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Influence of obesity on load-transfer mechanism, contact mechanics, and longevity of cemented acetabular cup
⁎Corresponding author: Subrata Mondal. smkittu1989@gmail.com
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
This investigation aimed to assess the impact of obesity on the load-transfer mechanism, longevity, and contact mechanics of cemented acetabular cups.
Three obesity scenarios were considered: obese case-I (100–110 kg), obese case-II (120–130 kg), and obese case-III (140–150 kg). Utilising six finite element models, the effects of different bodyweights on load transfer, contact mechanics, and cup longevity during normal walking conditions were assessed. Muscle forces and hip joint reaction forces were adjusted and linearly calibrated based on obesity cases.
Elevated stresses in cortical and cancellous bones, as well as the cement mantle, were observed in obese cases, suggesting a heightened risk of loosening and failure of the cemented fixation of the acetabular cup. Additionally, increased contact pressure and micromotion between articulating surfaces were noted in obese individuals, with a gradual escalation from obese case-I to obese case-III.
These findings highlight the significant negative impact of obesity on the performance of cemented acetabular cups, emphasizing the importance of considering bodyweight variations in the design and assessment of orthopaedic implants for optimal functionality and durability.
Abstract
Graphical abstract
Image 1
Keywords
Obesity
Contact mechanics
Wear
Longevity
Cemented acetabular cup
1 Introduction
Obesity significantly influences the biomechanical functionality, stability, and efficacy of cemented implants.1–4 The cemented acetabular cup is a prevalent choice in total hip arthroplasty (THA) for patients with hip joint arthritis or injuries. Comprising metal, polyethylene, and ceramic components, these cups are affixed to the pelvis using bone cement. Nevertheless, utilising cemented acetabular cups in obese individuals poses a specific challenge due to heightened stress on the joint.5 The surplus weight amplifies loads on the hip joint, elevating the risk of implant loosening or failure.5 Furthermore, this increased stress can disrupt load distribution within the joint, potentially affecting the stability and durability of the cemented acetabular cup.5 Additionally, obese patients might have weaker bone density and strength, further increasing the likelihood of loosening and interface failure of cemented implants.1–4 With the global surge in obesity, which affects over 1.9 billion adults,2,6–8 its repercussions on joint health, specifically regarding the biomechanical performance, contact mechanics, and durability of cemented acetabular cups in THA have become a growing concern.
The integrity of cemented acetabular cups is primarily compromised by mechanical factors with obesity playing a significant role in exacerbating these issues.4,5 Notably, obese individuals exhibit altered biomechanical dynamics that contribute to an increased risk of early hip replacement incidents1–4 and periprosthetic infections post-primary hip replacement.3 Studies have demonstrated that obesity induces heightened stress on both cortical and cancellous bones, as well as on the cement mantle, further predisposing to cup loosening and failure.4 Besides obesity, improper selection of implant materials, cement mantle thickness, quality of bone cement, bone quality, interfacial strength, and bone remodelling have also been implicated in the failure of cemented implants.5,9–13 Both experimental and clinical investigations have consistently highlighted the detrimental impact of obesity on the failure and loosening rates of cemented acetabular cups.14–16
In the analysis of cemented implants, researchers emphasize the significance of contact pressure and micromotion between the acetabular cup and femoral head, alongside the impact of varying body weights on contact mechanics.5,17–23 However, there remains a dearth of studies exploring the effects of obesity levels on load-transfer mechanisms, contact behaviour, and failure modes of cemented implants. This study aims to bridge this gap by examining the biomechanical response, contact characteristics, and failure patterns of cemented acetabular cups under different levels of obesity. Therefore, the subsequent objectives of this study were to (1) investigate the influence of obesity levels (ranging from obese-I to severely obese) on the load-transfer mechanism in bones, particularly cortical and cancellous stresses in the cement mantle, (2) analyse the contact mechanics between the acetabular cup and femoral head under varying body weights (100–150 kg) or levels of obesity, and (3) predict the failure modes and mechanisms of cemented acetabular cups subjected to different levels of obesity, with a focus on identifying critical stress points and failure patterns.
