Lag screws for hip fracture fixation: Evaluation of migration resistance under simulated walking

Abstract

Previous mechanical studies concerning cut-out of lag screws for pertrochanteric hip fractures have relied on static or dynamic uniaxial loading regimens to induce construct failure by varus collapse and superior cut-out. However, the hip is loaded in a multiplanar, dynamic manner during normal gait. We designed a hip implant performance simulator (HIPS) system to evaluate lag screw cut-out under multiplanar loading representative of normal gait. Five surrogate pertrochanteric fracture specimens with lag screw fixation were loaded up to 20,000 cycles using a biaxial rocking motion (BRM) gait simulation protocol. Anothpr five specimens were loaded using a standard uniaxial loading protocol. The BRM loading group exhibited combined varus collapse (5.4 ? 2.9") and backward rotation (7.2 2.8"). The uniaxial loading group exhibited four times less varus collapse (1.4 f 1.1 ") as compared to the BRM group, and only negligible rotation. For correlation of lag screw migration in surrogate specimens to that in native bone, six human cadaveric specimens were subjected to BRM loading. The degree of varus collapse (8.5 ? 7.7") and rotation (7.2 & 6.4") in cadaveric specimens were comparable to that in surrogate specimens, with the surrogate specimens showing significantly less variability. The results demonstrate that accounting for clinically realistic multiplanar loading vectors significantly affects implant migration, and therefore should be considered when evaluating the fixation strength of hip screw implants.

Hip screw migration testing: First results for hip screws and helical blades utilizing a new oscillating test method

“…Our results show the gait movement to have a significant influence on the migration behavior of the screws, supporting the necessity of including this movement into testing. Although Ehmke et al 13 and Sommers 19 both described significant femoral neck rotation, neither showed a large axial migration, since both set-ups used an axial head support, masking the axial displacement of the screws and limiting the generalizability to only stable fractures. A recent clinical study by Fitzpatrick et al 27 demonstrated the propensity of helical blade style devices to fail via axial penetration, supporting our results and test method.…”

“…They concluded that IM systems can sustain higher loads to failure, while blades sustain the highest failure loads when compared to screws. Ehmke et al 13 simulated dynamic multiplanar loading to induce lag screw migration. Our study built upon these models by using human gait simulation to compare different lag screws used in intertrochanteric fracture fixation.…”

“…The chance of screw cut-out can be increased if the sliding mechanism of a dynamic screw is impeded. 2 This support situation was created in contrast to the test used by Ehmke et al 13 where axial head support was given by a solid back-plate, which then rested on a neck support (Fig. 11).…”

“…Since we are unaware of any threshold values separating osteoporotic from normal strength bone, we believe this is similar to osteoporotic spongy bone. [12][13][14] Hip screws were inserted into the foam cylinders to a consistent depth of 37 mm using a custom-built test machine (Fig. 1).…”

Comparative study between proximal femoral nailing and dynamic hip screw with proximal femoral locking compression plates in intertrochanteric fracture of femur

“…Hence the interest in development of improvements in management of these fractures remains high. 8,9 Extracapsular proximal femoral fractures are those occurring in the region extending from extra-capsular basilar neck region to 5 cm below lesser trochanter. Proximal femoral fractures include intertrochanteric and sub-trochanteric fractures.…”