Eur J Orthop Surg Traumatol DOI 10.1007/s00590-014-1522-0

ORIGINAL ARTICLE

The effect of application time of two types of bone cement on the cement–bone interface strength Ziad Dahabreh • Hannah Kalpana Phillips Todd Stewart • Martin Stone



Received: 17 March 2014 / Accepted: 30 July 2014 Ó Springer-Verlag France 2014

Abstract The aim of this study was to investigate whether the application time of bone cement would have an effect on the cement–bone interface strength in two types of commercially available bone cements. CMW1 RadiopaqueÒ (CMW1) and SmartSetHVÒ (SmartSet) were applied to bovine cancellous bone specimens at 2 and at 4 min. Specimens were loaded to failure and the shear strength of the cement–bone interface was calculated. The mean shear strength (±standard deviation) of the cement–bone interface was 2.79 ± 1.29 MPa for CMW1 applied at 2 min; 1.35 ± 0.89 MPa for CMW1 applied at 4 min; 2.93 ± 1.21 MPa for SmartSet applied at 2 min and 3.00 ± 1.11 MPa for SmartSet applied at 4 min. Compared to all other groups, the cement–bone interface strength was significantly lower when CMW1 was applied to the bone specimens at 4 min (p \ 0.05). There was no significant difference in the cement–bone interface strength when SmartSet was applied to bone at 2 and at 4 min. Under these testing conditions, the

Z. Dahabreh Department of Trauma and Orthopaedics, Leeds General Infirmary, Great George Street, Leeds LS1 3EX, UK H. K. Phillips (&) Department of Trauma and Orthopaedics, Calderdale and Huddersfield NHS Trust, Acre Street, Lindley, Huddersfield HD3 3EA, UK e-mail: [email protected] T. Stewart The University of Leeds, Leeds LS2 9JT, UK M. Stone Chapel Allerton Hospital, Chapeltown Road, Leeds LS7 4SA, UK

cement–bone interface strength was not affected by the time of application of SmartSet to bone. However, it was significantly lower when CMW1 was applied to bone at 4 min. Keywords Bone cement  Aseptic loosening  Viscosity  Total knee arthroplasty  Polymethylmethacrylate

Introduction Aseptic loosening is a common mode of failure in cemented total knee arthroplasty (TKA) [1] and occurs more commonly in the tibial component [2]. The cement– bone interface plays an important role in aseptic loosening. Normal and shear loads are transferred to the bone by developing and maintaining a mechanical interlock between bone cement and cancellous bone [3, 4]. Under pressure, the ease of flow of cement into the pore openings of cancellous bone has an influence on interface properties [4–9]. Low-viscosity bone cements have a long-lasting liquid or low-viscosity wetting phase and usually stay sticky for 3 min. During the working phase, the viscosity increases continuously followed by hardening 1–2 min after the end of the working phase. High-viscosity bone cements have a short wetting phase. During a relatively long working phase, the viscosity remains unchanged and then slowly increases towards the end of this phase. Hardening occurs 1.5–2 min after the end of the working phase [10]. Under laboratory conditions, the use of a low-viscosity cement seems to improve the penetration of cement into cancellous bone and to increase the cement–bone interface strength, particularly if cement pressurisation techniques are employed [9, 11–14].

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Surgical cement used in TKA is known as polymethylmethacrylate (PMMA) is a brittle material, weak in tension and shear, but quite strong in compression [15– 17]. Cement at body temperature behaves as a viscoelastic solid undergoing both elastic and viscoelastic deformation (creep) under load. Cement creep relaxes cement stresses to create a more favourable stress distribution at the cement–bone interface [18]. Different brands of bone cement exhibit different creep behaviours under static loads [19]. Delayed application time of a certain bone cement has been shown to increase the creep strain from 1.66 % when injected at 90 s to 8.11 % when injected at 3 min [18]. Little is known about the influence of the application time of bone cement on the shear strength of the cement– bone interface. The first aim of this study was to compare the effect of applying bone cement at 2 and at 4 min on the cement–bone interface strength in two types of commercially available high-viscosity cements, CMW1 RadiopaqueÒ (CMW1) and SmartSetHVÒ (SmartSet). For each type of bone cement, the null hypothesis was that ‘there is no difference in the strength of the cement–bone interface between samples that received bone cement at 2 min and samples that received bone cement at 4 min’. The second aim was to determine the difference in the strength of the interface between the two types of cement at each application time (2 and 4 min). Therefore, the second null hypothesis was that ‘there is no difference in the strength of the cement–bone interface between samples that received CMW1 bone cement and samples that received SmartSet bone cement at each application time’.

