Inter-subject variability effects on the primary stability of a short cementless femoral stem

Influence of offset on osseointegration in cementless total hip arthroplasty: A finite element study

Early aseptic loosening is caused by deficient osteointegration of the femoral stem due to increased micromotions and represents a common mode of failure in uncemented total hip arthroplasty (THA). This study hypothesized that a higher femoral offset, a smaller stem size and obesity increase femoral micromotion, potentially resulting in early aseptic loosening. A finite element analysis was conducted based on computed tomography segmented model of four patients who received a THA with a triple-tapered straight stem (Size 1, 3, 6). The influence of femoral stem offset (short neck, standard, lateral), head length (S to XXL), femoral anteversion and obesity during daily activities of fast walking and stair climbing was analyzed. The micromotions for the femoral stem zones were compared to a threshold representing a value above which only partial osseointegration is expected. The minimum femoral offset configuration compared to the maximum offset configuration (short neck stem, S head vs. lateral stem, XXL head) leads to a relative mean micromotion increase of 24% for the upper stem zone. Increasing the body weight (body mass index 30-35 kg/m2 ) increases the micromotion by 20% for all stem zones. The obese population recorded threshold-exceeding micromotions for stem sizes 1 and 3 for all offset configurations during stair climbing. Higher femoral offset, a smaller stem size, and higher loading due to obesity lead to an increase in micromotion between the prosthesis and proximal femur and represent a risk configuration for impaired osseointegration of a triple-tapered straight stem, especially when these three factors are present simultaneously.

5 year follow up of a hydroxyapatite coated short stem femoral component for hip arthroplasty: a prospective multicentre study

Short stem, uncemented femoral implants for hip arthroplasty are bone conserving achieving stability through initial metaphyseal press-fit and biological fixation. This study aimed to evaluate the survivorship, mid-term function and health related quality of life outcomes in patients who have undergone total hip arthroplasty (THA) with a fully hydroxyapatite coated straight short stem femoral component with up to 5 years follow-up. 668 patients were recruited to a multicentre study investigating the performance of the cementless Furlong Evolution® stem for THA. 137 patients withdrew at various time points. The mean follow-up was 49 months. Clinical (Harris Hip Score (HHS), radiographic and patient-reported outcome measures-Oxford Hip Score (OHS) and EuroQol 5D (EQ-5D), were recorded pre-operatively and at 6 weeks, 6 months, 1 year, 3 year and 5 year follow ups. At 5-year follow-up, 12 patients underwent revision surgery, representing a cumulative revision rate of 1.8%. Median OHS, HHS and EQ5D scores improved significantly: OHS improved from a pre-operative median of 21 (IQR 14-26) to 47 (IQR 44-48) (p < 0.001). HHS improved from 52 (IQR 40-63) to 98 (IQR 92-100) (p < 0.001) and EQ5D improved from 70 (IQR 50-80) to 85 (IQR 75-95) (p < 0.001). This fully HA-coated straight short femoral stem implant demonstrated acceptable mid-term survivorship and delivered substantial improvements in function and quality of life after THA.

Patient-specific three-dimensional evaluation of interface micromotion in two different short stem designs in cementless total hip arthroplasty: a finite element analysis

Background

Evaluation of micromotion in various activities in daily life is essential to the assessment of the initial fixation of cementless short stems in total hip arthroplasty. This study sought to evaluate three-dimensionally the micromotion of two types of cementless short stems.

Methods

Two types of stems were used: the Fitmore stem with a rectangular cross-section (rectangular stem) and the octagonal-oval GTS stem with fins (finned stem). Finite element analysis was used to calculate the micromotion of two activities that place a heavy load on the stem (single-leg stance and stair climbing). Three values were measured: the magnitude of micromotion (mean and 95th percentile), the location of micromotion above the 95th percentile value, and the directions of the micromotion vector.

Results

1. There was no significant difference in the magnitude of the micromotion between the rectangular stem and finned stem groups for single-leg stance or stair climbing. 2. In both groups, the micromotion was greatest at the proximal and distal ends. 3. The direction of the micromotion was similar in both groups; internal rotation occurred from the distal to the middle of the stem during stair climbing.

