Analysis of bone-prosthesis interface micromotion for cementless tibial prosthesis fixation and the influence of loading conditions

Optimizing functionally graded tibial components for total knee replacements: a finite element analysis and multi-objective optimization study

The optimal design of complex engineering systems requires tracing precise mathematical modeling of the system's behavior as a function of a set of design variables to achieve the desired design. Despite the success of current tibial components of knee implants, the limited lifespan remains the main concern of these complex systems. The mismatch between the properties of engineered biomaterials and those of biological materials leads to inadequate bonding with bone and the stress-shielding effect. Exploiting a functionally graded material for the stem of the tibial component of knee implants is attractive because the properties can be designed to vary in a certain pattern, meeting the desired requirements at different regions of the knee joint system. Therefore, in this study, a Ti6Al4V/Hydroxyapatite functionally graded stem with a laminated structure underwent simulation-based multi-objective design optimization for a tibial component of the knee implant. Employing finite element analysis and response surface methodology, three material design variables (stem's central diameter, gradient factor, and number of layers) were optimized for seven objective functions related to stress-shielding and micro-motion (including Maximum stress on the cancellous bone, maximum and mean stresses on predefined paths, the standard deviation of mean stress on paths, maximum and mean micro-motions at the bone-implant interface and the standard deviation of mean micro-motion). Then, the optimized functionally graded stem with 6 layers, a central diameter of 5.59 mm, and a gradient factor of 1.31, was compared with a Ti6Al4V stem for various responses. In stress analysis, the optimal stem demonstrated a 1.92% improvement in cancellous bone stress while it had no considerable influence on the maximum, mean, and standard deviation of stresses on paths. In micro-motion analysis, the maximum, mean, and standard deviation of mean micro-motion at the interface were enhanced by 24.31%, 39.53%, and 19.77%, respectively.

Passive Biotelemetric Detection of Tibial Debonding in Wireless Battery-Free Smart Knee Implants

Aseptic loosening is the dominant failure mechanism in contemporary knee replacement surgery, but diagnostic techniques are poorly sensitive to the early stages of loosening and poorly specific in delineating aseptic cases from infections. Smart implants have been proposed as a solution, but incorporating components for sensing, powering, processing, and communication increases device cost, size, and risk; hence, minimising onboard instrumentation is desirable. In this study, two wireless, battery-free smart implants were developed that used passive biotelemetry to measure fixation at the implant-cement interface of the tibial components. The sensing system comprised of a piezoelectric transducer and coil, with the transducer affixed to the superior surface of the tibial trays of both partial (PKR) and total knee replacement (TKR) systems. Fixation was measured via pulse-echo responses elicited via a three-coil inductive link. The instrumented systems could detect loss of fixation when the implants were partially debonded (+7.1% PKA, +32.6% TKA, both p < 0.001) and fully debonded in situ (+6.3% PKA, +32.5% TKA, both p < 0.001). Measurements were robust to variations in positioning of the external reader, soft tissue, and the femoral component. With low cost and small form factor, the smart implant concept could be adopted for clinical use, particularly for generating an understanding of uncertain aseptic loosening mechanisms.

Quantifying the immediate post-implantation strain field of cadaveric tibiae implanted with cementless tibial trays: A time-elapsed micro-CT and digital volume correlation analysis during stair descent

Primary stability, the mechanical fixation between implant and bone prior to osseointegration, is crucial for the long-term success of cementless tibial trays. However, little is known about the mechanical interplay between the implant and bone internally, as experimental studies quantifying internal strain are limited. This study employed digital volume correlation (DVC) to quantify the immediate post-implantation strain field of five cadaveric tibiae implanted with a commercially available cementless titanium tibial tray (Attune, DePuy Synthes). The tibiae were subjected to a five-step loading sequence (0-2.5 bodyweight, BW) replicating stair descent, with concomitant time-elapsed micro-CT imaging. With progressive loads, increased compression of trabecular bone was quantified, with the highest strains directly under the posterior region of the tibial tray implant, dissipating with increasing distance from the bone-implant interface. After load removal of the last load step (2.5BW), residual strains were observed in all of the five tibiae, with residual strains confined within 3.14 mm from the bone-implant interface. The residual strain is reflective of the observed initial migration of cementless tibial trays reported in clinical studies. The presence of strains above the yield strain of bone accepted in literature suggests that inelastic properties should be included within finite element models of the initial mechanical environment. This study provides a means to experimentally quantify the internal strain distribution of human tibia with cementless trays, increasing the understanding of the mechanical interaction between bone and implant.

Outcomes of Short and Long Tibial Stems for Primary Total Knee Arthroplasty in a Population of Obese Patients at Two-Year Follow-Up: A Clinical and Biomechanical Study

BACKGROUND

Obesity can be a source of higher failure rates and inferior clinical outcomes after total knee arthroplasty (TKA). The aim of this study was to compare outcomes, failure rates, and stress distributions of TKA in obese patients using a short, long, or no tibial stem.

METHODS

A matching process based on the type of stem used and the age allowed included 180 patients who had a body mass index (BMI) > 30 and underwent a TKA between January 2010 and December 2019, with a minimum follow-up of 2 years. They were classified as moderately obese (MO: 30 < BMI < 35, N = 90) and severely obese (SO: BMI > 35, N = 90). For each, 3 subgroups were defined: thirty patients received a 30 mm short stem (SS), thirty received a 100 mm long stem (LS), and thirty received no stem (NS). Patients were assessed preoperatively and postoperatively using the Knee Society Score (KSS). A finite element model was developed to evaluate the biomechanical effects of the tibial stem on stress distribution in the subchondral bone based on BMI.

RESULTS

The SS patients had significantly higher postoperative KSS knee score [MO: 88.9 (SS) versus 79 (LS) versus 80.6 (NS); SO: 84.5 versus 72.4 versus 78.2] (P < .0001) and function score [MO: 90.4 (SS) versus 78.4 (LS) versus 68.5 (NS); SO: 85.5 versus 73 versus 61.8] (P < .0001) compared to LS and NS patients. The biomechanical study demonstrated a BMI-dependent increase in stress in the subchondral bone in contact with the tibial components. These stresses were mainly distributed at the tibial cut for NS and along the stem for SS and LS.

CONCLUSIONS

A short, cemented tibial stem offers better functional outcomes without increasing failure rates compared to a longer stem during primary TKA in a population of obese patients at two-year follow-up. A short tibial stem does not lead to increased stress compared to an LS, at least for certain BMI categories.

