Effect of lithotriptor treatment on the fracture toughness of acrylic bone cement Gladius Lewis Department
TN 38152, US.4
Extracorporeal shock wave lithotripsy (ESWL) has now been established as an efficacious non-invasive modality for the management of renal calculi and has shown promise for management of other types of stone, as well. Following on from these successes, ESWL has recently been proposed for use in the preliminary stages of revision of cemented total hip joint replacements as a means of breaking up the cement mantle. It is useful, therefore, to examine the effect of shock waves on pertinent mechanical properties of the cement. This study utilizes the chevron-notch short-rod specimen and a commercially available test system to obtain the values of one such property, namely fracture toughness, of Palacos Radiopaque@ bone cement before and after treatment with shock waves delivered from a lithotriptor. The fracture toughness drops by about 14% following the shock wave treatment, thus confirming the possibility that ESWL can be used, as indicated earlier, in revision arthroplasty. Keywords:
Received 22 February 1991; revised 6 June; accepted
In its clinical applications, extracorporeal shock wave lithotripsy (ESWL) involves four stepsI. First, shock waves are generated outside the body. Second, these waves are then suitably focused on to an area distant from the generation zone. Third, the waves are coupled into the body using a water bath or membrane. Fourth, once the target inside the body has been located and positioned into the focal area of the waves, the target is shattered by the waves. ESLW’s underlying principle is that the similarity between the acoustical impedances of the aqueous milieu and the body tissue allows the shock waves to traverse these media with very little energy loss. However, when the waves encounter the target (a solid object whose material has an acoustic impedance very different from that of the body tissue), a stress (exceeding the cohesive strength of the target material) is set up which fragments the target. The clinical use of ESWL is arguably the most innovative non-invasive endourologic modality this and results garnered century2, 3. With the experience from the use of commercially available lithotriptors (involving some 500 000 treatments carried out in over 300 centres in 32 countries since the technique was introduced clinically in Munich, Germany, in 19804), there is no doubt that ESWL has revolutionized the management of symptomatic renal calculi. It has been Correspondence
to Dr G. Lewis.
shown recently that this procedure may also be successfully applied in the fragmentation of ureteral stones5, common bile ductstone#, and gallstones7. Given this level of success of ESWL in current clinical endourologic practice, it is not surprising that potential new areas of application of the principle of the technique are now being identified. In this respect, ESWL has recently been suggested for use in the removal of the cement mantle in the first stages in the revision of cemented arthroplasties, particularly hip joint implants. Perhaps the impetus for this suggestion is the fact that current methods of fracturing the mantle are less than satisfactory and the newer approaches are unproven clinically. Thus, there have been many reports of inadvertent perforation of the femoral cortex’ when high-powered drills are used. Enough clinical data have not yet been obtained regarding the efficacy of or any side effects or problems associated with the use of ultrasonic tools for the cement removalg. Because of technical difficulties, the clinical use of carbon dioxide lasers has been very limited”. This potential new application of ESWL has been considered because it obviates the need for surgery and is expected to fracture the cement mantle with little adverse effect on the properties of the contiguous bone and with minimum patient morbidity. This expectation is based on the results of four reports8’“-‘3 regarding the use of shocks delivered from a lithotriptor to fracture the Biomaterials
1992, Vol. 13 No. 4
cement layer surrounding a stainless steel rod in cadaveric adult canine femora. Knowledge of the effect of shock waves delivered from a lithotriptor on the relevant mechanical properties of the bone cement [chief among them being tensile strength, compressive strength and fracture toughness] is crucial to an assessment of the potential role of ESWL in the revision of cemented arthroplasties. To date, the only data in this aspect are from the work of May et a1.l’ on Zimmer Regular@ bone cement. It is deemed important that a wider database be established so that the effects of lithotriptor treatment on bone cement can be unambiguously delineated. In this regard, fracture toughness is undoubtedly an important mechanical property. This is because, under service loading, components usually fracture due to the growth of pre-existing flaws. The most common direction and macroscopic mechanism of flaw advance is called mode I (or tensile opening] fracture. The state of the stress field in the vicinity of the flaw is reflected in the magnitude of the parameter called the stress intensity factor, K. The critical value of K to initiate fracture under mode I loading is termed K,,. The value of K,, is dependent on specimen thickness, with the lower limiting value being KI,, the mode-I-planestrain fracture toughness of the material. The wellestablished experimental procedure (whose details are given in the ASTM E399-83 specification] allows K,, to be determined. However, when a chevron-notch specimen is used to determine the plane-strain fracture toughness, the result is designated either K, (when the material exhibits a smooth-crack-growth fracture behaviour) or KIvi (when the material exhibits an unstable-crack-growth fracture behaviour known as crack-jump). In the former case, the load will at first increase as the crack moves from the chevron tip and then decrease after the crack has travelled about halfway through the chevron-shaped ligament that is formed by the two thin grooves in the sides of the specimen. In the latter case, the load required to initiate the crack at the point of the chevron slot is larger than the load required to advance the crack just after initiation, such that the crack suddenly and audibly jumps ahead at the time of its initiation. The details regarding the determination of KI, and KIvj are contained in the ASTM E1304-89 specification. While debate continues on the relation of KI, to K,, for a given material, the publication of several direct comparisons between these two parameters for many materials’4-‘7 is helping to clarify the relationship and, hence, establish K,, as a direct measure of mode-I-plane-strain fracture toughness. Cognizant of the comments in the preceding paragraph, the focus of the present work is thus an experimental determination of the mode-I-plane-strain fracture toughness of a commercial formulation of acrylic bone cement, Palaces Radiopaque@ (Smith and Nephew Richards Inc., Memphis, TN, USA), before and after exposure to shock waves, delivered by a commercially available lithotriptor.
of bone cement:
toughness specimens, lithotriptor treatment of specimens, determination of the fracture toughness of control (no shock-wave-treated) and shock-wave-treated specimens, and examination of the fracture surfaces of the specimens. Preparation of the cement mixture involved mixing 40 g of the copolymer powder and 20 ml of the monomer liquid, as per manufacturer’s instructions, followed by centrifugation (45 s at maximum speed of 3000 rev per min). The mixture was immediately poured into a Teflon@ mould (127 mm long and 12.90 mm inside diameter). After polymerization of the mixture, the hardened rod of cement was extruded from the mould and 19.07 mm long pieces were cut and sanded down to a diameter of about 12.8 mm. Six straight-sided chevron-notch short-rod (CNSR) specimens [Figure 1) were fabricated from these pieces using a special saw (model 4901@; Terra Tek Systems, Salt Lake City, UT, USA) and a water-dispersed cutting oil. The selection of the three specimens to receive the lithotriptor treatment and the three to serve as control samples was done on a randomized basis. The shock-wave-treated specimens were those specimens which had been subjected to 1000 shocks delivered from a commercial lithotriptor (Dornier HM3@;
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