Effect of Off-Axis Screw Insertion, Insertion Torque, and Plate Contouring on Locked Screw Strength

Abstract

Objectives This study quantifies the effects of insertion torque, off-axis screw angulation, and plate contouring on the strength of locking plate constructs. Methods Groups of locking screws (n = 6–11 screws) were inserted at 50%, 100%, 150%, and 200% of the manufacturer-recommended torque (3.2 Nm) into locking compression plates at various angles: orthogonal (control), 5-degree angle off-axis, and 10-degree angle off-axis. Screws were loaded to failure by a transverse force (parallel to the plate) either in the same (“+”) or opposite direction (“−”) of the initial screw angulation. Separately, locking plates were bent to 5 and 10-degree angles, with the bend apex at a screw hole. Locking screws inserted orthogonally into the apex hole at 100% torque were loaded to failure. Results Orthogonal insertion resulted in the highest average load to failure, 2577 ± 141 N (range, 2413–2778 N), whereas any off-axis insertion significantly weakened constructs (165–1285 N, at 100% torque) (P < 0.05). For “+” loading, torque beyond 100% did not increase strength, but 50% torque reduced screw strength (P < 0.05). Loading in the “−” direction consistently resulted in higher strengths than “+” loading (P < 0.05). Plate contouring of 5-degree angle did not significantly change screw strength compared with straight plates but contouring of 10-degree angle significantly reduced load to failure (P < 0.05). Conclusions To maximize the screw plate interface strength, locking screws should be inserted without cross-threading. The mechanical stability of locked screws is significantly compromised by loose insertion, off-axis insertion, or severe distortion of the locking mechanism.

Mechanical Evaluation of Locking, Nonlocking, and Hybrid Plating Constructs Using a Locking Compression Plate in a Canine Synthetic Bone Model

“…Multiple fracture lines and fragments can exist at the gap. Screws may need to be angled away from the fracture gap which weakens the construct and is not possible with locking screws …”

“…Screws may need to be angled away from the fracture gap which weakens the construct and is not possible with locking screws. 31 In conclusion, hybrid plating does not create a mechanically weaker construct, compared to the exclusive use of nonlocking screws in an LCP. Locking screws contribute most to the stiffness and strength of a construct when they are placed adjacent to the fracture gap.…”

Half-threaded holes markedly increase the fatigue life of locking plates without compromising screw stability

“…Common causes of screw loosening include improper screw tightening, unicortical purchase, and off-axis insertion. 19 , 20 However, decreasing the thread count, as proposed in the current study, may also jeopardize the screw plate bonding and cause loosening. In the literature, two methods have been used to assess the stability of locking screws.…”

“…In the literature, two methods have been used to assess the stability of locking screws. 19 , 21 , 22 The first involved measuring the removal torque of the screws before or after cyclic loading, and the second involved the bending strength of the screws. In the former method, the tightness of the screws is assessed as a prerequisite to maintaining the screw holding power, whereas in the latter approach, the screw holding power required to support the bone under load is utilized.…”

Internal Fixation of a Complicated Mandibular Fracture in a Filly Using a String-of-Pearls Locking Plate Assisted by a 3D Printed Model

“…In vitro experimentation has determined that contouring an angle greater than 10 degrees significantly reduces load to failure at the LCP screw-plate interface. 7,8 If the bending disrupts the integrity of the plate thread in the plate hole, there is consequent loss of the locking function. 9 If the surgeon over-estimates the angle and has to reverse bend the LCP, the resultant deformation of the constituent metal decreases the stiffness and strength of the implant and plate failure becomes more likely.…”