Varus malalignment of the lower limb increases the risk of femoral neck fracture: A biomechanical study using a finite element method.

INTRODUCTION: The understanding of the stresses and strains and their dependence on loading direction caused by an axial deformity is very important for understanding the mechanism of femural neck fractures. The hypothesis of this study is that lower limb malalignment is correlated with a substantial stress variation on the upper end of the femur. The purpose of this biomechanical trial using the finite element method is to determine the effect of the loading direction on the proximal femur regarding the malalignment of the lower limb, and also enlighten the relation between the lower limb alignment and the risk of a femoral neck fracture. METHODS: Ten segmentations of CT scans were considered. An axial compression load was applied to the femoral head to digitally simulate the physiological configuration in neutral position as well as in different axial positions in varus/valgus alignment. RESULTS: The stress at the proximal femur changes as the varus _valgus angle does. It can be observed the smaller absolute stress at angle 10° (valgus) and the higher absolute stress at angle -10° (varus). The mean maximum von Mises stress value was 14.1 (SD=±3.48) MPa for 0°, while the mean maximum von Mises stress value was 17.96 MPa (SD=4.87) for -10° in varus. The fracture risk indicator of the proximal femoral epiphyses changes inversely with angle direction. The FRI was the highest at -10° and the lowest at 10°. CONCLUSION: Based on the biomechanical findings and the fracture risk indicator determined in this preliminary study, varus malalignment increases the risk of femoral neck fracture. Consideration of other parameters such as bone mineral density and morphological parameters should also help to plan preventive medical strategy in the elderly.

Utility of cement injection to stabilize split-depression tibial plateau fracture by minimally invasive methods: A finite element analysis.

BACKGROUND: Treatment for fractures of the tibial plateau is in most cases carried out by stable fixation in order to allow early mobilization. Minimally invasive technologies such as tibioplasty or stabilization by locking plate, bone augmentation and cement filling (CF) have recently been used to treat this type of fracture. The aim of this paper was to determine the mechanical behavior of the tibial plateau by numerically modeling and by quantifying the mechanical effects on the tibia mechanical properties from injury healing. METHODS: A personalized Finite Element (FE) model of the tibial plateau from a clinical case has been developed to analyze stress distribution in the tibial plateau stabilized by balloon osteoplasty and to determine the influence of the cement injected. Stress analysis was performed for different stages after surgery. FINDINGS: Just after surgery, the maximum von Mises stresses obtained for the fractured tibia treated with and without CF were 134.9 MPa and 289.9 MPa respectively on the plate. Stress distribution showed an increase of values in the trabecular bone in the treated model with locking plate and CF and stress reduction in the cortical bone in the model treated with locking plate only. INTERPRETATION: The computed results of stresses or displacements of the fractured models show that the cement filling of the tibial depression fracture may increase implant stability, and decrease the loss of depression reduction, while the presence of the cement in the healed model renders the load distribution uniform.

Is early weight bearing resumption beneficial after total hip replacement?

OBJECTIVE: Current rehabilitation protocols of patients following total hip replacement recommend weight bearing on the operated extremity as early as possible. This strategy is likely to induce specific consequences on postural balance control; this study seeks to highlight these reactions to early loading. MATERIALS AND METHODS: Eight men and six women, ranging in age from 57 to 85 years, volunteered enrolling this study on their arrival at our rehabilitation center. This study assessed their postural behavior using a system composed of two separate force platforms under two loading situations; in the course of these tests they were required to minimize their body sway as much as possible by keeping their eyes open. In the natural uncontrolled situation, the subjects adopted a comfortable body weight distribution. In the imposed (IMP) situation, they had to load their operated extremity more than in regular natural conditions in order to distribute their body weight more evenly. Three successive 32-s trials (sampled at 64 Hz) with intermediate recovery periods of equivalent duration were performed allowing period of rest between each trial. The balance strategies were evaluated through a frequency analysis of the resultant and plantar centers of pressure (CP(Res)) of each foot and of the estimated trajectories of the vertical projection of the center of gravity (CG), and from the difference CP(Res)-CG. RESULTS: No difference was found for the plantar CP trajectories in the situation where body weight is spontaneously distributed, whereas loading the implanted extremity induced increased CP(Res), CG, and CP(Res)-CG trajectory amplitudes along the mediolateral axis. No effect was observed along the anteroposterior axis. Finally, when comparing the two limbs for each testing condition, the statistical analysis demonstrated greater displacements along the ML axis for the trajectories measured under the healthy leg than under the implanted extremity. DISCUSSION: Loading the operated extremity early in the rehabilitation process leads to less stability (an increase in the CG movements) and to increased energy expenditure (an increase in the CP(Res)-CG movements). These postural behavior alterations can be explained by various factors including a loss of muscle strength, residual apprehension due to the disuse of this limb, and persistent pain, all of which are increased by limb loading. These features should be taken into consideration when elaborating the rehabilitation protocol for these patients.

[Clinical and posturographic comparison of patients with recent total hip arthroplasty]. / Evolution comparée des tests cliniques et posturographiques lors de la rééducation des arthroplasties totales de hanche.

PURPOSE OF THE STUDY: To highlight the congruence of clinical and posturographic tests in patients undergoing hip arthroplasty. MATERIAL AND METHODS: Ten patients (six males and four females) were included in this study and tested when at admission and discharge from the rehabilitation department (12 and 27 days after surgery respectively). The patients were asked to stand undisturbed in the upright position, eyes closed on a system composed of two force platforms. Five successive 32s trials (sampled at 64 Hz) were conducted with rest intervals of similar duration between trials. The plantar center of pressure (CP), displacements, and resultant CP (CPRes) were then computed and analyzed in various ways. In parallel, various clinical tests, including muscular force, hip range of motion, walking speed, functional independence, pain, sensitivity, lateral reach, and get up and go aimed at evaluating global coordination. Correlations, using the non-parametric Spearman coefficient, were computed from the differences between clinical and posturographic parameters obtained at admission and discharge from the rehabilitation department. RESULTS: Certain statistically significant differences in postural behavior was observed both for clinical and posturographic tests between admission and discharge. Body weigh distribution over the two legs was largely asymmetric at onset and, though it declined, persisted at discharge. There was not difference for the mean positions of both the plantar CP and the resultant of the CP movements. On the contrary, it was noteworthy that the variances of CP displacements (data dispersion) were initially larger on the sound leg along the anteroposterior axis and that this compensatory feature disappeared at the end of the stay. At discharge, the variances computed from the sound and the prosthetic limb became equivalent. All the clinical tests demonstrated statistically significant improvements in results at discharge compared with admission. Several significant correlations involving clinical tests and mean positions along the anteroposterior axis, the degree of body weight asymmetry and variances along both the mediolateral and anteroposterior axes were found. DISCUSSION: These results enable a better understanding of strategies used by hip arthroplasty patients to keep balance. Even though their sensitivity was quite different, both clinical and posturographic measurements demonstrated their ability to assess recovery from surgery. These two evaluation techniques are complementary.