Comments to the Letter to the Editor of S. Shahsavari et al. concerning “Predicting medical complications in spine surgery: evaluation of a novel online risk calculator” by M. F. Kasparek et al. (Eur Spine J: doi:10.1007/s00586-018-5707-9) and the reply to the Letter to the Editor of S. Shahsavari et al. concerning “Predicting medical complications in spine surger y: evaluation of a novel online risk calculator” by M. F. Kasparek et al. (Eur Spine J: doi:10.1007/s00586-018-5707-9)