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

Tibial post loading increases the risk of aseptic loosening of posterior-stabilized tibial prosthesis

Aseptic loosening is the primary cause of failure following posterior-stabilized total knee arthroplasty. It is unclear whether tibial post loading of posterior-stabilized prosthesis increases the risk of aseptic loosening of the tibial prosthesis. The purpose of this study is to investigate the biomechanical effects of tibial post loading on the tibial prosthesis fixation interface during level walking, squatting, stair descent, and standing up-sitting down activities. In this paper, finite element models with and without post were established to compare the effects of tibial post loading on the von Mises stress of the proximal tibia, shear stress of the cement, and the bone-prosthesis interface micromotion during four physiological activities. The tibial post loading had an insignificant influence on tibial biomechanics and bone-prosthesis interface micromotion during leveling walking activity. However, compared to the insert without post condition, tibial post loading significantly increased the maximum tibial von Mises stress, the maximum shear stress in the medial of cement, and the bone-prosthesis interface peak micromotion by 912.84%, 612.77%, and 921.09%, respectively, at the moment of the maximum flexion angle for the stair descent activity, and 637.92%, 351.43%, and 519.13%, respectively, at the moment of the maximum flexion angle for the standing up-sitting down activity. Tibial post loading increased the risk of postoperative aseptic loosening of tibial prosthesis in patients with posterior-stabilized total knee arthroplasty, and it was recommended that the post-cam contact mechanism of posterior-stabilized prosthesis should be optimized to reduce the biomechanical impact of tibial post loading on tibial prosthesis fixation.

Biomechanical analysis of patient specific cone vs conventional stem in revision total knee arthroplasty

BACKGROUND

In revision total knee arthroplasty, addressing significant bone loss often involves the use of cemented or press-fit stems to ensure implant stability and long-term fixation. A possible alternative to stem was recently introduced utilizing custom-made porous metaphyseal cones, designed to reconstruct the missing tibial and femoral geometries. Early clinical and radiological assessments have shown promising results. The objective of this research was to biomechanically evaluate the performances of these custom-made cones.

METHODS

The biomechanical study was conducted using a validated finite element model. The bone geometries of a patient (selected for their history of four knee revisions due to infection and periprosthetic fractures, followed by a successful treatment with custom-made 3D-printed metaphyseal cones) were employed for the study. On these bone models, different revision scenarios were simulated and examined biomechanically: (A) custom-made cementless metaphyseal cones; (B) cemented stems; (C) press-fit stems; (D) distal femoral reconstruction with press-fit stem. All the models were analyzed at 0 °and 90 °of flexion, under physiological load conditions simulating daily activities; stress distribution, average Von-Mises stresses and risk of fracture were then analyzed and compared among configurations.

RESULTS

The use of custom-made 3D-printed cones exhibited the most favorable stress distribution in both femoral and tibial bones. Tibial bone stress was evenly distributed in custom-made cone configurations, while stress concentration was observed in distal regions for the other scenarios. Additionally, custom-made cones displayed overall homogeneity and lower stress levels, potentially contributing to limit pain. Symmetrical stress distribution was observed between the lateral and medial proximal tibia in custom-made cone models, whereas other scenarios exhibited uneven stress, particularly in the anterior tibial bone.

CONCLUSIONS

The biomechanical analysis of porous custom-made metaphyseal cones in re-revision arthroplasties is in agreement with the positive clinical and radiological outcomes. These findings provide valuable insights into the potential benefits of using custom-made cones, which offer more uniform stress distribution and may contribute to improve patient outcomes in revision TKA procedures. Further studies in this direction are warranted to validate these biomechanical findings.

Are Flexible Metaphyseal Femoral Cones Stable and Effective? A Biomechanical Study on Hinged Total Knee Arthroplasty

BACKGROUND

Cones currently available in the market are rigid, and unless they are custom-specific designed, are unable to correctly adapt to the shape of the patient's bone. Therefore, flexible metaphyseal cones have been recently introduced to reduce potential bone trauma during implantation. Even if a preliminary clinical study on their use has shown promising results, no biomechanical study evaluates and quantifies their mechanical efficacy and safety.

METHODS

Two commercial versions of flexible cones were analyzed in this study using finite element analysis, based on a previously validated model. Each cone geometry was modeled both as flexible and as rigid, and implanted following surgical guidelines. Three activities were simulated in this study and compared among configurations: surgical impaction, walking, and chair rise.

RESULTS

During impaction, results showed considerably reduced stress in the flexible cones in comparison with rigid ones; the stress resulted was also better distributed and more homogeneous all over the cortical bone, with lower bone peaks. Considering the 2 different activities, the analysis did not show any remarkable differences between flexible and rigid cones both in terms of bone stress and implant micromotion.

CONCLUSIONS

The findings demonstrate that metaphyseal flexible cones allow macrodeformation during impaction due to their flexibility, and therefore, are safer in comparison with rigid cones. However, for the daily tasks investigated, results showed no major differences between rigid and flexible cones in terms of bone stress, implant stability, and micromotion. Therefore, their mechanical performances can be considered similar to the rigid cone.

Biomechanical comparison of all-polyethylene total knee replacement and its metal-backed equivalent on periprosthetic tibia using the finite element method

BACKGROUND

Total knee arthroplasty (TKA) with all-polyethylene tibial (APT) components has shown comparable survivorship and clinical outcomes to that with metal-backed tibial (MBT). Although MBT is more frequently implanted, APT equivalents are considered a low-cost variant for elderly patients. A biomechanical analysis was assumed to be suitable to compare the response of the periprosthetic tibia after implantation of TKA NexGen APT and MBT equivalent.

