ASU KIN 412 - Disuse-induced deterioration of bone strength

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Journal of Biomechanics 34 (2001) 1631–1636Disuse-induced deterioration of bone strength is not stopped afterfree remobilization in young adult ratsHanna Trebacz*Department of Biophysics, Medical University of Lublin, Al. Racławickie 1, 20-059 Lublin, PolandAccepted 3 July 2001AbstractThe effect of unilateral hindlimb immobilization and subsequent free remobilization on mechanical properties of femur wasexamined in young adult rats. Right hindlimb of 17 weeks old male rats was immobilized for 2 weeks. Rats were sacrificed eitherdirectly after immobilization (E0) or after 4 weeks of free remobilization (E4). Mechanical properties in three-point bending as wellas dry mass (mdry), geometry, apparent density (dapp), and mineralization of dry bone tissue were measured post mortem in right andleft femora of experimental rats (E0, E4) and in right femora of age-matched controls (C0, C4). Differences between right femora ofexperimental and control animals and between right and left femora of experimental animals were analyzed. After immobilizationonly dappin E0 was significantly lower than in C0. Side-to-side differences in E0 were present only in mdryand dapp: Surprisingly, 4weeks after remobilization the differences between experimental and control femora were more pronounced. Mineralization, dapp;maximum bending moment (Mmax), yield bending moment (My) and stiffness of the right femur were lower in E4 than in age-matched C4. Side-to-side differences in remobilized rats (E4) were still significant for mdryand dapp: Additionally, the medullary areawas larger, and Mmax; My; stiffness and work to failure were lower in the right femur than in the left. It is concluded, that theprocesses of bone deterioration initiated during immobilization do not cease immediately after resumption of normal mechanicalloading. r 2001 Elsevier Science Ltd. All rights reserved.Keywords: Apparent density; Immobilization; Mineralization; Rat femur; Three-point bending1. IntroductionMechanical loading is essential for the normalfunctioning of bone tissue. Immobilization results inimbalance of bone metabolism followed by rapid boneloss and impairment of bone mechanical function(Kiratli, 1996). According to Frost’s theory of structuraladaptation to mechanical usage (Frost, 1990), decreasedmechanical usage depresses longitudinal growth andbone modeling-dependent bone gain, and stimulatesbone remodeling-dependent bone loss. Mechanismscontrolling gains and losses of bone mass are similarin young and aged rats and humans (Frost and Jee,1992; Jee et al., 1997), so in experiments concerning theproblem, rat models are popular.Unloading-induced depression of bone formation wasestablished on periosteal surfaces of cortical bone and incancellous bone tissue, both in intensively growing(Chen et al., 1992) and in adult rats (Dehority et al.,1999; Li et al., 1990; Li and Jee, 1991; Maeda et al.,1993). The overactive remodeling with increased resorp-tion was found in cancellous bone (Chen et al., 1992;Ijiri et al., 1995; Li et al., 1990; Maeda et al., 1993) andon endocortical envelope in diaphyses of long bones(Inman et al., 1999; Li and Jee, 1991; Maeda et al. 1993).The change of bone remodeling rate was followed by adecrease of bone mass, density (Ijiri et al., 1995; Li et al.,1990; Li and Jee, 1991; Maeda et al., 1993) andmechanical strength (Inman et al., 1999).The processes of bone resorption and formation areuncoupled during a transient phase of immobilization.At a steady state, bone mass and metabolic activitystabilize on a level adequate to the lower bone mass andloads (Frost, 1990; Jee et al., 1997). The transient phaselasts longer in adult (Ijiri et al., 1995; Li et al., 1990; Liand Jee, 1991) than in intensively growing animals(Chen et al., 1992). The question arises whethertermination of unloading, before a new steady state isestablished, would stop the bone loss.*Tel.: +48-81-5324501; fax: +48-81-5328903.E-mail address: [email protected] (H. Trebacz).0021-9290/01/$ -see front matter r 2001 Elsevier Science Ltd. All rights reserved.PII: S 0021-9290(01)00138-5Another fundamental question is whether the im-mobilization-induced osteopenia is temporary andreversible or nonrecoverable. The fact that the timeneeded for recovery is longer than the time needed toproduce osteopenic changes is proved both in intensivelygrowing (Kannus et al., 1996; Trebacz and Baj, 1999;Tuukkanen et al., 1992) and in adult animals (Kanepset al., 1997, Maeda et al., 1993). Moreover, the potentialfor complete and permanent recovery of an immobilizedbone is questionable (Kannus et al., 1996; Maeda et al.,1993; Tuukkanen et al., 1992).In the present work a unilateral hindlimb immobiliza-tion was applied to adult, but still growing rats. Theimmobilization was terminated after 2 weeks, suffi-ciently for initiation of substantial changes, but before anew steady state in bone turnover, was established, bothin cancellous and in cortical tissue (Chen et al., 1992;Ijiri et al., 1995; Jee et al., 1997; Li et al., 1990; Li andJee, 1991). The aim of the study was to evaluateeffects of a relatively short period of hindlimbimmobilization on mechanical properties, density andgeometry of femur and, what seems more interesting, toexamine the ability of bone to recover during freeremobilization.2. Materials and methodsAdult male Wistar rats were used in the study. After 2weeks of acclimatization to vivarium conditions, 17weeks old animals with mean body weight 412 g(S.D.=26 g), were randomly divided into four groups,ten animals per group. There were two control groups(C0, C4) and two study groups (E0, E4). Righthindlimbs of the study animals were immobilizedagainst the abdomen using bandages and padded tapeas previously described by Li et al. (1990). The fixationwas checked daily. All animals were housed in typicalwire cages (35  50 cm wide, 20 cm high), five animalsper cage. They were fed standard laboratory chow andwater ad libitum. After 2 weeks, the rats from the groupsE0 and C0 were sacrificed. Rats from the group E4 werereleased from tapes and bandages and allowed movingfreely for next 4 weeks. One day after the remobilizationthey used all legs while moving. At the end of 4th week,rats from the groups E4 and C4 were killed. All ratswere killed by cardiac puncture under ether anesthesia.The experiment was approved by the committee ofethics for animal experiments at the Medical Universityof Lublin.Both femora were removed


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