in a preferred embodiment of the invention , crystal growth on the calcified tissue surface is produced by interaction with a solution containing ionic species which result in an outgrowth of insoluble crystalline calcium salts which are bonded to the tissue surface . in order to confine the reaction to a specific area of tissue , the solution is often provided in a viscous or gel - like form to limit flow and surface reaction during application of the solution . after completion of crystal growth , which occurs rapidly , typically in about 30 seconds to about 6 minutes , the reactant solution is washed away and the tissue surface is dried prior to the application of resin or other bonding agent . in order to facilitate the crystal growth , the solution usually is mildly acidic in order to effect a slight chemical reaction with the tissue surface to release calcium ions therefrom which then form a salt with the crystal - forming ionic species in the solution , the salt depositing in crystalline form . the crystals are nucleated within the tissue surface and thereby are firmly attachd to it . the removal of calcium ions from the tissue surface to effect crystal formation involves considerably less damage to the surface when compared to the acid - etch technique . the crystal growth generally is needle - like and occurs in random directions with respect to the tooth surface in dense formation , causing the intermeshing of crystals which enhances the anchoring capability of the crystal growth . gypsum crystals usually are about 10 to about 25 micrometers long and about 2 to about 5 micrometers thick . in forming the crystal growth , too great a density should generally not be permitted to form , otherwise this inhibits penetration of the orthodontic resin to the tooth surface , and inadequate bond strengthens result as well as leakage at the resin - tooth interface . excess growth can , however , be washed away by vigorous washing action and the residual nucleated surface used to achieve the desired microchemical interlock with cured resin . one convenient agent for effecting the crystal formation procedure used in this invention is an aqueous solution of polyacrylic acid containing sulphate ions . the polyacrylic acid may have a molecular weight of from 1 , 000 to 100 , 000 and a concentration of from about 10 to about 60 %. in this instance , the crystal growth consists of gypsum crystals ( caso 4 . 2h 2 o ). in order to achieve significant crystal growth , the solution contains at least about 1 % of sulphate ions , with the upper limit of sulphate concentration being limited by solubility considerations and the desire to avoid over - etching of the tooth surface . the application of a drop of polyacrylic acid containing the sulphate ion to tooth enamel results in the formation of a dense growth of needle - shaped gypsum crystals having a spherulitic habit . the viscosity and reactivity of the solution may be varied by varying the concentration and / or molecular weight of the polyacrylic acid . the polyacrylic acid may be used as the carrier medium for other ionic species , such as , phosphate , oxalate or tartrate , or in some cases may be omitted entirely . the ionic species may be used in the form of their free acid , for example , sulphuric acid or oxalic acid , or in the form of their salts , for example , ammonium salts . materials other than polyacrylic acid can be used as a carrier medium for the ionic species and for retaining the solution at a desired location on the tooth surface . for example , high viscosity methyl cellulose may be used . solutions of polyacrylic acid of molecular weights varying from 5 , 000 ( 5t ) to 80 , 000 ( 80t ) of solution concentrations of 20 to 50 percent were formed by aqueous polymerization of acrylic acid using ammonium persulphate as initiator . these solutions were applied to calcified tissue . scanning electron microscopy showed a copious growth of gypsum crystals with all solutions containing more than about 1 percent sulphate ions . solutions containing less than this amount or dialysed free of sulphate ions ( d ) did not produce crystal growth . however , upon addition of the appropriate amount of sulphate ions , in the form of sulphuric acid or ammonium sulphate , crystal growth occurred . in all cases , crystal growth was complete in a period of 2 to 6 minutes . four of the sem &# 39 ; s at 2 , 000 × magnification for 5t polyacrylic acid solutions are reproduced in the following table i : table i______________________________________ solution treatment concentration timerun (%) ( mins ) ______________________________________1 20 12 30 13 40 14 40 5______________________________________ calcium release from the tooth enamel surface was also determined and compared with that obtained with phosphoric acid solutions . the results are reproduced in the following table ii : table ii______________________________________ total ca . sup .