A novel computational workflow to holistically assess total knee arthroplasty biomechanics identifies subject-specific effects of joint mechanics on implant fixation

Computational studies of total knee arthroplasty (TKA) often focus on either joint mechanics (kinematics and forces) or implant fixation mechanics. However, such disconnect between joint and fixation mechanics hinders our understanding of overall TKA biomechanical function by preventing identification of key relationships between these two levels of TKA mechanics. We developed a computational workflow to holistically assess TKA biomechanics by integrating musculoskeletal and finite element (FE) models. For our initial study using the workflow, we investigated how tibiofemoral contact mechanics affected the risk of failure due to debonding at the implant-cement interface using the four available subjects from the Grand Challenge Competitions to Predict In Vivo Knee Loads. We used a musculoskeletal model with a 12 degrees-of-freedom knee joint to simulate the stance phase of gait for each subject. The computed tibiofemoral joint forces at each node in contact were direct inputs to FE simulations of the same subjects. We found that the peak risk of failure did not coincide with the peak joint forces or the extreme tibiofemoral contact positions. Moreover, despite the consistency of joint forces across subjects, we observed important variability in the profile of the risk of failure during gait. Thus, by a combined evaluation of the joint and implant fixation mechanics of TKA, we could identify subject-specific effects of joint kinematics and forces on implant fixation that would otherwise have gone unnoticed. We intend to apply our workflow to evaluate the impact of implant alignment and design on TKA biomechanics.

Micromotion measurement at the interfaces of cemented tibial endoprosthetic replacements: A new standardized in vitro model using open-cell rigid foam

Early aseptic loosening following primary total knee arthroplasty related to several factors might appear at the interface implant-cement or cement-bone. A standardized in vitro model might provide information on the relevance of single variable parameter of cementation including technique and cement respectively bone structure on fixation strength. Micromotion measurement using different directions of load should detect the primary stability of the interfaces. An open-cell rigid foam model was used for cementation of PFC-Sigma tibial trays with Palacos®. Pins were applied to the model for continuous non-destructive measurement. Relative micromotions for rotation, valgus-varus and extension flexion stress were detected at the interfaces as well as cement penetration was measured. The reproducibility of the measurement could be shown for all interfaces in extension-flexion movements. For rotation a negative trend was shown for the interface cement-prosthesis and cement-bone concerning varus-valgus stress reflecting varying surgical cementation technique. More micromotion related to extension-flexion force might reflect the design of the implant. Measurement of relative micromotion and cement distribution appear accurate to detect small differences of movement at different interfaces of cemented tibial implants and the results are reproducible for most parameter. An increased number of specimens should achieve statistical relevance for all measurements.

The influence of body weight index on initial stability of uncemented femoral knee protheses: A finite element study

Background and objective

Obesity is one of the risk factors for osteoarthritis. The end-stage treatment for osteoarthritis is total knee arthroplasty (TKA). However, it remains controversial whether a high body mass index (BMI) affects the initial stability of the femoral prosthesis after TKA. Finite element analysis (FEA) was used to investigate this question in this study.

Methods

Four femur models that assembled with TKA femoral components were reconstructed and divided into high BMI group and normal BMI group. The three-dimensional femurs were modeled and assigned inhomogeneous materials based on computed tomography (CT) images. Then each FEA model was applied with gait and deep bend loading conditions to evaluate the maximum principal strain on the distal femur and the relative micromotion between the femur and prosthesis.

Results

The mean strain of the high BMI group increased by 32.7% (936.9 με versus 706.1 με) and 50.9% (2064.5 με versus 1368.2 με) under gait and deep bend loading conditions, respectively, compared to the normal BMI group. Meanwhile, the mean micromotion of the high BMI group increased by 41.6% (2.77 μm versus 1.96 μm) and 58.5% (62.1 μm versus 39.2 μm), respectively. Under gait condition, the maximum micromotion for high BMI group was 33.8 μm and would compromise the initial stability. Under deep bend condition, the maximum strain and micromotion exceeded -7300 με and 28 μm for both groups.

Conclusion

High BMI caused higher strain on the bone and higher micromotion between the prosthesis and the femur. Gait activities could be risky for prosthesis stability in high BMI group while be safe in normal group. Deep bend activities were highly dangerous for both groups with high BMI and normal BMI and should be avoided.

Impact of patient, surgical, and implant design factors on predicted tray-bone interface micromotions in cementless total knee arthroplasty

Micromotion magnitudes exceeding 150 µm may prevent bone formation and limit fixation after cementless total knee arthroplasty (TKA). Many factors influence the tray-bone interface micromotion but the critical parameters and sensitivities are less clear. In this study, we assessed the impacts of surgical (tray alignment, tibial coverage, and resection surface preparation), patient (bone properties and tibiofemoral kinematics), and implant design (tray feature and surface friction) factors on tray-bone interface micromotions during a series of activities of daily living. Micromotion was estimated via three previously validated implant-bone finite element models and tested under gait, deep knee bending, and stair descent loads. Overall, the average micromotion across the tray-bone cementless contact interface ranged from 9.3 to 111.4 µm, and peak micromotion was consistently found along the anterior tray edge. Maximizing tibial coverage above a properly sized tibial tray (an average of 12.3% additional area) had minimal impact on micromotion. A 1 mm anterior tray alignment change reduced the average micromotion by an average of 16.1%. Two-degree tibial angular resection errors reduced the area for bone ingrowth up to 48.1%. Differences on average micromotion from ±25% changes in bone moduli were up to 75.5%. A more posterior tibiofemoral contact due to additional 100 N posterior force resulted in an average of 79.3% increase on average micromotion. Overall, careful surgical technique, patient selection, and controlling kinematics through articular design all contribute meaningfully to minimizing micromotion in cementless TKA, with centralizing the load transfer to minimize the resulting moment at the anterior tray perimeter a consistent theme.

Changes in the Tibial Strain After Proximal Fibular Osteotomy: A Biomechanical Cadaveric Study

Total knee arthroplasty surgery is an increasingly common procedure for the treatment of uni- and tricompartmental knee osteoarthritis, particularly in advanced stages and in the older population. Its usage is being extended to younger patients, where implant longevity is of concern. In the younger age group, especially with early disease, other options merit consideration. On the other hand, it may not be possible for elderly patients with medical comorbidities to undergo joint replacement surgery. Proximal fibular osteotomy (PFO) has recently been advocated to treat medial knee osteoarthritis. Although there have been clinical reports showing promising outcomes, the biomechanical basis of this procedure is still unclear. We performed a cadaveric study to investigate the effect of PFO on proximal tibial strain. Eight unpaired cadaveric lower limb specimens were loaded in compression at 2 times body weight. Strain gauges were mounted on various sites on the proximal tibia and fibula. After PFO, there was a significant increase in the lateral tibial strain adjacent to the proximal tibiofibular joint (P<.05). There was moderate effect size reduction in the anteromedial tibial strain as well as moderate effect size increase in the posterior tibial strain. The strain reduction seen at the anteromedial tibia can offer a possible explanation for symptomatic relief after PFO. However, the increase in the lateral and posterior tibial strain raises concern about long-term accelerated wear in these regions. [Orthopedics. 2022;45(5):314-319.].

