Sex differences in cortical bone of humeral head region


 Background

Proximal humerus fractures are highly related to age, and are particularly high in women. The goal of this study was to investigate sex-related similarities and differences in humeral head structure.

Methods

Computed tomography (CT) scans of 147 healthy subjects (76 men /71 women) spanning a wide range of ages (20-88 years) were analyzed. Three groups of subjects were identified according to age: group A (aged 20-39 years), group B (aged 40-59 years), and group C (aged> 60 years). For each subject, cortical bone mapping (CBM) was applied to create color 3D thickness map for the proximal humerus. Regions of interest (ROI) were defined in three walls of humeral head which corresponded to the anterior, lateral and posterior aspects. Cortical parameters, including the cortical thickness (CTh), cortical mass surface density (CM), and the endocortical trabecular density (ECTD) were measured using CBM.

Results

In both sexes, the CTh value of proximal ROIs in lateral and anterior wall tended to decrease with age. No significant difference was detected between three age groups for each parameter in the posterior wall in men and women. In ROIs 1-9, the ECTD tended to show a notable decline with age in women. In contrast, the ECTD remained unchanged in men. When pooled across all decades, the CTh, CM and ECTD values of ROI 1-9 were approximately equivalent between men and women.

Conclusions

There was no sex difference in cortical bone parameters in the humeral head region. However, the endocortical trabecular density decreased significantly with age only in women.


Introduction
Proximal humerus fractures are the third frequent fragility fracture type in elderly patients, after wrist and hip fractures [1][2][3]. Although proximal humerus fractures recently increasing and associated with excessive mortality [3], the structural basis of the fracture remains unclear. Previous studies have concentrated on age-related changes of trabecular microstructure [4,5]. However, cortical bone makes up approximately 80% of the skeletal mass of an adult, particularly at appendicular sites where the cortex accounts for the majority of axial load transfer [6,7]. In the femoral neck, complete removal of trabecular bone leads to an only 7% reduction in load needed to fracture [7].Recent studies on radius, femur and humerus had found the vast majority of bone loss is cortical, not trabecular, and occurs age over 65 years by intracortical remodeling [8,9]. Marked thinning and high porosity of femoral neck cortex with advancing age had been shown to result in increased fracture incidence [10][11][12][13]. Structural property of cortical bone is a key element of bone strength, although trabecular bone may be the first site to display obvious microarchitectural weakness [8][9][10][11][12][13].
Similar to other primarily osteoporotic fractures, women have higher risk of low-energy fractures of the proximal humerus than men. It has been postulated that the sexual dimorphism was associated with smaller bone size, lower bone mass or to the greater bone loss in women [12,13]. There is steadily increasing evidence that focal weaknesses of cortical bone are linked to hip fracture [15][16][17][18][19][20].
Cortical thinning in superior surface of the mid-femoral neck was important in determining resistance to femoral neck fracture [13][14][15]. Focal differences in cortical bone properties were related to several factors, including age [14,15], drug treatment [17,18] and exercise regimes [19,20]. Meanwhile, superior femoral neck geometry between genders was significantly different [13,21]. The cortical thickness of super-anterior region in femoral neck was 27.4% lower in Chinese women, compared to men [21]. In a longitudinal study, the elderly women had found to lost thickness, total volumetric bone mineral density (vBMD) and cortical vBMD more rapidly than elderly men over a median followup of 5.1 years, especially in the superior region of femoral neck [13]. Those finding suggested that sex might play a significant role in the relationship between spatial bone properties and hip fracture susceptibility.
We previously found that cortical bone distribution is asymmetric in the humeral head region [22].
However, no data are available for sex differences in the spatial distribution of cortical bone in proximal humerus, one of most common site of osteoporotic fracture. The aim of this study was to examine sex-related similarities and difference of specific regions of the humeral head region using cortical bone mapping (CBM).

