Abstract

Abstract

Osteosynthesis of high-energy metaphyseal proximal tibia fractures is still challenging, especially in patients with severe soft tissue injuries and/or short stature. Although the use of external fixators is the traditional treatment of choice for open comminuted fractures, patients' acceptance is low due to the high profile and therefore the physical burden of the devices. Recently, clinical case reports have shown that supercutaneous locked plating used as definite external fixation could be an efficient alternative. Therefore, the aim of this study was to evaluate the effect of implant configuration on stability and interfragmentary motions of unstable proximal tibia fractures fixed by means of externalized locked plating. Based on a right tibia CT scan of a 48 years-old male donor, a finite element model of an unstable proximal tibia fracture was developed to compare the stability of one internal and two different externalized plate fixations. A 2-cm osteotomy gap, located 5 cm distally to the articular surface and replicating an AO/OTA 41-C2.2 fracture, was virtually fixed with a medial stainless steel LISS-DF plate. Three implant configurations (IC) with different plate elevations were modelled and virtually tested biomechanically: IC-1 with 2-mm elevation (internal locked plate fixation), IC-2 with 22-mm elevation (externalized locked plate fixation with thin soft tissue simulation) and IC-3 with 32-mm elevation (externalized locked plate fixation with thick soft tissue simulation). Axial loads of 25 kg (partial weightbearing) and 80 kg (full weightbearing) were applied to the proximal tibia end and distributed at a ratio of 80%/20% on the medial/lateral condyles. A hinge joint was simulated at the distal end of the tibia. Parameters of interest were construct stiffness, as well as interfragmentary motion and longitudinal strain at the most lateral aspect of the fracture. Construct stiffness was 655 N/mm (IC-1), 197 N/mm (IC-2) and 128 N/mm (IC-3). Interfragmentary motions under partial weightbearing were 0.31 mm (IC-1), 1.09 mm (IC-2) and 1.74 mm (IC-3), whereas under full weightbearing they were 0.97 mm (IC-1), 3.50 mm (IC-2) and 5.56 mm (IC-3). The corresponding longitudinal strains at the fracture site under partial weightbearing were 1.55% (IC-1), 5.45% (IC-2) and 8.70% (IC-3).

From virtual biomechanics point of view, externalized locked plating of unstable proximal tibia fractures with simulated thin and thick soft tissue environment seems to ensure favorable conditions for callus formation with longitudinal strains at the fracture site not exceeding 10%, thus providing appropriate relative stability for secondary bone healing under partial weightbearing during the early postoperative phase.

Summary Statement

Simulated increases in body weight led to increased displacement, von Mises stress, and contact pressure in finite element models of the extended and flexed knee. Contact shifted to locations of typical medial osteoarthritis lesions in the extended knee models.

Introduction. Being challenging, multifragmentary proximal tibial fractures in patients with severe soft tissue injuries and/or short stature can be treated using externalized locked plating. A recent finite element study, investigating the fixation stability of plated unstable tibial fractures with 2-mm, 22-mm and 32-mm plate elevation under partial and full weight-bearing, reported that from a virtual biomechanical point of view, externalized plating seems to provide appropriate relative stability for secondary bone healing under partial weight-bearing during the early postoperative phase. The aim of the current study was to evaluate the clinical outcomes of using a LISS plate as a definitive external fixator for the treatment of multifragmentary proximal tibial fractures. Methods. Following appropriate indirect reduction, externalized locked plating was performed and followed up in 12 patients with multifragmentary proximal tibial fractures with simple intraarticular involvement and injured soft tissue envelope. Results. Among all patients, the average follow up period was 22 months (range14–48 months), revealing uneventful healing in all of them. Time to fracture union was 21.8 weeks on average (range 16–28weeks). The mean HSS knee score was 87 (range 72–98) at 4 weeks postoperatively and 97 (range 88–100) at the final follow up. The average AOFAS score was 92 (range 84–100) at 4 weeks postoperatively and 98 (range 94–100) at the final follow up. Conclusions. Externalized locked plating seems to be a successful surgical alternative treatment in selected cases with unstable proximal tibial fractures and severe soft tissue injury, following appropriate indirect fracture reduction

