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Different polymethylmethacrylate (PMMA) reinforcement strategies for long bone osteoplasty procedures: a controlled laboratory comparison using the 4-point bending test

BMC Musculoskeletal Disorders volume 25, Article number: 1058 (2024) Cite this article

Abstract

Background

Cementoplasty has been successfully used for treating fractures in various parts of the human body, although the use in weight-bearing long bones is a subject of controversial debate. Strategies to improve the mechanical properties of polymethylmethacrylate-based bone cement (BC) comprise changing the chemical composition or the application of metal reinforcement strategies. In clinical practice reinforced bone cement is used despite biomechanical basic research regarding this topic being scare.

Objective

The aim of the present study was to evaluate the biomechanical properties of two different reinforcement strategies against non-reinforced polymethylmethacrylate-based BC subjected to bending stress.

Methods

In this controlled comparative laboratory analysis, we evaluated two types of reinforcement strategies in comparison to a control group (C). BC was reinforced with a Kirschner wire (group CW) or with a prestressed twinned steel cable (group CC); control group C was native polymethylmethacrylate-based BC. All the samples were prepared using a custom-made mould and underwent 4-point bending stress until fracture using a testing machine. Flexural strength, maximum strain, and Young’s modulus were assessed for the three groups and compared using the Kruskal‒Wallis test.

Results

The mean flexural strength in MPa was 48 ± 12 in C, 64 ± 6 in CW, and 63 ± 14 in CC. A significantly greater flexural strength of + 33% was found in both reinforced groups than in the C group (C vs. CW p = 0.011, C vs. CC p = 0.023). Regarding the flexural strength, no statistically significant difference could be found between the two reinforcement strategies CW and CC (p = 0.957). The maximum strain was 3.0% in C and CW and 3.8% in CC and no difference between the three groups was observed (p = 0.087). The Young’s modulus in GPa was 2.7 for C, 2.8 for CW, and 2.4 for CC. The comparison of Young’s module using the Kruskal-Wallis test showed no statistically significant difference between CC, CW and C (p = 0.051).

Conclusions

We detected an improvement in flexural strength in the reinforced groups. Both reinforcement through K-wire and prestressed cables promoted increased flexural strength. Furthermore, less material failure was observed with possible realignment and subsequent residual stability despite bone cement fracture. From a biomechanical view, the concept of macro metal reinforcement of osteoplasty is viable.

Peer Review reports

Introduction

Polymethylmethacrylate (PMMA)-based bone cement (BC) has been used extensively in orthopaedics for the past decades due to its good biocompatibility, processability and fixation stability to the bone [1]. Its main applications are arthroplasty (attaching prostheses to bone) [2, 3] and kyphoplasty (filling voids in defect fractures in the spine) [4]. In osteoplasty, BC stabilizes weak bone and alleviates pain from bone metastases. In an infection situation, antibiotic loaded BC acts as a drug delivery system [5]. BC is a thermoplastic polymer, and its properties change with ambient conditions such as temperature, fluid uptake and storage time [1]. PMMA-based BC consists of a solid component and a liquid component. The solid phase comprises approximately 89% polymers, a radio pacifier (approximately 10%), and potentially, antibiotics, dyes, or plasticizers. The liquid component makes up approximately 97% of the mixture, which contains mainly monomers, 2.7% activator, and an inhibitor [6]. Upon mixing, the two phases undergo free radical polymerization and curing. This exothermic chemical reaction generates heat and leads to volume shrinkage by 6–7% [1, 6].

The dough stage viscosity, defined as the working time, is crucial for moulding the bone cement in situ and lasts approximately 5 min [1]. The curing process and the duration of the phases are significantly influenced by ambient temperature, humidity and powder-to-liquid ratio [7].

Baseline characteristics of PMMA-based BC have been defined as: Compressive strength ≥ 70 MPa, bending strength ≥ 50 MPa and Young’s modulus of 2.4 GPa [8, 9]. Hence, PMMA is considered resistant to compressive forces but not traction or torsion [1, 10]. Mechanical properties are sensitive to several factors like powder size, molecular weight, monomer content, mixing technique and presence of porosity [11]. BC stability depends also on its fit with surrounding structures, e.g. shrinkage may inhibit interdigitation, heat may damage bone or implant. Mixing cement under atmospheric conditions increases the infiltration of air, resulting in pores decreasing material strength but counteracting shrinkage [11]. In vivo, water absorption of BC prevents most of the volume shrinkage [12, 13] and facilitates heat dissipation [1].

