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Purpose To evaluate if there was a correlation between in vivo kinematics of a medial-stabilized (MS) total knee arthroplasty (TKA) and post-operative clinical scores. We hypothesized that (1) a MS-TKA would produce a medial pivot movement and that (2) this speciic pattern would be correlated with higher clinical scores. Methods 18 patients were evaluated through clinical and functional scores evaluation (Knee Society Score clinical and functional, Womac, Oxford), and kinematically through dynamic radiostereometric analysis (RSA) at 9 months after MS-TKA, during the execution of a sit-to-stand and a lunge motor task. The anteroposterior (AP) Low Point translation of medial and lateral femoral compartments was compared through Student's t test (p < 0.05). A correlation analysis between scores and kinematics was performed through the Pearson's correlation coeicient r. Results A signiicantly greater (p < 0.0001) anterior translation of the lateral compartment with respect to the medial one was found in both sit-to-stand (medial 2.9 mm ± 0.7 mm, lateral 7.1 mm ± 0.6 mm) and lunge (medial 5.3 mm ± 0.9 mm, lateral 10.9 mm ± 0.7 mm) motor tasks, thus resulting in a medial pivot pattern in about 70% of patients. Signiicant positive correlation in sit-to-stand was found between the peak of AP translation in the lateral compartment and clinical scores (r = 0.59 for Knee Society Score clinical and r = 0.61 for Oxford). Moreover, we found that the higher peak of AP translation of the medial compartment correlated with lower clinical scores (r = − 0.55 for Knee Society Score clinical, r = − 0.61 for Womac and r = − 0.53 for Oxford) in the lunge. A negative correlation was found between Knee Society Score clinical and VV laxity during sit-to-stand (r = − 0.56) and peak of external rotation in the lunge motor task (r = − 0.66). Conclusions The MS-TKA investigated produced in vivo a medial pivot movement in about 70% of patients in both examined motor tasks. There was a correlation between the presence of medial pivot and higher post-operative scores. Level of evidence IV.

PurposeThe role of the anterior cruciate ligament (ACL) in knee biomechanics in vivo and under weight‐bearing is still unclear. The purpose of this study was to compare the tibiofemoral kinematics of ACL‐deficient knees to healthy contralateral ones during the execution of weight‐bearing activities. MethodsEight patients with isolated ACL injury and healthy contralateral knees were included in the study. Patients were asked to perform a single step forward and a single leg squat first with the injured knee and then with the contralateral one. Knee motion was determined using a validated model‐based tracking process that matched subject‐specific MRI bone models to dynamic biplane radiographic images, under the principles of Roentgen stereophotogrammetric analysis (RSA). Data processing was performed in a specific software developed in Matlab. ResultsStatistically significant differences (p < 0.05) were found for single leg squat along the frontal plane: ACL‐deficient knees showed a more varus angle, especially at the highest knee flexion angles (40°–50° on average), compared to the contralateral knees. Furthermore, ACL‐deficient knees showed tibial medialization along the entire task, while contralateral knees were always laterally aligned. This difference became statistically relevant (p < 0.05) for knee flexion angles included between 0° and about 30°. ConclusionACL‐deficient knees showed an abnormal tibial medialization and increased varus angle during single leg squat when compared to the contralateral knees. These biomechanical anomalies could cause a different force distribution on tibial plateau, explaining the higher risk of early osteoarthritis in ACL deficiency. The clinical relevance of this study is that also safe activities used in ACL rehabilitation protocols are significantly altered in ACL deficiency. Level of evidenceIII.

Purpose To assess, using model‐based dynamic radiostereometric analysis (RSA), the biomechanical behaviour of a new design posterior‐stabilized (PS) fixed‐bearing (FB) total knee arthroplasty (TKA) in vivo while patients performing two common motor tasks. The hypothesis was that model‐based dynamic RSA is able to detect different behaviour of the implant under weight‐bearing and non‐weight‐bearing conditions. Methods A cohort of 15 non‐consecutive patients was evaluated by dynamic RSA 9 months after TKA implantation. The mean age of patients was 73.4 (65–72) years. The kinematic evaluations were performed using an RSA device (BI‐STAND DRX 2) developed in our Institute. The patients were asked to perform two active motor tasks: sit‐to‐stand in weight‐bearing condition; range of motion (ROM) while sitting on the chair. The motion parameters were evaluated using the Grood and Suntay decomposition and the low‐point kinematics methods. Results The dynamic RSA evaluation showed a significant difference (p < 0.05) between the biomechanical behaviour of the prosthesis during the two motor tasks. When subjected to the patient weight (in the sit‐to‐stand) the low point of the medial compartment had a shorter motion (5.7 ± 0.2 mm) than the lateral (11.0 ± 0.2 mm). This realizes a medial pivot motion as in the normal knee. In the ROM task, where the patient had no weight on the prosthesis, this difference was not present: the medial compartment had a displacement of 12.7 ± 0.2 mm, while the lateral had 17.3 ± 0.2 mm. Conclusions Model‐based RSA proved to be an effective tool for the evaluation of TKA biomechanics. In particular, it was able to determine that the fixed‐bearing posterior‐stabilized TKA design evaluated in this study showed a medial pivoting movement under weight‐bearing conditions that was not present when load was not applied. Under loading conditions what drives the pattern of movement is the prosthetic design itself. By the systematic use of this study protocol future comparisons among different implants could be performed, thus contributing significantly to the improvement of TKA design. Level of evidence IV.

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