## 1 Introduction  
Injuries of the anterior cruciate ligament (ACL) of the knee lead to negative consequences for joint stability during sport and daily life [1]. There are studies which report that, in both the USA and Europe, around 30 people out of 100,000 inhabitants suffer this injury [2, 3]. The standard clinical evaluation of an ACL deficient knee comprises specific tests such as the Pivot-Shift or Lachman tests [4]. Unfortunately, they express instability only with a qualitative evaluation that is influenced by the technique employed by the surgeon and their expertise in grading the degree of instability [5].  
A precise quantitative evaluation of knee instability after an ACL rupture would be important in terms of both surgical planning and the postoperative rehabilitation program.  
Given that the Lachman test simply consists of anterior tibial plateau translation in the knee close to extension, the measure in millimeters of that translation provides a precise quantitative evaluation of that test. During the Pivot-Shift test, anterior subluxation of the lateral tibial plateau is firstly observed. Then, a spontaneous reduction occurs. Quantitative evaluation of the Pivot Shift test then could be achieved by measuring either anterior tibial translation or tibial reduction acceleration. The systems available for quantitative evaluation of the instability during the tests are not universally accessible. Additionally, most are expensive and present some disadvantages. Commercial accelerometers (such as KiRA [6]) can be used to quantify the instability during the Pivot-Shift or Lachman tests. However, it measures the absolute acceleration of the tibia with respect to the ground. It requires certain conditions to do the measurement. What is more, it may not be easy to interpret during the Pivot-Shift test evaluation. Worth considering is the fact that measurement with arthrometers such as KT-1000 (one of the most popular) might present an inadequate degree of reliability (both intra- and inter-examiner) as shown by several studies in the literature [7]. Fluoroscopy measurements would be the most reliable and accurate even though they are quite invasive and expensive [8].  
Image processing has become a low-cost and non-invasive method to measure the anterior-posterior (AP) translation of the knee during Lachman and Pivot-Shift tests. Nowadays, digital cameras used in smartphones and tablet PCs can capture high quality videos. In addition, the algorithms and libraries for image analysis, such as the open-source OpenCV Library [9], are available for everyone in different programming languages. [PERSON] et al. [10] demonstrated the possibility of the non-invasive evaluation of lateral translation during the Pivot-Shift test and Lachman test, respectively. The same authors developed an iPad application to capture and process data during the Pivot-Shift test [11]. However, as far as the authors know, no application is freely available, and neither is one available for Android.  
This article describes an Android application that can calculate AP translation during the Lachman and Pivot-Shift tests using the video capture of the tests. The application uses the open-source OpenCV library and can be installed on any Android device such as a smartphone or tablet PC. The results are presented for seven subjects. They show the differences between ipsilateral and contralateral legs. Since this method is non-invasive and does not require expensive instruments, it can be easily included in the testing routine in both clinics and operating rooms.  
It is important to extend the use of an objective, quantitative and open-source measurement method for knee instability to give everyone the opportunity to express their data with a single measurement parameter to make for effective global data comparisons.