Abstract: This review includes two studies. The previous review included one RCT with a parallel-group design, which was conducted in a dental hospital setting in the United Kingdom; our new search for this update identified one prospective cohort study conducted in the private sector in the USA. Primary outcome No eligible studies in this review reported the effects of removal compared with retention of asymptomatic disease-free impacted wisdom teeth on health-related quality of life Secondary outcomes We found only low to very low quality evidence of the effects of removal compared with retention of asymptomatic disease-free impacted wisdom teeth for a limited number of secondary outcome measures. One prospective cohort study, reporting data from a subgroup of 416 healthy male participants, aged 24 to 84 years, compared the effect of the absence (previous removal or agenesis) against the presence of asymptomatic disease-free impacted wisdom teeth on periodontitis and caries associated with the distal of the adjacent second molar during a follow-up period of three to over 25 years. Very low quality evidence suggests that the presence of asymptomatic disease-free impacted wisdom teeth may be associated with increased risk of periodontitis affecting the adjacent second molar in the long term. In the same study, which is at serious risk of bias, there is insufficient evidence to demonstrate a difference in caries risk associated with the presence or absence of impacted wisdom teeth. One RCT with 164 randomised and 77 analysed adolescent participants compared the effect of extraction with retention of asymptomatic disease-free impacted wisdom teeth on dimensional changes in the dental arch after five years. Participants (55% female) had previously undergone orthodontic treatment and had 'crowded' wisdom teeth. No evidence from this study, which was at high risk of bias, was found to suggest that removal of asymptomatic disease-free impacted wisdom teeth has a clinically significant effect on dimensional changes in the dental arch. The included studies did not measure our other secondary outcomes: costs, other adverse events associated with retention of asymptomatic disease-free impacted wisdom teeth (pericoronitis, root resorption, cyst formation, tumour formation, inflammation/infection) and adverse effects associated with their removal (alveolar osteitis/postoperative infection, nerve injury, damage to adjacent teeth during surgery, bleeding, osteonecrosis related to medication/radiotherapy, inflammation/infection). Insufficient evidence is available to determine whether or not asymptomatic disease-free impacted wisdom teeth should be removed. Although asymptomatic disease-free impacted wisdom teeth may be associated with increased risk of periodontitis affecting adjacent second molars in the long term, the evidence is of very low quality. Well-designed RCTs investigating long-term and rare effects of retention and removal of asymptomatic disease-free impacted wisdom teeth, in a representative group of individuals, are unlikely to be feasible. In their continuing absence, high quality, long-term prospective cohort studies may provide valuable evidence in the future. Given the lack of available evidence, patient values should be considered and clinical expertise used to guide shared decision making with patients who have asymptomatic disease-free impacted wisdom teeth. If the decision is made to retain asymptomatic disease-free impacted wisdom teeth, clinical assessment at regular intervals to prevent undesirable outcomes is advisable.

Plain-language summary: We searched the medical literature up to May 2016 and found one randomised controlled trial (RCT) and one prospective cohort study to include in this review. These studies involved 493 participants in total. The RCT conducted at a dental hospital in the UK included 77 adolescent male and female participants, and the cohort study conducted at a private dental clinic in the USA involved 416 men aged 24 to 84 years. Available evidence is insufficient to show whether or not asymptomatic disease-free impacted wisdom teeth should be removed. One study at serious risk of bias provided very low quality evidence suggesting that the presence of asymptomatic disease-free impacted wisdom teeth is associated with increased risk of periodontitis (infection of the gums) affecting the adjacent second molar (teeth directly in front of the wisdom teeth) in the long term. In the same study, no evidence was found to suggest that the presence of asymptomatic disease-free impacted wisdom teeth increases the risk of caries affecting the adjacent second molar. Another study, also at high risk of bias, found no evidence to suggest that removal of asymptomatic disease-free impacted wisdom teeth has an effect on crowding in the dental arch. The included studies did not measure our primary outcome - health-related quality of life. Nor did they measure our secondary outcomes - costs, other adverse events associated with retention of asymptomatic disease-free impacted wisdom teeth (pericoronitis, root resorption, cyst formation, tumour formation, inflammation/infection) and adverse effects associated with their removal (alveolar osteitis/postoperative infection, nerve injury, damage to adjacent teeth during surgery, bleeding, osteonecrosis related to medication/radiotherapy, inflammation/infection). Evidence provided by the two studies included in this review is of low to very low quality, so we cannot rely on these findings. High-quality research is urgently needed to support clinical practice in this area. In light of the lack of available evidence, patient values should be considered and clinical expertise used when treatment decisions are made with patients who have asymptomatic disease-free impacted wisdom teeth. If the decision is made to retain asymptomatic disease-free impacted wisdom teeth, clinical assessment at regular intervals is advisable to prevent undesirable outcomes.