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Biomechanical Comparison of a Locking Plate with Interfragmentary Screw Compression for Lapidus Arthrodesis versus a Locking Plate with Intraplate Compression Screw: A Cadaveric Study Joseph S. Baker, DPM, AACFAS 1 ; James M. Cottom, DPM FACFAS 2 1 – Desert Podiatric Medical Specialists, Tucson, AZ 2 - Florida Orthopedic Foot and Ankle Center, Sarasota, FL Ultimate Load (N) Moment (Nm) Stiffness (N/m 2 ) Plantar Lapidus Plate 197 ± 109 8 ± 4 17 ± 11 Medial Lapidus Plate with plantar lag screw 255 ± 155 10 ± 6 26 ± 16 Paired t test results p=0.402 p=0.402 p=0.243 Introduction The first-metatarsal-cuneiform arthrodesis traditionally required a prolonged period of non-weightbearing postoperatively, though this has been challenged in recent years and weight bearing advancement has been more aggressive recently (1-4). In order for patients to begin weightbearing earlier in the postoperative phase, the construct used for fusion must be strong enough to withstand this stress. In the quest for the ultimate construct for the Lapidus arthrodesis, the concept of applying fixation to the plantar aspect of the first metatarsal cuneiform joint has also been investigated, as it theoretically provides a stronger construct due to the fixation resisting bending on the torsion side of the joint (7-11). We sought to investigate in a laboratory setting the strength of a plantarly- applied Lapidus locking plate with intraplate lag screw technology. In doing so, we compared the strength of this construct to a previously described construct using a plantar lag screw and a medially-applied low profile locking plate (5, 11). Our hypothesis was that the plantarly-applied plate would have superior strength to the medially-applied plate. Materials and Methods Eight matched pairs of fresh cadaveric lower limbs were used in this experiment. In all specimens, a medial approach was used along the first metatarsal cuneiform joint. The joint was prepared using a curettage technique, keeping the subchondral plate intact. One specimen in each matched pair was then fixated with a medially-applied low-profile system (LPS) locking Lapidus plate (Arthrex, Naples, FL) with an additional 4.5-mm cannulated screw directed from plantar-distal to dorsal-proximal (Fig. 1). The second specimen in the matched pair was fixated using a plantarly-applied locking plate, which incorporates a 4.0-mm compression screw into a distal slot, placing the screw from plantar-distal to dorsal-proximal (Figs. 2,3). After surgical repair, the medial cuneiform and first metatarsal were dissected en bloc in all 16 specimens. A pilot hole was then created in the navicular using a .094” drill pin. Each specimen was then placed into a 1.5” schedule 40 PVC pipe. A .094” drill pin was then placed through the PVC pipe and the navicular bone. Specimens were then potted using polymethylmethacrylate (PMMA) bone cement, taking care to cover as much bone as possible without covering the surgical repair site. The potted specimens were then fixed to the base of the Instron 8871 Servo Hydraulic Materials Testing System (Instron, Canton, MA) by securing the PVC pipe in a V-block at approximately 15- degrees from horizontal in order to simulate an anatomic weight bearing position. A plunger with a rounded tip was positioned 4cm from the repair site. Specimens were then loaded to failure at a compressive rate of 5mm/min (Fig. 4, 5). The ultimate load to failure, moment, and stiffness of the constructs was recorded for analysis. Statistical analysis was then performed by way of a paired t test, with the confidence level set at ⍺=0.05. Results There were 8 matched pairs used in this study. One foot from each of the matched pairs underwent a first metatarsal cuneiform arthrodesis with a medial approach as described above in the methods section. In the first group, the arthrodesis site was fixated with a medially-applied LPS plate and a plantar-distal to dorsal-proximal 4.5-mm cannulated interfragmentary screw. The ultimate load to failure in this group after being tested was 255.38 ± 155.38 N. The moment at time of failure was 10.22 ± 6.22 Nm, and the stiffness was 25.73 ± 15.74 N/m 2 . The second group was fixated using a plantarly-applied anatomic Lapidus plate (Arthrex, Naples, FL). The ultimate load to failure in this group was 197.48 N ± 108.61. The moment at time of failure was 7.89 ± 4.34 Nm, and