Background: Headless screw is a standard implant for an osteochondral fragment fixation. With a threaded design, the screw head can be buried under the articular cartilage to prevent a post-traumatic arthritis. However, the screw is expensive and maybe not available in the emergency situation. The 2.4-mm AO locking screw also has a threaded head which is able to advance underneath the cartilage. This has been used for fixation of the osteochondal fracture clinically. We compared the pullout strength of 2.4-mm AO locking screw with those of Herbert screw and 2.0-mm AO cortical screw.
Material and Method: The studies performed by using Instron 4502 to measure the pullout strength in 12 models for each type of the screw. The pullout strength of the 2.4-mm AO locking screw from a corticocancellous bone model was compared with the pullout strength of the Herbert screw from a cancellous bone model and the 2.0-mm AO cortical screw from the corticocancellous bone model. The differences in pullout strength between the 2.4-mm AO locking screw and the other two screws were determined by independent t-test.
Results: The pullout strength of the 2.4-mm AO locking screw, Herbert screw and 2.0-mm AO cortical screw were 143.49+46.18 N, 72.83+16.64 N, and 80.38+1.42 N, respectively. The pullout strength of 2.4-mm AO locking screw was significantly higher than those of Herbert screw and 2.0-mm AO cortical screw (p<0.001).
Conclusion: According to the recent biomechanical study, the 2.4-mm AO locking screw in the corticocancellous bone model had pullout strength higher than the Herbert screw in the cancellous bone model and the 2.0-mm AO cortical screw in corticocancellous bone model. The 2.4-mm AO locking screw may use instead of headless screw for intra-articular fixation in a specific situation, such as when the headless screw is unavailable.

Keywords: Pullout strength, AO screw, Locking screw, Herbert screw