2 Materials and method
In the present numerical investigation, six finite element (FE) models of the hemipelvis were developed based on computed tomography (CT) scan data from a 62-year-old female patient with varying bodyweights ranging from 110 to 150 kg. These models, derived from the same patient, were individually modified to represent each specific bodyweight. Prior to the investigation, these FE models were validated against previous studies, using specific boundary and loading conditions relevant to each bodyweight and implant material combination.5,10,11 Mesh convergence analysis was performed using ten-noded tetrahedral elements to ensure computational efficiency and accuracy. The study ensured that the results, particularly the average stress values in the cortical and cancellous bone, remained unaffected by the number of elements in the FE models. The optimized number of elements (3,12,352 elements, elemental edge length found to vary between 0.5 and 3 mm) was selected to reduce computational costs without compromising accuracy. For the FE model, a uniform 3 mm cement mantle thickness was employed as shown in Fig. 1, consistent with an earlier published study.10 The numerical analysis considered a 450 inclination angle and 150 anteversion angle of the acetabular implant.5,10,11
2.1 Material properties of bones and implants
The FE modelling details were previously discussed in earlier published studies.5,10,11 A sandwich-like structure made up of layers of cancellous and cortical bone forms up the pelvic bone. Cortical bone was assumed to be homogeneous and isotropic based on previously published literature, indicating that isotropic modelling does not significantly affect the results compared to orthotropic modelling.24–28 Young's modulus and Poisson's ratio for cortical bone were taken as 17 GPa and 0.3, respectively, as per the previous study.5 On the other hand, cancellous bone was considered heterogeneous and isotropic, as mentioned in various earlier studies.5,10,11 The material properties of cancellous bone were distributed using a density-modulus power law (Table 1), similar to a previous study.5
| Components | Young's modulus (GPa) | Poisson's ratio |
| Cancellous bone (Spongy part of the bone) | Depends on spatial location (from CT scan data). Determined using density (ρ)-modulus (E) power law: E=2017.3ρ2.46 (MPa) and ρ = 0.022 + 0.001038 × HU | 0.2 |
| Ceramic (spherical femoral head and acetabular cup) | 350 | 0.26 |
| Cortical bone (denser bone) | 17 | 0.3 |
| Bone cement (PMMA) | 2 | 0.33 |
Ceramic was chosen as the implant material for the spherical femoral head and acetabular cup, as it has shown reduced failure rates compared to metallic implant materials.29,30 Moreover, ceramic implants have been found to enhance the longevity of the acetabular cup.30 Ceramic implants are being utilised more often to replace bone defects because they are extremely biocompatible, chemically comparable to bone tissue, bioactive, osteoconductive, osteoinductive, and have good mechanical properties.29 While ultra-high molecular weight polyethylene (UHMWPE) and metallic implants were also considered in joint arthroplasty,31–34 they exhibited issues in the long term.31–34 UHMWPE, due to its low stiffness and high deformability, resulted in a high volume of wear debris in joint arthroplasty.2,31,32 However, recent improvements in advanced and modern UHMWPE have improved wear resistance and long-term survivorship.31,32 Metallic implants were found to release harmful metallic ions and debris, causing long-term discomfort.2,33,34 Considering the longevity of cemented hip arthroplasty, ceramic was chosen as the implant material. For the present study, Young's modulus and Poisson's ratio for ceramic were set at 350 GPa and 0.26, respectively, as specified in Table 1. For the material properties of bone cement, the values for Young's modulus were 2 GPa and Poisson's ratio was 0.33. Detailed material properties of all the components of the cemented acetabular cup are shown in Table 1.
