title: Does Sclerostin Depletion Stimulate Fracture Healing in a Mouse Model?
abstract: BACKGROUND
  Sclerostin is a secreted glycoprotein that inhibits the intracellular Wnt signaling pathway, which, when inactivated, stimulates bone formation. This has been seen in fracture studies, which have shown larger and stronger calluses with accelerated fracture healing in sclerostin knockout and sclerostin antibody injection models. However, the effects of these two mechanisms have not been compared in the context of fracture healing.
  
  
  OBJECTIVE
  We sought to determine the degree to which sclerostin inhibition (Scl-Ab injection) and complete sclerostin depletion inhibit fracture healing in a mouse model as evaluated by (1) morphometric trabecular bone measures at the fracture site, and (2) fracture site structural strength.
  
  
  METHODS
  Ten-week-old male sclerostin knockout (n = 20) and wild type (n = 40) mice underwent insertion of a tibial intramedullary pin after which a midshaft tibial osteotomy was performed. The mice were divided in three groups: sclerostin knockout (n = 20), wild type with sclerostin antibody injection (intravenous dose of 100 mg/kg weekly) (n = 20), and wild type with saline injection (n = 20). The mice for each group where subdivided and euthanized at 14, 21, 28, and 35 days after surgery, at which time the fractured tibias were assessed with microCT (to assess morphometric trabecular bone measures: bone volume to total volume (BV/TV), trabecular thickness, trabecular number, and structural model index at the fracture site. Biomechanical testing in the form of three-point bending also was done to assess fracture site structural strength. A difference greater than 3.7% in our primary outcome (BV/TV) would be required to detect a difference between groups with a power of 80%, as per our power analysis.
  
  
  RESULTS
  The wild type with sclerostin antibody and the sclerostin knockout groups showed increased trabecular BV/TV at the fracture site compared with the wild type group with saline at all times, however no difference was seen between the treatment groups with the numbers available, except at 28 days postoperatively when the sclerostin knockout group showed greater BV/TV than the wild type sclerostin antibody group (47.0 ± 3.5 vs 40.1 ± 2.1; p < 0.05). On biomechanical testing the wild type sclerostin antibody showed increased stiffness at Days 14 and 28 compared with the wild type with saline group (70.9 ± 6.4 vs 14.8 ± 8.1; p = 0.001), (106.8 ± 24.3 vs 74.9 ± 16.0; p = 0.004); respectively. However, with the numbers available, no differences were detected between the wild type with sclerostin antibody and the sclerostin knockout groups in terms of whole-bone structural strength.
  
  
  CONCLUSIONS
  Sclerostin antibody injections showed promising results, which were not different with the numbers available, from results achieved with complete depletion of sclerostin, especially at earlier stages of the healing process, and therefore completed the healing process at an earlier time.
  
  
  CONCLUSIONS
  Sclerostin antibody injections appear to enhance fracture healing to a degree that is not different than complete sclerostin depletion, but larger animal studies are required to assess the accurate dosage and timing of administration in the fracture healing process to further evaluate its potential clinical utility to enhance fracture healing.
authors.affiliation: Shriners Hospital for Children, Montreal Children's Hospital, Montreal, QC, Canada. mohammad.alzahrani2@mail.mcgill.ca.|Division of Orthopaedic Surgery, McGill University, Montreal, QC, Canada. mohammad.alzahrani2@mail.mcgill.ca.|Department of Orthopaedic Surgery, University of Dammam, Dammam, Saudi Arabia. mohammad.alzahrani2@mail.mcgill.ca.|, 1529 Cedar Avenue, Montreal, QC, H3G 1A6, Canada. mohammad.alzahrani2@mail.mcgill.ca.
authors.forename: Mohammad M
authors.initials: MM
authors.lastname: Alzahrani
authors.affiliation: Shriners Hospital for Children, Montreal Children's Hospital, Montreal, QC, Canada.
authors.forename: Frank
authors.initials: F
authors.lastname: Rauch
authors.affiliation: Shriners Hospital for Children, Montreal Children's Hospital, Montreal, QC, Canada.|Division of Orthopaedic Surgery, McGill University, Montreal, QC, Canada.
authors.forename: Reggie C
authors.initials: RC
authors.lastname: Hamdy
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D000906:Antibodies / Q000008:administration & dosage; D001696:Biomechanical Phenomena; D016723:Bone Remodeling / Q000187:drug effects / Q000235:genetics; D004195:Disease Models, Animal; D017102:Fracture Healing / Q000187:drug effects / Q000235:genetics; D005838:Genotype; D006023:Glycoproteins / Q000037:antagonists & inhibitors / Q000172:deficiency* / Q000235:genetics / Q000276:immunology; D036341:Intercellular Signaling Peptides and Proteins; D008297:Male; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D010641:Phenotype; D013977:Tibia / Q000000981:diagnostic imaging / Q000187:drug effects / Q000378:metabolism* / Q000503:physiopathology / Q000601:surgery; D013978:Tibial Fractures / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D013997:Time Factors; D055114:X-Ray Microtomography
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000906:Antibodies; D006023:Glycoproteins; D036341:Intercellular Signaling Peptides and Proteins; C486030:Sost protein, mouse