Biomechanical interpretations of bone adaptation in biological reconstructions following bone tumors would be crucial for orthopedic oncologists, particularly if based on quantitative observations. This would help to plan for surgical treatments, rehabilitative programs and communication with the patients. In particular, outcomes of the Capanna technique, which combines bone allograft and vascularized fibula autograft, lead to stable and durable reconstructions [1, 2], and different remodeling patterns have been described [3] as a response to mechanical loading. However, there are several events that are not understood and require a biomechanical interpretation, as the evolution patterns can evolve towards conditions that threaten the strength of the reconstruction. We aimed to (i) analyze the biomechanical adaptation of a femoral reconstruction after Ewing sarcoma, in terms of morphological and densitometric evolution of bone from CT data, internal loads acting on the bone during movement, mechanical competence of the reconstruction, and (ii) relate in-progress bone resorption to the mechanical stimulus induced by different motor activities. Eight CT datasets of a patient (8 yrs at surgery using the Capanna technique) during 76-month follow-up were available. The evolution of bone morphology, density and moments of inertia was quantified. At the last control, the patient underwent gait analysis (walking, chair rise/sit, stair ascent/descent, squat). We created a multiscale musculoskeletal and finite element model from CT scans and motion analysis data at the end of follow-up, using state-of-the-art modeling workflows [4, 5], to analyze muscle and joint loads, and to compare the mechanical competence of the reconstructed bone with the contralateral limb, in the current real condition and in a possible revision surgery that removed proximal screws. Although there were no reconstruction complications and osteo-fusion with intense remodeling between allograft and autograft was shown, there was a progressive decrease in allograft cortical thickness and density. There were strategies of muscle coordination that led to differences in joint loads between limbs more marked in more demanding motor activities, and generally larger in the contralateral limb. The operated femur presented a markedly low ratio of physiological strain due to load-sharing with the metal implant, particularly in the lateral aspect. A possible revision surgery removing the three most proximal screws would help restore a physiological strain configuration, while the safety of the reconstruction would not be threatened. We suggest that bone resorption is related to load-sharing and to the internal forces exerted during movement, and the mechanical stimulus should be improved by adopting modifications in the surgical treatment and by promoting physical therapy aimed at specific muscle strengthening.
A population based finite element study that accounts for subject-specific morphology, density and load variations, suggests that osteoporosis does not markedly lower the mechanical compliance of the proximal femur to routine loads. Osteoporosis (OP) is a bone disease defined by low bone density and micro-architectural deterioration. This deterioration is neither uniform nor symmetric at the proximal femur. Evidence from analyses performed at the tissue level suggests that the cortical shell at the femoral neck is thinner in OP patients, especially in the superior regions, but not in the infero-anterior ones [Poole, Rubinacci]. Analogously, OP femurs show a higher anisotropy of the trabecular bone than controls [Ciarelli], suggesting a preservation of load bearing capacity in the principal loading direction vs. the transverse one. There is general consensus that the regions subjected to higher loads during walking, which is the predominant motor activity in the elderly, are mostly preserved. All these findings suggest that the OP femur should exhibit an almost normal mechanical competence during daily activities. This would be in accordance with the very low incidence of spontaneous fractures [Parker] and with the moderate fracture predictivity of BMD. Although reasonable, this hypothesis has never been tested at the organ level. Aim of the present study was to verify it with a population-based finite element (FE) study.Summary Statement
Introduction
A retrospective study on 98 patients shows that FE-based bone strength from CT data (using validated FE models) is a suitable candidate to discriminate fractured versus controls within a clinical cohort. Subject-specific Finite element models (FEM) from CT data are a promising tool to non-invasively assess the bone strength and the risk of fracture of bones in vivo in individual patients. The current clinical indicators, based on the epidemiological models like the FRAX tool, give limitation estimation of the risk of femoral neck fracture and they do not account for the mechanical determinants of the fracture. Aim of the present study is to prove the better predictive accuracy of individualised computer models based a CT-FEM protocol, with the accuracy of a widely used standard of care, the FRAX risk indicator.Summary
Introduction
In a retrospective study, FE-based bone strength from CT data showed a greater ability than aBMD to discriminate proximal femur fractures versus controls. Personalised Finite Element (FE) models from Computed Tomography (CT) data are superior to bone mineral density (BMD) in predicting proximal femoral strength Summary Statement
Introduction