Conclusions

The rectangular stem had comparable initial fixation to that of the finned stem. In both models, the micromotion was greater at the proximal and distal ends. The direction of the micromotion was not dependent on the stem shape but on the direction of the load on the artificial femoral head. These results will be important for stem selection and future stem development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-022-03329-5.

Designing and in vitro testing of a novel patient-specific total knee prosthesis using the probabilistic approach

In order to prevent failure as well as ensure comfort, patient-specific modelling for prostheses has been gaining interest. However, deterministic analyses have been widely used in the design process without considering any variation/uncertainties related to the design parameters of such prostheses. Therefore, this study aims to compare the performance of patient-specific anatomic Total Knee Arthroplasty (TKA) with off-the-shelf TKA. In the patient-specific model, the femoral condyle curves were considered in the femoral component's inner and outer surface design. The tibial component was designed to completely cover the tibia cutting surface. In vitro experiments were conducted to compare these two models in terms of loosening of the components. A probabilistic approach based on the finite element method was also used to compute the probability of failure of both models. According to the deterministic analysis results, 103.10 and 21.67 MPa von Mises stress values were obtained for the femoral component and cement in the anatomical model, while these values were 175.86 and 25.76 MPa, respectively, for the conventional model. In order to predict loosening damage due to local osteolysis or stress shield, it was determined that the deformation values in the examined cement structures were 15% lower in the anatomical model. According to probabilistic analysis results, it was observed that the probability of encountering an extreme value for the anatomical model is far less than that of the conventional model. This indicates that the anatomical model is safer than the conventional model, considering the failure scenarios in this study.

Primary stability of the Activ L® intervertebral disc prosthesis in cadaver bone and comparison of the keel and spike anchoring concept

Background

High primary stability is the key prerequisite for safe osseointegration of cementless intervertebral disc prostheses. The aim of our study was to determine the primary stability of intervertebral disc prostheses with two different anchoring concepts – keel and spike anchoring.

Methods

Ten ActivL intervertebral disc prostheses (5 x keel anchoring, 5 x spike anchoring) implanted in human cadaver lumbar spine specimens were tested in a spine movement simulator. Axial load flexion, extension, left and right bending and axial rotation motions were applied on the lumbar spine specimens through a defined three-dimensional movement program following ISO 2631 and ISO/CD 18192-1.3 standards. Tri-dimensional micromotions of the implants were measured for both anchor types and compared using Student’s T-test for significance after calculating 95 % confidence intervals.

Results

In the transverse axis, the keel anchoring concept showed statistically significant (p < 0.05) lower mean values of micromotions compared to the spike anchoring concept. The highest micromotion values for both types were observed in the longitudinal axis. In no case the threshold of 200 micrometers was exceeded.

Conclusions

Both fixation systems fulfill the required criteria of primary stability. Independent of the selected anchorage type an immediate postoperative active mobilization doesn’t compromise the stability of the prostheses.

Computational framework for population-based evaluation of TKR-implanted patellofemoral joint mechanics

Differences in patient anatomy are known to influence joint mechanics. Accordingly, intersubject anatomical variation is an important consideration when assessing the design of joint replacement implants. The objective of this study was to develop a computational workflow to perform population-based evaluations of total knee replacement implant mechanics considering variation in patient anatomy and to assess the potential for an efficient sampling strategy to support design phase screening analyses. The approach generated virtual subject anatomies using a statistical shape model of the knee and performed virtual implantation to size and align the implants. A finite-element analysis simulated a deep knee bend activity and predicted patellofemoral (PF) mechanics. The study predicted bounds of performance for kinematics and contact mechanics and investigated relationships between patient factors and outputs. For example, the patella was less flexed throughout the deep knee bend activity for patients with an alta patellar alignment. The results also showed the PF range of motions in AP and ML were generally larger with increasing femoral component size. Comparison of the 10-90% bounds between sampling strategies agreed reasonably, suggesting that Latin Hypercube sampling can be used for initial screening evaluations and followed up by more intensive Monte Carlo simulation for refined designs. The platform demonstrated a functional workflow to consider variation in joint anatomy to support robust implant design.

