Jan 15, 2001 - whereas the elastic modulus of bone is almost constant irrespective of collagen denaturation. These results ..... and osteoarthritic cartilage.
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Jfournal of Orthopaedic Research 19 (1001) 1021-1016
Journal of Orthopaedic Research www elsevier.com/loc~telort hres
The role of collagen in determining bone mechanical properties Xiaodu Wang
Ruud A. Bank ', Johan M. TeKoppele ', C. Mauli Agrawal
a hlechunicul Engineering. Uniwrsity of' Te.xus at Sun ilntonio, 6900 North Loop 1603 M'est, Sun '4nronio. T.Y 78-749. C'S.4 Musculoske1etul Biomgincwing Center. lrniocwit.v i f Tesas Hrcrlth Science Center ut Sun .Intonio, Sun .Inronio. T.Y 78249. 1;SA Division oj' 1ksculur und Connectiw Tissue Rrsrurch, TNO Prermtion tinu' H w l t h , Leiden, N~~tlitduntls
Accepted 1 5 January 2001
Abstract The hypothesis of this study was that collagen denaturation would lead to a significant decrease in the toughness of bone, but has little effect on the stiffness of bone. Using a heating model, effects of collagen denaturation on the biomechanical properties of human cadaveric bone were examined. Prior to testing, bone specimens were heat treated at varied temperatures (37-200°C) to induce different degrees of collagen denaturation. Collagen denaturation and mechanical properties of bone were determined using a selective digestion technique and three-point bending tests, respectively. The densities and weight fractions of the mineral and organic phases in bone also were determined. A repeated measures analysis of variance showed that heating had a significant effect on the biomechanical integrity of bone, corresponding to the degree of collagen denaturation. The results of this study indicate that the toughness and strength of bone decreases significantly with increasing collagen denaturation, whereas the elastic modulus of bone is almost constant irrespective of collagen denaturation. These results suggest that the collagen network plays an important role in the toughness of bone, but has little effect on the stiffness of bone. thereby supporting the hypothesis of this study. 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.
Introduction
Bone fracture is a major concern in the health care of the elderly population because it often causes morbidity and mortality. As a natural composite material, bone primarily comprises a hard mineral phase (mainly hydroxyapatite crystals) and a more compliant collagenous matrix (90% type I collagen) [lS]. From composite materials perspectives, the biomechanical properties of bone are dependent on the quality and the spatial arrangement of its constituents. Previous laboratory studies have suggested that changes in the collagen network may lead to significant variations in the mechanical integrity of bone. It has been shown that decrease in collagen cross-link concentration is associated with a decrease in bone stiffness and its capacity to absorb energy to fracture [14,18,24]. Using a laser autofluorescence technique, investigators from our group demonstrated that age-dependent changes in the collagenous matrix of baboon bone cor*Corresponding author. Tel.: +I-210-458-5565; fax: + 1-210-4585589. E-mail uddreJs: xwangputsa.edu (S.Wang).
relates with bone fracture toughness. [16]. Martin et al. [19] found that the longitudinal collagen fiber orientations correlate with the modulus and strength but not fatigue life of bone. Also, studies have shown that mutation of collagen molecules found in osteogenesis imperfects correlates significantly with the biomechanical properties of bone, such as the post-yield deformation and work to fracture [5,12,13,2S]. Also, recent studies have demonstrated that age-related reductions in the toughness of bone correlates significantly with changes in the integrity of the collagen network in bone (e.g., collagen denaturation and cross-links) [29,30]. However, how the integrity of the collagen network in bone relates with the bone mechanical properties is still not well understood. The hypothesis of the present study is that collagen denaturation may lead to a significant decrease in the toughness of bone, but has little effect on the stiffness of bone. To test the hypothesis, we heat treated bone specimens below and beyond the collagen denaturing temperatures and then investigated the effect of heatinduced denaturation of the collagen network on the strength, stiffness, and toughness of the bone specimens.
0736-0266/01/$ - see front matter 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved PII: S 0 7 3 6 - 0 2 6 6 ( 0 1 ) 0 0 0 4 7 - X
Materials and method Six fresh frozen human cadaveric femurs ( n = 6) were obtained from the Musculoskeletal Transplant Foundation (Edison, NJ). The bone samples were stored i n a freezer at -20°C prior to specimen preparation and testing. A 40 mm long section and a 4 mm thick slice were excised from the middle diaphysis of each femur. From each long section, nine bone coupons (30 1 2 mmf were prepared longitudinally for mechanical testing using a diamond saw and a benchtop milling machine. From the slice, nine small blocks of bone (4 > 1 = 2 nim) were prepared for determination of the amount of the denatured collagen in bone. One coupon and one small block specimen were randomly selected from each of the six femurs and heated for 1 h at one of nine different temperatures. Thus, at each temperature a total of 12 specimens (i.e., six for mechanical testing and six for collagen analysis) were prepared. The temperatures ranged between 37°C and 200°C (Table 1). This particular temperature range was selected for this study because the collagen network in bone denatures at certain specific temperatures: 60°C for the non-calcified collagen and 150°C for the calcified collagen [4,9,17,23]. Based on the results of previous studies using X-ray diffraction and scanning electron mici-oscopy techniques, at such low temperatures (
