Orthopedics

Feature Article 

Systematic Biomechanical Analysis of Prerequisites for Reliable Intraoperative Bonding of Polymethylmethacrylate Bone Cement in Preexisting Cement in Revision Arthroplasty

Dominik Kaiser, MD; Nora Zimmerli, MSc; Rebecca Hartmann, MD; Elias Bachmann, MSc; Klaus-Dieter Kühn, PhD; Dominik C. Meyer, MD

Abstract

Removal of a stable cement mantle may be invasive and time consuming and may result in unnecessary damage to bone and surrounding soft tissue. The goal of this study was to investigate the feasibility of the use of polymethylmethacrylate cement on preexisting cement as well as to explore the prerequisites for practical clinical use under standardized laboratory conditions. The strength of the cement–cement interface was evaluated with a 4-point bending to failure test, according to International Organization for Standardization 5833, as well as standardized shear strength, according to American Society for Testing and Materials D732. Various intraoperative cleaning agents were tested to remove simulated contamination with bone marrow. Contamination of the cement–cement interface with bone marrow decreases bending strength, modulus, and shear strength. Removal of the bone marrow with a degreasing agent significantly increases bending strength as well as bending modulus and can increase shear strength up to 9% compared with use of a nondegreasing agent. The cement–cement interface may reach up to 85% of bending strength, 92% of bending modulus, and comparable shear strength compared with a uniform cement block. Meticulous removal of fatty contaminant is important. Use of a degreasing agent further increases the stability of the cement–cement interface. With these precautions, it is safe to assume that the combined molecular and mechanical interlock is sufficient for most clinical applications and will not represent the weakest link in prosthetic revision. [Orthopedics. 2021;44(1):e55–e60.]

Abstract

Removal of a stable cement mantle may be invasive and time consuming and may result in unnecessary damage to bone and surrounding soft tissue. The goal of this study was to investigate the feasibility of the use of polymethylmethacrylate cement on preexisting cement as well as to explore the prerequisites for practical clinical use under standardized laboratory conditions. The strength of the cement–cement interface was evaluated with a 4-point bending to failure test, according to International Organization for Standardization 5833, as well as standardized shear strength, according to American Society for Testing and Materials D732. Various intraoperative cleaning agents were tested to remove simulated contamination with bone marrow. Contamination of the cement–cement interface with bone marrow decreases bending strength, modulus, and shear strength. Removal of the bone marrow with a degreasing agent significantly increases bending strength as well as bending modulus and can increase shear strength up to 9% compared with use of a nondegreasing agent. The cement–cement interface may reach up to 85% of bending strength, 92% of bending modulus, and comparable shear strength compared with a uniform cement block. Meticulous removal of fatty contaminant is important. Use of a degreasing agent further increases the stability of the cement–cement interface. With these precautions, it is safe to assume that the combined molecular and mechanical interlock is sufficient for most clinical applications and will not represent the weakest link in prosthetic revision. [Orthopedics. 2021;44(1):e55–e60.]

Acrylic bone cement is the most commonly used nonmetallic implant material in orthopedics, especially for stable fixation of endoprostheses.1–5 As the population ages, the number of revision surgeries of cemented endoprostheses continues to increase.6 Often a surgeon is confronted with a stable cement mantle around the prosthesis that must be explanted for other reasons, for example, to convert an anatomic shoulder prosthesis to a reverse shoulder prosthesis or to remove a femoral component for revision of a loose cup, recurrent dislocation secondary to component malposition, or debonding of the femoral component within an intact cement mantle.7 Complete removal of polymethylmethacrylate cement from the bone may be invasive and can cause major damage to the bone and surrounding soft tissue.8,9 In such a situation without infection, it appears far more attractive to leave all or part of the stable cement mantle in place and to cement a smaller shaft into the cement in place. However, the cement-in-cement interface has not been studied fully,10 and there is uncertainty under what conditions this practice is acceptable. Several reports support this strategy, whereas others advise against this procedure because of the potential for a poor mechanical bond between the old and new implanted cement, particularly if there is contamination with blood or bone marrow.8,9,11–21 Additionally, the roughness of the cement surface,22 the post-cure duration,15 and the porosity of the cement23 can affect mechanical quality. No systematic statistical data are available on the appropriate treatment of intraoperatively contaminated cement to achieve acceptable bonding strength with newly implanted cement.