METHODS

A standardised load model was used representing the highest load achieved during level walking. The geometry and material models were created using computed tomography data. In the analysis, a material model was created that represents a patient with osteopenia.

RESULTS

The equivalent strain distribution in the models of cancellous bone with an APT component showed values above 1000 με in the area below the medial tibial section, with MBT component were primarily localised in the stem tip area. For APT variants, the microstrain values in more than 80% of the volume were in the range from 300 to 1500 με, MBT only in less than 64% of the volume.

CONCLUSION

The effect of APT implantation on the periprosthetic tibia was shown as equal or even superior to that of MBT despite maximum strain values occurring in different locations. On the basis of the strain distribution, the state of the bone tissue was analysed to determine whether bone tissue remodelling or remodelling would occur. Following clinical validation, outcomes could eventually modify the implant selection criteria and lead to more frequent implantation of APT components.

Biomechanical analysis of different THA cementless femoral stem designs in physiological and osteoporotic bone during static loading conditions

BACKGROUND

The influence of THA stem design on periprosthetic femoral fractures (PFFs) risk is subject of debate. This study aims to compare the effects of different cementless stem designs on stress-strain distributions in both physiological and osteoporotic femur under various loading conditions.

MATERIALS

A biomechanical study using finite-element analysis was conducted. Four models were developed: three with implanted femurs and a native one chosen as control. Each model was analyzed for both healthy and osteoporotic bone. The following stem designs were examined: short anatomical stem with femoral neck preservation, double-wedge stem, and anatomical standard stem. Three loading conditions were assessed: gait, sideways falling, and four-point bending.

RESULTS

During gait in physiological bone, the anatomical stem and the short anatomical stem with femoral neck preservation showed stress distribution similar to the native model. The double-wedge stem reduced stress in the proximal area but concentrated it in the meta-diaphysis. In osteoporotic bone, the double-wedge stem design increased average stress by up to 10%. During sideways falling, the double-wedge stem exhibited higher stresses in osteoporotic bone. No significant differences in average stress were found in any of the studied models during four-point bending.

CONCLUSION

In physiological bone, anatomical stems demonstrated stress distribution comparable to the native model. The double-wedge stem showed uneven stress distribution, which may contribute to long-term stress shielding. In the case of osteoporotic bone, the double-wedge stem design resulted in a significant increase in average stress during both gait and sideways falling, potentially indicating a higher theoretical risk of PFF.

All-polyethylene versus metal-backed tibial components in total knee arthroplasty: a meta-analysis of randomized controlled trials

OBJECTIVES

The design of tibial trays for total knee arthroplasty (TKA) has been a topic of research for several decades. Although all-polyethylene trays were developed to address issues such as osteolysis and to enhance the longevity of the prosthesis, as well as knee range of motion, metal-backed designs have remained the most commonly used type of prosthesis. This meta-analysis aimed to compare the clinical, radiological, and survival outcomes of both designs.

METHODS

Five databases were searched from inception until October 1, 2020, for randomized controlled trials (RCTs) that compared the outcomes of all-polyethylene and metal-backed tibial components in TKA. The outcomes of interest included range of motion, knee society score, stairs climbing scores, radiostereographic analysis, survivorship and complication. This review was conducted in line with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Risk of bias was assessed using the Newcastle-Ottawa tool.

RESULTS

A total of 14 RCTs with 1367 TKA were included with a mean age of - years and - years for all-polyethylene and metal-backed tibial components groups, respectively. All-polyethylene group demonstrated statistically significant differences in five-year survivorship (OR 0.27; 95% CI 0.10-0.75; p value 0.01) and stairs climbing score (OR - 2.07; 95% CI - 3.27-0.87; p value 0.0007) when compared to the metal-backed group. The metal-backed design was significantly more radiographically stable in anterior-posterior, varus-valgus, and internal-external rotations at the 2-year follow-up compared to all-polyethylene tibias (OR - 0.09; 95% CI - 0.16 to - 0.02; p value 0.02) as per the pooled radiostereographic analysis. However, ten-year survivorship (OR 0.92; 95% CI 0.53-1.60; p value 0.78), range of motion (OR - 0.57; 95% CI - 2.00-0.85, p value 0.43), knee society scores (OR 1.38; 95% CI - 0.47-3.23, p value 0.14), and complications (OR 0.83; 95% CI 0.5-1.39, p value 0.48) were comparable between both groups.

CONCLUSIONS

While this meta-analysis suggests that all-polyethylene tibial components in total knee arthroplasty may offer advantages over metal-backed components in terms of five-year survivorship, and stairs climbing score, this finding should be considered in the context of potential confounding factors. Nonetheless, based on the results, the all-polyethylene implant should be considered a viable choice for primary knee replacement.

LEVEL OF EVIDENCE

I.

