The aim of the present study was to evaluate clinically and with MR imaging the outcome of a standard arthroscopic lysis and lavage of the TMJ in different ID of varying severity. Also the present study aimed to correlate between the preoperative clinical and imaging diagnosis using MRI and postoperative diagnosis with arthroscopic findings.Patients with unilateral or bilateral TMJ internal derangement who had unsuccessful conservative treatment for at least two months were randomly selected from the oral and maxillofacial surgery outpatient clinic, of Baptist hospital, Miami. Florida. USA. Exclusion criteria included patients with evidence of major jaw trauma, or deformities as well as systemic disease affecting the joint.Arthroscopic lysis and lavage was performed within one month of reevaluation after unsuccessful conservative treatment.Before the arthroscopic treatment, the patients were examined clinically as well as radiologically using magnetic resonance imaging. Based on the clinical and radiographic findings, a Wilkes staging was assigned to each joint preoperatively. Under intravenous sedation and local anesthesia or general anesthesia, arthroscopic lysis and lavage with mandibular manipulation was performed using a miniscope set of 1.9 mm diameter. After a double puncture arthroscopic examination of the superior joint space was performed. Then adhesions were detached, and lateral lysis was performed. The mandible was then manipulated. The joint space was irrigated with 100 ml of lactated Ringer’s solution.Post-surgical treatment comprised of a full - coverage occlusal stabilization splint and physical therapy. Three days after surgery, each patient started a training program for maximum opening, protrusive and lateral movements.All patients were assessed clinically at predetermined intervals regarding joint pain, mandibular movements, deviations on and maximal mouth opening.The number of treated patients was 197 patients; 28 (14.2%) males and 169 (85.8%) females. Mean age of the selected subjects was 40.3 years. The number of unilateral joint affection was 115 joints and bilateral joint affection was 82 joints with a total number of 279 joints. No perioperative or postoperative major complications occurred; none of the patients developed transient or permanent neurosensory deficits of the facial nerve. There were no signs of infection, bleeding, or hematoma.The main outcome variable of the current study was to assess the degree of joint pain at six months postoperatively. Results showed a significant statistical difference between the percentages of patients with successful outcome and those with failure outcome.Visual analog scale for joint pain and function showed a statistical significant improvement of both joint pain and function at the end of the follow up period.Other variables include, MIO, joint noise, joint loading sign and muscle pain also showed significant improvement of the treated joints.On the other hand the study showed a significant variation in accuracy between clinical and MRI diagnosis and Arthroscopic findings.Conclusion : The study concluded that arthroscopic lysis and lavage is an effective minimally invasive procedure for the treatment of some temporomandibular joint internal derangements refractory to conventional conservative therapy. Also the study showed a significant variation in accuracy between clinical and MRI diagnosis and arthroscopic findings.