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Femoral metaphyseal/diaphyseal blood flow was studied by 85Sr clearance in unfractured segments of dog femurs following compression plating and various control procedures (drilling with and without application of screws). The animals were tested as early as seven days and as late as two months after surgery. The only statistically significant change in strontium clearance (SrC) was an increase produced by the application of stainless-steel bone screws, with or without plates. The presence or absence of compression had no consistent effect on changes in the rate of SrC. During the period of study, no changes in bone mineralization, haversian porosity, or cortical bone thickness accompanied the SrC alterations.

The effects of each of the surgical stages involved in compression plating on the development of cortical thinning and porosity were assessed in the intact midshaft, stress-shielded femoral segments of adult mongrel dogs 6 months postoperatively. The data were evaluated in terms of a postsurgical tetracycline-based measure of remodeling and terminal 85Sr clearance (SrC) values for the plated segments of bone. Drilling had no effect on any parameter. Screw application was associated with minimal cortical thinning (p less than 0.05), while plate fixation clearly promoted thinning (p less than 0.01) and porosity (p less than 0.05). The percentage of labeled osteons, a measure of remodeling activity, increased only after plate fixation (p less than 0.05), and the labeling patterns suggested that most osteons had formed during the first 4 postsurgical months. That none of these changes were correlated with the 6-month SrC values suggests that the development of plate-induced osteopenia involves disparate histomorphometric time constants, rather than lack of any association.

We examined the effect of rigid plate application on the radiostrontium clearance of the intact canine femur at 6 months. We examined each of the component surgical steps. We calculated the clearance both for the whole bone and for each of the five transverse sections of the whole bone. Screw application, but not drilling, increased the clearance in the segment about the screw holes. Plate application produced an increase in the segment beneath the plate as well as around the screws and in the whole bone. These changes are accomplished not only by an absolute increase in clearance to the middle three segments, but by a relative diminution in clearance by the most proximal and distal segments. The histomorphometric changes in long bones following rigid plating may be accompanied by regional increases in bone blood flow.

The effects of rigid self-compressing plates were compared with the effects of less rigid internal contact splints on the healing characteristics of osteotomized canine femora. Radiographically, the rigid fixation afforded by the compression plate produced accelerated healing as compared with the less rigid fixation obtained with splints at two to four months. At six months there was little or no difference between the two groups. Comparison of the material biomechanical properties of the healing femora suggested that the repair tissue formed early after compression plating was stronger than the callus tissue that surrounded the less rigidly fixed femora. A comparison of the structural biomechanical properties of the bones suggested, conversely, that a stronger whole bone resulted from the less rigid fixation splint. The histomorphometric data showed that some cortical bone was lost under both types of fixation plates. However, the patterns of loss were not comparable. Use of rigid internal fixation led to loss of cortical bone after four to six months, while the less rigid fixation obtained with the Eggers splint produced mild intracortical resorption only. These observations point to the desirability of rigid internal fixation early in the healing process and the possible benefit of decreasing the rigidity of the fixation device as healing proceeds.