Superior Mechanical Strength - PALACOS®: Engineered for Success

After sixty years of experience as the inventor and sole manufacturer of PALACOS®-the gold standard in joint fixation1-Heraeus Medical has transitioned to be the exclusive provider for the PALACOS® cements family in the U.S (PALACOS® R/R+G, PALACOS® MV/MV+G, PALACOS® LV/LV+G, PALACOS® R/R+G pro). Characteristically green in color, PALACOS® doesn’t just claim to be the element of success in joint replacement, it continuously proves it as the most studied bone cement worldwide.2

Manufactured for Dependability

Due to the significant stresses encountered in vivo, the strength of a bone cement is one of the most important factors for guaranteeing long-term fixation in total knee arthroplasty (TKA) and total hip arthroplasty (THA).3 After implantation, patient movement exposes the cement mantle to a complex blend of compressive loading combined with bending, impact, tension, torsion and shear.

The International Organization for Standardization (ISO) has specified procedures to test bone cements for each of these strengths in isolation. In 2014, Klaus-Dieter Kuehn published novel comparative data on the strength of several brands of bone cements when tested according to ISO 5833. For these tests, Kuehn used nine total samples of each cement (3 samples each from 3 different batches).

  • Impact strength is the energy required to cause a fracture in the bone cement when struck by a sudden blow. In tests of impact strength, PALACOS® R possessed 16.9% and 30.4% greater impact strength than Simplex® P and CMW® 1, respectively.4
  • Bending strength is the force required to deform or break a bone cement under stress. In tests of bending strength, PALACOS® R+G was 7.1% stronger than Simplex® P+T and 14.5% stronger than Smartset GHV (Figure 2).5
  • Compressive strength is the maximum stress that a material can withstand before failure in compression. In compressive strength tests, PALACOS® R+G was 4.7% stronger than Simplex® P+T and 3.3% stronger than Smartset GHV (Figure 2).6
  • Taking the average of these findings, PALACOS® R+G is 11.8% stronger than the other leading brands of antibiotic loaded bone cement in the US.

A more complete picture of how bone cements hold up in vivo may result from fatigue testing in which a strip of bone cement is subjected to sinusoidal bending and/or compression until it fails. In fatigue testing, investigators have identified a number of factors that appear to affect the durability of bone cement:

  • One of the factors affecting the fatigue life of a bone cement is the co-polymer blend. The PALACOS® family of bone cements contain MA co-polymer which is more ductile than other copolymers (e.g. stryrene, pure PMMA base) and thus shows greater fatigue life and strength of the cement over cyclic testing than cements with these other copolymers.
  • Another factor affecting fatigue life is the method used to sterilize the bone cement. Constantly innovating to maintain product excellence, Heraeus pioneered the use of ethylene oxide (EtO) sterilization for bone cements. While more costly, EtO preserves the molecular weight of PMMA cements and enables long chains to form during the polymerization process-providing significant strength and durability to the finished PALACOS® products.

Consistency is Key

Cycle to Failure for PALACOS R (at 22 degrees Celsius)
Figure 4: Cycle to Failure for PALACOS R (at 22 degrees Celsius)

A third factor affecting the fatigue life of the cement stems from the creation of pores within the cement matrix during mixing. These pores serve as the starting point for cracks, which extend through the cement and lead to failure. Vacuum mixing reduces air pores in the cement, which leads to a stronger and more durable product.7

In 2003, Nicholas Dunne and colleagues at the Dublin City University prepared two bags of PALACOS® R bone cement in various ways to understand the impact of vacuum mixing on porosity and fatigue life.8 The investigators prepared eighteen test specimens according to ISO 527 for each mixing system and used optical microscopes to determine porosity before subjecting the samples to repeat loading under an upper stress of 22.9 MPa. They determined performance for three vacuum cartridge systems (2nd generation), two vacuum bowl systems (1st generation) and in an open bowl.

The average fatigue life for vacuum cartridge systems was 10,393 cycles versus 7,959 cycles for samples prepared in a vacuum bowl and 5,938 samples prepared in an open bowl. As such, canister or cartridge vacuum mixing systems (2nd generation) were found to provide 75% greater fatigue life for PALACOS® R than when cement was mixed in an open bowl; and 30.6% greater fatigue life than when cement was mixed in a vacuum bowl (1st gen) system.

Through multiple generations of mixing devices (1st gen: PALABOWL®; 2nd gen: PALAMIX®; 3rd gen: PALACOS® pro), Heraeus Medical has simplified the preparation of high quality cement.

Optimize Cement Strength with Pre-Mixed ALBCs

While manually mixing antibiotics in by hand may appear to be a cost saving solution, the surgeon may be causing more harm than good. Danielle Neut and colleagues at the University of Groningen in the Netherlands examined the elution of antibiotics from industrially preloaded and manually admixed samples of PALACOS® R.9 Manufactured PALACOS® R+G released a statistically significant greater proportion of gentamicin than PALACOS® R with gentamicin mixed in by hand (8.08% vs. 2.86%, p<0.001). Importantly, the cumulative gentamicin release curves from this study for PALACOS® R with gentamicin suggest that elution falls below 10 µg/cm2 within hours of preparation. In contrast, studies of PALACOS® R+G have found elution above 32 mg/L out to five days after preparation. Over 99% of MRSA and MSSA isolates are susceptible to gentamicin concentrations above 32 mg/L; below this minimum inhibitory concentration, research has shown that bacteria will bond to and develop biofilm on implanted metals leading to periprosthetic joint infection.10

In a similar vein, Sara Ferraris and colleagues at the University of Tourino looked at zones of inhibition in petri dishes of Staph. aureus for various antibiotic blends in PALACOS® R bone cement.11 PALACOS® R+G produced a zone of inhibition 19% greater than PALACOS® R with Gentamicin manually admixed. When Vancomycin was included in the blend, PALACOS® R+G with Vancomicin manually admixed produced a zone of inhibition 160% greater than PALACOS® R with Vancomycin manually admixed.

