Cement–Cement Interface Strength: Influence of Time toApposition

Sang-Hyun Park,1 Mauricio Silva,2 Jong-Seok Park,1 Edward Ebramzadeh,1 Thomas P. Schmalzried2,3

1 The J. Vernon Luck Sr. M.D. Orthopaedic Research Center at Orthopaedic Hospital, 2400 South Flower Street, LosAngeles, California 90007

2 Joint Replacement Institute at Orthopaedic Hospital, 2400 South Flower Street, Los Angeles, California 90007

3 Harbor–UCLA Department of Orthopaedic Surgery, 1000 West Carson Street, Torrance, California 90509

Received 4 May 2001; revised 17 July 2001; accepted 20 July 2001Published online 00 Month 2001; DOI 10.1002/jbm.0000

Abstract: Cement–cement interfaces were created under simulated operating-room condi-tions. In order to analyze the effect of time to apposition on interface strength, two cementsurfaces were brought together 1, 2, 4, and 6 min after 1 min of mixing and 45 s of waiting.Cement–cement interface strength was evaluated with the use of a three-point bending tofailur e test. Scanning electron microscopy (SEM) images of the failed interface were obtained.The mean interface strength decreased when the cement–cement interface was time delayed.Compared to bulk cement, interface strength in time-delayed groups decreased 8% after1-min delay (p5.037), 18% after 2-min delay (p5.0004), 20% after 4-min delay (p5.0005), and42% after 6-min delay (p<.0001). No statistically significant differences in interface strengthwere found between the2- and 4-min delayed groups (p5.73). SEM images revealed that after6-min delay, up to 50% of the cement surface can remain unbonded, explaining the decreasein strength of thecement–cement interfaceas afunction of timeto apposition. This laboratorystudy indicates that time to apposition plays a critica l role in cement–cement interfacestrength. I f any cementing technique involves the joinin g of two cement surfaces, it isrecommended that the two cement surfaces be mated together withi n 5 min and 45 safter thestart of mixing (1 min mixing; 45 s waiting; 4 min delay), in order to obtain a strongcement–cement interface bond. Delay beyond this can result in substantial reduction in thestrength of the cement–cement interface bond. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res(Appl Biomater) 58: 741–746, 2001

Keywords: Cement–cement interface; total knee replacement; cementing technique

INTRODUCTION

Although many aspectsof thesurgical technique in total kneereplacement have been extensively described, cementationmethodology has received relatively littl e attention. Routinecementing technique includes the use of doughy cement,because of the relative ease of handling, insertion, and pres-surization, to create asuitable cement–bone interface.1 How-ever, recent investigation has shown that the strength of thebond between a metallic surface and cement is strongestwhen wet cement is applied to the metallic surface soon aftermixing and decreases with increasing time between mixingand application as the cement becomes dry and moredoughy.2 Because loosening of the metallic components intotal knee replacement (TKR) can result in osteolysis and

clinical failure,3 it isdesirable to optimizeboth thestrength ofthe metal–cement and the cement–bone interfaces in TKR.

One technique that can be used in TKR is to apply wetcement to the metal surfaces, wait some time before pressingdoughy cement into the bone, and then insert the prosthesis,which presses the two cement surfaces together, in order toget a better combination of metal–cement interface strengthand cement intrusion into the bone. However, it is importantto recognizethat athird interface, acement–cement interface,is created. Bone cement is poorly adhesive to most surfaces,even to itself, as it becomes drier and more doughy.4 Thepurpose of the present study was to investigate the influenceof time on the bond strength of a cement–cement interface inorder to provide abetter cementing technique for TKR.

MATERIALS AND METHODS

Under stable-environment conditions intended to simulate ahospital operating room, with atemperature of 18–19°C anda relative humidity of 56–65%, ahalf-dose package (20 g of

Correspondence to: Dr. Sang-Hyun Park at the J. Vernon Luck Sr. M.D., Ortho-paedic Research Center

© 2001 John Wiley & Sons, Inc.

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powder and 10 ml of liquid) of surgical Simplex Pt bonecement (Stryker-Howmedica-Osteonics, Rutherford, NJ) wasmixed as per the manufacturer’s recommendations for 60 s.Once mixed, and after a period of 45 s (to simulate transfer ofcement from operating room personnel to the surgeon), thecement was injected into a plastic mold.

For the control group, the whole amount of mixed cementwas injected immediately into a 20320340-mm Peel-A Waytdisposable embedding mold (Polyscience, Warrington, PA) andthen cured in air for 1 h. For the test groups, half of the amount

of mixed cement was immediately injected into a 20320320mm Peel-A Wayt embedding mold. The remaining half ofmixed cement was injected into another plastic mold, of theexactly same dimensions, after a period of 1, 2, 4, or 6 min andthen packed with a gloved finger. The two cement-filled moldswere brought together under a force of 9.8 N, forming a cement–cement interface perpendicular to the longitudinal axis of thecement block. The cement was cured in air for 1 h.

After 1 h curing in air, the plastic molds were removed andall the cement blocks were stored in a saline solution at 37°C

Figure 1. Radiograph of 4-min delayed specimens. The cement–cement interface was easily identified asa dense line (arrows), with less than 0.5 mm in thickness, perpendicular to the longitudinal axis of cementspecimens. Similar cement–cement interfaces were observed in 1- and 2-min delayed groups.

Figure 2. Radiograph of 6-min delayed specimens. A cement–cement interface similar to the oneobserved in other groups was identified (black arrows). In addition, a radiolucent band of approximately2 mm in thickness was observed parallel and adjacent to the cement–cement interface, in the early-injected half block (double white arrows). This radiolucent band was not observed in other groups.

742 PARK ET AL.

for 10 days. To eliminate the cement skin, a 2-mm-thickcement skin layer was removed with the use of a diamondblade saw under water irrigation. From the remaining block,six specimens were obtained, with a final specimen dimen-sion of 7.934.8340 mm.

Radiographs of every specimen were obtained to localizethe cement–cement interface and to identify void defects. Inorder to avoid failure through a stress riser, those specimenswith voids larger than 1 mm were excluded from the study.After this selection, a total of 45 specimens (9 per group)were included for a bending test. A three-point bending tofailure test was performed on a servo-hydraulic testing ma-chine (858 Mini Bionix, MTS, Minneapolis, MN). The centerloading anvil was located at the previously identified cement–cement interface. Each specimen was loaded to failure at arate of 5 mm/min, according to standard recommendations(ISO 5833). Specimen bending strength was calculated fromthe load at the cement failure.

After specimen failure, the fractured surface was goldplasma coated and then surveyed under scanning electronmicroscopy (SEM) (Zeiss DSM 960, Oberkochen, Germany)in a topographic mode, using a backscattered electron detec-tor.

Statistical analysis for bending strength included one-wayANOVA followed by the least-significant-difference methodof multiple comparisons.

RESULTS

The cement–cement interface could not be seen with thenaked eye, but was easily identified on radiographs as a denseline, with less than 0.5 mm in thickness, perpendicular to thelongitudinal axis of cement blocks in all test groups (Figure1). In the 6-min delayed group, a radiolucent band of 2 mmin thickness was observed parallel and adjacent to the ce-ment–cement interface (Figure 2). This radiolucent band wasnot observed in other groups.

In the control group, a mean bending strength of 65.7 MPa(SD: 2.5 MPa) was found. The mean bending strength de-creased when the cement–cement interface was time-delayed(Table I). Compared to the control group, bending strength intime-delayed groups decreased 8% after 1-min delay(p5.037), 18% after 2-min delay (p5.0004), 20% after 4-mindelay (p50.0005), and 42% after 6-min delay (p,.0001)(Figure 3). There was no statistical significant difference inbending strength between 2- and 4-minutes delayed groups(p5.73).

SEM images from the control group demonstrated a flatfractured surface, characteristic of brittle failure, with manysmall-sized spherical pores (of less than 0.5 mm) surroundedby microcracks. As the time delaying increased to 1, 2, and 4min, the number of pores decreased progressively, with evi-dence of flat and smooth fractured surfaces (Figure 4). Incontrast, the 6-min delayed group demonstrated a rough sur-face, without any pores or cracks [Figure 5(a)]. At highermagnification (3200), the fractured surface of 6-min delayedgroup showed two different types of surfaces, indicatingdelamination, not seen in other groups: high-level areas,characterized by rough surfaces (corresponding to bondedcement), and low-level areas, characterized by shiny surfaces(corresponding to nonbonded cement) [Figure 5(b)]. In thisgroup, more than 50% of the cement–cement interface wasnot bonded.

DISCUSSION

There has been little investigation into the optimal use ofbone cement in TKR. Information obtained from research intotal hip arthroplasty has been extrapolated to knee arthro-plasty, because of similarities in the materials used in bothprocedures. In general, it has been recommended that cemen-tation should occur when the cement reaches a doughystate.5–10

This laboratory investigation indicates that cement–ce-ment interface strength decreases 18%, compared to the con-trol after 2-min delay. This level of interface strength isessentially maintained with a 4-min delay, with a furtherdecrease in strength of only 2%. No significant differenceswere found in cement–cement interface strengths betweenthe groups that were time delayed 2 and 4 min (p5 .73).According to this result, joining two cement surfaces within5 min and 45 s after the start of mixing (1-min mixing; 45-swaiting; 4-min delay) appears acceptable. However, after6-min delay, the cement–cement interface strength is 42%lower than the control.

Figure 3. Cement–cement interface strength decreases with time ofcementation (bars indicate standard errors).

