The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 87

Table 5.1 Scales of hierarchical organization within bone and cartilage
Unit (m) Bone [ 36 ] Cartilage [ 37 ]

10 10 -10 9 Ca 2+ ,PO 4 3 ,H 2 O Na+, Ca 2 +, SO-, COO ,H 2 O Nano-scale

10 8 -0 6 Ultra-scale Collagen Collagen

Hydroxyapatite Proteoglycans

10 7 -10 4 Micro-scale Lamella Cells

Cells Collagen fibril organization

10 4 -10 2 Tissue-scale Osteons Articular cartilage

Trabeculae Disc regions

10 2 -1

Macro-scale Whole bones Joints

by only a few researchers. Attempts to mimic functionality or specific features
of the natural tissue in tissue engineered constructs have met with limited success
[ 11 , 16 - 19 ]. Perhaps the greatest shortcoming in our ability to design improved
materials for osteochondral replacement lies in our current, narrow understanding
of how the biologic osteochondral interface is engineered in vivo to facilitate stress
transfer.
Structurally, the osteochondral interface possesses a hierarchical organization
(Table 5.1 ) that is functionally graded, where collagen fibers extend from the deep
zones of soft cartilage into a calcified region (termed “the zone of calcified
cartilage,” or ZCC) through a series of wavy tidemarks (Fig. 5.1 ). However, the
mechanisms of efficient load transfer through this region are not well understood.
Quantitative backscattered electron (qBSE) imaging of the human femoral head
sometimes shows a stepwise decrease in mineral density from bone to soft, hyaline
cartilage [ 20 ]. Yet qBSE and other techniques have also shown that many healthy
osteochondral regions include a ZCC that increases in mineral density immediately
adjacent to hyaline cartilage. A relatively sharp interface exists where the ZCC
meets the hyaline cartilage (Fig. 5.2 )[ 20 - 23 ]; however, such a sharp interface is
counterintuitive. While abrupt junctions between materials create a high localiza-
tion of stress and failure [ 24 - 27 ], the osteochondral region in healthy human tissues
rarely fails. Functional grading in natural [ 28 - 31 ] and engineered materials [ 32 - 35 ]
minimizes stresses at the interfaces of dissimilar materials, yet the mineral content
in the osteochondral region is not uniformly graded. This chapter will review the
biomechanical implications of the compositional and microstructural makeup of the
native osteochondral tissues and how these tissues are structured to effectively
transmit loads without failing.
Further, preparing undecalcified tissue sections across such a dissimilar
bimaterial interface is difficult and limits the range of possible analyses. Many
routine assays also require significant processing that alters tissue morphology or
properties. As such, the in vivo , micro- to nano-meter length scale histology and
mechanical properties that are relevant to the interface's function remain
understudied. The tissue organization has been well characterized via decalcified
tissue histology and other assays that require decalcification. However, removal of
the mineral phase eliminates the ability to study the role of the highest modulus
component. This chapter will describe existing knowledge gaps and explore what is