Initial stability of cementless total knee arthroplasty (TKA) tibial trays is necessary to facilitate biological fixation. Previous experimental and computational studies describe a dynamic loading micromotion test used to evaluate the initial stability of a design. Experimental tests were focused on cruciate retaining (CR) designs and walking gait loading. A FEA computational study of various constraints and activities found CR designs during walking gait experienced the greatest micromotion. This experimental study is a continuation of testing performed on CR and walking gait to include a PS design and stair descent activity. The previously described experimental method employed robotic loading informed by a custom computational model of the knee. Different TKA designs were virtually implanted into a specimen specific model of the knee. Activities were simulated using in-vivo loading profiles from instrumented tibia implants. The calculated loads on the tibia were applied in a robotic test. Anatomically designed cementless tibia components were implanted into a bone surrogate. Micromotion of the tray relative to the bone was measured using digital image correlation at 10 locations around the tray. Three PS and three CR samples were dynamically loaded with their respective femur components with force and moment profiles simulating walking gait and stair descent activities. Periods of walking and stair descent cycles were alternated for a total of 2500 walking cycles and 180 stair descent cycles. Micromotion data was collected intermittently throughout the test and the overall 3D motion during a particular cycle calculated. The data was normalized to the maximum micromotion value measured throughout the test. The experimental data was evaluated against previously reported computational finite element model of the micromotion test.Introduction
Methods
In knee biomechanics the concept of the envelope of motion (EOM) has proven to be a powerful method to characterize joint mechanics and the effect of surgical interventions. It is furthermore indispensable for numerical model validation. While commonly used for tibiofemoral kinematics, there is very little report of applying the concept to patellofemoral kinematics. EOM measurements require precise and reproducible displacement and load control in all degrees of freedom (DOF), which robotic testing has proven to provide. The objectives of this study were therefore to (1) develop a robotic method to assess patellofemoral EOM as a function of tibiofemoral EOM, (2) compare resulting patellofemoral kinematics to published data, and (3) determine which DOFs in the tibiofemoral EOM mostly account for the patellofemoral EOM. The developed robotic (KUKA KR140 comp) method was evaluated using 8 post-mortem human leg specimens of both genders (age: 55±11 years, BMI: 23±5). Firstly, tibiofemoral neutral flexion was established as well as the EOM by applying anterior-posterior (±100 N), medial-lateral (±100 N), internal-external (±4 Nm) and varus-valgus (±12 Nm) loads under low compression (44 N) at 7 flexion angles. Secondly, patellofemoral flexion kinematics and EOM were measured during a robotic playback of the previously established tibiofemoral kinematics. During these measurements, the quadriceps tendon was loaded with a hanging weight (20 kg) via a pulley system directing the force to the anterior superior iliac spine. Kinematics were tracked optically (OptiTrack) and registered to CT scans using co-scanned aluminum cylinders and beads embedded in the patella. The overall patellofemoral EOM was calculated as the extent of patellar motion observed during manipulating the tibia inside the tibiofemoral EOM in all DOFs. Additionally, patellofemoral EOMs were calculated for tibial manipulations along individual DOFs to analyze the importance of these DOFs.Introduction
Material and Methods
Total-knee-arthroplasty (TKA) is a well-established method to restore the joint function of the human knee. Different types of TKA designs are clinically available which can be divided in two main groups, the posterior-cruciate- ligament (PCL) sacrificing and retaining group. However, pre-operatively it is often difficult to plan for one or the other. Therefore, the research question was: Is it possible to develop a TKA bearing design which works for both the cruciate sacrificing and retaining technique? A medial-congruent (MC) bearing design was developed, characterized by a high medial sagittal conformity and lower lateral sagittal conformity, which can be used for both cruciate ligament states. This study compares the laxity and kinematics of this MC design to a contemporary PS design for the cruciate sacrificing technique and to a contemporary CR design for the cruciate retaining technique. Four specimen-specific computer models of the human knee, consisting of a femur, tibia and fibula bone as well as the contribution of the ligaments and capsule, were virtually implanted with three TKA designs in four constellations: 1) MC without PCL, 2) MC with PCL, 3) contemporary PS without PCL and 4) contemporary CR with PCL following the design specific surgical technique and tibia slopes. Laxity tests in internal-external rotation (moment ± 4 Nm) were performed with the implanted models for a weight bearing case (500N compression). In addition, a high demanding activity (lunge) was simulated. The resulting averaged laxities and kinematics were analysed and compared to each other.Introduction