Because of the massive number of surgical procedures involving revision of cemented prostheses and the invasiveness of removing solidly implanted cement, the authors recognized the need to specify the requirements for preparing and preserving a stable cement mantle.

The goal of this study was to investigate the cement-in-cement interface in a laboratory setting by determining bending strength and bending modulus, according to the International Organization for Standardization (ISO),24 in a 4-point bending test, and determining shear strength, according to the American Society for Testing and Materials (ASTM),25 in a standardized shear test. The authors sought to identify the effect of surface roughness and contamination with porcine bone marrow with different methods of subsequent mechanical and chemical cleaning.

Materials and Methods

Cement Preparation

Palacos R (Heraeus Medical GmbH) acrylic bone cement was used in this study. The doughy mass was used to fill specific molding templates. Preparation time for the cement until application was less than 4 minutes.12

Specimen Fabrication

Specimen Fabrication for 4-Point Bending Tests. Uniform rectangular specimens that were 90.0 mm long, 10.0 mm wide, and 3.3 mm high were produced and post-cured for 24 hours at 23 °C in a dry environment.26 The first 6 specimens underwent no further treatment and were used as the control group. The next batches of specimens were cut in half with a band saw and further treated as described later.

Specimen Fabrication for Shear Tests. For the shear specimens, hollow cement cylinders with outer diameter of 50.8 mm, inner diameter of 25.0 mm, and height of 40.0 mm were prepared by filling premixed cement in a custom-made polyoxymethylene mold with a centrally placed casting rod.15 After a curing time of at least 15 minutes, the hollow cement cylinders were removed and the surface treatments were applied to the inner cylinder surface as described later.

Surface Treatments. For specimen group p40, the surface was treated with sandpaper with grain size of 40 to establish a rough surface, rinsed with Ringer's solution, and wiped with gauze to remove residue.

For specimen group p400, the surface was treated in the same way as described for specimen group p40, but with sandpaper with grain size of 400.

For specimen group p40+brush, the surface was treated in the same way as described for specimen group p40 and also brushed with a surgical brush (Medi-Scrub; Rovers Medical Devices).

For specimen group bone marrow, the surface was treated in the same way as described for specimen group p40 and then contaminated with porcine bone marrow to simulate realistic contamination.

For specimen group Ringer, the surface was treated in the same way as described for specimen group bone marrow, thoroughly cleaned 3 times for 3 seconds each with a surgical brush soaked in Ringer's solution, and rinsed with Ringer's solution.

For specimen group H202, the surface was treated in the same way as described for specimen group Ringer, with the brush soaked in 3% hydrogen peroxide solution, and then rinsed with Ringer's solution.

For specimen group ethanol, the surface was treated in the same way as described for specimen group Ringer, with the brush soaked in Softasept N (B Braun), a solution containing 654.3 mg ethanol and 83.0 mg/1 mL isopropanol, and then rinsed with Ringer's solution.

For specimen group soap, the surface was treated in the same way as described for specimen group Ringer, with the brush soaked in Lifosan Soft soap (B Braun), and then rinsed with Ringer's solution.

For specimen group control, a uniform block of acrylic bone cement was used without further treatment.

Mechanical Testing

Four-Point Bending Tests According to ISO 5833 (2002). The treated specimen halves were reinserted into the casting mold. The other half of the casting mold was filled with fresh acrylic bone cement, according to the protocol described earlier. After curing for at least 15 minutes, the specimens were removed from the casting mold and stored for 24 hours at 23 °C.

Shear Tests According to ASTM D732 (1993). The treated cylinders were reinserted into the mold, and fresh cement was placed into the cavity where the casting rod had been located previously, resulting in solid cylinders. After 15 minutes of curing, the solid cylinders were removed from the mold, and an 11-mm–diameter drill hole was placed centrally with a bench drill. The cylinders were then cut into 5-mm–thick slices.

