Biomechanical comparison of osteoporotic distal radius fractures fixed by distal locking screws with different length

Abstract

Objectives: To evaluate the postoperative stability of osteoporotic distal radius fractures fixed with distal locking screws with different length.

Methods: A comminuted extra-articular dorsally unstable distal radius fracture, treated with volar locking plate system, was created. The 18 specimens were randomized into 3 groups based on distal locked screws with different length: Group A had unicortical screws with 50% length to the dorsal cortex. Group B had unicortical screws with 75% length to the dorsal cortex. Group C had bicortical screws. Axial compression and bending loads were imposed on the models before and after cycling testing as well as load to clinical and catastrophic failure.

Results: Minimum change in stiffness was observed before and after fatigue for all groups. The final stiffness to bending forces was statistically similar in all groups, but stiffness to axial compression was statistically significant different: Group A approached significance with respect to groups B and C (P = 0.017, 0.009), whereas stiffness in group B and C was statistically similar (P = 0.93). Load to clinical failure was significantly less for group A (456.54±78.59 N) compared with groups B (580.24±73.85 N) and C (591.07±38.40 N). Load to catastrophic failure was statistically similar between groups, but mean values for Group A were 18% less than means for Group C.

Conclusions: The volar locking plate system fixed with unicortical locking screws with at least 75% length not only produced early stability for osteoporotic distal radius fractures, but also avoided extensor tendon complications due to dorsal screw protrusion.

Figure 1. Different fixations in the distal fragment according to groups:

(A) screws and plates used in the experiment, (B) fixation with bicortical screws, (C) fixation with 75% unicortical screws, (D) fixation with 50% unicortical screws.

Figure 2. The specimens tested under different loading conditions in a materials testing machine:

(A) A specimen was in double plastic wrapping, (B) volar bending, (C) dorsal bending, (D) axial compression, (E) A specimen under axial fatigue test.

Figure 3. Axial stiffness before and after cyclic testing by different fixation groups.

Mean stiffness of each group under axial loading condition were not significantly different (P>0.05). Numbers over bar indicated p values which were conducted through Independent-Samples T Test.

Figure 4. Mean stiffness results under volar and dorsal bending between groups.

Stiffness values of each group under volar and dorsal bending load were not significantly different (P>0.05). P values were determined by way of independent sample test.

Figure 5. Comparison of postcycling axial stiffness among different groups.

The stiffness were significantly different in all groups (P = 0.002) and P values were determined using One-way analysis of variance. While mean axial stiffness in group B and group C was statistically similar (P = 0.93).

Figure 6. Comparison of mean load to clinical and catastrophic failure after cyclic conditioning by different fixation groups.

As for load to clinical failure, there were statistically significant differences among all groups (P = 0.013), while the differences were not statistically significant for load to catastrophic failure (P = 0.483).

Figure 7. Catastrophic failure modes seen in specimens after failure testing:

(A) A specimen failed by collapse of the location of distal radius diaphysis and slight deformation of mid-plate, (B) A specimen failed by collapse of the location of distal radius diaphysis and uncoupling of the proximal locking screw from the screw hole, (C) A specimen failed by collapse of the location of distal radius diaphysis.