title: Experimental models for cancellous bone healing in the rat.
abstract: OBJECTIVE
  Cancellous bone appears to heal by mechanisms different from shaft fracture healing. There is a paucity of animal models for fractures in cancellous bone, especially with mechanical evaluation. One proposed model consists of a screw in the proximal tibia of rodents, evaluated by pull-out testing. We evaluated this model in rats by comparing it to the healing of empty drill holes, in order to explain its relevance for fracture healing in cancellous bone. To determine the sensitivity to external influences, we also compared the response to drugs that influence bone healing.
  
  
  METHODS
  Mechanical fixation of the screws was measured by pull-out test and related to the density of the new bone formed around similar, but radiolucent, PMMA screws. The pull-out force was also related to the bone density in drill holes at various time points, as measured by microCT.
  
  
  RESULTS
  The initial bone formation was similar in drill holes and around the screw, and appeared to be reflected by the pull-out force. Both models responded similarly to alendronate or teriparatide (PTH). Later, the models became different as the bone that initially filled the drill hole was resorbed to restore the bone marrow cavity, whereas on the implant surface a thin layer of bone remained, making it change gradually from a trauma-related model to an implant fixation model.
  
  
  CONCLUSIONS
  The similar initial bone formation in the different models suggests that pull-out testing in the screw model is relevant for assessment of metaphyseal bone healing. The subsequent remodeling would not be of clinical relevance in either model.
authors.affiliation: a Orthopaedics, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University , Linköping, Sweden.
authors.forename: Magnus
authors.initials: M
authors.lastname: Bernhardsson
authors.affiliation: a Orthopaedics, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University , Linköping, Sweden.
authors.forename: Olof
authors.initials: O
authors.lastname: Sandberg
authors.affiliation: a Orthopaedics, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University , Linköping, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Aspenberg
mesh_terms: D000818:Animals; D015519:Bone Density / Q000502:physiology; D001863:Bone Screws; D004195:Disease Models, Animal; D005593:Fracture Fixation, Internal; D017102:Fracture Healing / Q000502:physiology; D050723:Fractures, Bone / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D010012:Osteogenesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D013978:Tibial Fractures / Q000503:physiopathology / Q000601:surgery