A need for enhanced durability and stability remains especially for the use in the weight bearing parts of the skeletal system [14]. Different reinforcement techniques have been proposed to improve the mechanical properties of BC, either by altering the chemical composition based on knowledge in materials science [15,16,17,18], or by applying macro-metal reinforcement strategies in clinical practice [19,20,21,22,23,24,25].

Percutaneous cementoplasty (PC), a new and minimally invasive technique [26], has been proven to be an effective therapy for bone metastatic lesions in the spine, pelvis, and proximal femur [27,28,29,30,31,32]. Koirala et al. investigated on percutaneous reinforced osteoplasty for long bone metastases and concluded that reinforced osteoplasty might be applicable for the prevention or treatment of pathological fractures [33]. Cazzato et al. concluded that K-wires osteoplasty augmentation does not improve the resistance of diaphyseal bone to bending stresses [34], while Nakata et al. showed when using bone marrow nails where robust enough to restore a long bone fracture although the achieved bending strength was unsatisfactory [35]. Kitridis et al. reviewed 12 studies involving nearly 350 patients who underwent either augmented or nonaugmented percutaneous osteoplasty [36]. Roux et al. concluded that minimally invasive augmented osteoplasties appear to be the gold standard alternative to total hip arthroplasty in cancer patients unfit for surgery [37]. Biomechanical basic research regarding this topic is scarce [34]. To evaluate in vivo application of augmented BC, biomechanical testing of the new material in a controlled environment is essential before conclusions about mechanical improvements can be made.

We hypothesize that BC reinforced with either wires or prestressed cables will exhibit significantly enhanced biomechanical properties compared to native BC. We evaluated the bending strength, Young’s modulus and specimen characteristics of non-reinforced BC (control group C), wire-reinforced BC (group CW), and prestressed cable-reinforced BC (group CC) through a 4-point bending test in a comparative analysis. The hypothesis was that with the appropriate reinforcement strategy BC can resist higher flexural stress. As a second hypothesis, we assumed that prestressed cables are superior to wires as they should counteract the deflection of the specimen.

Materials and methods

Sample size

Thirty cuboid PMMA specimens were moulded each from 1.5 units of PMMA Palacos ® (Heraeus, Hanau, Germany) and divided into three groups of 10. A sample size of a minimum of 5 samples is required by the “D6272– Standard Test Method for Flexural Properties” of the American Society for Testing and Materials (ASTM) [38]. The control group C consisted solely of compact PMMA, while group CW had a 2.0 mm Kirschner wire (K-wire) embedded, and group CC was reinforced with a prestressed 2.0 mm twinned steel cerclage cable.

Sample dimensions

Since ISO 5833 “implants for surgery – Acrylic resin cements“ [39] does not specify reinforcement requirements the EN 1992-1-1:2004 “Eurocode 2: Design of concrete structures Sect. 9.5.2 Longitudinal reinforcement” specifies that the cross-section of the reinforcement should lie between 0.2% and 4% of the specimen cross Sect. [40]. Reinforcements had a diameter of 2.0 mm and a circular cross section of 3.14 mm². The proportion requirements of the Eurocode 2 were fulfilled given a reinforcement section of 3.14 mm² occupying 2.1% of the specimen total cross section of the sample (150 mm²) [38]. The volume of the reinforcement (0.628 cm³) to the total volume (30 cm³) ratio was 2.1%. The ASTM D6272 [38] guidelines defines the span to be sixteen times the thickness (160 mm) and the width should not exceed one fourth of the span (15 ≤ 40 mm). Minimum length should be 110% of the span (192 mm). As per specification, the dimensions of the specimens were chosen to be 15 × 10 × 200 mm (w × h × l) (Fig. 1) with tolerance ± 1.

Fig. 1
figure 1

Schematic drawing of the specimen. The dimensions are given In mm, where r1 indicates the ø 2.0 mm reinforcement. The unreinforced control group consisted of compact pmma

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A structure under a bending load exhibits maximum tensile stress on one side and maximum compressive stress on the opposite side, with a neutral axis where no longitudinal stresses or strains occur. Reinforcements should be placed within areas of maximum stress, while maintaining sufficient surrounding cement material to prevent fractures, similar to prestressed concrete in civil engineering [41, 42].