Effect of varus alignment on the bone-implant interaction of a cementless tibial baseplate during gait

Component alignment in total knee arthroplasty is a determining factor for implant longevity. Mechanical alignment, which provides balanced load transfer, is the most common alignment strategy. However, a retrospective review found that varus alignment, which could lead to unbalanced loading, can happen in up to 18% of tibial baseplates. This may be particularly burdensome for cementless tibial baseplates, which require low bone-implant micromotion and avoidance of bone overload to obtain bone ingrowth. Our aim was to assess the effect of varus alignment on the bone-implant interaction of cementless baseplates. We virtually implanted 11 patients with knee OA with a modern cementless tibial baseplate in mechanical alignment and in 2° of tibial varus alignment. We performed finite element simulations throughout gait, with loading conditions derived from literature. Throughout the stance phase, varus alignment had greater micromotion and percentage of bone volume at risk of failure than mechanical alignment. At mid-stance, when the most critical conditions occurred, the average increase in peak micromotion and amount of bone at risk of failure due to varus alignment were 79% and 59%, respectively. Varus alignment also resulted in the decrease of the surface area with micromotion compatible with bone ingrowth. However, for both alignments, this surface area was larger than the average area of ingrowth reported for well-fixed implants retrieved post-mortem. Our findings suggest that small varus deviations from mechanical alignment can adversely impact the biomechanics of the bone-implant interaction for cementless tibial baseplates during gait; however, the clinical implications of such changes remain unclear.

Polka dot cementless talar component in enhancing total ankle replacement fixation: A parametric study using the finite element analysis approach

The primary stability of a total ankle replacement (TAR) is essential in preventing long-term aseptic loosening failure and could be quantified based on micromotion at the bone-prosthesis interface subjected to physiological loading during the normal walking. A 3D finite element analysis was conducted to investigate the current commercial STAR™ Ankle TAR bone-prosthesis interface relative micromotion (BPIRM) with addition of the talus bone minimum principal bone stresses (MPBS). Comparison was made to the proposed polka dot designs with the hemispheric feature that was demonstrated to enhance BPIRM. Parametric studies were conducted on the hemispheric features with changes in its diameter, length and shape. The FE results indicated high BPIRM at the talar component was primarily contributed by de-bonding (in the normal direction) between the talus bone and talar component. The MPBS were found to be most significant in the superior anterior and superior medial regions of the talus bone. When the pin length was increased from 1.5 to 3 mm, the BPIRM was predicted to fall below 50 μm in favour of bone in-growth. Based on the practicality of the prosthesis implantation during the surgical procedure, the final design that incorporated both the initial polka dot and 3 mm pin length in a crisscross manner was deemed to be a favorable design with reduced BPIRM and MPBS hence lowering the risk of long-term aseptic loosening.

No effect in primary stability after increasing interference fit in cementless TKA tibial components

Cementless total knee arthroplasty (TKA) implants rely on interference fit to achieve initial stability. However, the optimal interference fit is unknown. This study investigates the effect of using different interference fit on the initial stability of tibial TKA implants. Experiments were performed on human cadaveric tibias using a low interference fit of 350 μm of a clinically established cementless porous-coated tibial implant and a high interference fit of 700 μm. The Orthoload peak loads of gait and squat were applied to the specimens with a custom-made load applicator. Micromotions and gaps opening/closing were measured at the bone-implant interface using Digital Image Correlation (DIC) in 6 regions of interest (ROIs). Two multilevel linear mixed-effect models were created with micromotions and gaps as dependent variables. The results revealed no significant differences for micromotions between the two interference fits (gait p = 0.755, squat p = 0.232), nor for gaps opening/closing (gait p = 0.474, squat p = 0.269). In contrast, significant differences were found for the ROIs in the two dependent variables (p < 0.001), where more gap closing was seen in the posterior ROIs than in the anterior ROIs during both loading configurations. This study showed that increasing the interference fit from 350 to 700 μm did not influence initial stability.

Primary Stability in Cementless Rotating Platform Total Knee Arthroplasty

Highly porous ingrowth surfaces have been introduced into tibial tray fixation to improve long-term survivorship in cementless total knee arthroplasty. This study was designed to evaluate the effect of porous ingrowth surface on primary stability in the implanted cementless tibial component. Three tibial tray designs possessing sintered bead or roughened porous coating ingrowth surfaces were implanted into a foam tibia model with primary stability assessed via digital image correlation during stair descent and condylar liftoff loading. Follow-up testing was conducted by implanting matched-pair cadaveric tibias with otherwise identical trays with two iterations of ingrowth surface design. Trays were loaded and micromotion evaluated in a condylar liftoff model. The sintered bead tibial tray exhibited slightly lower micromotion than the roughened porous coating in stair descent loading. However, no significant difference in primary stability was observed in condylar liftoff loading in either foam or cadaveric specimens. Cementless tibial trays featuring two different iterations of porous ingrowth surfaces demonstrated both good stability in cadaveric specimens with less than 80 microns of micromotion and 1 mm of subsidence under cyclic loading. While improved ingrowth surfaces may lead to improved biological fixation and long-term osteointegration, this study was unable to identify a difference in primary stability associated with subsequent ingrown surface design iteration.

Micromotion and Migration of Cementless Tibial Trays Under Functional Loading Conditions

BACKGROUND

The outcome of cementless total knee arthroplasty (TKA) relies on successful bony ingrowth into the implant surfaces. Failures due to aseptic loosening are still reported, especially in younger and more active patients. The objective of this study is to quantify the micromotion of a commercially available design of cementless tibial tray under loading conditions simulating walking and stair descent.

METHOD

A commercially available design of cementless total knee arthroplasty was implanted in 7 cadaveric knees which were preconditioned with 500 cycles of 0°-100° flexion under a vertical load of 1050 N in a custom-built, multiaxial functional activity simulator. This was followed by application of the peak forces and moments occurring during walking and stair descent. During each loading procedure, 3-dimensional motion at the bone-prosthesis interface was measured using digital image correlation.

RESULTS

The tray migrated 101 ± 25 μm on average during preconditioning, which was dominated by rotation in the sagittal plane (92% of total migration), combined with posterior translation (28%) and minimal rotation in the transverse plane (14%). The migration varied 2.7-fold (61-167 μm) between the 6 measurement zones. Stair descent produced significantly higher total micromotion than walking in zone #5 (62 ± 9 vs 51 ± 10 μm, P < .05) and zone #6 (68 ± 17 vs 37 ± 10 μm, P < .05). In addition, during stair descent, the tray exhibited significantly more tilting (anterior zones: 31 ± 17 vs -16 ± 20 μm, P < .05; posterior zones: -60 ± 8 vs -40 ± 7 μm, P < .05) and more anteroposterior displacement in the anterior zones (-25 ± 3 vs -13 ± 2 μm, P < .05) when compared to walking.

CONCLUSION

The relative motion at the bone-prosthesis interface varied substantially around the periphery of the cementless tray. Under the loading conditions evaluated, the tray primarily underwent a rocking motion in the sagittal plane. Compared with walking, stair descent produced significantly more micromotion, especially in the posterior zones.

Biomechanical evaluation of total ankle arthroplasty. Part II: Influence of loading and fixation design on tibial bone-implant interaction

Finite element (FE) models to evaluate the burden placed on the interaction between total ankle arthroplasty (TAA) implants and the bone often rely on peak axial forces. However, the loading environment of the ankle is complex, and it is unclear whether peak axial forces represent a challenging scenario for the interaction between the implant and the bone. Our goal was to determine how the loads and the design of the fixation of the tibial component of TAA impact the interaction between the implant and the bone. To this end, we developed a framework that integrated robotic cadaveric simulations to determine the ankle kinematics, musculoskeletal models to determine the ankle joint loads, and FE models to evaluate the interaction between TAA and the bone. We compared the bone-implant micromotion and the risk of bone failure of three common fixation designs for the tibial component of TAA: spikes, a stem, and a keel. We found that the most critical conditions for the interaction between the implant and the bone were dependent on the specimen and the fixation design, but always involved submaximal forces and large moments. We also found that while the fixation design influenced the distribution and the peak value of bone-implant micromotion, the amount of bone at risk of failure was specimen dependent. To account for the most critical conditions for the interaction between the implant and the bone, our results support simulating multiple specimens under complex loading profiles that include multiaxial moments and span entire activity cycles.