2.2 Applied loading and boundary conditions
For loading conditions, 21 muscle forces and the hip joint reaction force were considered, consistent with several published studies.5,10,11 The sacroiliac joint and pubic symphysis were considered fixed constraints in this numerical investigation.5,10,11 The hip joint reaction force was applied through the center of the spherical femoral head. The muscle forces and hip joint reaction force were calculated and linearly calibrated based on different bodyweight and obesity cases (obese case-I to obese case-III (severe obesity)) of the patient for normal walking conditions, similar to values presented in earlier published studies.5,10,24,35 These calibrated values for 21 muscle forces and hip joint reaction forces were applied to the 3-D FE model of the hemipelvis (Table 2). Body mass index (BMI) was calculated for different bodyweights and an average height of 1.82 m. The cases of obesity were categorised based on BMI values, following a similar approach in a previously published study.4 The numerical investigation considered three different cases of obesity: obese case-I (100–110 kg, BMI 30.2–33.2 kg/m2), obese case-II (120–130 kg, BMI 36.2–39.2 kg/m2), and obese case-III (140–150 kg, BMI 42.3–45.3 kg/m2).
| Muscle and Hip joint force | Obese case-I | Obese case-II | Obese case-III | |||
| 100 kg | 110 kg | 120 kg | 130 kg | 140 kg | 150 kg | |
| Hip Joint Force | 3083 | 3391 | 3699 | 4008 | 4316 | 4624 |
| Adductor brevis | 163 | 179 | 195 | 212 | 228 | 244 |
| Adductor longus | 126 | 138 | 151 | 163 | 176 | 189 |
| Adductor magnus | 0 | 0 | 0 | 0 | 0 | 0 |
| Biceps femoris | 289 | 317 | 346 | 375 | 404 | 433 |
| Gemellus superior | 126 | 138 | 151 | 163 | 176 | 188 |
| Gemellus inferior | 0 | 0 | 0 | 0 | 0 | 0 |
| Gluteus maximus | 1329 | 1461 | 1594 | 1727 | 1860 | 1993 |
| Gluteus medius | 1504 | 1654 | 1805 | 1956 | 2106 | 2256 |
| Gluteus minimus | 200 | 220 | 240 | 260 | 280 | 300 |
| Gracilis | 0 | 0 | 0 | 0 | 0 | 0 |
| Iliopsoas | 0 | 0 | 0 | 0 | 0 | 0 |
| Obturator externus | 0 | 0 | 0 | 0 | 0 | 0 |
| Obturator internus | 176 | 193 | 211 | 228 | 246 | 263 |
| Pectineus | 0 | 0 | 0 | 0 | 0 | 0 |
| Piriformis | 250 | 275 | 300 | 325 | 350 | 375 |
| Quadratus femoris | 137 | 150 | 164 | 178 | 192 | 206 |
| Rectus femoris | 176 | 193 | 211 | 228 | 246 | 264 |
| Sartorius | 126 | 138 | 151 | 163 | 176 | 189 |
| Semimembranosus | 526 | 578 | 631 | 683 | 736 | 788 |
| Semitendinosus | 200 | 220 | 240 | 260 | 280 | 300 |
| Tensor fasciae latae | 189 | 207 | 226 | 245 | 264 | 283 |
2.3 Interface conditions
Cemented implants consist of two major interfaces: bone-cement and cement-implant interfaces. In this study, both interfaces of the cemented acetabular cup were modelled as fully bonded. However, frictionless contact was considered between the acetabular cup and the femoral head. The contact parameters between the implants were optimized to avoid affecting the study's aim and scope while also reducing computational costs. Six-noded surface-to-surface contact elements were used to analyse the contact performance of the cemented implant.5 The selection of contact parameters, such as normal penalty stiffness and penetration tolerance factor, significantly influenced contact performance. Hence, these parameters were chosen to achieve convergence of the non-linear solution with the least computational time.5 The numerical investigations were performed using ANSYS v19 (ANSYS Inc. PA, USA). The overall workflow of the current numerical investigation is presented in Fig. 2.
