SLR - March 2013 - Doyle
Reference: Hunt K, Barr C, Lindsey D, Chou L. Locked Versus Nonlocked Plate Fixation for First Metatarsophalangeal Arthrodesis: A Biomechanical Investigation. Foot & Ankle International. Vol. 33, No. 11 November 2012. DOI: 10.3113/FAI.2012.0984
Scientific Literature Review
Reviewed by: Dustin Doyle, DPM
Residency Program: St. Vincent Hospital, Worcester, MA
Podiatric Relevance:
The two most common disorders of the first metatarsophalangeal joint (MTPJ) are hallux valgus and hallux rigidus. One salvage option for failed procedures is first MTPJ fusion, the primary goals of which are to reduce pain and restore function. Fusion can be accomplished by a variety of fixation techniques, such as compression screws, crossed K-wires, staples, dorsal plates, and various combinations of the previously mentioned techniques. Cup-and-cone reaming of the first MTPJ and fixation with a lag screw has been shown to produce the greatest stability in many articles. However, nonunion rates and hardware failure still remain a problem.
Methods:
Nine pairs of fresh-frozen below-the-knee extremity cadaveric specimens were obtained and evaluated both visually and radiographically to ensure absence of gross deformity, previous operation, or fracture. First ray dissection was performed with care to leave ligaments intact at the MTPJ. All MTPJs were prepared using the standard arthrodesis procedure with the Wright Medical Technology cup-and-cone reamer system. The hallux was placed in 20 degrees of dorsiflexion and 10 degrees of valgus, and then the first MTPJ was temporarily stabilized with crossed K-wires. Next, a single cross-compression Synthes 3.5mm screw was placed from distal-medial to proximal-lateral across the joint. After randomization by a computer program, matched pairs of cadaveric first MTPJs then received placement of a seven-hole 2.7mm dorsal stainless steel LCP T-plate with locking or nonlocking cortical screws.
Each metatarsal base was then embedded in PVC tubing filled with PMMA that was mounted with a custom fixture on a servohydraulic materials testing machine. A perpendicular load was then applied to the plantar surfaces of the proximal phalanx at the level of the distal condyles. Each specimen was loaded in a cyclic sinusoidal fashion between five and 90 N at a rate of three Hz for a total of 250,000 cycles, replicating six weeks of postoperative weight bearing in a short leg walking cast. The load and amount of plantar MTPJ gap were recorded for the first 100 cycles, then every 100th cycle to 1,000 cycles, then every 1,000 cycles until the 250,000th cycle test was completed. Testing was terminated early if the plantar MTPJ gap reached five millimeters. Stiffness (N/mm) was calculated from the final load-to-failure test using the linear portion of the load-displacement curve, and mode of failure was recorded.
Results:
Four nonlocked plate specimens failed before reaching 250,000 cycles, while three locked plate specimens failed before reaching the cycle end. The locked plate group demonstrated significantly less plantar gapping from cycles 10,000 through 250,000 as compared with the nonlocked plate construct. There was no statistically significant difference in the ultimate load to failure between the two methods of fixation. Six of the nine specimens in the locked plate group failed by bone fracture, and six of the nine specimens in the nonlocked group failed by plate bending. Mean stiffness was significantly greater in the locked plate group compared to the nonlocked plate group. There was no significant difference in the bone mineral density between the two groups.
Conclusions:
The results demonstrate that first MTPJ arthrodesis locked plates exhibit significantly greater stiffness in load-to-failure testing and significantly less plantar gapping after 10,000 cycles of fatigue endurance testing. However, no difference was found in ultimate load to failure between the groups. Further in vivo studies are needed to determine whether locked or nonlocked plates lead to any difference in ultimate fusion rates.