Unique myological changes associated with ossified fabellae: a femorofabellar ligament and systematic review of the double-headed popliteus

Abstract

Introduction The fabella is a sesamoid bone embedded in the tendon of the lateral head of the gastrocnemius. It is the only bone in the human body to increase in prevalence in the last 100 years. As the fabella can serve as an origin/insertion for muscles, tendons, and/or ligaments (e.g., the oblique popliteal and fabellofibular ligaments), temporal changes in fabella prevalence could lead to temporal changes in “standard” knee anatomy. The aim of this study was to investigate unique myological changes to the posterolateral corner knee associated with ossified fabella presence and perform a systematic review to contextualize our results. Methods Thirty-three fresh frozen cadaveric knees were considered. As the knees were all used for previous experimentation, the knees were in variable levels of preservation. Those with adequate preservation were used to determine ossified fabella presence/absence. When ossified fabellae were present, unique myologies associated with the fabella were recorded. A systematic review was performed on the double-headed popliteus to investigate possible correlations between this anatomical variant and the fabella. Results Of the 33 knees, 30 preserved enough soft tissue to determine fabella presence/absence: 16/30 knees had fabellae (five cartilaginous and 11 ossified). Eight of the eleven knees with ossified fabellae retained enough soft tissue to investigate the posterolateral knee anatomy. Of these, 4/8 exhibited unique myological changes. One knee had a double-headed popliteus muscle where one head originated from the medial side of a large, bulbous fabella. A systematic review revealed double-headed popliteus muscles are rare, but individuals are 3.7 times more likely to have a fabella if they have a double-headed popliteus. Another knee had a large, thick ligament stretching from the lateral edge of the fabella to the inferoposterior edge of the lateral femoral epicondyle, deep to the lateral collateral ligament (LCL) and near the popliteal sulcus. We found no mention of such a ligament in the literature and refer to it here as the “femorofabellar ligament”. In all four knees, the plantaris and lateral gastrocnemius appeared to share a common tendinous origin, and the fabella was located at/near the junction of these muscles. In the case of the double-headed popliteus, the fabella clearly served as an origin for the plantaris. Conclusions Despite being found in an average of 36.80% of human knees, most standard anatomical models fail to account for the fabella and/or the unique myological changes associated with fabella presence. Although our sample is small, these data highlight aspects of human biological variability generally not considered when creating generalized anatomical models. Further work is needed to identify additional changes associated with ossified fabellae and the functional consequences of omitting these changes from models.

Sensitivity of musculoskeletal models to variation in muscle architecture parameters

“…While the Charles et al (2019) data includes pennation angle, we lacked the data to relate the orientations of the AnyBody muscle elements to the Charles et al full muscles. We also did not address other areas of interindividual variation that could impact joint reaction and muscle forces, including variability in bone shape (e.g., pelvic shape (Cox, 2021), femoral torsion (De Pieri et al, 2021)), muscle origin and insertion locations (e.g., tibialis anterior (Zielinska et al, 2021), popliteus (Berthaume, Barnes, Athwal, & Willinger, 2020)), or even the presence or absence of anatomical elements (e.g. fabella (Berthaume & Bull, 2020), plantaris (Olewnik et al, 2020)).…”

“…pelvic shape (Cox, 2021 ), femoral torsion (De Pieri et al, 2021 )), muscle origin and insertion locations (e.g. tibialis anterior (Zielinska et al, 2021 ), popliteus (Berthaume, Barnes, Athwal, & Willinger, 2020 )), or even the presence or absence of anatomical elements (e.g. fabella (Berthaume & Bull, 2020 ), plantaris (Olewnik et al, 2020 )).…”

Biomechanics in anthropology

“…In the case of humans this often means leaving out small, occasionally present sesamoid and accessory bones and variations in muscle origins, insertions, force production potential, and even the number of heads (e.g., doubleheaded popliteus muscles). 74,75 Accessibility constraints prevent access to subjects, specimens, material, equipment, and knowledge, which may influence accuracy, precision, and resolution. 76 There are, and continue to be, ethical concerns over access to data and precious, rare, (sub)fossil material.…”

“…These cadavers are usually older and have begun to show signs of degeneration, and do not encompass species‐level biological variation. In the case of humans this often means leaving out small, occasionally present sesamoid and accessory bones and variations in muscle origins, insertions, force production potential, and even the number of heads (e.g., double‐headed popliteus muscles) 74,75 …”

X-ray Imaging Versus Anatomical Dissection for Identification of the Fabella

“…The traditional interpretation of the fabella function is that of an atavistic structure more useful in quadrupeds, whose knees undergo more rotation and must balance mobility with stability [ 7 , 8 ]. More recent functional speculation has focused on the role of the fabella in reinforcing the soft tissues of the posterolateral corner of the knee, in particular in conjunction with the patellofemoral ligament [ 2 , 9 ].…”