Four-Point Bending Tests According to ISO 5833. The 4-point bending test was performed with a Universal material testing machine (Zwick 1456; Zwick GmbH) mounted with a 20-kN load cell (K-Series; Gassmann Theiss Messtechnik GmbH) in a laboratory setting. The 4-point testing rig had a 25-mm load span and a 75-mm support span, which deviates from the norm (20-mm load span, 60-mm support span) (Figure 1A). The tests were performed with a constant crosshead speed of 5.00 mm/min until failure occurred. After failure occurred, the fracture site was examined for irregularities. The bending modulus was calculated according to the method of Kuehn et al.26

Four-point bending test setup to test bending strength and modulus of the cement–cement interface (A). Shear test setup to test shear strength of the cement–cement interface (B). Both tests were performed with a Universal material testing machine (Zwick 1456; Zwick GmbH).

Figure 1:

Four-point bending test setup to test bending strength and modulus of the cement–cement interface (A). Shear test setup to test shear strength of the cement–cement interface (B). Both tests were performed with a Universal material testing machine (Zwick 1456; Zwick GmbH).

Shear Tests According to ASTM D732. Shear testing was performed with the same machine, and 6 samples per group were tested. Before testing, the thickness of each specimen was measured. Subsequently, the disks were rigidly mounted on the punch and positioned in the support fixture before the specimens were loaded with a crosshead speed of 1.27 mm/min (Figure 1B). Shear strength of each specimen was determined by dividing the load required to shear the specimen by the product of the thickness of the specimen and the circumference of the punch, according to the applied standard.

For an additional laboratory test, the authors combined 2 rectangular Palacos R specimens. One specimen was approximately 15 years old, and the other specimen had been freshly prepared. The fresh cement was set within a mold on the long side of the old specimen. The mechanical stability of the bond was tested manually.

Statistical Analysis

Statistical analysis was performed with one-way analysis of variance and Tukey's multiple comparisons test for bending strength, bending modulus, and shear strength. Differences were considered statistically significant at P<.05. Results are reported as mean, standard deviation, and associated P value, if not stated otherwise.

Results

Bending strength and bending modulus of the uniform cement block were statistically significantly greater compared with all groups except the p40 group (Tables 12; Figure 2). Bending strength of the p40, p400, p40+brush, and soap groups was significantly greater than that of the Ringer and H2O2 groups. Bending strength of the ethanol group was markedly higher, 35.98 MPa, than that of the Ringer (24.63 MPa) and H202 (24.69 MPa) groups; however, the difference was not statistically significant P=.10 (vs Ringer) and P=.11 (vs H2O2). Bending modulus of the p40, p400, p40+brush, soap, and ethanol groups was significantly greater than that of the Ringer and H2O2 groups.

Summary of the Results of the 4-Point Bending Test According to International Organization for Standardization 5833a

Table 1:

Summary of the Results of the 4-Point Bending Test According to International Organization for Standardization 5833

Results of the Shear Test According to American Society for Testing and Materials D732a

Table 2:

Results of the Shear Test According to American Society for Testing and Materials D732

Box plots showing bending strength (A), bending modulus (B), and shear strength (C) of the cement–cement interface of all of the studied groups.

Figure 2:

Box plots showing bending strength (A), bending modulus (B), and shear strength (C) of the cement–cement interface of all of the studied groups.

Shear strength of the uniform cement block was significantly greater than that of only the bone marrow group (Tables 12; Figure 2). Mean shear strength of the bone marrow group was 21.01 MPa and significantly less than that of all other groups. As seen in Table 2, the other groups did not show statistically significantly differences. The greatest shear strength was seen in the ethanol group, 44.61 MPa, followed by the soap group, 41.95 MPa.

Manual testing of the mechanical stability of a cement plate consisting of old and freshly added new cement showed a fracture not at the interface but within the cured fresh cement (Figure 3).

Result of manual testing of a cement specimen consisting of old and fresh cement (simulated cement-in-cement application). The fracture surface was not at the contact surface of the old and newly added cement.

Figure 3:

Result of manual testing of a cement specimen consisting of old and fresh cement (simulated cement-in-cement application). The fracture surface was not at the contact surface of the old and newly added cement.