The eccentricity limit describes in a prestressed structure the section with maximum stress as normal compression and the bending moment are present simultaneously [42].

The reinforcement was placed in the tension zone, to reinforce the structure while bending. It also has to be placed above the bottom edge to account for enough bone cement cover and anchorage requirements. A cement mantle thicker than 1 mm is recommended [43]. The eccentricity limit from the bottom to maximize compressive resistance during bending calculated with the following formula:

$$\:e=\:\frac{h\:}{6}$$

where h is the height of the sample [42]. Considering the height of the sample of 10 mm e resulted to be 1.6 mm Considering a triangular stress distribution the neutral line stays at 3.4 mm (h/2-h/6). Considering the diameter of the wire (2 mm) which was placed below the neutral line a resulting distance from the reinforcement and the bottom of the specimen of 1.4 mm was obtained.

Sample preparation

PMMA was stored in a dry and cool place at room temperature and mixed following the manufacturer guidelines using the Palamix® vacuum mixing system (Heraeus, Hanau, Germany). For reinforced samples, the K-Wire or cable was pulled through assigned holes of the mould`s side plate, assuring precise positioning, while filling in the BC. A Tensioner of the Dall Miles Cable System (Stryker, Kalamazoo, Michigan, US) was used to prestress the cable with 445 N, held in place by two clamps attached to both ends of the cable before administering the BC. BC was filled and the mould was closed by tightening six M5 bolts to 10 N m to compensate for dilation and ensure uniform size. After 30 min of polymerization in the mould samples were allowed to cool and consolidate for another 30 min at room temperature under a steel plate (height 8 mm) fixed with C-clamps to prevent deflection. To achieve precise dimensions within the tolerance limit of ± 1 according to ASTM D6272 [38] all specimens were milled longitudinally to a height of 10 mm using a fine mile (FF 500/BL-CNC, Precision 0.05 mm, Proxxon S.A., Wecker, Luxembourg)]. Specimens were prepared over two days at 23 °C.

Quality control of samples

All samples underwent quality control by visual inspection of defects and by performing a multi-slice Computed Tomography (CT) scan after 12 to 24 h for each specimen (100 kV; 300 mA; 1 Sect. 0.625/0.312; bone IQ Enhance; 0.53; 10.62 mm/rotation GE Medical Systems Light Speed VCT, Chicago, IL-US). Radiodensity measurements [HU] were conducted using a volume of interest analysis on the whole sample volume using Syngo.via VB40B_HF01 (Siemens Healthcare GmbH, Erlangen, DE). Radiodensity (HU) was also analysed for a cement-only section of the reinforced specimen’s volume.

For seven days, all the samples were incubated at 37 °C and 95% humidity to allow sufficient time after polymerization. Mass and geometry variations were ascertained before (t0) and after (t1) the incubation, (Δt) represents the difference between t0 and t1. Mass was measured using a high-precision balance (SI-603, balance precision: 0.001 g, Denver Instrument, Bohemia, NY, USA) before and after the incubation period. The volume (V) was determined before and after the incubation by measuring the length, width, and depth three times per sample with a calliper. With a calliper, the mid-specimen elevation e (distance from the deepest point to a line connecting both endpoints of the sample) was measured, and the respective central angle \(\:\theta\:\) of the circular segment was calculated based on the length of specimen l at t0 and t1.

$$\:\theta\:=\:4*\:arctan\:*\:\frac{2\:*e\:}{\stackrel{-}{l}}$$

Mechanical testing

A servo-hydraulic material testing machine (MTS, 858 MiniBionix II, US-MN) performed the four-point bending test with a 2.5 kN load cell using suitable hardware (sampling frequency 100 Hz). All samples were inserted horizontally with a 10 N preload (Fig. 2), the reinforced specimens were placed in the testing machine with the reinforcement oriented downward to optimize their performance under bending stress. According to the guidelines of mechanical testing, a load span to support span ratio of one third was chosen. Hence, the four-point test rig featured a load span of 53 mm and a support span of 160 mm. Meeting the requirements stated in the D6272, the loading noses and supports had cylindrical surfaces and its radii were 5.0 mm [38]. All specimens were loaded with a constant crosshead speed of 5 mm/minute [39]. The specimen was deflected until failure of the material, which was defined as breaking load (Fbreak) and determined by load drop exceeding 50 N. The movement of the loading noses relative to the supports was used to measure the displacement. The maximal force Fmax was determined as the maximum peak. Stress (S) and strain (R) were calculated for every point on the load‒displacement curve by the following equations:

$$\:S=\frac{F*L}{b*{d}^{2}}\:*\:{10}^{-6}\:$$
Fig. 2
figure 2

Cable-reinforced specimen undergoing the four-point bending test. (1) Load sensor, (2) loading noses, (3) specimen, (4) supports. The distance between the loading points and the supports were 53 mm each

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where.

