Straight proximal humeral nails are surrounded by more bone stock in comparison to bent nails in an experimental cadaveric study
© Günther et al.; licensee BioMed Central Ltd. 2014
Received: 13 February 2014
Accepted: 6 April 2014
Published: 22 April 2014
In the management of proximal humeral fractures intramedullary implants with bent and straight shape of the proximal part of nail are available. Based on data from previous studies on bone distribution in the humeral head, we hypothesized, that higher densities might exist in the bone stock surrounding straight nails in comparison to their angulated counterparts. With a known positive correlation between bone density and mechanical stability, this could indicate potentially higher rigidity of osteosyntheses done with straight implants.
We performed high resolution peripheral quantitative computed tomographies of the potential straight and bent implant bearing regions of 27 cadaveric proximal humeri. The acquired data were analyzed for differences between straight and bent Volumes of Interest as well as intra- and interindividual bone stock distribution.
For both straight and bent volumes of interest a considerably declining bone mineral density was found in craniocaudal direction. Mean densities of bent volumes were significantly lower in comparison to their corresponding straight counterparts (p < 0.01) Intra-individual comparison yielded high bivariate correlations of the corresponding Volumes of Interest of the right and the left side (p < 0.01).
Based on the volumetric data a statistically relevant biomechanical superiority of straight shaped implants can be assumed. Since we found a rapid decrease of bone density in cranio-caudal direction, intramedullary implants should be anchored as proximally in the subcortical area as possible to minimize the risk of displacement or cutout. The high correlation between the Volumes of Interest of the corresponding right and left sides could aid in preoperative planning when considering an intra- or extramedullary approach.
KeywordsBent vs. straight proximal humeral nails Bone stock Reduce risk of implant failure
Proximal humerus fractures are the second most common fracture of the upper extremity  with an incidence ranging age- and sex-dependent between 6 and 440 per 100.000 person-years [2–4] with an exponential increase after the 5th decade and a female predominance of 1.5:1 up to 3:1 [3, 4]. 15-40% of these fractures need to be addressed operatively [5, 6]. During the process of aging a substantial, systemic loss of bone mass and microstructural integrity occurs, concurring with an increasing fracture risk in general as well as locally in the area of the proximal humerus . Epidemiologic distribution of age and sex in patients with proximal humeral fractures which are typically low energy traumata suggest an at least partially relevant relation to osteoporosis. As early as 1947 Berndt described age and sex dependent differences of bone structure in proximal humeri based on radiologic findings . Barvencik et al. found lowered bone density in the lateral humeral head, which could be an explanation for the high incidence of avulsion fractures of the greater tubercle in postmenopausal women . The presence of osteoporotic bone stock impairs the outcome of osteosyntheses by means of insufficient implant fixation putting the patient at risk for subsequent revision procedures. While bone structure is essential for the stability of the bone-implant-construct , bone quality is an often neglected factor in preoperative planning. Good correlation between bone density and the mechanical stability of cancellous bone is well documented in different regions of the human and bovine skeleton [11–15]. Along with plates, the most commonly used implants in the management of proximal humerus fractures are intramedullary nails. These come in two variations: straight nails, which are inserted at the apex of the humeral head, offering the significant advantage of passing through the muscular portion of the supraspinatus tendon and thus doing less harm to the rotator cuff. On the other hand, insertion can be difficult since the direct insertion path is blocked by the acromion necessitating manipulation of the fracture zone. This lead to the development of proximally angulated nails, which facilitate insertion, but bear the risk of furtherly displacing fractures running through the greater tubercle or even causing iatrogenic fractures. However, for both implants one of the most important aspects is the primary stability of the osteosynthesis preventing implant failure, especially in the osteoprotic bone. While both implant designs are readily available and are widely used, it is unclear which of them offers higher stability. Since the screws and the distal part of the central rods are the common denominator of both designs, the main factor influencing anchoring stability should be the bone stock surrounding the proximal portion of the nails. Though the aforementioned studies already provide valuable data, they involve an arbitrary selection of the analyzed regions with little comparability to the realistic circumstances of intramedullary nails. In addition, the previously used radiologic techniques were either of a two-dimensional nature or lacked sufficient resolution rendering a detailed analysis of the regions or volumes of interest impossible. Thus the purpose of the current study was to gather high resolution quantitative data (bone volume vs. total volume, BV/TV) of the cancellous bone surrounding the cranial parts of proximal humeral nails, allowing estimates towards the mechanical properties of each design. These data should lead to a recommendation for straight or bent implants, contributing to the effort of achieving high surgical success rates. Based on previous studies we hypothesized that the bone stock decreases in craniocaudal direction. Also we expected the surroundings of the straight nails to have higher densities in comparison to the bent nails due to a known area of lower density and biomechanical stability around the greater tuberosity .
