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. 2015 Sep 8:10:139.
doi: 10.1186/s13018-015-0283-8.

The influence of distal screw length on the primary stability of volar plate osteosynthesis--a biomechanical study

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The influence of distal screw length on the primary stability of volar plate osteosynthesis--a biomechanical study

Sebastian F Baumbach et al. J Orthop Surg Res. .

Abstract

Background: Extensor tendon irritation is one of the most common complications following volar locking plate osteosynthesis (VLPO) for distal radius fractures. It is most likely caused by distal screws protruding the dorsal cortex. Shorter distal screws could avoid this, yet the influence of distal screw length on the primary stability in VLPO is unknown. The aim of this study was to compare 75 to 100% distal screw lengths in VLPO.

Methods: A biomechanical study was conducted on 11 paired fresh-frozen radii. HRpQCT scans were performed to assess bone mineral density (BMD) and bone mineral content (BMC). The specimens were randomized pair-wise into two groups: 100% (group A) and 75% (group B) unicortical distal screw lengths. A validated fracture model for extra-articular distal radius fractures (AO-23 A3) was used. Polyaxial volar locking plates were mounted, and distal screws was inserted using a drill guide block. For group A, the distal screw tips were intended to be flush or just short of the dorsal cortex. In group B, a target screw length of 75% was calculated. The specimens were tested to failure using a displacement-controlled axial compression test. Primary biomechanical stability was assessed by stiffness, elastic limit, and maximum force as well as with residual tilt, which quantified plastic deformation.

Results: Nine specimens were tested successfully. BMD and BMC did not differ between the two groups. The mean distal screw length of group A was 21.7 ± 2.6 mm (range: 16 to 26 mm), for group B 16.9 ± 1.9 mm (range: 12 to 20 mm). Distal screws in group B were on average 5.6 ± 0.9 mm (range: 3 to 7 mm) shorter than measured. No significant differences were found for stiffness (706 ± 103 N/mm vs. 660 ± 124 N/mm), elastic limit (177 ± 25 N vs. 167 ± 36 N), maximum force (493 ± 139 N vs. 471 ± 149 N), or residual tilt (7.3° ± 0.7° vs. 7.1° ± 1.3°).

Conclusion: The 75% distal screw length in VLPO provides similar primary stability to 100% unicortical screw length. This study, for the first time, provides the biomechanical basis to choose distal screws significantly shorter then measured.

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Figures

Fig. 1
Fig. 1
Outline of the 100 % (group A) and 75 % distal screw length (group B) setups. A unicortical, 100 % distal screw length (Group A). B 75 % distal screw length (Group B). 1 Skyline view, 2 lateral radiograph
Fig. 2
Fig. 2
Illustration of the biomechanical setup (one half of the aluminium jigs were removed to show the embedding). A Photography of the final setup; A1 custom-made aluminium-jigs, A2 multiaxial load cell, A3 Zebris motion tracking system. B Schematic drawing of the final setup illustrating the load axis
Fig. 3
Fig. 3
Analysis of the load-displacement curves and the motion tracking system. A Analysis of a typical force-displacement curve. B Analysis of the residual tilt using the Zebris motion tracking system

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