Aims: The shape and the structure of cortical bone of the diaphysis is the result of the continual resorption/ replacement process where the two phases have well regulated temporal sequence and spatial localization. Different methods can be employed to measure the appositional growth but their possible application to structural studies has not been so far considered.

The broad interest of this study was addressed to the mechanisms which control the structural modelling of cortical bone in the course of the long growth and development, therefore a morphometric evaluation appeared the more suitable method for the possibility to examine large segments of the bone.

Methods: The study was carried out on the femurs of four male New Zealand white rabbits. The left femurs were prepared with the techniques for undecalcified bone and studied in incedent fluorescent light. The right femurs were decalcified, prepared in sections and studied in bright field, in polarized light and in phase contrast.

Ditigital microscope images were analyzed utilizing the software Cell D: the cortical area was measured and the number of vascular canals was counted and expressed as a function of the cortical area (n/mm²). The total cortical area, the density of vascular canals and the frequency distribution for area classes in the cortex of mid-shaft and distal-shaft was compared with paired student t test and Pearson chi-square test respectively.

Results and Conclusions: The canals distribution for area classes showed a significant prevalence for actually structuring osteons in the distal shaft: these data demonstrate the higher rate of bone remodelling of the most recent apposed bone at the extremities of the shaft. On the contrary there were not significant differences among sectors at levels of the mid-shaft and distal-shaft.

A stiff prosthetic stem in the proximal femur alters the load-bearing model af the bone and leads to changes of the lamellar organization, known as “stress shielding”.

To avoid this problem in the 70th a stem with a higher elastic modulus was developed and implanted in patiens (isoelastic stem of Mathys and Morscher).

A group of 17 isoelastic stems of this type which have remained in the femur from 6 to 8 years, were removed and are the object of the study. Analysis of the polyacetile surface was carried out with a low enlargement microscope. The stems were then radiographed and cut with a low speed saw to investigate structural changes between the polyacetile cover and the steinless steel core.

Two type of material lesions were observed:

- loss of polyacetile material from the surface (wear)

Distribution and frequency of each type of lesion was registerd in relation to four zones of the stem: a) cone with the metallic head; b) proximal stem (metaphyseal); c) body of the stem (diaphyseal); a)apex of the stem.

Wear of the polyacetile stem is evidence that no fixation of the stem was achieved, therefore a fraction of the load energy was dissipated at the prosthesis-endosteal surface.

However the presence of structural yielding lesions demonstrate that another fraction of the load energy was carried out by the stem. The topography of these lesions allowed to understand the biomechanical behavior of the “isoelastic” stem inside the femur with vertical longitudinal (piston) and rotational in the horizontal plane movements.

Failure of polyacetile and decoupling of polyacetile cover and stainless steel core were the other main findings.

The theory of an isoelastic stem is appealing by a biomechanical point of view, however two practical problems caused failure in this series: 1) lack of fixation at the implant-bone interface; 2) failure of materials.