The development of the tensor vastus intermedius during the human embryonic period and its clinical implications.

The tensor vastus intermedius (TVI) is a newly discovered muscle located in the anterolateral thigh area and is considered the fifth component of the quadriceps femoris muscle. There have been several papers describing its anatomical and morphological features in detail; however, many features of this muscle, such as its ontology or kinetic functions, remain unknown. The purpose of this study was to determine the initial appearance of the TVI muscle in human embryonic development and to investigate its growth and development. Histological observations were performed on 30 lower limbs of 15 human embryos from Carnegie stage (CS) 21, 22, and 23 (with crown-rump length ranging from 18.7 to 28.7 mm). Myocyte clusters of the TVI were observed between the vastus lateralis and intermedius muscles in 7 out of 10 limbs in CS 22, indicating that the TVI arises during this stage. In CS 23, the TVI was clearly present in all specimens except one. However, neither the aponeurosis nor the tendonous structure of the TVI were observed in these embryonic stages. Formation of the conventional four components of the quadriceps muscle is completed within CS 21; therefore, our results suggest that the TVI is the last element to develop in the quadriceps femoris complex. It is posited that after the embryonic period, the TVI continues to grow, while forming the tendinous structure toward the patella and receiving vascular supply from certain vascular branches. The clinical significance of these findings is that orthopedists and plastic surgeons who perform surgical procedures within the anterolateral thigh (ALT) area should be aware of the anatomy and development of the TVI in order to reduce surgical complications. Our present research aims to contribute to a deeper understanding of the morphogenesis of the TVI and the other femoral extensor muscles.