You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Outcome", "Participant", "Intervention"].

Please return the output as a JSON object of the format: {"Participant" : ["patients with COPD", ...], "Intervention" : ["Atenolol", ...], "Outcome" : ["blood pressure", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

The influence of pre- and intraoperative positioning of the condyle in the centre of the articular fossa on the position of the disc in orthognathic surgery . A magnetic resonance study . OBJECTIVE We investigated the changes in the temporomandibular joint ( TMJ ) after bilateral sagittal split osteotomy of the mandible for orthognathic surgery and the influence of positioning of the condylar process in the centre of the articular fossa before and during the operation for preventing changes in the TMJ postoperatively . STUDY DESIGN A total of 28 patients with mandibular retrognathism had bilateral sagittal split osteotomies for mandibular advancement . In one group of 14 patients ( 28 TMJ ) , the condyles were placed in the centre of the articular fossa before and during the operation , and in the other group they were not . Differences on magnetic resonance imaging ( MRI ) were calculated and the results were evaluated . RESULTS The main differences were found at maximal mouth opening . 15/28 TMJs ( 54 % ) that had not been positioned changed the position of the disc from physiological to anterior disc derangement with and without reduction postoperatively . In the 28 that had been positioned , changes were found in only 3 TMJs ( 11 % ) postoperatively . CONCLUSIONS Fixing the condylar process in the centre of the articular fossa intraoperatively before bilateral sagittal split osteotomy is a factor in preventing postoperative structural changes in the temporomandibular joint .
{"Intervention": ["Fixing the condylar process in the centre of the articular fossa intraoperatively", "magnetic resonance imaging ( MRI )", "bilateral sagittal split osteotomy of the mandible", "orthognathic surgery .", "condyles were placed in the centre of the articular fossa before and during the operation , and in the other group they were not", "bilateral sagittal split osteotomies for mandibular advancement .", "positioning of the condylar process", "pre- and intraoperative positioning of the condyle"], "Outcome": ["Differences on magnetic resonance imaging ( MRI )", "TMJs", "maximal mouth opening"], "Participant": ["28", "temporomandibular joint ( TMJ )", "3", "TMJs", "orthognathic", "15/28", "bilateral sagittal split osteotomy", "mandibular retrognathism", "bilateral sagittal split osteotomies", "TMJ", "14"]}