Application strategy of finite element analysis in artificial knee arthroplasty

Artificial knee arthroplasty, as the most effective method for the treatment of end-stage joint diseases such as knee osteoarthritis and rheumatoid arthritis, is widely used in the field of joint surgery. At present, Finite element analysis (FEA) has been widely used in artificial knee replacement biomechanical research. This review presents the current hotspots for the application of FEA in the field of artificial knee replacement by reviewing the existing research literature and, by comparison, summarizes guidance and recommendations for artificial knee replacement surgery. We believe that lower contact stress can produce less wear and complications when components move against each other, in the process of total knee arthroplasty (TKA), mobile-bearing prostheses reduce the contact surface stress of the tibial-femoral joint compared with fixed-bearing prostheses, thus reducing the wear of the polyethylene insert. Compared with mechanical alignment, kinematic alignment reduces the maximum stress and maximum strain of the femoral component and polyethylene insert in TKA, and the lower stress reduces the wear of the joint contact surface and prolongs the life of the prosthesis. In the unicompartmental knee arthroplasty (UKA), the femoral and tibial components of mobile-bearing prostheses have better conformity, which can reduce the wear of the components, while local stress concentration caused by excessive overconformity of fixed-bearing prostheses should be avoided in UKA to prevent accelerated wear of the components, the mobile-bearing prosthesis maintained in the coronal position from 4° varus to 4° valgus and the fixed-bearing prosthesis implanted in the neutral position (0°) are recommended. In revision total knee arthroplasty (RTKA), the stem implant design should maintain the best balance between preserving bone and reducing stress around the prosthesis after implantation. Compared with cemented stems, cementless press-fit femoral stems show higher fretting, for tibial plateau bone defects, porous metal blocks are more effective in stress dispersion. Finally, compared with traditional mechanical research methods, FEA methods can yield relatively accurate simulations, which could compensate for the deficiencies of traditional mechanics in knee joint research. Thus, FEA has great potential for applications in the field of medicine.

The optimal size of screw for using cement-screw technique to repair tibial defect in total knee arthroplasty: A finite element analysis

Cement can be reinforced with cancellous screws for repairing tibial defect in total knee arthroplasty (TKA). However, it is still unknown which size (diameter, length) of screws is better, and the purpose of this study was to perform a finite element analysis (FEA) to determine it. Twelve FEA models were set to represent the cement-screw technique with different diameters (3.5 mm, 5 mm and 6.5 mm) and lengths (20 mm, 25 mm, 30 mm and 35 mm). Contact stresses on the surface of cancellous bone in different areas were calculated. Compared to screws with diameter of 3.5 mm, stresses on the surface of bone defect reduced 5.75% of 5 mm and 10.68% of 6.5 mm for the screw length of 20 mm, 4.23% of 5 mm and 9.16% of 6.5 mm for 25 mm, 6.65% of 5 mm and 12.30% of 6.5 mm for 30 mm, and 5.05% of 5 mm and 12.16% of 6.5 mm for 35 mm. Compared to screws with diameter of 5 mm, stresses on the surface of defect reduced 5.24%, 5.15%, 6.05%, and 7.49% of 6.5 mm for the screw length of 20, 25, 30, and 35 mm. However, it did not show any significant difference in other comparisons. For the treatment of tibial defect in TKA with cement-screw technique, longer screw may not achieve better stability, but the thicker screw can reduce more stresses on the surface of tibial defect and achieve better stability. However, the depth of bone defect must be considered when making a choice.

Biomechanical analysis of different levels of constraint in TKA during daily activities

BACKGROUND

Numerous total knee prosthetic implants are currently available on the orthopedic market, and this variety covers a set of different levels of constraint: among the various models available, a significant role is covered by mobile bearing cruciate-retaining design with an ultra-congruent insert, mobile bearing cruciate-retaining design, fixed-bearing posterior stabilized prosthesis and fixed-bearing constrained condylar knee. A biomechanical comparative study among them could therefore be helpful for the clinical decision-making process. This study aimed to compare the effect of these different levels of constraint in the knee biomechanics of a patient, in three different configurations representing the typical boundary conditions experienced by the knee joint during daily activities.

METHOD

The investigation was performed via finite element analysis with a knee model based on an already published and validated one. Four different types of prosthesis designs were analyzed: two mobile-bearing models and two fixed-bearing models, each one having a different level of constraint. The different designs were incorporated in to the 3D finite element model of the lower leg and analyzed in three different configurations reproducing the landing and the taking-off phases occurring during the gait cycle and chair-rising. Implant kinetics (in terms of polyethylene contact areas and contact pressure), polyethylene and tibial bone stresses were calculated under three different loading conditions for each design.

RESULTS

The tibial stress distribution in the different regions of interest of the tibia remains relatively homogeneous regardless of the type of design used. The main relevant difference was observed between the mobile and fixed-bearing models, as the contact areas were significantly different between these models in the different loading conditions. As a consequence, significant changes in the stress distribution were observed at the interface between the prosthetic components, but no significant changes were noted on the tibial bone. Moreover, the different models exhibited a symmetrical medial and lateral distribution of the contact areas, which was not always common among all the currently available prostheses (i.e. medial pivot designs).

CONCLUSION

The changes of the prosthetic implant did not induce a big variation of the stress distribution in the different regions of the tibial bone, while they significantly changed the distribution of stress at the interface between the prosthetic components.

All-polyethylene tibia components have the same functional outcomes and survival, and are more cost-effective than metal-backed components in patients 70 years and older undergoing total knee arthroplasty : propensity match study with a minimum five-year follow-up

AIMS

The tibial component of total knee arthroplasty can either be an all-polyethylene (AP) implant or a metal-backed (MB) implant. This study aims to compare the five-year functional outcomes of AP tibial components to MB components in patients aged over 70 years. Secondary aims are to compare quality of life, implant survivorship, and cost-effectiveness.

METHODS

A group of 130 patients who had received an AP tibial component were matched for demographic factors of age, BMI, American Society of Anesthesiologists (ASA) grade, sex, and preoperative Knee Society Score (KSS) to create a comparison group of 130 patients who received a MB tibial component. Functional outcome was assessed prospectively by KSS, quality of life by 12-Item Short-Form Health Survey questionnaire (SF-12), and range of motion (ROM), and implant survivorships were compared. The SF six-dimension (6D) was used to calculate the incremental cost effectiveness ratio (ICER) for AP compared to MB tibial components using quality-adjusted life year methodology.

