## 4 Discussion  
The most important finding of this study was that the PCL envelope lack sign (PELS) was an effective arthroscopic diagnostic tool for confirmation of chronic PCL insufficiency. It is a direct sign of PCL deficiency, which can be observed on the ligament itself. Currently available arthroscopic tests to assess the integrity of the PCL, such as the \"slack ACL sign\", the posteromedial drive-through sign, and others, are only indirect signs and thus vulnerable to inaccuracy [1,6,8]. For example, the posteromedial drive-through sign, reported by [PERSON] et al., in our opinion may be false-positive in the case of general joint laxity or medial knee instability [10]. It may also be false-negative in the presence of osteophytes or if the position of the anterolateral portal is very high and constrains one to change the angle of visualization, therefore demanding higher force to perform trans-notch maneuver [10]. Another indirect sign, cartilage lesions in the patellofemoral joint or medial compartment, observed in chronic PCL instability, in fact may be caused also by traumatic events, or even without any instabilities as a common pattern in OA development [11]. This is the reason why we searched for direct arthroscopic signs independent of other factors, which could be strong evidence for the orthopedic surgeon in directly evaluating for a PCL tear. In the case of a positive PELS sign, (1) the PCL adheres to the proximal tibia adjacent to the medial meniscal posterior root attachment, (2) there is a loss of the natural free space between the PCL and posterior tibia, and (3) there is a loss of soft tissue cuff around the PCL attachment. While it is unclear why these three phenomena occur, possible explanation can be due to the posterior subluxation of tibia after PCL healing in elongation. In this situation, contact area between the PCL and posterior tibia may change and natural movements of PCL may alter, leading to PELS in two distinct mechanisms. The first mechanism of PELS formation can be pressing of elongated PCL against the posterior part of the tibia, which provides no natural free space anterior to the tibial part of the PCL. A second possible cause is formation of scar tissue between the soft tissue cuff and PCL substance, where there should be a free space in the healthy knee. It is also worth noting that natural PCL  
Figure 3: (**a**) Arthroscopic view from an anterolateral portal in a right knee joint after a PCL reconstruction. Trans-notch maneuver. The recreated PCL “envelope” can be observed between PCL graft and tibia. (**b**) Arthroscopic view from a posteromedial portal in a right knee joint after a PCL reconstruction. The recreated PCL envelope can be observed in the knee with a posttraumatic PCL graft tear. The probe is inserted through the anterolateral portal and intercondylar notch. MFC—medial femoral condyle, MM—medial meniscus. \"envelope\" may be recreated after PCL reconstruction, suggesting that PCL \"envelope\" may be necessary or is a natural effect of proper PCL functioning. It is important to recognize that complete PCL tears reportedly take a median of 21 months from the index injury to the diagnosis for the requirement for operative treatment [12,13]. However, it has been reported that PCL reconstruction for posterior knee instability improves patient subjective functional scores and objectively restores knee stability [4,7,14,15]. This leads to the conclusion that a weak point in PCL injury management still remains in terms of making an accurate diagnosis [12,14]. Whereas physical examination and MRI may not be sufficient in chronic PCL insufficiency, stress radiographs and arthroscopy (indirect signs) remain a confirmatory tool for suspected PCL injury. As surgeons, we continuously pursue to offer optimal diagnostic solutions on the basis of more than just a single clinical test or diagnostic modality. We think that PELS can be a helpful confirmatory tool during arthroscopy to increase assurance that the preoperative diagnosis of the chronic PCL insufficiency is correct.