Abstract. Objectives. Bone shape and internal architecture are accepted as optimised to resist joint contact and muscle forces the skeleton is subjected to through daily living and more demanding activities. Finite element studies to predict bone architecture, either using continuum or structural approaches have made assumptions common in structural optimisation, that lead to trabecular bone effectively being modelled as a truss-type structure, with compressive or tensile strains, present due to axial forces driving adaptation. These models are successful in predicting bone fracture, and trends in bone degradation associated with disuse or unloading osteopenia but tend to overpredict bone mineral density reduction compared to clinical observations. Methods. A new structural model of bone adaptation, including both trabeculae (element) cross-section adaptation in response to axial force and biaxial bending moments, and alteration of joint (node) positions within the trabecular network, was developed using a Voronoi space partition to define the initial network. This was compared to results from a structural bone adaptation using a truss-type network generated by connecting each node to its nearest 16 neighbours [1]. Results. Relative density (bone volume divided by total volume) was higher in the predicted structure from the Voronoi network, compared to the truss-type network, with elements close to nodes adapting to resist higher bending moments. Bone promoting strains were found to be spread throughout the Voronoi network in contrast to the truss-type network. Predicted bone degradation in the Voronoi network was lower than in the truss-type network when load cases were removed from the loading envelope. Conclusion. It is hypothesised that bone is optimised for robustness as well as stiffness, with trabecular architecture allowing a wide range of load cases to cause bone promoting strains across the network, reducing the impact of reduced activity or altered loading. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Summary Statement. 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. Introduction. 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. Patients & Methods. Whole femur Computed Tomography (CT) scans of 200 patients (115 women) with normal femoral anatomy were retrieved from a repository of the Istituto Ortopedico Rizzoli. The database is representative of an adult Italian population (mean 57yrs, range 23–84), and spans a wide range of morphological and densitometric characteristics (CT-simulated T-score of femoral neck BMD ranging from 1 to −4.6). Personalised FE models of all femurs were built from CT data using a validated procedure [Schileo]. A personalised estimate of the variability of loads acting on the proximal femur during normal walking (NW) and stair climbing (SC) was obtained by querying an indexed and searchable database of joint and muscle loads obtained from musculoskeletal models of 90 subjects. 78 possible loading combinations for NW and 50 for SC were defined for each subject, taking into account individual characteristics (height, weight, femoral antetorsion, CCD angle and neck length). Risk of fracture (RF) was defined for each subject as the maximum principal strain / limit strain (1.04% compressive, 0.73% tensile) ratio over the whole loading spectrum. Results and Discussion. No fracture was predicted by the FE models throughout the entire population, yielding an average safety coefficient of between 4 and 5, which is consistent with experimentally determined failure loads in the single leg stance configuration (around 11 BW [Cristofolini]). While a general inverse association was observed with R2∼0.2, no clear correlation was present between the fracture risk and the T-score. The hypothesis that OP does not macroscopically influence the mechanical competence of the femur for daily activities was therefore corroborated, suggesting that the highest risk of fracture in OP patients might be related to a lower OP induced compliance to accidental loads

Recently finite element studies have incorporated bone remodelling algorithms in an attempt to simulate bone's mechano-adaptation to loading conditions. In order to simplify these analyses, bone is usually considered to be isotropic, which does not explain the directionality of its internal structures; neither the orthotropic properties measured at the continuum level. Furthermore, simplified loading is usually applied to the bone models, which can result in an unrealistic remodelling stimulus. However, free boundary condition modelling of the femoral and pelvic constructs has been shown to produce more physiological stress and strain distributions. This paper describes the application of a 3D remodelling algorithm (with bone modelled as a strain-adaptive continuum with local orthotropic material properties) to a free boundary model of the femoral construct, where the hip and knee joints, as well as muscles and ligaments crossing the joints were included explicitly. Two load cases were analysed: single leg stance and standing up. Material properties and directionality distributions were produced for the whole femur, showing good agreement with observed structures from clinical studies. This indicates that the loading conditions modelled correspond to those experienced in vivo. In addition, the impact of the different load cases in bone structure modelling could be compared. Observations of the material properties distribution and orientation for standing up indicate that it promotes changes in bone stiffness in the anterior regions of the femoral neck and cortical shaft and the posterior side of the condyles. Development of this approach to modelling and bone structure prediction can lead to a better understanding of bone's mechanical behaviour and to the development and public release of orthotropic heterogeneous models for different constructs. These can be applied in many areas of interest in orthopaedic biomechanics, such as the study of bone-implant interfaces, improvement of the currently used surgical tools and techniques and the influence of certain activities in affecting local bone strength and mineralisation

Objectives

Preservation of both anterior and posterior cruciate ligaments in total knee arthroplasty (TKA) can lead to near-normal post-operative joint mechanics and improved knee function. We hypothesised that a patient-specific bicruciate-retaining prosthesis preserves near-normal kinematics better than standard off-the-shelf posterior cruciate-retaining and bicruciate-retaining prostheses in TKA.

The cortical strains on the femoral neck and proximal femur were measured before and after implantation of a resurfacing femoral component in 13 femurs from human cadavers. These were loaded into a hip simulator for single-leg stance and stair-climbing. After resurfacing, the mean tensile strain increased by 15% (95% confidence interval (CI) 6 to 24, p = 0.003) on the lateral femoral neck and the mean compressive strain increased by 11% (95% CI 5 to 17, p = 0.002) on the medial femoral neck during stimulation of single-leg stance. On the proximal femur the deformation pattern remained similar to that of the unoperated femurs.

The small increase of strains in the neck area alone would probably not be sufficient to cause fracture of the neck However, with patient-related and surgical factors these strain changes may contribute to the risk of early periprosthetic fracture.