Modification of regional bone mineral density due to femoral rasping in cementless proximally fixed total hip arthroplasty

BACKGROUND

Three-dimensional planning (3DP) in total hip arthroplasty using computed tomography (CT) to analyze bone mineral density (BMD) at the stem-femur interface has a high reported accuracy and excellent mid-term results in the literature. However, 3DP does not take into account the effect of femoral rasping on BMD distribution within the rasped cavity. Characterizing the impact of femoral rasping on BMD may help avoid mechanical failures, but this data is not accurately investigated. Therefore, we set out a cadaveric study to identify if: (1) Femoral rasping modified regional BMD in areas considered critical for bone anchorage of cementless metaphyseally fixed anatomic stems. (2) In areas of bone-implant contact with an initial high BMD, does femoral rasping increase BMD?

HYPOTHESIS

Femoral rasping increases BMD in some zones considered critical for bone anchorage of cementless metaphyseally fixed anatomic stems within the rasped femoral cavity.

METHODS

Four cadaveric femurs were selected to undergo a rasping procedure similar to surgical techniques used for metaphyseally fixed anatomic stems. Images of femurs before and after rasping were obtained with a micro-CT scanner (pixel size 35μm). BMD values before and after rasping were compared in a trabecular bone ring of 3mm thickness around the cavity created by the rasps, in a region extending 3cm above and 2cm below the middle of the lesser trochanter.

RESULTS

Average BMD increased significantly after rasping in 3 of the 4 femurs (13% (0.27 to 0.30) (p=0.004)), 12% (0.32 to 0.36 (p=0.034)) and 15% (0.4 to 0.46 (p=0.001)), while there was no significant variation in the last femur (0.32 to 0.32 (p>0.05)). Increases in regional BMD were significantly higher in the lateral and medial areas, as well as in the most distal femoral regions. There were significantly lower variations of BMD in regions with initially higher BMD.

DISCUSSION

Current opinion considers trabecular bone debris from femoral rasping to have an impact on final stem position and outcome. Our study has demonstrated an overall positive effect of femoral rasping on BMD in the rasped cavity. Understanding this in the context of 3DP may help avoid mechanical failures such as, suboptimal implant fit, fill, and stability as well as femoral fractures during stem implantation.

LEVEL OF EVIDENCE

IV, Prospective in vitro study.

Effect of anatomical variability on stress-shielding induced by short calcar-guided stems: Automated finite element analysis of 90 femora

Short stem hip implants are becoming increasingly popular since they preserve bone stock and presumably reduce stress-shielding. However, concerns remain whether they are suitable for a wide range of patients with varying anatomy. The aim of this study was to investigate how femoral anatomy influences stress-shielding induced by a short calcar-guided stem across a set of 90 CT-based femur models. A computational tool was developed that automatically selected the optimal size and position of the stem. Finite element models of the intact and implanted femurs were constructed and subjected to walking loads. Stress-shielding was evaluated in relevant volumes of interest of the proximal femur. After a detailed anatomical analysis, linear regression was performed to find potential correlations between anatomy and stress-shielding. Stress-shielding was found to be highest in the proximal regions on the medial and posterior side. A highly significant negative relationship was observed between stress-shielding and bone density; a strong positive relationship was observed with stem size and the valgus orientation of the stem with respect to the femur. The results reveal how anatomy influences stress-shielding, and they highlight the importance of evaluating new implant designs across a large population taking into account the anatomical variability. The study demonstrates that such large population studies can be conducted in an efficient way using an automated workflow. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-8, 2019.