The effect of different interference fits on the primary fixation of a cementless femoral component during experimental testing

Cementless femoral total knee arthroplasty (TKA) components use a press-fit (referred to as interference fit) to achieve initial fixation. A higher interference fit could lead to a superior fixation, but it could also introduce more damage to the bone during implantation. The purpose of the current study was to investigate the effect of interference fit on the micromotions and gap opening/closing at the bone-implant interface. Experimental tests were performed in six pairs of cadaveric femurs implanted with femoral components using a low interference fit of 350 μm and a high interference fit of 700 μm. The specimens were subjected to the peak loads of gait and squat, based on the Orthoload dataset. Digital Image Correlation (DIC) was used to measure the micromotions and opening/closing in different regions of interest (ROIs). Two linear mixed-effect statistical models were created with micromotions and gap opening/closing as dependent variables. ROIs, loading conditions, and implant designs as independent variables, and cadaver specimens as random intercepts. The results revealed no significant difference between the two interference fit implants for micromotions (p = 0.837 for gait and p = 0.065 for squat), nor for the gap opening/closing (p = 0.748 for gait and p = 0.561 for squat). In contrast, significant differences were found between loading and most of the ROIs in both dependent variables (p < 0.0001). Additionally, no difference in bone deformation was found between low and high interference fit. Changing interference between either 350 μm or 700 μm did not affect the primary stability of a femoral TKA component. There could be an interference fit threshold beyond which fixation does not further improve.

Biomechanical comparison between metal block and cement-screw techniques for the treatment of tibial bone defects in total knee arthroplasty based on finite element analysis

BACKGROUND

Managing bone defects is a critical aspect of total knee arthroplasty. In this study, we compared the metal block and cement-screw techniques for the treatment of Anderson Orthopaedic Research Institute type 2A tibial bone defects from the biomechanical standpoint.

METHOD

The metal block and cement-screw techniques were applied to finite element models of 5- and 10-mm tibial bone defects. Biomechanical compatibility was evaluated based on the stress distributions of the proximal tibia and tibial tray. The displacement of the tibial tray and maximum relative micromotion between the tibial stem and tibia were analyzed to assess the stability of the implant.

RESULTS

The maximum stress in both the proximal tibia and tibial tray was greater with the cement-screw technique than with the metal block technique. The stress of the proximal lateral tibia with the cement-screw technique was significantly larger than with the metal block technique (p < 0.05). For the 5-mm bone defect, the maximum relative micromotion was lower than the critical value of 150 μm. For the 10-mm defect, the maximum relative micromotion was 128 μm with the metal block technique and 155 μm with the cement-screw technique, with the latter exceeding the critical value.

CONCLUSIONS

The cement-screw technique showed superior biomechanical compatibility to the metal block technique and is more suitable for 5-mm bone defects. However, as it may reduce the fixation strength in 10-mm bone defects, the metal block technique is more appropriate in this case.

Finite-Element analysis of a lateral femoro-tibial impact on the total knee arthroplasty

BACKGROUND AND OBJECTIVE

Total knee arthroplasty (TKA) is a routine surgery performed to treat patients with severe knee osteoarthritis. The success of a TKA depends strongly on the initial stability of the prosthetic components and its long-term osseointegration due to the optimal distribution of mechanical stresses in the surrounding bones under the effect of the different biomechanical loads applied to the Femur-TKA-Tibia system. The purpose of this study is to analyze the level and the distribution of the induced stresses in a Femur-TKA-Tibia system subjected to combined triaxial forces, which mimic a femoral mechanical shock.

METHODS

In this study, complex TKA system implanted in both femoral and tibial bones has been analyzed numerically with a three-dimensional finite-element method. A virtual model is designed to examine in silico the effect of the combined triaxial forces acting on this prosthesis in femoral region. Anatomical three-dimensional finite-element models of both femoral and tibial bones were constructed to calculate the interfacial stresses around the TKA components. The 3D finite-element processing program ABAQUS was used to perform the analysis.

RESULTS

The stresses propagated in the bone regions adjacent to the TKA osseointegrated components, and the decreased in their magnitude to the outer region. These stresses reached the highest level in the cortical bone areas that are right next to the proximal upper attachment portions of the TKA osseointegrated components. The magnitude of the stresses in the tibial component is higher than that in the femoral component. Finally, it is very important to emphasize the role of the polyethylene articulating spacer in the shock absorption of bone support sections. Thus, this component should be preserved mechanically from the impact of high shocks in order to maintain healthy TKA systems.

CONCLUSIONS

Optimizing TKA model by controlling the biomechanical stresses distributed within its both components and supporting bones is a valid approach to achieving favorable long-term outcomes. The 3D finite-element analysis provides an effective pre-operative method for planning patient-specific TKA prostheses, and for designing future models that preserves the biomechanical function of the Femur-TKA-Tibia system.

Validation and sensitivity of model-predicted proximal tibial displacement and tray micromotion in cementless total knee arthroplasty under physiological loading conditions

The initial fixation of cementless tibial trays after total knee arthroplasty is crucial to bony ingrowth onto the porous surface of the implants, as micromotion magnitudes exceeding 150 μm may inhibit bone formations and limit fixation. Experimental measurement of the interface micromotions is still very challenging. Thus, previous studies investigated micromotions at the bone-tray interface via finite element methods, but few performed direct validation via in vitro cadaveric testing under physiological loading conditions. Additionally, previous models were validated by solely considering relative displacements of the marker couples placed around the tray-bone interface. In this paper, we present an experimental-computational validation framework for investigating micromotions at the tray-bone interface under physiological conditions. Three cadaveric specimens were implanted with cementless rotating-platform implants and tested under gait, deep knee bending, and stair descent loads. Corresponding subject-specific finite element models were developed and used to predict the marker (tray-bone) relative displacements and tibial surface displacements. Experimental measurements were used to validate model estimations. Subsequent sensitivity analyses were performed on implantation and friction parameters to represent model uncertainties. The models appropriately differentiated between locations, activities, and specimens. The average root-mean-square (RMS) differences and correlations between measured marker relative displacements and predictions from the 'best-matching' models were 13.1 μm and 0.86. RMS differences and correlations between measured surface displacements and predictions were 78.9 μm and 0.84. Full-field interface micromotions were investigated and compared with predicted marker relative displacements. The marker relative displacements underestimated the actual interface micromotions. Initial tray-bone alignment in anterior-posterior, flexion-extension, and varus-valgus degrees of freedom have a considerable impact on the interface micromotions. The validated cadaveric models can be further used for pre-clinical assessments of new TKR tray design. The outcomes of the sensitivity analyses provide further insights into reducing interface micromotions via clinical techniques.