Materials and methods All experiments were carried out using a standardised technique at the Faculty of Mechanical Engineering laboratories, University of Leeds (Leeds, United Kingdom) at a constant temperature of 21 °C. Bone specimens Cancellous bone was obtained from the proximal tibial metaphysis of bovine tibiae. Cylindrical specimens of bone were cored out using a manufactured powered core cutter. Each bone specimen had a radius of 12 mm, length of 2 cm in the direction of the long axis of the tibia and a cross sectional surface area of 4.52 9 10-04 m2. Twentyfour bone specimens were prepared and frozen at -20 °C on the same day. The specimens were allowed to thaw to room temperature and then reach 37 °C prior to testing. The surface preparation of bone segments included

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Rubber band

Bone specimen

Plasticine

Plastic spacer

Fig. 1 Bone specimen attached to the plastic spacer using plasticine then placed inside the syringe. A rubber band encircled around the end of the bone specimen near surface onto which bone cement will be applied. The plastic plunger is utilised to introduce bone cement to the bone specimen under constant pressure

irrigation with saline followed by vacuum suction and drying with a swab to mimic surgical conditions. The decision to use bovine bone was based on pore size and porosity that is believed to be close to that of human bone. However, the porosity of cancellous bone of the tibial plateau is known to be variable and could not be controlled [4]. Coring out of bone specimens was carried out from a matching area of each bovine tibial plateau to minimise variability. Each bone specimen was attached to a plastic spacer of an equal diameter using plasticine in order to minimise unnecessary motion of the specimens inside the syringe while cement was being applied. A rubber band was placed around the other end of each bone specimen in order to localise the cement application to the bone surface. Each plastic spacer with an attached bone specimen was placed inside a 60-ml syringe (Fig. 1).

Eur J Orthop Surg Traumatol

Bone cements Two commercially available clinical high-viscosity bone cements were used. They were stored according to manufacturer’s instructions below 25 °C and away from direct light. Both cements were stored at a constant temperature of 23 °C for at least 24 h prior to use.

62N Weight Metal cylinder

CMW1 Radiopaque CMW1Ò Radiopaque (DePuy International Ltd, Blackpool, England), LOT 1894021. Expiry date: May 2008. Bone cement packs were supplied with the radiopaque agent, barium sulphate, incorporated directly into the powder component. The powder component weighed 40 g, and each ampoule of liquid monomer weighed 18.37 g.

Metal holder

Fig. 2 Syringe placed into a metal holder after cement had been applied to the bone specimen (right). A weight of 62 N centralised over the plunger utilising a metal cylinder allowed constant pressure to be applied (left)

SmartSet HV SmartSet HVÒ (DePuy International Ltd, Blackpool, England), LOT 1905292. Expiry date: June 2007. Bone cement packs were supplied with the radiopaque agent, zirconium dioxide, incorporated directly into the powder component. The powder component weighed 40 g, and each ampoule of liquid monomer weighed 18.88 g. All cement mixing was carried out according to manufacturer’s instructions under vacuum using CEMVACÒ vacuum mixing system (DePuy CMW, Blackpool, FY4 4QQ, England-LOT 1897595). A timer was started as soon as the polymer powder was added to the monomer liquid. The duration of mixing was 30 s when SmartSet was used and 35 s when CMW1 was used.

Cement application Each bone cement was applied to bone at 2 and at 4 min from the initiation of mixing producing four independent groups with six samples in each. These times were selected to represent early and late cementing and were guided by published working times of CMW1 and SmartSet [20], as well as documented times in the product leaflet [21]. Cement was applied to bone as per described surgical methods of cement application to tibial components [22, 23]. After applying cement to the bone specimens, the plunger was inserted into the syringe and a continuous load was applied over the plunger to simulate continuous pressure over the cement during surgery. A pressure of 0.137 MPa was achieved using a weight of 62 N over the top of the plunger (Fig. 2). This pressure has been reported to represent manual cement pressurisation values during surgery that would be adequate for satisfactory cement penetration into bone [4, 14, 24, 25]. Pressure was

Cement

Bone

Fig. 3 Cement–bone construct inside the plastic syringe after the syringe had been removed from the metal holder

maintained for 5 min in all experiments. The use of fixed weights over the syringe plungers ensured equal pressure was applied to all specimens. The specimens were then immersed in a phosphate buffered saline solution maintained at a constant temperature of 37 °C and were allowed to cure for 24 ± 2 h before mechanical testing. Mechanical testing All mechanical tests were carried out at a laboratory temperature of 21 °C. The cement–bone construct was removed from the plastic syringe and mounted into a universal testing machine (Figs. 3, 4a). The cement portion of each specimen was fixed in a custom test jig. The jig allowed for accurate placement of the cement–bone interface against the shearing force, as well as providing a tight fit to minimise any bending during the test. A metal attachment was fixed to the moving part of the testing machine. This attachment had a conforming semicircular surface to the bone specimen in order to minimise out-ofplane forces on the test specimens during loading (Fig. 4b). Specimens were loaded to failure at a displacement rate of 1 mm/min using displacement control loading. The applied

