Does rotational strain at screw tightening affect the attainment or maintenance of osseointegration?

Objectives: This study investigated whether rotational strain affects osseointegration. Material and Methods: A total of 135 male rats were divided into five groups: 2-w rotation, 4-w rotation, 8-w rotation, 12-w rotation and control. Two hundred and seventy implants were inserted in rat tibia. The control group received no strain, while the 2-w, 4-w, 8-w and 12-w rotation groups received rotational strain at 2, 4, 8 and 12 weeks after implant placement, respectively. Removal torque (N cm) was measured in vivo. Bone contact rate (%) was calculated histomorphologically. Immunostaining for osteonectin (ON), osteopontin (OPN) and osteocalcin (OCN) was performed. The removal torque and bone contact rate were analyzed using one-way analyses of variance and the Scheffé method. Results: At 4 weeks, the torque was significantly higher in the 2-w rotation group (1.30 ± 0.44 N cm) than in the control group (0.79 ± 0.67 N cm). From 8 to 16 weeks, the strained groups showed no significant differences from the control group. From the bone contact rates, bone formation was larger in the 4-week rotation group (62.9 ± 10.7%) than in the control group (42.1 ± 17.9%) at 8 weeks. The 4-week rotation group showed higher bone contact rate (61.1 ± 11.3%) compared with the other strained groups and maintained this higher value until 16 weeks, showing no significant difference from the control group (72 ± 5.2%). At the implant-bone interface, OPN was widely distributed and OCN was detected at a low level; however, ON could not be observed in any group. Conclusions: The bone contact rate changed when rotational strain was exerted at different periods after implant placement. However, the removal torque and distribution of extracellular matrix proteins were not adversely affected by the rotational strain.