Biomechanical Comparison between Preloaded Positional Screwing and Lag Screwing in Fracture Fixation
Date Issued
2015
Date
2015
Author(s)
Ho, Chi-Yuan
Abstract
Humeral condylar fracture is a common fracture in canine. In order to preserve the joint function, surgical management for reduction and steady fixation via transcondylar lag screw is recommended to achieve the direct bone healing. The advantage of lag screws is fixing the fracture and generating a compression effect concurrently. However, disadvantages of lag screws, such as increased loss of bone tissue, larger area for surgical approach, and creating more damage to the surrounding soft tissues, may delay the bone healing. In the literature, preloading compression effect has been proposed and applied in the fracture fixation for long bone. Prior to installing screws, a preloading compression device is applied to provide the interfragmentary compression effect, which would be maintained after screw installation and device removal. The aims of the study is to evaluate the inter-fragmentary compression effect and biomechanical strength between preloading positional screws (PPS) and traditional lag screws (LS) in fracture fixation. In the first stage of the study, fresh porcine rib was used to create the bone fracture model. The rib was transversely sawed at the midpoint, and randomly one of the paired segments was assigned to the LS group and the other to the PPS group. In each segment, a semi-cylindrical-shaped, 2-cm length, free bone fragment was created. A wireless film compressive sensor was placed in the fracture gap to record the real-time inter-fragmentary mechanical force, followed by screw installation. Three experiments were carried out: a) 0.2 Nm toque was applied to install screws in both groups in 7 paired samples, b) 0.4 Nm toque was applied to install screws in both groups in 5 paired samples, and c) 0.2 Nm toque was applied in LS group and 0.4 Nm torque in PPS group in 10 paired samples. Data from the film compressive sensor were analyzed by Mann-Whitney test. P value of less than 0.05 was considered statistically significant. In the second stage of the study, condylar fracture was created in paired humerus from canine cadaver and fixed by a LS or PPS. Biomechanical testing and radiographic evaluation was performed. When applied 0.2 Nm torque, no significant difference was detected between two groups regarding the interfragmentary force and pressure. When applied 0.4 Nm torque, all LS failed due to stripped-thread so did not generate interfragmentary force and pressure. In the PSS group, the interfragmentary force and pressure was 3.16±0.82 kg and 1.83±0.47 MPa respectively. There were significant differences between two groups regarding both the interfragmentary force and pressure (p<0.01). In the third experiment, the interfragmentary force and pressure were 1.80±0.91 kg and 1.04±0.53 MPa in the LS group under 0.2 Nm torque, 3.07±1.25 kg and 1.77±0.74 MPa in the PPS group under 0.4 Nm torque. Significant differences were detected between two groups regarding the interfragmentary force and pressure (p<0.05). When comparing the interfragmentary force and pressure between conventional unpreloaded lag screws and preloaded lag screws after the removal of the preloading device, the results showed no significant difference between two groups no matter in force (force: 3.01±1.06 kg verse 3.88±1.17 kg, p=0.75; pressure: 1.74±0.61 MPa verse 2.24±0.67 MPa, p=0.75). However, when comparing the interfragmentary force and pressure before and after the removal of the preloading device in the group of preloaded lag screws, significant difference was detected in force (6.06±1.72 kg verse 3.01±1.06 kg, p<0.01) and pressure (3.50±0.99 MPa verse 1.74±0.61 MPa, p<0.01). There was a pilot study in the second stage of the study, two pairs of humerus from a 7-kg and a 22-kg canine cadaver were examined. Radiographic evaluation revealed less interfragmentary gap in the PPS group. In addition, the displacements of LS and PPS were 7.96 mm and 4.11 mm respectively in 7-kg dog model before implant failure. The 60% of the body weight were 1.66 mm and 0.68 mm, and 130% of the body weight were 2.44 mm and 0.68 mm. In 22-kg dog model, the displacements were 17.54 mm and 7.97 before implant failure. The 60% of the body weight were 5.74 mm and 1.37 mm, and 130% of the body weight were 9.64 mm and 3.97 mm. Less device displacement was also noted in the PPS group under the maximal force prior to implant failure and the force equal to 60% and 130% of the body weight. In conclusion, compared with traditional lag screws, preloading positional screw provides larger interfragmentary force and pressure, tolerates larger toque when installing, has lower chance of thread stripping, and creates less interfragmentary displacement during the biomechanical testing. However, larger sample size for the radiographic evaluation and biomechanical testing is needed to more dogs cadaver models further confirm the current observation and needed living patient in clinical application outcome.
Subjects
preloading compression effect
positional screw
lag screw
condylar fracture
Type
thesis
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