Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 108

on anatomic studies in cadavers, and has proven to be a great aid in bio-
mechanical analyses of forces during various motions of limb segments.

PROBLEM 5.1
Suppose that a muscle with a belly cross section of 10 cm 2 contracts by
20%.

1. What is the maximum external force that the muscle can resist at
this contraction?

2. If the attached tendon is assumed to have a cross-sectional area of
1.0 cm 2 , what strain will it experience?

ANSWER:

1. 260 N. This is obtained by determining that the maximum force at
20% contraction (80% of resting length) is (from Figure 5.5) 65%
of peak (= 40 N/cm 2 ). Thus:

0.65 × 40 × 10 = 260 N

2. 0.03 (3%). The stress in the tendon is 260/10 −4 or 2.6 MPa. From
Figure 5.2, this stress corresponds to 3% strain, near the physio-
logic limit for tendon. Training would increase the cross-sectional
area (hypertrophy) and reduce both the stress and the resulting
strain.

articular cartilage This extraordinary tissue is responsible for the easy rotational and trans-

lational movement of synovial joints. It is very well hydrated, contain-
ing 70%-85% water by weight. Its solid components are approximately
50% type II collagen and 50% glycosaminoglycans (GAGs). The GAGs
consist of a protein backbone with highly charged polysaccharide side
chains. The collagen forms a network, in tension under unloaded physi-
ologic conditions, to contain the highly hydrophilic GAGs and its associ-
ated water. The result is a markedly viscoelastic tissue.
Complex theories have been proposed to explain the stress-strain and
associated stress-electrical potential behavior of articular cartilage at
a local level, depending on the properties of its components and their
relative presence. These theories arise from the recognition that articu-
lar cartilage has a variable composition from the high-collagen-content
superficial layer (lamina splendens) to the more cellular regions near
the bone-cartilage junction. Moreover, the collagen in the surface layer
tends to be elongated along it and to be oriented parallel to the direction
of normal movement of the joint from which the cartilage is taken. This
superficial layer represents 10% to 20% of the total thickness of articu-
lar cartilage. In contrast, the collagen fibrils in the middle zone, which
comprises approximately 40% to 60% of the total thickness, are more
randomly oriented. This transitions to the deep zone of the cartilage,
which forms approximately 30% of the cartilage thickness, where the
fibers are woven together and oriented perpendicular to the surface. The