Methods
Total-knee-arthroplasty (TKA) is used to restore knee function and is a well-established treatment of osteoarthritis. Along with the widely used fixed bearing TKA design, some surgeons opt to use mobile bearing designs. The mobile-bearing TKA is believed to allow for more freedom in placement of the tibial plate, greater range of motion in internal-external (IE) rotation and greater constraint through the articular surface. This current study evaluates 1) the kinematics of a high constraint three condyle mobile bearing TKA, 2) the insert rotation relative to the tibia, and 3) compares them with the intact knee joint kinematics during laxity tests and activities-of-daily-living (lunge, level walking, stairs down). We hypothesize that 1) in contrast to the intact state the anterior-posterior (AP) stability of the implanted joint increases when increasing compression level while 2) maintaining the IE mobility, and that 3) the high constraint does not prevent differential femorotibial rollback during lunge. Six fresh-frozen human cadaveric knee joints with a mean donor age of 64.5 (±2.4) years and BMI of 23.3 (±7.3) were tested on a robot (KR140, KUKA) in two different states: 1) intact, 2) after implantation of a three condyle mobile bearing TKA. The tibia plateau and the insert of each tested specimen were equipped with a sensor to measure the insert rotation during testing. Laxity tests were done at extension and under flexion (15°, 30°, 45°, 60° 90°, 120°) by applying subsequent forces in AP and medial-lateral (ML) of ±100N and moments in IE and varus-valgus (VV) rotation (6Nm/4Nm, 12 Nm/-). Testing was performed under low (44N) and weight bearing compression (500N). Loading during the lunge, level walking and stairs descent activity was based on in-vivo data. Resulting data was averaged and compared with the kinematics of the intact knee.Introduction
Methods
Partial knee arthroplasty (PKA) has demonstrated the potential to improve patient satisfaction over total knee arthroplasty. It is however perceived as a more challenging procedure that requires precise adaptation to the complex mechanics of the knee. A recently developed PKA system aims to address these challenges by anatomical, compartment specific shapes and fine-tuned mechanical instrumentation. We investigated how closely this PKA system replicates the balance and kinematics of the intact knee. Eight post-mortem human knee specimens (age: 55±11 years, BMI: 23±5, 4 male, 4 female) underwent full leg CT scanning and comprehensive robotic (KUKA KR140 comp) assessments of tibiofemoral and patellofemoral kinematics. Specimens were tested in the intact state and after fixed bearing medial PKA. Implantations were performed by two experienced surgeons. Assessments included laxity testing (anterior-posterior: ±100 N, medial-lateral: ±100 N, internal-external: ±3 Nm, varus- valgus: ±12 Nm) under 2 compressive loads (44 N, 500 N) at 7 flexion angles and simulations of level walking, lunge and stair descent based on in-vivo loading profiles. Kinematics were tracked robotically and optically (OptiTrack) and represented by the femoral flexion facet center (FFC) motions. Similarity between intact and operated curves was expressed by the root mean square of deviations (RMSD) along the curves. Group data were summarized by average and standard deviation and compared using the paired Student's T-test (α = 0.05).Introduction
Materials and Methods
Clinical studies have shown that the knee tends to experience laterally higher AP motion (posterior directed) than medially (Asano at al., 2001; Dennis et al., 2005; Hill et al., 2000; Moro Oka et al., 2007). Traditional posterior stabilized (PS) total knee arthroplasty (TKA) designs allow deep flexion stability and femoral rollback once cam/spine engagement occurs, however mechanical stability provided by tibial bearing conformity during early to mid-flexion is highly variable. In this study a computer knee model is used to compare AP kinematics in PS TKA designs while evaluating multiple sagittal tibia bearing conformities. We hypothesized that highly conforming designs would be necessary to promote AP stability prior to cam/spine engagement. A specimen specific