++ treatment time released ( mean ) acid ( min ) ( microg . ) ______________________________________50 t 5 17250 t 15 9250 t d 5 12250 t d 15 1585 t 5 2265 t d 5 14850 % h . sub . 3 po . sub . 4 5 103465 % h . sub . 3 po . sub . 4 5 1518______________________________________ it will be seen from the above table ii that the calcium released by acid - etch is many multiples of the calcium released by the polyacrylic acid solutions used in this invention . the calcium release values correspond approximately to an etch depth of about 7 micrometers for the polyacrylic acid and approximately 64 micrometers for the phosphoric acid . solutions of polyacrylic acid of molecular weight 12 , 000 ( 12t ) containing at least 1 % so 4 = were used to treat surfaces of extracted human premolar teeth for four minutes followed by washing and drying . orthodontic brackets were bonded to the prepared tooth surfaces using two commercial orthodontic bonding resins . similar specimens were prepared using the acid - etch technique using 37 % orthophosphoric acid for 90 seconds . details of the specimens are reproduced in the following table iii : table iii______________________________________ toothtooth group conditioning bonding agent______________________________________1 20 % 12 t + 3 . 9 % so . sub . 4 adhesive a . sup . 12 20 % 12 t + 3 . 9 % so . sub . 4 adhesive b . sup . 23 40 % 12 t adhesive a4 40 % 12 t adhesive b5 37 % phosphoric acid adhesive a6 37 % phosphoric acid adhesive b______________________________________ notes : . sup . 1 adhesive a is a commercial orthodontic bonding resin sold under th trademark &# 34 ; orthomite iis &# 34 ; by rocky mountain orthodontics , colorado . sup . 2 adhesive b is a commercial orthodontic bonding resin sold under th trademark &# 34 ; autotach &# 34 ; by l . d . caulk co ., milford . after the specimens were stored at 37 ° c . and 100 % relative humidity for 24 hours , the tensile bond strengths were determined using an instron testing machine . the bond test results , in terms of the absolute force required to achieve fracture along the bracket - tooth interfacial bond and the force per unit area of bracket - tooth interfacial bond , are reproduced in the following table iv : table iv__________________________________________________________________________ fracture force coeff . of mean range variation bond strengthtooth group no . in group ( kg ) s . d . ( kg ) (%) kg / cm . sup . 2 psi__________________________________________________________________________1 10 7 . 84 1 . 81 5 . 7 to 11 . 1 23 . 08 25 . 13 359 . 22 10 10 . 64 3 . 12 5 . 8 to 16 . 0 29 . 32 34 . 1 487 . 73 10 5 . 67 1 . 08 4 . 5 to 7 . 2 19 . 05 18 . 17 259 . 84 10 4 . 33 0 . 92 3 . 0 to 5 . 7 21 . 25 13 . 88 198 . 55 10 11 . 11 1 . 64 7 . 8 to 12 . 6 14 . 76 35 . 6 509 . 26 10 14 . 38 1 . 91 11 . 3 to 17 . 1 13 . 28 46 . 09 659 . 1__________________________________________________________________________ the results of table iv show that the bond strength can be varied as desired depending on the conditions of formation of the crystal growth and can be made to approach that of the acid - etch technique . a solution of polyacrylic acid of molecular weight 12 , 000 ( 12t ) was prepared as described in example 1 and adjusted to a concentration of 35 %. the sulphate concentration was 3 . 7 %. this solution was used to treat the labial surfaces of bovine teeth and orthodontic attachments were bonded as described in example 2 . the bonded assemblies were stored in water at 37 ° c . for varying periods of time up to 4 weeks before testing the tensile bond strength . the results , in terms of the force per unit area of attachment - tooth interfacial bond required to achieve fracture , are reproduced in the following table v : table v______________________________________ bond strength treatment coefficient time bond - sam - oftreat - ( water ing ple mean variationment storage ) agent size ( kg / cm . sup . 