Abstract
BackgroundLag screw cut-out failure following fixation of unstable intertrochanteric fractures in osteoporotic bone remains an unsolved challenge. This study tested if resistance to cut-out failure can be improved by using a dual lag screw implant in place of a single lag screw implant. Migration behavior and cut-out resistance of a single and a dual lag screw implant were comparatively evaluated in surrogate specimens using an established laboratory model of hip screw cut-out failure.MethodsFive dual lag screw implants (Endovis, Citieffe) and five single lag screw implants (DHS, Synthes) were tested in the Hip Implant Performance Simulator (HIPS) of the Legacy Biomechanics Laboratory. This model simulated osteoporotic bone, an unstable fracture, and biaxial rocking motion representative of hip loading during normal gait. All constructs were loaded up to 20,000 cycles of 1.45 kN peak magnitude under biaxial rocking motion. The migration kinematics was continuously monitored with 6-degrees of freedom motion tracking system and the number of cycles to implant cut-out was recorded.ResultsThe dual lag screw implant exhibited significantly less migration and sustained more loading cycles in comparison to the DHS single lag screw. All DHS constructs failed before 20,000 cycles, on average at 6,638 ± 2,837 cycles either by cut-out or permanent screw bending. At failure, DHS constructs exhibited 10.8 ± 2.3° varus collapse and 15.5 ± 9.5° rotation around the lag screw axis. Four out of five dual screws constructs sustained 20,000 loading cycles. One dual screw specimens sustained cut-out by medial migration of the distal screw after 10,054 cycles. At test end, varus collapse and neck rotation in dual screws implants advanced to 3.7 ± 1.7° and 1.6 ± 1.0°, respectively.ConclusionThe single and double lag screw implants demonstrated a significantly different migration resistance in surrogate specimens under gait loading simulation with the HIPS model. In this model, the double screw construct provided significantly greater resistance against varus collapse and neck rotation in comparison to a standard DHS lag screw implant.
Highlights
Operative treatment for hip fractures was introduced in the 1950s with the expectation of improved functional outcome and a reduction of the complications associated with immobilisation and prolonged bed rest [1,2,3].Since a variety of different implants has been used either extramedullary or intramedullary in nature
Ropars et al in a recent study, compared two minimally invasive implants; one with dual, and one with single cephalic lag screws [24]. They concluded that both implants have biomechanical properties which are as favourable as conventional hip screws, and that loading and mode of failure were found to be similar
LFaigusrcere4w failure modes Lag screw failure modes: a) DHS varus collapse and subsequent cut-out failure, b) lag screw bending in absence of cut-out, and c) axial migration of distal Endovis screw, leading to cut-out despite minimal varus collapse
Summary
Operative treatment for hip fractures was introduced in the 1950s with the expectation of improved functional outcome and a reduction of the complications associated with immobilisation and prolonged bed rest [1,2,3].Since a variety of different implants has been used either extramedullary or intramedullary in nature. The most commonly used extramedullary implant is the sliding hip screw (SHS) with side plate It is currently considered the gold standard for fixation of extracapsular hip fractures as well as the implant that any new design should be compared with [4,5,6]. Ropars et al in a recent study, compared two minimally invasive implants; one with dual, and one with single cephalic lag screws [24] They concluded that both implants have biomechanical properties which are as favourable as conventional hip screws, and that loading and mode of failure were found to be similar. In both studies and for static and cyclic loading the specimens were only loaded in the vertical direction. Migration behavior and cut-out resistance of a single and a dual lag screw implant were comparatively evaluated in surrogate specimens using an established laboratory model of hip screw cut-out failure
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