Prophylactic augmentation of the proximal femur: an investigation of two techniques

How accurately do finite element models predict the fall impact response of ex vivo specimens augmented by prophylactic intramedullary nailing?

Hip fracture prevention approaches like prophylactic augmentation devices have been proposed to strengthen the femur and prevent hip fracture in a fall scenario. The aim of this study was to validate the finite element model (FEM) of specimens augmented by prophylactic intramedullary nailing in a simulated sideways fall impact against ex vivo experimental data. A dynamic inertia-driven sideways fall simulator was used to test six cadaveric specimens (3 females, 3 males, age 63-83 years) prophylactically implanted with an intramedullary nailing system used to augment the femur. Impact force measurements, pelvic deformation, effective pelvic stiffness, and fracture outcomes were compared between the ex vivo experiments and the FEMs. The FEMs over-predicted the effective pelvic stiffness for most specimens and showed variability in terms of under- and over-predicting peak impact force and pelvis compression depending on the specimen. A significant correlation was found for time to peak impact force when comparing ex vivo and FEM data. No femoral fractures were found in the ex vivo experiments, but two specimens sustained pelvic fractures. These two pelvis fractures were correctly identified by the FEMs, but the FEMs made three additional false-positive fracture identifications. These validation results highlight current limitations of these sideways fall impact models specific to the inclusion of an orthopaedic implant. These FEMs present a conservative strategy for fracture prediction in future applications. Further evaluation of the modelling approaches used for the bone-implant interface is recommended for modelling augmented specimens, alongside the importance of maintaining well-controlled experimental conditions.

Proximal femoral nailing for intertrochanteric fracture combined with contralateral femoral neck local osteo-enhancement procedure (LOEP) for severe osteoporotic bone loss: An original Italian case series

Proximal femoral fractures in elderly women are a major cause of morbidity and mortality worldwide and a public health concern. Although pharmacological therapies have shown potential in improving bone mineral density (BMD) and decreasing fracture risk, the current research effort is focused on developing a procedure that can ensure both immediate and long-term efficacy. A minimally-invasive surgical approach, known as AGN1 local osteo-enhancement procedure (LOEP), has been recently developed to promote bone augmentation. The procedure implies the preparation of an enhancement site, a specific location where new bone is required within a local bony area weakened by osteoporotic bone loss, and the insertion of a triphasic, resorbable, calcium-based implant material. The results of this procedure have shown a significant and sustainable long-term increase in the proximal femur BMD and consequently in bone strength, thereby improving the femoral neck's resistance to compression and distraction forces that may result in fall-related fractures. A preliminary case series of ten women, suffering from intertrochanteric fracture and contralateral proximal femur severe osteoporotic bone loss, who underwent a combined procedure of proximal femoral nailing and AGN1 local osteo-enhancement procedure, has been developed over the course of a year of clinical and radiological data collection.

Evaluating femoral augmentation to prevent geriatric hip fracture: A scoping review of experimental methods

Various femoral augmentation designs have been investigated over the past decade for the prevention of geriatric hip fracture. The experimental methods used to evaluate the efficacy of these augmentations have not been critically evaluated or compared in terms of biofidelity, robustness, or ease of application. Such parameters have significant relevance in characterizing future clinical success. In this study we aimed to use a scoping review to summarize the experimental studies that evaluate femoral augmentation approaches, and critically evaluate commonly applied protocols and identify areas for concordance with the clinical situation. We conducted a literature search targeting studies that used experimental test methods to evaluate femoral augmentation to prevent geriatric fragility fracture. A total of 25 studies met the eligibility criteria. The most commonly investigated augmentation to date is the injection of bone cement or another material that cured in situ, and a popular subsequent method for biomechanical evaluation was to load the augmented proximal femur until fracture in a sideways fall configuration. We noted limitations in the clinical relevance of sideways fall scenarios being modeled and large variance in the concordance of many of the studies identified. Our review brings about recommendations for enhancing the fidelity of experimental methods modeling clinical sideways falls, which include an improved representation of soft tissue effects, using outcome metrics beyond load-to-failure, and applying loads inertially. Effective augmentations are encouraging for their potential to reduce the burden of hip fracture; however, the likelihood of this success is only as strong as the methods used in their evaluation.