TABLE I. Cement–Cement Interface Bond Strength

Groups Mean (MPa) SD (MPa)

Control 65.7 2.51 min 60.2 6.82 min 53.8 7.64 min 52.4 8.96 min 38.0 4.1

743CEMENT–CEMENT INTERFACE STRENGTH

Similar decrease in cement–cement interface strength hasbeen shown in two previous mechanical studies.11,12 Flivik,Yuan, Ryd, Juliusson, and Lidgren,11 analyzing the influenceof time on the tensile strength of cement–cement interfacescreatedin vitro, reported a 12% reduction on the interfacetensile strength when the two cement surfaces were broughttogether after 4 min of the start of mixing. This result iscomparable to the one obtained in our 2-min delayed group (1

min of mixing, 45 s of waiting, and 2 min delay). However,the greatest reduction in strength occurs later, and was notaccounted in that study. Gruen, Markolf, and Amstutz12

tested specimens with either dry or blood-laminated inter-faces for tensile and shear strength. Their testing conditionsdiffer slightly from the present ones, in that they mixed thecement for 3 min and their lamination time was only 1–3 min.This may explain why their cement–cement interface

Figure 4. SEM photographs of the fractured surface of a 4 min delayed specimen, showing brittlefailure characteristics. (a) Low magnification (318). (b) High magnification (350).

744 PARK ET AL.

strengths were about 60% to 70% weaker than Flavik’s andour data at comparable times. The length of curing and thestorage environment of the cement block, which may affectthe interface conditions, were not described in previous stud-ies.

Reduction of the interface strength with increasing time toapposition could be the result of evaporation of methyl

methacrylate monomer from the surface over time. As aresult, the surface of PMMA has a reduced amount of mono-mer available for polymerization, creating less bonding be-tween the surfaces. A lower amount of monomer would alsoreduce the heat generated at the interface. Supporting evi-dence for this position is the radiographic dense lines ob-served in time-delayed specimens as well as the reduced

Figure 5. SEM photographs of the fractured surface of a 6-min delayed specimen. (a) Low magnifi-cation (318), showing rough surfaces without evidence of any pores or cracks. (b). High magnification(3200), showing delamination of the cement–cement interface. The black arrow shows unbondedcement, and the white arrow shows bonded cement.

745CEMENT–CEMENT INTERFACE STRENGTH

number of pores detected with the SEM on the fracturesurfaces, which are generated by monomer boiling.

Radiographic analysis of the cement–cement interface re-vealed no major differences when cementation was 1-, 2-, or4-min delayed. However, after 6 min, a radiolucent band of 2mm in thickness, not present in other groups, was observedparallel and adjacent to the cement–cement interface in theearly-injected half block. It is hypothesized that air entrappedin the cement rises to the top of the cement block in allspecimens. When the two molds were brought together underpressure at 1, 2 or 4 min, some cement was displaced from theinterface, taking away the most superficial layers of bothsides and, with them, the entrapped air. At 6 min, however,the cement was too doughy, and the amount of cementdisplaced from the interface was smaller, retaining the en-trapped air in the sample.

This study has some limitations. This study simulatesconditions of specific temperature and humidity, common tomany operating rooms. The results would likely vary depend-ing upon differences in temperature and humidity, and thespecific type of cement used. Moreover, it has the conve-nience of being done under laboratory conditions and doesnot account for further reduction in strength than can resultfrom blood or other fluid at the cement–cement interface,11,12

as can occur in surgery. Furthermore, because blocks ofcement were used, the polymerization process in this modelcould be different than the one that occursin vivo because ofdifferences in the mass of the cement and the consequentdifference in temperature at the cement–cement interface. Inaddition, the metallic prosthesis could act as a heat sink,reducing the temperature at the interface and delaying thepolymerization process. Finally, the forces that cement expe-riencesin vivo are complex. Cyclic compression and tension,as well as shear forces, that were not tested in the presentstudy, may influence the integrity of the cement–cementinterface in vivo. However, the methodology used in thislaboratory experiment is reproducible and is a valid measureof cement–cement interface strength.

In conclusion, this laboratory study indicates that time toapposition plays a critical role in cement–cement interfacestrength. If any cementing technique involves the joining oftwo cement surfaces, it is recommended that they be mated

together within 5 min and 45 s after the start of mixing, inorder to obtain a strong cement–cement interface bond. De-lay beyond this can result in substantial reduction in thestrength of the cement–cement interface bond.

The authors would like to acknowledge Stryker-Howmedica-Osteonics (Rutherford, NJ) for providing the cement used in thepresent experiment.

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8. Wixon RL, Lautenschlager EP. Methyl methacrylate. In: Cal-laghan JJ, Rosenberg AG, Rubash HE, editors. The adult hip.Philadelphia: Lippincott-Raven; 1998. p 135–157.

9. Mulroy WF, Estok DM, Harris WH. Total hip arthroplasty withuse of so-called second-generation cementing techniques. Afifteen-year-average follow-up study. J Bone Joint Surg Am1995;77-A:1845–1852.

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12. Gruen TA, Markolf KL, Amstutz HC. Effects of laminationsand blood entrapment on the strength of acrylic bone cement.Clin Orthop 1976;119:250–255.

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