Concept of variable angle locking--evolution and mechanical evaluation of a recent technology.

Applications for fracture-adapted screw positioning offered by variable angle locking screws are increasing. The locking strength of the variable angle locking mechanism at different insertion angles was compared to conventional fixed angle locking screws. Stainless steel (S) and titanium (Ti) variable and fixed angle 2.4 mm locking screws, inserted at different inclinations (0°-15°), and locked at 0.8 Nm were subjected to a load-to-failure test. Ultimate failure moment at the screw-head interface and failure mode of the screws were determined. Significant differences were detected by one-way ANOVA (p < 0.05). Stainless steel and titanium variable angle locking screws inserted at 2°-10° inclination exhibited a failure moment comparable to the 0° position (S 1.67± 0.04 Nm; Ti 1.67 ± 0.14 Nm) and failed predominantly at the screw neck, with the head remaining fixed to the plate. Inserted at 15° inclination, screws revealed a lower failure moment (S 1.33 ± 0.06 Nm, Ti 1.58 ± 0.05Nm), and failed predominantly by breakout of the head thread. Fixed angle locking screws inserted at an inclination >2° did not lock properly in the plate hole, providing insufficient locking strength. Variable angle locking screws offer a stable head-locking mechanism at different inclinations, comparable to the locking strength of orthogonal inserted fixed angle locking screws. Marginal inclinations >15° should be used with care.

Mechanical behavior of fixation components for periprosthetic fracture surgery.

BACKGROUND: Reliable periprosthetic fracture treatment needs detailed knowledge on the mechanical behavior of the fixation components used. The holding capacity of three conventional fixation components for periprosthetic fracture treatment was systematically investigated under different loading directions. METHODS: Locking compression plates were fixed to a 7 cm long part of diaphyseal fresh frozen human femur with either a single 1.7 mm cerclage cable, a 5.0mm monocortical or a bicortical locking screw (n=5 per group). Constructs were loaded in lateral, torsional and axial direction with respect to the bone axis in a load-to-failure test. Corresponding stress distribution around the screw holes was analyzed by finite element modeling. FINDINGS: Both screw fixations revealed significantly higher stiffness and ultimate strength in axial compression and torsion compared to the cerclage (all P<0.01). Ultimate strength in lateral loading and torsion was significantly higher for bicortical screws (mean 3968 N SD 657; mean 28.8 Nm SD 5.9) compared to monocortical screws (mean 2748 N SD 585; mean 14.4 Nm SD 5.7 Nm) and cerclages (mean 3001N SD 252; mean 3.2 Nm SD 2.0) (P≤0.04). Stress distribution around the screw hole varied according to the screw type and load direction. INTERPRETATION: Fixation components may be combined according to their individual advantages to achieve an optimal periprosthetic fracture fixation.

Biomechanical performance of different cable and wire cerclage configurations.

PURPOSE: Cerclage technology is regaining interest due to the increasing number of periprosthetic fractures. Different wiring techniques have been formerly proposed and have hibernated over years. Hereby, they are compared to current cerclage technology. METHODS: Seven groups (n = 6) of different cable cerclage (Ø1.7 mm, crimp closure) configurations (one single cerclage looped once around the shells, one single cerclage looped twice, two cerclages each looped once) and solid wire cerclages (Ø1.5 mm, twist closure) (same configurations as cable cerclages, and two braided wires, twisted around each other looped once) fixed two cortical half shells of human femoral shaft mounted on a testing jig. Sinusoidal cyclic loading with constantly increasing force (0.1 N/cycle) was applied starting at 50 N peak load. Cerclage pretension (P), load leading to onset of plastic deformation (D) and load at total failure (T) were identified. Statistical differences between the groups were detected by univariate ANOVA. RESULTS: Double looped cables (P442N ± 129; D1334N ± 319; T2734N ± 330) performed significantly better (p < 0.05) than single looped cables (P292N ± 56; D646N ± 108; T1622N ± 171) and were comparable to two single cables (P392N ± 154; D1191N ± 334; T2675N ± 361). Double looped wires (P335N ± 49; D752N ± 119; T1359N ± 80) were significantly better (p < 0.05) than single looped wires (P181N ± 16; D343N ± 33; T606N ± 109) and performed similarly to single looped cables. Braided wires (P119N ± 26; D225N ± 55; T919N ± 197) exhibited early loss of pretension and plastic deformation. CONCLUSION: Double looped cerclages provided a better fixation stability compared to a single looped cerclage. Double looped wires were comparable to a single looped cable. The use of braided wires could not be recommended mechanically.

Underneath the cerclage: an ex vivo study on the cerclage-bone interface mechanics.

INTRODUCTION: Cerclages regain interest due to a rising number of periprosthetic fractures. The contact distribution at the circumferential cerclage-bone interface is still unknown. Local interface pressure depends on the amount of contact area. Cortical damage at the interface would provoke cerclage loosening. Therefore, the contact area, the bone pressure along the interface and the cortical resistance underneath loaded cerclages were determined in an ex vivo model. MATERIALS AND METHODS: Human diaphyseal femoral bone was used with differing cross-sectional geometry. Bone contact points of fixed 1.5 mm wire and 1.7 mm cable cerclages were identified from axial radiographs. Pressure distribution at the cerclage-bone interface was recorded with a pressure-measuring film using a distraction setup with two cortical half shells. Bone shells with installed cerclages were separated with up to 400 N force and were subsequently analyzed histologically to detect cortical damage. RESULTS: Both cerclage types exhibited a point contact fixation with non-loaded spanned zones in-between. Cables cover larger contact areas. Both cerclages exhibited an inhomogeneous interface pressure distribution depending on the bone surface geometry. Histology revealed intact cortical bone without cortical affection after loading of both cerclage types. CONCLUSION: Point contact fixation of the cerclages installs non-loaded, spanned zones where the periosteum is not compressed, rendering a strangulation of the blood supply unlikely. Cortical bone withstands static concentric pressure produced by the cerclage. Cortical groove formation is attributed to instability under functional load and not to weakness of the cortex itself.