Sampling strategies for approximating patient variability in population-based finite element studies of total hip replacement

Total hip replacements must be robust to patient variability for long-term success in the population. The challenge during the design process is evaluating an implant in a diverse population but the computational cost of simulating a population of subject-specific finite element (FE) models is not practical. We examined five strategies to generate representative subsets of subjects from a cohort of 103 implanted hip joint FE models to approximate the variability in output metrics. Comparing with the median and distribution of the 95^th percentile composite peak micromotion (CPM) and polar gap in the full cohort (CPM median: 136 μm, interquartile range [IQR]: 74-230 μm) (Polar Gap median: 467 μm, IQR: 434-548 μm), the Anatomic Sampling strategy (12 subjects) achieved the best balance of computational cost and approximation of the output metrics (CPM median: 169 μm, IQR: 78-236 μm) (Polar Gap median: 469 μm, IQR: 448-537 μm). Convex hull sampling (41 subjects) more closely captured the output metrics (CPM median: 99 μm, IQR: 70-191 μm) (Polar Gap median: 456 μm, IQR: 418-533 μm) but required over three times the number of subjects. Volume reduction of the convex hull captured the extremes of variability with subsets of 5 to 20 subjects, while the largest minimum-distance strategy captured the variability toward the middle of the cohort. These strategies can estimate the level of variability in FE model output metrics with a low computational cost when evaluating implants during the design process.

Virtual trial to evaluate the robustness of cementless femoral stems to patient and surgical variation

Primary stability is essential for the success of cementless femoral stems. In this study, patient specific finite element (FE) models were used to assess changes in primary stability due to variability in patient anatomy, bone properties and stem alignment for two commonly used cementless femoral stems, Corail® and Summit® (DePuy Synthes, Warsaw, USA). Computed-tomography images of the femur were obtained for 8 males and 8 females. An automated algorithm was used to determine the stem position and size which minimized the endo-cortical space, and then span the plausible surgical envelope of implant positions constrained by the endo-cortical boundary. A total of 1952 models were generated and ran, each with a unique alignment scenario. Peak hip contact and muscle forces for stair climbing were scaled to the donor's body weight and applied to the model. The primary stability was assessed by comparing the implant micromotion and peri-prosthetic strains to thresholds (150 μm and 7000 µε, respectively) above which fibrous tissue differentiation and bone damage are expected to prevail. Despite the wide range of implant positions included, FE prediction were mostly below the thresholds (medians: Corail®: 20-74 µm and 1150-2884 µε, Summit®: 25-111 µm and 860-3010 µε), but sensitivity of micromotion and interfacial strains varied across femora, with the majority being sensitive (p < 0.0029) to average bone mineral density, cranio-caudal angle, post-implantation anteversion angle and lateral offset of the femur. The results confirm the relationship between implant position and primary stability was highly dependent on the patient and the stem design used.

Evaluating the primary stability of standard vs lateralised cementless femoral stems - A finite element study using a diverse patient cohort

BACKGROUND

Restoring the original femoral offset is desirable for total hip replacements as it preserves the original muscle lever arm and soft tissue tensions. This can be achieved through lateralised stems, however, the effect of variation in the hip centre offset on the primary stability remains unclear.

METHODS

Finite element analysis was used to compare the primary stability of lateralised and standard designs for a cementless femoral stem (Corail®) across a representative cohort of male and female femora (N = 31 femora; age from 50 to 80 years old). Each femur model was implanted with three designs of the Corail® stem, each designed to achieve a different degree of lateralisation. An automated algorithm was used to select the size and position that achieve maximum metaphyseal fit for each of the designs. Joint contact and muscle forces simulating the peak forces during level gait and stair climbing were scaled to the body mass of each subject.

FINDINGS

The study found that differences in restoring the native femoral offset introduce marginal differences in micromotion (differences in peak micromotion <21 μm), for most cases. Nonetheless, significant reduction in the interfacial strains (>3000 με) was achieved for some subjects when lateralized stems were used.

INTERPRETATION

Findings of this study suggest that, with the appropriate size and alignment, the standard offset design is likely to be sufficient for primary stability, in most cases. Nonetheless, appropriate use of lateralised stems has the potential reduce the risk of peri-prosthetic bone damage. This highlights the importance of appropriate implant selection during the surgical planning stage.

