Less loosening with bicortical locking screws vs. nonlocking fixation

Plates fitted with all unicortical screws proved the weakest constructs during torsion testing.

Bicortical locking plates may provide similar stiffness and greater resistance to loosening compared to nonlocking plates for osteoporotic fractures.

Researchers examining a nonlocking plate construct and five locked plate constructs discovered that plates completely fixed with bicortical locking screws withstood significantly greater torsional loading than conventional plating. Plates attached with all unicortical locked screws proved the weakest constructs during torsion testing.

“Interestingly, unicortical locked constructs were significantly weak in torsion. However, we found that the addition of a single bicortical locked screw to an otherwise all unicortical construct is beneficial,” said Daniel C. Fitzpatrick, MD, an orthopedist at Orthopedic Healthcare Northwest in Oregon. He and his colleagues discovered that if a bicortical locked screw was added, the plates could withstand up to 6.92 ± 1.19 Nm of loading before onset of loosening vs. 0.59 ± 0.18 Nm with all unicortical screws. Fitzpatrick presented the findings at the Orthopaedic Trauma Association 21st Annual Meeting.

Six plating combinations

The researchers studied six plate and screw constructs with the following:

three nonlocking screws;
three bicortical screws;
three unicortical screws; and
three locked plate combinations consisting of two unicortical locked screws and one bicortical screw.

They performed the study using surrogate models that mimicked the characteristics of osteoporotic bone. “Our surrogate specimens were manufactured from 30 PCF foam with a 30 mm outside diameter and a 3 mm wall thickness,” Fitzpatrick said.

The investigators fitted the specimens with screws in the first, third and fifth plate holes. In the mixed unicortical/bicortical locked groups, they placed one bicortical screw in the first, third or fifth hole. All of the locked plate constructs were positioned 2 mm from the bone.

The researchers examined the constructs during axial loading, 4-point bending and torsion testing. “We looked at five specimens tested for each of the six constructs under the three loading scenerios for a total of 90 specimens,” Fitzpatrick said. They also noted initial stiffness, load failure and onset of loosening. Specimen fracture or plate pull-off constituted a failure, while the researchers termed onset of loosening as a 10% decrease in initial stiffness. The nonlocking construct served as the gold standard for comparison.

Adding a bicortical screw

Torsion testing revealed that plates with all bicortical locking screws upheld more than 10 Nm of loading before onset of loosening compared to 2.88 ± 0.45 Nm for plates fixed with all nonlocking screws. “We found that bicortical locked constructs showed superior resistance to loosening,” Fitzpatrick said. Loosening loads for all unicortical screw constructs only reached 0.59 ± 0.18 Nm, but the strength significantly improved with a bicortical screw. “Unicortical screws are especially weak in torsion and should be avoided in situations in which high torsional loads are expected,” the researchers wrote in their abstract.

“In axial loading, we saw a significant benefit from locked plating across all constructs,” Fitzpatrick said. Four of the five locked constructs withstood more than 600 N before onset of loosening vs. 284 ± 8 N for the nonlocking group. The researchers also found no significant differences regarding 4-point bending across the constructs.

Although the study highlights the benefits of bicortical locking plates, Fitzpatrick noted the specificity of the findings. “I should mention that these results are really limited to osteoporotic large diameter bone,” he said.

For more information:
Fitzpatrick DC, Bruehl M, Ehmke L. Relative strength of conventional and locking plate fixation in osteoporotic bone. #39. Presented at the Orthopaedic Trauma Association 21st Annual Meeting. Oct. 19-22, 2005. Ottawa.