S = stress in the outer fibre throughout the load span [MPa].

F = load at a given point on the load‒deflection curve [N].

L = length of support span [m]

b = width of specimen [m].

d = depth of specimen [m]

$$\:R=\frac{4.70*D*d}{{L}^{2}}$$

where:

R = strain in the outer fibres [m/m].

D = deflection under the supports [m].

L = length of support span [m].

d = depth of specimen [m].

Young’s modulus during bending was computed as follows:

$$\:{E}_{B}=\frac{0.21*{L}^{3}*m}{b{*d}^{3}}\:*\:{10}^{-6}$$

where:

EB = Young’s modulus in bending [MPa].

L = length of support span [m].

b = width of specimen [m].

d = depth of specimen [m].

m = slope of the tangent to the initial straight line in the load‒displacement curve.

m was ascertained using the trendline option in Excel in the range of 50 N (preload + 40) until Fmax–40.

Statistical analysis

All the statistical analyses and graphical representations were performed with GraphPad Prism 8.0.1 software (GraphPad Software, Inc., La Jolla, USA). The data was presented in mean ± standard deviation (SD). The relative change of bending strength by reinforcement was calculated as a proportion of its control group magnitude, normalized to a percentage scale [%]. A positive result conveys the extent of the increase relative to the initial value. P values of p < 0.05 were considered to indicate statistical significance.

The Shapiro‒Wilk test was conducted to assess the normality of the data. Kruskal‒Wallis tests with Dunn’s multiple comparison test were used to compare the three groups. The Wilcoxon matched pairs-signed rank test was used to assess changes over the samples’ incubation time (Δt) without a normal distribution.

Results

The polymerization process showed no significant change in volume or density after 7 days of consolidation and fulfilled the dimensional criterion of ± 1 tolerance according to ASTM D6272 [38]. No significant differences in the dimensions of the groups were found. The samples showed no normal distribution concerning the volume curvature or CT radiodensity of the full sample (p < 0.05), and analysis of the cement-only section showed a normal distribution (p > 0.05).

Assessment of the consolidation process

No statistically significant differences could be found in the sample volume among the three groups (CW, CC, and C) at either t0 (p > 0.05) or t1 (p > 0.05). Both reinforced groups showed a significantly greater mass than did group C (C vs. CC: p = 0.029 and C vs. CW: p = 0.001), while no difference was found between CW and CC (p = 0.966). The same behaviour could be observed after the consolidation period t1 (C vs. CC: p = 0.027 and C vs. CW: p < 0.001, CW vs. CC: p = 0.929). Considering the mass of the BC (excluding the reinforcement), no statistically significant differences could be found between CW and CC (p = 0.999). The comparison between C and CW (p = 0.058) and between C and CC (p = 0.041) at t0 were very close to the significance level of p > 0.05 and might indicate a difference. However after the consolidation period t1, no statistically significant difference was found between the groups concerning the mass of the BC (p = 0.158) indicating that there was no change in mass between the three groups.

Assessment of geometrical changes

The reinforced samples exhibited a statistically significant difference in curvature at t0 (C vs. CW: p = 0.002; C vs. CC: p < 0.001) in comparison to C, while there was no difference between CW and CC (p > 0.999) (Table 1). At t1, the curvature was numerically greater in CC in comparison to C (p = 0.009). No statistically significant difference in curvature could be found between C and CW (p = 0.058) and between CC and CW (p > 0.999) at t1 (Table 1). Over the incubation period (Δt), we found no significant change in volume (C: p = 0.971, CW: p = 0.528, CC: p = 0.315) or in curvature (CΔt: p = 0.105, CWΔt: p = 0.695, CCΔt: p = 0.695) (Table 1).

