JOURNAL OF MATERIALS SCIENCE:MATERIALS IN MEDICINE 7 (I996) 207-213 A quantitative technique for comparing synthetic porous hydroxyapatite structures and cancellous bone K. O'KELLY, D. TANCRED, B. McCORMACK, A. CARR Department of Mechanical Engineering, University College Dublin, Ireland There are many different materials currently available for cancellous bone grafting. There is however, very little information relating the morphology of these materials to cancellous bone. Work was undertaken to develop a quantitative method for comparing synthetic hydroxyapatite bone structures with cancellous bone. The bases for comparison were mean plate thickness, mean plate distance, mineral area fraction, mineral volume fraction and plate orientation coupled with mechanical tests. The aim of this work was to develop a protocol for assessing whether these critical parameters which influence the success of bone implants were achieved in the synthetic materials. The methodology is successful in providing quantitative information about the mineral area fraction, the mean plate distance or pore size and the intercept frequency as a function of angle. Combining these three provides a quantitative measure of how much mineral there is and how it is distributed and oriented. The mechanical tests yield strengths and moduli values based on apparent density. The results of the mechanical tests can also be plotted as functions of the more discrete structural features such as those quantified in the image analysis to allow for even more equitable and systematic comparisons of different porous materials. 1. Introduction It has long been accepted that cancellous bone micro- structure has a profound influence on the rate of in- growth into bone grafts. Similarly, material composi- tion also plays a significant role in the process [1-7]. Thus most research into bone grafting has focused on duplicating the structure and material of cancellous bone using natural and synthetic structures. For all such materials, it is necessary to quantify the structural, material and mechanical properties so that comparisons can be made to natural bone for selection purposes. DeHoff [8] classified the geometric properties of three-dimensional structures into: Class I-standard stereological properties, estimated without geometric assumptions, Class II-properties that require geo- metric assumptions for their estimation and Class III-properties that cannot be estimated stereologi- cally. Although Class I measurements are the most straightforward, they provide information that is too specific for the purposes of classifying cancellous bone and grafting materials by key structural features. The Class I measurements would yield an enormous num- ber of types when applied to the range of architectures. These would have to be rationalized and generalized to yield a manageable group of structural families. Class II properties do compromise the specificity of Class I measurements by requiring some geometric assumptions to be made but are far more useful when trying to classify an almost infinite number of bone structures. Class III properties are almost impossible 0957-4530 0 ]996 Chapman & Hall to quantify and so are used almost exclusively for qualitative descriptions. There has been a great deal of work done in trying to identify key structural features that sufficiently dif- ferentiate the types of cancellous bone. The ap- proaches have used all three types of geometric prop- erties and have included porous block models [9], node-strut models [-10, 11] and star volume models [12]. Singh [13] classified cancellous bone into seven types based on shapes, thicknesses and orientations of the rods and plates. The qualitative nature of these classifications is useful in making general comparisons between cancellous bone in different loci but makes it difficult to compare structures of different materials. Other methods include using three-dimensional con- nectivity (Euler number/tissue volume) [14]; the tra- .becular bone channel density (no. of channels/cm 2 of tissue area) [5]; the ratio of nodes to free ends and lengths of different strut types expressed as a percent- age of total strut lengths [15]; a probability distribu- tion function of porosity and an autocorrelation func- tion describing the probability of a point being part of the matrix [16]. Most work, however, has centred on the Class II type properties of bone: mean trabecular plate thick- ness, mean trabecular plate distance or mean pore size, bone area fraction, bone volume fraction and principal axes or orientation. The distinguishing as- pect of research in this area has been the methods used to measure and derive these quantities [7,17-19]. 207