RESULTS

The AP group had a mean KSS-Knee of 83.4 (standard deviation (SD) 19.2) and the MB group a mean of 84.9 (SD 18.2; p = 0.631), while mean KSS-Function was 75.4 (SD 15.3) and 73.2 (SD 16.2 p = 0.472), respectively. The mental (44.3 vs 45.1; p = 0.464) and physical (44.8 vs 44.9; p = 0.893) dimensions of the SF-12 and ROM (97.9° vs 99.7°; p = 0.444) were not different between the groups. Implant survivorship at five years were 99.2% and 97.7% (p = 0.321). The AP group had a greater SF-6D gain of 0.145 compared to the MB group, with an associated cost saving of £406, which resulted in a negative ICER of -£406/0.145 = -£2,800. Therefore, the AP tibial component was dominant, being a more effective and less expensive intervention.

CONCLUSION

There were no differences in functional outcomes or survivorship at five years between AP and MB tibial components in patients aged 70 years and older, however the AP component was shown to be more cost-effective. In the UK, only 1.4% of all total knee arthroplasties use an AP component; even a modest increase in usage nationally could lead to significant financial savings.Cite this article: Bone Jt Open 2022;3(12):969-976.

The optimal angle of screw for using cement-screw technique to repair tibial defect in total knee arthroplasty: a finite element analysis

BACKGROUND

The cement-screw technique is a convenient method to repair tibial plateau defects in primary and revision total knee arthroplasty (TKA). However, the optimal angle of screw insertions is unknown. This study aimed to perform a finite element analysis (FEA) to determine the optimal screw angle for the repair of tibial plateau defects in TKA.

METHODS

Seven FEA models were set and two common different defects (defect 1: area < 12%, depth < 12 mm; defect 2: area > 12%, depth > 12 mm) were simulated. One screw was used in defect 1, and one or two screws were used in defect 2. Screws were parallel to the proximal cortical bone (oblique screw) or perpendicular to the upper surface (vertical screw) of the tibia. Contact stresses on cancellous bone in different areas were determined. Maximum principal stress on the cancellous bone around each screw was also compared.

RESULTS

The FEA models showed that stresses on the surface of cancellous bone in tibial defect (0.13-0.39 MPa) and stress focus spot (0.45 MPa) around the screw were lower when one vertical screw was used in defect 1. The stresses on the surface of cancellous bone in tibial defect (0.09-0.44 MPa), stresses in the medial tibial plateau (0.14-0.21 MPa), and stress focus spot around the screws were lowest (0.42 MPa and 1.37 MPa) when two vertical screws were used in defect 2, followed by of one vertical and one oblique (0.16-0.48 MPa; 0.15-0.21 MPa; 1.63 MPa and 1.11 MPa). No other statistically significant differences were found.

CONCLUSIONS

Either for one or two screws, those perpendicular to the upper surface achieve better stability than those parallel to the proximal cortical bone of the tibia. If two vertical screws cannot be performed, one vertical and one oblique is also acceptable.

Biomechanical Analysis of the Use of Stems in Revision Total Knee Arthroplasty

Adequate fixation is fundamental in revision total knee arthroplasty; consequently, surgeons must determine the correct set-up for each patient, choosing from numerous stem solutions. Several designs are currently available on the market, but there are no evidence-based quantitative biomechanical guideline yet. Therefore, several stems were designed and analyzed using a previously-validated finite-element model. The following parameters were studied: stem design characteristics (length and shape), added features (straight/bowed stem), fixation technique, and effect of slots/flutes. Bone stress and Risk of Fracture (RF) were analyzed in different regions of interest during a squat (up to 120°). For the femoral stem, the results indicated that all parameters influenced the bone stress distribution. The maximum von Mises stress and RF were always located near the tip of the stem. The long stems generated stress-shielding in the distal bone. Regarding the tibial stem, cemented stems showed lower micromotions at the bone-tibial tray interface and at the stem tip compared to press-fit stems, reducing the risk of implant loosening. The results demonstrated that anatomical shapes and slots reduce bone stress and risk of fracture, whereas flutes have the opposite effect; no relevant differences were found in this regard when alternating cemented and press-fit stem configurations. Cemented tibial stems reduce antero-posterior micromotions, preventing implant loosening.

Biomechanical analysis of all-polyethylene total knee arthroplasty on periprosthetic tibia using the finite element method

BACKGROUND AND OBJECTIVE

Total knee arthroplasty (TKA) with modern all-polyethylene tibial (APT) components has shown high long-term survival rates and comparable results to those with metal-backed tibial components. Nevertheless, APT components are primarily recommended for older and low-demand patients. There are no evidence-based biomechanical guidelines for orthopaedic surgeons to determine the appropriate lower age limit for implantation of APT components. A biomechanical analysis was assumed to be suitable to evaluate the clinical results in patients under 70 years. The scope of this study was to determine biomechanically the appropriate lower age limit for implantation of APT components.

METHODS

To generate data of the highest possible quality, the geometry of the computational models was created based on computed tomography (CT) images of a representative patient. The cortical bone tissue model distinguishes the change in mechanical properties described in three parts from the tibial cut. The cancellous bone material model has a heterogeneous distribution of mechanical properties. The values used to determine the material properties of the tissues were obtained from measurements of a CT dataset comprising 45 patients.