Table 1 Baseline characteristics of experimental specimen. Volume refers to specimen volume after milling. Curvature indicates the longitudinal bending of the sample measured by central angulation. t0 is before the 7 days incubation period, t1 after. CT radiodensity describes the mean attenuation of the whole sample volume and a cement only section
Full size table

Radiodensity analysis

Analysis of the radiodensity values (HU) revealed a highly significant difference among the groups in both the whole sample (p < 0.001) as well as cement-only section (p < 0.001). In the whole sample analysis, cement-only group C displayed the lowest radiodensity, followed by the CC group. The highest radiodensity was found in the CW group. All groups differed significantly from each other (C vs. CW: p < 0.001, C vs. CC: p = 0.033, CW vs. CC: p = 0.033) (Table 1). Cement-only section comparison revealed a greater radiodensity in the CW and CC reinforced groups than in group C (C vs. CW: p < 0.001, C vs. CC: p < 0.001). CW and CC demonstrated similar radiodensities (p = 0.862) (Table 1).

Mechanical properties

Flexural strength, Young’s modulus as well as maximum where not normally distributed in most of the cases (p < 0.05). A pairwise comparison revealed a significantly greater flexural strength for both reinforced groups (CW and CC) than in the cement-only group C (C vs. CW p = 0.011, C vs. CC p = 0.023) (Fig. 3). With a mean of 64.45 MPa, the CW group displayed the highest flexural strength, exceeding that of group C by 34%. The flexural strength of the CC group was 31% greater than that of the C group. No statistically significant difference could be found for the flexural strength between CW and CC (p = 0.957). No significant differences were found in the maximum strain (p = 0.087) (Fig. 4) or Young’s modulus (p = 0.051) (Fig. 5).

Fig. 3
figure 3

Boxplot diagrams display the means and standard deviation (SDs) with the error bars reflecting the minimum and maximum of the flexural strength in [MPa] of the three groups: C, CW and CC. Auxiliary symbols were used to visualize significance levels as follows: * indicates 0.05 > p > 0.01

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Fig. 4
figure 4

Boxplot diagrams display the means and standard deviation (SDs) with the error bars reflecting the minimum and maximum of the maximum strain in [%] of C, CW and CC. no statistically significant difference was found

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Fig. 5
figure 5

Boxplot diagrams display the means and standard deviation (SDs) with the error bars reflecting the minimum and maximum of the young’s modulus in [GPa] of C, CW and CC. No statistically significant difference was found

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Fracture location assessment

In group C, fracture localization was central in six samples and off center in 4 samples. In group CW, cement fracture occurred in all nine samples. The wire was bent at the fracture point, which was located centrally (where maximum bending stress was applied) in seven cases (Fig. 6a) and off center in the remaining cases (Fig. 6b). Group CC displayed fracture lines in nine samples with four incomplete fractures (Fig. 6c). Reinforcement fracture occurred in three samples (Fig. 6d), while samples with intact reinforcement returned to its original shape. Three samples showed counterrotated fracture parts (Fig. 6e). Eight samples had central fracture lines. Table 2 summarizes the flexural strength, maximum strain, and Youngs modulus values for each group with the respective fracture characteristics regarding sample fracture, reinforcement fracture, fracture location and displacement.

Fig. 6
figure 6

Images of the corresponding fracture sites indicated with arrows. Doted lines show the loading points applied by the 4-point bending test. a) Fractured in center of the point of maximum bending stress group CW, b) Fractured off center from the maximum bending stress group CW. c) Fractured centrally at the point of maximum bending stress group CC, d) Incomplete fractured off center from the maximum bending stress group CC, e) Fractured of center with counterrotated fracture parts of the sample group CC

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Table 2 Flexural strength, maximum strain, elastic modulus, and fracture characteristics across experimental groups
Full size table

Discussion

Despite the clinical inadequacy of stand-alone osteoplasty for stabilisation of long bone impending fractures [1, 2] and the subsequent implementation of multiple alternative techniques (51,Table 1.5), the biomechanical basis of these procedures has hardly been evaluated [34]. The aim of this study was to assess the effects of metal reinforcement on the flexural properties of BC. Both the CW- and CC-reinforced groups exhibited 33% greater flexural strength than did the cement-only samples in group C. No significant difference was detected in the flexural strength between the two reinforced groups CW and CC groups or in the strain or Young’s modulus among all three groups. Group C scarcely met the ISO requirements of 50 MPa [39], while both the reinforced groups CW and CC, were well above the threshold with a bending strength of 64 MPa and 63 MPa, respectively. The Young’s modulus ranged between 2.4 and 2.8 in all three groups, meeting the requirements of the ISO standard with a minimum bending modulus of 1.8 GPa [39]. In literature the flexural strength and the Young’s modulus values reported lie around 2.4 GPa, respectively [9]. cr372 mm while meeting the testing requirements of ISO 6833, it was necessary to increase the sample dimensions. The scale effect regarding sample geometry produces a greater probability of having material defects, and the material tends to be more brittle due to the higher cement volume [9, 45, 46].

