Greenwood 1994.

Study characteristics
Methods	Study design: RCT (split‐mouth) Conducted in: Department of Oral and Maxillofacial Surgery, The University of Manchester, UK
Participants	Inclusion criteria: "150 patients undergoing third molar removal under general anaesthesia were entered into the study. Cases were selected so that the left and right sides were close to identical for tooth position and degree of difficulty" Exclusion criteria: none described Age: not stated Number randomised: 150 Number evaluated: 150
Interventions	Howarth's elevator versus broad retractor for lingual nerve protection Group A (n = 150): lingual flap retraction with Howarth's elevator Group B (n = 150): lingual flap retraction using broad retractor All procedures performed under general anaesthesia, all required bone removal with either drill or chisel. Operators had varying experience, from house officers to consultant. Both extractions for each participant were completed by the same operator. Follow‐up: 1 month
Outcomes	Verbal self‐assessment of lingual nerve function, immediately and at 10 and 30 days postoperatively
Notes	Sample size calculation: not reported E‐mail sent 15 January 2003, and reply received 20 January 2003 with unpublished data.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "Left and right sides were then allocated at random..." Comment: we contacted the author who provided the following further information: "the randomisation code was computer generated"
Allocation concealment (selection bias)	Low risk	In the same letter, the author notes that "the allocation was concealed until surgery".
Blinding (performance bias and detection bias) patient	Low risk	All procedures were performed under general anaesthetic, so it can be assumed that the participants were blinded as to which side received the broader retractor; however, this is not specified in the paper.
Blinding (performance bias and detection bias) assessor	Low risk	Quote: "...the nurse and surgeon assessors were blinded" (from private correspondence)
Incomplete outcome data (attrition bias) All outcomes	Low risk	No withdrawals
Selective reporting (reporting bias)	Low risk	All prespecified outcomes reported on.
Other bias	High risk	The method of bone removal was not standardised: "the tooth was then removed employing either drill or chisel for bone removal, according to the operator's personal preference". Bone removal technique may possibly have confounded the results, and it was not recorded how many in each group had bone removal by each technique. Howarth's elevator was used to raise the initial flap for both sides, and then the broad retractor was introduced to 1 side.