RESULTS

Computational modeling showed that in the majority of the periprosthetic volume, the von Mises strain equivalent ranges from 200 to 2700 με; these strain values induce bone modeling and remodeling. The highest measured deformation value was 2910 με. There was no significant difference in the induced mechanical response between bone models of the 60-year and 70-year age groups, and there was <3% difference from the 65-year age group.

CONCLUSIONS

Considering in silico limitations, we suggest that APT components could be conveniently used on a bone with mechanical properties of the examined age categories. Under defined loading conditions, implantation of TKA with APT components is expected to induce modeling and remodeling of the periprosthetic tibia. Following clinical validation, the results of our study could modify the indication criteria of the procedure, and lead to more frequent implantation of all-polyethylene TKA in younger patients.

Biomechanical analysis of femoral stems in hinged total knee arthroplasty in physiological and osteoporotic bone

BACKGROUND AND OBJECTIVE

Adequate fixation is a requisite for hinged Total Knee Arthroplasty (TKA): consequently, several stem solutions are currently available. However, there are no evidence-based biomechanical guidelines for surgeons to determine the appropriate stem length and whether to use cemented or press-fit fixation. The objective of this study is therefore to compare, using a validated finite-element model, bone stresses and implant micromotions in different configurations.

METHODS

The 3D bone geometries were obtained from CT-scans reconstruction and the 3D model components of an Endo-Model Rotating Hinge (WALDEMAR LINK GmbH & Co. KG, Hamburg, Germany) were generated from industrial designs provided by the manufacturer. Sixteen configurations were investigated considering four stem lengths (50, 95, 120, 160 mm), cemented and press-fit fixation and physiological and osteoporotic bone properties. A further configuration without stem was analyzed as control. Average Von-Mises stresses, risk of fracture and micromotions were extracted in several regions of interest at 0° and 90° of flexion, under physiological load conditions.

RESULTS

Generally, longer stems guarantee better fixation compared to short ones; however, they induce higher stress-shielding effect in the distal region of the femur (even greater for press-fit stems, with values up to 38.5% greater than cemented ones). The cemented configurations, especially in case of 50 mm and 95 mm lengths, induce lower micromotions (down to 16% lower) compared to their respective press-fit configurations. The osteoporotic RF values were greater than the physiological ones (up to 20.5%), but always below the bone limit of fracture.

CONCLUSIONS

According to this study, when surgeons need to select a femoral stem in a hinged TKA aiming to proper stability and bone stress, the preferable option would be short cemented stems.

All-Polyethylene Tibia: An Opportunity for Value-Based Care in Bundled Reimbursement Initiatives

Surgeons play a critical role in making cost-effective decisions that maintain high-quality patient outcomes, which is the current focus of the Centers for Medicare & Medicaid Services. All-polyethylene tibial (APT) components often cost less during total knee arthroplasty (TKA). The authors sought to determine the relative cost savings of APT, as well as their effect on 90-day quality outcome metrics. This was a retrospective review of primary TKAs performed at a single tertiary referral center participating in the Comprehensive Care for Joint Replacement model, by 2 surgeons, from 2015 to 2017. Patient demographic data and direct hospital costs were collected, and patients were stratified by APTs vs metal-backed components. Univariable and multivariable analyses were performed for all outcome metrics. A total of 188 primary TKAs were included (92 APT, 96 metal-backed). Patients receiving APT components were older (P<.001) and had a lower body mass index (P<.001), but there was no difference in sex or American Society of Anesthesiologists score between groups. Operative time was significantly less (mean, 13 minutes) and direct surgery costs were significantly lower for APTs (P<.001). A multivariable regression model for surgical costs demonstrated significant savings (P<.001), and total hospital cost demonstrated a 6.2% average savings with APT. There was no difference in 90-day emergency department visits or re-admissions. This study demonstrates that the use of an APT is able to significantly affect not only the surgical cost but also the total hospital admission cost while maintaining equivalent 90-day outcome metrics. Strategies like this should be considered in appropriately selected patients as the incidence of TKA continues to expand. [Orthopedics. 2021;44(1):e114-e118.].

Use of porous custom-made cones for meta-diaphyseal bone defects reconstruction in knee revision surgery: a clinical and biomechanical analysis

INTRODUCTION

Although the practice of metaphyseal reconstruction has obtained successful clinical and radiological results in revision total knee surgery, off-the-shelf devices aren't an effective solution for all patients as they do not cover the full range of clinical possibilities. For this reason, during severe knee revisions, custom-made porous titanium cementless metaphyseal cones are nowadays employed as alternative to traditional surgeries. The aim of this study is to understand the benefits gained by the use of the custom-made cones against the performance of more traditional techniques, such as the use of cemented or cementless stems. Thus, a retrospective study on eleven patients and a biomechanical finite element analysis (FEA) was developed, based upon three clinical cases of the clinical analyzed cohort.

MATERIALS AND METHODS

Eleven patients underwent staged total knee arthroplasty revision with the use of 16 custom-made cones to correct severe femoral and tibial meta-diaphyseal bone defects. Clinical scores and range of movement were observed during the follow-up period (mean follow-up 26 ± 9.4 months). Reason for surgery was periprosthetic joint infection (PJI) in eight patients and post-traumatic osteomyelitis in the other three patients. Three patients previously affected by PJI were selected among the eleven patients of the clinical population. For those patients, bone geometries and implants during surgery were replicated in silico and analyzed during different daily activities. For the same patients, as alternative solution for surgery, the use of cemented or cementless stems was also simulated by FEA. Stress patterns in different region of interest and risk of fracture in the bone were calculated and compared.

