Micro-mechanical modeling of the cement-bone interface: the effect of friction, morphology and material properties on the micromechanical response

Biomechanical study of using patient-specific diaphyseal femoral cone in revision total knee arthroplasty (rTKA)

Background

The primary objective of revision total knee surgery is to achieve solid bone fixation. Generally, this could be accomplished using sleeves and long stems, which require substantial remaining bone stock and may increase the risk of stem tip pain. An alternative approach involves the use of customized diaphyseal cones, which can preserve the integrity of the bone canal. This study evaluates the impact of employing femoral diaphyseal cones with various stem lengths on stress distribution and relative motion.

Methods

CT scan data from five patients were used to generate the 3D model of the femur, cement, customized stems, and cones, along with assigning patient-specific material for each candidate's femur. Three different stem lengths, both with and without the customized cone, were assessed under three gait loading conditions to compare the resulting Von Mises stress distribution and relative motion.

Results

Analysis indicated that the use of customized femoral cones moderately increases stress distribution values up to 30 % while significantly reducing relative motion at the femoral canal-cone interface by nearly 60 %. The presence of the cone did not significantly alter relative motion with varying stem lengths, although stem length variation without a cone substantially affected these values.

Conclusion

Incorporating cones alongside stems enhances metaphyseal fixation, reduces stress shielding, potentially allowing for the use of shorter stems. Furthermore, cones promote osseointegration by minimizing relative motion, ultimately improving prosthetic stability.

Finite element study on the micromechanics of cement-augmented proximal femoral nail anti-rotation (PFNA) for intertrochanteric fracture treatment

Blade cut-out is a common complication when using proximal femoral nail anti-rotation (PFNA) for the treatment of intertrochanteric fractures. Although cement augmentation has been introduced to overcome the cut-out effect, the micromechanics of this approach remain to be clarified. While previous studies have developed finite element (FE) models based on lab-prepared or cadaveric samples to study the cement-trabeculae interface, their demanding nature and inherent disadvantages limit their application. The aim of this study was to develop a novel 'one-step forming' method for creating a cement-trabeculae interface FE model to investigate its micromechanics in relation to PFNA with cement augmentation. A human femoral head was scanned using micro-computed tomography, and four volume of interest (VOI) trabeculae were segmented. The VOI trabeculae were enclosed within a box to represent the encapsulated region of bone cement using ANSYS software. Tetrahedral meshing was performed with Hypermesh software based on Boolean operation. Finally, four cement-trabeculae interface FE models comprising four interdigitated depths and five FE models comprising different volume fraction were established after element removal. The effects of friction contact, frictionless contact, and bond contact properties between the bone and cement were identified. The maximum micromotion and stress in the interdigitated and loading bones were quantified and compared between the pre- and post-augmentation situations. The differences in micromotion and stress with the three contact methods were minimal. Micromotion and stress decreased as the interdigitation depth increased. Stress in the proximal interdigitated bone showed a correlation with the bone volume fraction (R2 = 0.70); both micromotion (R2 = 0.61) and stress (R2 = 0.93) at the most proximal loading region exhibited a similar correlation tendency. When comparing the post- and pre-augmentation situations, micromotion reduction in the interdigitated bone was more effective than stress reduction, particularly near the cement border. The cementation resulted in a significant reduction in micromotion within the loading bone, while the decrease in stress was minimal. Noticeable gradients of displacement and stress reduction can be observed in models with lower bone volume fraction (BV/TV). In summary, cement augmentation is more effective at reducing micromotion rather than stress. Furthermore, the reinforcing impact of bone cement is particularly prominent in cases with a low BV/TV. The utilization of bone cement may contribute to the stabilization of trabecular bone and PFNA primarily by constraining micromotion and partially shielding stress.

[Cement in revision arthroplasty-what about the "glacier effect"? : Case studies viewed from different perspectives]

BACKGROUND

The number of operations concerning revision arthroplasty has been increasing continuously in recent years, and it can be assumed that they will continue to increase in the future. If an arthroplasty implant becomes loose, it must be changed. The question often arises as to how the new implant should be fixed in the bone.

ADVANTAGES

Revision implants can be inserted into the bone without cement. In the subsequent period, a secondary osseointegration of the implant takes place. Another possibility is to anchor the implant by using bone cement. The advantage of cemented anchorage is that the implant is firmly fixed in the bone, in principle, immediately, and it is possible to fully load the implant directly. Direct postoperative full weight bearing is helpful, especially for older and multimorbid patients, in order to achieve rapid mobilization.

PREREQUISITES AND CHALLENGES

When using cement in revision cases, however, there are a few prerequisites and challenges that the surgeon should definitely take into account. In the case of revision, the bone in the former implant bed is often deficient and appears thinned and sclerosed. It is, therefore, important to analyze the bone quality preoperatively on radiographic images and to include it in the planning of the anchoring strategy. In addition, the individual bone quality of the patient must also be taken into account intraoperatively. In any case, it must be clarified whether the basic prerequisites for the sufficient bond strength of the cement with the bone to be formed can still be met. Furthermore, the principles of cementing technique must be strictly observed, and the goal of a perfect cement mantle must be aimed for. If the indication for this is overstated, early loosening of the cemented revision arthroplasty is very likely.

Micromechanical interlocking structure at the filler/resin interface for dental composites: a review

Dental resin composites (DRCs) are popular materials for repairing caries or dental defect, requiring excellent properties to cope with the complex oral environment. Filler/resin interface interaction has a significant impact on the physicochemical/biological properties and service life of DRCs. Various chemical and physical modification methods on filler/resin interface have been introduced and studied, and the physical micromechanical interlocking caused by the modification of fillers morphology and structure is a promising method. This paper firstly introduces the composition and development of DRCs, then reviews the chemical and physical modification methods of the filler/resin interface, mainly discusses the interface micromechanical interlocking structures and their enhancement mechanism for DRCs, finally give a summary on the existing problems and development potential.

