Knee joint instability, which is a primary reason for TKA revision surgeries, is typically caused by deficiency in the knee ligaments [1, 2]. Managing ligament deficiency and restoring joint stability continues to be one of the greatest challenges for revision surgeries [3]. To treat such patients, revision TKA implants frequently incorporate a constrained post and cam mechanism to provide enhanced varus-valgus constraint to supplement the function of the collateral ligaments. The aim of this study was to evaluate knee kinematics during a weight bearing deep knee bend for both a primary TKA system and its complimentary revision system. The hypothesis of the study was that the revision tibial insert would demonstrate improved knee stability, in the form of a reduced range of motion under out-of-plane loading, when compared to the primary system Eight cadaveric knees (age: 59±10 years, BMI 23.3±3.5) were implanted with an ATTUNE™ revision femoral component and a primary posterior stabilized tibial component. Each knee was mounted and aligned into the Kansas Knee Simulator (Fig. 1) [4]. A deep knee bend was performed between 10° and 110° flexion with no out-of-plane loading. Additional deep knee bends were performed with constant 6Nm external and 6Nm internal torques about the tibial long axis, and with 40N medial and 40N lateral loads applied at the ankle sled. The 40N medial and 40N lateral loads produce approximately 15Nm adduction and abduction moments at the knee, respectively. The primary tibial insets were then replaced with revision tibial inserts from the same TKA system and the deep knee bend cycles were repeated. The revision tibial inserts included a larger tibial post intended to constrain the varus-valgus rotation of the knee. The change in knee kinematics of the revision tibial insert compared to the primary insert was calculated and student t-tests were performed to identify significant differences between the two tibial insert types for each loading condition.Introduction
Methods
Tibiofemoral constraint in patients with total knee replacements (TKR) is dependent on both implant geometry and the surrounding soft tissue structures. Choosing more highly constrained geometries can reduce the contribution of soft tissue necessary to maintain joint stability [1]. Often when knee revision surgeries are required, the soft tissue and bone are compromised leading to the use of more constrained implants to ensure knee stability [2]. The current study quantifies the differences in varus-valgus (VV) and internal-external (IE) constraint between two types of total knee revision systems: SIGMA® TC3© and ATTUNE® REVISION. Nine cadaveric knees (7 male, age 64.0 ± 9.8 years, BMI 26.28 ± 4.92) were implanted with both fixed-bearing SIGMA TC3 and ATTUNE REVISION knee systems. Five knees received the TC3 implant first, while the remaining 4 received the ATTUNE implant first. The knees were mounted in an inverted position, and a six degree-of-freedom force-torque sensor (JR3, Woodland, CA) was rigidly secured to the distal tibia (Fig. 1). A series of manual manipulations applying IE and VV torques was performed through the flexion range [3]. Each specimen was then revised to the alternate revision system, and the manual manipulations were repeated. Joint loads were calculated, and tibiofemoral kinematics were described according to the Grood-Suntay definition [4]. VV and IE kinematics were calculated as a function of flexion angle, VV torque, and IE torque as has been described previously [3]. The knees were analysed at ±6 Nm VV and ±4 Nm IE, and the kinematics were normalized to the zero load path. A paired t-test (p < .05) was employed to identify significant differences between the kinematics of the two knee systems at 10º flexion increments.Introduction
Methods
During primary total knee arthroplasty (TKA), surgeons occasionally encounter compromised bone and fixation cannot be achieved using a primary femoral component. Revision knee replacement components incorporate additional features to improve fixation, such as modular connection to sleeves or stems, and feature additional varus-valgus constraint in the post-cam mechanism to compensate for soft tissue laxity. The revision femoral component can be used in place of the primary femur to address fixation challenges; however, it is unclear if additional features of the revision femoral components adversely affect knee kinematics when compared to primary TKA components. The objective of this study was to compare weight-bearing tibiofemoral and patellofemoral kinematics between primary and revision femoral component with the primary tibial insert for a single knee replacement system. The hypothesis of the study was that kinematics for revision femoral components will be