Author Correction: Tibio-Femoral Contact Force Distribution is Not the Only Factor Governing Pivot Location after Total Knee Arthroplasty.

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Tibio-Femoral Contact Force Distribution is Not the Only Factor Governing Pivot Location after Total Knee Arthroplasty.

Total knee arthroplasty aims to mimic the natural knee kinematics by optimizing implant geometry, but it is not clear how loading relates to tibio-femoral anterior-posterior translation or internal-external pivoting. We hypothesised that the point of pivot in the transverse plane is governed by the location of the highest axial force. Tibio-femoral loading was measured using an instrumented tibial component in six total knee arthroplasty patients (aged 65-80y, 5-7y post-op) during 5-6 squat repetitions, while knee kinematics were captured using a mobile video-fluoroscope. In the range of congruent tibio-femoral contact the medial femoral condyle remained approximately static while the lateral condyle translated posteriorly by 4.1 mm (median). Beyond the congruent range, the medial and lateral condyle motions both abruptly changed to anterior sliding by 4.6 mm, and 2.6 mm respectively. On average, both the axial loading and pivot position were more medial near extension, and transferred to the lateral side in flexion. However, no consistent relationship between pivoting and load distribution was found across all patients throughout flexion, with R2 values ranging from 0.00 to 0.65. Tibio-femoral kinematics is not related to the load distribution alone: medial loading of the knee does not necessarily imply a medial pivot location.

Mediolateral force distribution at the knee joint shifts across activities and is driven by tibiofemoral alignment.

AIMS: Tibiofemoral alignment is important to determine the rate of progression of osteoarthritis and implant survival after total knee arthroplasty (TKA). Normally, surgeons aim for neutral tibiofemoral alignment following TKA, but this has been questioned in recent years. The aim of this study was to evaluate whether varus or valgus alignment indeed leads to increased medial or lateral tibiofemoral forces during static and dynamic weight-bearing activities. PATIENTS AND METHODS: Tibiofemoral contact forces and moments were measured in nine patients with instrumented knee implants. Medial force ratios were analysed during nine daily activities, including activities with single-limb support (e.g. walking) and double-limb support (e.g. knee bend). Hip-knee-ankle angles in the frontal plane were analysed using full-leg coronal radiographs. RESULTS: The medial force ratio strongly correlated with the tibiofemoral alignment in the static condition of one-legged stance (R² = 0.88) and dynamic single-limb loading (R² = 0.59) with varus malalignment leading to increased medial force ratios of up to 88%. In contrast, the correlation between leg alignment and magnitude of medial compartment force was much less pronounced. A lateral shift of force occurred during activities with double-limb support and higher knee flexion angles. CONCLUSION: The medial force ratio depends on both the tibiofemoral alignment and the nature of the activity involved. It cannot be generalised to a single value. Higher medial ratios during single-limb loading are associated with varus malalignment in TKA. The current trend towards a 'constitutional varus' after joint replacement, in terms of overall tibiofemoral alignment, should be considered carefully with respect to the increased medial force ratio. Cite this article: Bone Joint J 2017;99-B:779-87.

Loading of the knee joint during activities of daily living measured in vivo in five subjects.

Detailed knowledge about loading of the knee joint is essential for preclinical testing of implants, validation of musculoskeletal models and biomechanical understanding of the knee joint. The contact forces and moments acting on the tibial component were therefore measured in 5 subjects in vivo by an instrumented knee implant during various activities of daily living. Average peak resultant forces, in percent of body weight, were highest during stair descending (346% BW), followed by stair ascending (316% BW), level walking (261% BW), one legged stance (259% BW), knee bending (253% BW), standing up (246% BW), sitting down (225% BW) and two legged stance (107% BW). Peak shear forces were about 10-20 times smaller than the axial force. Resultant forces acted almost vertically on the tibial plateau even during high flexion. Highest moments acted in the frontal plane with a typical peak to peak range -2.91% BWm (adduction moment) to 1.61% BWm (abduction moment) throughout all activities. Peak flexion/extension moments ranged between -0.44% BWm (extension moment) and 3.16% BWm (flexion moment). Peak external/internal torques lay between -1.1% BWm (internal torque) and 0.53% BWm (external torque). The knee joint is highly loaded during daily life. In general, resultant contact forces during dynamic activities were lower than the ones predicted by many mathematical models, but lay in a similar range as measured in vivo by others. Some of the observed load components were much higher than those currently applied when testing knee implants.