Multi-objective design optimization of dental implant geometrical parameters

In-silico investigations are becoming an integral part of the development of novel biomedical devices, including dental implants. Using computer simulations can streamline the process by tuning different geometrical and structural features, emphasizing the osseointegration of the implant design a priori, leading to the optimal designs in preparation for in-vivo trails. This research aims to elucidate the interrelationship between 12 geometrical variables that holistically define the shape of the implant. The approach to achieve optimality hinged on coupling the finite element analysis results with the fractional factorial design method. The latter was used to determine the most influential variables during the screening process, followed by the parameter optimization process using the response surface method, regarding four different objectives, namely: bone-implant contact area, volume of trabecular bone dead cells, volume of cortical bone dead cells, and axial displacement. This resulted in reducing the number of virtual experiments and substantially decreasing the computational cost without compromising the accuracy of the solution. It was found that the optimized values improved the performance significantly. The validity of all models was verified by comparing optimized responses with simulation results. A sensitivity analysis was performed on all five optimized models to address the effect of friction coefficient on the implant-bone joint interaction. It was shown that the mechanical behavior of implant-bone would be independent in higher friction coefficients. The significance of this study is demonstrated in determining the most effective and optimized values of all possible geometrical parameters considering their singular or interactive effects.

Effect of stem position and length on bone-stem constructs after cementless hip arthroplasty

Aims

There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning.

Methods

In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition.

Results

Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition.

Conclusion

The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem.

Cite this article: Bone Joint Res 2021;10(4):250–258.

A Multiscale Finite Element Analysis of Balloon Kyphoplasty to Investigate the Risk of Bone-Cement Separation In Vivo

BACKGROUND

During the past decade there has been a significant increase in the number of vertebral fractures being treated with the balloon kyphoplasty procedure. Although previous investigations have found kyphoplasty to be an effective treatment for reducing patient pain and lowering cement-leakage risk, there have been reports of vertebral recollapse following the procedure. These reports have indicated evidence of in vivo bone-cement separation leading to collapse of the treated vertebra.

METHODS

The following study documents a multiscale analysis capable of evaluating the risk of bone-cement interface separation during lying, standing, and walking activities following balloon kyphoplasty.

RESULTS

Results from the analysis found that instances of reduced cement interlock could initiate both tensile and shear separation of the interface region at up to 7 times the failure threshold during walking or up to 1.9 times the threshold during some cases for standing. Lying prone offered the best protection from interface failure in all cases, with a minimum safety factor of 2.95.

CONCLUSIONS

The results of the multiscale analysis show it is essential for kyphoplasty simulations to take account of the micromechanical behavior of the bone-cement interface to be truly representative of the in vivo situation after the treatment. The results further illustrate the importance of ensuring adequate cement infiltration into the compacted bone periphery during kyphoplasty through a combination of new techniques, tools, and biomaterials in a multifaceted approach to solve this complex challenge.

The impact of tourniquet on tibial bone cement penetration in different zones in primary total knee arthroplasty: a meta-analysis

Background

Cement mantle penetration and the cement–bone interface strength were critical to a successful primary total knee arthroplasty (TKA). It remained unclear whether decreased blood and fat in the cancellous bone achieved with the use of a tourniquet increases tibial cement mantle penetration in different zones on AP and lateral view in TKA according to criteria defined by the Knee Society Scoring System (KSS). The purpose of this study was to determine whether tourniquet use influences tibial cement mantle penetration in different zones on AP and lateral view in TKA according to KSS.

Methods

We conducted a meta-analysis to identify studies involving the impact of tourniquet use and no tourniquet use on tibial bone cement penetration in primary TKA in electronic databases, including Web of Science, Embase, PubMed, Cochrane Controlled Trials Register, Cochrane Library, Highwire, CBM, VIP, Wanfang database, up to January 2021. Finally, we identified 1231 patients (1231 knees) assessed in twelve studies.

Results

Tourniquet use increases the cumulative cement mantle penetration (P < 0.00001), mean cement mantle penetration (P = 0.004), and cement mantle in zone 3(P < 0.0001) on AP view. However, there were no significant differences in cement mantle in zone 1(P = 0.5), zone 2(P =0 .54), zone 4(P = 0.07) on AP view, and zone 1(P = 0.32), zone 2(P = 0.38) on lateral view between two groups. There were also no significant differences in length of surgery(P = 0.7), change in hemoglobin(P = 0.4), transfusion rates(P = 0.47), and complications such as muscular calf vein thrombosis(P = 0.21), superficial infection (P = 0.72), and deep vein thrombosis (P = 0.66) between two groups.

Conclusion

The application of a tourniquet increases the thickness of the tibial bone cement penetration—the increase in the thickness of bone cement penetration mainly located in zone 3 on the anteroposterior (AP) view.

Reliability of two different measuring techniques with computer tomography for penetration and distribution of cement in the proximal tibia after total knee arthroplasty

BACKGROUND

To evaluate the reliability of two different techniques for measuring penetration and distribution of the cement mantle in the proximal tibia after total knee arthroplasty (TKA) with Computer Tomography (CT) in vivo.

METHODS

Standardized CT scans of the proximal tibia were taken 1 to 2 years after total knee arthroplasties implanted with a surface cementing technique. These prospectively acquired transversal CT images of the surface of the proximal tibia were divided into four quadrants and were assessed once manually and once with a numerical computing program (MATLAB® Update 2, The MathWorks, Inc.) based on Hounsfield Units by one of the researchers. The assessments were repeated by the same and a second researcher. The ratio cement/trabecular bone was calculated 1, 3 and 5 mm distal of the tibia tray per quadrant. Kruskall-Wallis tests with multiple pairwise comparisons (Dunn's test) were used to determine differences between the quadrants. Intra- and inter-rater reliability as well as the inter method reliability were assessed with the Intraclass Correlation Coefficient (ICC) per level of depth and with Bland-Altman plots.

RESULTS

A total of 92 CT scans were included. The intra- and inter-rater reliability of the manual method ranged from 0.22 and 0.52. The intra- and inter-rater reliability of the matlab method varied between 0.98 to 0.99. The median percentage cement measured with the matlab method 1 mm underneath the tibial tray varied between 82 and 88%; at 3 mm depth between 38 and 54% and at 5 mm between 15 and 25%. There was significantly (p < 0.05) less cement in the antero-medial quadrant compared to the antero-lateral and postero-lateral quadrant at 3 mm and 5 mm depth.