computer model consisting of the femur, tibia and fibula, as well as the contribution of the ligaments and capsule was virtually implanted with TKA designs of the appropriate size at 5° tibia slope with the posterior cruciate ligament sacrificed. A single PS femoral component was evaluated with five PS tibia bearing designs with variable sagittal conformity ratios ranging from 1.05:1 to 2.2:1 (conformity ratio = tibia bearing sagittal radius / femur sagittal condylar radius). Designs were fully conforming frontally, with cam/spine engagement beyond 90° flexion. In all designs, lateral conformity ratios were increased relative to medial conformity ratios to facilitate lateral femoral rollback. Resultant AP kinematic predictions were obtained for femoral Low Points (LP) during 1) envelope of motion during internal external (IE) laxity evaluation and 2) knee bend functional activity.INTRODUCTION
METHOD
The intact, healthy human knee joint is stable under anterior-posterior (AP) loading but allows for substantial internal-external (IE) laxity. In vivo clinical studies of the intact knee consistently demonstrate femoral rollback with flexion (Hill et al., 2000, Dennis et al., 2005). A tri-condylar, posterior stabilized (PS) total knee arthroplasty (TKA) with a rotating platform bearing (TKA-A) has been designed to address these characteristics of the intact knee. The third condyle is designed to guide the femoral component throughout the entire flexion arc (AP stability and femoral rollback with flexion), while the rotating platform bearing allows for IE rotation. This study used a computer model to compare the AP and IE laxity of a new TKA-A to that of two clinically established TKAs (TKA-B: rotating PS TKA, TKA-C: fixed PS TKA) and to demonstrate improvements in AP stability, IE rotation, and femoral rollback. A specimen-specific, robotically calibrated computer knee model (Siggelkow et al., 2012), consisting of the femur, tibia and fibula as well as the kinetic contribution of the ligaments and capsule was virtually implanted with appropriate sizes of TKA-A, TKA-B and TKA-C adhering to the respective surgical techniques. A similar extension gap was targeted for all designs. The following kinematic data resulting from applied loads and moments were analyzed: 1) Passive AP and IE laxity (AP load: ± 50 N, IE moment: ± 6 Nm) of the midpoint between the flexion facet centers (Iwaki et al., JBJS, 2000) under low compression (44 N), 2) AP position of the medial and lateral low points (LP) of the femoral component during a lunge motion (Varadarajan et al., 2008).INTRODUCTION
METHODS
Advancements in knee surgery require a profound understanding of knee mechanics. However, there are seemingly contradicting reports regarding certain aspects of normal knee function, such as the location of the pivot of internal-external rotation in the transverse plane. Among others, it has been suggested to be located close to the knee center or in the medial compartment. We hypothesized that this apparent contradiction is a result of different studied knee motions and that it can be explained by the underlying envelopes of motion. The study objective was to characterize normal knee behavior in-vitro with an emphasis on pivot location. Thirty-four cadaveric human knee specimens (Age: 61±8 years, BMI: 25±7) underwent CT and MR imaging and load controlled in-vitro testing using an industrial robot (KUKA, Augsburg, Germany). The robot simulated passive knee flexion and assessed the envelopes of motion through anterior-posterior (AP, ±100 N), medial-lateral (ML, ±100 N) and internal-external (IE, ±6 Nm) laxity testing at five flexion angles. Kinematics were expressed by the femoral flexion facet centers (FFC). The pivot location was determined for IE laxity testing and passive flexion by computing the center of transverse femoral rotation in a least squares sense. Groups were compared by one-way ANOVA (α = 0.05). Results are stated as average ± standard deviation.Introduction
Methods
Smaller increments in the antero-posterior dimensions of femoral components allows significant improvements in balancing of the knee after TKA with restoration of more normal soft-tissue stability. The soft-tissue stability of the knee after TKA is often compromised by the fact that only a finite set of implantable component sizes is available to match bony anatomy. While this could be overcome with custom components, a more practical solution is a set of femoral components with smaller increments in the antero-posterior (AP) dimension. However, this results in a larger assortment of sizes of both implants and trial components. This study was performed to determine whether smaller increments in the AP sizing of knee prostheses would lead to real benefits in restoration of normal knee function and stability after TKA.Summary:
Introduction:
