The effects of nail rigidity on fracture healing in rats with osteoporosis.
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.
Sha
Mo
M
Department of Orthopedics, Xijing Hospital, Fourth Military Medical University, Xi'an, China.
Guo
Zheng
Z
Fu
Jun
J
Li
Jing
J
Yuan
Chao Fan
CF
Shi
Lei
L
Li
Shu Jun
SJ
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