This virtual realtiy was implemented with MarVAJED® (Marmara Visual Auditory Joint Education Device) system which was developed by Marmara University, Department of Physiotherapy and Rehabilitation, Istanbul,Turkey. MarVAJED® is a system that evaulates the range of motion of the joints, analyzes the sensation of joint position, provides biofeedback support to increase joint control and the same time allows exercises to be controlled. This device analyzes the joint motion with the help of small sensors (Figure 2). The obtained data transfers to your mobile phone, to the tablet or to your personal computer. It stores the data by downloading it to the central server via internet for storage.

Conventional physiotherapy consists of electrotherapy and exercise programs. Hotpack or coldpack, therapeutic ultrasound (US) and conventional TENS were applied as electrotherapy program.

Virtual reality-based rehabilitation was implemented via MarVAJED system. At first the sensors of MarVAJED® were placed under and above the knee joint and then it was calibrated. The content of the game programs included in the MarVAJED system and how to perform these games with the exercises were explained to the participants. We used games called "Crazy Wings and Blasting Ball" included in the MarVAJED®.There was a bird figure in Crazy Wings game and represented knee joint of participants. Although the bird figure moved upwards during knee flexion, it moved downward in knee extension, there were some obstacles on the way of the bird went. The subjects were asked to follow the bird figure visually and to pass through the obstacles by flexing or extending their knees when they encountered obstacles.

PEDALO® Sensamove Balance System was used to evaluate postural balance of subjects.It has a balance board in which consists of sensors to record and process data about balance performance to a computer.

Severity of the knee Osteoarthritis of participants was evaulated with "Western Ontario and McMaster Universities Arthritis" (WOMAC LK 3.1). The WOMAC LK has 24 questions in three subscales (pain , stiffness, physical functions) and a total of 24 questions. Each item is scored 0 to 4. Maxiumum score is 96. the higher the scores indicate that the severity of the disease is worse.

Kinesiophobia was assessed with TAMPA Scale for Kinesiophobia (TSK). TSK is a 17-item measure that assesses kinesiophobia (fear of movement). Each item is evaluated on a 4-point Likert scale with scoring alternatives ranging from 'strongly disagree' to 'strongly agree. Total scores range from 17 to 68, with higher scores reflecting greater kinesiophobia.

Muscle Strength was evaulated with Biodex System Pro 4 Isokinetic Dynamometer (Biodex Medical Systems, Inc, Shirley, NY. USA). The subjects were seated in such a position that knee and hip were at 900 flexion, and shoulders, ankle and pelvis were fixed to the machine. The centre of the knee joint was aligned with the centre of the dynamometer by using a laser-pointing device. Before testing, 5-min warming programme was performed by using treadmill. Isokinetic test was performed with 5 repetitions at 1200/s and 2400/s angular velocities. Tests were performed in a standardized order and all subjects have received identical instructions and encouragements.