The best location for proximal locking screw for femur interlocking nailing: A biomechanical study

Abstract

Background:Proximal locking screw deformation and screw fracture is a frequently seen problem for femur interlocking nailing that affects fracture healing. We realized that there is lack of literature for the right level for the proximal locking screw. We investigated the difference of locking screw bending resistance between the application of screws on different proximal femoral levels.Materials and Methods:We used a total of 80 proximal locking screws for eight groups, 10 screws for each group. Three-point bending tests were performed on four types of screws in two different trochanteric levels (the lesser trochanter and 20 mm proximal). We determined the yield points at three-point bending tests that a permanent deformation started in the locking screws using an axial compression testing machine.Results:The mean yield point value of 5 mm threaded locking screws applied 20 mm proximal of lesser trochanter was 1022 ± 49 (range 986–1057) (mean ± standard deviation, 95% confidence interval). On the other hand, the mean yield point value of the same type of locking screws applied on the lesser trochanteric level was 2089 ± 249 (range 1911–2268). Which means 103% increase of screw resistance between two levels (P = 0.000). In all screw groups, on the lesser trochanter line we determined 98–174% higher than the yield point values of the same type of locking screws in comparison with 20 mm proximal to the lesser trochanter (P = 0.000).Conclusion:According to our findings, there is twice as much difference in locking screw bending resistance between these two application levels. To avoid proximal locking screw deformation, locking screws should be placed in the level of the lesser trochanter in nailing of 1/3 middle and distal femur fractures.

Tibia Intramedüller Çivileme Sisteminde Proksimal Kilitleme Vidalarının Dirençlerinin Karşılaştırılması: Biyomekanik Çalışma

“…The same experimental design was used by many researches with polyethylene tube [7][8][9], aluminium tube [13] -12] to obtain yield strength and yield point. With the polyethylene tube and aluminium tube to obtain true yield point was not likely as it was unreliable if the deformation that appears on the stress-strain graph belonged to polyethylene tube and its holes deformations or the locking screw deformation itself [10][11][12].…”

“…It was stated that the stress on 1-mm deformation was defined as "yielding strength" instead of "true yield point [7-9, 14, 15]. It was concluded that due to the trapping of screws within the tube holes, the diameter of the metal tube hole must be 7 mm in order to clearly determine the true yield point [10][11][12].…”

“…Then we calculated the inner and outer diameter of the stainless steel tube used in the experiment. We used a stainless steel tube (inner diameter of 34 mm, outer diameter 39 mm and 420 mm in length) that represented the level of proximal tibia, like the same test system used by previous studies [7][8][9][10][11][12]. 18 mm below of the stainless steel tube upper tip were two opposite holes in diameters of 7 mm.…”

“…On the tibia interlocking nail there is one proximal locking screw oblong hole 6,5 mm in diameter and 16 mm in length. The tibia interlocking nail was supported from its proximal and distal part by two metal ring shaped devices that don't allow movement of the nail to sides on the stainless tube [10][11][12]. Metal rings had external diameter of 32 mm and internal diameter of 14 mm and were 12 mm high simulating the cross sectional area of a standart tibia.…”

“…The screws passed through the metal tube holes of 7 mm diameter and proximal locking screw oblong hole of the tibia nail. The speed of loading on 3 -point bending tests was 1 mm/ minute resampling previous studies [7][8][9][10][11][12]. Loading was conducted from the upper tip of the tibia nail by axial compression machine ( Figure 2).…”