Despite the fact that nearly 60 percent of anatomical texts and atlases as well as over 90 percent of specialty journal articles state that the distal semimembranosus tendon contributes fibers to the oblique popliteal ligament; none have hypothesized that this structure is itself a tendon [1
]. A macroanalysis using deep dissection of the posterior knee revealed that the OPL's distal (medial) attachment originated from the SMTU in 100% of the knees. This provided evidence in support of the author's hypothesis; however, a microanalysis was also necessary to propitiate these findings. This study was the first to conduct a histological microanalysis of the OPL.
There have been previous studies that have used various staining protocols on the deep tissue of the knee, namely, the cruciate ligaments, menisci, and the medial collateral ligament [33
]. The majority of this research conducted histological studies specifically targeting the morphology of nerve endings in these tissues. This was the first known study to use immunohistochemistry staining with an antibody specific to neuronal axons in the deep tissue of the knee. This was also the first study to utilize any staining protocol on the OPL.
The microanalysis of the tendon properties using rabbit anti-PGP9.5/goat anti-rabbit biotinylated immunohistochemistry staining revealed neuronal axons in both the SMTU and the OPL and displayed similar histological patterns in both structures [33
]. The LCL did not display a positive result for this stain and had a markedly different histology to both the OPL and SMT. Furthermore, the positive stain for neuronal axons provides grounds that Golgi tendon organs, nervous tissue specific to tendons, may be located in the OPL. These facts confirm the author's hypothesis that this structure is a tendon.
The authors are not aware of a stain specific to Golgi tendon organs. However, in pursuit of providing increased evidential proof for a change in terminology, the authors conducted a different, more definitive immunohistochemistry stain for neuronal axons using neuronal class III β-tubulin (NCT) with rabbit anti-NCT/goat anti-rabbit biotinylated. Though the histology of both the SMTU and the OPL was once again quite similar and vastly different from that of the LCL, the stain revealed a positive stain for neural tissue in all three structures: the OPL, SMTU, and LCL. This result does not nullify the results obtained from the PGP9.5 stains; however, it forced the authors to question whether or not immunohistochemistry staining for neural tissue within these structures is the most viable method for differentiating tendon from ligament.
The macroanalysis of the distal SMTU provides undeniable evidence that the OPL is indigenous to this tendon. The immunohistochemistry used in this study is proven to provide definitive results for neuronal axons within tissue samples [42
] and was the first to demonstrate that there is nervous tissue within the OPL. Despite the inconclusive results of the final immunohistochemistry stains, the macro- and microevidence that the oblique popliteal ligament is not a ligament at all is overwhelming. This evidence has led the authors to propose a nomenclature change for this structure, naming it the oblique popliteal tendon.