1 Introduction

The antibiotic bone cement spacer is considered a significant treatment procedures for the periprosthetic joint infections.¹ Periprosthetic total hip replacement infection is a commonly encountered condition in the field of orthopedic and trauma surgery, with a reported incidence ranging from 0.5 % to 3 %.² The management of infected hip prostheses involves the removal of the affected prosthesis, followed by the placement of a molded-cement spacer (CS) for a specific duration, 3–6 months in general. This interim stage is imperative prior to the subsequent reimplantation of the second hip prosthesis. Traditionally, surgeons have relied on manual technique to mold the CS. Despite its effectiveness in eradicating infections, the manual molding technique lacks the ability to provide precise-shape designs and has been associated with notable complications, such as cement fracture and hip dislocation, as reported in several previous studies.^3,4,5 In recent years, there has been a heightened focus on achieving precise fabrication of hip cement spacers, contributing to the rising popularity of the hip cement spacer molding block as an effective tool. In spite of the advancements in precise molding, the spacer fracture and subsequent dislocation continue to be prevalent issues.⁵ To address the problem of CS fracture, it is crucial to investigate the mechanical behavior of the CS for daily human activity. Most previous studies have focused on describing the result of incorporating a metallic endoskeleton into the CS and the quality of the cement mixing method that impacts CS strength. To our knowledge, there has been limited investigation into the influence of CS geometry design on CS strength.^6,7,8,9,10 This investigation will enable the development of enhanced hip spacer geometry designs that can reliably endure the mechanical loads encountered during daily activities.

This study aimed to investigate the mechanical behavior of the CS under static loading conditions using a finite element model (FEM). Additionally, static compression testing was conducted to determine the ultimate failure load associated with mechanical failure.

2 Materials and methods

2.1

2.1 Model design and material properties

The three-dimensional solid model of CS was designed according to the orthopedic surgeon's requirement through an engineering program (Solid works). The CS consisted of two components, including the femoral stem and the femoral head (head diameter, 42 mm; stem length, 150 mm), as shown in Fig. 1. In order to reduce the risk of cement spacer fracture, special attention is given to the design of the femoral neck. The femoral neck is intentionally designed to be shorter and thicker, with a length of 15 mm and a width of 14 mm. This design differs from the typical dimensions of femoral prostheses, aiming to enhance its strength and durability in the critical femoral neck area. The material properties for creating finite element model, including Polymethyl-Methacrylate(PMMA) density, Young's modulus, Poisson's ratio, and ultimate tensile strength of PMMA, have been provided in Table 1^1,11.

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Fig. 1 The Cement spacer Geometry (mm; millimeter).

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Table 1 Material properties of bone cement^1,11

Material	Density (kg/m3)	Young's Modulus (MPa)	Poisson's Ratio	Ultimate Tensile Strength (MPa)10
PMMA	1188	2500	0.35	50

2.2

2.2 Loading, boundary conditions and mesh model

The CS model was tested with three static-loading values of 35 kg, 52.5 kg and 70 kg, covering a range of patient weights from 50 kg to 70 kg. the load vector is applied vertically to the femoral head, replication the effect of the body weight on the femoral head, while fixed points are connected around a stem, simulating its insertion into the femoral bone (Fig. 2a). The tetrahedral mesh type was chosen and the optimal mesh size was verified for consistency with the calculated values based on the theoretical equivalent strain (Fig. 2b).

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Fig. 2 The loading conditions (Fig. 2a) and the mesh elements (Fig. 2b) (Kg; kilogram, N; newton, S; second, mm; millimeter).

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2.3

2.3 Mechanical test conditions

To obtain the ultimate failure load and evaluate the number of cycles on cemented hip replacement. The compression testing and fatigue testing were performed in the study. The molded-ACS sample for testing was formed by an aluminum hip-shape mold under the temperature of 19 °C with the procedures as follows: 20 ml liquid monomer is mixed with 40 g of Polymethy-methacrylate powder polymer (PMMA, PALACOS®) by stirring for about 30 s, followed by 1-min waiting period. The PMMA bone cement was shaped into a molded spacer and waited for hardening, approximately 12–14 min (Fig. 3). Subsequently, the molded-ACS was extracted from the aluminum mold to be used in mechanical testing.

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Fig. 3 The forming cemented hip replacement procedure (PMMA; Polymethyl-Methacrylate powder polymer).