RESULTS

No loosening, component migration, or mismatches between preoperative planning and intraoperative findings were clinically registered. Biomechanical results demonstrated that the use of custom-made cones induces a more homogeneously distributed bone stress than the other two techniques that concentrate the stress in spotted regions. The risk of fracture is comparable between the use of custom-made cones and cemented technique, while press-fit configurations increase the risk of fracture (more than 35%).

CONCLUSIONS

Based upon the clinical evidence and the findings after the FEAs, the practice of porous custom-made metaphyseal cones in severe revisions of knee arthroplasties is showing promising biomechanical results. The homogeneous stresses distributions and the lower bone stress gradient could justify a reduction of bone fractures and the risk of implant loosening which could be the explanation to the successful clinical outcomes.

Change in knee biomechanics during squat and walking induced by a modification in TKA size

The purpose of this study is to analyze the effects of TKA under-dimensioning during daily activities. A regular ("control") size and an undersized design of the same fixed bearing asymmetric PS prosthesis were analyzed during walking and squat using finite element analysis. The two models showed similar internal-external rotations and antero-posterior displacements during both activities. Slightly higher displacements, wider contact areas and lower contact pressure were found in the control size. Post-cam engagement angles were similar on both sizes. Changes in TKA size slightly affected knee kinematics and kinetics, with post-cam related differences leading to minor changes in kinetic patterns.

Influence of material properties and boundary conditions on patient-specific models

Patient-specific finite element models (PSFEM) are becoming more and more used. Different methods for assigning their material properties were studied on PSFEMs of 9 tibias along with the minimal required length of the CT acquisition window. Material properties are generally attributed to the PSFEM using relationships linking the grayscale of CT scans to the elasticity moduli. Using cortical-specific and trabecular-specific relationships or a generic one, did not result in significant differences. However, the use of homogeneous elastic moduli in the cortical and trabecular regions led to considerable differences. The result highlight that the PSFEM must comprise at least 40% of the tibia to ensure consistent results in the proximal 20%.

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

BACKGROUND

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

METHOD

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

RESULTS

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

CONCLUSIONS

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

High congruency MB insert design: stabilizing knee joint even with PCL deficiency

PURPOSE

PCL management and choice of insert design and mobility in total knee arthroplasty are still debated in the literature. Consequently, the purpose of this study was to analyze the biomechanics of a fixed and a mobile bearing total knee arthroplasty with conventional and ultra-congruent insert during walking and squat activities, using finite element analysis, and to check the performance in a knee with healthy and deficient PCL.

METHODS

The study was based on an already validated and published knee model. Fixed bearing and mobile bearing cruciate-retain designs were selected for this study. Implant kinematics and kinetics were calculated, following previously experimental tests, during a walking cycle and a loaded squat in a knee with intact and with deficient PCL.

RESULTS

Mobile bearing design, due to its higher congruency, was able to complete the task in intact and deficient PCL conditions, with similar internal-external femoral rotation and with a slight higher anterior translation of the one of the intact knees. Such outcomes were also in agreement with the results of different experimental studies of native knee specimens under similar boundary conditions. Contrariwise, fixed bearing design was able to accomplish the task only in healthy PCL conditions.

CONCLUSION

Results demonstrated how the high congruency of the mobile bearing design is able to guarantee proper knee stability and kinematics even when the PCL is deficient. Instead, the fixed bearing insert, with lower congruency, is not able, in the absence of the PCL, to stabilize the joint inducing irregular kinematic pattern and component dislocation. Surgeons will have to consider these findings to guarantee the best outcome for the patient and the related change in stability in case of PCL deficiency.

EVALUATING THE EFFECTS OF EXPERIMENTAL SETTINGS DURING ISO 7206-4 ENDURANCE AND PERFORMACE TESTS: A FINITE ELEMENT ANALYSIS

ISO 7206-4:2010 is used to determine the endurance properties and performance of stemmed femoral components of hip prosthesis. Experimental set-ups are allowed for tolerances with respect to the desired settings. The effects of the acceptable accuracies are not known nowadays. For this reason, this study aims at evaluating how and how much the precision of the experimental settings can affect the outputs under ISO 7206-4:2010 boundaries. Thus, a finite element model was first defined and verified against experimental test. Then, a sensitivity analysis involving accepted variations for potting level, angle in the frontal plane, angle in the lateral plane and material in use for the cement block, was performed. The results of sensitivity analysis show that both stress and deformation outputs were affected up to 100[Formula: see text]MPa and 0.5[Formula: see text]mm, respectively, for varus configurations. The material properties of the cement block had a main effect on the displacement of the head. The collected information through the performed sensitivity analysis on the verified model against experimental test has a double benefit. It supports the understanding of the potential effects during the experimental set-ups and it is also helpful in case of determining ranges for verification of developed numerical models.

Analysis of Biomechanical Differences Between Condylar Constrained Knee and Rotating Hinged Implants: A Numerical Study

BACKGROUND

Different levels of constraint for total knee arthroplasty can be considered for revision surgeries. While prior studies have assessed the clinical impact and patient outcomes of condylar constrained knee (CCK) and rotating hinged (RTH) implants, nowadays little is known about the biomechanical effects induced by different levels of constraint on bone stress and implant micromotions.

METHODS

CCK and RTH implant models were analyzed using a previously validated numerical model. Each system was investigated during a squat and a lunge motor task. The force in the joint, the bone and implant stresses, and micromotions in this latter were analyzed and compared among designs.

RESULTS

Different activities induced similar bone stress distributions in both implants. The RTH implant induces mostly high stress compared to the CCK implant, especially in the region close to tip of the stem. However, in the proximal tibia, the stresses achieved with the CCK implant is higher than the one calculated for the RTH design, due to the presence of the post-cam system. Accordingly, the condylar constrained design shows higher implant micromotions due to the greater torsional constraint.

