Biomechanical Evaluation of Hybrid Bicortical and Unicortical Screw and Bone Plate Fixation in Humeral Mid-Shaft Fractures: A Study on Cadaveric Bone

Abstract

Internal bone plate fixation is currently considered the optimal bone fixation method for open fractures of long bone. This study aims to investigate the effects of non-locking bicortical and unicortical orthopedic bone screws and bone plate configuration on an open fracture of a long bone, the humerus. Eight cadaver humerus bones (n=8) were collected and an open gap, 1.5 cm mid-shaft transverse fracture, was modeled in each one. They were fixated by a 4.5mm non-locking (D.C) plate with two different configurations of 4.5mm non-locking bone screws: Group 1 and Group 2. The orthopedic non-locking bone plate had a total of 7 holes with 3 holes on either side of the fracture site. Therefore, 2 bicortical screws (30 mm long) and 4 unicortical screws (14mm long) were implemented into the fixation in both groups. Group 1 was defined as the bicortical screws being placed close to the fracture gap while Group 2 had the bicortical screws placed further away from fracture gap. Unicortical screws were placed in the other four holes in each setup. In order to complete mechanical evaluation, fatigue and strength of the bone plate construct was assessed by using the Material Testing System (858 Mini Bionix) to simulate cyclic loading and axial failure tests. The cyclic failure test was conducted on the two group bone specimens for 4000 cycles with the cyclic loading being approximately 240N. All bone plate and bone screw setups passed cyclic loading without any problem or failures. The axial failure test was completed by conducting an axial compression force with MTS’s run rate of 0.1mm/sec and was performed on the two bone groups. The average maximum axial failure force was observed at 1064±188 (N) for Group 1 and 1446 ±467 (N) for Group 2. A two-tail t-test revealed that the strength of Group 2 was statistically significant. Thus, it was shown that Group 2’s configuration was the better option for long bone fixation.