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
Removal of primary components during revision TKA procedure can damage underlying bone, resulting in defects that may need filled for stability of the revision reconstruction. Special revision components including cones and/or augments are often used to compensate for the missing bones. Little work has been done to characterize metaphyseal geometry in the vicinity of the knee joint, however, in order to motivate proper size and shape of cones and augments. The objective of this study was to use statistical shape modelling to evaluate variation in endosteal anatomy for revision TKA. Digital models of the femur and tibia were generated through segmentation of computed tomography scans, for the femur and the tibia (n∼500). Custom software was used to perform virtual surgery and statistical shape analysis of the metaphyseal geometry. A representative and appropriately sized revision femoral component was placed on each bone, assuming anterior referencing with an external rotation of 3 degrees from the posterior condyle axis. The outer and inner boundaries of the cortical bone were determined at the resection level and at 5 mm increments proximally, up to 40 mm. Similar analyses were performed on the tibia, using a typical revision resection (0 degrees medial and posterior slope), with outer and inner boundaries of the cortical bone were determined in 5 mm increments up to 40mm distal to the resection. Metaphyseal contours were exported relative to the central fixation feature of the implant, and average geometries were calculated based on size, and across the entire cohort. Principal Component Analysis (PCA) was used to quantify the variability in shape, specifically to evaluate the +/− 1 and 2 standard deviation geometries at each cross section level of Principal Component 1 (PC1).Introduction
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Total ankle replacement (TAR) success has improved since first-generation implants, but patient satisfaction continues to be less than knee and hip replacements. Little is known about variations in distal tibia anatomy between genders and across ethnicities; therefore it is unclear the extent to which current TAR prostheses accommodate variability in patient size and shape. This study quantified distal tibia morphometrics relevant to TAR design, and assessed differences between ethnicities and genders. The hypotheses were: (1) The anterior-posterior (AP) location of the dwell point of the tibia is centralized; (2) The sagittal radius of curvature of the tibial articulation increases with bone size; (3) Differences in dwell point location or sagittal radii between genders and ethnicities can be attributed to size differences between those populations. Tibial CT scans were obtained from cadavers or individuals of various ethnicities (Table 1). Landmarks were defined on digital models created from the scans, including medial and lateral edges of the distal tibial articulation (Figure 1a), and sagittal contours of the articulation (Figure 1b). The articulation center was defined as the average center point of all contours (Figure 1c). The AP center and AP length at the level of a distal tibial resection for TAR were determined, and the AP offset of the articulation center was calculated (Figure 1c). Differences in metrics for each ethnic and gender group were determined using a one-way Anova (P<.05) with Tukey's method for differentiating groups. Regression fits of AP offset, average medial radius, and average lateral radius were determined. Utilizing AP length as a covariate, ANCOVA was utilized to assess differences in AP offset and sagittal radii between gender and ethnic groups (P<.05).Introduction/Purpose
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Aseptic loosening is the most common failure mode for Total Elbow Arthroplasty (TEA) and is considered to be associated with accelerated polyethylene bearing wear [1, 2]. This study aimed to evaluate three commercially available implant designs under loads associated with daily living. The hypothesis was that more recent designs (Discovery and Nexel) provide greater articular contact areas resulting in lower polyethylene stresses compared to the Coonrad/Morrey (CM). Motion tracking was performed on a healthy volunteer during elbow flexion at 0, 45, and 90° shoulder abduction because most daily activities occur with some shoulder abduction [3] resulting in varus stress about the elbow. This kinematic data was used in an OpenSim upper extremity musculoskeletal model [4] to estimate muscle and joint reaction loads with 5lb in hand, consistent with the common clinical restrictions following TEA. Computer aided assemblies of the smallest size implants for each system were imported to ANSYS for finite element analysis. Metallic components were treated as rigid and polyethylene components were modeled using a nonlinear elastoplastic constitutive model calibrated to material data. Articular contacts were frictional. Physiologic joint reaction forces and moments quantified in OpenSim were applied and the resulting peak articular contact area and peak bearing von Mises stresses were assessed.INTRODUCTION
METHODS
