SLR - April 2014 - Jennifer C. Van
Reference: Bishop JA, Behn AW, Castillo TN. J Orthop Trauma. 2014 Feb; 28 (2): 114 – 117
Scientific Literature Review
Reviewed By: Jennifer C. Van, DPM
Residency Program: Temple University Hospital, Philadelphia, PA
Podiatric Relevance: A vast array of procedures in podiatric surgery, including arthrodesis, osteotomies, and open reduction with internal fixation, requires the use of screws to generate compression. Particularly with fracture fixation, washers can be used with lag screws to optimize compression and minimize the risk of unintentional penetration, or intrusion, of the screw head through cortical bone during screw insertion.
Washers work by distributing the compressive force applied by a screw head over a greater area, allowing more compression before the waster penetrates the cortex of the bone. This article attempts to detail the biomechanical consequences of washer use and screw intrusion, defined as violation of a cortex by a screw head or washer.
Methods: Biomechanical analysis was carried out using synthetic bone blocks fabricated to simulate a thin cortex and cancellous bone of normal density. 7.0 mm partially threaded cannulated screws (Synthes, West Chester, PA) were inserted and tightened at a rate of 5 degrees per second rotation until intrusion of the screw head or washer occurred. The compressive force measured was recorded at 10 Hz. Next, the maximum compressive force and the postintrusion compressive force were determined for each test. Screw intrustion was defined as a failure of the synthetic cortical bone at the screw head interface, which was accompanied by a sustained drop in compressive load.
Twenty-four screws were tested with a washer (n=8), without a washer (n=8), and with a washer after initially being intruded without a washer (n=8). The third scenario was characterized as a “salvage screw.” Statistical analysis was performed using the student t tests and analysis of variance with SPSS version 20.0 software. Significance was set at P = 0.05.
Results: Partially threaded cannulated screws inserted with washers generated a mean compressive force of 2916 N (SD: 116 N) before intrusion and 2669 N (SD: 117 N) after intrusion. Screws without washers generated a mean of 1167 N (SD: 157 N) before intrusion and 1032 N (SD: 174 N) after intrusion. Salvage screws produced a mean compressive force of 2616 N (SD: 280 N) before intrusion and 2497 N (SD: 258 N) after intrusion.
The differences between preintrusion and postintrusion compressive force were statistically significant for each test condition. Moreover, the difference in both preintrusion and postintrusion compressive force between screws inserted with and without washers was statistically significant (P < 0.001). Though the mean preintrusion compressive force produced by salvage screws was greater than that of screws inserted without washers, it was significantly less than the mean compressive force produced by screws inserted with washers primarily (P = 0.02).
Conclusions: This study quantifies the biomechanical advantage of placing screws with washers as providing approximately 2.5 times more compressive force than screws placed without washers. Moreover, regardless of washer use, the screw intrusion during screw insertion compromises fixation quality with a loss of compressive force of 7-16 percent if a washer was not used, 7-9 percent if a washer is used, and 1-7 percent if a salvage washer is used. This study further shows that washers can be used in a salvage fashion after screw intrusion to produce approximately 2.5 times more compressive force than an intruded screw or 2.25 times more compressive force than an appropriately inserted screw without washer.