In posterior fixation for deformity correction and spinal fusion, there is increasing discussion around auxiliary rods secured to the pedicle screws, sharing the loads, and reducing stresses in the primary rods. Dual-rod, multiaxial screws (DRMAS) provide two rod mounting points on a single screw shaft to allow unique constructs and load-sharing at specific vertebrae. These implants provide surgical flexibility to add auxiliary rods across a pedicle subtraction osteotomy (PSO) or over multiple vertebral levels where higher bending loads are anticipated in primary rods. Other options include fixed-angle devices as multiple rod connectors (MRC) and variable-angle dominoes (VAD) with a single-axis rotation in the connection. The objective in this simulation study was to assess rod bending in adult spinal instrumentation across an osteotomy using constructs with DRMAS, MRC, or VAD multi-rod connections.

The study was performed using computer biomechanical models of two adult patients having undergone posterior instrumented spinal fusion for deformity. The models were patient-specific, incorporating the biomechanics of the spine, have been calibrated to assess deformity correction and intra- and postoperative loads across the instrumented spine. One traditional bilateral-rod construct was used as a control for six multi-rod configurations. Spinal fixation scenarios from T10 through S1 with the PSO at L4 were simulated on each patient-specific model. The multi-rod configurations were bilateral and unilateral DRMAS at L2 through S1 (B-DRMAS and U-DRMAS), bilateral DRMAS at L3 and L5 (Hybrid), bilateral MRC over L3-L5, bilateral and unilateral VAD over L3-L5 (B-VAD and U-VAD). Postoperative gravity plus 8-Nm flexion and extension loads were simulated and bending moments in the rods were computed and compared.

In the simulated control for each case (#1 & #2), average rod bending moments (of the right and left rods) at the PSO level were 6.7Nm & 5.5Nm, respectively, in upright position, 8.8Nm & 7.3Nm in 8-Nm flexion, and 4.6Nm & 3.7Nm in 8-Nm extension. When the primary rods of the multi-rod constructs were normalized to this control, the bending moments in the primary rods of Case #1 & #2 were respectively 57% & 58% (B-DRMAS), 54% & 62% (B-VAD), 60% & 61% (MRC), 72% & 69% (Hybrid), 81% & 70% (U-DRMAS), and 81% & 73% (U-VAD). Overall, the reduction in primary rod bending moments ranged from 19–46% for standing loads. Under simulated 8-Nm functional moments, the primary rod moments were reduced by 18–46% in flexion and 17–48% in extension. More rods and stiffer connections produced the largest reductions for the primary rods, but auxiliary rods had bending moments that varied from 49% lower to 13% higher than the primary ones.

Additional rods through DRMAS, MRC, and VAD connections noticeably reduced the bending loads in the primary rods compared with a standard bilateral-rod construct. Yet, bending loads in the auxiliary rods were higher or lower than those in the primary rods depending on the 3D spinal deformity and stiffness of the auxiliary rod connections. Additional studies and patient-specific analyses are needed to optimize instrumentation parameters that may improve load-sharing in these constructs.

Background

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Summary

Timing for the application and use of fentanyl patches for pre-emptive analgesia and sedation is crucial to obtain good clinical outcomes. Placement and timing is important to maximise clinical effect and apparent levels of analgesia.

Load-controlled knee simulators, representing the passive constraints and joint loads observed in the natural knee, have been developed to assess device-dependent kinematics and wear damage of total knee replacements (TKR) in a controlled mechanical environment. Using a finite element model (FEM) to represent the simulator, our objective in this study was to quantify the variations in kinematics, contact stresses, and contact areas that occur with variations in the ‘soft-tissue’ spring stiffness and coefficient of friction for a conforming knee design.

A finite element model was created of the Insall-Burstein Posterior-Stabilized II knee system. The model conditions corresponded with the International Standards Organisation (ISO) test protocol #14243-1 and consisted of the prescribed flexion angle, the axial compressive load, the anterior-posterior (AP) force, the internal-external (IE) moment, and linear springs mounted to provide AP and IE restraints. This setup has been validated as a reasonable equivalent system for this design in the Instron-Stanmore knee simulator. The linear spring constant was set at 7.24 N/mm and the coefficient of friction was 0.01; both values were then varied by an order of magnitude. The implant kinematics and the maximum contact stress and areas of contact over the loading cycle were determined.

Varying the spring constant by a factor of two changed the AP motions and IE rotations of the tibial insert by about 20%. The maximum contact stresses, occurring during peak loads and moments, varied by 40%, while the area of contact over the full cycle changed by 30%. Changing the coefficient of friction had little effect upon the dependent variables. Wear is a function of both stresses and kinematics. This study indicates that stresses in this design are more sensitive than kinematics to changes in ‘soft-tissue’ stiffness. Therefore, both must be considered to determine wear potential.