Influence of collars on the primary stability of cementless femoral stems: A finite element study using a diverse patient cohort

For cementless femoral stems, there is debate as to whether a collar enhances primary stability and load transfer compared to collarless designs. Finite Element (FE) analysis has the potential to compare stem designs within the same cohort, allowing for subtle performance differences to be identified, if present. Subject-specific FE models of intact and implanted femora were run for a diverse cohort (21 males, 20 females; BMI 16.4-41.2 kg/m² , age 50-80 yrs). Collared and collarless versions of Corail^® (DePuy Synthes, Warsaw, IN) were sized and positioned using an automated algorithm that aligns the femoral/stem axes, preserves the head-center location, and maximizes metaphyseal fit. Joint contact and muscle forces simulating peak forces in level gait and stair climbing and were scaled to the body mass and applied to each subject. Three failure scenarios were assessed: Potential for peri-prosthetic fibrous tissue formation (stem micromotion), potential for peri-prosthetic bone damage (equivalent strains), and calcar bone remodeling (changes in strain-energy density). Comparisons were performed using paired t-tests. Only subtle differences were found (mean 90th percentile micromotion: Collared = 86 µm, collarless = 92.5 µm, mean 90th percentile interface strains: Collared = 733 µϵ, collarless = 767 µϵ, and similar remodeling stimuli were predicted). The slight differences observed were small in comparison with the inter-patient variability. Statement of clinical significance: Our results suggest that the presence/absence of a collar is unlikely to substantially alter the bone-implant biomechanics nor the initial mechanical environment. Hence, a collar is likely to have minimal clinical impact. Analysis using different femoral stem designs is recommended before generalising these findings. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1185-1195, 2018.

Primary stability of a cementless acetabular cup in a cohort of patient-specific finite element models

The primary stability achieved during total hip arthroplasty determines the long-term success of cementless acetabular cups. Pre-clinical finite element testing of cups typically use a model of a single patient and assume the results can be extrapolated to the general population. This study explored the variability in predicted primary stability of a Pinnacle^® cementless acetabular cup in 103 patient-specific finite element models of the hemipelvis and examined the association between patient-related factors and the observed variability. Cups were inserted by displacement-control into the FE models and then a loading configuration simulating a complete level gait cycle was applied. The cohort showed a range of polar gap of 284-1112 μm and 95th percentile composite peak micromotion (CPM) of 18-624 μm. Regression analysis was not conclusive on the relationship between patient-related factors and primary stability. No relationship was found between polar gap and micromotion. However, when the patient-related factors were categorised into quartile groups, trends suggested higher polar gaps occurred in subjects with small and shallow acetabular geometries and cup motion during gait was affected most by low elastic modulus and high bodyweight. The variation in primary stability in the cohort for an acetabular cup with a proven clinical track record may provide benchmark data when evaluating new cup designs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1012-1023, 2018.

Modal analysis for the assessment of cementless hip stem primary stability in preoperative THA planning

This numerical vibration finite element (FE) study introduces resonance three-dimensional planning (RP3D) to assess preoperatively the primary stability of a cementless stem for total hip arthroplasty. Based on a patient's CT-scan and a numerical model of a stem, RP3D aims at providing mechanical criteria indicative of the achievable primary stability. We investigate variations of the modal response of the stem to changes of area and apparent stiffness of the bone-implant interface. The model is computationally cheap as it does not include a mesh of the bone. The apparent stiffness of the bone is modeled by springs attached to the nodes of the stem's mesh. We investigate an extended range of stiffness values while, in future works, patient's specific Hounsfield values could be used to define stiffness. We report modal frequencies, shapes, and a ratio of elastic potential energies (rEPE) that quantifies the proximal motion that should be minimum for a stable stem. The modal response exhibits a clear transition between loose and tight contact as area and stiffness of the interface increase. rEPE thresholds that could potentially discriminate preoperatively between stable and unstable stems are given for a Symbios SPS^® size C stem.

