Se. Utvag et O. Reikeras, EFFECTS OF NAIL RIGIDITY ON FRACTURE-HEALING - STRENGTH AND MINERALIZATION IN RAT FEMORAL BONE, Archives of orthopaedic and trauma surgery, 118(1-2), 1998, pp. 7-13
The present study was undertaken to assess the effects of the rigidity
of nails on the healing of fractures as evaluated by mechanical stren
gth and rate of bone mineralisation. Ten rats supplied biomechanical d
ata at the start of the experiment and another 100 rats were randomly
assigned to 5 groups. In 20 rats no intervention was performed, and th
ey served as a reference group of normal values. Thus, 80 rats underwe
nt a standardised partial osteotomy in the left femur diaphysis prior
to manual fracture, reaming and intramedullary nailing. One group rece
ived rigid steel nails (group I)and another, cannulated steel nails (g
roup 2). Group 3 received semi-rigid titanium nails and group 4, soft
polyethylene nails. Ten rats in each group were evaluated at 6 and 12
weeks after fracture, and radiographs and callus diameter measurements
were performed. Dual-energy X-ray absorptiometry (DEXA) of the bones
was performed, and the degree of mineralisation in the callus segment,
distal diaphysis and total femur was calculated by the scanner. Subse
quently, the bones were tested mechanically by a three-point bending t
est in a Mini Bionix (MTS) testing system. Radiographs revealed clearl
y visible fractures at 6 weeks in the intervention groups. At 12 weeks
there were no signs of bridging callus in group 4, while the other gr
oups presented scarcely visible osteotomies. The callus area in group
4 was significantly larger at both 6 and 12 weeks than in groups 1-3.
In these groups there was a significant decrease in callus area betwee
n 6 and 12 weeks. Both maximum bending load, bending rigidity and frac
ture energy increased significantly in groups 1-3 between 6 and 12 wee
ks, while bending load and fracture energy increased in group 4. At 6
weeks no significant differences were detected between groups 1-3 in m
echanical strength. Group 4 had a significantly reduced maximum bendin
g load and fracture energy at 6 and 12 weeks as well as bending rigidi
ty at 12 weeks. At 12 weeks group 3 had a significantly increased maxi
mum bending load and fracture energy compared with groups 1 and 2. Bon
e mineral count (BMC) in the callus region in group 4 was significantl
y reduced at both 6 and 12 weeks. BMC in the callus showed no signific
ant differences between groups 1-3, either at 6 or at 12 weeks. BMC in
the distal diaphysis was significantly reduced in groups 1 and 4 comp
ared with group 2 at 12 weeks. Bone mineral density (BMD) in the callu
s region revealed no significant differences between groups 1-3 at 6 w
eeks, while at 12 weeks BMD was higher in group 3 than in group 1. BMD
in the callus region and total femur in group 4 was significantly red
uced at 6 weeks. This study demonstrates that diaphyseal fractures tre
ated with titanium nails with a bending rigidity similar to the intact
femur have a higher maximum bending load and fracture energy at 12 we
eks than both rigidly or softly nailed fractures. BMD in the callus re
gion was also highest after titanium nailing at 12 weeks. BMD in the d
istal diaphysis and total femur indicates that the degree of stiffness
of the nails have little influence on total bone mineralisation at 12
weeks.