Total hip arthroplasty (THA) is a commonly performed procedure to relieve arthritis or traumatic injury. However, implant failure can occur from implant loosening or crevice corrosion as a result of inadequate seating of the femoral head onto the stem during implantation. There is no consensus—either by manufacturers or by the surgical community—on what head/stem assembly procedure should be used to maximize modular junction stability. Furthermore, the role of “off-axis” loads—loads not aligned with the stem taper axis—during assembly may significantly affect modular junction stability, but has not been sufficiently evaluated. The objective of this study was to measure the three-dimensional (3D) head/stem assembly loads considering material choice—metal or ceramic—and the surgeon experience level.Introduction
Objective
Studies have shown that increased implant conformity in total knee arthroplasty (TKA) has been linked to increased constraint and thus rotational torque at the bone/implant interface. Anterior stabilized (AS) tibial inserts were designed to compensate for excessive AP motion in less-conforming cruciate-retaining (CR) tibial inserts. However, increased constraint may affect implant loading. Therefore, the purpose of this study is to model rotational prosthesis constraint based on implant-specific data and to compare rotational torque and 3D contact forces in implants with CR-lipped and AS tibial inserts during normal gait. A previously reported knee joint contact model was updated to include rotational torque due to prosthesis constraint (ASTM F1223(14)). Piecewise multiple linear regression with manually selected cutoff points was used to determine estimates of AP force, ML force, and rotation torque as functions of AP displacement, ML displacement, knee external rotation, respectively, and knee flexion angle from standard data. These functions were used to estimate total moment contribution of the prosthesis from measured knee displacement/rotation angles. Estimates were incorporated into the contact model equilibrium equations as needed by the model. As the model parametrically varies muscle activation coefficients to solve for the range of physiologically possible forces at each time point, the reported force/torque values are the mean across all solutions at each time point. Rotational torque and three dimensional contact forces were calculated for 14 informed-consented subjects, five with AS tibial inserts (1/4 m/f, 67±10 years, 29.2±4.4 BMI, 1/4 right/left) and nine with CR-lipped TKRs (2/7 m/f, 64±6 years, 30.6±5.8 BMI, 4/5 right/left). Rotational torque waveforms were compared using statistical nonparametric mapping; 3D contact forces were compared at mean timing of the flexion/extension moment peaks using independent samples t-tests.Introduction
Methods
Improper seating during head/stem assembly can lead to unintended micromotion between the femoral head and stem taper—resulting in fretting corrosion and implant failure. There is no consensus—either by manufacturers or by the surgical community—on what head/stem taper assembly method maximizes modular junction stability in total hip arthroplasty (THA). A 2018 clinical survey found that orthopedic surgeons prefer applying one strike or three, subsequent strikes when assembling head/stem taper. However, it has been suggested that additional strikes may lead to decreased interference. Additionally, the taper surface finish—micro-grooves—has been shown to affect taper interference and may be influenced by assembly method. The objective of this study was to employ a novel, micro-grooved finite element (FEA) model of the hip taper interface and assess the role of head/stem assembly method—one vs three strikes—on modular taper junction stability.Introduction
Objective
Modular junctions in total hip replacement (THR) have been a primary source of fretting and corrosion which can lead to implant failure. Fretting is a result of unintended micromotion between the femoral head and stem tapers and is suspected to result after improper taper seating during assembly. Two design factors known to influence in-vitro taper assembly mechanics are relative taper alignment—mismatch angle—and the surface finish—micro-grooves. However, these factors have not been systematically evaluated together. The objective of this study was to employ a novel, micro-grooved finite element (FEA) model of the hip taper interface and assess the role of taper mismatch angle and taper surface finish—smooth and rough—on the modular junction mechanics during assembly.Introduction
Objective
The lifetime of total hip replacements (THR) is often limited by adverse local tissue reactions to corrosion products generated from modular junctions. Two prominent damage modes are the imprinting of the rougher stem topography into the smoother head taper topography (imprinting) and the occurrence of column-like troughs running parallel to the taper axis (column damage). It was the purpose of this study to identify mechanisms that lead to imprinting and column damage based on a thorough analysis of retrieved implants. 776 femoral heads were studied. Heads were visually inspected for imprinting and column damage. Molds were made of each head taper and scanned with an optical coordinate measuring machine. The resulting intensity images were used to visualize damage on the entire surface. In selected cases, implant surfaces were further analyzed by means of scanning electron microscopy (SEM) and white light interferometry. The alloy microstructure was characterized for designs from different manufactures.INTRODUCTION
METHODS
