Stability of Compression Screws and Locking Plate for Lapidus Arthrodesis: A Biomechanical Comparison of Plate Position

David Drummond DPM, JPS Health Network
Travis Motley DPM, MS, FACFAS, JPS Health Network
Victor Kosmopoulos PhD, UNT Health Science Center

Abstract

Objectives

To mechanically compare the resulting motion at the fracture site, stiffness, and strength of identical locking plate constructs fixed at various anatomical positions around the 1st tarsometatarsal joint on composite bone models.

Methods and Measures

Using composite bone models of the medial cuneiform and 1st metatarsal, divided into 2-3 groups each being fixated with a different construct comprised of a combination lag screw/locking plate. Fixation constructs applied to the various groups would be identical with regard to screw placement and vary only in the position of the plate -i.e., plantar, medial, and dorsal.

Fixation will consist of 2 crossing 4.0mm Cannulated Screws 40mm in length along with a 2.5mm Straight Hand Fracture High Strength Plate cut to 5 holes in length. 1 non-locking 2.5mmx20mm screw and 3 locking 2.5mmx20mm screws will be used to fix the F3 straight plate to the bone model.

Each construct will be tested with a material testing system (MTS) using a four point bending configuration (fracture site centered). Initially, 20 preconditioning cycles (range 10-40 N, 1Hz) will be applied followed by a monotonically increasing load from 10 to 100N to determine initial construct stiffness. The construct will then be fatigue tested sinusoidally between 25-100 N at 5Hz for 3000 cycles. The fatigue test will be followed by a monotonically increasing displacement at a rate of 5mm/min while measuring load. Strength will be determined at failure, defined as the force at either a (1) 3-mm gap at the fracture site as measured by an extensometer; (2) fractured bone model; or (3) a sudden drop in the load-displacement curve5 signaling a loss of fixation. Stiffness will be calculated before and after cyclic loading from the slope of the load-versus-deformation curve. Note, load ranges as described here are based on the literature. Preliminary testing using our specific setup however, will finalize the load ranges assumed.

Results (TBD, research ongoing)

Outcome measures will be (1) pre- and post-fatigue stiffness; (2) failure strength; and (3) motion at the fracture site

Conclusions

To either accept or reject our hypothesis: Locking plate anatomic position does not significantly affect strength and stiffness of the overall fixation construct in Lapidus arthrodesis.

 

Stability of Compression Screws and Locking Plate for Lapidus Arthrodesis: A Biomechanical Comparison of Plate Position

Objectives

To mechanically compare the resulting motion at the fracture site, stiffness, and strength of identical locking plate constructs fixed at various anatomical positions around the 1st tarsometatarsal joint on composite bone models.

Methods and Measures

Using composite bone models of the medial cuneiform and 1st metatarsal, divided into 2-3 groups each being fixated with a different construct comprised of a combination lag screw/locking plate. Fixation constructs applied to the various groups would be identical with regard to screw placement and vary only in the position of the plate -i.e., plantar, medial, and dorsal.

Fixation will consist of 2 crossing 4.0mm Cannulated Screws 40mm in length along with a 2.5mm Straight Hand Fracture High Strength Plate cut to 5 holes in length. 1 non-locking 2.5mmx20mm screw and 3 locking 2.5mmx20mm screws will be used to fix the F3 straight plate to the bone model.

Each construct will be tested with a material testing system (MTS) using a four point bending configuration (fracture site centered). Initially, 20 preconditioning cycles (range 10-40 N, 1Hz) will be applied followed by a monotonically increasing load from 10 to 100N to determine initial construct stiffness. The construct will then be fatigue tested sinusoidally between 25-100 N at 5Hz for 3000 cycles. The fatigue test will be followed by a monotonically increasing displacement at a rate of 5mm/min while measuring load. Strength will be determined at failure, defined as the force at either a (1) 3-mm gap at the fracture site as measured by an extensometer; (2) fractured bone model; or (3) a sudden drop in the load-displacement curve5 signaling a loss of fixation. Stiffness will be calculated before and after cyclic loading from the slope of the load-versus-deformation curve. Note, load ranges as described here are based on the literature. Preliminary testing using our specific setup however, will finalize the load ranges assumed.

Results (TBD, research ongoing)

Outcome measures will be (1) pre- and post-fatigue stiffness; (2) failure strength; and (3) motion at the fracture site

Conclusions

To either accept or reject our hypothesis: Locking plate anatomic position does not significantly affect strength and stiffness of the overall fixation construct in Lapidus arthrodesis.