CONCLUSIONS

Distribution and penetration of cement in the proximal tibia in a total knee arthroplasty can be measured reliably with CT in combination with the matlab method presented in this manuscript. This method can be used for clinical purposes as well as for scientific research.

TRIAL REGISTRATION

METC-nr: 06-104 Dossier NL14807.098.06/versie 06.

Cementing Osseointegration Implants Results in Loosening: Case Report and Review of Literature

Skeletal transcutaneous osseointegration was performed on a 54-year-old female transfemoral amputee. None of the available osseointegration implants achieved press-fit stability, so an implant was cemented in position. Although initially stable, by six months the patient reported painful loading and radiographs revealed cement mantle lucency. The osseointegration implant was removed, antibiotics were delivered via implanted spacer and intravenously, and revision osseointegration three months later achieved appropriate immediate press-fit stability. Cemented transcutaneous osseointegration implants loosen within one year. Osseointegration is only successful when bone grows directly onto the implant.

Sensitivity analysis of the material properties of different soft-tissues: implications for a subject-specific knee arthroplasty

Introduction

While developing a subject-specific knee model, different kinds of data-inputs are required. If information about geometries can be definitely obtained from images, more effort is necessary for the in vivo properties. Consequently, such information are recruited from the literature as common habit. However, the effects of the combined sources still need to be evaluated.

Methods

This work aims at developing an intact native subject-specific knee model for performing a sensitivity analysis on soft-tissues. The impacts on the biomechanical outputs were analysed during a daily activity for which articular knee kinetics and kinematics were compared among the different configurations. Prior to the sensitivity analysis, experimental and literature data were checked for the model reliability.

Results

Average values of mixed sources allowed the agreement with experimental data for personalized outputs. From the sensitivity analysis, knee kinematics did not significantly change in the selected ranges of properties for the soft-tissues (in rotation less than 0.5°), while contact stresses were greatly affected, especially for the articular cartilage (with differences in the results more than 100%).

Conclusion

In conclusion, during the development of a personalized knee model, the selection of the correct material properties is fundamental because wrong values could highly affect the numerical results.

Level of evidence

III a.

The effect of tourniquet usage on cement penetration in total knee arthroplasty: A prospective randomized study of 3 methods

BACKGROUND

It is a randomized study to compare cement penetration on x-rays after total knee arthroplasty (TKA) among 3 different ways to use tourniquets; application during the surgery, application only with implantation prosthesis and with no tourniquet use.

METHODS

A total 69 knees of 59 patients were included in the study in a quasirandom manner. Each patient had physical exams and standard radiographic evaluations at 6 weeks after the TKA procedure. Outcome evaluations included visual analog scale (VAS) scores, Knee Society Scores (KSS), blood transfusion, and drainage status after surgery for all groups. For radiographic review, the tibial plateau was divided into zones in the anterior-posterior and lateral views, according to the Knee Society Scoring System.

RESULTS

The average age of the patients who were eligible for the study was 65.05 (range 46-81) years. All 59 patients included in the study were female patients. Group 1 consisted of 24 patients who had TKA with use of a tourniquet during the entire operation. Group 2 consisted of 20 patients who had TKA with use of tourniquet only at the time of cementing and group 3 consisted of 25 patients with no use tourniquet. There is no significant difference in early cement penetration among the groups (group 1 2.50 mm, group 2 2.28 mm, group 3 2.27 mm; group 1 vs 2 P = .083, group 1 vs 3 P = .091, group 2 vs 3 P = .073). There is no significant difference for postoperative drainage among the 3 groups (group 1 245 mL, group 2 258.76 mL, group 3 175.88 mL; group 1 vs 2 P = .081, group 1 vs 3 P = .072, group 2 vs 3 P = .054). There was no need to transfuse more than 1 unit in any patient. The VAS score was significantly higher (group 1 3.58, group 2 1.55, group 3 1.52; group 1 vs 2 P = .022, group 1 vs 3 P = .018, group 2 vs 3 P = .062) and KSS was significantly lower in the tourniquet group (group 1 63, group 2 79, group 3 82; group 1 vs 2 P = .017, group 1 vs 3 P = .02, group 2 vs 3 P = .082).

CONCLUSION

We do not suggest long-duration tourniquet use, which can lead higher pain scores and reduce functional recovery after total knee arthroplasty.

Cementing technique affects the rate of femoral component loosening after high flexion total knee arthroplasty

BACKGROUND

The purpose of this study was to determine the effects of different cementing techniques on the rate of early femoral loosening of high-flexion total knee arthroplasties (TKAs).

METHODS

A total of 734 knees from 486 patients treated with high-flexion design TKA between July 2001 and July 2010 were divided into two groups based on the cementing technique used. For 403 knees (group N), cement was applied onto the distal and anterior cut surfaces of the femur and the posterior flanges of the femoral component without pressurization. For 331 knees (group P), cement was applied onto distal and anterior femoral cut surfaces with digital pressurization and whole cement surfaces of the femoral component. Two groups were subjected to clinical and radiological evaluation with a minimum five year follow-up period. Cox proportional hazards model with revision surgery of the prosthesis or radiological loosening as an endpoint was used to evaluate the effect of the cementing technique and other covariates.

RESULTS

The pressurizing and bi-surface cementing technique resulted in significant reduction in femoral radiological loosening incidence compared to that without pressurization (0.3% vs. 2.5%, P=0.015) and revision rate for aseptic causes (0.9% vs. 3.2%, P=0.032). Cox proportional hazard regression analysis revealed a significant difference in component survival rate between the two groups if femoral radiological loosening was considered as failure (hazard ratio, 4.229, 95% confidence interval (CI): 1.256-14.243, P=0.020).

CONCLUSION

Pressurizing and bi-surface cementation can reduce the occurrence rate of early loosening around the femoral component in high-flexion TKAs.

Finite Element Analysis of porously punched prosthetic short stem virtually designed for simulative uncemented Hip Arthroplasty

Background

There is no universal hip implant suitably fills all femoral types, whether prostheses of porous short-stem suitable for Hip Arthroplasty is to be measured scientifically.