Finite element analysis of bone-prosthesis interface micromotion for cementless talar component fixation through critical loading conditions

The total ankle replacement (TAR) survivability rate is still suboptimal, and this leads to many orthopaedic surgeons opting arthrodesis as a better option for the ankle arthritis patients. One of the fundamental reasons is due to the lack of primary stability of the prosthesis fixation at the bone-prosthesis interface hence leading to long-term aseptic loosening of the talar component. The commercially available Scandinavian Total Ankle Replacement (STAR) Ankle design and several additional design features (including trabecular metal, side fin, double fin, and polka-dot designs) were studied using finite element analysis, and the bone-prosthesis interface relative micromotion (BPIRM) and talar bone minimum principal stresses were examined and analysed. Three loading conditions at a gait cycle of heel strike, midstance, and toe off with different meniscal bearing displacement were also included as part of the study parameters. The results were correlated to in vitro cadaveric measurements and reported clinical studies. Simulated results showed that the de-bonding relative distance between the bone and prosthesis upon loading (COPEN defined by the simulation software) was the main reason constituting to the high interface micromotion between the talar component and talus bone (which could lead to long-term aseptic loosening). The polka-dot design was shown to induce the lowest BPIRM among all the designs studied.

A Vibrational Technique for In Vitro Intraoperative Prosthesis Fixation Monitoring

OBJECTIVE

In this paper, a new vibrational modal analysis technique was developed for intraoperative cementless prosthesis fixation evaluation upon hammering.

METHODS

An artificial bone (Sawbones)-prosthesis system was excited by sweeping of a sine signal over a wide frequency range. The exponential sine sweep technique was implemented to the response signal in order to determine the linear impulse response. Recursive Fourier transform enhancement (RFTE) technique was applied to the linear impulse response signal in order to enhance the frequency spectrum with sharp and distinguishable peak values indicating distinct high natural frequencies of the system (ranging from 15 kHz to 90 kHz). The experiment was repeated with 5 Sawbones-prosthesis samples. Upon successive hammering during the prosthesis insertion, variation of each natural frequency was traced.

RESULTS

Compared to classical Fast Fourier Transform, RFTE provided a better tracing and enhancement of frequency components during insertion. Three different types of frequency evolving trends (monotonically increasing, insensitive, and plateau-like) were observed for all samples, as confirmed by a new finite element simulation of the prosthesis dynamic insertion. Two main mechanical phenomena (i.e., geometrical compaction and compressive stress) were shown to govern these trends in opposite ways. Follow-up of the plateau-like trend upon hammering showed that the frequency shift is a good indicator of fixation.

CONCLUSION

Alongside the individual follow-up of frequency shifts, combinatorial frequency analysis provides new objective information on the mechanical stability of Sawbone-prosthesis fixation.

SIGNIFICANCE

The proposed vibrational technique based on RTFE can provide the surgeon with a new assistive diagnostic technique during the surgery by indicating when the bone-prosthesis fixation is acceptable, and beyond of which further hammering should be done cautiously to avoid bone fracture.

Finite-element analysis of the proximal tibial sclerotic bone and different alignment in total knee arthroplasty

Background

Despite potential for improving patient outcomes, studies using three-dimensional measurements to quantify proximal tibial sclerotic bone and its effects on prosthesis stability after total knee arthroplasty (TKA) are lacking. Therefore, this study aimed to determine: (1) the distribution range of tibial sclerotic bone in patients with severe genu varum using three-dimensional measurements, (2) the effect of the proximal tibial sclerotic bone thickness on prosthesis stability according to finite-element modelling of TKA with kinematic alignment (KA), mechanical alignment (MA), and 3° valgus alignment, and (3) the effect of short extension stem augment utilization on prosthesis stability.

Methods

The sclerotic bone in the medial tibial plateau of 116 patients with severe genu varum was measured and classified according to its position and thickness. Based on these cases, finite-element models were established to simulate 3 different tibial cut alignments with 4 different thicknesses of the sclerotic bone to measure the stress distribution of the tibia and tibial prosthesis, the relative micromotion beneath the stem, and the influence of the short extension stem on stability.

Results

The distribution range of proximal tibial sclerotic bone was at the anteromedial tibial plateau. The models were divided into four types according to the thickness of the sclerotic bone: 15 mm, 10 mm, 5 mm, and 0 mm. The relative micromotion under maximum stress was smallest after MA with no sclerotic bone (3241 μm) and largest after KA with 15 mm sclerotic bone (4467 μm). Relative micromotion was largest with KA and smallest with MA in sclerotic models with the same thickness. Relative micromotion increased as thickness of the sclerotic bone increased with KA and MA (R = 0.937, P = 0.03 and R = 0.756, P = 0.07, respectively). Relative micromotion decreased with short extension stem augment in the KA model when there was proximal tibial sclerotic bone.

Conclusions

The influence of proximal tibial sclerotic bone on prosthesis’s stability is significant, especially with KA tibial cut. Tibial component’s short extension stem augment can improve stability.

Biomechanical behaviours of the bone-implant interface: a review

In recent decades, cementless implants have been widely used in clinical practice to replace missing organs, to replace damaged or missing bone tissue or to restore joint functionality. However, there remain risks of failure which may have dramatic consequences. The success of an implant depends on its stability, which is determined by the biomechanical properties of the bone-implant interface (BII). The aim of this review article is to provide more insight on the current state of the art concerning the evolution of the biomechanical properties of the BII as a function of the implant's environment. The main characteristics of the BII and the determinants of implant stability are first introduced. Then, the different mechanical methods that have been employed to derive the macroscopic properties of the BII will be described. The experimental multi-modality approaches used to determine the microscopic biomechanical properties of periprosthetic newly formed bone tissue are also reviewed. Eventually, the influence of the implant's properties, in terms of both surface properties and biomaterials, is investigated. A better understanding of the phenomena occurring at the BII will lead to (i) medical devices that help surgeons to determine an implant's stability and (ii) an improvement in the quality of implants.

Vascular endothelial growth factor pathway promotes osseointegration and CD31hiEMCNhi endothelium expansion in a mouse tibial implant model: an animal study

AIMS

It is increasingly appreciated that coordinated regulation of angiogenesis and osteogenesis is needed for bone formation. How this regulation is achieved during peri-implant bone healing, such as osseointegration, is largely unclear. This study examined the relationship between angiogenesis and osteogenesis in a unique model of osseointegration of a mouse tibial implant by pharmacologically blocking the vascular endothelial growth factor (VEGF) pathway.

MATERIALS AND METHODS

An implant was inserted into the right tibia of 16-week-old female C57BL/6 mice (n = 38). Mice received anti-VEGF receptor-1 (VEGFR-1) antibody (25 mg/kg) and VEGF receptor-2 (VEGFR-2) antibody (25 mg/kg; n = 19) or an isotype control antibody (n = 19). Flow cytometric (n = 4/group) and immunofluorescent (n = 3/group) analyses were performed at two weeks post-implantation to detect the distribution and density of CD31^hiEMCN^hi endothelium. RNA sequencing analysis was performed using sorted CD31^hiEMCN^hi endothelial cells (n = 2/group). Osteoblast lineage cells expressing osterix (OSX) and osteopontin (OPN) were also detected with immunofluorescence. Mechanical pull-out testing (n = 12/group) was used at four weeks post-implantation to determine the strength of the bone-implant interface. After pull-out testing, the tissue attached to the implant surface was harvested. Whole mount immunofluorescent staining of OSX and OPN was performed to determine the amount of osteoblast lineage cells.

