title: The effects of nail rigidity on fracture healing in rats with osteoporosis.
abstract: OBJECTIVE
  Stress shielding from rigid internal fixation may lead to refracture after removal of the osteosynthesis material. We investigated the effect of a low-rigidity (Ti-24Nb-4Zr-7.9Sn) intramedullary nail regarding stress shielding and bone healing of osteoporotic fractures in the rat.
  
  
  METHODS
  40 female Sprague-Dawley rats, aged 3 months, were divided into the following groups: sham-operation (SHAM) (n = 10), ovariectomized (OVX) (n = 10) and OVX-fracture (n = 20). 10 SHAM rats and 10 OVX rats were killed after 12 weeks to provide biomechanical data. Ovariectomy was performed 12 weeks before fracturing both femurs in 20 rats. The left fracture was stabilized with a high-rigidity titanium alloy pin (Ti-6Al-4V; elastic modulus 110 GPa) and the right with a low-rigidity (Ti-24Nb-4Zr-7.9Sn; elastic modulus 33 GPa). The bony calluses were examined by micro-CT at 6 and 12 weeks after fracture, bone volume (BV) and total volume (TV) were determined at the callus region (ROI1) and the total femur (ROI2). Subsequently, the bones were tested mechanically by a three-point bending test.
  
  
  RESULTS
  In the low-rigidity group, TV (ROI1) increased at 6 weeks, but BV (ROI1), BV (ROI2) were similar but maximum load increased. At 12 weeks, the maximum load and also BV (ROI1, ROI2) were increased in the low-rigidity group.
  
  
  CONCLUSIONS
  The low-rigidity nail manufactured from Ti-24Nb-4Zr-7.9Sn showed better external callus formation, seemed to reduce effects of stress shielding, and reduced bone resorption better than the stiffer nail. The low-rigidity nail was strong enough to maintain alignment of the fracture in the osteoporotic rat model without delayed union.
authors.affiliation: Department of Orthopedics, Xijing Hospital, Fourth Military Medical University, Xi'an, China.
authors.forename: Mo
authors.initials: M
authors.lastname: Sha
authors.forename: Zheng
authors.initials: Z
authors.lastname: Guo
authors.forename: Jun
authors.initials: J
authors.lastname: Fu
authors.forename: Jing
authors.initials: J
authors.lastname: Li
authors.forename: Chao Fan
authors.initials: CF
authors.lastname: Yuan
authors.forename: Lei
authors.initials: L
authors.lastname: Shi
authors.forename: Shu Jun
authors.initials: SJ
authors.lastname: Li
mesh_terms: D000818:Animals; D001858:Bone Nails; D004195:Disease Models, Animal; D005260:Female; D005594:Fracture Fixation, Intramedullary / Q000009:adverse effects / Q000379:methods; D017102:Fracture Healing; D006801:Humans; D015663:Osteoporosis, Postmenopausal / Q000150:complications; D051381:Rats; D017207:Rats, Sprague-Dawley; D013314:Stress, Mechanical