Joshua E. Johnson

Barefoot plantar pressure assessment commonly involves discrete regional sampling from maximum pressure distributions measured over the stance phase of gait. This approach fails to capture spatiotemporal variation in plantar pressures across a spectrum of foot morphologies. Here we introduce a framework for systematically evaluating spatiotemporal plantar pressure distributions. Plantar pressures measured over stance are linearly interpolated into 5%-time instance pressure distributions. The foot progression angle is determined using the maximum pressure distribution and used to rotate each interpolated 5%-time instance pressure distribution. A bounding region capturing the entiret maximum pressure distribution is used to scale each 5% distribution to a standardized 50 x 18 grid using a force preserving approach. Reliability was assessed in individuals with structurally altered (n=10) and healthy (n=10) feet. Intraclass Correlation Coefficients (ICCs) were calculated for foot progression angle, minimum detectable change (MDC) for maximum pressure at each grid location, and a linear mixed effects model with Bland-Altman plots for mean pressures between original and aligned intervals. All ICC values were greater than 0.85 with an MDC of 9.05 ± 10.79 kPa across the entire grid. The linear mixed effect model and Bland-Altman plots demonstrated a significant systematic framework bias of 9.91 ± 2.05 kPa (p<0.001) and 8.58 ± 1.88 kPa (p<0.001) for healthy and structurally altered feet, independent of stance interval, foot size, or foot progression angle. These data suggest the framework can be used to reliably assess plantar pressures across varied foot morphologies, enabling robust comparison of spatial pressure distributions during gait.

Arthroplasty is most often performed to alleviate pain and restore function in patients with end‐stage degenerative joint disease. Uncemented implant fixation is increasingly used in total knee and total ankle arthroplasty. Implantation using interference press‐fit is a manufacturer‐recommended guideline for achieving stable primary fixation in uncemented applications, which is important to prevent long‐term implant failure due to aseptic loosening. However, when evaluating implant–bone interfacial mechanics, many studies have not modeled press‐fit implantation. This can lead to gross underestimation of primary implant fixation stability, limiting the clinical applicability of findings. The goal of this paper is to highlight the importance of simulating press‐fit implantation when evaluating primary orthopedic implant stability using finite element analysis in uncemented arthroplasty. Experiences gained in modeling press‐fit implantation in total knee and total ankle arthroplasty by two different active research groups are shared in this context.

Background: Primary implant stability is important for successful outcomes after uncemented total ankle replacement (TAR). However, the influence of patient-specific bone density on TAR performance is poorly understood, especially for implants that rely on press-fit for stable primary fixation. Our goal was to evaluate how bone density influences implant-bone interfacial micromotions in 3 press-fit tibial component designs by sampling from a TAR preoperative planning database using finite element analysis (FEA). Methods: FEA was conducted in 4 TAR patients with relatively low-density (n = 2, lowest 10% of a sample including 58 patients) and average-density (n = 2, midrange of sample) bone as assessed from deidentified patient CT scans. Three tibial implant designs were evaluated: a bone-sparing resurfacing implant, a cortex-sparing anterior approach monoblock stemmed implant, and a distal-reaming modular stemmed implant. Implants were inserted into tibia geometries obtained from the CT scans. Press-fit implantation was modeled first, followed by loadings from the stance phase of gait, and the associated micromotions were computed from the FEA output. Results: In general, patients with average-density bone had FEA predicted lower micromotions than patients with low-density bone. FEA suggests that implant fixation features had less influence on micromotions in patients with average-density bone, with peak micromotions ranging from 2 to 23 µm (3.1 ± 1.3 µm average micromotion). For patients with low-density bone, interfacial regions are predicted to experience micromotions exceeding the bony ingrowth threshold of 50 µm only for the resurfacing implant. Conclusion: We investigated the influence of bone density on implant-bone micromotions with varying primary fixation features using FEA. The model predicts that micromotions are less in average-density bone, regardless of implant fixation features. However, both stemmed devices showed lower micromotions in less-dense bone, albeit with the corresponding clinical trade-off of requiring more tibial bone removal. Clinical Relevance: The results presented here implicate the complementary role that local bone density plays in the primary fixation stability of uncemented TAR.