Methods

Ten specimens of femurs scanned by CT were input onto Mimics to rebuild 3D models; their *stl format dataset were imported into Geomagic-Studio for simulative osteotomy; the generated *.igs dataset were interacted by UG to fit solid models; the prosthesis were obtained by the same way from patients, and bored by punching bears designed by Pro-E virtually; cements between femora and prosthesis were extracted by deleting prosthesis; in HyperMesh, all compartments were assembled onto four artificial joint style as: (a) cemented long-stem prosthesis; (b) porous long-stem prosthesis; (c) cemented short-stem prosthesis; (d) porous short-stem prosthesis. Then, these numerical models of Finite Element Analysis were exported to AnSys for numerical solution.

Results

Observed whatever from femur or prosthesis or combinational femora-prostheses, “Kruskal-Wallis” value p > 0.05 demonstrates that displacement of (d) ≈ (a) ≈ (b) ≈ (c) shows nothing different significantly by comparison with 600 N load. If stresses are tested upon prosthesis, (d) ≈ (a) ≈ (b) ≈ (c) is also displayed; if upon femora, (d) ≈ (a) ≈ (b) < (c) is suggested; if upon integral joint, (d) ≈ (a) < (b) < (c) is presented.

Conclusions

Mechanically, these four sorts of artificial joint replacement are stabilized in quantity. Cemented short-stem prostheses present the biggest stress, while porous short-stem & cemented long-stem designs are equivalently better than porous long-stem prostheses and alternatives for femoral-head replacement. The preferred design of those two depends on clinical conditions. The cemented long-stem is favorable for inactive elders with osteoporosis, and porously punched cementless short-stem design is suitable for patients with osteoporosis, while the porously punched cementless short-stem is favorable for those with a cement allergy. Clinically, the strength of this study is to enable preoperative strategy to provide acute correction and decrease procedure time.

Is cement penetration in TKR reduced by not using a tourniquet during cementation? A single blinded, randomized trial

Despite suggestions that tourniquet inflation during total knee replacement reduces bleeding and hence improves cement penetration, no studies exist supporting this widely held belief. In this single-blinded, single-surgeon, randomized controlled trial, the tourniquet inflation during cementation group ( n = 20) did not have greater tibial cement penetration compared to a no tourniquet group ( n = 20). No statistically significant differences in semiautomatic digitally measured average and central radiographic tibial plateau penetration values were observed between the two groups ( p = 0.93; p = 0.84). Tourniquet inflation during cementation does not appear to improve tibial cementation penetration.

Compression and flexural strength of bone cement mixed with blood

PURPOSE

To assess the compression and flexural strength of bone cement mixed with 0 ml, 1 ml, or 2 ml of blood.

METHODS

High viscosity polymethyl methacrylate (PMMA) loaded with or without gentamicin was used. Blood was collected from total knee arthroplasty patients. In the same operating room, one pack of cement each was mixed with 0 ml (control), 1 ml, or 2 ml of blood for 1 minute during the dough phase. The dough was extruded into cylindrical and rectangular moulds for 20 minutes of setting, and then cured in phosphate buffered saline at 37±1ºC for 7 days. The samples were visually inspected for fractures and areas of weakness, and then scanned using microcomputed tomography. 48 gentamicin-loaded and 59 non-gentamicin-loaded samples mixed with 0 ml (control), 1 ml, or 2 ml of blood were randomised for flexural and compression strength testing; each group had at least 6 samples.

RESULTS

In samples loaded with or without gentamicin, the flexural and compressive strength was highest in controls, followed by samples mixed with 1 ml or 2 ml of blood. In samples mixed with 2 ml of blood, the flexural strength fell below the standard of 50 MPa. In samples mixed with 2 ml of blood and all gentamicin-loaded samples, the compressive strength fell below the standard of 70 MPa. Microcomputed tomography revealed areas of voids and pores indicating the presence of laminations and partitions within.

CONCLUSION

The biomechanical strength of PMMA contaminated with blood may decrease. Precautions such as saline lavage, pack drying the bone, change of gloves, and prompt insertion of the implant should be taken to prevent blood from contaminating bone cement.

All-polyethylene tibial components generate higher stress and micromotions than metal-backed tibial components in total knee arthroplasty

PURPOSE

Most total knee arthroplasty tibial components are metal-backed, but an alternative tibial component made entirely of polyethylene (all-polyethylene design) exists. While several clinical studies have shown that all-poly design performs similarly to the metal-backed, the objective of this study is to perform a biomechanical comparison.

METHODS

Loads, constraints and geometries during a squat activity at 120° of flexion were obtained from a validated musculoskeletal model and applied to a finite element model. Stresses in the tibia and micromotions at the bone-implant interface were evaluated for several implant configurations: (1) three different thicknesses of the cement penetration under the baseplate (2, 3 and 4 mm), (2) the presence or absence of a cement layer around the stem of the tibial tray and (3) three different bone conditions (physiological, osteopenic and osteoporotic bone).

RESULTS

All-polyethylene tibial components resulted in significantly higher (p < 0.001) and more uneven stress distributions in the cancellous bone under the baseplate (peak difference: +128.4 %) and fivefold increased micromotions (p < 0.001). Performance of both implant designs worsened with poorer bone quality with peaks in stress and micromotion variations of +40.8 and +54.0 %, respectively (p < 0.001). Performance improvements when the stem was cemented were not statistically significant (n.s.).

CONCLUSION

The metal-backed design showed better biomechanical performance during a squat activity at 120° of flexion compared to the all-polyethylene design. These results should be considered when selecting the appropriate tibial component for a patient, especially in the presence of osteoporotic bone or if intense physical activity is foreseen.

Biomechanical effects of morphological variations of the cortical wall at the bone-cement interface

BACKGROUND

The integrity of bone-cement interface is very important for the stabilization and long-term sustain of cemented prosthesis. Variations in the bone-cement interface morphology may affect the mechanical response of the shape-closed interlock.

METHODS

Self-developed new reamer was used to process fresh pig reamed femoral canal, creating cortical grooves in the canal wall of experimental group. The biomechanical effects of varying the morphology with grooves of the bone-cement interface were investigated using finite element analysis (FEA) and validated using companion experimental data. Micro-CT scans were used to document interlock morphology.

