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


CONCLUSIONS
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.
Department of Trauma Surgery, University Hospital of Munich (LMU), Campus Innenstadt, Nußbaumstrasse 20, 80336, Munich, Germany. sebastian.baumbach@med.uni-muenchen.de.
Sebastian F
SF
Baumbach
Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Getreidemarkt 9, 1060, Vienna, Austria.
Alexander
A
Synek
Center of Anatomy and Cell Biology, Department of Systematic Anatomy, Medical University Vienna, Währinger Straße 13, 1090, Vienna, Austria.
Hannes
H
Traxler
Department of Trauma Surgery, University Hospital of Munich (LMU), Campus Innenstadt, Nußbaumstrasse 20, 80336, Munich, Germany.
Wolf
W
Mutschler
Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Getreidemarkt 9, 1060, Vienna, Austria.
Dieter
D
Pahr
Department of Orthopedic Surgery, Physical Medicine and Rehabilitation, University Hospital of Munich (LMU), Campus Großhadern, Marchioninistrasse 15, 81377, Munich, Germany.
Yan
Y
Chevalier
D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology*; D001860:Bone Plates; D001863:Bone Screws; D005260:Female; D005593:Fracture Fixation, Internal / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D008297:Male; D011885:Radius Fractures / Q000473:pathology* / Q000601:surgery*