Inadequate stability of the baseplate is a leading cause of revision within reverse total shoulder arthroplasty (rTSA). Micromotion between baseplate and bone is commonly used as a pre-clinical indicator for clinical stability (ASTM F2028-14). Finite element analysis (FEA) has been shown to accurately predict baseplate-bone micromotion, but results may be critically dependent on several modeling assumptions. Here, FEA was used to assess the impact of key modeling assumptions related to screw-bone interactions on various rTSA configurations. FEA with Ansys ver. 16 was used to simulate a fixation experiment. Baseplates of two different sizes (25mm and 28mm diameter), each with a central screw and four peripheral screws, were virtually implanted in a synthetic bone block. Each baseplate was analyzed using 1.5mm and 3.5mm superior-inferior (SI) offsets of the glenosphere center, as well as using four (‘4S’) and two (‘2S’) peripheral screws. A clinically relevant loading of 756N was applied in compression as well as in inferior-to-superior shear direction through the glenosphere (Figure 1A, 1B). Screw-bone block interactions were modeled in three different ways: (1) Threads were defeatured from the peripheral screws, which were bonded to the bone block (b-nt); (2) Threads were modeled, while still assuming bonded contact (b-t); (3) Threads were modeled, with frictional contact between threads-bone block (f-t). Micromotion results (Figure 1C) from all 24 simulations (3 screw-bone interactions × 2 baseplate diameters × 2 SI offsets × 2 screw configurations) were compared.Introduction
Methods
Good outcomes in reverse shoulder arthroplasty (RSA) rely in part on stability of the humeral component. Traditionally humeral components have been cemented, however there has been recent interest in press-fit fixation of humeral components in RSA. Lateralization of the head center in RSA can impart larger moments on the humeral component than for anatomic reconstructions, increasing the importance of distal humeral canal preparation for implant stability. To date, the primary stability of any type of press-fit humeral prosthesis has been largely unexplored. The goal of this study is to evaluate the effect of over-reaming the distal humeral canal in a press-fit humeral component in RSA. Computed tomography (CT) data of the shoulder were obtained from 55 shoulders. Images were segmented to produce digital models of the humerus. Humeral components for RSA (2mm diameter size increments) were sized and placed per the surgical technique, including preparation of the humerus with the appropriate reamers (1mm increments). Finite element models for each specimen were created with heterogeneous bone properties derived from the CT scan. Pressfit between the bone and stem was resolved to quantify the initial contact pressure on the stem; each stem was then loaded at 566N oriented 20° lateral and 45° anterior. Overall motion of the stem was measured, as well as interfacial micromotion in the porous coating region (Fig. 1). The effect of line-to-line (L2L) reaming and over-reaming by 1 mm was evaluated using an unpaired Student's t-test, with significance defined at p<0.05.Introduction
Methods
Comprehensive research and retrieval analyses of metal on metal / metal on polyethylene hip fretting and corrosion have been reported. Design choices such as modularity, material couples, geometry and offsets, as well as surgical variability and patient sensitivity have been cited as factors contributing to revision. Findings are informing new designs, surgical techniques and patient testing. However, similar efforts have not been performed on the shoulder. Do reduced joint reaction forces imply lower risk of fretting and corrosion? In this study we designed an accelerated corrosion fatigue (ACF) test specific for the shoulder to allow for evaluation of varying designs, and compared results to a reported shoulder retrieval study [Day ORS 2015]. Anatomic configuration and reverse shoulder ACF tests were developed with loads and orientations determined from instrumented shoulder data and reported literature. Scaled loads of 1480 N and 962 N were applied to anatomic (Fig 1.A) and reverse (Fig 1.B) prostheses, respectively (n=5 each, with additional assembly control), in potential worse case loading directions (α=25°, β=20°: anatomic; α=0°, β=0°: reverse), at 5 Hz for 3.0 Mc with R=0.1. Test environment included 0.9% NaCl solution at elevated temperature (50° C) and a decreased pH (3.5). Mass, roughness (Ra) and taper damage (modified Goldberg scoring system) measures were taken before and after testing. Taper connections were assembled at impact loads of 3600 N +/− 20% based on cadaveric studies. Goldberg scores for 79 humeral heads and 61 stems from an IRB approved collection served as the comparator.Introduction
Methods
During total knee arthroplasty (TKA), the pursuit of accurate alignment, proper bone cuts, and good soft tissue balancing sometimes can result in the overhang of the femoral component, especially in smaller-sized Asian knees. As size and shape of the distal femur are highly variable, component designs that offer increased shape and size offerings may be desirable to fit the distal femur. This study tested the hypothesis that increased shape and size offerings in TKA femoral designs may improve their fit to the Japanese femur compared to designs that offer only one shape and limited sizes. Five contemporary femoral component designs were evaluated (Designs A-E). Design A has multiple mediolateral (ML) size offerings for a specific component anteroposterior (AP) size, and the finest increment (2mm) in AP sizes among all the designs. Designs B-E have single ML offerings across component AP sizes. For each design, virtual TKA resections were performed on the digital surfaces of 82 Japanese distal femora, each sized by selecting the component AP size that most closely matched but did not exceed the femoral AP dimension (Fig 1A,B). The aspect ratio (ML/AP) of the resected femora was regressed against the aspect ratio of their properly sized components per design. The closeness of each design to the perfect shape match was evaluated by the root-mean-square deviation (RMSD) of the deviations between the femoral bone and components. Differences in ML dimensions (overhang/underhang) between component and resected femora were calculated (Fig1C,D). The incidence of clinically significant femoral overhang (>3mm), in which component downsizing is required, were analyzed.INTRODUCTION
