Objective—To test the effects of bone diameter and
eccentric loading on fatigue life of 2.7-mm-diameter
cortical bone screws used for locking a 6-mm-diameter
Sample Population—Eighteen 2.7-mm-diameter
cortical bone screws.
Procedure—A simulated bone model with aluminum
tubing and a 6-mm-diameter interlocking nail was
used to load screws in cyclic 3-point bending. Group
1 included 6 screws that were centrally loaded within
19-mm-diameter aluminum tubing. Group 2 included
6 screws that were centrally loaded within 31.8-mmdiameter
aluminum tubing. Group 3 included 6
screws that were eccentrically loaded (5.5 mm from
center) within 31.8-mm-diameter aluminum tubing.
The number of cycles until screw failure and the
mode of failure were recorded.
Results—An increase in the diameter of the aluminum
tubing from 19 to 31.8 mm resulted in a significant
decrease in the number of cycles to failure (mean ± SD,
761,215 ± 239,853 to 16,941 ± 2,829 cycles, respectively).
Within 31.8-mm tubing, the number of cycles of
failure of eccentrically loaded screws (43,068 ± 14,073
cycles) was significantly greater than that of centrally
loaded screws (16,941 ± 2,829 cycles).
Conclusions and Clinical Relevance—Within a
bone, locking screws are subjected to different loading
conditions depending on location (diaphyseal vs
metaphyseal). The fatigue life of a locking screw centrally
loaded in the metaphyseal region of bone may
be shorter than in the diaphysis. Eccentric loading of
the locking screw in the metaphysis may help to
improve its fatigue life. (Am J Vet Res 2003;64:569–573)
Objective—To investigate the effect of laser shock
peening on the fatigue life and surface characteristics
of 3.5-mm-diameter cortical bone screws.
Sample Population—32 stainless steel, 3.5-mm-diameter
cortical bone screws.
Procedure—Screws were randomly assigned to an
untreated control group or 2 power-density treatment
groups, 6 gigawatts (GW)/cm2 and 8.5 GW/cm2, for laser
shock peening. Number of cycles to failure and findings
on scanning electron microscopy-assisted morphometric
evaluation, including the mode of failure, surface
debris, surface damage, and thread deformation, were
compared between control and treated screws.
Results—The 6 GW/cm2 treated screws had a significant
(11%) improvement in fatigue life, compared
with untreated control screws. The 8.5 GW/cm2 treated
screws had a significant (20%) decrease in fatigue
life, compared with control screws. A mild but significant
increase in thread deformation was evident in all
treated screws, compared with control screws. The
8.5 GW/cm2 treated screws had significantly more
surface irregularities (elevations and pits), compared
with control or 6 GW/cm2 treated screws.
Conclusions and Clinical Relevance—A modest positive
increase in fatigue strength was produced by this
design of laser shock peening on the midshaft of cortical
bone screws. High laser shock peening power densities
were detrimental, decreasing screw fatigue strength
probably resulting from structural damage. Greater
fatigue life of cortical bone screws can be generated
with laser shock peening and could reduce screw breakage
as a cause of implant failure; however, future studies
will be necessary to address biocompatibility, alternative
cleaning techniques, alterations in screw strength
and pullout characteristics, and effects on susceptibility
to corrosion. ( Am J Vet Res 2004;65:972–976)
Objective—To compare biodegradable magnesium phosphate cement (Mg-cement), calcium phosphate cement (Ca-cement), and no cement on bone repair, biocompatibility, and bone adhesive characteristics in vivo in horses.
Animals—8 clinically normal adulthorses.
Procedures—Triangular fragments (1-cm-long arms) were created by Y-shaped osteotomy of the second and fourth metatarsal bones (MTII and MTIV, respectively). Fragments were replaced in pairs to compare Mg-cement (MTII, n = 8; MTIV, 8) with Ca-cement (MTIV, 8) or with no cement (MTII, 8). Clinical and radiographic evaluations were performed for 7 weeks, at which time osteotomy sites were harvested for computed tomographic measurement of bone density and callus amount, 3-point mechanical testing, and histologic evaluation of healing pattern and biodegradation.
Results—All horses tolerated the procedure without clinical problems. Radiographically, Mg-cement secured fragments significantly closer to parent bone, compared with Ca-cement or no treatment. Callus amount and bone remodeling and healing were significantly greater with Mg-cement, compared with Ca-cement or no cement. Biomechanical testing results and callus density among treatments were not significantly different. Significantly greater woven bone was observed adjacent to the Mg-cement without foreign body reac-tion, compared with Ca-cement or no cement. The Mg-cement was not fully degraded and was still adhered to the fragment.
Conclusions and Clinical Relevance—Both bone cements were biocompatible in horses, and Mg-cementmay assistfracture repair by osteogenesis and fragmentstabilization. Fur ther studies are warranted on other applications and to define degradation characteristics.
Objective—To compare biomechanical strength, interface quality, and effects of bone healing in bone-implant interfaces that were untreated or treated with calcium phosphate cement (Ca-cement), magnesium phosphate cement (Mg-cement), or polymethylmethacrylate (PMMA) in horses.
Animals—6 adult horses.
Procedures—4 screw holes were created (day 0) in each third metacarpal and third metatarsal bone of 6 horses. In each bone, a unicortical screw was placed in each hole following application of Ca-cement, Mg-cement, PMMA, or no treatment (24 screw holes/treatment). Screws were inserted to 2.82 N m torque. Horses were euthanized and bones were harvested at day 5 (16 screw holes/treatment) or day 182 (8 screw holes/treatment). Radiography, biomechanical testing, histomorphometry, and micro–computed tomography were performed to characterize the bone-implant interfaces.
Results—Use of Mg-cement increased the peak torque to failure at bone-implant interfaces, compared with the effects of no treatment and Ca-cement, and increased interface toughness, compared with the effects of no treatment, Ca-cement, and PMMA. Histologically, there was 44% less Ca-cement and 69% less Mg-cement at the interfaces at day 182, compared with amounts present at day 5. Within screw threads, Ca-cement increased mineral density, compared with PMMA or no treatment. In the bone adjacent to the screw, Mg-cement increased mineral density, compared with PMMA or no treatment. One untreated and 1 Ca-cement–treated screw backed out after day 5.
Conclusions and Clinical Relevance—In horses, Mg-cement promoted bone-implant bonding and adjacent bone osteogenesis, which may reduce the risk of screw loosening.