We read with interest the paper by Mourão et al.  assessing dental injury after conventional direct laryngoscopy, in which they found a high incidence of dental injury (134/536, 25%) when the patient is examined carefully postoperatively. This study is an important contribution in our efforts to define the frequency and risk factors for peri-operative dental injury. However, in our view, there are several aspects of this study that should be discussed and clarified.
First, oral examination to determine a tooth injury was performed between 12 and 36 hours following anaesthesia and all dental injuries were attributed solely to direct laryngoscopy performed to aid tracheal intubation. Although direct laryngoscopy is a common cause of peri-operative dental injury, insertion, manipulation or removal of almost any airway, bite block or suction device can also result in dental injury . For example, oral airways are designed to lie in the midline so that if a patient bites on one, the forces will tend to push the anterior teeth forward and may dislodge or fracture them . Similarly, biting and grinding of teeth during anaesthesia recovery may cause dental injuries. A retrospective study including 598 904 patients showed that the incidence of dental injuries associated with anaesthesia was 1:4537 and 50% of injuries occurred during laryngoscopy, 23% following extubation, 8% during emergence, and 5% in the context of regional anaesthesia . Therefore, when assessing peri-operative dental injuries, other risk factors must also be taken into account.
Second, when patients with dental injury were compared with patients without dental injury in tooth 22, the number of intubation attempts was found to be associated with dental injury, but difficult intubation was not associated with dental injury. In the Methods Section the authors stated that the definition of difficult airway used in their study was consistent with the Practice Guidelines of the ASA Task Force for Managing the Difficult Airway , in which difficult tracheal intubation is defined as when tracheal intubation requires multiple attempts. Because this definition does not state a specific number of attempts, it can be interpreted differently by practitioners; for example, multiple may mean more than one, two, three, etc. Therefore, it may have been more appropriate to provide a specific number of attempts for definition of difficult tracheal intubation used in this study.
Third, the study design did not include the details of tracheal intubation. Consequently, it is difficult to estimate the degree to which intubation technique may have influenced outcomes. It is generally acknowledged that poor laryngoscopy technique can give rise to excessive forces on teeth, resulting in an increased risk of dental injury . The attending anaesthetist determined both the anaesthetic technique and the airway management plan in this study, but it was unclear whether experienced anaesthetists performed all laryngoscopies and intubations. Furthermore, we should like to know whether an optimal/best attempt at conventional laryngoscopy was achieved during the study, with the patient's head and neck always placed in an optimal sniffing position during laryngoscopy, optimal external laryngeal manipulation used immediately if a poor laryngoscopic view was obtained, and appropriate laryngoscopic blades used in all patients .
We believe that addressing these confounding factors would further clarify the transparency of this study.