METHODS
Balancing accurate rotational alignment, minimal overhang, and good coverage during total knee arthroplasty (TKA) often leads to compromises in tibial component fit, especially in smaller-sized Asian knees. This study compared the fit and surgical compromise between contemporary anatomic and non-anatomic tibial designs in Japanese patients. Size and shape of six contemporary tibial component designs (A:anatomic, B:asymmetric, C-F:symmetric) were compared against morphological characteristics measured from 120 Japanese tibiae resected following TKA surgical technique. The designs were then digitally placed on the resected tibiae. Each placement selected the largest possible component size, while ensuring <1mm overhang and proper alignment (within 5° of neutral rotational axis). When a compromise on either alignment or overhang was required (due to smaller-sized component unavailable), the design was flagged as “no suitable component fit” for that bone. Tibial coverage was compared across designs. Next, 32 femora were randomly selected from the dataset onto which each design was evaluated in two placements, the first maximizing coverage without attention to rotation and the second enforcing rotational accuracy. Downsizing was identified if in the second placement, enforcing rotational accuracy, required a smaller component size compared the first placement. The degree of mal-alignment while maximizing coverage, the incidence of downsizing, and difference in coverage between the two placements were compared across designs. Statistical significance was defined at p<0.05.INTRODUCTION
METHODS
Successful tibial component placement during total knee arthroplasty (TKA) entails accurate rotational alignment, minimal overhang, and good bone coverage, each of which can be facilitated with a tibial component that matches the resected tibial surface. Previous studies investigated bony coverage of multiple tibial component families on digitized resections. However, these studies were based on manual placement of the component that may lead to variability in overhang and rotational alignment. An automated simulation that follows a consistent algorithm for tibial component placement is desirable in order to facilitate direct comparison between tibia component designs. A simulation has been developed and applied to quantify tibial coverage in multiple ethnicities, including Japanese, Indian, and Caucasian. Here, this approach is taken to evaluate tibial coverage of five contemporary tibial designs in Chinese subjects. Digital models of 100 healthy Chinese tibiae (50 male, 50 female; age 68 ± 3 years; stature 1.65 ± 0.10 m) were virtually resected at 5° posterior slope referencing the anterior border of the proximal tibia, 0° varus/valgus rotation referencing the tibial mechanical axis, and 8 mm off the unaffected plateau (reflecting a 10 mm surgical cut, assuming a cartilage thickness of 2 mm). Neutral internal/external (I/E) alignment axis was derived from the medial third of the tubercle and the PCL attachment site. Five commercial tibial designs (Design A, Deluxe™, Montagne, Beijing, China; Designs B-E contemporary market-established symmetric designs from four US manufacturers) were virtually placed on the resected tibiae following an automated algorithm, which maximizes component size while ensuring proper rotational alignment (within 5° I/E) and minimizing overhang (<1 mm in zones 1–4, Fig 1). Tibial coverage (posterior notch excluded, zone 5 in Fig 1) and distance from the component to the exterior cortex of the tibia in four clinically relevant anatomical zones (anterior medial, anterior lateral, posterior medial, and posterior lateral, zones 1–4, Fig 1) were calculated. Statistical significance was defined at p < 0.05.INTRODUCTION:
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Tibial components that match the resected proximal tibia may promote accurate rotational alignment and maximize coverage while minimizing overhang in total knee arthroplasty (TKA). Tibial component designs have traditionally been evaluated utilizing an overall anterior-posterior (AP)/medial-lateral (ML) ratio. However, since the tibial plateau is irregularly shaped, such a metric has drawbacks. Here, a detailed set of morphological metrics is used to evaluate six contemporary tibia designs against a multi-ethnic bone database. Tibial surfaces from 347 subjects, including 97 Indian (50m/47f), 99 Japanese (44m/55f), and 151 Caucasian (85m/66f), were virtually resected following a specific TKA procedure, as previous publications have shown surgical variability minimally impacts tibial resection morphology. Medial and lateral AP dimensions (MAP and LAP), ML width (ML), and medial and lateral anterior radii (MAR and LAR) were measured in a coordinate system constructed on the resected surface based on the neutral rotational axis (Fig. 1A). These metrics, along with anterior radius asymmetry (MAR/LAR), were regressed against ML for each ethnicity. The regressions were then compared with similar measurements obtained from tibial components in six contemporary TKA systems (one asymmetric: Design A; four symmetric: Designs B-E; and one anatomic: Design F).Introduction
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