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Results

A Conforming moving part of testing machine Cement-bone interface Cement specimen in custom test jig

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Conforming moving part of testing machine

Bone specimen

Fig. 4 a After removing the cement–bone construct from the syringe, the cement specimen is fixed into the custom test jig. Loading applied through conforming moving part of testing machine. b Conforming custom jig and moving part designed to minimise out-of-plane forces on the specimens during loading

load was measured using the load cell from a validated materials testing machine (Howden, Warwickshire, United Kingdom), (Fig. 5).

Macroscopic appearance confirmed that failure after shear loading had occurred at the cement–bone interface with no evidence of fracture through the cement or the bone components. In all specimens, the load versus displacement response for the cement–bone interface for shear loading exhibited a linear elastic response followed by yielding and complete failure of the cement–bone interface (Figs. 6, 7, 8, 9). The shear strength of the cement–bone interface (peak load divided by the cross sectional surface area) was calculated for all specimens (Table 1). The shear strength was significantly lower when CMW1 was applied to the bone specimens at 4 min (p = 0.048) compared to the shear strength of the cement–bone interface when CMW1 was applied to the bone specimens at 2 min. There was no significant difference in the shear strength of the cement– bone interface between specimens in which SmartSet was applied to the bone at 2 min and specimens in which SmartSet was applied to the bone at 4 min (p = 0.918). The shear strength of the cement–bone interface was significantly lower for CMW1 applied at 4 min when compared to the shear strength for SmartSet applied at 4 min (p = 0.017) and when compared to the shear strength for SmartSet applied at 2 min (p = 0.028). There was no significant difference in the shear strength of the cement– bone interface between specimens in which CMW1 was applied to the bone at 2 min and specimens in which SmartSet was applied to the bone at either 2 min (p = 0.852) or at 4 min (p = 0.770).

Discussion Data analysis Continuous recording of the resultant applied load versus displacement produced a load versus displacement curve. Failure was marked by a sudden drop in load. The maximum load was recorded and used to calculate the shear strength of the cement–bone interface. The cement–bone interface strength was calculated as peak applied load divided by the cross sectional surface area [6–8] of the cement–bone interface (4.52 9 10-04 m2). Statistical analysis was performed using SPSS version 13.0 for Windows. Normal distribution was confirmed using the Shapiro–Wilk test. Equality of variance was confirmed using Levene’s test. As the groups were independent, the data continuous and following a normal distribution, an independent samples t test was used to test the significance of the difference between the means of each two groups analysed. Significance was set at p value \ 0.05.

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The shear strength was significantly lower when CMW1 was applied to bone at 4 min compared to at 2 min (p = 0.048). However, it was not affected by the time of application of SmartSet to bone. The shear strength was significantly higher in the specimens in which SmartSet was applied to bone at 4 min compared to specimens in which CMW1 was applied to bone at 4 min (p \ 0.05). Therefore, it is possible to reject the null hypothesis of the first aim of the study for CMW1 but not for SmartSet. Similarly, it is possible to reject the null hypothesis of the second aim for an application time of 4 min but not for an application time of 2 min. The reduction in shear strength of the cement–bone interface observed when CMW1 was applied to bone at 4 min may be related to a reduction of the ease of flow of cement into the pore openings of the cancellous bone specimens as the cement viscosity increases. This would result in a reduction in the mechanical interlock between

Eur J Orthop Surg Traumatol Fig. 5 Howden testing machine attached to a computer to measure and analyse load versus displacement

Conforming custom jig and moving part of testing machine

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Fig. 6 Load (N) versus displacement (mm) curves for CMW1 applied at 2 min. Graphs produced after analysing data obtained from the computer attached to testing machine. Y-axis: Load (N) applied to the cement–bone interface via the conforming moving part of the testing machine. Maximum load identified as highest peak (N) before failure used to calculate the shear strength of cement–bone interface. X-axis: Displacement (mm) at the cement–bone interface at a displacement rate of 1 mm/min

the cement and bone, thus reducing the shear strength of the interface. The viscosity of CMW1 may have risen to such a level at 4 min that would not allow for satisfactory penetration of bone cement into cancellous bone. Such an assumption would be supported by the findings of previous