RESULTS

The contact area of the bone-cement interface was greater (P < 0.05) for the experimental group (5470 ± 265 mm(2)) when compared to the specimens of control group (5289 ± 299 mm(2)). The mechanical responses to tensile loading and anti-torsion showed that the specimens with grooves were stronger (P < 0.05) at the bone-cement interface than the specimens without grooves. There were positively significant correlation between the contact area and the tensile force (r (2) = 0.85) and the maximal torsion (r (2) = 0.77) at the bone-cement interface. The volume of cement of the experimental group (7688 ± 278 mm(3)) was greater (P < 0.05) than of the control group (5764 ± 186 mm(3)). There were positively significant correlations between the volume of cement and the tensile force (r (2) = 0.90) and the maximal torsion (r (2) = 0.97) at the bone-cement interface. The FEA results compared favorably to the tensile and torsion relationships determined experimentally. More cracks occurred in the cement than in the bone.

CONCLUSIONS

Converting the standard reaming process from a smooth bore cortical tube to the one with grooves permits the cement to interlock with the reamed bony wall. This would increase the strength of the bone-cement interface.

Computer Simulation and Analysis on Flow Characteristics and Distribution Patterns of Polymethylmethacrylate in Lumbar Vertebral Body and Vertebral Pedicle

This study was designed to analyze the flow and distribution of polymethylmethacrylate (PMMA) in vertebral body through computer simulation. Cadaveric lumbar vertebrae were scanned through electron beam tomography (EBT). The data was imported into Mimics software to build computational model. Vertebral body center and junction of pedicle and vertebral body were chosen as injection points. Silicone oil with viscosity of 100,000 cSt matching with PMMA bone cement was chosen for injection. The flow and distribution of silicone oil were analyzed using Fluent software. In vertebral body, silicone oil formed a circle-like shape centered by injection point on transverse and longitudinal sections, finally forming a sphere-like shape as a whole. Silicone oil diffused along lateral and posterior walls forming a circle-like shape on transverse section centered by injection point in pedicle, eventually forming a sphere-like shape as a whole. This study demonstrated that silicone oil flowed and diffused into a circle-like shape centered by injection point and finally formed a sphere-like shape as a whole in both vertebral body and pedicle. The flow and distribution of silicon oil in computational model could simulate PMMA distribution in vertebral body. It may provide theoretical evidence to reduce PMMA leakage risk during percutaneous vertebroplasty.

Peri-implant stress correlates with bone and cement morphology: Micro-FE modeling of implanted cadaveric glenoids

Aseptic loosening of cemented joint replacements is a complex biological and mechanical process, and remains a clinical concern especially in patients with poor bone quality. Utilizing high resolution finite element analysis of a series of implanted cadaver glenoids, the objective of this study was to quantify relationships between construct morphology and resulting mechanical stresses in cement and trabeculae. Eight glenoid cadavers were implanted with a cemented central peg implant. Specimens were imaged by micro-CT, and subject-specific finite element models were developed. Bone volume fraction, glenoid width, implant-cortex distance, cement volume, cement-cortex contact, and cement-bone interface area were measured. Axial loading was applied to the implant of each model and stress distributions were characterized. Correlation analysis was completed across all specimens for pairs of morphological and mechanical variables. The amount of trabecular bone with high stress was strongly negatively correlated with both cement volume and contact between the cement and cortex (r = -0.85 and -0.84, p < 0.05). Bone with high stress was also correlated with both glenoid width and implant-cortex distance. Contact between the cement and underlying cortex may dramatically reduce trabecular bone stresses surrounding the cement, and this contact depends on bone shape, cement amount, and implant positioning.

Numerical Methodology to Evaluate the Effects of Bone Density and Cement Augmentation on Fixation Stiffness of Bone-Anchoring Devices

Bone quality is one of the reported factors influencing the success of bone anchors in arthroscopic repairs of torn rotator cuffs at the shoulder. This work was aimed at developing refined numerical methods to investigate how bone quality can influence the fixation stiffness of bone anchors. To do that bone biopsies were scanned at 26-μm resolution with a high-resolution microcomputer tomography (micro-CT) scanner and their images were processed for virtual implantation of a typical design of bone anchor. These were converted to microfinite element (μFE) and homogenized classical FE models, and analyses were performed to simulate pulling on the bone anchor with and without cement augmentation. Quantification of structural stiffness for each implanted specimen was then computed, as well as stress distributions within the bone structures, and related to the bone volume fraction of the specimens. Results show that the classical method is excellently correlated to structural predictions of the more refined μFE method, despite the qualitative differences in local stresses in the bone surrounding the implant. Predictions from additional loading cases suggest that structural fixation stiffness in various directions is related to apparent bone density of the surrounding bone. Augmentation of anchoring with bone cement stiffens the fixation and alters these relations. This work showed the usability of homogenized FE (hFE) in the evaluation of bone anchor fixation and will be used to develop new methodologies for virtual investigations leading to optimized repairs of rotator cuff and glenoid Bankart lesions.

Need for CT-based bone density modelling in finite element analysis of a shoulder arthroplasty revealed through a novel method for result analysis

Treatment of common pathologies of the shoulder complex, such as rheumatoid arthritis and osteoporosis, is usually performed by total shoulder arthroplasty (TSA). Survival of the glenoid component is still a problem in TSA, whereas the humeral component is rarely subject to failure. To set up a finite element analysis (FEA) for simulation of a TSA in order to gain insight into the mechanical behaviour of a glenoid implant, the modelling procedure and the application of boundary conditions are of major importance because the computed result strongly depends upon the accuracy and sense of realism of the model. The goal of this study was to show the influence on glenoid stress distribution of a patient-specific bone density distribution compared with a homogenous bone density distribution for the purpose of generating a valid model in future FEA studies of the shoulder complex. Detailed information on the integration of bone density properties using existing numerical models as well as the applied boundary conditions is provided. A novel approach involving statistical analysis of values derived from an FEA is demonstrated using a cumulative distribution function. The results show well the mechanically superior behaviour of a realistic bone density distribution and therefore emphasise the necessity for patient-specific simulations in biomechanical and medical simulations.

