title: Muscle loading is necessary for the formation of a functional tendon enthesis.
abstract: Muscle forces are essential for skeletal patterning during development. Eliminating muscle forces, e.g., through paralysis, leads to bone and joint deformities. Botulinum toxin (BtxA)-induced paralysis of mouse rotator cuffs throughout postnatal development closely mimics neonatal brachial plexus palsy, a significant clinical condition in infants. In these mice, the tendon-to-bone attachment (i.e., the tendon enthesis) presents defects in mineral accumulation and fibrocartilage formation, presumably impairing the function of the tissue. The objective of the current study was to investigate the functional consequences of muscle unloading using BtxA on the developing supraspinatus tendon enthesis. We found that the maximum endurable load and stiffness of the supraspinatus tendon attachment decreased after four and eight weeks of post-natal BtxA-muscle unloading relative to controls. Tendon cross-sectional area was not significantly reduced by BtxA-unloading, while, strength, modulus, and toughness were decreased in the BtxA-unloaded group compared to controls, indicating a decrease in tissue quality. Polarized-light microscopy and Raman microprobe analysis were used to determine collagen fiber alignment and mineral characteristics, respectively, in the tendon enthesis that might contribute to the reduced biomechanical performance in BtxA-unloaded shoulders. Collagen fiber alignment was significantly reduced in BtxA-unloaded shoulders. The mineral-to-matrix ratio in mineralized fibrocartilage was not affected by loading. However, the crystallographic atomic order of the hydroxylapatite phase (a measure of crystallinity) was reduced and the amount of carbonate (substituting for phosphate) in the hydroxylapatite crystals was increased. Taken together, these micrometer-scale structural and compositional changes partly explain the observed decreases in the mechanical functionality of the tendon enthesis in the absence of muscle loading.
authors.affiliation: Department of Orthopaedic Surgery, Washington University, St Louis, MO 63110, USA.
authors.forename: A G
authors.initials: AG
authors.lastname: Schwartz
authors.forename: J H
authors.initials: JH
authors.lastname: Lipner
authors.forename: J D
authors.initials: JD
authors.lastname: Pasteris
authors.forename: G M
authors.initials: GM
authors.lastname: Genin
authors.forename: S
authors.initials: S
authors.lastname: Thomopoulos
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001696:Biomechanical Phenomena / Q000187:drug effects / Q000502:physiology; D001842:Bone and Bones / Q000187:drug effects / Q000503:physiopathology*; D019274:Botulinum Toxins, Type A / Q000494:pharmacology; D024022:Fibrillar Collagens / Q000378:metabolism; D006801:Humans; D051379:Mice; D009132:Muscles / Q000187:drug effects / Q000473:pathology / Q000503:physiopathology*; D010243:Paralysis / Q000503:physiopathology; D013710:Tendons / Q000187:drug effects / Q000473:pathology / Q000503:physiopathology*; D016474:Weight-Bearing / Q000502:physiology
chemical_list: D024022:Fibrillar Collagens; D019274:Botulinum Toxins, Type A
references.citation: Bone. 2006 Feb;38(2):257-64
references.pmid: 16185943
references.citation: J Biomed Mater Res A. 2010 Mar 1;92(3):1048-56
references.pmid: 19301272
references.citation: Biophys J. 2005 Feb;88(2):765-77
references.pmid: 15596492
references.citation: J Orthop Res. 2007 Sep;25(9):1154-63
references.pmid: 17506506
references.citation: J Bone Miner Res. 2003 Jun;18(6):1012-9
references.pmid: 12817753
references.citation: J Bone Joint Surg Am. 2003 Feb;85(2):316-22
references.pmid: 12571311
references.citation: Tissue Eng. 2007 Jun;13(6):1219-26
references.pmid: 17518715
references.citation: Ann N Y Acad Sci. 2007 Nov;1116:444-60
references.pmid: 17584988
references.citation: J Perinatol. 2006 Feb;26(2):85-8
references.pmid: 16407959
references.citation: J Biomech Eng. 2003 Apr;125(2):280-7
references.pmid: 12751291
references.citation: J Orthop Res. 2011 Feb;29(2):281-8
references.pmid: 20803483
references.citation: J Biomech Eng. 2005 Oct;127(5):742-50
references.pmid: 16248303
references.citation: Appl Spectrosc. 2008 Dec;62(12):1285-94
references.pmid: 19094386
references.citation: J Bone Joint Surg Am. 2009 Apr;91(4):879-91
references.pmid: 19339573
references.citation: Biomaterials. 2007 Jul;28(19):3043-54
references.pmid: 17382382
references.citation: Biophys J. 2005 Feb;88(2):778-89
references.pmid: 15596491
references.citation: Bone. 2010 Jan;46(1):18-23
references.pmid: 19857614
references.citation: J Appl Physiol (1985). 2007 Aug;103(2):459-63
references.pmid: 17412787
references.citation: J Biomech. 2006;39(10):1842-51
references.pmid: 16024026
references.citation: J Bone Miner Res. 2009 Jul;24(7):1271-81
references.pmid: 19210217
references.citation: Brain. 2002 Jan;125(Pt 1):113-22
references.pmid: 11834597
references.citation: Bone. 2004 Mar;34(3):443-53
references.pmid: 15003792
references.citation: Int J Solids Struct. 2009 May 15;46(10):2136-2150
references.pmid: 23874001
references.citation: FASEB J. 2012 Mar;26(3):1110-8
references.pmid: 22125315
references.citation: Biophys J. 2009 Aug 19;97(4):976-85
references.pmid: 19686644
references.citation: Biophys J. 1997 Oct;73(4):2164-72
references.pmid: 9336212
references.citation: Poult Sci. 2007 May;86(5):931-6
references.pmid: 17435028
references.citation: Bone. 2008 Mar;42(3):476-82
references.pmid: 18187375
references.citation: Anat Rec A Discov Mol Cell Evol Biol. 2005 Feb;282(2):147-56
references.pmid: 15627985
references.citation: J Biomech. 2011 Jan 11;44(2):277-84
references.pmid: 21074774
references.citation: Bone. 2010 Aug;47(2):392-9
references.pmid: 20450992
references.citation: J Bone Joint Surg Br. 2005 Feb;87(2):231-5
references.pmid: 15736749
references.citation: J Orthop Res. 2010 Oct;28(10):1391-8
references.pmid: 20225283
references.citation: J Bone Joint Surg Am. 2004 Apr;86(4):787-93
references.pmid: 15069145
references.citation: Biomaterials. 2009 Jun;30(17):3118-27
references.pmid: 19324407
references.citation: J Biomech. 2012 Aug 9;45(12):2061-5
references.pmid: 22776688
references.citation: PLoS One. 2012;7(11):e48630
references.pmid: 23152788
references.citation: J Appl Physiol (1985). 2010 Dec;109(6):1762-8
references.pmid: 20947711
references.citation: J Bone Miner Res. 2007 Jul;22(7):1037-45
references.pmid: 17402847
references.citation: Mech Mater. 2012 Jan;44:
references.pmid: 24285911
references.citation: J Orthop Res. 2003 May;21(3):413-9
references.pmid: 12706013