A Cadaver Biomechanical Comparison of the Plate Construct for Open Lapidus Fusion versus Percutaneous Lapidus Fusion Using Subchondral Fixation

Alastair Younger, Stephen Steinlauf, Nathan Webb, Brian Thoren, Douglas Linton

Introduction: Tarsometatarsal (TMT) fusions traditionally use bridge plates or compression screws through open incisions to correct deformity and achieve bone healing. A new percutaneous technique can remove cartilage, preserve the subchondral bone and transfix the fusion site. Intraosseus, headless, chamfered, full-thread, non-variable pitch screws are used to transfix all available bone. This study compared the biomechanical properties of the plate construct versus the percutaneous construct.

Method: TMT joints of six cadavers (12 limbs) were stabilized with a locking plate and 3.5 mm compression screw on one limb, and with a percutaneous Lapidus construct on the matched limb. An extensometer was placed on the plantar side of the TMT. The first metatarsal was point loaded cyclically from 9 to 90 N at 60 mm from the TMT joint at 3 Hz. Testing was stopped if the extensometer reached 7 mm of plantar gap, or when 250,000 cycles were reached. Specimens were then statically loaded to failure.

Results: Percutaneous screw specimens reached more cycles to failure at 226,000 ± 58,000 versus 30,000 ± 52,000 for the open plate construct (p<.001). Plantar gap was higher in the open plate construct at all cycle counts from 10 to 10,000 cycles (>4 mm) versus the percutaneous group (<1 mm). Maximum load was higher in the percutaneous group (343.3 ± 92.8 N versus 247.3 ± 28.3 N) (p<.05). Stiffness of the percutaneous group was higher (40.9 ± 13.4 N/mm versus 16.0 ± 5.3 N/mm) (p<.01). No differences were found in bone quality testing. A Cox proportional hazard model identified the mode of fixation as the only significant covariant (p<.05) in predicting cycles to failure.

Conclusion: The percutaneous subchondral fixation fusion construct with full-thread, non-variable pitch, intraosseous screws create a stiffer and stronger fixation construct than dorsal plates and cross-screws.