Cementing techniques for the tibial component in primary total knee replacement

The optimum cementing technique for the tibial component in cemented primary total knee replacement (TKR) remains controversial. The technique of cementing, the volume of cement and the penetration are largely dependent on the operator, and hence large variations can occur. Clinical, experimental and computational studies have been performed, with conflicting results. Early implant migration is an indication of loosening. Aseptic loosening is the most common cause of failure in primary TKR and is the product of several factors. Sufficient penetration of cement has been shown to increase implant stability. This review discusses the relevant literature regarding all aspects of the cementing of the tibial component at primary TKR.

On stabilization of loosened hip stems via cement injection into osteolytic cavities

BACKGROUND

Cement injection into osteolytic areas around the cement mantle is a technique for refixation of loose hip implants for patients who cannot undergo standard revision surgery. Preliminary clinical results show the improvement in walking distance, patients' independence and pain relief.

METHODS

In this study, we use a detailed finite element model to analyze whether cement injection into osteolytic areas contributes to the overall implant stability. We study the effect of various factors, like location and size of osteolytic areas, interface conditions and bone stiffness on bone-cement relative motion.

FINDINGS

Presented results demonstrate that the procedure is most effective for the osteolytic areas located in the proximal region of the femur, while factors like a thin layer of residual fibrous tissue around the injected cement, that was not removed during the surgery, combined with reduced bone stiffness reduce the efficiency of the procedure.

INTERPRETATION

Cement injection is able to stabilize loosened hip prostheses. However, it is important to remove the fibrous tissue layer completely, as even a thin layer will negatively influence stabilization. We will focus our research efforts on developing fibrous tissue removal techniques in order to optimize this minimally invasive treatment.

Interface micromechanics of transverse sections from retrieved cemented hip reconstructions: an experimental and finite element comparison

In finite element analysis (FEA) models of cemented hip reconstructions, it is crucial to include the cement-bone interface mechanics. Recently, a micromechanical cohesive model was generated which reproduces the behavior of the cement-bone interface. The goal was to investigate whether this cohesive model was directly applicable on a macro level. From transverse sections of retrieved cemented hip reconstructions, two FEA-models were generated. The cement-bone interface was modeled with cohesive elements. A torque was applied and the cement-bone interface micromotions, global stiffness and stem translation were monitored. A sensitivity analysis was performed to investigate whether the cohesive model could be improved. All results were compared with experimental findings. That the original cohesive model resulted in a too compliant macromechanical response; the motions were too large and the global stiffness too small. When the cohesive model was modified, the match with the experimental response improved considerably.

Differences in the stress distribution in the distal femur between patellofemoral joint replacement and total knee replacement: a finite element study

Background

Patellofemoral joint replacement is a successful treatment option for isolated patellofemoral osteoarthritis. However, results of later conversion to total knee replacement may be compromised by periprosthetic bone loss. Previous clinical studies have demonstrated a decrease in distal femoral bone mineral density after patellofemoral joint replacement. It is unclear whether this is due to periprosthetic stress shielding. The main objective of the current study was to evaluate the stress shielding effect of prosthetic replacement with 2 different patellofemoral prosthetic designs and with a total knee prosthesis.

Methods

We developed a finite element model of an intact patellofemoral joint, and finite element models of patellofemoral joint replacement with a Journey PFJ prosthesis, a Richards II prosthesis, and a Genesis II total knee prosthesis. For each of these 4 finite element models, the average Von Mises stress in 2 clinically relevant regions of interest were evaluated during a simulated squatting movement until 120 degrees of flexion.

Results

During deep knee flexion, in the anterior region of interest, the average Von Mises stress with the Journey PFJ design was comparable to the physiological knee, while reduced by almost 25% for both the Richards II design and the Genesis II total knee joint replacement design. The average Von Mises stress in the supracondylar region of interest was similar for both patellofemoral prosthetic designs and the physiological model, with slightly lower stress for the Genesis II design.

Conclusions

Patellofemoral joint replacement results in periprosthetic stress-shielding, although to a smaller degree than in total knee replacement. Specific patellofemoral prosthetic design properties may result in differences in femoral stress shielding.

Trabecular level analysis of bone cement augmentation: a comparative experimental and finite element study

The representation of cement–augmented bone in finite element (FE) models of vertebrae following vertebroplasty remains a challenge, and the methods of the model validation are limited. The aim of this study was to create specimen-specific FE models of cement–augmented synthetic bone at the microscopic level, and to develop a new methodology to validate these models. An open cell polyurethane foam was used reduce drying effects and because of its similar structure to osteoporotic trabecular bone. Cylindrical specimens of the foam were augmented with PMMA cement. Each specimen was loaded to three levels of compression inside a micro-computed tomography (μCT) scanner and imaged both before compression and in each of the loaded states. Micro-FE models were generated from the unloaded μCT images and displacements applied to match measurements taken from the images. A morphological comparison between the FE-predicted trabecular deformations and the corresponding experimental measurements was developed to validate the accuracy of the FE model. The predicted deformation was found to be accurate (less than 12% error) in the elastic region. This method can now be used to evaluate real bone and different types of bone cements for different clinical situations.

Micro-mechanical damage of trabecular bone-cement interface under selected loading conditions: a finite element study

In this study, two micro finite element models of trabecular bone-cement interface developed from high resolution computed tomography (CT) images were loaded under compression and validated using the in situ experimental data. The models were then used under tension and shear to examine the load transfer between the bone and cement and the micro damage development at the bone-cement interface. In addition, one models was further modified to investigate the effect of cement penetration on the bone-cement interfacial behaviour. The simulated results show that the load transfer at the bone-cement interface occurred mainly in the bone cement partially interdigitated region, while the fully interdigitated region seemed to contribute little to the mechanical response. Consequently, cement penetration beyond a certain value would seem to be ineffective in improving the mechanical strength of trabecular bone-cement interface. Under tension and shear loading conditions, more cement failures were found in denser bones, while the cement damage is generally low under compression.

