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AimsIn Asia and the Middle-East, people often flex their knees deeply in order to perform activities of daily living. The purpose of this study was to investigate the 3D kinematics of normal knees during high-flexion activities. Our hypothesis was that the femorotibial rotation, varus-valgus angle, translations, and kinematic pathway of normal knees during high-flexion activities, varied according to activity.Materials and MethodsWe investigated the in vivo kinematics of eight normal knees in four male volunteers (mean age 41.8 years; 37 to 53) using 2D and 3D registration technique, and modelled the knees with a computer aided design program. Each subject squatted, kneeled, and sat cross-legged. We evaluated the femoral rotation and varus-valgus angle relative to the tibia and anteroposterior translation of the medial and lateral side, using the transepicodylar axis as our femoral reference relative to the perpendicular projection on to the tibial plateau. This method evaluates the femur medially from what has elsewhere been described as the extension facet centre, and differs from the method classically applied. ResultsDuring squatting and kneeling, the knees displayed femoral external rotation. When sitting cross-legged, femurs displayed internal rotation from 10° to 100°. From 100°, femoral external rotation was observed. No significant difference in varus-valgus angle was seen between squatting and kneeling, whereas a varus position was observed from 140° when sitting cross-legged. The measure kinematic pathway using our methodology found during squatting a medial pivoting pattern from 0° to 40° and bicondylar rollback from 40° to 150°. During kneeling, a medial pivot pattern was evident. When sitting cross-legged, a lateral pivot pattern was seen from 0° to 100°, and a medial pivot pattern beyond 100°.ConclusionThe kinematics of normal knees during high flexion are variable according to activity. Nevertheless, our study was limited to a small number of male patients using a different technique to report the kinematics than previous publications. Accordingly, caution should be observed in generalizing our findings.Cite this article: Bone Joint J 2018;100-B:50–5.

Purpose Bicruciate‐stabilised total knee arthroplasty (BCS‐TKA) uses a dual‐post‐cam mechanism as a substitute for the anterior cruciate ligament and posterior cruciate ligament (PCL), with the surface geometry providing additional guidance for axial rotation and posterior translation. However, the effect of weight‐bearing on the kinematics of BCS‐TKA has not been investigated. Therefore, the aim of this study was to clarify the effect of weight‐bearing on the kinematics of BCS‐TKA during high‐flexion activities. Methods The kinematics of 11 BCS‐TKAs were evaluated under fluoroscopy, with two‐ and three‐dimensional image registration, during squatting weight‐bearing and active‐assisted knee flexion non‐weight‐bearing. The following variables were measured: knee range of motion, axis of femoral rotation relative to the tibial component, anteroposterior (AP) translation of the medial contact point, kinematic path of the joint surfaces, lateral femorotibial contact point, and anterior and post‐cam engagement. Results The weight‐bearing condition did not influence the range of knee flexion, axis of femoral rotation, medial pivot pattern during early flexion, or bicondylar rollback beyond mid‐flexion. With regard to AP translation, both the medial and lateral contact points had more posterior locations in weight‐bearing than in non‐weight‐bearing at the mid‐flexion angle. Anterior engagement was identified in 6/11 knees (54.5%) in weight‐bearing and 4/11 knees (36.3%) in non‐weight‐bearing. Post‐cam engagement was observed in all knees, with the angle of flexion at engagement being larger in weight‐bearing than in non‐weight‐bearing. Conclusion Although weight‐bearing resulted in a posterior location of the medial and lateral contact points of the BCS‐TKA design compared with non‐weight‐bearing in the mid‐range of knee flexion, the amount of anterior translation was small overall. Therefore, BCS‐TKA provides good stability during high‐flexion weight‐bearing activities. However, BCS‐TKA is associated with high rate of the anterior engagement during early flexion. Therefore, understanding a patient's activities is an important factor when selecting the most appropriate TKA method.

Purpose This study aimed to clarify the natural knee kinematics provided by bicruciate‐retaining total knee arthroplasty (BCR‐TKA) compared with those of unicompartmental knee arthroplasty (UKA) and normal knees. Methods Volunteers and patients who had undergone UKA and BCR‐TKA with anatomical articular surface performed squatting motion under fluoroscopy. To estimate the knee's spatial position and orientation, a two‐dimensional/three‐dimensional registration technique was used. The rotation angle and anteroposterior translation of the medial and lateral sides of the femur relative to the tibia in each flexion angle were directly evaluated using the same local coordinate system and their differences amongst the three groups were analysed using two‐way analysis of variance and Bonferroni post hoc pairwise comparison. Results From 0° to 10° of flexion, the femoral external rotation angle of BCR‐TKA knees was significantly greater than that of normal and UKA knees and the medial side of BCR‐TKA knees was significantly more anteriorly located than that of normal and UKA knees. From 40° to 50° of flexion, the medial side of UKA knees was significantly more posteriorly located than that of normal and BCR‐TKA knees. From 30° to 120° of flexion, the lateral side of BCR‐TKA knees was significantly more anteriorly located than that of normal and UKA knees. Conclusion The in vivo kinematics of BCR‐TKA knees reproduces those of normal knees to a lower extent than those of UKA knees. Thus, BCR‐TKA with anatomical articular surface reproduces in vivo kinematics of normal knees to a lower extent than UKA. Level of evidence III.