RESULTS

Flow cytometry revealed that anti-VEGFR treatment decreased CD31^hiEMCN^hi vascular endothelium in the peri-implant bone versus controls at two weeks post-implantation. This was confirmed by the decrease of CD31 and endomucin (EMCN) double-positive cells detected with immunofluorescence. In addition, treated mice had more OPN-positive cells in both peri-implant bone and tissue on the implant surface at two weeks and four weeks, respectively. More OSX-positive cells were present in peri-implant bone at two weeks. More importantly, anti-VEGFR treatment decreased the maximum load of pull-out testing compared with the control.

CONCLUSION

VEGF pathway controls the coupling of angiogenesis and osteogenesis in orthopaedic implant osseointegration by affecting the formation of CD31^hiEMCN^hi endothelium. Cite this article: Bone Joint J 2019;101-B(7 Supple C):108-114.

Influence of stems and metaphyseal sleeve on primary stability of cementless revision tibial trays used to reconstruct AORI IIB defects

Metaphyseal augments, such as sleeves, have been introduced to augment the fixation of revision total knee replacement (rTKR) components, and can be used with or without a stem. The effect of sleeve size in combination with stems on the primary stability and load transfer of a rTKR implant in AORI type IIB defects where the defect involves both condyles are poorly understood. The aim of this study was to examine the primary stability of revision tibial tray augmented with a sleeve in an AORI type IIB defect which involves both condyles with loss of cortical and cancellous bone. Finite element models were generated from computed tomography (CT) scans of nine individuals. All the bones used in the study had an AORI type IIB defect. The cohort included eight females (mean weight: 64 kg, height: 1.6 m). Material properties were sampled from CT data and assigned to the FE model. Joint contact forces for level gait, stair descent, and squat were applied. Stemless sleeved implants under various loading conditions were shown to have adequate primary stability in all AORI type IIB defects investigated. Adding a stem only marginally improved the primary stability of the implant but reduced the strain in the metaphysis compared to stemless implants. Once good initial mechanical stability was established with a sleeve, there was no benefit, in terms of primary stability or bone strains, from increasing sleeve size. This study suggests that metaphyseal sleeves, without a stem, can provide the required primary stability required by a rTKR tibial implant, to reconstruct an AORI type IIB defect. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Mechanical performance of cementless total knee replacements: It is not all about the maximum loads

Finite element (FE) models are frequently used to assess mechanical interactions between orthopedic implants and surrounding bone. However, FE studies are often limited by the small number of bones that are modeled; the use of normal bones that do not reflect the altered bone density distributions that result from osteoarthritis (OA); and the application of simplified load cases usually based on peak forces and without consideration of tibiofemoral kinematics. To overcome these limitations, we undertook an integrated approach to determine the most critical scenario for the interaction between an uncemented tibial component and surrounding proximal tibial bone. A cementless component, based on a modern design, was virtually implanted using computed-tomography scans from 13 patients with knee OA. FE simulations were performed across a demanding activity, stair ascent, by combining in vivo experimental forces from the literature with tibiofemoral kinematics measured from patients who had received the same design of knee component. The worst conditions for the bone-implant interaction, in terms of micromotion and percentage of interfacial bone mass at risk of failure, did not arise from the maximum applied loads. We also found large variability among bones and tibiofemoral kinematics sets. Our results suggest that future FE studies should not focus solely on peak loads as this approach does not consistently correlate to worst-case scenarios. Moreover, multiple load cases and multiple bones should be considered to best reflect variations in tibiofemoral kinematics, anatomy, and tissue properties. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:350-357, 2019.

Additive manufactured push-fit implant fixation with screw-strength pull out

Additive manufacturing offers exciting new possibilities for improving long-term metallic implant fixation in bone through enabling open porous structures for bony ingrowth. The aim of this research was to investigate how the technology could also improve initial fixation, a precursor to successful long-term fixation. A new barbed fixation mechanism, relying on flexible struts was proposed and manufactured as a push-fit peg. The technology was optimized using a synthetic bone model and compared with conventional press-fit peg controls tested over a range of interference fits. Optimum designs, achieving maximum pull-out force, were subsequently tested in a cadaveric femoral condyle model. The barbed fixation surface provided more than double the pull-out force for less than a third of the insertion force compared to the best performing conventional press-fit peg (p < 0.001). Indeed, it provided screw-strength pull out from a push-fit device (1,124 ± 146 N). This step change in implant fixation potential offers new capabilities for low profile, minimally invasive implant design, while providing new options to simplify surgery, allowing for one-piece push-fit components with high levels of initial stability. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:1508-1518, 2018.

Comparison between uncemented and cemented fixation for the tibial component in distal femoral replacement: a clinical and radiological study

PURPOSES

We performed a retrospective, cohort study to compare uncemented tibial fixation with cemented tibial fixation in distal femoral replacement (DFR).

METHODS

Sixty-two cases with uncemented tibial fixation and 58 cases with cemented tibial fixation were included. Inter-group comparisons were performed for baseline data, oncological and prosthetic outcomes, and changes of cortical thickness of tibial diaphysis. Radiological signs of bone adaptations around the uncemented tibial stem were identified through evaluation of plain films during follow-up.

RESULTS

Uncemented tibial fixation shortened operative duration by 26 minutes, achieved equivalent oncological and prosthetic outcomes, and helped preserve anterior cortical thickness of tibia compared with the cemented counterpart after a mean follow-up of over 40 months. Radiological signs of osseointegration and reactive line were observed in 64.3 and 17.9% cases with uncemented tibial fixation. The two signs had different patterns of distribution and no significant predisposing factors could be identified.

CONCLUSIONS

For DFR, the uncemented tibial fixation was safe and effective in functional reconstruction and in preservation of anterior cortex of tibial diaphysis. It could achieve osseointegration and might permit adaptive micromotion of the tibial stem post-operatively.

LEVEL OF EVIDENCE

level III Therapeutic.

Quantification of interfacial motions following primary and revision total knee arthroplasty: A verification study versus experimental data

Motion at the bone-implant interface, following primary or revision knee arthroplasty, can be detrimental to the long-term survival of the implant. This study employs experimentally verified computational models of the distal femur to characterize the relative motion at the bone-implant interface for three different implant types; a posterior stabilizing implant (PS), a total stabilizing implant (TS) with short stem (12 mm × 50 mm), and a total stabilizing implant (TS) with long offset stem (19 mm × 150 mm with a 4 mm lateral offset). Relative motion was investigated for both cemented and uncemented interface conditions. Monitoring relative motion about a single reference point, though useful for discerning global differences between implant types, was found to not be representative of the true pattern and distribution of motions which occur at the interface. The contribution of elastic deformation to apparent reference point motion varied based on implant type, with the PS and TSSS implanted femurs experiencing larger deformations (43 and 39 μm, respectively) than the TSLS implanted femur (22 μm). Furthermore, the pattern of applied loading was observed to greatly influence location and magnitude of peak motions, as well as the surface area under increased motion. Interestingly, the influence was not uniform across all implant types, with motions at the interface of long stemmed prosthesis found to be less susceptible to changes in pattern of loading. These findings have important implications for the optimization and testing of orthopedic implants in vitro and in silico. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:387-396, 2018.