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The compression testing was carried out by using the standard mechanical testing machine (Cometech, NTS Technology CO., LTD., load cell 20 kN). The CS sample was embedded into a specifically designed jig and subjected to loading until it reached the point of breakage, in order to obtain the ultimate compressive force, as shown in Fig. 4. To assess the dynamic load, fatigue testing was performed on the CS using a pneumatic system. The setup consisted of a bending load cell, capable of handling loads ranging from 5 to 500 kg, an air cylinder was used to apply pressure on the femoral head surface, and a Graphic User Interface (GUI) system for measuring force and cycle time at a sampling rate of 1000 values per second. The entire system was controlled by an Arduino program running on the Nano PLC version 2.0. To evaluate the fatigue life, the CS was initially subjected to a load of 35 kg, followed by an additional total of 50 kg, representing 50 % partial weight bearing ambulation of the patients weighing between 70 and 100 kg.

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Fig. 4 Mechanical testing on cemented hip replacement set up in the side view (Fig. 4a) and the front view (Fig. 4b).

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3 Results

3.1

3.1 Finite element model results

The finite element method results illustrate the distribution of Von-Mises stress and the factor of safety (FOS) across the CS model (Fig. 5). For load cases of 35 kg, 52.5 kg, and 70 kg, the maximum Von-Mises stresses were 18.16 MPa, 27.24 MPa, and 36.32 MPa, and the corresponding FOS values were 2.75, 18.3 and 1.37, respectively. Notably, the highest Von-Mises stress was observed at the medial side femoral head-neck junction.

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Fig. 5 Analysis of simulated mechanical behavior on static loading results (MPa; mega pascal).

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In order to verify the equivalent elastic strain from simulation testing, the independent components of the strain tensor⁴ were defined to calculate the equivalent stress of these components. There were nine components used to determine strain tensor, which consisted of six independent components of strain: εxx, εyy, εzz, εxy, εyz, εxz, representing the relative displacement. Regarding the extensional strain components (εxx, εyy, εz) which represented the change in length, while the shear strain components (εxy, εyz, εxz) indicated the change in angles between any two linear elements (as shown in the strain tensor matrix). The equivalent strain value (εeqv) of these components was calculated according to the equation as shown in Fig. 6

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Fig. 6 The equation for the equivalent elastic strain calculation.

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An equivalent elastic strain and the plane shear strain were also depicted in Fig. 7. The maximum equivalent elastic strain of 0.01507 mm/mm was detected at the femoral neck. The calculated equivalent strain values were presented in Table 2.

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Fig. 7 The equivalent elastic strain and plane shear strain (S; second, mm; millimeter).

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Table 2 A comparing equivalent strain results between software and calculation.

Element size (mm)	Number of elements	Number of nodes	Equivalent strain result (εt) (mm/mm)		Engineering strain (εe) (mm/mm)
			Ansys software	Calculation
2	584,233	842,743	0.01507	0.01473	0.01518

3.2

3.2 Fatigue and compression results

The dynamic load mechanical testing was conducted using a Cometech-pneumatic system machine. The test was designed to subject the femoral head to an initial load of 35 kg for 300,000 cycles, followed by 50 kg for an additional 300,000 cycles, as illustrated in Fig. 8. The results of the fatigue testing, consisting of a total of 600,000 cycles, demonstrate the absence of damage in the CS.

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Fig. 8 The number of cycles within 1 s of each load entering (Kg; kilogram).

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The mechanical testing under dynamic loading was studied as well. The designed cemented hip replacement was molded for fatigue and compression testing, then the fatigue testing was undertaken through a pneumatic system. The result in Fig. 8a represents putting the first load at 35 kg that performs on the femoral head, followed by 50 kg within 1 s (Fig. 8b). The loading weight at 35 kg has been tested a total of 300,000 times, and then increased to 50 kg, which employed an equal number of cycles to evaluate deterioration. It can be concluded that fatigue testing results in a total of 600,000 cycles not being able to build damage on cemented hip replacement, which could lead to destructive testing, such as compressive testing.

To determine the ultimate compressive force, the static loading was performed in the study using a compression test. A CS sample was subjected to loading until failure, and the relationship between the applied force and the displacement of CS was recorded. The results showed that the ultimate load at the failure point was 4354 N (444 kg, Fig. 9). This indicates that the CS, which was designed based on finite element model analysis, effectively withstood high-load conditions.

