Reduction of pullout strength caused by reinsertion of 3.5-mm cortical screws

Osteosynthesis of the tibia, tibial plafond, and calcaneus is commonly performed with plates and 3.5-mm self-tapping cortical screws. Screw insertion and reinsertion within the same hole in the bone may occur during surgery. Therefore, the purpose of this study was to evaluate the pullout strength of 3.5-mm self-tapping screws with up to 5 re-insertions in the diaphysis of the tibia, metaphysis of the distal tibia, calcaneus, and a polyurethane synthetic bone model.

Screws were inserted into a synthetic bone model and 5 pairs of human cadaveric diaphyseal tibiae, distal tibiae, and calcanei. The bone was predrilled, and then 3.5-mm cortical self-tapping 316 L stainless steel screws with a washer were inserted bicortically. Screws were inserted from 1 to 5 times at each location. The screws were grasped and subjected to 5-mm/min tensional force via the biaxial material testing systems machine. Statistical significance was determined using a paired 2-tailed t test.

There was a significant difference in the pullout strength of the tibial diaphysis (1710 ± 550 N), tibial metaphysis (471 ± 266 N), and calcaneus (238 ± 90 N; P < 0.01). The tibial diaphysis pullout strength was 1710 ± 550 N for one insertion differing significantly relative to the groups with 4 (average 1030 ± 543 N, P = 0.004) or 5 (average 364 ± 209 N, P < 0.001) insertions. The tibial metaphyseal pullout strength for the single insertion group was 471 ± 266 N and differed significantly relative to the 3 (P = 0.026), 4 (P = 0.044), and 5 (P = 0.042) insertion groups. The calcaneal pullout strength for the single insertion group was 238 ± 90 N with a significant difference of the 1, 3, and 4, versus the 5 insertion group (P = 0.027, 0.040, and 0.033, respectively). The synthetic bone model pullout strength decreased significantly from the one insertion group relative to all other insertion groups (group 1, 1167 ± 263 N; group 2, 768 ± 199 N; group 3, 694 ± 295 N; group 4, 662 ± 356 N; and group 5, 154 ± 183 N; P < 0.02).

There is a significant decrease in relative pullout strength of 3.5-mm self-tapping cortical screws when comparing the tibial diaphysis, tibial metaphysis, and calcaneus. There is also a significant decrease in 3.5-mm self-tapping screw pullout strength after repeated reinsertions in the synthetic bone model, mid-shaft tibia, metaphyseal tibia, and calcaneus. We recommend that during osteosynthesis, careful screw insertion, and minimal reinsertion be performed.