Page 186 - Advances in Biomechanics and Tissue Regeneration

182 9. COMPUTATIONAL MUSCULOSKELETAL BIOMECHANICS OF THE KNEE JOINT

into a single sagittal plane. In fact, joints are commonly simplified with constrained kinematics. In most MS models the
human knee joint is either described as an idealized frictionless spherical joint with rotations only [29] or a simple
planar joint [16]. OpenSim, developed in the 1990s, is a popular interactive software for the development and analysis
of human MS systems [16]. This publically available software is regularly used by many researchers [30, 31]. Another
more recently developed commercially available MS package is the AnyBody Modeling System (AnyBody Technology
A/S, Aalborg, Denmark) that is also used for the estimation of muscle and joint contact forces [32–34].
Our recent lower extremity hybrid kinematics-driven MS model investigations, by explicit incorporation of a val-
idated detailed FE model of the entire knee joint, offer improved estimations both at the global (muscle and joint forces)
and local knee joint (tissue stresses and strains, contact pressure, and center) levels [15, 19, 20, 22, 23, 35–37] by full
consideration of the active–passive synergy (though currently only at the knee joint). This chapter starts with a short
description of the joint passive tissues and our knee joint FE and lower extremity MS models followed by some sample
results and validation of model predictions.

9.2 METHODS

9.2.1 Passive Tissues
9.2.1.1 Cartilage
As a fluid-saturated nonhomogeneous composite tissue, articular cartilage provides smooth articulation at joint sur-
faces, absorbs impacts, and redistributes applied stresses to the underlying bone. Articular cartilage is made mainly of
collagen (type II) (50%–73% dry weight), proteoglycans (15%–30% dry weight), and water (58%–78% weight). The
composition and structure of articular cartilage change with depth, from the joint surface to the anchorage at bone
[38–45]. At the superficial zone, collagen fibrils are horizontally oriented parallel to the articular surface, whereas they
become rather random in the transitional zone and finally turn perpendicular to the subchondral bone–cartilage inter-
face in the deep zone [40, 46, 47]. Collagen content is highest in the superficial and deep zones of articular cartilage and
lowest in the middle zone [41]. In tandem with proteoglycan content, the equilibrium modulus of the nonfibrillar solid
matrix increases downward along with the depth from the articular surface to the subchondral junction [41].
This layered structure along with the nonlinearity and tension-compression differences in the collagen fibril prop-
erties results in a highly nonlinear, nonhomogeneous composite fibrous tissue [48–52]. The tissue is saturated with
water, the flow of which (mobile portion) gives rise to the time-dependent viscoelastic behavior leading to common
stress relaxation (drop in stress under constant strain) and creep (increase in deformation under constant loads) effects.
The water content, as in other similar fibrous tissues such as intervertebral discs and menisci, varies with time depend-
ing on the tissue composition, external load history, and osmolality of surrounding media and as such plays a crucial
role in the load bearing especially in the transient (short-term) periods where fluid pressurization plays a crucial
mechanical role in supporting applied loads in nondegenerate conditions [53].

9.2.1.2 Ligaments
Similar to other biological soft tissues, the ligaments of the knee are made of a ground substance reinforced with
collagen fibers (mainly type I) [54]. Four major ligaments (ACL, anterior cruciate ligament; PCL, posterior cruciate
ligament; LCL, lateral collateral ligament; and MCL, medial collateral ligament), among others, stiffen and control
the relative movements of TF joint. ACL and PCL can each be separated into two distinct bundles: anteromedial
(am) and posterolateral (pl) in ACL and anterolateral (al) and posteromedial (pm) in PCL. These bundles experience
different patterns of length changes during active/passive knee flexion [55, 56]. Cadaver studies have confirmed the
primary roles of ACL-am in high flexion and ACL-pl at near extension angles during passive knee flexion or under
anterior tibial forces [57–59]. Depending on loading conditions, one or more of these ligaments act as primary restraints
in resisting applied force movements and enhancing joint stability. Though modulated by joint flexion angle, MCL and
LCL are the primary constraints in adduction-abduction rotations, whereas AP translations and internal-external rota-
tions are resisted primarily by cruciate ligaments [59, 60].
9.2.1.3 Meniscus
Meniscus is a semilunar shaped fibrocartilaginous tissue with extremities inserted into the intercondylar eminence
at the proximal tibial plateau. It is composed mainly of a dense network of collagen fibrils (mainly type I), proteogly-
cans, and water. Similar to cartilage, its fluid content gives rise to tissue time-dependent response. While the collagen
fibrils at the top, bottom, and peripheral surfaces show no major preferred orientations, they are nevertheless

I. BIOMECHANICS