One of the foremost findings is the significantly lower bone volume fraction of the bent tubular quarters which reach only 11-57% of their corresponding straight counterparts. Though to the best of our knowledge no comparable study exists for the humeral head in terms of resolution and specificity of the volumes of interest, our results in general compare favorably to the literature [
9,
17,
18]. With a strong correlation between radiologic bone densities and mechanical properties of cancellous bone documented in the literature [
19], these findings could be interpreted as an indication of mechanical superiority of straight implants. Another considerable advantage of straight nails is their more medially located entry point, allowing them to penetrate though the muscular, well vascularized portion of supraspinatus theoretically reducing the rates of shoulder pain. Also, there have been reports about iatrogenic extension of fracture zones and generation of avulsion fractures of the greater tubercle in the insertion area of bent intramedullary systems [
16,
20,
21] as well as the risk of further dislocation in the process of bent nail insertion [
20,
22]. The intraregional analysis revealed a rapid decline in bone mineral density moving from cranial to caudal regions. While the decrease is continuous for the straight volumes, the most distal bent quarter shows a slight increase for both sexes. Though this could be attributed to outliers, there is also the possibility of an interference of the bent distal volumes with a persisting epiphyseal growth plate scar [
23] which consists of more dense bone tissue than the rest of the humeral head. The culmination of bone density in subcortical areas puts emphasis on correct positioning of an intramedullary nail. While too proximal positioning bears the risk of irritation or even rupture of the rotator cuff, placing the implant too low could cause disadvantages regarding the resulting stability of the osteosynthesis [
24‐
26]. As we expected, we encountered significantly higher densities for male donors in comparison to females for both bent and straight designs looking at all four subvolumes together. Separate analyses of individual subvolumes yielded significance for the proximal three quarters of the straight, but only the most proximal of the bent VOIs, possibly caused by very low absolute densities in that area. Through greater diameters of the proximal implant part, the bone-metal interface area could be increased, allowing for better distribution of transmitted forces and thus better stability of the osteosynthesis itself. On the other hand bigger implants reduce the biologic fraction of the construct, increasing the risk for avascular necrosis and infection. Preoperatively the surgeon has to choose an adequate implant based on the fracture type and quality of bone. Since the ipsilateral side is compromised in structure and there is no way to reliably measure the local bone density the surgeon has to resort to a comparable area. Total VOIs show high correlation regarding sides for both bent and straight designs (see Table
1), suggesting one would very well be able measure the uninjured side by means of pQCT, facilitating the choice between an intra- vs. an extramedullary approach. Of course, several other factors such as variations in different manufacurers’ implant design and the fact, that modern implants offer an increasing number of customizable locking options, have an impact on the biomechanical stability of the osteosynthesis. Quantity, type and placement of the locking screws or the presence of a locking blade generate a multitude of scenarios. The present study focuses on the isolated situation around the central core of the implant. Subsequent biomechanical testing of the specimens used in the study could give additional information about the stability of the osteosynthesis and allow for correlation of the BV/TV values and failure loads. Also, finite element modelling based on the HR-pQCT images could be used to simulate the bone – implant interaction, with diverse nails. Previous studies have indicated that higher predictive strength regarding stability of cancellous bone can be obtained taking parameters of trabecular microstructure like direction, girth, quantity and interconnectivity into consideration [
27,
28]. As a consequence a three dimensional morphometric analysis of the respective parameters using the present data sets should follow. In addition, an even more realistic estimation of the bone implant interface region in humeral heads could be achieved taking the surrounding of the locking screws into the equation.
Table 1
Intraindividual correlations between right and left humeri
All VOIs (1st-4th quarter) | 0,923** | 0,919** |
1st quarter | 0,885** | 0,539 |
2nd quarter | 0,918** | 0,631* |
3rd quarter | 0,953** | 0,771** |
4th quarter | 0,888** | 0,825** |
In the literature both concordant [
2,
3] and separate [
16] age peaks for the incidence of proximal humerus fractures have been reported. However, interpreting the results of our study, one has to take into account, that the male donors in the sample are considerably younger than the female ones. Also the small group sizes of 14 male and 13 female donors should be considered. For statistical workup of the volumes of interest an arbitrary segmentation of the entire VOI is necessary. Increasing the number of sub-VOIs can provide a better understanding of the local distribution of trabecular bone. However time and effort consumed by the analysis increase along with the overall complexity of a higher number of subvolumes. While the resolution of the HR-pQCT scanner is one of the best available today, the overall accuracy of the segmentation is limited by its absolute spatial resolution. Since information on handedness and individual activities of daily life was not available for the specimens used in this study, we cannot rule out possible influence of these parameters. Though a direct correlation exists between radiologic features, bone mineral density and mechanical stability of a osteosynthesis construct, it would be premature to disregard other factors such as general condition of the patient, comorbidities, medication, preexisting level of activity, fracture morphology, quality of reduction, soft tissue damage, form and intensity of forces acting on the osteosynthesis and postoperative rehabilitation regime.