Contribution of hydroxyapatite to the tensile strenght of the isobutyl-2-cyanoacrylate-bone bond

Contribution hydroxyapatiteto the of tensile strength of the isobutyl-Z cyanoacrylate-bonebond Frank J. Papatheofanis Bone Metabolism Laboratory, Depanment of Orthopaedics. University of Illinois, Chicago. IL 60680. (Received 9 April 1988; revised 18 July 1988; accepted 18 July 1988J USA The bonding strength between bone and a-2cyanoacrylata polymers, with or without the addition of powdered hydroxyapatite, was determined. The tensile strength of a bone-cyanoacrylate bond was measured for each polymer: 4.31 f 0.88 MPa (methyl-), 5.74 k 0.62 MPe (ethyl-), and 8.33 + 0.41 MPa (isobutyl-). The tensile strength of the isobutyl-2-cyanoacrylate bond increased to 12.03 + 0.72 MPa with the addition of 10% (w/v) hydroxyapatite before decreasing to 7.89 f 0.59 MPa on addition of 15% (w/v) hydroxyapatite. An optimal concentration of hydroxyapatite significantly increased the tensile strength of a bone-cyanoacrylate bond in vitro in a manner comparable to reinforced bone replacement materials. Keywords: Hydroxyapatite, bone, cyanoactylate. adhesives The a-2-cyanoacrylates have been used in experimental bone fracture repair for many years’. Recent applications include repair of osteochondral fractures of the femoral condyle in the dog2 and transverse fractures of the tibia and femur in the rat and rabbit, respectively3. Recent studies have also reported on the bonding strength4 and bond durability5 of cyanoacrylates with bone, and on the contribution of bone remodelling to the strength of the adhesive bond6. In part, the difficulty associated with the application of this class of adhesive to bone fracture repair is due to the elastic behaviour of cortical bone. The complex biomechanical properties of cortical bone are especially evident in the development of bone composite analogues7. Some success has been obtained in the development of bone replacement materials by increasing the stiffness of carbon fibre and glass fibre-reinforced polymers with the addition of particulate hydroxyapatite. In addition, calcium phosphate ceramics have been employed as bone tissue replacements in a variety of cases*. The purpose of the present study was to determine whether addition of hydroxyapatite would enhance the bonding strength of cyanoacrylate adhesives to bone when quantitated by measurement of tensile strength. rat forequarter. After dissection, the fresh bone specimens were cleaned and milled to 8 mm diameter cylinders. Further machining and processing of the bone was performed according to a previously published method5. MATERIALS The test for tensile strength of adhesives to bone, as previously introduced by researchers at the National Bureau of Standards, was performed on the bone specimens5. Five bone samples were tested in each experimental condition and the results were reported as the mean and standard deviation for the five determinations. AND METHODS Specimens The source of bone was the distal tibia1 segment of the adult Correspondence to Dr F.J. Papatheofanis. 0 1989 Adhesives Methyl-, ethyl-, and isobutyl-2-cyanoacrylate were purchased from Vigor Co., New York, NY. Hydroxyapatite (Ca,, (PO,), (OH,)) was purchased from Fisher Scientific Co. (Fairlawn, NJ) as an ACS certified reagent. An electric mill was used to grind the hydroxyapatite crystals to approximately 50pm diameter, as confirmed by electron microscopy. The adhesives were uniformly applied as a thin film to the bone ends to be joined, and cured for 5 min at r.t.. The surfaces of the bone ends were not primed or altered following machining. The glued bones were kept in an ice-cold balanced salt solution until testing (up to 2 h). Addition of 10% (w/v) hydroxyapatite to isobutyl-2-cyanoacrylate did not significantly alter spreadability. The sessile-drop method (Lorentzen-Wettres goniometer) was used to measure the contact angle: 55 + 2” (isobutyl-2-cyanoacrylate) and 5 1 + 2” (composite), for triplicate determinations. Measurement and testing Butterworth Et Co (Publishers) Ltd. 0142-9612/89/030185-02$03.00 Biomaterials 1989, Vol 10 April 185 ffydm~apatjte and cyanoactyfate &he&es: Ten&a strvngtb of a-2-cyanoacrylate Tebla 7 bone EJ. Papatheofanis and ~droxyap8t~e-bonded Adhesive Tensile strength (MPa)a Methyl-2-cyanoacrylate Ethyl-2-cyanoacrylate Isobutyl-2-cyanoacrylate + 1% (w/v) hydroxyapatite + 5% (w/v) hydroxyapatite + 10% (w/v) hydroxyapatite + 15% (w/v) hydroxyapatite 4.3 1 5.74 8.33 8.50 10.24 12.03 7.89 + 0.88b Y!Z 0.62 + 0.41 + 0.3 1 it: 0.39 + 0.72 + 0.58 ‘1 MFb = 10.2 kgf/cm* = 145 psi. ‘E~~rimental determination representing mean t standard deviation of five samples. RESULTS Asummaryof tensile strengths obtained from the analysis of bone with cx-2-cyanoactylates and hydroxyapatite is illustrated in Table 1. An increase in tensile strength was observed as the polymers increased in chemical complexity: methyl(4.31 + 0.88 MPa), ethyl- (5.74 f 0.62) and isobutyl(8.33 f 0.41). The bonding strength of the isobutyl polymer was almost twice that for the methyl homologue. Furthermore, addition of increasing amounts of hydroxyapatite led to an increase in tensile strength up to I,O% (w/v). The bonding strength of isobutyl-2cyanoacrylate was increased by approximately 44% on addition of 10% (w/v) hydroxyapatite. However, addition of 15% (w/v) hydroxyapatite resulted in a 5% decrease in bonding strength of this polymer when compared to the unmodified adhesive. than dense calcium phosphate ceramics (i.e. 4.8 MPa (porous) versus 38-196 MPa (dense))g. Significantly, the mechanical properties of porous ceramics decrease with increasing amounts of micro- and macropores”. Perhaps, in the present case, the relative amount of micropores or macropores reached a threshold at approximately 10% (w/v) hydroxyapatite; and addition of more hydroxyapatite resulted in increased micropore formation that ultimately resulted in decreased adhesive-bone tensile strength (i.e. at 15% (w/v) hydroxyapatite). The results suggested that hydroxyapatite-reinforce bone bonding with cyanoacrylate adhesives may be useful in applications where high tensile forces are encountered in the repair of skeletal structures. For example, one application of a hydroxyapatite-cyanoacrylate composite may be in the area of fracture repair in long weight-bearing bones (e.g. femur), or in the repair of fractures in the bones of the foot and hand. Furthermore, other natural or synthetic materials, more suitable for other tissues, may be useful in enhancing the bonding strength of the alkyl-2-cyanoacrylates in a range of biological applications. Further investigation is directed to this end. ACKNOWLEDGEMENTS The author is grateful to Riad Barmada, MD, Professor and Head, Department of Orthopaedics for his support. REFERENCES DISCUSSION 1 The results from this study agreed closely with previous measurements of the tensile strength of a-2cyanoacrylate bonds with bone; and, previous values reported forthis bond were 6.60 It 1 .I 3 MPa (ethyl-) and 6.62 ?I 1.73 MPa (isobutyl-)5. In comparison, the present results for the same adhesives were 5.74 -t 0.62 MPa (ethyl-) and 8.33 + 0.41 MPa (isobutyl-), representing a difference from the previous report of 1 3% and 20%. respectively. The present results indicated that the strongest bond was obtained with the isobutyl polymer. Significantly, the isobutyl polymer has been used in the most successful applications of adhesives to bone fracture repair in vivu2~3. Addition of hydroxyapatite to isobu~l-2-cyanoac~late resulted in increased ending strength. The bonding strength observed with the addition of 5 and 10% (w/v) hydroxya~tite, 10.24 + 0.39 and 12.03 f 0.72 MPa, respectively, was higher than that reported for any of the cyanoacrylate homologues tested in this manner to date4*5. The added hydroxyapatite may have formed a matrix or scaffold to support the adhesion. In this regard, the composite adhesive may have formed a microstructure similar to that of porous calcium phosphate ceramics. Porous calcium phosphate ceramics display lower tensile strengths 2 186 Biomaterials 1989, Vol 10 April 3 4 5 6 7 8 9 10 Meyer, G., Muster, D., Schmitt, D., Jung, P. and Jaeger, J.H., Bone bonding through bioadhesives: present status, ffiomat. Med. D8v. Aft. Org. 1979.7, 55-71 Harper. 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Williams), CRC Press, Boca Raton, 1985; 89-l 02 Williams, D.F., The biocompatibility and clinical uses of calcium phosphate ceramics, in Biocompatibi/ity of Tissue Analogues, Vol. N (Ed. D.F. Williams), CRC Press, 8oca Raton. 1985; 43-70 VanRaemdonck, W., Ducheyne, P. and DeMeester, P., Calcium phosphate ceramics, in Metal and Ceramic Biomaterials, Vol. N (Ed. P. Ducheyne and G.W. Hastings), CRC Press, 8oca Fiaton, 1984; 143-166 Rao, R.W.R. and Boehm, R.F.. A study of sintered apatite, J. Deer. Res. 1974,53,1351-1354