Neurosensory deficit is a well-reported complication after lower third molar surgery. It is useful to know the outcomes of the available treatments for neurosensory deficit after third molar surgery. It is more important to prevent nerve injury from third molar surgery. This thesis aims1) to evaluate the outcomes of treatments for neurosensory deficit after lower third molar surgery; 2) to investigate the effect of permanent neurosensory deficit from the patient’s perspective;3) to identify radiographic signs as predictors of inferior alveolar nerve (IAN) deficit in third molar surgery; 4)to monitor the long-term root movement and morbidities of the retained roots following coronectomy of the lower third molars. (1) A systematic search on treatment modalities and their outcomes of neurosensory deficit after lower third molar surgery was performed. 4 surgical treatments and 2 non-surgical treatments were identified. Significant improvement in sensation was found in the majority of the subjects who received surgical or non-surgical treatment. Complete recovery was uncommon in all kinds of available treatments. (2) Forty-eight subjects (24 cases) were recruited in a prospective case-control study comparing the general and oral health-related quality of life (QoL), life satisfaction and depression symptoms of patients with persistent lingual nerve (LN)or IAN neurosensory deficit (12 months or more) after third molar surgery with those who did not have such deficit. It was found that patients with persistent neurosensory deficit after third molar surgery have significantly poorer general and oral health-related QoL, worse life satisfaction and more depression symptoms than those without such deficits. (3) Twelve patients with neurosensory deficit after lower third molar surgery (10 LN, 2 IAN) who received microsurgical repair of the affected nervewere recruited in a prospective longitudinal observational study of the treatmentoutcomes. Most patients with pain wererecovered after surgery. Subjective symptoms including numbness, taste sensation and speech were improved after LN repair. Improvement was noted in all three objective neurosensory tests at post-operative 12 months. (3) Twelve patients with neurosensory deficit after lower third molar surgery (10 LN, 2 IAN) who received microsurgical repair of the affected nervewere recruited in a prospective longitudinal observational study of the treatment outcomes. Most patients with pain were recovered after surgery. Subjective symptoms including numbness, taste sensation and speech were improved after LN repair. Improvement was noted in all three objective neurosensory tests at post-operative 12 months. (4) 178lower third molars with one or more of the five radiographic signs suggesting of close proximity of their roots to the IAN were analyzed. It was found that radiographic signs of “darkening of root(s)” and “displacement of inferior alveolar canal by the root(s)” were associated with increased risk of intraoperative IAN exposure. In addition, “darkening of the root(s)” or co-existing radiographic signs were associated with an increased risk of post-operative IAN deficit. (5) A phase 4 clinical trial with 612 lower third molar coronectomies was conducted to monitor the long term safety of the treatment. It was demonstrated that the technique has minimal morbidity in terms of infection, pain, dry socket or development of pathologies. Most retained roots (90.9%) migrated upward with the highest migration rate in the first 6 months, which gradually slowed down and stopped to migrate at 24months. 2.3% of the roots became exposed in the oral cavity and required removal. Re-operation to remove the exposed root did not cause any IAN deficit. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy