Influence of tibial slope on extension and flexion gaps in total knee arthroplasty: increasing the tibial slope affects both gaps

Abstract

Purpose Increasing the tibial slope is often performed if the flexion gap is narrower than the extension gap. The main hypothesis of this study is that increasing the tibial slope coincidentally enlarges the extension gap. Methods Twenty formalin-fixed cadaveric knees were obtained for study. After CT in full extension and 90°flexion, the data of each specimen were entered into a standardized coordinate system and virtual bone cuts were performed with incrementally increasing the posterior slope. Gaps were measured at tibiofemoral contact points in 90°-flexion and full extension in the medial and lateral compartment. Results Increasing the tibial slope did significantly widen both the extension and the flexion gaps (p<0.001). In extension, the opening rates, i.e. the gap increase per degree of slope increase, were equal medially and laterally (0.5 mm ±0.1) medial vs 0.6 mm (±0.0) lateral), whereas in flexion the lateral gap did open significantly more than the medial one (0.6 mm ±0.1) medial vs 0.9 mm (±0.1) lateral (p<0.001), resulting in a significantly greater flexion gap laterally.Conclusions Increasing the tibial slope beyond the preoperative planning in order to widen a tight flexion gap intra-operatively is not recommended as doing so will increase the extension gap simultaneously and will make the medial and lateral flexion gaps unequal.

Systematic effects of femoral component rotation and tibial slope on the medial and lateral tibiofemoral flexion gaps in total knee arthroplasty

“…Under distraction, the typical flexion-extension gap would occur assuming that the ligament tension would not be affected by the resection. This phenomenon can be simplified by the equation [ 15 ] flexion-extension gap ​= ​deformation of the surrounding soft tissue ​+ ​bony resection gap. Assuming an ideal soft tissue mantle, perfect orientation of the components and a neutral alignment, a prosthesis would take up the entire gap around the tibiofemoral contact points throughout the entire movement cycle.…”

“…In this investigational cadaver study, as a continuative project of the study of Nowakowski et al. from 2014 [ 15 ], the same samples consisting of 19 formalin-fixed knee cadaver specimens (all Caucasian) were further analysed from the repository of the Department of Anatomy. The convenience sample included 10 male and nine female donors [five left and five right knees for men, four left and five right knees for women; mean ​± ​1 standard deviation (SD), age 81.0 ​± ​12.1 years; body mass 68.7 ​± ​13.9 ​kg; height 165.2 ​± ​10.4 ​cm].…”

“…The sagittal plane was defined orthogonally to the frontal plane containing the tibia reference point, the transverse plane by the flexion/extension axis orthogonal to the other two planes and the origin as the intersection of all planes. Pairs of data sets in extension and flexion were rendered by matching the identical voxel data of the tibia to obtain a combined data set oriented along the coordinate system [ 15 ]. The medial and lateral tibiofemoral contact points were identified in both extension (0°) and flexion (90°) according to Wretenberg et al.…”

“…[ 20 ] and as previously described by Nowakowski et al. [ 15 ]. Briefly, the tibia was sectioned orthogonally to the coordinate axis defined above, at a resection depth of 6 ​mm (the average tibia plateau component thickness), which was defined as a 0° slope.…”

“…The longest mediolateral diameter in the 0° tibial slope plane was identified, and its midpoint was measured and projected orthogonally onto the ventral tibia edge to obtain the centre of rotation for the simulated tibial slopes. Virtual tibial slopes of 0°, 3.5°, 7° and 10° were generated [ 15 ].…”