CONTRIBUTION OF COLLAGEN NETWORK AND FIXED CHARGE TO THE CONFINED COMPRESSION MODULUS OF
ARTICULAR CARTILAGE
*Chen, S S (A-NIH); +*Sah, R L (A-NASA, NIH, NSF)
+*Department of Bioengineering, UC-San Diego. 9500 Gilman Dr., MC 0412, La Jolla, CA 92093-0412, 858-534-0412, Fax: 858-534-6896, rsah@ucsd.edu
INTRODUCTION
A variety of studies of articular cartilage indicate that the fixed charge
(FC), contributed by anionic glycosaminoglycan (GAG) constituents,
contributes primarily to the swelling and compressive properties, while the
collagen network (CN) provides cohesive properties (resisting the FC swelling
pressure) and strength in tension.7,8 The balance of forces between the FC and
CN is believed to be central to the properties of articular cartilage, with
increased tissue hydration in osteoarthritis attributed to the weakening of the
CN.7,8 Quantitative experimentation and theoretical analysis of isotropic
osmotic compression has allowed assessment of the biomechanical properties
of the CN.1 Tissue-level biomechanical (confined compression) and
biochemical data is now available for articular cartilage. Also, correlative
studies have recently implicated both FC and CN as contributors to the
confined compression modulus (HA0);11 however, the exact contribution of
molecular components to HA0 remains unclear. The present study tested the
hypothesis that both FC and CN contribute to the HA0 of articular cartilage.
Theoretical and experimental analysis of bovine and human cartilage in
confined compression were performed to assess the swelling stress of FC
(πFC), the stress on CN (σCN), and the contributions to HA0 of FC and CN (i.e.,
HA0FC and HA0CN).
METHODS
Confined Compression Experiments: Stress-strain data from previous
experiments on macroscopically normal articular cartilage was analyzed.
These included studies on cartilage from (1) fetal, calf, and adult bovines from
the femoral condyle and groove (1.0 mm thick),11 (2) femoral condyle
cartilage from young (20-40 yo) and aged (60-80 yo) humans (full thickness),
and aged (>60 yo) human femoral head cartilage (~1.5 mm thick).3 The HA0 of
the 1.0 mm thick articular layers, and the depth-varying HA0 of aged femoral
heads in four successive 125 µm layers (layer I-IV) from the articular surface
and on additional four 250 µm layers (layer V-VIII)3 was calculated using a
finite deformation model.5 Samples were also analyzed biochemically for FC9
or GAG4 (FC [mEq] = 3.98 [mEq/g] * GAG [g]), and also collagen.10
“Balance of Stress” Theory in Confined Compression: The
extracellular matrix was assumed to be composed of extrafibrillar (EF) and
intrafibrillar (IF) compartments.1 Since the PG are excluded from the IF
spaces within the collagen, fixed charge density was calculated based on the
EF water, FCDEF. The dependence of πFC on FCDEF and of collagen hydration,
HCN (IF water content/g of dry collagen), on πFC were from Basser.1 At
equilibrium, the balance of stresses during confined compression is:
applied
FC
CN
(1)
σ
= π +σ
At each uncompressed and compressed state (-ε=0–0.3), πFC was calculated
from the measured collagen, FC (or GAG), and water contents. Then σCN was
calculated from equation (1). HA0FC was calculated by fitting the πFC vs. ε
data.4 HA0CN was calculated as HA0-HA0FC.
Statistical Analysis: Multivariate ANOVA and the Tukey post hoc test
were used. Data are shown as mean±SE.
RESULTS
The distribution of σapplied between πFC and σCN with increasing
compressive strain showed distinct patterns during bovine development
(Fig. 1A-C) and different human joints and aging (Fig. 1E-F). In free-swelling
cartilage (ε=0), the tensile stress on CN (–σCN) was slightly lower (by ~23%)
in the fetus than the calf or adult bovine (p<0.001) but not different with aging
and for hip and knee human cartilage (p=0.08). With increasing compression
of bovine cartilage, πFC increased; however, the tension on the CN remained
nearly unchanged in the fetal tissue, while it decreased in both calf and adult
tissue. With compression of human cartilage, πFC increased in all tissues, but
the effect on tension on the CN was distinct. In both young and aged femoral
condyle, the –σCN decreased but remained tensile; however, in the aged
femoral head samples, it became zero at ~10% strain and began to sustain
compressive stress.
The relative contributions of FC and CN to HA0 varied markedly for the
superficial 1.0 mm layer of articular cartilage. HA0, HA0FC, and HA0CN were
each significantly lower for fetal cartilage than calf or adult bovine cartilage
(Fig. 2A, p<0.05). The contribution of FCEF to HA0, i.e. HA0FC / HA0, decreased
from fetal (100%) to adult (58%) (p<0.01). In human cartilage, HA0, HA0FC,
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and HA0CN were not different between young and aged femoral condyle
samples (p=0.4-0.7), but were lower in aged femoral condyle samples than in
the aged femoral head samples (Fig. 2B) (p<0.05). HA0FC / HA0 also varied,
being high ( 64% and 74% respectively) in young and aged femoral condyle,
and low (44 %) in the aged femoral head.
In addition, HA0, HA0FC, and HA0CN increased significantly with depth in
aged femoral head samples (p<0.001). However, HA0FC / HA0, remained at
~20% in each layer (Fig. 3).
DISCUSSION
The present study delineates, for the first time, the contribution of the
FC and CN constituents to the confined compression modulus of articular
cartilage. Factors that affect the results, determined from experimental studies
and a theoretical model, include the dependence of πFC and HCN on FCDEF.
However, slight variations in the assumed relationships 1 do not markedly
affect the findings described here (data now shown). Other relationships for
GAG-induced swelling and osmotic effects on collagen could also be
investigated (e.g., in Buschmann2 and Lai6).
The molecular basis for the substantial HA0CN remains to be delineated.
Since the model does account for the space-filling effect of collagen,
modulating the EF volume in which the FC resides, the results imply that the
CN directly contributes to the compressive properties of cartilage. It is
possible that molecules modifying CN structure, such as crosslinks or
collagen-binding small proteoglycans, as well as collagen fibril orientation,
may underlie such CN properties.
REFERENCES
1 Basser+, Arch Biochem Biophys 351:207, 1998. 2 Buschmann+, J Biomech
Engng 117:179, 1995. 3 Chen+, Trans Orthop Res Soc 24:643, 1999. 4
Farndale+, Biochim Biophys Acta 883:173, 1986. 5 Kwan+, J Biomechanics
23:145, 1990. 6 Lai+, J Biomech Engng 113:245, 1991. 7 Maroudas, Nature
260:808, 1976. 8 Maroudas, Sem Arthritis Rheum 11:36, 1981. 9 Maroudas+,
Biochim Biophys Acta 215:214, 1970. 10 Stegemann+, Clin Chim Acta
18:267, 1967. 11 Williamson+, Int Cart Repair Soc
3, 2000.
Poster Session - Cartilage Mechanics - Hall E
47th Annual Meeting, Orthopaedic Research Society, February 25 - 28, 2001, San Francisco, California