Hwabok Wee

Tangential Bicortical Locked Fixation Improves Stability in Vancouver B1 Periprosthetic Femur Fractures: A Biomechanical Study.

Objectives: The biomechanical difficulty in fixation of a Vancouver B1 periprosthetic fracture is purchase of the proximal femoral segment in the presence of the hip stem. Several newer technologies provide the ability to place bicortical locking screws tangential to the hip stem with much longer lengths of screw purchase compared with unicortical screws. This biomechanical study compares the stability of 2 of these newer constructs to previous methods. Methods: Thirty composite synthetic femurs were prepared with cemented hip stems. The distal femur segment was osteotomized, and plates were fixed proximally with either (1) cerclage cables, (2) locked unicortical screws, (3) a composite of locked screws and cables, or tangentially directed bicortical locking screws using either (4) a stainless steel locking compression plate system with a Locking Attachment Plate (Synthes) or (5) a titanium alloy Non-Contact Bridging system (Zimmer). Specimens were tested to failure in either axial or torsional quasistatic loading modes (n = 3) after 20 moderate load preconditioning cycles. Stiffness, maximum force, and failure mechanism were determined. Results: Bicortical constructs resisted higher (by an average of at least 27%) maximum forces than the other 3 constructs in torsional loading (P < 0.05). Cables constructs exhibited lower maximum force than all other constructs, in both axial and torsional loading. The bicortical titanium construct was stiffer than the bicortical stainless steel construct in axial loading. Conclusions: Proximal fixation stability is likely improved with the use of bicortical locking screws as compared with traditional unicortical screws and cable techniques. In this study with a limited sample size, we found the addition of cerclage cables to unicortical screws may not offer much improvement in biomechanical stability of unstable B1 fractures.

Peri-implant stress correlates with bone and cement morphology: Micro-FE modeling of implanted cadaveric glenoids.

Aseptic loosening of cemented joint replacements is a complex biological and mechanical process, and remains a clinical concern especially in patients with poor bone quality. Utilizing high resolution finite element analysis of a series of implanted cadaver glenoids, the objective of this study was to quantify relationships between construct morphology and resulting mechanical stresses in cement and trabeculae.

Time course of peri-implant bone regeneration around loaded and unloaded implants in a rat model

The time‐course of cancellous bone regeneration surrounding mechanically loaded implants affects implant fixation, and is relevant to determining optimal rehabilitation protocols following orthopaedic surgeries. We investigated the influence of controlled mechanical loading of titanium‐coated polyether‐ether ketone (PEEK) implants on osseointegration using time‐lapsed, non‐invasive, in vivo micro‐computed tomography (micro‐CT) scans. Implants were inserted into proximal tibial metaphyses of both limbs of eight female Sprague–Dawley rats. External cyclic loading (60 or 100 μm displacement, 1 Hz, 60 s) was applied every other day for 14 days to one implant in each rat, while implants in contralateral limbs served as the unloaded controls. Hind limbs were imaged with high‐resolution micro‐CT (12.5 μm voxel size) at 2, 5, 9, and 12 days post‐surgery. Trabecular changes over time were detected by 3D image registration allowing for measurements of bone‐formation rate (BFR) and bone‐resorption rate (BRR). At day 9, mean %BV/TV for loaded and unloaded limbs were 35.5 ± 10.0% and 37.2 ± 10.0%, respectively, and demonstrated significant increases in bone volume compared to day 2. BRR increased significantly after day 9. No significant differences between bone volumes, BFR, and BRR were detected due to implant loading. Although not reaching significance (p = 0.16), an average 119% increase in pull‐out strength was measured in the loaded implants.

Dynamic Analysis of a Spread Cell Using Finite Element Method

The dynamic analysis of a cultured cell using Finite Element Analysis is presented to understand the effect of vibration on a cell structure. The model of a spread cell on a culturing plate has been developed as a continuum model and a cellular tensegrity model. Using Finite Element modal analysis, natural frequencies and mode shapes of both models were obtained and compared with each other. Finite Element harmonic response analysis was carried out to investigate the dynamic response of a spread cell exposed to vibration in the frequency range of 1–60 Hz with 1 G acceleration. Both continuum model and tensegrity model showed that the first three natural frequencies appeared in range of 18 ~ 27 Hz and they were in the effective vibration frequency range for bone cell growth. In mode 1–3 the major oscillation was observed in horizontal direction and the resonance occurred when the base vibration frequency was closed to the calculated natural frequency. It is presumed that the optimal frequency for bone cell growth is closely related the natural frequency of cell structures and associated with the resonance of cellular structures. For better understanding resonance of cell structure future studies will consider the damping capability of cell structures.

Modal analysis of a spreading osteoblast cell in culturing

In bone cell culturing environment, mechanical vibration has been utilized to study the effects of various frequency and amplitude conditions on the cells. In this study, a spreading single cell was modeled as a simple dome to investigate the dynamic properties of a cell in the culturing environment. The finite element modal analysis was applied to evaluate the natural frequency and mode shapes of a spreading bone cell. The variance of cell shapes and mechanical properties such as Youngs modulus were utilized. Simulation results show that the range of the natural frequency for a spreading cell of various sizes (135×30, 100×40, 80×50, and 67×60 μm with 10 μm height) is 9.95-211.05 Hz and cell shapes do not affect the modal analysis results. However the natural frequency is increased proportionally to the increase in Youngs modulus.

Dynamic response of human foot and ankle system to vertical vibration

The transmissibility and the phase shift of the foot and ankle system (FAS) under the vertical sinusoidal vibration were studied as a function of the applied load. It was found that the increase of the applied load leads to the increase of the observed resonant frequency and stiffness of the system case.

Foot and ankle system identification with black box models

System identification based on the black box models was utilized for understanding of the transfer function of the foot and ankle exposed to vertical excitation. Various linear polynomial structures and state-space model were tested by comparing the measured acceleration at the base, the Medial Malleolus, and the Tibial Tuberosity respectively for input and output. Identified models are well matched with the measured data when the high model order was selected. The Output Error model has generally shown the best fit to the data, while in several cases the state-space model presents better fit. These results can be used as a starting point for the nonlinear black box model and the grey box model.