Subjects and study design
Our analytical sample included 147 healthy subjects, with the goal of studying sex-related variations in cortical bone of humeral head region in the dominant upper extremity. Data were acquired as part of the aging and osteoporotic proximal humerus fracture Study, a single-center prospective ongoing population study of Chinese men and women. The early part of study has been reported [22]. The subjects consisted of 71 men and 76 women, with range from 20 to 88 years (mean, 49.79 years).
Arm dominance was determined as the arm with which subjects would throw a ball. Subjects with a history of or evidence of metabolic bone disease, as well as those receiving chronic treatment that may affect bone metabolism were excluded from the study. Three groups of volunteers were The cortical parameter measurement and mapping technique has been previously described [14,15].
Cortical thickness measurement was performed using CBM, implemented by a freely available inhouse program called Stradwin (http://mi.eng.cam.ac.uk/~rwp/stradwin/). First, an approximate segmentation of each proximal humerus from the CT data was performed using Stradwin, and results in a triangulated surface mesh with ~10 4 vertices distributed uniformly over the proximal humerus surface. Secondly, the CT data was sampled at each vertex of the mesh using 18 mm lines perpendicular to and passing through the humeral cortex and trabeculae. Finally, a model that accounts for the imaging blur was fitted to the data samples. This validated model-based deconvolution process allows the measurement of much smaller features than would normally be visible in the CT data. This process was repeated at all vertices. As a result, color maps on the proximal humerus were created for accurately estimating the cortical thickness (CTh, mm) and cortical mass surface density (CM, the cortical mass per unit cortical surface area, HUmm), as well as endocortical trabecular density (ECTD, HU). ECTD is the average trabecular density immediately adjacent to the cortex, to a depth of about 8 mm, excluding any effects due to blurring of the endocortical edge.

Definition of the regions of interest for cortical bone distribution assessment
For the evaluation of the bone morphometric analysis, specific regions of interest (ROIs) were defined within the proximal end of the humerus. The specific methodology has been described in detail previously and will be briefly outlined here [22].The cortical bone in humeral head region was defined as anterior, lateral and posterior wall. In anatomical perspective, the anterior wall is equivalent to the less tuberosity. The lateral and posterior part of greater tuberosity corresponds to lateral and posterior wall. After creation of a single 3D thickness map, the humeral head height was determined by measuring the distance between the highest point of the humeral head and the most distal margin of the articular surface (Fig. 1). The height of humeral head was then quartered by parallel plane 1-3.
In each slice, to obtain more details of cortical bone tissue, the longest line (Line 1) between the joint surface and greater tuberosity was drawn; this line was divided into a medial and a lateral segment by line 2 (Fig.2). The intersection of the above two lines with cortical shell in each plane was the 6 measurement point (ROI 1-9) (Fig.3).

Statistical Analysis
The age group difference within the same gender was compared using one-way ANOVA for normally distributed values and Kruskal-Wallis Test for non-normally distributed values. Gender and age differences were evaluated using generalized linear models (GLMs). Correlation between cortical parameters and age in ROI 1-9 was studied by linear regression analysis. All statistical analysis was performed using IBM SPSS Statistics for Windows version 20.0 (IBM SPSS Inc., Chicago, IL).
Significance level was set at P<0.05 for all statistical tests.

Results
In the lateral wall, the CTh and CM value of ROI 1-3 tended to decrease with age in men, whereas the age-related decrement was seen only in ROI 1 in women. In the anterior wall, the CTh of ROI4 appeared to have a significantly negative association with age in both sexes, whereas no significant relationship was observed in ROI5-6. In the posterior wall, no statistically significant difference was detected between three age groups for each parameter in both sexes (Table.1).
In ROI1-9, the ECTD tended to show a notable decline with age in women, although the value in ROI 4 and 8 didn't reach the significant level. But in contrast, the ECTD remained unchanged in men (Table.1).
When pooled across all decades, the CTh, CM and ECTD values of ROI 1-9 were approximately equivalent between men and women. Moreover, cortical parameters in ROIs tended to decline with age, except the CTh and CM in ROI7-9, ECTD in ROI4 (Tab.2, Fig. 4). We also found that age-sex interaction was not significant in cortical parameters of ROIs in regression models (Tab.2).