similar to kinematics of the primary femoral components Eight cadaveric knees (age: 59±10 years, BMI 23.3±3.5) were implanted with a primary TKA system (ATTUNE™ Posterior Stabilized Total Knee Replacement System). Each knee was mounted and aligned in the Kansas Knee Simulator (Fig. 1) [1]. A deep knee bend was performed which flexed the knee from full extension to 110° flexion, while the medial-lateral translation, internal-external, and varus-valgus rotations at the ankle were unconstrained. The femoral component was then replaced with a revision femoral component of the same TKA system, articulating on the same primary insert component, and the deep knee bend was repeated. The translations of the lowest points (LP) of the medial and lateral femoral condyles along the superior-inferior axis of the tibia were calculated. In addition, tibiofemoral and patellofemoral kinematics were calculated for each cycle based on the Grood-Suntay coordinate system [2] [1]. The change in LP and patellofemoral kinematics from the primary to revision femurs were calculated. Student t-tests were performed at 5° increments of knee flexion to identify significant differences between the two implant types.Introduction
Methods
Quadriceps weakness, which is often reported following total knee arthroplasty (TKA), affects patients' abilities to perform activities of daily living [1]. Implant design features, particularly of the patella-femoral joint, influence the mechanical advantage of the extensor mechanism. This study quantifies the changes in extensor mechanism moment arms due to different patellar resurfacing options during TKA. Posterior-stabilized TKR surgery was performed on seven cadaveric knees which were subsequently mounted in the Kansas Knee Simulator (KKS) [2]. A dynamic physiological squat was simulated between 5° and 80° knee flexion at 50% body weight while knee kinematics, including the lines of action of the rectus femoris (RF) muscle and patellar tendon (PT), were recorded using an optical tracking system. The simulation was performed after three patella treatment options: 1) leaving the native patella Unresurfaced, 2) resurfaced with a medialized Dome patella, and 3) resurfaced with a medialized Anatomic patella which included a conforming lateral facet. Moment arms from the tibio-femoral helical axis to the line of action of the PT and the RF were calculated for each patella condition.Introduction
Methods
Design phase evaluation of potential implant designs requires verified computational and experimental models. Computational models are important where parametric evaluation of geometric features experimentally is both cost and time-prohibitive due to the need to manufacture complex parts, and provide information not easily measured experimentally, such as internal stresses/strains in the implant or bone. However, before implementation into the design process, a thorough verification/validation is required. In this study, a finite element model of the Kansas knee simulator (KKS) was developed and a systematic verification of predicted joint kinematics was performed by comparison with experimental measurements, including evaluating the patellofemoral joint first in isolation, followed by whole joint kinematic comparisons. Four unmatched, healthy cadaver knees (average age 63 yrs) were mounted in the KKS to reproduce a simulated gait and deep knee bend activity in their natural and implanted states. Finite element models of the KKS assembly and the four cadaver specimens in their natural and implanted states were created. Isolated patellofem-oral kinematics were initially verified during simulated deep knee bend. Average RMS differences between predicted and experimental natural patellar kinematics were less than 3.1° and 1.7 mm for rotations and translations, respectively, while differences in implanted kinematics were less than 2.1° and 1.6 mm between 10 and 110° femoral flexion. Similar agreement was found with the subsequent whole joint simulations. Deep knee bend tibiofemoral internal-external (IE) and varus-valgus (VV) rotations had average RMS differences from experimental measurements of 1.5 ± 0.4° and 0.9 ± 0.5°, respectively. Anterior-posterior (AP), inferior-superior (IS) and medial-lateral translations matched within 1.8 ±0.8 mm, 1.2 ±0.7 mm, and 0.6 ±0.1 mm, respectively. The experimental and verified computational tools can be used in harmony for pre-clinical assessment of implant designs; the computational model allows rapid screening of implant geometry or alignment issues and provides additional insight into joint mechanics such as implant stresses or bone strains, while the experimental simulator can subsequently be utilized to assess in cadavera only the most promising designs or features identified.
