SLR - May 2012 - Jennifer L. Buchanan
Reference: Orr JD, Glisson RR, Nunley JA. Jones Fracture Fixation: A Biomechanical Comparison of Partially Threaded Screws Versus Tapered Variable Pitch Screws. Am J Sports Med 40:3, 691-698, 2012.
Scientific Literature Review
Reviewed by: Jennifer L. Buchanan, DPM
Residency Program: Cambridge Health Alliance
Podiatric Relevance:
The Jones fracture of the fifth metatarsal has a high tendency for nonunion for several theorized reasons. Surgical fixation is commonly employed; the most common procedure is intramedullary screw fixation, most often utilizing partially threaded cancellous screws. Recently, the use of tapered variable pitch screws has gained popularity, however there are no comparative studies pertaining to the biomechanical effects of these two fixation techniques. The goal of this study was to directly compare conventional partially threaded screws to tapered variable pitched screws in a new biomechanical model.
Methods:
Twenty-three matched pairs of cadaveric fifth metatarsals were dissected of soft tissue attachments. Average age of donors was 57 years. Bone density was quantified using DEXA. Radiographic imaging was performed and the inner cortical diameter, outer cortical diameter and 70 percent length of the metatarsal were utilized to select optimal screw size. Computer randomization was used to determine type of screw to be used for fixation which included the stainless steel 4.5 and 6.5 partially threaded cancellous screws (Synthes), titanium 5.0 partially threaded cancellous screws (DePuy), and titanium Acutrak 4/5 and Acutrack Plus tapered fully threaded variable pitched titanium screws (Acumed). The fracture site was created using a saw, removing 4 mm of bone. One metatarsal from each pair received a partially threaded screw and the contralateral metatarsal received an appropriately sized Acutrack screw. Initial compression, fracture site compression, angulation and bending stiffness were compared between the two groups with cyclical loading of 0-12 N applied to the metatarsal heads in the sagittal plane over 1000 cycles. A newly designed testing apparatus was utilized to apply compressive and angular forces to simulate weight bearing activity.
Results:
There was no difference in mean bone density between the groups. Mean initial compression at fracture site obtained by conventional screws was significantly greater than that of the tapered variable screws, respectively 92.1 N and 59.4 N (p = 0.029). Mean fracture site compression was significantly greater for conventional screws compared to variable pitch screws at all 13 of the cyclic load increments (p<0.001), although there was little loss of compression on cyclical loading with both groups. The mean angulation at the fracture site with variable screw fixated metatarsals exceeded that of conventional screw fixation beginning at load cycle 10 through 1000 (0.00002> p <0.044). Although not significantly different, there was a trend toward improved bending stiffness with conventional screw fixation.
Conclusions:
This is the largest cadaveric study to date comparing conventional partially threaded screws to tapered variable pitch screws in Jones fractures. This study used new biomechanical methods to assess compression, angulation and stiffness in simulated weight bearing cycling. The results suggest initial fracture site compression and maintenance of compression throughout cyclic loading is superior with conventional screw fixation compared to variable compression screw fixation. This may affect clinical outcomes; however correlation between these laboratory data and in vivo bone healing needs further clinical studies to determine actual outcomes.