The effect of bone ingrowth depth on the tensile and shear strength of the implant-bone e-beam produced interface

New technologies, such as selective electron beam melting, allow to create complex interface structures to enhance bone ingrowth in cementless implants. The efficacy of such structures can be tested in animal experiments. Although animal studies provide insight into the biological response of new structures, it remains unclear how ingrowth depth is related to interface strength. Theoretically, there could be a threshold of ingrowth, above which the interface strength does not further increase. To test the relationship between depth and strength we performed a finite element study on micro models with simulated uncoated and hydroxyapatite (HA) coated surfaces. We examined whether complete ingrowth is necessary to obtain a maximal interface strength. An increase in bone ingrowth depth did not always enhance the bone-implant interface strength. For the uncoated specimens a plateau was reached at 1,500 μm of ingrowth depth. For the specimens with a simulated HA coating, a bone ingrowth depth of 500 μm already yielded a substantial interface strength, and deeper ingrowth did not enhance the interface strength considerably. These findings may assist in optimizing interface morphology (its depth) and in judging the effect of bone ingrowth depth on interface strength.

Morphology based cohesive zone modeling of the cement-bone interface from postmortem retrievals

In cemented total hip arthroplasty, the cement-bone interface can be considerably degenerated after less than one year in vivo service; this makes the interface much weaker relative to the direct post-operative situation. It is, however, still unknown how these degenerated interfaces behave under mixed-mode loading and how this is related to the interface morphology. In this study, we used a finite element (FE) approach to analyze the mixed-mode response of the cement-bone interface taken from postmortem retrievals. We investigated whether it was feasible to generate a fully elastic and a failure cohesive model based on only morphological input parameters. Computed tomography-based FE-models of postmortem cement-bone interfaces were generated and the interface morphology was determined. The models were loaded until failure in multiple directions by allowing cracking of the bone and cement components and including periodic boundary conditions. The resulting stiffness was related to the interface morphology. A closed form mixed-mode cohesive model that included failure was determined and related to the interface morphology. The responses of the FE-simulations compare satisfactorily with experimental observations, albeit the magnitude of the strength and stiffness are somewhat overestimated. Surprisingly, the FE-simulations predict no failure under shear loading and a considerable normal compression is generated which prevents dilation of the interface. The obtained mixed-mode stiffness response could subsequently be related to the interface morphology and subsequently be formulated into an elastic cohesive zone model. Finally, the acquired data could be used as an input for a cohesive model that also includes interface failure.

Comparison of patellar bone strain in the natural and implanted knee during simulated deep flexion

Instances of anterior knee pain and patellar fracture are significant complications following total knee replacement (TKR). Bone strain measured in the patella can provide an indication of patellar fracture risk and may also be related to anterior knee pain. The objective of this study was to develop subject-specific finite element models of the patellofemoral (PF) joint including density-mapped material properties to gain insight into the patellar bone strain distribution in the natural and implanted knee. In eight subjects, the volume of bone experiencing strains >0.5% in the implanted condition was ∼200% larger, on average, than the natural condition. An inverse relationship with a correlation of -0.74 was established between postoperative bone volume and strain in the implanted specimens, suggesting that patellar geometry may be a useful indicator of postoperative strain. Comparing strains between regions (superior, inferior, medial, and lateral), it was found that although highly strained bone was evenly distributed between medial and lateral regions in the natural case, the implanted specimens demonstrated significantly larger volumes of highly strained bone medially as a result of substantially lower modulus bone in the medial compartment. Understanding distributions of PF strain may aid in preoperative identification of those patients at risk for patellar fracture or anterior knee pain, guidance regarding altered component placement for at-risk patients, and design of components considering the implications of PF load transfer and patellar strain distribution.

Mixed-mode loading of the cement-bone interface: a finite element study

While including the cement-bone interface of complete cemented hip reconstructions is crucial to correctly capture their response, its modelling is often overly simplified. In this study, the mechanical mixed-mode response of the cement-bone interface is investigated, taking into account the effects of the well-defined microstructure that characterises the interface. Computed tomography-based plain strain finite element analyses models of the cement-bone interface are built and loaded in multiple directions. Periodic boundaries are considered and the failure of the cement and bone fractions by cracking of the bulk components are included. The results compare favourably with experimental observations. Surprisingly, the analyses reveal that under shear loading no failure occurs and considerable normal compression is generated to prevent interface dilation. Reaction forces, crack patterns and stress fields provide more insight into the mixed-mode failure process. Moreover, the cement-bone interface analyses provide details which can serve as a basis for the development of a cohesive law.

Modelling cement augmentation: a comparative experimental and finite element study at the continuum level

Subject-specific computational models of anatomical components can now be generated from image data and used in the assessment of orthopaedic interventions. However, little work has been undertaken to model cement-augmented bone using these methods. The purpose of this study was to investigate different methods of representing a trabecular-like material (open-cell polyurethane foam, Sawbone, Sweden) augmented with poly(methyl methacrylate) (PMMA) bone cement in a finite element (FE) model. Three sets of specimens (untreated, fully augmented with cement, partially augmented with cement) were imaged using micro computed tomography (microCT) and tested under axial compression. Subject-specific continuum level FE models were built based on the microCT images. Using the first two sets of models, the material conversion factors between image greyscale and mechanical properties for the pure synthetic bone and cement-augmented composite were determined iteratively by matching the FE predictions to the experimental measurements. By applying these greyscale related mechanical properties to the FE models of the partially augmented specimens, the predicted stiffness was found to be more accurate (approximately 5 per cent error) than using homogeneous properties for the augmented and synthetic bone regions (approximately 18 per cent error). It was also found that the predicted stiffness using the modulus of pure cement to define the augmented region was overestimated, and generally the apparent elastic modulus was dominated by the properties of the synthetic bone.

The effect of cement creep and cement fatigue damage on the micromechanics of the cement-bone interface

The cement-bone interface provides fixation for the cement mantle within the bone. The cement-bone interface is affected by fatigue loading in terms of fatigue damage or microcracks and creep, both mostly in the cement. This study investigates how fatigue damage and cement creep separately affect the mechanical response of the cement-bone interface at various load levels in terms of plastic displacement and crack formation. Two FEA models were created, which were based on micro-computed tomography data of two physical cement-bone interface specimens. These models were subjected to tensile fatigue loads with four different magnitudes. Three deformation modes of the cement were considered: 'only creep', 'only damage' or 'creep and damage'. The interfacial plastic deformation, the crack reduction as a result of creep and the interfacial stresses in the bone were monitored. The results demonstrate that, although some models failed early, the majority of plastic displacement was caused by fatigue damage, rather than cement creep. However, cement creep does decrease the crack formation in the cement up to 20%. Finally, while cement creep hardly influences the stress levels in the bone, fatigue damage of the cement considerably increases the stress levels in the bone. We conclude that at low load levels the plastic displacement is mainly caused by creep. At moderate to high load levels, however, the plastic displacement is dominated by fatigue damage and is hardly affected by creep, although creep reduced the number of cracks in moderate to high load region.