3 Results
The findings of this study were reported in terms of stresses in the cortical bone, cancellous bone, and the cement mantle to understand the biomechanical performance and load-transfer mechanism of the cemented cup. The von Mises stress was utilised to predict stress in the cortical and cancellous bone, while the 1st principal stress was determined for the cement layer of the cemented acetabular cup, as illustrated in Fig. 3(a) and (b). According to a prior study,4 the 1st principal stress proved more reliable in forecasting failure in the cement Additionally, contact pressure and micromotion between the bearing surfaces (acetabular cup and femoral head) were evaluated to predict the contact mechanics of the cemented acetabular cup (Fig. 3(c) and (d)) The average values of von Mises stresses, contact pressure and micromotion subjected to three different obesity cases is presented in Fig. 3.
Results of this analysis showed that the average von Mises stress in cortical and cancellous bone enhanced as bodyweight increases from obese case-I to obese case-III (Fig. 3(a)). Similarly, the average 1st principal stress in cement mantle also enhances as the bodyweight increases from obese case-I to obese case-III (Fig. 3(b)). The values of the average von Mises stress in cortical bone were found to increase from 8.87 MPa to 12.81 MPa as the bodyweight increased from 100 kg to 150 kg (obese case-I to obese case-III) (Fig. 3(a)). Similarly, the average von Mises stress in cancellous bone found to be increased from 0.641 MPa to 0.866 MPa as the bodyweight increases from obese case-I level (100, and 110 kg) to obese case-III level (severely obese) (140, and 150 kg) (Fig. 3(a)). The value of the average 1st principal stress in the cement layer was found to be increased from 1.71 MPa to 2.738 MPa (Fig. 3(b)) with the bodyweight from obese-I to obese-III level. The average value of contact pressure between articulating surfaces (acetabular cup and spherical femoral head) increased from 3.52 MPa to 5.54 MPa as the bodyweight increased from obese-I to obese-III category of obesity (Fig. 3(c)). Similarly, the micromotion between articulating surfaces was also found to increase from 0.09 mm to 0.14 mm with the bodyweight from obese-I to obese-III level (Fig. 3(d)).
4 Discussions
Elevated stress levels in the cortical and cancellous bone, as well as within the cement mantle, play a pivotal role in influencing the durability and ultimate failure of cemented fixation in the acetabular cup.4,5 The presence of high contact pressure and micromotion at the articulating surface of the cemented acetabular cup significantly contributes to increased wear and potential failure of the components.5,20–23 These combined factors underscore the critical importance of mitigating stress concentrations and minimizing motion to enhance the longevity and performance of cemented acetabular cup fixations.4,5,20–23 The findings of this study offer valuable insights into the biomechanical performance and load-transfer mechanism of cemented acetabular cups under varying degrees of obesity. By assessing stresses in cortical bone, cancellous bone, and the cement mantle, as well as contact mechanics between articulating surfaces, this investigation sheds light on the effects of obesity on the functionality and longevity of orthopaedic implants.