Discussion

In aseptic revision arthroplasty, preserving a stable cement mantle may decisively decrease morbidity and surgical time. However, concerns about bonding strength between polymerized and newly added cement have been raised, particularly if the bone cement is contaminated with bone marrow and blood.8,22 Therefore, the most important result of this study was the introduction of the use of a degreasing agent in combination with brushing and roughening of the cement surface, which provided bending strength of 65%, bending modulus of 90%, and shear strength comparable to that of a uniform cement block, despite contamination with bone marrow. The important contribution of this work is that tests were performed according to international testing protocols and assessed molecular bonding strength between the 2 materials individually. The strong mechanical bond between 2 flat cement surfaces at least partially contradicts the assumption that bone cement is not glue. Clinically, when a prosthesis is inserted into a preexisting cement mantle, in addition to the chemical bond, an important structural interlock may further improve stability. Therefore, the authors consider cementation into a preexisting cement mantle a safe procedure for most practical situations, and meticulous cleaning of the surface is of great importance.

Intermediate- and long-term clinical results with cement-in-cement arthroplasty have been promising for revision hip and shoulder arthroplasty, and short-term advantages include decreases in risk of bone loss, operating time, blood loss, and infection rate.9,11,17,18,20,21,27

Cement-in-cement arthroplasty is not an option in septic revision because all foreign particles, including cement, must be removed.28

Literature on the cement–cement interface is scarce.8,10,12–15 To the best of the authors' knowledge, standardized testing as described by Kuehn et al26 has not been performed.

The authors noted a tendency toward higher interface strength for the rough surface comparable to the findings in the literature.12,22 Nonetheless, even with a smooth surface, bending strength of 75%, bending modulus of 91%, and shear strength of 107% were achieved compared with a uniform cement block, and bending strength of 87%, bending modulus of 99%, and shear strength of 101% were achieved compared with the rough surface. This finding supports the hypothesis that mechanisms other than mechanical interlock must play a significant role in the cohesive bond formed between new and old polymethylmethacrylate, as, for example, molecular interdigitation.15,29

As discussed earlier, the goal of this study was to identify an intraoperatively feasible, reliable, and practical method to reverse the detrimental effect of bone marrow, a common intraoperative contaminant, on the cement–cement interface.

The authors confirmed the detrimental effect of bone marrow on interface strength, as previously described.8 Bending strength and bending modulus were too low to be tested in this study because the bond did not withstand removal from the casting mold. Shear strength was significantly lower compared with all groups, at 21.01 MPa (Tables 12).

After mechanical cleaning with Ringer's solution and H202, bending strength was increased to a mean of 24.63 and 24.69 MPa, bending modulus to a mean of 2008.01 and 1936.30 MPa, and shear strength to a mean of 41.01 and 40.91 MPa, respectively.

However, by cleaning the contaminated interface with intraoperatively applicable agents with degreasing properties (ethanol and soap groups), bending strength was further increased significantly in the soap group compared with the Ringer (P=.0062) and H2O2 groups (P=.0065). In addition, bending modulus was further significantly increased in the ethanol and soap groups compared with the Ringer and H202 groups. After treatment with degreasing agents, bending strength and bending modulus were no longer significantly different compared with the p40 and p400 groups.

In all groups, shear strength was significantly increased compared with the bone marrow group. Although no significant difference was found between the different cleaning agents, mean shear strength in the ethanol group was 44.61 MPa and 9% greater than in the H2O2 and Ringer groups. Thus, cleaning the cement interface with a degreasing agent, an easy intraoperative step, may increase the strength of the cement–cement interface.

Surprisingly, all of the treated interfaces showed higher shear strength than the uniform cement block, although this finding was comparable to reports in the literature.8 To minimize the risk of a batch error, measurements of the control group were repeated with nearly identical values and a small standard deviation.

Compared with the study of Li et al,8 the current authors investigated not only shear strength of the cement–cement interface but also bending strength and bending modulus, according to ISO 5833. The authors believe that the combination of these tests more accurately represents the complex stress placed on this interface in the body. Additionally, the authors showed that thorough cleaning of the cement with simple degreasing agents led to results that were comparable to a bond with a spotless clean interface and even a uniform bone cement block.

