Several lines of preclinical evidence suggest that estrogen plays an essential physiological role during bone development and the bone remodeling process [4]. The administration of estrogen in a sustained way was shown to be able to reverse the decline in bone strength and to re-establish bone quality in rats [8, 24, 25]. Furthermore, long-term ovariectomized rats showed impaired fracture healing [26]. Moreover, histomorphological analyses demonstrated a delay in fracture callus healing with poor development of mature bone in these animals [27]. These findings suggested a possible role of the hormone in bone fracture healing, as the remodeling of the fracture callus is essential during this process [4, 28]. It has also been demonstrated that cells involved in fracture healing such as osteoblasts, osteocytes and osteoclasts express functional estrogen receptors, ER-alpha and ER-beta, and that ER alpha supports callus formation and thus fracture healing [29]. These receptors are also expressed in bone marrow stromal cells, the precursors of osteoblasts. In addition, Boden et al. (1989) [30] reported that rat estrogen receptor genes are expressed in fracture callus, implying that estrogen could play a role in the normal fracture healing process. Finally, it was shown in a mouse model that estrogen plays a crucial role in fracture healing [4], and that estrogen may enhance the fracture healing process in animal models such as rabbits [9]. Collectively, these findings suggested that estrogen treatment may benefit postmenopausal patients with osteoporotic fractures [31].
However, there are reports on severe side effects of estrogen treatment of human. High doses of estrogen can cause incident coronary heart disease, stroke, venous thromboembolism, breast cancer, colorectal cancer, endometrial cancer or hip fractures [32, 33, 34]. It has recently been shown that estrogen-alone hormone therapy does not increase the risk of breast cancer in postmenopausal women as outlined in an updated report of the Women's Health Initiative (WHI) estrogen-alone trial [35]. Doses of 0.625 mg/day of conjugated equine estrogen (CEE) were applied to 10,739 post hysterectomy women showing that they caused other adverse reactions [36]. This dose was about twice as high as applied in the animal model organisms where beneficial effects on bone fracture healing were reported, but as observed in humans, strong side effects were also observed [4].
Studies analyzing the dose responses of estrogen with respect to bone healing, its dose-dependent side effects and clinical studies with lower doses of estrogen on post hysterectomy women have not yet been reported. To avoid any potential estrogen-dependent complications [32, 33, 34] in our study, we used a sufficiently low estrogen dose to reach the endogenous systemic level of estrogen equivalent to the level in premenopausal women. This was achieved by applying a weekly transdermal dose of 4 mg estradiol over a period of four weeks. This treatment increased the endogenous estrogen level in the blood of the patients from about 16 ng/ml to a blood concentration of ≤ 30 ng/ml estrogen, corresponding to the endogenous estrogen level of women during the first half of the menopause cycle [37].
Our findings showing that there is no statistically significant beneficial effect of estrogen treatment on bone fracture healing are in apparent contrast to observations with preclinical models. They show that estrogen administration, although at much higher doses however, strengthens indeed the properties of bones of ovariectomized animals after femoral fracture [4, 8, 9, 11, 12, 13, 14, 38]. However, there are no studies yet which determine the minimal dose of estrogen that causes a beneficial effect on bone healing in the preclinical model organisms without producing the known side effects of estrogen. It could well be that a high estrogen dose as applied to the preclinical models would also have beneficial effects on human bone fractures, but causes otherwise adverse side reactions. It is noteworthy that mechanostimulation by low-magnitude high frequency vibration was shown to provoke anabolic effects on the intact skeleton of both mice and human. This effect is estrogen-dependent and shown to be mediated primarily via the estrogen receptor alpha in mice, and vibration-induced effects on fracture healing in combination with estrogen treatment have also been shown to improve the osteopenic bone structure and increased the bone stiffness in preclinical studies [14, 39, 40, 41]. Thus, it is possible that such a combination of treatments could improve the healing of bone fractures rather than estrogen treatment alone.
We observed that callus formation after bone fracture was age-dependent. The CALCBD levels decreased with the age of the patients, i.e. the on average 60-year-old patients had a CALCBD level of 250 mg/cm3, whereas the CALCBD level of 90-year-old patients was about 20% lower. This finding suggests a tendency for the reduction of callus formation with increasing age.