A biomechanical study comparing polyaxial locking screw mechanisms

Head‐locking durability of fixed and variable angle locking screws under repetitive loading

“…As previously shown in quasi‐static tests, tilted fixed angle locking screws failed at a low bending moment and were therefore not considered in the current study . Loosening of fixed angle locking screw heads is mainly induced by misinsertion of the screws in a tilted position, not orthogonal to the hole threads, where only a cross‐threading but no locking of the screw head occurs . Although a certain self‐adjustment effect is observed for deviations up to 3° for 2.4 mm fixed angle locking screws with subsequent correct locking, other screw types (e.g., 7.3 mm locking screws) exhibit disengagement with cross‐threading already at marginal inclinations of 2° .…”

“…Failure mode depends on the head locking mechanism: In the current study, all locking screws failed at the shaft‐head junction. Breakout of the head thread, being described as failure mode of other polyaxial locking systems, using a hoop of softer grade II titanium or a star petal interface for head locking, was not observed, indicating a lasting head locking mechanism in the allowed inclination range of the variable angle locking screws . Overloading the end cap polyaxial locking system, which is based on frictional coupling, exhibited a slippage of the screw head under the end cap at a failure moment comparable to locked fixed angle locking screws but with subsequent relocking when the moment was decreased …”

“…3,8,10 Loosening of fixed angle locking screw heads is mainly induced by misinsertion of the screws in a tilted position, not orthogonal to the hole threads, where only a cross-threading but no locking of the screw head occurs. 3,[8][9][10] Although a certain self-adjustment effect is observed for deviations up to 3˚for 2.4 mm fixed angle locking screws with subsequent correct locking, 3 other screw types (e.g., 7.3 mm locking screws) exhibit disengagement with cross-threading already at marginal inclinations of 2˚. 7 In contrast, an inclined position of the variable angle locking screws did not induce a head thread loosening although tilting of variable angle locking screws without cyclic loading resulted in a slight drop of the removal torque.…”

“…Variable angle locking screws provide a good screw head purchase with an ultimate strength comparable to orthogonally inserted fixed angle locking screws . However, head loosening of variable angle locking screws has not been investigated yet and no information is gained about the durability of the locked screw head under repetitive loading. Screw loosening is a continuous process requiring application of cyclic activity .…”

Minimally invasive surgery with locking plate for periprosthetic femoral fractures: technical note

“…The clinical advantage has been stressed, but some authors [21] have experienced better and more encouraging clinical results with a minimally invasive approach than with a mini-open approach [34]. Most manufacturers allow a 30°cone (15°in each direction), though the optimal cone to avoid harming assembly stability is around 20°(10°in each direction) [35].…”