High inter- and intraindividual differences in medial and lateral posterior tibial slope are not reproduced accurately by conventional TKA alignment techniques

Purpose The purpose of this study was to describe the medial and lateral posterior tibial slope (MPTS and LPTS) on 3D-CT in a Caucasian population without osteoarthritis. It was hypothesised that standard TKA alignment techniques would not reproduce the anatomy in a high percentage of native knees. Methods CT scans of 301 knees [male:female = 192:109; mean age 30.1 (+/- 6.1)] were analysed retrospectively. Tibial slope was measured medially and laterally in relation to the mechanical axis of the tibia. The proportion of MPTS and LPTS was calculated, corresponding to the "standard PTS" of 3 degrees-7 degrees. The proportion of knees accurately reproduced with the recommended PTS of 0 degrees-3 degrees for PS and 5 degrees-7 degrees for CR TKA were evaluated. Results Interindividual mean values of MPTS and LPTS did not differ significantly (mean (range); MPTS: 7.2 degrees (-1.0 degrees-19.0 degrees) vs. LPTS: 7.2 degrees (-2.4 degrees-17.8 degrees), n.s.). The mean absolute intraindividual difference was 2.9 degrees (0.0 degrees-10.8 degrees). In 40.5% the intraindividual difference between MPTS and LPTS was > 3 degrees. When the standard slope of 3 degrees-7 degrees medial and lateral was considered, only 15% of the knees were covered. The tibial cut for a PS TKA or a CR TKA changes the combined PTS (MPTS + LPTS) in 99.3% and 95.3% of cases, respectively. Conclusion A high interindividual range of MPTS and LPTS as well as considerable intraindividual differences were shown. When implementing the recommended slope values for PS and CR prostheses, changes in native slope must be accepted. Further research is needed to evaluate the impact of altering a patient's native slope on the clinical outcome.