CEMENTLESS MIS MINI-KEEL PROSTHESIS REDUCES INTERFACE MICROMOTION VERSUS STANDARD STEMMED TIBIAL COMPONENTS

Fixation strength of the cementless knee prostheses is dependent on the initial stability of the fixation and minimal relative motion across the prosthesis–bone interface. Broad mini-keels have been developed for tibial components to allow minimally invasive knee arthroplasty, but the effect of the change in fixation design is unknown. In this study, bone–prosthesis interface micromotions of the mini-keel tibial components (consisting of two designs; one is stemless and another with a stem extension of 45[Formula: see text]mm) induced by walking and stair climbing were investigated by finite element modeling and compared with standard stemmed design. The prosthesis surface area amenable for bone ingrowth for the mini-keel tibial components (both stemmed and unstemmed) was predicted to be at least 67% larger than the standard stemmed implant, thereby reducing the risk of long-term aseptic loosening. It was also found that while different load patterns may have led to diverse predictions of the magnitude of the interface micromotions and the extent of osseointegration onto the prosthesis, the outcome of design change evaluation in cementless tibial fixations remains unchanged. The mini-keel tibial components were predicted to anchor onto the periprosthetic bone better than the standard stemmed design under all loading conditions investigated.

FE analysis of the effects of simplifications in experimental testing on micromotions of uncemented femoral knee implants

Experimental testing of orthopaedic implants requires simplifications concerning load application and activities being analyzed. This computational study investigated how these simplifications affect micromotions at the bone-implant interface of an uncemented femoral knee implant. As a basis, validated in vivo loads of the stance phase of gait and a deep knee bend were adopted. Eventually, three configurations were considered: (i) simulation of the complete loading cycle; (ii) inclusion of only tibiofemoral loads (ignoring patellofemoral loads); and (iii) applying only a single peak tibiofemoral force. For all loading conditions the largest micromotions found at the proximal anterior flange. Without the patellofemoral force, peak micromotions increased 6% and 22% for gait and deep knee bend, respectively. By applying a single peak tibiofemoral force micromotions were overestimated. However, the peak micromotions corresponded to the maximum tibiofemoral force, and strong micromotion correlations were found between a complete loading cycle and a single peak load (R(2)  = 0.73 and R(2)  = 0.89 for gait and deep knee bend, respectively). Deep knee bend resulted in larger micromotions than gait. Our study suggests that a simplified peak force can be used to assess the stability of cementless femoral components. For more robust testing, implants should be subjected to different loading modes. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:812-819, 2016.

Micromotion at the tibial plateau in primary and revision total knee arthroplasty: fixed versus rotating platform designs

OBJECTIVES

Initial stability of tibial trays is crucial for long-term success of total knee arthroplasty (TKA) in both primary and revision settings. Rotating platform (RP) designs reduce torque transfer at the tibiofemoral interface. We asked if this reduced torque transfer in RP designs resulted in subsequently reduced micromotion at the cemented fixation interface between the prosthesis component and the adjacent bone.

METHODS

Composite tibias were implanted with fixed and RP primary and revision tibial trays and biomechanically tested under up to 2.5 kN of axial compression and 10° of external femoral component rotation. Relative micromotion between the implanted tibial tray and the neighbouring bone was quantified using high-precision digital image correlation techniques.

RESULTS

Rotational malalignment between femoral and tibial components generated 40% less overall tibial tray micromotion in RP designs than in standard fixed bearing tibial trays. RP trays reduced micromotion by up to 172 µm in axial compression and 84 µm in rotational malalignment models.

CONCLUSIONS

Reduced torque transfer at the tibiofemoral interface in RP tibial trays reduces relative component micromotion and may aid long-term stability in cases of revision TKA or poor bone quality.Cite this article: Mr S. R. Small. Micromotion at the tibial plateau in primary and revision total knee arthroplasty: fixed versus rotating platform designs. Bone Joint Res 2016;5:122-129. DOI: 10.1302/2046-3758.54.2000481.

A Novel Method to Simulate the Progression of Collagen Degeneration of Cartilage in the Knee: Data from the Osteoarthritis Initiative

We present a novel algorithm combined with computational modeling to simulate the development of knee osteoarthritis. The degeneration algorithm was based on excessive and cumulatively accumulated stresses within knee joint cartilage during physiological gait loading. In the algorithm, the collagen network stiffness of cartilage was reduced iteratively if excessive maximum principal stresses were observed. The developed algorithm was tested and validated against experimental baseline and 4-year follow-up Kellgren-Lawrence grades, indicating different levels of cartilage degeneration at the tibiofemoral contact region. Test groups consisted of normal weight and obese subjects with the same gender and similar age and height without osteoarthritic changes. The algorithm accurately simulated cartilage degeneration as compared to the Kellgren-Lawrence findings in the subject group with excess weight, while the healthy subject group’s joint remained intact. Furthermore, the developed algorithm followed the experimentally found trend of cartilage degeneration in the obese group (R² = 0.95, p < 0.05; experiments vs. model), in which the rapid degeneration immediately after initiation of osteoarthritis (0–2 years, p < 0.001) was followed by a slow or negligible degeneration (2–4 years, p > 0.05). The proposed algorithm revealed a great potential to objectively simulate the progression of knee osteoarthritis.

Intermittent Parathyroid Hormone Enhances Cancellous Osseointegration of a Novel Murine Tibial Implant

BACKGROUND

Long-term fixation of uncemented joint implants requires early mechanical stability and implant osseointegration. To date, osseointegration has been unreliable and remains a major challenge in cementless total knee arthroplasty. We developed a murine model in which an intra-articular proximal tibial titanium implant with a roughened stem can be loaded through the knee joint. Using this model, we tested the hypothesis that intermittent injection of parathyroid hormone (iPTH) would increase proximal tibial cancellous osseointegration.

METHODS

Ten-week-old female C57BL/6 mice received a subcutaneous injection of PTH (40 μg/kg/day) or a vehicle (n = 45 per treatment group) five days per week for six weeks, at which time the baseline group was killed (n = 6 per treatment group) and an implant was inserted into the proximal part of the tibiae of the remaining mice. Injections were continued until the animals were killed at one week (n = 7 per treatment group), two weeks (n = 14 per treatment group), or four weeks (n = 17 per treatment group) after implantation. Outcomes included peri-implant bone morphology as analyzed with micro-computed tomography (microCT), osseointegration percentage and bone area fraction as shown with backscattered electron microscopy, cellular composition as demonstrated by immunohistochemical analysis, and pullout strength as measured with mechanical testing.

