The tendon of the vastus intermedius showed a complex multi-layered structure. It converged towards the patella and divided into a lateral and medial part. The lateral part of the vastus intermedius aponeurosis formed the deepest layer of the quadriceps tendon. These lateral fibers were oriented towards the medial femoral condyle (Fig. 2). The medial part of the vastus intermedius aponeurosis separated into a superficial and deep medial layer with an orientation towards the lateral femoral condyle (Figs. 2 and 3a). The fibers of the deep medial vastus intermedius aponeurosis were located above the lateral part of the vastus intermedius aponeurosis. Five to 10 mm medial to the mid-line of the quadriceps tendon, the superficial and deep layer of the medial vastus intermedius aponeurosis fused with the aponeurosis of the tensor vastus intermedius and vastus lateralis respectively. Together they formed the two-layered intermediate layer of the quadriceps tendon. Therefore, the vastus intermedius contributed first to the deep layer of the quadriceps tendon through its lateral part of the vastus intermedius aponeurosis and second to the two-layerd intermediate layer of the quadriceps tendon by its superficial and deep medial VI aponeurosis (Fig. 3). Jointly the different fibers continued towards the patella. The fusion point of the fibers for the deep intermediate layer was on average 56 mm (range, 30 to 90 mm, SD +/− 21) proximal to the patella (Fig. 3b). In the distal aspect the superficial medial layer of the vastus intermedius aponeurosis turned into a tendinous gliding layer of the vastus medialis. The latter, in turn, extended to the medial proximal margin of the patella, and in the deeper aspect to the tendon of the rectus femoris. The superficial medial layer of the vastus intermedius aponeurosis that fused with the aponeurosis of the vastus lateralis (superficial intermediate layer) met 33 mm (range, 13 to 53 mm, SD +/− 14) above the patella (Fig. 3b). The fibers of the vastus lateralis were often composed of bundles of individual thin fiber strands (Fig. 4). The meeting point of the superficial intermediate layer was always distal (23 mm, range, 12 to 41 mm, SD +/− 0.9) to the meeting point of the deep intermediate layer (Fig. 3b).Fig. 2Orientation of the multilayered structure of the extensor apparatus of the knee joint. The distal aspect of a right thigh is shown. Red stickers mark the medial and lateral femoral condyles above the knee joint space. The vastus medialis is released from its insertion into the vastus intermedius and rectus femoris (reflected laterally) freeing the view to the complex multi-layered aponeurosis of the vastus intermedius. The lateral part of the vastus intermedius aponeurosis form the deepest layer of the quadriceps tendon. The medial part of the vastus intermedius aponeurosis separates into a superficial and deep medial layer with an orientation towards the lateral femoral condyle (lateral red stick). The fibers of the medial vastus intermedius aponeurosis are located above the lateral part of the vastus intermedius aponeurosis. Generally, lateral fibers are oriented towards the medial femoral condyle (medial red stick). The blue dots mark the fusing points of the intermediate layer of the quadriceps tendon and the superior base of the patella. For better visualization the fusing points are underlined with black paper. The white dotted lines indicate the fiber direction towards the condyles Fig. 3Architecture of the multilayered quadriceps tendon. The distal aspect of a right thigh a with corresponding distal section of the quadriceps tendon b is shown. a The architecture of the quadriceps tendon consisting of the rectus femoris (R), vastus lateralis (VL), tensor vastus intermedius (TVI), vastus intermedius (VI) is shown. The medial part of the VI aponeurosis separates into a superficial and deep medial layer. Sartorius (S), patella (P). The insertions of the vastus medialis (VM) into the patella and capsule of the knee joint (blue arrow), rectus femoris (red arrow) and vastus intermedius (green arrow) is marked. Anterior insertion of the vastus medialis in the the vastus intermedius (VM ant), posterior insertion of the vastus medialis into the vastus intermedius (VM post). b The proximal two blue dots (F1 and F2) mark the fusing points of the intermediate layer of the quadriceps tendon. All portions of the extensor apparatus fuse over a region ranging from 13 to 90 mm proximal to the superior pole of the patella (distal blue dot). Lateral portions of the vastus intermedius (lateral VI) form the deepest layer of the quadriceps tendon. The superficial and deep layer of the medial vastus intermedius aponeurosis (black dotted arrows) fuse 56 mm (range, 30 to 90 mm) and 33 cm (range, 13 to 53 mm) proximal to the patella with the aponeurosis of the tensor vastus intermedius (TVI) and vastus lateralis (VL) respectively. Together they built the two-layered intermediate layer of the quadriceps tendon. The tendon of the rectus femoris (R) forms the superficial layer of the quadriceps tendon. For better visualization the fusing point are underlined with black paper. The vastus medialis is released from its insertion into the vastus intermedius and rectus femoris. Depending on the level of virtual transection one finds two, three or four layers (white arrows with numbers). An oblique transection could lead to the impression of a complex multi-layered arrangement of the quadriceps tendon Fig. 4Distal aspect of the extensor apparatus of the knee joint above the patella. The rectus femoris muscle is reflected medially displaying the anterior insertion of the vastus medialis into the vastus intermedius (see also Fig. 3a). The vastus medialis further inserts into the rectus femoris and patella. The medial deep and medial superficial layer of the vastus intermedius aponeurosis are covered by the inserting muscle fibers of the vastus medialis and therefore not visible. The aponeurosis of the vastus lateralis is separated into two fiber bundles. The fibers of the vastus lateralis and the tensor vastus intermedius are oriented towards the medial femoral condyle. Together with parts of rectus tendon the vastus medialis occupies the supero-medial half of the upper semi-circle of the patella (green dotted line). The strong muscle belly of the vastus lateralis inserts at the supero-lateral semi-circle of the patella (red dotted line)