A mechanical comparison of the locking compression plate (LCP) and the low contact-dynamic compression plate (DCP) in an osteoporotic bone model.

OBJECTIVE

To determine if locking compression plates (LCP) are mechanically advantageous compared to low-contact dynamic compression plates (DCP) when used as a bridging plate in a synthetic model of osteoporotic bone.

METHODS

Five synthetic bars (Synbone Osteoporotic bone) were initially tested in compression and the Young's modulus determined. It was found to be comparable to that of tibial cancellous bone in an 80-year-old woman; thus, the synthetic bars were deemed usable to simulate some properties of osteoporotic bone. Six bars were then instrumented with an 8-hole narrow large fragment DCP with six 4.5-mm cortical screws (placed in holes 1-3 and holes 6-8). Six bars were instrumented with a narrow 8-hole LCP using four 5-mm locking screws (placed in the 1st, 3rd, 6th, and 8th holes). In a third group, 6 synbone bars were instrumented with a narrow 8-hole large fragment DCP. Cortical screws were placed in holes 2-3 and holes 6-7. In holes 1 and 8, two 6.5 fully threaded cancellous screws were inserted. A 1-cm osteotomy was created in the Synbone at the center of each plate to represent a comminuted fracture. Initially, quasi-static testing was carried out on all specimens in compression to a maximum load of 450 N. Then 4-point bend tests were carried out in two planes (0 degrees and 90 degrees) with the maximum bending moment at 3.5 Nm. Finally, torsional testing was done to a maximum load of 3.5 Nm. The specimens were then cycled in axial compression 350 N at 5 Hz for 30,000 cycles. The static nondestructive tests were repeated. The slope of the load deformation curve indicated the relative stiffness of the construct. The slopes were determined pre- and postcycling and the loss of stiffness in each group compared. Statistical analysis was carried out using the paired t-test. The specimens were then loaded to failure in compression.

RESULTS

There was no statistical difference in the stiffness of the LCP or in the osteotomy gap postcycling. All specimens in the DCP groups failed initial static testing in axial compression. No fatigue testing could be undertaken in this group.

CONCLUSIONS

In a synthetic model, the LCP was mechanically superior to the DCP when used as a bridging plate and tested in axial compression.