Fig. 7 Load (N) versus displacement (mm) curves for CMW1 applied at 4 min. For details, see Fig. 6

research that showed a direct relationship between the depth of penetration of bone cement into the microstructure of cancellous bone and the integrity, and strength, of the cement–bone interface [4, 26]. Under laboratory conditions, a lower viscosity of cement has been shown to improve the penetration of cement into bone and to increase the cement–bone interface strength [9, 11–13]. The results of this study suggest that a delayed application time of CMW1 into cancellous bone may have a negative influence on the shear strength of the cement–bone

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Fig. 9 Load (N) versus displacement (mm) curves for SmartSet applied at 4 min. For details, see Fig. 6 Table 1 Peak load and shear strength of the cement–bone interface (SD) Specimen and application time

Peak load (N)

Shear strength of the cement–bone interface (MPa)

CMW1 applied at 2 min

1,261.36 (548.6)

2.79 (1.29)

CMW1 applied at 4 min

609.83 (401.1)

1.35 (0.89)

SmartSet applied at 2 min

1,323.80 (548.3)

2.93 (1.21)

SmartSet applied at 4 min

1,355.81 (501.89)

3.00 (1.11)

interface. However, for SmartSet bone cement, between 2 and 4 min the viscosity is likely to have remained within a range that still allowed for adequate penetration of bone cement into bone. This would have achieved sufficient mechanical interlock between the cement and the bone specimens at both application times and consequently resulted in comparable shear strengths of the cement–bone interface. The measured shear strength of the cement–bone interface in this study agrees with other published studies [4–7, 9]. The findings of this study are relevant to the clinical use

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of high-viscosity bone cements in TKA. Factors affecting cement intrusion into cancellous bone which are under the control of the surgeon include the time of application of bone cement to bone. Ideally, a cement would display a low and constant viscosity during its working phase, followed by a relatively short time to full polymerisation and setting [27]. This would result in a material in which the handling characteristics and workability remain uniform, hence minimising variability in the surgical technique. This working phase must also provide adequate time to deliver the cement to the appropriate site, achieve cement interdigitation with the bone and insert the prosthesis. The findings of this study suggest that the working phase of SmartSet may fulfil such requirements, at least up to 4 min after mixing. The application times of 2 and 4 min were selected to represent a substantial delay in cement application in a clinical setting. In contrast to CMW1, it seems that SmartSet retains a sufficient level of viscosity between 2 and 4 min that would allow for adequate penetration into bone even when applied at 4 min. The experiments in this study were carried out at a laboratory temperature of 21 °C. This was found to be the mean temperature of eight elective operating theatres dedicated to lower limb joint arthroplasty in the North of England where this study was conducted. Bone cements in general have a longer working time at lower temperatures, and the rate of increase in cement viscosity during polymerisation increases at higher temperatures. Therefore, the results presented in this study are specific to a workingenvironment temperature of 21 °C. The in vitro nature of the study also limits the ability to accept generalising the results into clinical practice. Other factors requiring consideration before the findings of this study can be applied to other settings include the three-dimensional microstructure of bovine bone, which remains ill-defined. Similarly, differences between normal and osteoarthritic tibial plateau cancellous bone porosity and microstructure remain to be well studied. Bleeding from exposed bone surfaces, the type of bone cement used, mixing techniques and the amount of pressure applied during cement application are further variables that would be interesting to study in future research. Although more challenging to control, future studies could also utilise human tibial bone specimens or test specific tibial components. Further analysis of the penetration depth of cement into bone may be performed with the aid of X-ray microtomography (Micro-CT) and would be beneficial to analyse the correlation between the depth of cement penetration and shear strength of the cement–bone interface in each specific setting. The findings of this study suggest that a delayed application time of CMW1 into cancellous bone may have a detrimental effect on the cement–bone interface strength.

Eur J Orthop Surg Traumatol

On the other hand, it seems that SmartSet retains a sufficient level of viscosity throughout the working phase (between 2 and 4 min) that would allow for adequate penetration into bone even when applied at 4 min. Therefore, SmartSet would be more likely to provide the surgeon with adequate time to ensure optimal conditions are achieved prior to applying cement to bone and accurately implanting TKA components. An understanding of the factors that may influence cement penetration into bone and consequently the strength of the cement–bone interface will improve surgical technique, and ultimately, TKA survivorship. Acknowledgments All the authors would like to acknowledge Mr D. Derby’s invaluable help in ensuring all the experiments were carried out safely and effectively. Conflict of interest

Nothing to disclose from any of the authors.

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The effect of application time of two types of bone cement on the cement-bone interface strength.

The aim of this study was to investigate whether the application time of bone cement would have an effect on the cement-bone interface strength in two...
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