Twenty-seven humeral specimens (complete humeri, 14 male specimens, 13 female specimens, 13 corresponding pairs) were harvested from 14 post-mortem donors of our anatomical institute, immediately frozen and stored at −20°C. The overall mean age was 71.1 (SD ± 12.9; range 42–98) years. While the mean age of the male donors was 62.8 (SD ± 10.7; range 42–74) years, the female donors mean age was 78.9 (SD ± 9.6; range 68–98) years. Prior to the radiologic analyses the specimens were thawed for 24 hours at 4°C. No humeri with a donor history of cancer, diabetes, bone metabolism altering medication, severe liver or kidney disease, prolonged immobilization, signs of recent or earlier fractures or operative interventions as well as indications of malignancy were included. This trial conforms to the Helsinki Declaration and to local legislation. The ethics committee of the University Hospital of Munich (LMU) approved the trial.
The radiologic analysis was done using the high resolution peripheral quantitative computed tomography (HR-pQCT) imaging system (XtremeCT, Scanco Medical, Brüttisellen, Switzerland) which is equipped with a 70 μm focal spot. The x-ray tube was operated at 60 kVp and 900 μA. The integration time was set to 300 milliseconds. Two-dimensional CT images were acquired, and reformats in 1536 × 1536 pixel matrices from 750 projections using a standard convolution-backprojection procedure were performed. Images were stored in 3-dimensional arrays with an isotropic voxel size of 82 μm. For scanning, the proximal humerus was fixed horizontally with the lesser tubercle in a 12-o’clock position. The complete proximal end of the humerus was scanned, resulting in approximately 550 to 600 microtomographic slices, corresponding to a length of 50 mm. All scans, image processing and analysis were performed by the same person using the proprietary software package provided by the scanner manufacturer (Image Processing Language, Scanco Medical, Brüttisellen, Switzerland). A Gaussian filter with a sigma of 0.7 and support of 1 voxel was primarily used to suppress noise.
Selection of volumes of interest, segmentation, bone volume calculation
Statistic processing of data was done using SPSS Statistics (Version 17.0.0, SPSS Inc., Chicago, IL). Normal distribution was falsified for all groups with the exception of the cortical discs using Shapiro-Wilk- and Kolmogorov-Smirnov-tests with Lilliefors’ correction of significance. Thus, for subsequent analyses, we used non-parametric tests. The comparison of adjacent tubular quarters as well as the comparison of positional corresponding straight vs. bent quarters was done using Mann–Whitney-U-tests. For analysis of right vs. left sides and sex differences we used a Kruskal-Wallis-ANOVA. For side comparison we excluded the single unilateral humerus, reducing the resulting number of specimens of that test to n = 26 (13 pairs).
Intraindividual correlations between right and left humeri
All VOIs (1st-4th quarter)
In the literature both concordant [2, 3] and separate  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.
In conclusion, the present study demonstrates bone density distributions in the human humeral head which highlight regional differences in material quantity. These differences in turn suggest that the anchoring stability of proximal humeral nails may be significantly influenced by the shape of the implant. Isolating the bone stock around the nails, our data suggest that implants with straight design might offer higher biomechanical stability in comparison with bent versions. Since we found the highest BV/TV values for both straight and angled VOIs subcortically with a rapid decrease in craniocaudal direction, intramedullary implants, regardless of the design, should be anchored as proximally as possible to minimize risk of osteosynthesis failure. We found high correlations between VOIs of corresponding right and left sides, which could aid in preoperative planning when considering an intra- or extramedullary approach. An analysis of the local 3-dimensional trabecular structures could further aid to optimize implant design and positioning as well as predicting the risk of osteosynthesis failure.
Quantitative computed tomography
Peripheral quantiativ computed tomography
Volume of interest
Analysis of variance
Statistical package for the social sciences.
The acquisition of the data was performed by Sven Duda at the AO Research Institute Davos (Davos, Switzerland). None of the authors received any funding for the trial or the preparation of the manuscript.
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