Our study showed that the bending strength of standardized BC samples can be increased trough 2 mm of reinforcement, while the strain and Young’s modulus were not affected. Since we could not detect a difference between the CW and CC, the characteristics of the reinforcement (wire vs. cable) seemed to play a minor role in the flexural strength, yet both the material failure and post fracture alignment were influenced by the type of reinforcement. The majority of all the samples fractured in the center and showed no defects, indicating BC distribution among all the study groups and a macroscopically intact BC-reinforcement interface. Interestingly, internal reinforcement not only augmented the BC but also resulted in decreased material failure. In 16 reinforced samples (9 CW, 7 CC), the respective reinforcement remained intact despite fracture of the surrounding BC, maintaining residual stability. Only in three CC samples did the reinforcement also fracture. This could be improved by using other commercially available materials for orthopedic cables/wires, e.g., solid stainless steel 316 L, Ti-Al6-Nb7, Ti-C-P or Co-Cr-N-Wi. We observed incomplete fracture lines and axially realigned fracture parts with residual stability provided by the cable in the CC group. Even higher residual stability due to the more stable wire was observed in the CW group. No material failure occurred there, but all the samples remained angulated. These results show that the metal reinforcement of BC leads to less material failure and, in the case of cement fracture, a high chance for residual stability through the reinforcement.

The samples were prepared randomly on two consecutive days and had equal dimensions and volumes. After moulding and processing, all the samples fulfilled the required inclusion criteria for dimension and volume in the range of ± 10%. A significant average curvature of \(\:\theta\:\)= 2.2° was detected in the reinforced samples with similar standard deviations of 1.8°. The eccentrically placed reinforcement seemed to generate tension in the sample, leading to deformation during the initial curing time, which developed after sample preparation and did not progress during the incubation period Δt. The nonsignificant difference in group CW at t1 may be explained by a slightly broader standard deviation. In both the wire- and cable-reinforced groups, buckling away from the reinforcement was observed. Prestressing of the cable seems to have a minor effect than internal stresses due to the bounding of BC to the metal surface.

The reinforcement was placed closer to the bottom than to the top of the sample to withstand the maximum compression in the sample, resulting from the compression stress (prestressed samples) and the bending moment [47]. This concept is widely used in prestressed concrete used in civil engineering [47] and mimicked in this study in the CW group, which featured a prestressed cerclage cable. After 30 min of in-mould hardening, the prestressed cable was loosened, and the sample was retrieved. Nevertheless, it does not explain the curvature in the wire reinforcement group CW. As Kühn et al. described, higher temperatures lead to faster polymerization and consecutive faster after-polymerization, completing the curing process [12]. Subsequently, the polymerization speed decreases with increasing distance from the reinforcement. This might explain why the native BC samples in group C did not display a curvature. Further experiments are needed to reproduce and assess this effect. To generate equal starting points and therefore comparable displacement between the groups, we preloaded every sample with 10 N m to compensate for any possible curvature.

The inserted reinforcement accounted for 2% of the sample volume, and the adaptation of the BC volume administered per sample was neglected. Therefore, all the samples were moulded with the same amount of BC, regardless of the presence of a reinforcement. This leads to a marginal volume effect in the reinforced samples, resulting in a slightly greater cement density, reflected by higher HU values as a surrogate for the reinforced groups CW and CC than for the cement-only group C. The lower density, thus allowing a higher air content, in group C may also explain the 4 off-center fractures. K-wires are denser and therefore more stable than the threaded cerclage cables with a silk core that we used in this study. This explains the HU ranking in the whole-sample comparison of CW > CC > C. Further radiographic investigations revealed multiple minor defects in all samples and all groups, following no specific pattern, despite the use of vacuum mixed BC and a press-fit mould [4]. This observation is consistent with previous studies in which quantitative defect analysis revealed that defects due to air trapping can still be found using a vacuum mixing system [1, 9, 18]. No significant air pockets around the reinforcement could be observed. Manual attempts to displace the reinforcement of the BC were futile. Therefore, a sufficient connection between BC and metal may be assumed [47]. Minor differences in radiodensity between the reinforced groups and native BC could be observed.

