兎下腿骨延長における高齢と幼若個体間での骨格筋適応の違い

Diverse Muscular Adaptation between Aged and Young Groups in a Rabbit Model of Tibial Lengthening

要旨：骨延長術において骨格筋は過剰伸張され，骨格筋はサルコメア数を増やすなどして適応する．本研究の目的は兎骨延長モデルを用いて，年齢による筋適応の違いを明らかにすることである．白色家兎，高齢5羽（月齡10～43m），幼若6羽（手術時月齡3～4m）を用い，下腿骨で1日1.4mmで2週間延長を行った．延長終了直後に両下肢を摘出，膝・足関節90度にてホルマリン固定した．下腿の代表的5筋について，筋腹長，筋線維長，サルコメア長を計測し，サルコメア数，筋内腱長を計算した．筋腹長は両群において非手術側と比べて全筋で増加を示した．筋内腱長において高齢兎は1筋，幼若兎は3筋で有意な増加を認めた．サルコメア長は両群で延長側が長い傾向があった．サルコメア数で有意増加を認めたのは高齢兎で3筋，幼若兎では2筋で，その増加量は前者が顕著であった．高齢ではサルコメア数増加が筋適応の主役で，幼若個体では筋内腱伸張寄与が多かった．

Abstract : Skeletal muscles are overstretched following limb lengthening procedures. Muscles can adapt to this lengthening by adding new sarcomeres in series. Recent developments in limb lengthening provide adult patients more opportunities to undergo limb lengthening procedures. The purpose of this study was to clarify the difference in muscle adaptation between adult and young groups using a rabbit model of limb lengthening. Five mature (10-43 month old) and 6 immature (3-4 month old) white rabbits underwent tibial osteotomy. After a 1-week lag phase, tibial lengthening was applied at a rate of 1.4 mm/day for 2 weeks. Animals were euthanized after the completion of lengthening. Both hindlimbs were immersed in buffered formalin with the ankle and knee at a right angle. Muscle belly length, muscle fiber bundle length and sarcomere length were measured, and sarcomere number and internal tendon length were calculated in five representative muscles around the lengthened segment. Muscle belly length increased in all the lengthened muscles compared with the corresponding contralateral muscles regardless the group. Aponeurosis length increased significantly in one muscle for the adult group and three muscles for the young group. Sarcomere length tended to be longer or was significantly longer in the lengthened muscles. Serial sarcomere number significantly increased in 3 lengthened muscles in the adult group with a more conspicuous increase in the amount, while this occurred in 2 muscles in the young group with only a tiny increase. The predominant processes of skeletal muscle adaptation to the limb lengthening are sarcomere number addition in muscle fibers for the adult group and elongation of internal tendon for the young group.