AGN1 implant material to treat bone loss: Resorbable implant forms normal bone with and without alendronate in a canine critical size humeral defect model

BACKGROUND

Fractures secondary to osteoporosis, particularly those of the hip and spine, are a major public health concern with high social and economic costs. The Local Osteo-Enhancement Procedure (LOEP) is an approach intended to strengthen skeletal areas that are at the highest risk for fracture due to osteoporosis. LOEP involves the implantation of AGN1, a triphasic, calcium-based, osteoconductive material which is then resorbed and replaced by bone. Since alendronate is the most prescribed osteoporotic treatment, the purpose of this canine study is to determine if the newly formed bone has the same properties as normal bone and whether alendronate treatment impacts AGN1 resorption and replacement with bone.

METHODS

Sixty skeletally mature male hounds (24-38 kg) were evenly divided between alendronate (0.2 mg/kg/day) and non-alendronate treatment groups. A critical-size core bone defect created in one proximal humerus was implanted with AGN1 while the contralateral non-operated humerus served as a paired control in each animal. Animals were sacrificed 13, 26, and 52 weeks post-operatively (10 per treatment per timepoint). The control and treatment site bone specimens from each animal were examined using radiographic, histomorphometric, and biomechanical techniques. Results between alendronate-treated and non-alendronate-treated animals were compared as groups.

RESULTS

AGN1 implant material was consistently resorbed and replaced by bone in all animals. At 52 weeks, only minimal residual implant material could be detected (0.9 ± 2.3% non-alendronate group; 2.2 ± 3.1% alendronate group), and new bone filled the defects in both the non-alendronate and alendronate groups. At 13 and 26 weeks, microCT revealed the newly formed bone in the defects had significantly higher trabecular bone volume and number connectivity than control bone in both groups. Mechanical testing demonstrated that the new bone had ultimate compressive strength and modulus equivalent to control bone as early as 13 weeks post-surgery which was maintained to 52 weeks in both groups.

CONCLUSIONS

In this canine critical-sized humeral core defect model, AGN1 was progressively replaced by normal bone as evaluated by all outcome measures. Concurrent alendronate therapy did not significantly impact AGN1 resorption or new bone formation. These results demonstrate that AGN1 can be used in conjunction with alendronate in non-osteoporotic animals.

CLINICAL RELEVANCE

This study suggests that the AGN1 implant material demonstrates potential for local restoration of bone in critical-size core defects, and that the material is compatible with alendronate drug therapy. Further studies will be required to determine if these results apply to other osteoporosis medications.

Prophylactic augmentation implants in the proximal femur for hip fracture prevention: An in silico investigation of simulated sideways fall impacts

Femoral fractures from sideways falls in the elderly are associated with significant rates of morbidity and mortality. Approaches to prevent these catastrophic injuries include pharmacological treatments, which have limited efficacy. Prophylactic femoral augmentation systems are a promising alternative that are gaining prominence by addressing the most debilitating osteoporosis-related fracture. We have developed finite element models (FEMs) of a novel experimental sideways fall simulator for cadavers. By virtue of the range of specimens and injury outcomes, these FEMs are well-suited to the evaluation of such implants. The purpose of this study was to use the FEMs to evaluate the mechanical effectiveness of three different prophylactic femoral augmentation systems. Models of the Y-Strut® (Hyprevention®, Pessac, France), Gamma Nail® (Stryker, Kalamazoo, USA), and a simple lag screw femoral fracture implant systems were placed into FEMs of five cadaveric pelvis-femur constructs embedded in a soft tissue surrogate, which were then subject to simulated sideways falls at seven impact velocities. Femur-only FEMs were also evaluated. Peak impact forces and peak acetabular forces were examined, and failure was evaluated using a strain-based criterion. We found that the femoral augmentation systems increased the peak forces prior to fracture, but were unable to prevent fracture for severe impacts. The Gamma Nail® system consistently produced the largest strength increases relative to the unaugmented femur for all five specimens in both the pendulum-drop FEMs and the femur-only simulations. In some cases, the same implant appeared to cause fractures in the acetabulum. The femur-only FEMs showed larger force increases than the pendulum-drop simulations, which suggests that the results of the femur-only simulations may not represent sideways falls as accurately as the soft tissue-embedded pendulum-drop simulations. The results from this study demonstrate the ability to simulate a high energy phenomenon and the effect of implants in an in silico environment. The results also suggest that implants could increase the force applied to the proximal femur during impact. Fracture outcomes from the tested implants can be used to inform the design of future devices, which reaffirms the value of modelling with biofidelic considerations in the implant design process. To the authors' knowledge, this is the first paper to use more complex biofidelic FEMs to assess prophylactic femoral augmentation methods.

