Range of Movement for Impingement and Dislocation Avoidance in Total Hip Replacement Predicted by Finite Element Model.

Dislocation is a serious complication in total hip replacement (THR). An inadequate range of movement (ROM) can lead to impingement of the prosthesis neck on the acetabular cup; furthermore, the initiation of subluxation and dislocation may occur. The objective of this study was to generate a parametric three-dimensional finite element (FE) model capable of predicting the dislocation stability for various positions of the prosthetic head, neck, and cup under various activities. Three femoral head sizes (28, 32, and 36 mm) were simulated. Nine acetabular placement positions (abduction angles of 25°, 40° and 60° combined with anteversion angles of 0°, 15° and 25°) were analyzed. The ROM and maximum resisting moment (RM) until dislocation were evaluated based on the stress distribution in the acetabulum component. The analysis allowed for the definition of a "safe zone" of movement for impingement and dislocation avoidance in THR: an abduction angle of 40°-60° and anteversion angle of 15°-25°. It is especially critical that the anteversion angle does not fall to 10°-15°. The sequence of the RM is a valid parameter for describing dislocation stability in FE studies.

Biomechanical evaluation of syndesmotic fixation techniques via finite element analysis: Screw vs. suture button.

BACKGROUND AND OBJECTIVE: Tibiofibular syndesmotic injuries may cause degenerative changes, reduction in ankle function and compromising ankle stability. Different fixation techniques try to restore its functionality. Screw-fixation is the gold-standard. Recently, suture-button fixation has aroused the attention because it allows for physiologic micromotion while maintaining an accurate reduction. The aim of this study is to compare the biomechanical behaviour of both fixation techniques using the finite element method. METHODS: A three-dimensional finite element model of the tibiofibular joint was reconstructed simulating the intact ankle and the injured syndesmosis. Then, different methods of syndesmosis fixation were analysed: screws (number of cortices, number of screws and distance between screws) and suture buttons (single, double parallel and double divergent with a sensitivity analysis on the pretension forces) configuration. Ligaments and cartilages were included and simulated as spring elements. Physiological loads during stance phase were simulated. RESULTS: Syndesmosis widening and von Mises stresses were computed. Syndesmosis widening in the injured configuration compromised joint stability (2.06 mm), whereas using a single quadricortical screw (0.18 mm) stiffened the joint. Syndesmosis widening using suture-buttons were closer to syndesmosis widening of the intact ankle configuration (0.97 mm). Von Mises stresses were higher for the titanium screws than for the suture buttons. CONCLUSIONS: A detailed biomechanical comparison among different syndesmotic fixation was performed. Suture buttons have advantages with regard to syndesmosis widening in comparison to screw fixation. This fact supports the good long-term clinical results obtained with suture buttons fixation. The proposed methodology could be an efficient tool for preoperative planning.

Computational comparison of reamed versus unreamed intramedullary tibial nails.

We compared, via a computational model, the biomechanical performance of reamed versus unreamed intramedullary tibial nails to treat fractures in three different locations: proximal, mid-diaphyseal, and distal. Two finite element models were analyzed for the two nail types and the three kinds of fractures. Several biomechanical variables were determined: interfragmentary strains in the fracture site, von Mises stresses in nails and bolts, and strain distributions in the tibia and fibula. Although good mechanical stabilization was achieved in all the simulated fractures, the best results were obtained in the proximal fracture for the unreamed nail and in the mid-diaphyseal and distal fractures for the reamed nail. The interlocking bolts, in general, were subjected to higher stresses in the unreamed tibial nail than in the reamed one; thus the former stabilization technique is more likely to fail due to fatigue.

Biomechanical evaluation of tibial bone adaptation after revision total knee arthroplasty: A comparison of different implant systems.

The best methods to manage tibial bone defects following total knee arthroplasty remain under debate. Different fixation systems exist to help surgeons reconstruct knee osseous bone loss (such as tantalum cones, cement, modular metal augments, autografts, allografts and porous metaphyseal sleeves) However, the effects of the various solutions on the long-term outcome remain unknown. In the present work, a bone remodeling mathematical model was used to predict bone remodeling after total knee arthroplasty (TKA) revision. Five different types of prostheses were analyzed: one with a straight stem; two with offset stems, with and without supplements; and two with sleeves, with and without stems. Alterations in tibia bone density distribution and implant Von Mises stresses were quantified. In all cases, the bone density decreased in the proximal epiphysis and medullary channels, and an increase in bone density was predicted in the diaphysis and around stem tips. The highest bone resorption was predicted for the offset prosthesis without the supplement, and the highest bone formation was computed for the straight stem. The highest Von Mises stress was obtained for the straight tibial stem, and the lowest was observed for the stemless metaphyseal sleeves prosthesis. The computational model predicted different behaviors among the five systems. We were able to demonstrate the importance of choosing an adequate revision system and that in silico models may help surgeons choose patient-specific treatments.

Influence of high-frequency cyclical stimulation on the bone fracture-healing process: mathematical and experimental models.

Mechanical stimulation affects the evolution of healthy and fractured bone. However, the effect of applying cyclical mechanical stimuli on bone healing has not yet been fully clarified. The aim of the present study was to determine the influence of a high-frequency and low-magnitude cyclical displacement of the fractured fragments on the bone-healing process. This subject is studied experimentally and computationally for a sheep long bone. On the one hand, the mathematical computational study indicates that mechanical stimulation at high frequencies can stimulate and accelerate the process of chondrogenesis and endochondral ossification and consequently the bony union of the fracture. This is probably achieved by the interstitial fluid flow, which can move nutrients and waste from one place to another in the callus. This movement of fluid modifies the mechanical stimulus on the cells attached to the extracellular matrix. On the other hand, the experimental study was carried out using two sheep groups. In the first group, static fixators were implanted, while, in the second one, identical devices were used, but with an additional vibrator. This vibrator allowed a cyclic displacement with low magnitude and high frequency (LMHF) to be applied to the fractured zone every day; the frequency of stimulation was chosen from mechano-biological model predictions. Analysing the results obtained for the control and stimulated groups, we observed improvements in the bone-healing process in the stimulated group. Therefore, in this study, we show the potential of computer mechano-biological models to guide and define better mechanical conditions for experiments in order to improve bone fracture healing. In fact, both experimental and computational studies indicated improvements in the healing process in the LMHF mechanically stimulated fractures. In both studies, these improvements could be associated with the promotion of endochondral ossification and an increase in the rate of cell proliferation and tissue synthesis.