Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 68

seen in direct insertions. Here, the collagen fibers meet the bone at an acute angle,
and there is a tidemark separating the mineralized and non-mineralized tissue.
The distal (tibial) attachment of the MCL to the tibia is a good example of an
indirect insertion (Fig. 4.1b ). Here, superficial fibers are attached to the periosteum
while deeper fibers are directly attached to the bone at acute angles [ 1 ].
4.2.1.3 Vascular and Nerve Supply
The insertion sites of ligaments and tendons are relatively avascular, as the blood
supply to the soft tissues comes from the joint capsules. Commonly, the vessels in
the marginal part of the insertion area of the ligament or tendon connect with those
of the periosteum. Therefore, the vessels of the external peritendineum and of the
periosteum are the major vessels disrupted in avulsion injuries at the bone surface.
Superficial blood vessels in the anastomase merge with vessels of the periosteum in
most tendon-bone junctions, but intratendinous vessels remain separated from the
vasculature of bone except in circumscribed diphysoperiosteal insertions, charac-
terized by those to spinae, trabeculae, or tuberosities of bone.
The nerve supply has a pattern similar to that of the blood supply in that there are
no nerves that cross the zone of fibrocartilage to innervate both sides of the insertion
site. However, bones, tendons, and ligaments all have abundant neural elements
that may transmit important information in the analysis of joint motion, position,
and acceleration. Nerve endings and Pacini and Ruffini receptors have been found
in the interfascicular connective tissue. These receptors seem to possess a “limit-
detection function” of the physiological tolerable limit of movement or may control
small movements via the perception of acceleration [ 2 ]. Pacini and Ruffini
receptors, Golgi tendon organ-like receptors, and free nerve endings were found
in proximity to the insertions of the anterior cruciate ligament (ACL) [ 3 ]. Further-
more, Grigg and associates described that the afferents in the posterior articular
nerve (PAN) from the posterior capsule of the knee joint were sensitive to linear
stress, and that their role is to signal the limit of the joint motion in extension, in
addition to the afferents coming from the knee ligaments [ 4 - 6 ].
4.2.2 Biomechanical Function
The insertion site functions to transmit a load between the flexible yet strong
tension-bearing ligament or tendon into rigid, less compliant bone without damag-
ing the soft tissue [ 7 ]. However, the load magnitude varies greatly between tendons
and ligaments as well as between different joints. Typically, in vivo forces on
tendon insertions are large because of muscular contractions, while forces in
ligaments tend to be relatively small. To facilitate this load transfer and prevent
tearing at the attachment site, a gradual change in tissue composition across the
insertion site between soft tissue and bone is needed [ 8 , 9 ].