The objective of this work was to determine the effects of using simplified finite element (FE) mesh geometry in the process of performing reverse iterative fitting to estimate cartilage material parameters from in situ indentation testing. Six bovine tibial osteochondral explants were indented with sequential 5 % step-strains followed by a 600 s hold while relaxation force was measured. Three sets of porous viscohyperelastic material parameters were estimated for each specimen using reverse iterative fitting of the indentation test with (1) 2D axisymmetric, (2) 3D idealized, and (3) 3D specimen-specific FE meshes. Variable material parameters were identified using the three different meshes, and there were no systematic differences, correlation to basic geometric features, nor distinct patterns of variation based on the type of mesh used. Implementing the three material parameter sets in a separate 3D FE model of 40 % compressive strain produced differences in von Mises stresses and pore pressures up to 25 % and 50 %, respectively. Accurate material parameters are crucial in any FE model, and parameter differences influenced by idealized assumptions in initial material property determination have the potential to alter subsequent FE models in unpredictable ways and hinder the interpretation of their results.

The success of uncemented total ankle replacement (TAR) is linked to initial stability because bony ingrowth depends upon limited early micromotion. Tibial implant design fixation features resist micromotion aided by bony sidewall retention and interference fit. Our goal was to investigate factors influencing implant‐bone micromotion in TAR. Two TAR tibial components were virtually inserted into CT‐derived computer models of two distal tibias from patients with end‐stage ankle arthritis. Density‐based inhomogeneous material assignment was used to model bone compaction during press‐fit. Finite element analysis (FEA) was used to simulate three fixation cases: (1) no sidewalls + line‐to‐line fit, (2) sidewalls + line‐to‐line fit, and (3) sidewalls + 50, 100, or 200 µm interference fit. Kinetic profiles from the stance phase of gait were simulated and micromotions computed from FEA output. Without sidewalls or interference fit, micromotions were largest in early and late stance, with largest micromotions (averaging ~150–250 µm) observed near heel strike. Micromotions decreased 39%–62% when sidewalls were retained. When interference fit was also modeled, micromotions decreased another 37%–61% to ~10 µm. Micromotion differences between patients persisted with sidewall retention but largely disappeared with interference fit. This study presents new insights into the effects of TAR fixation features on implant‐bone micromotion. Stability appeared to be influenced by surrounding bone quality, but this influence was greatly diminished when interference fit was introduced. More complete understanding of TAR implant features and performance is needed, but our results show the importance of bone quality and interference fit in the stability of uncemented TAR.

Many patients with metastatic bone disease (MBD) of the femur undergo prophylactic surgical fixation for impending pathologic fractures; intramedullary nailing (IMN) being the most common fixation type. However, surgeons often question if IMN fixation provides sufficient improvements in mechanical strength for particular metastatic lesions. Our goal was to use patient-specific finite element (FE) modeling to computationally evaluate the effects of simulated IMN fixation on the mechanics of femurs affected with MBD. Computed tomography (CT) scans were available retrospectively from 48 patients (54 femurs) with proximal femoral metastases. The CT scans were used to create patient-specific, non-linear, voxel-based FE models of the femur, simulating the instant of peak hip joint contact force during normal walking. FE analyses were repeated after incorporating virtual IMN fixation (Smith and Nephew, TRIGEN INTERTAN) into the same femurs. Femur strength and load-to-strength ratio (LSR; lower LSR indicates lower fracture risk) were compared between untreated and IMN conditions using statistical analyses. IMN fixation resulted in a very modest average 10% increase in mechanical strength (p<0.001), which was associated with a slight 7% reduction in fracture risk (p10%), IMN hardware directly passed through a considerable section of that femur's metastatic lesion. Femurs with lytic (10%) and diffuse (9%) metastases tended to have greater reductions in fracture risk compared to femurs with blastic (5%) and mixed (4%) metastases (p=0.073). Given the mechanically strong baseline condition of most femurs in this cohort, evident by the low fracture risk at the time of CT scanning, the relative increase in stiffness with the addition of the IMN hardware may not make a substantial contribution to overall mechanical strength. The mechanical gains of IMN fixation in femurs with MBD appear most beneficial when the hardware traverses an adequate section of the lesion. Level of Evidence: III.

