B2O3/SiO2 substitution in 55S4.3 bioactive glasses with 5 mol% of SrO has been synthesized and characterized to understand their structure and bioactivity as a function of composition by combining experimental and computer simulation techniques. Raman spectrometry, X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR) were utilized to characterize the structural changes induced by boron content and to identify the formation of hydroxyapatite (HAp). In vitro bioactivity tests were performed in simulated body fluid with a fixed glass mass to solution volume ratio and a particle size range. Needle-like HAp was found to form on the surface of the 55S4.3 with SrO sample from scanning electron microscopy and confirmed from XRD and FTIR. In addition to the experimental efforts, these glasses were also simulated using classical molecular dynamics simulations with partial charge potentials and recently developed parameters for boron oxide to understand their short- and medium-range structures. The glasses from simulations were analyzed in terms of the local structure around the glass network formers, especially the boron coordination number, and found to agree well with theoretical models. The medium-range structural information such as Q(n) distribution and network connectivity was also obtained and used to understand the compositional dependence of property and bioactivity. The results show that additional boron oxide increased the network connectivity of the 55S4.3 glass and inhibited or delayed the formation of HAp in vitro.