the stiffness was 17.33 ± 11.49 N/m 2 . Statistical analysis was then performed using a paired t test, with the confidence level set at ⍺=0.05. No significant difference was found between the two groups with respect to ultimate load to failure (p=0.402), moment at time of failure (p=0.402), or stiffness (p=0.243). The results of the experiment are summarized in Table 1. Discussion While we hypothesized that the plantarly-applied plate would be stronger than the medially-applied plate, the results show that the opposite was true, though this difference did not reach statistical significance. Plantar plating for Lapidus arthrodesis has been an attractive option for foot and ankle surgeons, with more anatomic plates becoming available commercially (9). Adding to the popularity of the plantarly-applied plates are several studies comparing the strength of fixation to 3.5mm screws alone (6) or to dorsally-applied plates (8). Saxena et al. also looked at plantarly-applied fixation in a clinical study and commented on the importance of fixating the plantar, or tension side, of the joint (7). While we expected the plantar plate to in fact be stronger than the medial plate, this was not actually the case. One possible reason for this observation is the notion that perhaps the tension side of the first metatarsal cuneiform joint in a foot with hallux valgus is not truly on the plantar side, but actually lies on the medial-plantar side. The concept of the plantar aspect of the joint being the tension side comes from the AO/ASIF principles of fracture management (12). This theory of the plantar aspect of the first metatarsal cuneiform joint being the tension side of the joint may represent a simplified application of this concept, using two-dimensional diagrammatic thinking and applying it to a three-dimensional concept. Further research directed at determining the three-dimensional aspect of the first metatarsal cuneiform joint and its tension and compression side may be warranted, though it is beyond the scope of the current investigation. References: 1. Sorensen MD, Hyer CF, Berlet GC. Results of Lapidus arthrodesis and locked plating with early weightbearing. Foot Ankle Spec 2:227-233, 2009. 2. Basile P, Cook EA, Cook JJ. Immediate weight-bearing following modified Lapidus arthrodesis. J Foot Ankle Surg 49:459-464, 2010. 3. Donnenworth MP, Borkosky SL, Abicht BP, Plovanich EJ, Roukis TS, Rate of nonunion after first metatarsal-cuneiform arthrodesis using joint curettage and two crossed compression screw fixation: a systematic review. J Foot Ankle Surg 50:707-709, 2011. 4. Prissel MA, Hyer CF, Grambart ST, Bussewitz BW, Brigido SA, DiDomenico LA, Lee MS, Reeves CL, Shane AM, Tucker DJ, Weinraub GM. A multicenter, retrospective study of early weughtbearing for modified Lapidus arthrodesis. J Foot Ankle Surg 55:226-229, 2016. 5. Cottom JM, Rigby RB. Biomechanical comparison of a locking plate with intraplate compression screw versus locking plate with plantar interfragmentary screw for Lapidus arthrodesis: a cadaveric study. J Foot Ankle Surg 52:339-342, 2013. 6. Marks RM, Parks BG, Schon LC. Midfoot fusion technique for neuroarthropathic feet: biomechanical analysis and rationale. Foot Ankle Int 19:507-510, 1998. 7. Saxena A, Nguyen A, Nelson E. Lapidus bunionectomy: early evaluation of crossed lag screws versus locking plate with plantar lag screw. J Foot Ankle Surg 48:170-179, 2009. 8. Klos K, Simons P, Hajduk A, Hoffmeier KL, Gras F, Fröber R, Hofmann GO, Mückley T. Plantar versus dorsomedial locked plating for Lapidus arthrodesis: a biomechanical comparison. Foot Ankle Int 32:1081-1085, 2011. 9. Plaass C, Claassen L, Daniilidis K, Funmy M, Stukenborg-Colsman C, Schmiedl A, Ettinger S. Placement of plantar plates for Lapidus arthrodesis: anatomical considerations. Foot Ankle Int 37:1-6, 2016. 10. Klos K, Wilde CH, Lange A, Wagner A, Gras F, Skulev HK, Mückley T, Simons P. Modified Lapidus arthrodesis with plantar plate and compression screw for treatment of hallux valgus with hypermobility of the first ray: A preliminary report. Foot Ankle Surg 19: 239-244, 2013. 11. Cottom JM, Vora AM. Fixation of Lapidus arthrodesis with a plantar interfragmentary screw and medial locking plate: a report of 88 cases. J Foot Ankle Surg 52:465-469, 2013. 12. Rüedi P, Buckley R, Moran C: AO Principles of Fracture Management, Stuttgart, Thieme, 2007 Figure 1 Figure 2 Figure 3 Figure 4 Figure 5
