Yannick Baril

Improving greater trochanteric reattachment with a novel cable plate system.

Cable-grip systems are commonly used for greater trochanteric reattachment because they have provided the best fixation performance to date, even though they have a rather high complication rate. A novel reattachment system is proposed with the aim of improving fixation stability. It consists of a Y-shaped fixation plate combined with locking screws and superelastic cables to reduce cable loosening and limit greater trochanter movement. The novel system is compared with a commercially available reattachment system in terms of greater trochanter movement and cable tensions under different greater trochanteric abductor application angles. A factorial design of experiments was used including four independent variables: plate system, cable type, abductor application angle, and femur model. The test procedure included 50 cycles of simultaneous application of an abductor force on the greater trochanter and a hip force on the femoral head. The novel plate reduces the movements of a greater trochanter fragment within a single loading cycle up to 26%. Permanent degradation of the fixation (accumulated movement based on 50-cycle testing) is reduced up to 46%. The use of superelastic cables reduces tension loosening up to 24%. However this last improvement did not result in a significant reduction of the grater trochanter movement. The novel plate and cables present advantages over the commercially available greater trochanter reattachment system. The plate reduces movements generated by the hip abductor. The superelastic cables reduce cable loosening during cycling. Both of these positive effects could decrease the risks related to grater trochanter non-union.

Fatigue Properties of Superelastic Ti-Ni Filaments Used in Braided Cables for Bone Fixation

Superelastic 0.1mm diameter Ti-Ni filaments are used to manufacture braided orthopedic cable for bone fixation. Biomechanical conditions for this application generally have a cyclic nature, and therefore it becomes important to evaluate the influence of the installation (mean) strain on the fatigue life of these filaments. Uniaxial tension cyclic testing of Ti-Ni filaments is performed in a water bath at 37°C with a 2Hz frequency of to 100 000 cycles. Strain-controlled testing conditions are as follows: alternating strain magnitude varies between 0.64 and 3.64% with mean strain range between 1.32 and 7.1%. Based on the premises that the minimum strain should be high enough to prevent any loss of tension in the tested specimen and that the maximum strain should not bring the specimen to failure during the first loading cycle, the total strain magnitude is encompassed between 0.68 and 8.94%. The results obtained provide a better understanding of the impact mean strain has on the fatigue life of superelastic Ti-Ni alloys.

Median sternotomy: Comparative testing of braided superelastic and monofilament stainless steel sternal sutures

Abstract A new device to reduce the risk of post-operative complications following median sternotomy is proposed, made of a superelastic shape memory alloy and called a braided tubular superelastic (BTS) suture. This study compares the viability of the BTS suture with that of the standard monofilament stainless steel (MSS) suture. A custom test bench was developed to perform comparative testing of the two sternal closure systems. Sternal models made of polyurethane were closed using common wiring configurations. Static and dynamic tensile separation forces, up to a maximum of 1200 N, were then applied to the closed sternums. The MSS and BTS sutures are compared in terms of the force required to open completely the sternum, the compression force at the sternum midline, and the permanent sternum opening. With a smaller sternum opening and a higher tensile separation force, the MSS suture showed greater rigidity than the BTS suture. The BTS suture, however, displayed a better capacity to reapply compression forces at the sternum midline following the repetitive application and release of tensile separation forces. These results confirm the potential of the BTS suture technology, but further studies using cadaveric sterna are needed to attest definitely to the benefits of using the BTS suture to improve bone healing.

Testing System for the Comparative Evaluation of Greater Trochanter Re‐attachment Devices

The scope of this study was to propose and validate a specialized test bench that applies biaxial forces on an orthopedic model of Greater Trochanter (GT) re-attachment with integrated cable tension measurement. Stability of the GT fragment is evaluated using a custom triplanar video movement-analysis system with the first camera’s field of view (FOV) corresponding to the GT osteotomy plane and the second and third camera’s FOVs corresponding to the median plane of the femur in frontal and posterior views, respectively. A typical experimentation and its critical analysis conclude the paper.

Force relaxation and sprinback of novel elastic orthopedic cables

Cerclage cables have proven to be very useful in the orthopedic field for bones stabilization and plate fixation but the initial enthusiasm for metallic cables has declined with their high complication rates. Metal materials provide limited elastic deformation compromising their ability to maintain compression. This study compares the mechanical properties of new elastic cables with cobalt-chrome and stainless-steel cables. Methods: Stainless-steel, cobalt-chrome, nylon and nickel-titanium cables were first loaded up to 356 N, then elongation was maintained for 12 hours, next unloaded and finally reloaded to failure. Initial elongation (%), Relative force relaxation (% loss of initial load after a 12h), elastic springback (%) and force to failure (N) were extracted from force-elongation curves. Findings: Initial elongation was the highest for nylon cables (9%), followed by the nickel-titanium (4%) and both metallic cables (0.3%). During 12 hours, no relaxation was observed for the nickel-titanium and the cobalt-chrome cables, whereas 28 and 45% of the tension was lost respectively for the stainless-steel and the nylon cables. The elastic springback of the nickel-titanium and nylon cables (4.4 and 4.7% respectively) was 20 times higher than that of the stainless-steel and cobalt-chrome cables (0.12 and 0.16% respectively). The force to failure of the stainless steel and cobalt-chrome cables was twice that of the nickel-titanium cables. Interpretation: Multi-braided stainless-steel and cobalt-chrome cables have a high-stiffness with limited ability to tolerate displacement, leading to early cable loosening. Novel low-stiffness cables made of nylon or nickel-titanium offer significant elastic springback improving binding stability.