Assessment of the effect of high-frequency loading on osteoporotic bone and on implant osseointegration

Summary

Postmenopausal osteoporosis is a serious and often underestimated disease. It occurs as a consequence of an imbalance between the bone formation and the bone resorption process thereby resulting in fragile bones that are porous in nature and susceptible to fractures. More than 40% of women are affected by this disease and as the population of aged people is increasing worldwide, this number is going to steadily grow in the coming years. From the patient’s point of view, debilitating fractures with slow healing rate resulting in loss of mobility often brings about a major deterioration in the quality of life. Hence, osteoporosis poses as a public health issue due to catastrophic effects and high predominance in the population. 

Anti-resorptive agents such as bisphosphonates, calcitonins, selective estrogen receptor modulators and estrogens are the common treatment options for osteoporosis. Their mechanism of action works on decreasing the bone resorption rate by inhibiting the osteoclastic activity and thereby prohibiting trabecular bone loss. Among the bisphosphonates alendronate is one of the most commonly used drugs to treat osteoporosis due to its low cost when compared with the other drugs on the market. It mainly utilizes its anti-resorptive property by interfering in apoptosis. However, the success of these anti-resorptive agents in osteoporosis depends on early diagnosis and the progression of the disease. Moreover, these treatments are not effective during later stages of the disease in which the fracture incidence is high. Unfortunately, although pharmaceutical interventions are the first choice of treatment, it has been seen that over the years long-term treatment results in severe side effects that are detrimental. Hence, there is a need to develop other novel agents that are non-toxic in nature as an effective treatment alternative for osteoporosis.

For this reason, the main purpose of this PhD was to study non-pharmaceutical treatment options for osteoporosis. In this aspect, mechanical signals arising from the mechanical stimulation of the bone has shown to be beneficial in increasing bone mass and quality, thereby paving the way for the introduction of biomechanical therapies for treating osteoporosis. Most notably, mechanical signals generated by high frequency (HF) loading have shown profound positive effects in improving bone strength and even recovering the bone mass that is lost as a result of osteoporosis. Whole body vibration (WBV) is one such in vivo system employing high-frequency loading that has been evidenced successfully and is used in this PhD to study its role as a possible mechanical treatment intervention for osteoporosis. Therefore, the main aim of this thesis was to assess the effect on the osteoporotic bone micro-environment when subjected to mechanical and pharmaceutical influences both on the jaw bone and long bone. How these interventions respond to and affect the bone remodelling pattern as well as the implant osseointegration process was the primary focus of the thesis.

Osteoporosis bone loss in the jaw has been of critical interest especially in the field of implant dentistry to check if it can be used as a predictor for early diagnosis of osteoporosis. It can offer to act as an early indicator for the onset of the disease and can be useful in devising effective treatment protocols. In this regard, the study on the micro-architectural changes in the jaw bones that occur during the prevalence of osteoporotic conditions is of special interest. Moreover, how bisphosphonates interact and affect the jaw bones had not been investigated. Therefore, in the first part of this thesis, the micro-structural characteristics of the osteoporotic jaw bone and its interaction in the presence on absence of alendronate was assessed. In order to do so, first a literature search on the most appropriate regions of interest susceptible to bone changes in the jaw bones was undertaken. The search yielded that the molar region of the maxilla, mandible, the maxillary tuber region and the mandibular condylar head with the subchondral region were the most affected by bone changes due to experimental interventions. Next, a gap was observed in identifying the exact methodological steps required to measure these regions of interest in a most standardized and reproducible manner. This is quite crucial in determining accurately the trabecular and cortical regions of the bone that changed due to the experimental interventions. Hence an appropriate methodology that is capable of measuring and quantifying accurately the different morphometric parameters from these regions of interest was devised. For the four anatomical areas of the jaw bone both, semi-automatic and manual methods were employed for the delineation of the bone. Image filtration/segmentation techniques along with the addition of well-defined landmarks and references for the positioning of the regions of interest were also included for easy measurements of these regions. Finally, by utilizing the methodology developed, it was possible to analyse the micro-architecture of the jaw bone and the effect of bisphosphonate treatment. The study observed that in the maxilla the trabecular bone density was compromised in osteoporotic conditions but was however able to recover due to the alendronate treatment. In the mandible, the trabecular bone surrounding the first and third molar and the condylar head were the most affected as a result of osteoporosis and once again alendronate treatment was able to improve the bone density.

With time the anabolic effects of HF WBV has gained popularity as a possible non-pharmaceutical treatment option for osteoporosis. It was interesting to examine its effect both individually or together with the bisphosphonate treatment. In this regard, the second part of the thesis focussed on studying the effect of HF WBV and ALN treatment on the osteoporotic long bone in vivo. The hypothesis was that this combined treatment could be synergetically anabolic on the osteoporotic long bone. The main findings from this study indicated that there was a differing response to this mechanical treatment observed relative to the different bone regions. The cortical region responded positively to the HF WBV stimulus in the osteoporotic condition over the trabecular regions. Furthermore, the synergistic effect that was expected did not occur. This inadequacy was anticipated to be due to the dosage of the alendronate which was probably too high and could have masked a potential synergistic response from being observed. Hence another study was initiated with 2 different doses of alendronate (2mg/kg and 1mg/kg) to make sure that the synergistic effect if present could be revealed, as these doses corresponded to moderate or reduced concentrations. However, the results of the study showed that once again no synergy could be observed. The reduced dose of alendronate fared better in the trabecular bone region when compared to the first study where effects were only seen in the cortical region. The effect of alendronate in spite of the reduced dosage still seemed to mask any potential effects of the HF WBV treatment. Thus, although HF-WBV improved the cortical and trabecular regions of the bone, no synergistic positive effects in response to bisphosphonates were observed on the osteoporotic bone. To further increase our understanding on the bone anabolic effects of HF WBV and alendronate treatment, it was important to understand what are the molecular and cellular reactions that occur in vitro contributing to this positive effect. In addition, it was also worthwhile to assess the effect of AMPK, which has recently been shown to be an attractive therapeutic target for osteoporosis. The findings from this study revealed that there was no osteogenic effect on the progenitor cells observed through HF WBV and alendronate treatment with the exception that the levels of RUNX2 were found to be decreased. It was speculated that there are other cellular/molecular factors that bring about the anabolic response observed in vivo such as AMPK which was not able to be observed in this in vitro study. Even though the in vitro study was unable to disclose novel cellular responses implicated in the bone response to mechanical stimuli, one can anticipate that there are other cellular targets involved worth investigating.

Osteoporosis has shown to be a risk factor that prevents favourable osseointegration. Hence, there is need for the development of optimal loading protocols to aid in successful implant integration especially in these compromised bone conditions. Therefore, the last part of the thesis was dedicated to investigate the effect of HF WBV on the peri-implant bone response in 3 conditions of the bone micro-environment namely healthy vs compromised vs pharmaceutically treated. The results from this study showed that indeed the peri-implant bone response to healing in the osteoporotic condition is compromised but mainly in the trabecular region of the bone compared to the cortical region. Regardless of the existing bone environment, both HF WBV as well as the alendronate treatment were able to counteract and impact the implant osseointegration process of the compromised bone condition in a positive manner.

Based on the different studies performed in the due course of this thesis our understanding on the specific bone micro-architectural, cellular and biomaterial integration level related to HF WBV treatment has been enhanced. Even though the current thesis highlights the influence of HF WBV and bisphosphonate treatment on the jaw bone, long bone and implant osseointegration, there are still questions unanswered which can serve as the basis for future studies in this field. Eventually, these results can pave the way to optimize clinical loading protocols in implant dentistry.