Fixation of HA-coated unicortical locking screws in a sheep gap model: a comparative biomechanical study

To evaluate whether fixing a locking compression plate (LCP) with hydroxyapatite (HA)-coated screws provides improved biomechanical fixation and healing compared with standard screws under loaded conditions.

Ten sheep were randomly divided into 2 groups. A resection osteotomy of 8 mm was performed in the sheep's right tibiae. Five tibiae were fixed with a 9-hole LCP and 8 standard unicortical locking screws (Group Non-HA), and 5 tibiae were fixed with a 9-hole LCP and 8 HA-coated unicortical locking screws (Group HA). All screws were implanted at the same insertion torque of 4000 Nmm. Three months after surgery, all the sheep were euthanized. Bone segments after screw removal were randomly chosen from each group for histologic analysis (Group Non-HA=5, Group HA=5).

Mean screw extraction torque was 438+/-288 Nmm in Group Non-HA (n=40) and 2317+/-657 Nmm in Group HA (n=40) (P<0.0005). The tibial torque resistance of the resected tibiae was 24+/-8 Nm in Group Non-HA (n=5) and 31+/-3 in Group HA (n=5) (P=0.045). In Group Non-HA, histology showed bone resorption and fibrous tissue encapsulation in all the samples, but this was not found in any of the Group HA samples.

This study shows that an LCP with HA-coated screws provides improved biomechanical fixation than an LCP with similar standard screws as shown by a 5-fold greater screw extraction torque (P<0.0005). Furthermore, the higher tibial torque resistance is potentially beneficial for improved gap healing, as shown by higher tibial torque resistance.