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Fig. 9 The force-displacement of PMMA bone cement under compression testing (N; newton, mm; millimeter).

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The fracture analysis of the CS revealed that, following the ultimate load compression testing, the CS sample fractured into three separate parts at two distinct fracture locations, as shown in Fig. 10. The fracture occurred at the neck and proximal part of the stem of the CS. Regarding neck fracture, the fracture was a vertical shearing type that extended from the superior-lateral part and ran down to the inferior-medial part of the neck. On the other hand, the proximal part of the CS stem initially broke at the lateral side in a transverse fracture pattern and then extended medially in a short oblique fracture type. These findings indicate that the femoral neck area primarily experienced shearing forces, while the proximal part of the CS stem underwent combined bending and shearing force.

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Fig. 10 The fracture characteristics after compression testing.

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4 Discussion

The use of antibiotic cement spacer remains the most effective approach for managing chronic periprosthetic hip infections. Nonetheless, the occurrence of cement spacer fractures during the interim period remains a challenging issue¹. In order to decrease the occurrence of CS fractures, it is crucial that the cement spacer possesses enough strength to withstand the physiological load for daily activity. Numerous studies have examined methods to enhance the mechanical strength of the CS, such as reinforcing with metal wire, incorporating a metal rod, and utilizing commercially available hip cement spacers ^8,12. To the best of our knowledge, there is a scarcity of studies investigating the impact of geometric design through both finite element analysis and mechanical testing.

The finite element analysis revealed that the femoral head surface experiences the maximum deformation at the compression site, with deformation gradually decreasing towards the base of the femoral neck, where the lowest deformation occurs. In terms of von Mises stress, the highest stress occurs at the medial side of the femoral head-neck junction and spreads throughout the femoral neck area, while the femoral head and femoral stem exhibit the lowest von Mises stress distribution.

The cement spacer in this study was fabricated using a specific molding block technique, ensuring a more consistent compression of the bone cement when compared to the manual hand molding technique. consequently, the structural models underwent evaluation with an ultimate tensile strength of 50 MPa, as reference by Jaeblon et al.¹¹ None of the maximum Von-Mises stress values in the load cases exceeded the specified criteria value. Furthermore, the FOS consistently remained above 1.0, indicating that the structural performance is deemed acceptable. Under a 70 kg load, the maximum Von-Mises stress recorded was 37 MPa, which is below the previously described limit of 35 MPa.⁷ This leads to concerns when contemplating the application of this CS model for patients weighing over 100 kg (bearing 50 % of the load). However, to enhance the structural performance of the CS, potential improvements could focus on either diminishing stress values at the femoral head-neck junction of strengthening the FOS through geometric design.

This outcome suggests that the femoral neck area experiences significant stress with less deformation capability, highlighting the importance of specific design modifications to enhance the failure load. This finding aligns with numerous previous studies, such as work by Hao Ge et al., which describes the highest compressive stress on the medial femoral neck and the use of annular metallic endoskeleton to distribute forces on the cement spacer.^13,7 However, there is still conflicting evidence as the results from mechanical testing show that the cement spacer without metal reinforcement has the highest ultimate failure load¹⁰.

Based on the compression loading test, the CS exhibited durability with the toleration up to 50 kg over the total of 600,000 cycles without failure. These findings suggest that our designed cement spacer, which incorporates an increased thickness and width in the neck part, can withstand approximately 3333–6666 walking steps during the 12-week interim period with 50 % partial weight bearing for patients weighing between 70-100 kg. The present study revealed an ultimate failure load of 4344 N, which was lower than the previously reported value of 5475.42 N in prior study¹⁰. This difference can be attributed to the variances in cement mixing techniques. In our study, manual cement mixing techniques were utilized without the use of a cement gun. On the contrary, Kaku N et al.’s employed modern cement mixing techniques, including a vacuum cement mixing system and the use of a cement gun¹⁰. These advanced techniques help reduce air porosity in the cement mantle, thereby enhancing the strength of the cement spacer¹⁰. However, these modern techniques come with higher costs and are often unavailable in many developing countries. Nevertheless, the designed CS utilizing manual cement mixing demonstrates a load to failure of up to 434 kg, indicating its ability to withstand the typical daily activities.

