The most common complication of cemented total hip arthroplasty is aseptic loosening of prosthetic components. The failure of the cement mantle is the most common mode of aseptic loosening of cemented components. The large voids entrapped
within
the
cement by current hand mixing technique may contribute to poor mechanical properties and early fatigue failure of bone cement.
Two major methods including centrifugation and vacuum mixing had been studied to roduce porosity or voids and to improve mechanical properties of acrylic bone cements. A common method of reducing the viscosity of acrylic bone cements or of
prologing
the period of lower viscosity state is to prepare of the bone cements using monomer that has been chilled to 0-4。C.
The object of this experiment is to study the effect. Of vacuum mixing, and the effect of monomer chilling on the tensile and Fatigue strength of acrylic bone cement. The bone cements used was Palacos R(r) (Schering-Plough International
Inc.
USA). The monomer of Palcos R(r) was kept at 4。c and 21。C prior to mixing. Two mixing methods were used : hand stirring in the atmosphoric pressure and under a vacuum. Hand mixing method was prepared by mixing forty grams of polymer with
twenty
milliliters of monomer for fifty seconds at a stirring approximately 2 times at one second in an open plastic mixing bowel with a plastic spatula. For vacuum mixing, vacuum mixing system from Scientific Development Compony (Germany) was used.
According
to manufacturer's instruction, twenty milliliters of monomer and forty grams of polymer were mixed at the constant pressure of four hundred mmHg for fifty seconds.
After preparation by a hand mixing method and a vacuum mixing method, all mixing cements poured into ASTM standard(KSB 0802) cylindric mold.
Eighty molded specimens were made for experiments, and all specimens were tested at 21。C. Experiments were divided into 4 groups:
Group 1: Hand mixed specimens with monomer keeping at 21。C
Group 2: Hand mixed specimens with monomer chilling at 4。C
Group 3: Vacuum mixed specimens with monomer keeping at 21。C
Group 4: Vacuum mixed specimens with monomer chilling at 4。C
Each experimental group was comprised of twenty specimens. Static tensile strength test was carried out on each group of ten specimens of 4 experimental groups using Instron Model 1127 testing machine at a strain rate of 5mm per minute with
gauge
length of 50mm. Fatigue strength test was carried out on each group of ten specimens of 4 experimental groups using MTS Model 810 at stress of 25MPa and a freguency of 2.5 hertz. The number of cycles to failure was recorded. Statistical
analysis
was
done with the ANOVA method and the log rank test. The probability of survival of fatigue strength was analysed by gamma distribution.
@ES The results obtained were as follows:
@EN 1. The mean ultimate tensile strength of vacuum mixed specimens was increased more than hand mixed specimens regardless of prechilling of the monomer. The mean ultimate tensile strength of vacuum mixed specimens with monomer
chilling
at
4。C increased thirty-two percentage greater than hand mixed specimens with monomer keeping at 21。C
2. The ultimate tensile strength of hand mixed speciemns was not influenced by prechilling of the monomer.
3. The mean fatigue strength of vacuum mixed specimens was increased more than hand mixed specimens regardless prechilling of the monomer. The mean fatigue strength of vacuum mixed specimens with monomer chilling at 4。C was 9.5 times
greater
than hand mixed specimens with monomer keeping at 21。C.
4. The mean fatigue strength of hand mixed specimens with monomer keeping at 21。C was 1.7 times greater than hand mixed specimens with monomer chilling at 4。C but it has no statistical difference.
5. The survival probability of fatigue strength at 3,000 cycles gamma distribution showed longer fatigue life for vacuum mixed specimens than hand mixed specimens regardless of prechilling of monomer.
6. The scanning electromicrographic findings of fracture surface of specimens in 4 experi mental groups showed a greater number and larger diameter of air voids in the hand mixed specimens than vacuum mixed specimens regardless of
prechilling
of
the monomer.
Above results suggest that vacuum mixed method with monomer chilling at 4。C before mixing of cement increase tensile and fatigue strength of acrylic bone cement, and decrease larger or small voids, and improve the longevity of cemented total
hip
arthroplasty.
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