Abstract Knee osteoarthritis (OA) is one of the most common diseases among the elderly people. Typically, medical attention is not sought until the disease has progressed to a point at which it is not possible to treat effectively, often due to concerns over the cost of detection at an earlier stage. Ultrasound (US) imaging has a number of advantages as an imaging technique; apart from being a low cost diagnostic method, it is also non-invasive, utilizes non-ionizing radiation and portable. Due to progression of knee OA, the cartilage will experience a significant change in shape, and it becomes degenerated. After image processing using US medical imaging, it is possible to detect the cartilage shape change of the knee joint. Low contrast ratio and speckle noise are two main disadvantages of US imaging. The aim of this paper is to present a method for enhancing the contrast of the US image of knee joint cartilage in detecting early stages of knee OA. Conventional contrast enhancing methods are known to have some limitations. The objective of this paper is to propose a new contrast enhancing method which can overcome the limitations of the conventional contrast enhancing method. Most conventional contrasts enhancing methods emphasize only on one character. In the proposed method, the optimum value of contrast, brightness and detail preservation are considered. The proposed method is applied to find out the optimum separating point for segmenting the histogram of US image, for which optimum value of contrast, brightness and detail preservation will be preserved. In this method as well, three metrics, named as Preservation of Brightness Score function (PBS), Optimum Contrast Score function (OCS) and Preservation of Detail Score function (PDS), are defined.