Background Achieving balanced gaps is a key surgical goal in total knee arthroplasty, yet most methods rely on subjective surgeon feel and experience to assess and achieve knee balance intraoperatively. Our objective was to evaluate the ability to quantitatively plan and achieve a balanced knee throughout the range of motion using robotic-assisted instrumentation in a tibia-first, gap-balancing technique. Methods A robotic-assisted, gap-balancing technique was used in 121 consecutive knees. After resection of the proximal tibia, a computer-controlled tensioning device was inserted into the knee joint and the pre-femoral-resection knee gaps were acquired dynamically throughout flexion under controlled load. Predicted gap profiles were used to plan the femoral implant by adjusting the implant alignment and position within certain boundaries to achieve a balanced knee throughout the range of flexion. Femoral cuts were then made according to this plan using a miniature robotic-assisted cutting guide. The tensioning device used to measure the pre-femoral-resection gaps was then reinserted into the joint to quantify the final gap balance under known tension. The final gap profiles were then compared with the predictive gap plans. Results The overall root mean square error between the predicted and achieved gaps was 1.3 mm and 1.5 mm for the medial and lateral sides, respectively. Use of robotic assistance resulted in over 90% of knees having mediolateral balance within 2 mm across the flexion range. Gaps at 0° flexion were 2 mm smaller than the gaps at 90°. This difference decreased to less than 1 mm when comparing the tibiofemoral gaps at 10°, 45°, and 90°. Conclusions Imageless, robotic-assisted total knee arthroplasty accurately predicts postoperative gaps before femoral resections. This allows surgeons to virtually plan femoral implant alignment and optimize gap balance throughout the range of motion. The accurate prediction of gaps throughout the arc of motion combined with precise, robotically assisted femoral resection produces accurate postoperative ligament balance consistently.

Healthy patellofemoral (PF) joint mechanics are critical to optimal function of the knee joint. Patellar maltracking may lead to large joint reaction loads and high stresses on the articular cartilage, increasing the risk of cartilage wear and the onset of osteoarthritis. While the mechanical sources of PF joint dysfunction are not well understood, links have been established between PF tracking and abnormal kinematics of the tibiofemoral (TF) joint, specifically following cruciate ligament injury and repair. The objective of this study was to create a validated finite element (FE) representation of the PF joint in order to predict PF kinematics and quadriceps force across healthy and pathological specimens. Measurements from a series of dynamic in-vitro cadaveric experiments were used to develop finite element models of the knee for three specimens. Specimens were loaded under intact, ACL-resected, and both ACL and PCL-resected conditions. Finite element models of each specimen were constructed and calibrated to the outputs of the intact knee condition, and subsequently used to predict PF kinematics, contact mechanics, quadriceps force, patellar tendon moment arm, and patellar tendon angle of the cruciate resected conditions. Model results for the intact and cruciate resected trials successfully matched experimental kinematics (avg. RMSE 4.0°, 3.1 mm) and peak quadriceps forces (avg. difference 5.6%). Cruciate resections demonstrated either increased patellar tendon loads or increased joint reaction forces. The current study advances the standard for evaluation of PF mechanics through direct validation of cruciate-resected conditions including specimen-specific representations of PF anatomy.

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Purpose Achieving a balanced knee is accepted as an important goal in total knee arthroplasty; however, the deinition of ideal balance remains controversial. This study therefore endeavoured to determine: (1) whether medio-lateral gap balance in extension, midlexion, and lexion are associated with improved outcome scores at one-year post-operatively and (2) whether these relationships can be used to identify windows of optimal gap balance throughout lexion. Methods 135 patients were enrolled in a multicenter, multi-surgeon, prospective investigation using a robot-assisted surgical platform and posterior cruciate ligament sacriicing gap balancing technique. Joint gaps were measured under a controlled tension of 70-90 N from 10°-90° lexion. Linear correlations between joint gaps and one-year KOOS outcomes were investigated. KOOS Pain and Activities of Daily Living sub-scores were used to deine clinically relevant joint gap target thresholds in extension, midlexion, and lexion. Gap thresholds were then combined to investigate the synergistic efects of satisfying multiple targets. Results Signiicant linear correlations were found throughout extension, midlexion, and lexion. Joint gap thresholds of an equally balanced or tighter medial compartment in extension, medial laxity ± 1 mm compared to the inal insert thickness in midlexion, and a medio-lateral imbalance of less than 1.5 mm in lexion generated subgroups that reported signiicantly improved KOOS pain scores at one year (median ∆ = 8.3, 5.6 and 2.8 points, respectively). Combining any two targets resulted in further improved outcomes, with the greatest improvement observed when all three targets were satisied (median ∆ = 11.2, p = 0.002). Conclusion Gap thresholds identiied in this study provide clinically relevant and achievable targets for optimising soft tissue balance in posterior cruciate ligament sacriicing gap balancing total knee arthroplasty. When all three balance windows were achieved, clinically meaningful pain improvement was observed. Level of Evidence Level II.

Quantifying the mechanical environment at the knee is crucial for developing successful rehabilitation and surgical protocols. Computational models have been developed to complement in-vitro studies, but are typically created to represent healthy conditions, and may not be useful in modeling pathology and repair. Thus, the objective of this study was to create finite element (FE) models of the natural knee, including specimen-specific tibiofemoral (TF) and patellofemoral (PF) soft tissue structures, and to evaluate joint mechanics in intact and ACL-deficient conditions. Simulated gait in a whole joint knee simulator was performed on two cadaveric specimens in an intact state and subsequently repeated following ACL resection. Simulated gait was performed using motor-actuated quadriceps, and loads at the hip and ankle. Specimen-specific FE models of these experiments were developed in both intact and ACL-deficient states. Model simulations compared kinematics and loading of the experimental TF and PF joints, with average RMS differences [max] of 3.0°[8.2°] and 2.1°[8.4°] in rotations, and 1.7[3.0] and 2.5[5.1] mm in translations, for intact and ACL-deficient states, respectively. The timing of peak quadriceps force during stance and swing phase of gait was accurately replicated within 2° of knee flexion and with an average error of 16.7% across specimens and pathology. Ligament recruitment patterns were unique in each specimen; recruitment variability was likely influenced by variations in ligament attachment locations. ACL resections demonstrated contrasting joint mechanics in the two specimens with altered knee motion shown in one specimen (up to 5 mm anterior tibial translation) while increased TF joint loading was shown in the other (up to 400 N).