CONCLUSION

Different levels of constraint in revision arthroplasty were always associated with different biomechanical outputs. RTH implants are characterized by higher tibial stress especially in the region close to the stem tip; condylar implants, instead, increase the proximal tibial stress and therefore implant micromotions, as a result of the presence of the post-cam mechanism. Surgeons will have to consider these findings to guarantee the best outcome for the patient and the related change in the bone stress and implant fixation induced by different levels of constrain in a total knee arthroplasty.

Asymmetric polyethylene inserts promote favorable kinematics and better clinical outcome compared to symmetric inserts in a mobile bearing total knee arthroplasty

PURPOSE

This study aims at comparing the effects of symmetric and asymmetric designs for the polyethylene insert currently available and also for mobile bearing total knee arthroplasty (TKA). The investigation was performed both clinically and biomechanically through finite element analysis.

METHODS

303 patients, with a mobile bearing TKA, were analyzed retrospectively. All patients received the same femoral and tibial components; for the insert, 151 patients received a symmetric design (SD) and 152 an asymmetric design (AD). Additionally, a 3D finite element model of a lower leg was developed, resurfaced with the same TKAs and analysed during gait and squat activities. TKA kinematics, and bone-stresses were investigated for the two insert solutions.

RESULTS

After surgery, patients' average flexion improved from 105°, with 5° of preoperative extension deficit, to 120° (AD-group) and 115° (SD-group) at the latest follow-up. There was no postoperative extension deficit. No pain affected the AD-group, while an antero-lateral pain was reported in some patients of the SD-group. Patients of the AD-group presented a better ability to perform certain physical routines. Biomechanically, the SD induced higher tibial-bone stresses than the AD. Both designs replicated similar kinematics, comparable to literature. However, SD rotates more on the tray, reducing the motion between femoral and polyethylene components, while AD permits greater insert rotation.

CONCLUSION

The biomechanical analysis justifies the clinical findings. TKA kinematics is similar for the two designs, although the asymmetric solution shows less bone stress, thus resulting as more suitable to be cemented, avoiding lift-off issues, inducing less pain. Clinically, and biomechanically, an asymmetric mobile bearing insert could be a valid alternative to symmetric mobile bearing insert.

LEVEL OF EVIDENCE

Case-control study retrospective comparative study, III.

Biomechanical Analysis of Augments in Revision Total Knee Arthroplasty

Augments are a common solution for treating bone loss in revision total knee arthroplasty and industry is providing to surgeons several options, in terms of material, thickness and shapes. Actually, while the choice of the shape and the thickness is mainly dictated by the bone defect, no proper guidelines are currently available to select the optimal material for a specific clinical situation. Nevertheless, different materials could induce different bone responses and, later, potentially compromise implant stability and performances. Therefore, in this study, a biomechanical analysis is performed by means of finite element modelling about existing features for augment designs. Based upon a review of available products at present, the following augments features were analyzed: position (distal/proximal and posterior), thickness (5, 10 and 15 mm) and material (bone cement, porous and solid metal). For all analyzed configurations, bone stresses were investigated in different regions and compared among all configurations and the control model for which no augments were used. Results show that the use of any kind of augment usually induces a change in bone stresses, especially in the region close to the bone cut. The porous metal presents result very close to cement ones; thus it could be considered as a good alternative for defects of any size. Solid metal has the least satisfying results inducing the highest changes in bone stress. The results of this study demonstrate that material stiffness of the augment should be as close as possible to bone properties for allowing the best implant performances.

Finite element analysis of the tibial bone graft in cementless total knee arthroplasty

Background

Achieving stability of the tibial implant is essential following cementless total knee arthroplasty with bone grafting. We investigated the effects of bone grafting on the relative micromotion of the tibial implant and stress between the tibial implant and adjacent bone in the immediate postoperative period.

Methods

Tibial implant models were developed using a nonlinear, three-dimensional, finite element method. On the basis of a preprepared template, several bone graft models of varying sizes and material properties were prepared.

Results

Micromotion was larger in the bone graft models than in the intact model. Maximum micromotion and excessive stress in the area adjacent to the bone graft were observed for the soft and large graft models. With hard bone grafting, increased load transfer and decreased micromotion were observed.

Conclusions

Avoidance of large soft bone grafts and use of hard bone grafting effectively reduced micromotion and undue stress in the adjacent area.

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.

All-Polyethylene Tibial Component Use for Elderly, Low-Demand Total Knee Arthroplasty Patients

BACKGROUND

Potential advantages combined with lower cost have prompted a renewed interest in modern all-polyethylene tibial designs.

METHODS

A total of 317 Natural Knee total knee arthroplasties with an all-polyethylene tibial component that was performed since 1993 were retrospectively reviewed to confirm that cost savings were not associated with inferior clinical results. They were primarily used in elderly, low-demand patients. The average age at the time of surgery was 80.8 years.

RESULTS

227 knees with a minimum of 2-year follow-up were identified. The average follow-up was 5.6 years (range, 2-20 years). Clinical and radiographic outcomes were evaluated. At the latest follow-up, the average Knee Society and functional score was 94.2 and 57.2. The average postoperative range of motion was 1.6°-115.4°. No patient required a revision for aseptic or septic loosening, wear, or instability. Only 5 knees had a partial nonprogressive cement/bone radiolucency. There was an approximate 33% cost savings for the implants when the all-polyethylene component was used.

CONCLUSION

An all-polyethylene tibial component of this design provides excellent results in the elderly population along with a significant cost savings.

