Background: We aimed to design a micromotion-balancing drilling system on the basis of the existing locking plate to maintain the balance of the micromotion of the cortex on both sides of a fracture region. We tested the system by subjecting it to a few biomechanical tests.
Methods: According to the shape of screw holes on the cortex, the fixed fracture models were divided into a control group (standard screw hole group X126, 6 cases) and an experimental group (elliptical screw hole group N, 36 cases). The experimental group was further divided into 6 subgroups with 6 cases in each (N126, N136, N1256, N1356, N12356, N123456) on the basis of the number and distribution of the screws on the proximal fracture segment. The control, N126, and N136 groups were subjected to 500-N axial load, and other groups were subjected to 1000-N axial load. The displacements of the kinetic head, distal cortex, and proximal cortex were measured. The integral structural stiffness of the model and the proximal cortical strain were calculated. The data of each group were analyzed by paired t-tests.
Results: When the distal cortical strains were 2%, 5%, and 10%, the proximal cortical strains in group N126 were 0.96%, 2.35% and 4.62%, respectively, which were significantly higher than those in the control group (X126) (p<0.05). When the distal cortical strains were 2%, 5% and 10%, the proximal cortical strain in group N126 was significantly higher than that in group N136 (p<0.05). However, there was no significant difference between the proximal cortical strains in the two groups with 4 screws (p>0.05). The proximal cortical strain in the 3-screw groups was significantly higher than that in the 4-screw groups (p<0.05), and there was no significant difference in the proximal cortical strain in the 4-, 5-, and 6-screw groups (p>0.05).
Conclusions: The new drill and the matching sleeves enabled a conventional locking compression plate to be transformed into an internal fixation system and improved the balanced motion of the distal and proximal cortices. Thus, the strain on a fracture site can be controlled by adjusting the drill diameter and sleeve eccentricity.