Studies of retrieved TKR components demonstrate that Eleven retrieved ultra-high molecular weight polyethylene (UHMWPE) cruciate-retaining tibial liner components from ten separate patients (implantation time = 8.6±5.6 years) had matching gait trials of normal level walking for each knee. Volume loss on retrieved components was calculated using a coordinate measuring machine and autonomous reconstruction. Motion analysis of normal level walking gait had been conducted between 1986 and 2005 for various previous studies and stored in a consented Human Mechanics Repository, ranging from pre-operative to long-term post- operative testing. Contact location between the femoral component and the tibial component on the medial and lateral plateaus were calculated throughout stance. A previously validated and fine-tuned parametric numerical model was used to calculate TKR contact forces for each gait trial. Vertical contact forces and contact paths on the medial and lateral plateaus were input as normal force and sliding distance to a simplified Archard equation for wear with material wear constant averaged from literature (2.42 × 10−7 mm3/Nm) to compute average wear per gait cycle. Wear rates were calculated using linear regression, and Pearson correlation examined correlations between modeled and measured wear.INTRODUCTION
METHODS
Generic walking profiles applied to mechanical knee simulators are the gold standard in wear testing of total knee replacements (TKRs). Recently, there was a change in the international standard (ISO) for knee wear testing (ISO 14243-3): the direction of motion in the anterior/posterior (AP) and internal/external (IE) directions were reversed. The effects of this change have not been investigated, therefore it is not known whether results generated by following this new standard can be compared to historical wear tests which used the old standard. Using a finite element analysis (FEA) model of a TKR in parallel with an energy based wear model and adaptive remeshing, we investigated differences in wear between the newest ISO standard developed in 2014, and the previous ISO standard developed in 2004. CAD models of a left sided NexGen Cruciate Retaining (CR) TKR (Zimmer, Warsaw, IN) were used to create the FEA model (Figure 1). The loads and motions specified by simulator standards ISO 14243-3(2004) and ISO 14243-3(2014) were applied to the model. Analyses were run using ABAQUS v6.13-2 Standard (Dassault Systèmes, Waltham, MA). 8 node hexahedral elements were used to model the UHMWPE component. The contact was modeled as penalty contact, with the friction coefficient set to 0.04 on the articular surface. The cobalt chromium molybdenum femoral component was modeled as a rigid surface, utilizing a mix of 2nd order quadrilaterals and tetrahedrons. Wear of the polyethylene (PE) component was predicted to 1,000,000 cycles using a previously published frictional energy-based wear model. The wear model, developed from data generated in wheel-on-flat tests, utilizes two parameters defining the frictional energy required to remove a unit volume of material both parallel (3.86E8 J/mm3) and perpendicular (3.55E7 J/mm3) to the primary polyethylene fibril direction. Primary fibril direction for the analysis was set to the AP direction. Wear for each simulation of a gait cycle was scaled to 500,000 cycles. Two gait cycles were simulated representing 1,000,000 cycles in total. Adaptive remeshing was driven by the wear model, with the mesh being updated every time increment to simulate material ablation. The time step size was variable with a maximum of 0.01s. The FEA predicted higher wear rates for the newest ISO standard (7.34mg/million cycles) compared to the previous standard (6.04mg/million cycles) (Figure 2). Comparing the predicted wear scars (Figure 3), the new version of the standard covered a larger percentage of the total articular surface, with wear being more spread out as opposed to localized. This is more similar to what is seen in patient retrievals. The results of the study suggest that major differences between the old and the new ISO standard exist and therefore historical wear results are not comparable to newly obtained results. In addition, this study demonstrates the utility of FEA in wear analysis, though the wear model needs further work and validation before it can be used as a supplement to simulator testing. Validation of the wear model against simulator tests and pin-on-disk experiments is currently underway.
Failure of total knee replacements due to the generation of polyethylene wear debris remains a crucial issue in orthopedics. Unlike the hip, it is difficult to accurately determine knee implant wear rates from retrieved components. Several studies have relied on thickness measurements to estimate penetration, but the complicated geometry of contemporary tibial liners poses a challenge to accurately assess wear. In this study we address the question whether linear penetration can serve as a surrogate measure for volumetric material loss. Eighty-one retrieved UHMWPE NexGen cruciate-retaining tibial liners (Zimmer, Warsaw, IN) with an average time in situ of 5.27±2.89 years were included in the study. Metrology data for the surfaces of the tibial liners were obtained with a coordinate measuring machine (OGP, Rochester, NY). Using a laser scanner with two micrometer depth accuracy, at least 400,000 measurement points were taken by investigator #1. Areal thickness changes were mapped for the lateral and medial sides with the help of an autonomous mathematical reconstruction algorithm and volume loss was calculated based on wear scar area and local thickness change. Investigator #2, blinded from these results, measured the minimum thickness of the medial and lateral tibial plateau using a dial indicator with a spherical tip radius of 3mm. Twenty-three short term retrievals (3 to 4 per implant size), removed due to infection and without any signs of wear, served as “unused” reference. Linear penetration was then calculated by subtracting the minimum thickness of each plateau from the average thickness of the reference components.Introduction
Methods
