Effect of rotational prosthetic alignment variation on tibiofemoral contact pressure distribution and joint kinematics in total knee replacement.

In total knee replacement surgery, implant alignment is one of the most important criteria for successful long-term clinical outcome. During total knee replacement implantation, femoral and tibial alignment are determined through appropriate bone resections, which could vary based on patient anatomy, implant design and surgical technique and further influence loading conditions and clinical outcomes. The current research focused on three critical alignment parameters for total knee replacement insertion: femoral component internal/external (I/E) rotation, varus-valgus tibiofemoral angulation and posterior tibial slope. A computational finite element model of total knee replacement implant was developed and validated comparing with kinematic outputs generated from experimentally simulated knee joint motion. The FE model was then used to assess 12 different alignment scenarios based on previous case reports. Postoperative knee kinematics and joint contact pressure during simulated gait motion were assessed. According to the parametric study, FE model cases with femoral rotation revealed extra tibial I/E rotation in the predefined direction but negligible change in tibial anterior-posterior translation; cases with increased tibial slope showed notably increased tibial external rotation and anterior translation; cases with varus tibiofemoral angle presented slightly more tibial external rotation, whereas cases with valgus angle presented an observable increase in tibial internal rotation at the middle phase of the gait cycle. Finally, the response surface obtained from the postprocessing study demonstrated good statistical correlation with existing case study results, providing reliable estimation of peak tibiofemoral contact pressure affected by combinations of alignment parameters. The observations indicate that femoral external alignment should be favored clinically for enhanced patellar tracking and reduced contact pressure concentration for better long-term performance. Posterior tibial slope enables deep knee flexion. Extra femoral internal rotation as well as tibiofemoral varus-valgus alignment could be avoided in surgery due to deficiency in patellar tracking and high pressure concentration.

Bi-unicondylar knee replacement laxity with changes to simulated soft tissue constraints.

Unicondylar knee replacement systems have been shown to perform comparably to total knee replacements, while being much less surgically invasive. Proper ligament balancing, as well as knee laxity, has been shown to play an important role in optimizing kinematic behavior of these implant systems and improving long-term survival of the implant. This study investigates the effect of different simulated ligament laxity conditions of the anterior cruciate ligament and the posterior cruciate ligament on the resulting anteroposterior and mediolateral contact kinematics for medial and lateral pairs of UKR implants with flat and symmetric ultrahigh-molecular-weight polyethylene inserts during force-controlled ISO-14243-1 knee testing simulation. A novel method of capturing the tibiofemoral lowest point contact path was used to calculate the shear plane lowest point contact path kinematics in both the anteroposterior and the mediolateral directions. The results illustrated that multiple clinically relevant soft tissue configurations produce statistically different measured knee kinematics in unicondylar knee replacement systems than is seen in accepted "standard" knee simulator protocols with 95% confidence interval. The observed kinematic differences in anteroposterior and mediolateral movement from what was observed using standard wear testing protocols could aid in the development of unicondylar knee replacement design enhancements that are resistant to varying soft tissue deficiencies.

In vitro acute load to failure and eyelet abrasion testing of a novel veterinary screw-type mini-anchor design.

OBJECTIVE: To determine acute load to failure (ALF) and suture abrasion (SA) at 0° and 90° for a novel screw-type mini-anchor design. STUDY DESIGN: Biomechanical in vitro study. SAMPLE POPULATION: Synthetic bone. METHODS: Twenty mini-anchors were inserted into synthetic bone blocks assigned to 1 of 2 groups (0° ALF, 90° ALF). Pullout was performed at 5 mm/min. ALF, yield strength and stiffness were calculated. SA constructs were created with 4 groups of 5 anchors each with either 30 lb nylon leader line (NLL), 40 lb NLL, #2 Fiberwire or #5 Fiberwire. SA was performed at 0° and 90° with a sinusoidal wave form at 0.5 Hz and 10 N load for 1000 cycles or until failure. Data were summarized as mean ± SD. ALF data were analyzed using t-tests. SA data were analyzed using log rank, Tukey-adjusted pairwise comparisons and sign tests. Significance was set at P = .05. RESULTS: Mean ± SD ALF at 0° and 90° was 431.8 ± 70.8 N and 683 ± 48.7 N, respectively. 90° ALF was significantly higher. Yield strength and stiffness were not significantly different at 0° and 90°. #5 and #2 Fiberwire survived significantly more cycles than 40 lb and 30 lb NLL at 90°. At 0°, 30 lb NLL survived significantly less cycles than either Fiberwire size. Suture orientation did not have a significant effect on SA for Fiberwire constructs. CONCLUSION: The novel mini-anchor has ALF comparable to other mini-anchors. Fiberwire survived more cycles in the novel anchor eyelet than NLL and FW suture orientation in the eyelet did not affect SA.