Multiple clinical studies have demonstrated the benefits of PC in patients with bone metastases [48, 49]. While cement augmentation alone has provided pain relief, reinforcement and cementoplasty, in addition, have been shown to be effective in reducing the incidence of repeated fractures from daily life activities [33]. A few cadaveric studies showed little or no benefit in the reinforcement of BC regarding biomechanical strength [5,6,7]. In vivo, a successful consolidation depends on a connection between metal reinforcement, BC and bone stock sufficient enough to transfer the loads working upon the construct [47]. Long bones are constantly subjected to a significant bending moment e.g. in the single food stance face during walking. To improve the flexural stability of a reinforced construct air inclusions should be avoided in any way as this is the primary cause of failure of the construct [45]. Also based on the rational of civil engineering the reinforcement must be surrounded by a significant layer of BC. The adhesion to metal surfaces of the BC leads to internal tensions, which explains the bending of our samples during the preparation process even after polymerization was concluded. In a load bearing situation this might be used in favour of withstanding loads if used in an appropriate way [8]. A careful selection of patients regarding the location and extent of the defect is highly relevant to the success of a cementoplasty [9]. Further investigations are required, especially in an in vivo setting.

Limitations

Limitations of this study are the in vitro testing environment and its limited transferability into a clinical setting. Standardized samples might not reflect in vivo conditions, where geometry and size might vary significantly. Also, mechanical behaviour of BC changes, when in contact with body fluids. Fatigue behaviour was not evaluated in this experiment. We measured the deflection as the difference between the loading noses and the supports rather than the difference between the loading noses and the lowest point mid-sample. Finally, HU values might not accurately represent density and lack a scaling element.

Conclusion

In summary, our study showed that the flexural strength of BC can be improved through the insertion of metal reinforcement, while Young’s modulus and strain remain unchanged. The type of reinforcement (wire vs. cable) seemed to play a minor role in enhancing the bending properties of BC. However, wires were superior in providing residual stability (no wire fractures occurred) compared to cables (3 out of 10 complete fractures occurred in our study). Furthermore, the reinforcement of BC led to less material failure and, in an optimal setting, to realigned samples with residual stability despite BC fracture. These early findings suggest that the presence of scaffolding material along with BC has the potential to improve the mechanical properties of fractured bone. From a biomechanical view, our study supports augmented osteoplasty, especially in the context of compound osteosynthesis in a palliative setting, yet further in vivo investigations are essential.

Research on the complex loading of different long bones could help clinicians gain knowledge about influencing factors and favourable lesions. Investigating the reinforcement direction, positioning and appropriate access methods is crucial for comparing the advantages and drawbacks of both stiff and elastic reinforcements with either smooth or structured surfaces to improve osteoplasty procedures.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. This study was funded with internal funds.

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Author notes

  1. David Putzer and Valentina Egger contributed equally to this work.

Authors and Affiliations

  1. Experimental Orthopaedics, Department of Orthopaedics and Traumatology, Medical University of Innsbruck, Innsbruck, 6020, Austria

    David Putzer, Valentina Egger & Michael Nogler

  2. Department of Surgery, Spital Zollikerberg, Zurich, 8125, Switzerland

    Valentina Egger

  3. Helios Klinikum, Arthroplasty Center Munich West, 81241, Munich, Germany

    Martin Thaler

  4. Center of Orthopaedics, Trauma Surgery and Rehabilitation Medicine, University of Greifswald, 17489, Greifswald, Germany

    Martin Thaler

  5. Department of Orthopaedics and Traumatology, Medical University of Innsbruck, Anichstrasse 35, Innsbruck, 6020, Austria

    Johannes Pallua & Werner Schmölz

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Contributions

The first two authors contributed equally to the manuscript and should be both considered first authors. All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by VE, DP, JP and WS. The first draft of the manuscript was written by DP and VE and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Putzer, D., Egger, V., Pallua, J. et al. Different polymethylmethacrylate (PMMA) reinforcement strategies for long bone osteoplasty procedures: a controlled laboratory comparison using the 4-point bending test. BMC Musculoskelet Disord 25, 1058 (2024). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12891-024-08148-9

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BMC Musculoskeletal Disorders

ISSN: 1471-2474

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