Category: Ankle Arthritis Introduction/Purpose: Patient outcomes and construct longevity after total ankle replacement (TAR) are both linked to initial implant stability. Limited early micromotion between the implant and bone encourages stability. Tibial component design fixation features play a critical role in determining the initial stability, with retention of medial and lateral bone sidewalls and interference press-fit used to supplement fixation. The goal of this study was to characterize micromotion between the tibia and baseplate using finite element analysis (FEA) to determine how baseplate design features influence stability. Methods: Three commercially available TAR tibial component baseplate designs were virtually inserted into a computer model of the distal tibia of an end-stage ankle arthritis patient (Fig. 1a). Implant insertion was simulated using FEA for each tibial component with two different fixation cases: (1) no sidewalls+line-to-line fit and (2) sidewalls retained+50µm interference press-fit. Tibia models were derived from patient CT scans, with element-by-element material properties assigned based upon CT Hounsfield units. The tibial components were assigned titanium alloy properties. Following simulated implantation, FEA was performed using kinematic and kinetic profiles representing the stance phase of gait. Loads/moments scaled to patient body weight were applied to the implant (Fig. 1b, 1c), while the proximal tibia was held fixed. Micromotions were defined as the displacement difference between bone-implant closest nodal pairs, computed from FEA output using custom-written code. Results: Amongst tibial component designs inserted without sidewalls and with line-to-line fit, relatively large micromotions were observed. Micromotion differences at heel strike between Peg Designs 1 and 2 may be attributable to sidewall height, while Peg Design 3 was less susceptible to the proximal forces and dorsiflexion moments dominant in early stance (Fig. 1d), potentially because of its angled anterior pegs. Micromotions in Peg Design 3 were largest near toe off, where the plantarflexion moment and decreased proximal force may have permitted motion along the pegs. Differences in micromotion between the different implant designs over the stance phase of gait were greatly reduced when interference press-fit was modeled (Fig. 1e). Conclusion: This study provides new insight on the relative initial stability afforded by different TAR fixation features across designs. In the absence of interference press-fit, all designs showed micromotions large enough to be concerning. However, when interference press-fit was modeled, all designs performed similarly in terms of initial stability during simulated gait. While a better understanding of the relationships between TAR implant features, patient bone density profiles, and initial stability is still needed, our findings suggest that implant interference press-fit plays an important and dominant role in device performance.

Background and objectives: Femurs affected by metastatic bone disease (MBD) frequently undergo surgery to prevent impending pathologic fractures due to clinician-perceived increases in fracture risk. Finite element (FE) models can provide more objective assessments of fracture risk. However, FE models of femurs with MBD have implemented strain- and strength-based estimates of fracture risk under a wide variety of loading configurations, and “physiologic” loading models typically simulate a single abductor force. Due to these variations, it is currently difficult to interpret mechanical fracture risk results across studies of femoral MBD. Our aims were to evaluate (1) differences in mechanical behavior between idealized loading configurations and those incorporating physiologic muscle forces, and (2) differences in the rankings of mechanical behavior between different loading configurations, in FE simulations to predict fracture risk in femurs with MBD. Methods: We evaluated 9 different patient-specific FE loading simulations for a cohort of 54 MBD femurs: strain outcome simulations—physiologic (normal walking [NW], stair ascent [SA], stumbling), and joint contact only (NW contact force, excluding muscle forces); strength outcome simulations—physiologic (NW, SA), joint contact only, offset torsion, and sideways fall. Tensile principal strain and femur strength were compared between simulations using statistical analyses. Results: Tensile principal strain was 26% higher ( R ² = 0.719, P < .001) and femur strength was 4% lower ( R ² = 0.984, P < .001) in simulations excluding physiologic muscle forces. Rankings of the mechanical predictions were correlated between the strain outcome simulations (ρ = 0.723 to 0.990, P < .001), and between strength outcome simulations (ρ = 0.524 to 0.984, P < .001). Conclusions: Overall, simulations incorporating physiologic muscle forces affected local strain outcomes more than global strength outcomes. Absolute values of strain and strength computed using idealized (no muscle forces) and physiologic loading configurations should be used within the appropriate context when interpreting fracture risk in femurs with MBD.