modes for the tibia were able to explain 80% of variance. Errors for specificity and generalization for both SSMs remained below 1 mm. The modes of variation of the SSMs were able to truthfully depict the three-dimensional osseous shape variations of the distal femur and proximal tibia.
The ligament attachment sites projected from the segmented specimens onto the fitted femoral SSM shapes indicated a mean of 28.4 (range 14-45) and 63.6 (range 40-92) correspondence points as respectively ACL and PCL insertion. On the tibial SSM, a mean of 53.5 (range 25-98) and 79.2 (range 43-138) correspondence points were indicated as ACL and PCL insertion, respectively, by the projected ligament attachment sites. These numbers indicate the small variation of insertion sites after adjustment for three-dimensional bony geometry, with the femoral attachments of cruciate ligaments having an even more constant attachment site than the tibial attachments.
By determining the correspondence points constantly indicating the same insertion, 8 and 52 correspondence points on the femoral SSM, and 28 and 51 correspondence points on the tibial SSM were defined as respectively ACL and PCL insertion regions. These correspondence points on the SSMs defined as the insertions adapted position and distance towards each other with the different modes of variation. Thus, along with the SSMs' modes of variation, these areas defined as ligament insertions varied in location, shape and size. This reflected the claimed variation of the ACL and PCL insertions on bony surfaces.
Conclusion
The presumed variation of the attachment sites of cruciate ligaments is only miminal after adjustment for three-dimensional osseous morphology. Therefore, it is feasible the three-dimensional osseous geometry of the distal femur and proximal tibia considerably determine the insertion sites of the cruciate ligaments.
Identifying the ligament attachment sites by referencing to the three-dimensional bony morphology may thus be a more reliable and better reproducible method, than referencing to landmarks which may have a variable relation to the insertion sites. Therefore the three-dimensional SSM of either the proximal tibia or the distal femur might be used as template when planning anatomic ACL or PCL reconstruction, or when assessing tunnel position after surgery.