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Rotational Stability of One vs Two Intramedullary Screws for Proximal Phalanx Fractures: A Biomechanical Study
Andrew L O'Brien, MD, MPH
1; Zakkary J Walterscheid, MD
2; Manaswini Chennoju, MS
2; Pooyan Abbasi, MS
2; Kenneth R. Means, Jr., MD
2; Aviram M. Giladi, MD, MS
21The Ohio State University Medical Center, Columbus, OH; 2The Curtis National Hand Center, Baltimore, MD
IntroductionProximal phalanx fractures are common injuries and numerous osteosynthesis constructs have been used for their fixation. Intramedullary screws in varying single or double screw configurations have been increasingly used for the treatment of these fractures, offering limited soft tissue disruption compared to other forms of open fixation. Despite their increasing popularity, little is known about the biomechanical stability of different intramedullary screw configurations in proximal phalanx fractures; however, there are meaningful cost and potentially rehabilitation considerations based on how much hardware is used. This study serves to assess the rotational stability of one- vs two-screw constructs in the fixation of transverse proximal phalanx fractures.Materials & MethodsMatched pairs of fresh frozen cadaver upper extremities were used. Proximal phalanges were harvested and transverse mid-diaphyseal osteotomies were made. Alternating digits and laterality, one- and two-screw constructs were created. One-screw sizing was determined by manual chatter experienced when drilling the isthmus of the phalanx; sizes ranged from 2.4-3.5mm. Two-screw constructs were placed in "V” or "lambda” configurations such that both screws crossed the fracture; all screws were 1.8mm in core diameter. Specimens were then placed in an MTS Frame and rotated at a rate of 2 degrees/sec. Outcomes included torque (N.m) to 15 degrees of rotation that we considered clinical failure, and torque to construct failure.ResultsSix matched pairs of phalanges from two cadaver specimens were prepared. At all degrees of rotation, two-screw constructs required more mean torque to displacement than one screw constructs; however, there was no statistically significant difference between two- and one-screw constructs in torque to 15 degrees of rotation (0.32 vs 0.21 Nm respectively, p=0.27). On average, two screw constructs required almost twice as much torque as one-screw to rotational failure of the model, 0.66 vs 0.35 Nm respectively (p=0.04). There was no difference in the degrees of rotation at failure between the two constructs, which occurred at 63 degrees (two-screw), and 66 degrees (one-screw), p=0.88.ConclusionIntramedullary screws are an increasingly popular modality for fixation of proximal phalanx fractures, and case series have reported good clinical outcomes. Our study demonstrates a biomechanical advantage of two screws vs one to protect against rotational deformity to total failure; however, the two-screw construct did not provide nearly as substantial a benefit in protecting from clinical failure of 15-degree rotation.
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