EFFECT OF TIBIAL SLOPE ON FLEXION AND FEMORAL ROLLBACK IN TOTAL KNEE ARTHROPLASTY: A CADAVERIC STUDY

Purpose (a): Reduced posterior tibial slope (PTS) and posterior tibiofemoral translation (PTFT) in posterior cruciate retaining (PCR) total knee arthroplasty (TKA) has been shown to result in suboptimal postoperative knee flexion due to the occurrence of tibiofemoral impingement. Although reduced PTS and PTFT have been shown independently to negatively affect total knee flexion following TKA, there has never been a study to our knowledge that has shown the effect of PTS on PTFT. We evaluated the relationship between PTS, PTFT, and total knee flexion in a cadaveric model after TKA. Methods (b): We obtained nine transfemoral fresh frozen cadaver specimens and preformed a balanced PCR TKA. The pre-operative and post-operative PTS were precisely measured with c arm fluoroscopy and the post-operative PTS was changed in 1 degree increments using custom shims for the TKA trial components. We successively measured the total flexion using a motion tracking system in response to a 25 lb force applied to the hamstrings at 1 degree increments of posterior tibial slope (1-10 degrees). Relative PTFT was measured at maximal flexion with C-arm fluoroscopy. Results (c): We used Tukey ANOVA test to determine significant changes in flexion and PTFT as a function of PTS. We found that there was an average increase in flexion of 2.3 o per degree increase of PTS from 1o (1 degree) to 5 o (p. Conclusions (d): Small increases in PTS in the range of 1o to 5o appear to significantly increase knee flexion and PTFT. As the PTS is further increased above 5 o, these findings suggest that flexion and PTFT do not continue to increase significantly. This is the first study to find a direct relationship between PTS and PTFT. These findings may be explained by changes in PCL tension with different PTS. As the flexion gap is loosened above a threshold (5 o) with increased PTS, the relatively lax PCL likely fails to initiate PTFT and subsequent total knee flexion is subsequently decreased due to posterior tibiofemoral soft tissue impingement. Additionally, we did not observe a correlation between native PTS and optimal degree of post-operative PTS. Although these results suggest that increasing PTS above 5o does not improve flexion or PTFT, clinical judgment and proper flexion gap balancing remain paramount in maximizing post-operative knee flexion. In vivo studies will be necessary to further substantiate these conclusions.