The mechanical effects of different levels of cement penetration at the cement-bone interface

The mechanical effects of varying the depth of cement penetration in the cement-bone interface were investigated using finite element analysis (FEA) and validated using companion experimental data. Two FEA models of the cement-bone interface were created from micro-computed tomography data and the penetration of cement into the bone was varied over six levels each. The FEA models, consisting of the interdigitated cement-bone constructs with friction between cement and bone, were loaded to failure in tension and in shear. The cement and bone elements had provision for crack formation due to excessive stress. The interfacial strength showed a strong relationship with the average interdigitation (r(2)=0.97 and r(2)=0.93 in tension and shear, respectively). Also, the interface strength was strongly related with the contact area (r(2)=0.98 and r(2)=0.95 in tension and shear, respectively). The FEA results compared favorably to the stiffness-strength relationships determined experimentally. Overall, the cement-bone interface was 2.5 times stronger in shear than in tension and 1.15 times stiffer in tension than in shear, independent of the average interdigitation. More cracks occurred in the cement than in the bone, independent of the average interdigitation, consistent with the experimental results. In addition, more cracks were generated in shear than in tension. In conclusion, achieving and maintaining maximal infiltration of cement into the bone to obtain large interdigitation and contact area is key to optimizing the interfacial strength.

Finite element simulation of cement-bone interface micromechanics: a comparison to experimental results

Recently, experiments were performed to determine the micromechanical behavior of the cement-bone interface under tension-compression loading conditions. These experiments were simulated using finite element analysis (FEA) to test whether the micromechanical response of the interface could be captured in micromodels. Models were created of experimental specimens based upon microcomputed tomography data, including the complex interdigitated bone-cement morphology and simulated frictional contact at the interface. The models were subjected to a fully reversed tension-compression load, mimicking the experimental protocol. Similar to what was found experimentally, the simulated interface was stiffer in compression than in tension, and the majority of displacement was localized to the cement-bone interface. A weak correlation was found between the FEA-predicted stiffness and the stiffness found experimentally, with average errors of 8 and 30% in tension and compression, respectively. The hysteresis behavior found experimentally was partially reproduced in the simulation by including friction at the cement-bone interface. Furthermore, stress analysis suggested that cement was more at risk of fatigue failure than bone, concurring with the experimental observation that more cracks were formed in the cement than in the bone. The current study provides information that may help explain the load transfer mechanisms taking place at the cement-bone interface.

Fatigue creep damage at the cement-bone interface: an experimental and a micro-mechanical finite element study

The goal of this study was to quantify the micromechanics of the cement-bone interface under tensile fatigue loading using finite element analysis (FEA) and to understand the underlying mechanisms that play a role in the fatigue behavior of this interface. Laboratory cement-bone specimens were subjected to a tensile fatigue load, while local displacements and crack growth on the specimen's surface were monitored. FEA models were created from these specimens based upon micro-computed tomography data. To accurately model interfacial gaps at the interface between the bone and cement, a custom-written erosion algorithm was applied to the bone model. A fatigue load was simulated in the FEA models while monitoring the local displacements and crack propagation. The results showed the FEA models were able to capture the general experimental creep damage behavior and creep stages of the interface. Consistent with the experiments, the majority of the deformation took place at the contact interface. Additionally, the FEA models predicted fatigue crack patterns similar to experimental findings. Experimental surface cracks correlated moderately with FEA surface cracks (r(2)=0.43), but did not correlate with the simulated crack volume fraction (r(2)=0.06). Although there was no relationship between experimental surface cracks and experimental creep damage displacement (r(2)=0.07), there was a strong relationship between the FEA crack volume fraction and the FEA creep damage displacement (r(2)=0.76). This study shows the additional value of FEA of the cement-bone interface relative to experimental studies and can therefore be used to optimize its mechanical properties.

Can medio-lateral baseplate position and load sharing induce asymptomatic local bone resorption of the proximal tibia? A finite element study

Background

Asymptomatic local bone resorption of the tibia under the baseplate can occasionally be observed after total knee arthroplasty (TKA). Its occurrence is not well documented, and so far no explanation is available. We report the incidence of this finding in our practice, and investigate whether it can be attributed to specific mechanical factors.

Methods

The postoperative radiographs of 500 consecutive TKA patients were analyzed to determine the occurrence of local medial bone resorption under the baseplate. Based on these cases, a 3D FE model was developed. Cemented and cementless technique, seven positions of the baseplate and eleven load sharing conditions were considered. The average VonMises stress was evaluated in the bone-baseplate interface, and the medial and lateral periprosthetic region.

Results

Sixteen cases with local bone resorption were identified. In each, bone loss became apparent at 3 months post-op and did not increase after one year. None of these cases were symptomatic and infection screening was negative for all. The FE analysis demonstrated an influence of baseplate positioning, and also of load sharing, on stresses. The average stress in the medial periprosthetic region showed a non linear decrease when the prosthetic baseplate was shifted laterally. Shifting the component medially increased the stress on the medial periprosthetic region, but did not significantly unload the lateral side. The presence of a cement layer decreases the stresses.

Conclusion

Local bone resorption of the proximal tibia can occur after TKA and might be attributed to a stress shielding effect. This FE study shows that the medial periprosthetic region of the tibia is more sensitive than the lateral region to mediolateral positioning of the baseplate. Medial cortical support of the tibial baseplate is important for normal stress transfer to the underlying bone. The absence of medial cortical support of the tibial baseplate may lead to local bone resorption at the proximal tibia, as a result of the stress shielding effect. The presence of a complete layer of cement can reduce stress shielding, though. Despite the fact that the local bone resorption is asymptomatic and non-progressive, surgeons should be aware of this phenomenon in their interpretation of follow-up radiographs.