A Biomechanical Comparison of a Dorsal Plate and Cross Screw Versus Full Thread Screws and Subchondral Fixation for First Tarsometatarsal Fusion

Foot and ankle joint fusion fixation traditionally uses bridging plates or compression screws. However plates require incisions unsuitable for percutaneous surgery. Therefore in creating a percutaneous Lapidus procedure a new form of fusion fixation was designed. The concept preserves subchondral bone after complete cartilage removal. Fixation is with intra osseus full thread non variable pitch screws. The purpose of this study was to compare the biomechanical properties of the open plate and cross screw construct vs. the percutaneous construct. The cadaveric study goal was to compare the cycles to failure, load to failure and stiffness of each fixation type, and the plantar gap measured during each cycle throughout the test. Loading conditions replicated 6 weeks of walking in a short-leg walking cast. The TMT joint of six matched cadaveric legs were fixated with a Lapidus procedure: Six limbs with a locking plate and 3.5 mm cross compression lag screw (“plated treatment”) and six with a percutaneous Lapidus procedure, preserving subchondral bone (“MIS treatment”). The three medial metatarsals, cuneiforms, and the navicular were excised from each cadaveric limb and potted, with the TMT joint exposed ~1 cm, then frozen. Specimens were thawed, an extensometer was placed on the plantar side of the TMT, and the 1st metatarsal was loaded 5 cm distal to the TMT joint, from 9N to 90N compression at 3 Hz. Testing was stopped if the extensometer reached 7 mm of plantar gap, or 250,000 cycles were reached. Following fatigue loading, specimens were statically loaded to failure. The maximum load and stiffness were recorded. Bone screw pullout strength and compressive modulus of the distal metatarsal were measured. MIS-treated specimens reached significantly-higher cycles to failure (221,452 cycles) compared to plate-treated specimens (34,941 cycles). Plantar gap was significantly higher at all cycle counts from 10 to 10000 cycles (> 4mm) in the plate-treated group than in the MIS-treated group (The maximum load observed in the cantilever static test following fatigue testing was significantly higher in the MIS-treated group (343.3 N) compared to the plate-treated group (247.3 N). The stiffness of the MIS-treated group was also significantly higher than the plate treated group (40.9 N/mm vs 16.0 N/mm). No statistical differences in the screw pullout strength nor compressive modulus were observed. A Cox Proportional Hazard model identified the mode of fixation as the only significant covariant (p=0.03) when predicting cycles-to-failure. Our data indicate the percutaneous Lapidus construct had significantly lower plantar gap during cyclic loading, higher load-to-failure following cyclic loading, higher stiffness, and higher number of cycles required to reach 7 mm of displacement compared to the plate. The implant type (MIS or Plate) was the only covariant which predicted fatigue loading cycles to failure. Stratifying the baseline hazards by implant type in the hazard model, no other covariant reached statistical significance. The percutaneous subchondral fixation fusion construct with full thread non variable pitch intraosseous screws allows stronger fixation than traditional plates and cross screws.