Full Inhibition Zone
Figure 5: Full Inhibition Zone

In addition to lacking the elution needed to prevent periprosthetic joint infection, manually admixed bone cements demonstrates significantly poorer strength and fatigue life. In 2007, Nicholas Dunne and colleagues examined the fatigue life for PALACOS® R+G and PALACOS® R with gentamicin mixed in by hand.14 They used a turbo blender at speeds of 1500 rpms to blend the antibiotics in by hand in an attempt to increase homogeneity and thus strength of the admixed cement. Unfortunately, even when using this technique, they found that manual addition of gentamicin lowered the compressive and bending strengths of the cement by 15% and 18%, respectively. Of most concern, the mean number of cycles to failure for PALACOS® R+G was 48,417 versus 19,861 (a 59% reduction) for PALACOS® R with 1 g of Gentamicin manually admixed.

Fatigue Life
Figure 6: Fatigue Life

Importantly, the surgeon assumes product liability when they add antibiotics into bone cement by hand.15 The lack of benefit due to poor elution and the very real possibility of harm due to weakening of the cement may cause a manually admixed strategy to be much more costly than realized.

Literature:
1 Olerud et.al.: Comparison of Refobacin Bone Cement and Palacos with Gentamicin in total hip arthroplasty; an RSA study with two years follow-up, Hip int., 2013:1.
2 PMMA Cements, Klaus-Dieter Kuhn. Springer Medizin, 2014. Pages 30-31.
3 Kuehn KD. PMMA Cements P 184.
4 Kuehn KD. PMMA Cements. P 187.
5 Kuehn KD. PMMA Cements. P 215.
6 Kuehn KD. PMMA Cements. P 215.
7 Kuehn KD. PMMA Cements. P 221.
8 Dunne NJ, Orr JF, Mushipe MT, Eveleigh RJ. The relationship between porosity and fatigue characteristics. Biomaterials 2003; 24: 239-245.
9 Neut D, van de Belt H, van Horn JR, van der Mei HC, Busscher HJ. The effect of mixing on gentamicin releases from polymethylmethacrylate bone cements. Acta Orthop Scand 2003; 74(6): 670-676.
10 Cerca N, Martins S, Pier GB, Oliveira R, Azeredo J. The relationship between inhibition of bacterial adhesion to a solid surface by sub-MICs of antibiotics and subsequent development of a biofilm. Res Microbiol 2005; 156(5-6): 650-55.
11 Ferraris S, Miola M, Bistolfi A, Fucale G, Crova M, Masse A, Verne E. In vitro comparison between commercially and manually mixed antibiotic-loaded bone cements. J Appl Biomater Biomech 2010; 8(3): 166-174.
12 Postak PD, Greenwald AS. The Influence of Antibiotics on the Fatigue Life of Acrylic Bone Cement. AAOS 2005.
13 DeLuise M, Scott CP. Addition of Hand-Blended Generic Tobramycin in Bone Cement: Effect of Mechanical Strength. Orthopedics 2004; 27(12): 1289-1291.
14 Dunne N, Hill J, Mcafee P, Todd K, Kirkpatrick R, Tunney M, Patrick S. In vitro study of the efficacy of acrylic bone cement loaded with supplementary amounts of gentamicin: effect on mechanical properties, antibiotic release, and biofilm formation. Acta Orthopaedic 2007; 78(6): 774-785.
15 Starnes BW. A surgeon’s perspective regarding the regulatory, compliance, and legal issues involved with physician-modified devices. J Vasc Surg 2013; 57: 829-31.

Figures:
Figure 1: Kuhn KD. PMMA Cements. Noval Comparative Data (Table 12.1). Berlin: Springer; 2014.
Figure 2-3: Kuhn KD. PMMA Cements. Noval Comparative Data (Table 13.3). Berlin: Springer; 2014.
Figure 4: Dunne NJ, Orr JF, Mushipe MT, Eveleigh RJ. The relationship between porosity and fatigue characteristics. Biomaterials 2003; 24: 239-245.
Figure 5: Ferraris S, Miola M, Bistolfi A, Fucale G, Crova M, Masse A, Verne E. In vitro comparison between commercially and manually mixed antibiotic-loaded bone cements. J Appl Biomater Biomech 2010; 8(3): 166-174.
Figure 6: Dunne N, Hill J, McAfee P, Todd K, Kirkpatrick R, Tunney M, Patrick S. In vitro study of the efficacy of acrylic bone cement loaded with supplementary amounts of gentamicin. Acta Orthopaedica 2007; 78(6): 774-85.

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