2 ) s . d . (%) ______________________________________none 24 hrs . sup . a . sup . 1 25 21 . 7 16 . 8 79 . 7 ( con - . sup . b . sup . 2 28 18 . 7 16 . 6 88 . 8trol ) acid 24 hrs a 16 107 . 1 26 . 6 24 . 8etch b 25 117 . 0 26 . 0 22 . 212 t 24 hrs a 18 113 . 0 31 . 9 28 . 2 b 23 118 . 0 23 . 9 20 . 212 t 1 wk a 15 127 . 4 29 . 1 22 . 8 b 15 118 . 1 13 . 9 11 . 812 t 2 wk a 12 83 . 5 25 . 5 30 . 5 b 15 110 . 8 22 . 3 20 . 112 t 4 wk a 12 78 . 4 24 . 4 31 . 1 b 14 107 . 8 36 . 2 33 . 6acid 1 wk a 14 117 . 1 34 . 1 29 . 1etch b 15 130 . 3 20 . 6 15 . 8acid 2 wk a 12 100 . 9 26 . 4 26 . 1etch b 17 129 . 4 18 . 7 14 . 5acid 4 wk a 12 87 . 7 38 . 1 43 . 4etch b 15 112 . 2 18 . 3 16 . 3______________________________________ notes : . sup . 1 bonding agent a is a commercial orthodontic bonding resin sold under the trademark &# 34 ; orthomite iis &# 34 ; by rocky mountain orthodontics , colorado . sup . 2 bonding agent b is a commercial orthodontic bonding resin sold under the trademark &# 34 ; concise &# 34 ; by 3m company , minnesota the results of the above table v illustrate that comparable results for the crystal bond technique are achieved when compared to the acid etch technique even after 4 weeks of water storage . a bovine enamel surface was ground flat , washed and dried . the prepared surface was treated for 30 seconds with a 35 wt % solution of polyacrylic acid of molecular weight about 12 , 000 and containing 3 wt % of sulphate ion . the resulting crystal growth on the surface was washed and dried . an orthodontic bonding resin , namely that sold under the trademark &# 34 ; solotach &# 34 ; by l . d . caulk co ., milford , del ., was mixed and manipulated according to the manufacturer &# 39 ; s instructions and used to bond an orthodontic mesh based attachment to the prepared surface . after 24 hours storage in water at 37 ° c ., the breaking load in tension of the bond of the attachment to the tooth was determined using an instron universal testing machine at a crosshead rate of 0 . 5 cm / min and the tensile bond strength calculated . five samples were tested and the mean bond strength result calculated to be 104 kg / cm 2 , which is a very satisfactory result for the bonding of orthodontic attachments . the procedure of example 4 was repeated , except that a 2 . 6 wt % aqueous solution of sulphuric acid was used in place of the polyacrylic acid solution . crystal growth again occurred and the tensile bond strength of an orthodontic resin to the surface was determined in example 4 . a mean bond strength of 102 kg / cm 2 was found . the procedure of example 4 again was repeated , in this case using a viscous methyl cellulose solution containing the 2 . 6 wt % sulphuric acid . a mean tensile bond strength of 92 kg / cm 2 resulted . the procedure of example 4 was repeated , except that an aqueous methyl cellulose solution containing 1 wt % oxalic acid was used in place of the polyacrylic acid solution . crystal growth believed to be calcium oxalate occurred . the mean tensile bond strength of an orthodontic resin to the surface , determined as described in example 4 , was 22 kg / cm 2 . while this value is relatively low , it is satisfactory for application where temporary bonding only is required . the procedure of example 4 was repeated using a saturated aqueous solution of ammonium oxalate in place of the polyacrylic acid . the mean tensile bond strength for an orthodontic resin adhered to the resulting crystal growth was 44 kg / cm 2 . in summary of this disclosure , the present invention provides a new method of bonding to teeth and other calcified tissues which is a substantial improvement over the conventional acid - etch technique . modifications are possible within the scope of this invention .
bonding to calcified tissues , including human teeth , tooth dentine and bone , is achieved through the medium of crystal growth adhered to the tissue . the crystals preferably comprise gypsum crystals which are formed by contacting the tissue surface with a mildly acidic solution containing sulphate ions . an uncured liquid resin is applied to the crystal growth and is cured to achieve adhesion by micromechanical interlock with the crystal growth and by superficial penetration of the tissue surface .