Limitations of this study included storage of the bone cement, which was tested dry and at 23 °C. Kuehn et al26 showed that the mechanical properties of bone cement may vary after full water saturation at 37 °C. In this study, tests were performed with only 1 specific bone cement without antibiotic loading. Bone cement loaded with antibiotic may show marginally weaker mechanical properties.15 Because degreasing agents may adversely affect cell walls, they must be applied only in the cement mantle, thoroughly irrigated, and removed completely from the wound.

Conclusion

This study showed that the cement-in-cement interface may reach 85% of bending strength, 92% of bending modulus, and comparable shear strength compared with a uniform cement block and that this interface may not be the site of failure (Figure 3). Meticulous removal of fatty contaminant is of great importance. The use of a degreasing agent further increases the stability of the cement–cement interface. If these precautions are taken carefully, it is safe to assume that the combined molecular and mechanical interlock is sufficient for most clinical applications and will not represent the weakest link in prosthetic revision. Partial or total preservation of a stable, aseptic cement mantle helps to reduce both operative time and unnecessary collateral damage.

References

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Summary of the Results of the 4-Point Bending Test According to International Organization for Standardization 5833a

GroupNo. of specimens testedMean±SDStatistically significantly (P<.05) superior toMean±SD bending modulus,c MPaStatistically significantly (P<.05) superior to

Force at failure, NSpecimen cross-section, mm2Bending strength,b MPa
Control696.73±6.3736.20±1.0255.45±1.17p400, Ringer, H2O2, p40+brush, soap, ethanol2548.06±138.45Ringer, H2O2, p40+brush
p40684.69±17.0836.00±0.9946.91±7.05Ringer, H2O22353.94±316.40Ringer, H2O2, p40+brush
p400675.52±5.4237.34±1.7440.72±4.56Ringer, H2O22333.29±160.07Ringer, H2O2
p40+brush661.15±8.3035.35±0.4436.55±4.81Ringer, H2O22151.87±72.32
Bone marrow6Not applicableNot applicableNot applicable
Ringer640.87±12.3435.25±0.5824.63±7.592008.01±106.75
H2O2640.97±7.9435.21±0.6724.69±4.741936.30±82.22
Ethanol659.31±9.2435.12±0.4135.98±5.592338.58±108.64Ringer, H2O2
Soap669.15±19.9936.52±0.6038.63±11.06Ringer, H2O22338.58±108.64Ringer, H2O2

Results of the Shear Test According to American Society for Testing and Materials D732a

GroupNo. of specimens testedMean±SDStatistically significantly (P<.05) superior to

Shear strength,b MPaThickness, mm
Control636.86±1.715.08±0.13Bone marrow
p40639.07± 3.394.94±0.04Bone marrow
p400639.70±1.264.97±0.06Bone marrow
p40+brush640.79±2.084.90±0.07Bone marrow
Bone marrow621.01±11.224.92±0.05
Ringer641.01±2.284.94±0.06Bone marrow
H2O2640.91±3.054.94±0.04Bone marrow
Ethanol644.61±1.084.91±0.06Bone marrow
Soap641.95±1.714.91±0.06Bone marrow
Authors

The authors are from the Department of Orthopedics (DK, RH, DCM), Balgrist University Hospital, University of Zurich, and the Institute for Biomechanics (NZ, EB), ETH Zurich, Zurich, Switzerland; and the Department of Orthopedic Surgery (K-DK), Medical University of Graz, Graz, Austria.

The authors have no relevant financial relationships to disclose.

The authors thank Tony Kaiser and Beda Rutishauser for their support and Heraeus for providing the bone cement. Institutional funding of the University of Zurich is gratefully acknowledged.

Correspondence should be addressed to: Dominik Kaiser, MD, Department of Orthopedics, Balgrist University Hospital, University of Zurich, Universitätsklinik Balgrist, Forchstrasse 340, 8008 Zurich, Switzerland ( dominik14k@gmail.com).

Received: August 28, 2019
Accepted: October 24, 2019
Posted Online: October 22, 2020

10.3928/01477447-20201007-05

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