RESULTS

Preimplantation iPTH increased the epiphyseal bone volume fraction by 31.6%. When the data at post-implantation weeks 1, 2, and 4 were averaged for the iPTH-treated mice, the bone volume fraction was 74.5% higher in the peri-implant region and 168% higher distal to the implant compared with the bone volume fractions in the same regions in the vehicle-treated mice. Additionally, the trabecular number was 84.8% greater in the peri-implant region and 74.3% greater distal to the implant. Metaphyseal osseointegration and bone area fraction were 28.1% and 70.1% higher, respectively, in the iPTH-treated mice than in the vehicle-treated mice, and the maximum implant pullout strength was 30.9% greater. iPTH also increased osteoblast and osteoclast density by 65.2% and 47.0%, respectively, relative to the values in the vehicle group, when the data at post-implantation weeks 1 and 2 were averaged.

CONCLUSIONS

iPTH increased osseointegration, cancellous mass, and the strength of the bone-implant interface.

CLINICAL RELEVANCE

Our murine model is an excellent platform on which to study biological enhancement of cancellous osseointegration.

FINITE ELEMENT ANALYSIS OF A RETRIEVED CUSTOM-MADE KNEE PROSTHESIS

Custom-made knee prostheses have been widely used to reconstruct the function of the lower limb in bone tumor resections. A custom-made tumor knee prosthesis was retrieved on account of prosthesis loosening post-surgery. Misalignment between the anatomical axis of the femur and the axis of the femoral stem as well as the material loss at the posterior region of the tibial plateau were considered to be the primary causes of the failure. Based on this hypothesis, finite element analysis was performed to investigate the contact mechanics of the prosthesis while implanted in vivo. The maximum deformation at the femur was 0.59 and 1.17 mm when the misalignment angle was 3° and 6°, respectively. Besides, the maximum contact pressure at the tibial plateau was 44.88 MPa at an extremely high flexion of angle 135° during squatting or kneeling. Uneven stress distribution at the femur, which came from the misalignment, was the main cause of loosening, which was aggravated indirectly with the material loss at the posterior region of the tibial plateau. Optimized prosthesis design and appropriate selection, with accurate surgical positioning and targeted rehabilitation training programme are important considerations for prolonging life-span of prostheses in vivo.

Digital volume correlation and micro-CT: An in-vitro technique for measuring full-field interface micromotion around polyethylene implants

Micromotion around implants is commonly measured using displacement-sensor techniques. Due to the limitations of these techniques, an alternative approach (DVC-μCT) using digital volume correlation (DVC) and micro-CT (μCT) was developed in this study. The validation consisted of evaluating DVC-μCT based micromotion against known micromotions (40, 100 and 150 μm) in a simplified experiment. Subsequently, a more clinically realistic experiment in which a glenoid component was implanted into a porcine scapula was carried out and the DVC-μCT measurements during a single load cycle (duration 20 min due to scanning time) was correlated with the manual tracking of micromotion at 12 discrete points across the implant interface. In this same experiment the full-field DVC-μCT micromotion was compared to the full-field micromotion predicted by a parallel finite element analysis (FEA). It was found that DVC-μCT micromotion matched the known micromotion of the simplified experiment (average/peak error=1.4/1.7 μm, regression line slope=0.999) and correlated with the micromotion at the 12 points tracked manually during the realistic experiment (R(2)=0.96). The DVC-μCT full-field micromotion matched the pattern of the full-field FEA predicted micromotion. This study showed that the DVC-μCT technique provides sensible estimates of micromotion. The main advantages of this technique are that it does not damage important parts of the specimen to gain access to the bone-implant interface, and it provides a full-field evaluation of micromotion as opposed to the micromotion at just a few discrete points. In conclusion the DVC-μCT technique provides a useful tool for investigations of micromotion around plastic implants.

Different femorotibial contact on the weight-bearing: midflexion between normal and varus aligned knees after total knee arthroplasty

PURPOSE

The influence of residual malalignment on biomechanical analysis after total knee arthroplasty (TKA) is currently uncertain. The hypothesis is that postoperative alignment would influence the in vivo kinematics after TKA, under weight-bearing conditions but not under non-weight-bearing condition. The purpose of the present study was to compare weight-bearing and non-weight-bearing conditions and to evaluate the effect of the postoperative alignment on the in vivo kinematics after posterior cruciate ligament-retaining TKA during midflexion using 2-dimensional/3-dimensional registration.

METHODS

Thirty knees of 30 patients with pre-operative varus deformity were divided into 2 groups according to their postoperative alignment: the normal alignment group (N = 21) and the varus alignment group (N = 9).

RESULTS

Under weight-bearing conditions, the varus alignment group showed a significant posterior displacement of the medial femoral condyle (flexion: 80°, 90° P < 0.05) and a significant anterior displacement of the lateral femoral condyle (flexion: 10° P < 0.01, 20° P < 0.05, and extension: 10°, 20° P < 0.01, 30°, 40° P < 0.05) as compared with the normal alignment group. In contrast, no significant difference in the medial and lateral femoral condyle positions under non-weight-bearing conditions was observed between the normal and varus alignment groups.

CONCLUSION

The postoperative alignment influenced knee kinematics under weight-bearing conditions. The weight load influenced knee kinematics through posterior tibial slope and induced greater lateral femoral condyle mobility, which might explain the better clinical and functional outcome. These findings contribute to gaining a proper understanding of the in vivo kinematics of the postoperative varus alignment and might be useful for orthopaedic surgeons in the achievement of patient satisfaction.

LEVEL OF EVIDENCE

III.

Four decades of finite element analysis of orthopaedic devices: where are we now and what are the opportunities?

Finite element has been used for more than four decades to study and evaluate the mechanical behaviour total joint replacements. In Huiskes seminal paper "Failed innovation in total hip replacement: diagnosis and proposals for a cure", finite element modelling was one of the potential cures to avoid poorly performing designs reaching the market place. The size and sophistication of models has increased significantly since that paper and a range of techniques are available from predicting the initial mechanical environment through to advanced adaptive simulations including bone adaptation, tissue differentiation, damage accumulation and wear. However, are we any closer to FE becoming an effective screening tool for new devices? This review contains a critical analysis of currently available finite element modelling techniques including (i) development of the basic model, the application of appropriate material properties, loading and boundary conditions, (ii) describing the initial mechanical environment of the bone-implant system, (iii) capturing the time dependent behaviour in adaptive simulations, (iv) the design and implementation of computer based experiments and (v) determining suitable performance metrics. The development of the underlying tools and techniques appears to have plateaued and further advances appear to be limited either by a lack of data to populate the models or the need to better understand the fundamentals of the mechanical and biological processes. There has been progress in the design of computer based experiments. Historically, FE has been used in a similar way to in vitro tests, by running only a limited set of analyses, typically of a single bone segment or joint under idealised conditions. The power of finite element is the ability to run multiple simulations and explore the performance of a device under a variety of conditions. There has been increasing usage of design of experiments, probabilistic techniques and more recently population based modelling to account for patient and surgical variability. In order to have effective screening methods, we need to continue to develop these approaches to examine the behaviour and performance of total joint replacements and benchmark them for devices with known clinical performance. Finite element will increasingly be used in the design, development and pre-clinical testing of total joint replacements. However, simulations must include holistic, closely corroborated, multi-domain analyses which account for real world variability.