The analysis of fracture characteristics revealed that fractures occurred at the femoral neck and proximal part of the stem. Fractures in the neck area exhibited an oblique type, indicating a primarily shearing load. Conversely, fractures in the proximal stem displayed a transverse-oblique type, suggesting a combination of bending and shearing loads. To enhance the mechanical properties of the cement spacer, numerous studies supported the utilization of metal reinforcement within the spacer, such as metal wire, a Rush pin, or incorporated with a hip compression screw device ^6,8,9. However, a recent study revealed that the incorporation of metal reinforcement with metal wire or pin smaller than 6 mm in size resulted in lower mechanical strength compared to the use of standard cement alone¹⁰. The choice of material is another factor in reducing the risk of microbial infection on the metal surface, as titanium alloys demonstrate lower affinity for microbial adhesion compared to other material.¹⁴ When utilizing metal reinforcement, the authors suggested that using the titanium pin reinforcement larger than 6 mm could increase the ultimate failure load. However, this would result in a decrease amount of antibiotic within the cement, consequently diminishing the ability to eliminate bacteria¹⁵. Considering these factors, the optimization of CS geometry design could be the most favorable approach for enhancing the ultimate failure load while maintaining the efficacy of the antibiotic component.

The findings from the present study propose that enhancing the neck-stem angle which increases the CS straightness can help convert shearing forces into compression loads, resulting in a higher resistance to failure loads. Additionally, increasing the width and thickness of the neck, proportionate to the size of the femoral head, should be considered.

The study has certain limitations. Firstly, the finite element study and dynamic load test are not able to completely replicate the typical biomechanics and kinematics of the hip joint. Nonetheless, the findings from this study offer insights into the failure mode and can contribute to an improvement of the geometric design of the CS, enabling it to better withstand such failure. A second limitation is that this study represented the results of the bone CS without the addition of antibiotics. The addition of a substantial amount of antibiotics to the bone cement can adversely affect its strength. Nevertheless, previous literatures suggest that when the antibiotic concentration is below 5 % of the total 40 g bone cement (referred to a low-dose antibiotic cement spacer), it does not significantly impact the strength of the bone cement^16,17. Further research is required to conduct mechanical loading tests specifically on high-dose antibiotic cement spacers. Lastly, this study is primarily descriptive in nature and does not involve any comparative analysis. Further research on clinical outcomes should be conducted. The main objective of the study was to examine and enhance the geometric design of the CS in order to withstand the forces generated during typical daily activities.

5 Conclusion

The designed hip cement spacer demonstrates the ability to withstand compressive forces exceeding 400 kg, as well as repetitive weights of 35 kg and 50 kg over a total of 600,000 cycles, without displaying any signs of fatigue failure. This suggests that the designed CS can be utilized for patients weighing between 70 and 100 kg, enabling 50 % partial weight-bearing over a period of 4–6 months, while taking into account a daily activity level of 3000–5000 steps. The femoral neck area is recognized as a crucial area due to its exposure to the significant von Mises stress and limited deformation capacity. To achieve optimal geometric design for the hip cement spacer, careful attention should be given to the key factors. Firstly, increasing the femoral neck-shaft angle can effectively decrease the shearing force that impacts the femoral neck. Secondly, it is essential to maximize the diameter of the femoral neck to ensure it can withstand the substantial tensile and compressive forces. Furthermore, it is crucial to proportionally increase the size of the femoral neck diameter in relation to the femoral head size.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding statement

This research was supported internal funding support by The Medical Innovation of Mankind Center (MIND), Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Thailand.

Consent for publication

All authors have read the final version and give consent for the article to be published.

Ethical statement

The research does not require ethical committee approval. The author confirms that the study does not involve the use of nonhuman animals or human.

Patients consent

The study does not involve in human research.

CRediT authorship contribution statement

Eakkachai Warinsiriruk: Conceptualization, Methodology, Investigation, (finite element analysis and mechanical load test). Nutchanat Thongchuea: Investigation, (finite element analysis and mechanical load test). Nachapan Pengrung: provide important intellectual content. Chavarat Jarungvittayakon: provide important intellectual content. Paphon Sa-Ngasoongsong: revise manuscript and approved the version to be published. Kulapat Chulsomlee: Conceptualization, Methodology, Writing – original draft, preparation, revise manuscript and approved the version to be published.