Fixation techniques and stem dimensions in hinged total knee arthroplasty: a finite element study

INTRODUCTION

No evidence-based guidelines are available to determine the appropriate stem length, and whether or not to cement stems in revision total knee arthroplasty (TKA). Therefore, the objective of this study was to compare stresses and relative movement of cemented and uncemented stems of different lengths using a finite element analysis.

MATERIALS AND METHODS

A finite element model was created for a synthetic tibia. Two stem lengths (95 and 160 mm) and two types of fixation (cemented or press fit) of a hinged TKA were examined. The average compressive stress distribution in different regions of interest, as well as implant micromotions, was determined and compared during lunge and squat motor tasks.

RESULTS

Both long and short stems in revision TKA lead to high stresses, primarily in the region around the stem tip. The presence of cement reduces the stresses in the bone in every region along the stem. Short stem configurations are less affected by the presence of cement than the long stem configuration. Press-fit stems showed higher micromotions compared to cemented stems.

CONCLUSIONS

Lowest stresses and micromotion were found for long cemented stems. Cementless stems showed more micromotion and increased stress levels especially at the level of the stem tip, which may explain the clinical phenomenon of stem-end pain following revision knee arthroplasty. These findings will help the surgeon with optimal individual implant choice.

Limb-salvage treatment of en-block resected distal femoral tumors with endoprosthesis of all-polyethylene tibial component: a 9-year follow-up study

OBJECTIVE

To evaluate the medium-term outcome of limb-salvage surgery using all-polyethylene tibial endoprosthetic replacement following en-block resection for distal femoral tumors.

METHODS

Forty-nine patients with distal femoral tumor were treated between June 2006 and June 2012. The follow-up period was 6-110 months (average 53.4 months). The prosthetic survival was analyzed using the Kaplan-Meier method. The classification of failure of limb salvage after reconstructive surgery for bone tumors was adapted. Limb function was evaluated with the scoring system of the Musculoskeletal Tumor Society (MSTS).

RESULTS

Complications were observed in six cases (12.2%). Four suffered infection around the prosthesis, of which two cases were treated with debridement, drainage, and antibiotics without removal of the prosthesis, and the other two cases underwent amputation. Two cases were identified as radiographically loose at 7 year follow-up and did not require revision surgery. One patient underwent amputation due to local recurrence. Failure of limb salvage occurred in nine cases (18.4%), of which two cases were of type 1A, two cases of type 2B, three cases of type 4A, one case of type 4B, and one case of type 5A. The mean MSTS score was 84.3%. Twelve cases died due to distant metastases (24.5%), and the average survival time for these patients was 13.5 months. Thirty-seven patients survived (75.5%), for whom the average follow-up time was 66.3 months and the 5-year prosthetic survival rate was 88.2%.

CONCLUSION

The outcome of medium-term and long-term clinical follow-up was satisfactory. All-polyethylene tibial endoprosthetic replacement following en-block resection can be an alternative method of limb salvage for distal femoral tumors.

Finite element analysis: a comparison of an all-polyethylene tibial implant and its metal-backed equivalent

PURPOSE

The hypothesis of this study is that all-polyethylene (APE) tibial implants offer a biomechanical profile similar to metal-backed tray (MBT). There are significant financial implications, in selected patient groups, if APE can be deemed to perform as well as MBT.

METHODS

Using a finite element analysis of CAD models provided by DePuy (Leeds), stress distributions were investigated for both an APE and MBT tibial implant. The performance was assessed for cancellous bone at 700 MPa (normal) and at 350 MPa (less stiff). Plots were recorded along the length of the tibia, showing the loads carried by the bone (cortical and cancellous), the implant interface, cement interface and the stem. von Mises stress distributions and percentage volumes were used to assess bone resorption and hence potential for failure (fracture).

RESULTS

Higher stress shielding (resorption) occurred around the keel and stem of the MBT revealing greater potential for bone loss in these areas. APE had no areas of bone resorption (being more flexible resulting in less stress shielding). The stiffer MBT carries a higher proportion of the load down the stem. MBT stress in cancellous bone is lower than APE, as load is distributed to the cortical rim. APE has a marginally favourable strain state in cancellous bone and spreads loads more at the cement interface than MBT.

CONCLUSION

Modern-day APE bearings may be superior to previously designed implants due to improvements in manufacturing. In the correct patient group, this could offer substantial cost savings.

A Finite-Element Study of Metal Backing and Tibial Resection Depth in a Composite Tibia Following Total Knee Arthroplasty

Prosthetic alignment, patient characteristics, and implant design are all factors in long-term survival of total knee arthroplasty (TKA), yet the level at which each of these factors contribute to implant loosening has not been fully described. Prior clinical and biomechanical studies have indicated tibial overload as a cause of early TKA revision. The purpose of this study was to determine the relationship between tibial component design and bone resection on tibial loading. Finite-element analysis (FEA) was performed after simulated implantation of metal backed (MB) and all-polyethylene (AP) TKA components in 5 and 15 mm of tibial resection into a validated intact tibia model. Proximal tibial strains significantly increased between 13% and 199% when implanted with AP components (p < 0.05). Strain significantly increased between 12% and 209% in the posterior tibial compartment with increased bone resection (p < 0.05). This study indicates elevated strains in AP implanted tibias across the entirety of the proximal tibial cortex, as well as a posterior shift in tibial loading in instances of increased resection depth. These results are consistent with trends observed in prior biomechanical studies and may associate the documented device history of tibial collapse in AP components with increased bone strain and overload beneath the prosthesis.