Characterization of Femoral Component Initial Stability and Cortical Strain in a Reduced Stem-Length Design

BACKGROUND

Short-stemmed femoral components facilitate reduced exposure surgical techniques while preserving native bone. A clinically successful stem should ideally reduce risk for stress shielding while maintaining adequate primary stability for biological fixation. We asked (1) how stem-length changes cortical strain distribution in the proximal femur in a fit-and-fill geometry and (2) if short-stemmed components exhibit primary stability on par with clinically successful designs.

METHODS

Cortical strain was assessed via digital image correlation in composite femurs implanted with long, medium, and short metaphyseal fit-and-fill stem designs in a single-leg stance loading model. Strain was compared to a loaded, unimplanted femur. Bone-implant micromotion was then compared with reduced lateral shoulder short stem and short tapered-wedge designs in cyclic axial and torsional testing.

RESULTS

Femurs implanted with short-stemmed components exhibited cortical strain response most closely matching that of the intact femur model, theoretically reducing the potential for proximal stress shielding. In micromotion testing, no difference in primary stability was observed as a function of reduced stem length within the same component design.

CONCLUSION

Our findings demonstrate that within this fit-and-fill stem design, reduction in stem length improved proximal cortical strain distribution and maintained axial and torsional stability on par with other stem designs in a composite femur model. Short-stemmed implants may accommodate less invasive surgical techniques while facilitating more physiological femoral loading without sacrificing primary implant stability.

Full-field measurement of micromotion around a cementless femoral stem using micro-CT imaging and radiopaque markers

A good primary stability of cementless femoral stems is essential for the long-term success of total hip arthroplasty. Experimental measurement of implant micromotion with linear variable differential transformers is commonly used to assess implant primary stability in pre-clinical testing. But these measurements are often limited to a few distinct points at the interface. New techniques based on micro-computed tomography (micro-CT) have recently been introduced, such as Digital Volume Correlation (DVC) or markers-based approaches. DVC is however limited to measurement around non-metallic implants due to metal-induced imaging artifacts, and markers-based techniques are confined to a small portion of the implant. In this paper, we present a technique based on micro-CT imaging and radiopaque markers to provide the first full-field micromotion measurement at the entire bone-implant interface of a cementless femoral stem implanted in a cadaveric femur. Micromotion was measured during compression and torsion. Over 300 simultaneous measurement points were obtained. Micromotion amplitude ranged from 0 to 24µm in compression and from 0 to 49µm in torsion. Peak micromotion was distal in compression and proximal in torsion. The technique bias was 5.1µm and its repeatability standard deviation was 4µm. The method was thus highly reliable and compared well with results obtained with linear variable differential transformers (LVDTs) reported in the literature. These results indicate that this micro-CT based technique is perfectly relevant to observe local variations in primary stability around metallic implants. Possible applications include pre-clinical testing of implants and validation of patient-specific models for pre-operative planning.

Finite Element-Derived Surrogate Models of Locked Plate Fracture Fixation Biomechanics

Internal fixation of bone fractures using plates and screws involves many choices-implant type, material, sizes, and geometric configuration-made by the surgeon. These decisions can be important for providing adequate stability to promote healing and prevent implant mechanical failure. The purpose of this study was to develop mathematical models of the relationships between fracture fixation construct parameters and resulting 3D biomechanics, based on parametric computer simulations. Finite element models of hundreds of different locked plate fixation constructs for midshaft diaphyseal fractures were systematically assembled using custom algorithms, and axial, torsional, and bending loadings were simulated. Multivariate regression was used to fit response surface polynomial equations relating fixation design parameters to outputs including maximum implant stresses, axial and shear strain at the fracture site, and construct stiffness. Surrogate models with as little as three regressors showed good fitting (R ² = 0.62-0.97). Inner working length was the strongest predictor of maximum plate and screw stresses, and a variety of quadratic and interaction terms influenced resulting biomechanics. The framework presented in this study can be applied to additional types of bone fractures to provide clinicians and implant designers with clinical insight, surgical optimization, and a comprehensive mathematical description of biomechanics.