Discussion
This study presented the sex differences in cortical thickness, cortical mass surface density and endocortical trabecular density in specific regions of the humeral head region measured in a Chinese cohort by CBM technique. Our principal findings are (1) cortical parameters in ROIs were approximately equivalent between men and women. (2) the endocortical trabecular density in ROIs was found to decrease with age in women, but not men; (3) the CTh of proximal ROIs of lateral and anterior wall was significantly associated with age in both sexes; (4) CTh and CM value of cortex in posterior wall didn't change with advancing age in both men and women.
Cortical bone is integral to the mechanical strength of bone, with good correlation between bone density and compressive strength [7,8,22]. Our early regional analysis revealed that cortical structure of the humeral head region varied dramatically in cross-section, with focal cortical thinning in the posterior wall [22]. Fractures occur when mechanical stress is disproportionately concentrated on a portion of the bone relative to the surrounding area. Majed found that simple head split or threepart fracture pattern emanated from the greater tuberosity in a biomechanical testing [23]. Hasan analyzed the common location and orientation of the proximal humeral fracture lines using three dimension computed tomography. They found that fractures frequently split the greater tuberosity and were closely related to the intervals of the rotator cuff attachments [24]. Combined with these data, we believed that cortical thinning in the posterior wall may be the starting point for a cascade of changes that ultimately culminate in proximal humeral fractures. Previous studies had mainly focused on identify spatial differences in proximal femoral vBMD and cortical bone properties [13,21]. The thumbnail-sized patches of cortical thinning in the superior femoral neck were found to be associated with femoral neck fracture in vivo. Substantially thinned cortex was more pronounced in majority of elderly women, compared to men [12,13,25]. However, no sex difference was detected in the posterior wall regarding cortical parameters in our study. A possible explanation for this discrepancy is that mechanical environment of the humeral head is different from that of femoral neck.
Noticeably, our results indicated the endocortical trabecular density appeared to be negatively associated with age in women while the ECTD remained unchanged in men. These findings were in agreement with earlier histomorphometric analysis [4]. The pronounced decrease of density of trabecular bone adjacent to cortical bone in posterior wall points to a structural weakness, which can partly explain the higher incidence of fractures of the proximal humerus in women.
Age, osteoporosis, inactivity or pathology of the shoulder joint may influence cortical bone quality.
Proximal humerus fragility in old age is usually attributed to age-related bone loss [4,9]. However little is known about the cortical changes in specific regions of the humeral head. In this study, we 8 found that the CTh of proximal ROIs of lateral and anterior wall was significantly associated with age in men and women, but not cortex in posterior wall. The results showed significant locationdependence in bone loss with aging. Anatomically, the supraspinatus, infraspinatus, and teres minor muscles attach to the greater tuberosity and the subscapularis tendon attaches to the lesser tuberosity. Our results lead to the conclusion that age-related bone loss of proximal portion of lateral and anterior wall is not entirely prevented by routine rotator cuff activity.
Our study has several limitations. The most obvious limitation is the cross-sectional nature of the study which limits the ability to reflect age-related changes in bone geometry. A longitudinal study or randomized controlled trial (RCT) is needed to predict changes in proximal humerus bone health. Second, we have evaluated age-and gender-related cortical bone effects in Chinese cohort. The current data are not directly translatable to individuals of other racial or ethnic backgrounds since previous data suggesting structural differences of the proximal femur between Asian and other ethnicities [16,21,26]. Finally, microarchitectural changes of cortical bone in humeral head region could not been analyzed in the study. Some authors recently reported that cortical porosity and thickness has significant impact on bone loss and mechanical stability [8,9,11]. So our data were limited in predicting the risk of proximal humeral fractures.

Perspectives And Significance Section
In this study, no sex difference was found in cortical bone parameters in the humeral head region.
However, the endocortical trabecular bone decreased significantly with age only in women. Cortical thickness in proximal ROIs of lateral and anterior wall, as measured by CBM, exhibits significant agerelated decrease in both sexes. Obvious reduction of ECTD value in posterior wall with aging in women may contribute materially to greater risk of proximal humerus fracture in elderly women than men.

Ethics Approval And Consent To Participate
Institutional Review Board (IRB) approval was obtained from the ethics committee of Tianjin Hospital, and signed informed consent form was obtained from the participants prior to participation. All the participants were admitted to Tianjin hospital due to shoulder trauma. CT examination was performed 9 in the emergency department to exclude minor fractures.

Availability of data and materials
The datasets generated and analyzed during the current study are available in the tables as provided in this manuscript. Any additional information on the data can be requested from the corresponding author.

Author contributions
Yeming wang designed the study and prepared the first draft of the paper. He is guarantor. Jian li and Jianhua Yang contributed to the experimental work. Jingming dong was responsible for statistical analysis of the data. All authors revised the paper critically for intellectual content and approved the final version. All authors agree to be accountable for the work and to ensure that any questions relating to the accuracy and integrity of the paper are investigated and properly resolved.