Firstly, our analysis revealed a consistent increase in von Mises stress within cortical and cancellous bone as body weight escalated from obese case-I to obese case-III. This escalation suggests a heightened risk of bone overloading and potential failure, particularly in severely obese individuals. The observed disparity in stress magnitude between cortical and cancellous bone underscores the importance of considering bone quality and distribution of loading forces in implant design and fixation strategies. While the average stress values align with previous studies,4,5 the slightly higher stresses observed in this investigation may be attributed to the consideration of 21 muscle forces,35 indicating their significant influence on load distribution within the acetabular cup. The stress magnitude in the cortical bone was much larger than that of the cancellous bone, indicating that the cortical bone bears a higher load.4,5 Furthermore, the stress distribution in the cement mantle, as indicated by the first principal stress, exhibited a similar trend of increase with escalating body weight.5 This finding corroborates the notion that obesity exacerbates stresses within the cement layer, potentially compromising its structural integrity and leading to implant loosening or failure.2,4,5 The observed agreement between our results and previous studies underscores the reliability of utilising the first principal stress as a predictor for cement failure.5 The average stress values in the cement mantle also showed good agreement with previously published data.4,5
Contact pressure and micromotion between articulating surfaces have been established as crucial parameters influencing the longevity, loosening, and failure analysis of the acetabular cup.5,36 The role of these contact parameters in the contact mechanics of the cemented acetabular cup is of paramount importance.5,20–23 In addition to stress distribution, our analysis of contact mechanics parameters, including contact pressure and micromotion between articulating surfaces, revealed notable increases with rising body weight. These findings underscore the critical role of contact mechanics in implant longevity and performance, particularly in obese patients. Higher contact pressures and increased micromotion not only elevate the risk of wear and frictional damage but also exacerbate stresses within the implant-bone interface, contributing to accelerated implant failure.10,37–42 Moreover, the influence of obesity on gait patterns and activity levels further complicates the biomechanical response of cemented acetabular cups, necessitating tailored implant designs and surgical techniques to accommodate varying patient profiles.37,38
4.1 Clinical significance
Obesity exerts a notable influence on various aspects of THA involving cemented acetabular cups. Biomechanical performance, analysis of contact mechanics parameters, and the longevity of these cups are all significantly impacted.1–8 Additionally, the patient's obesity can dictate alterations in the surgical technique employed during cup implantation.1–8 Obese individuals undergoing THA are at heightened risk of encountering surgical complications such as infections, blood loss, and nerve damage.6–8 These complications not only jeopardize the immediate success of the implant but also elevate the probability of subsequent revision surgery, aseptic loosening, and periprosthetic fracture.4–8 The elevated stresses on bones and higher contact mechanics parameters between articulating surfaces in obese patients can collectively lead to a reduced lifespan of the cemented acetabular cup.4–8
4.2 Limitations and future scopes
Several limitations are notable in this numerical analysis. Firstly, this investigation primarily examined static loading conditions, with an acknowledgment that stresses within the cemented acetabular cup components and contact parameters may escalate under fatigue loading. The model did not incorporate the microstructural details of cortical bone, nor did it account for morphological changes in bones due to varying bodyweights (obese-I to obese-III). Furthermore, the study relied on data from a single patient, and considering data from multiple patients with different bodyweights or obesity levels could yield more comprehensive results. However, gathering and differentiating such individual-specific data could be time-consuming and challenging. Despite these limitations, various clinical studies have already established that obesity adversely affects bone and joint longevity, and it is linked with conditions such as type 2 diabetes, cardiovascular disease, sleep apnea, osteoarthritis, and cancer.43,44 Some studies even consider obesity as a disease in itself.43,44 To address these limitations, future research and investigations should focus on developing different implant designs and surgical techniques that can better accommodate obese patients and improve the longevity and success of cemented acetabular cups.
5 Conclusion
The findings of this investigation strongly indicate that obesity has a detrimental impact on the longevity of cemented acetabular cups. The likelihood of failure in the cemented acetabular cup increases with obesity. As the bodyweight of the patient escalates from obese-I to obese-III (100 kg–150 kg), there is a substantial increase in stresses observed in the bones and the layer of bone cement. Moreover, obesity is associated with higher contact mechanics parameters, leading to elevated contact pressure and micromotion between articulating surfaces, which ultimately contributes to the high wear and failure of the cemented acetabular cup. Taking proactive measures to address obesity can significantly improve the success and longevity of cemented acetabular cups in total hip arthroplasty.
Ethical statement
The authors hereby declare that proper approval has been obtained from the patient for the submission of this research publication in accordance with institutional rules and procedures. Informed consent was given by the patient receiving treatment for the collection and use of CT data for study.
Funding statement
None.
Guardian/patient's consent
Not Applicable.
CRediT authorship contribution statement
Ajay Kumar: Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Writing – original draft. Subrata Mondal: Investigation, Methodology, Validation, Writing – review & editing. Rajesh Ghosh: Guidance, Supervision, Resources, Conceptualization, Investigation, Methodology, Software, Validation, Writing – review & editing.
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