PROXIMAL FEMORAL FIXATION DEVICES: IS LAG SCREW SLIDING ALTERED BY THE NUMBER OF LAG SCREWS OR BARREL LENGTH?

Abstract

Purpose: A variety of second generation femoral interlocking intramedullary nails, in which the proximal lag screw is engaged in the femoral head, are now available for the treatment of complex comminuted pertrochanteric femoral fractures. Jamming of the lag screw results in a rigid device which is more likely to cut-out of the femoral head. The aim of this study was to determine the sliding characteristics and jamming potential of the lag screws of five different devices used to treat these fractures.

Method: The devices examined include; the single lag screw devices: the DHS, the Gamma nail and the Intramedullary hip screw (IMHS), and the double lag screw devices: the Russell-Taylor Reconstruction nail (RTN) and the Austofix Hip nail. The devices were mounted in a servo-hydraulic testing apparatus and examined by two different techniques. The first set-up looked at lag screw motion with respect to loads applied which were representative of the single limb stance phase of gait (SLSPOG). The second set-up which, was first described by Kyle in 1980, looked at the forces required to initiate sliding.

Results: For the first set up (SLSPOG), all single lag screw devices demonstrated sliding across the normal physiological range of applied load. The Russell Taylor Reconstruction nails demonstrated conflicting results with the lag screws of two nails sliding and one nail jamming. All the Austofix nails jammed at the higher angles of the normal physiological range (1590, 1640).

Using the Kyle set-up, the forces required to initiate sliding were found to be lowest with the Synthes DHS (42.33±5.77N), Zimmer CHS (52.67±26.56N), and the IMHS (45.33±10.97N). These were closely followed by the Gamma nail (79.33±8.39N) and the Richards Classic hip screw (82.00±16.37N). The highest forces were for the RTN (98.00±18.52N) and the Austofix hip nail (283.00±70.62N). These results were significantly different. (p< 0.001, ANOVA)

Conclusion: The results demonstrate that double lag screw implants require greater loads to initiate sliding and have a greater potential for jamming. Whilst all single lag screw nails slide, barrel length does alter the forces required to initiate sliding. Further testing using a lubricant is currently being undertaken.