Multiscale Modeling of Ligaments and Tendons - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 105

less than 1 mm [ 245 ], and consist of a distinct right-angle boundary where deep
collagen fibrils extend out of the ground substance matrix and become fibrocar-
tilage tissue, mineralized fibrocartilage tissue and then bone [ 47 ]. Indirect insertion
sites occur over a larger area where superficial collagen fibers gradually blend into
the periosteum at more acute angles. Deep collagen fibers also make attachments
at indirect insertion sites, but the connections are fewer than at direct insertion
sites, occur at more acute angles and without the fibrocartilagenous transitional
zone observed in direct insertions [ 26 ]. Some ligaments have the same type of
insertion site at both ends, while other ligaments have different types of insertion
sites on opposing ends. The ACL in the knee, for example, has direct insertion
sites on both ends, while the MCL in the knee attaches to the femur with a direct
insertion, but attaches to the tibia with an indirect insertion.
2.2 Tendon
Tendons connect muscles to bones and transmit the forces generated by the
muscles to the bones. Tendons generally stretch more than ligaments during use,
with some tendons being very efficient at storing and recovering energy [ 23 , 40 ,
48 , 71 , 100 , 116 , 139 , 151 , 155 , 204 ]. These elastic properties allow tendons to
passively modulate forces during locomotion, providing additional stability with
no active work [ 71 , 116 , 119 , 139 ]. The length of a tendon significantly contributes
to these characteristics. Shorter tendons allow for more muscle mass [ 173 , 178 ],
but longer tendons provide more elastic recovery of stored energy [ 30 , 71 , 116 ,
152 , 153 , 155 , 177 , 204 ]. The primary function of tendons like the rotator cuff,
finger tendons, and animal extensor tendons is to transfer load generated by their
associated muscles [ 23 , 27 , 28 , 123 , 177 , 178 , 258 ]. In contrast, tendons like the
Achilles tendon and animal flexor tendons store substantial amounts of energy and
are thought to act like biological springs [ 30 , 71 , 116 , 152 , 153 , 155 , 177 , 204 ].
Structure and composition vary between different tendons and between different
locations within individual tendons [ 28 , 121 , 190 ]. Similar to ligaments, tendons
connect to bones with either direct or indirect insertion sites. These attachments
are complex, are often the site of injury and as such are an area of extensive
continued research [ 8 , 22 , 60 , 79 - 81 , 150 , 151 , 214 , 215 , 246 , 255 ].
2.3 Hierarchical Structure of Ligament and Tendon
Tendons and ligaments are multiphasic biological composites. The extracellular
matrix (ECM) is composed of a fluid phase and a solid phase, with the bulk of the
tissue consisting of the fluid phase (i.e. water). The solid phase consists primarily
of type I collagen, which is organized into a complex hierarchy where tropocol-
lagen monomers form fibrils at the nanoscale, fibrils form fibers at the microscale,