Augmentation of core decompression with synthetic bone graft does not improve mechanical properties of the proximal femur

Core decompression is a minimally invasive surgical technique used to treat patients with avascular necrosis of the femoral head. The procedure requires an entry hole in the lateral cortex of the femur which potentially leaves patients susceptible to subtrochanteric fractures. The purpose of this study was to determine if filling the core decompression tract with synthetic bone-graft mechanically strengthens the proximal femur. Twenty composite synthetic femurs underwent a core decompression procedure; ten were augmented with synthetic bone-graft (PRO-DENSE™, Wright Medical) and ten femurs were left unfilled as a control group. Compressive testing to failure was performed using a mechanical testing machine. Stiffness, fracture load, and toughness did not significantly differ between groups. More subtrochanteric fractures were seen in the control group (6 of 10 specimens) compared to the bone-graft augmented group (2 of 10 specimens). In conclusion, augmentation of a core decompression tract does not improve mechanical properties in a synthetic bone model but may be protective of subtrochanteric fracture.

In vitro injection of osteoporotic cadaveric femurs with a triphasic calcium-based implant confers immediate biomechanical integrity

Current pharmaceutical therapies can reduce hip fractures by up to 50%, but compliance to treatment is low and therapies take up to 18 months to reduce risk. Thus, alternative or complementary approaches to reduce the risk of hip fracture are needed. The AGN1 local osteo-enhancement procedure (LOEP) is one such alternative approach, as it is designed to locally replace bone lost due to osteoporosis and provide immediate biomechanical benefit. This in vitro study evaluated the initial biomechanical impact of this treatment on human cadaveric femurs. We obtained 45 pairs of cadaveric femurs from women aged 77.8 ± 8.8 years. One femur of each pair was treated, while the contralateral femur served as an untreated control. Treatment included debridement, irrigation/suction, and injection of a triphasic calcium-based implant (AGN1). Mechanical testing of the femora was performed in a sideways fall configuration 24 h after treatment. Of the 45 pairs, 4 had normal, 16 osteopenic, and 25 osteoporotic BMD T-scores. Altogether, treatment increased failure load on average by 20.5% (p < 0.0001). In the subset of osteoporotic femurs, treatment increased failure load by 26% and work to failure by 45% (p < 0.01 for both). Treatment did not significantly affect stiffness in any group. These findings provide evidence that local delivery of the triphasic calcium-based implant in the proximal femur is technically feasible and provides immediate biomechanical benefit. Our results provide strong rationale for additional studies investigating the utility of this approach for reducing the risk of hip fracture. © 2019 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society.

Numerical investigation of the effect of bone cement porosity on osteoporotic femoral augmentation

Femoroplasty is the injection of bone cement into the proximal femur, enhances the bone load capacity, and is typically applied to osteoporotic femora. To minimize the required injected volume of bone cement and maximize the load capacity enhancement, an optimization problem must be solved, where the modulus of elasticity of the augmented bone is a key element. This paper, through the numerical investigation of a fall on the greater trochanter of an osteoporotic femur, compares different ways to calculate this modulus and introduces an approach, based on the concept of bone cement porosity, which provides results statistically similar to those obtained with other considerations. Based on this approach, the present paper quantifies the correlation between degree of osteoporosis and optimum volume of bone cement. It concludes with an exhaustive search that reveals the effect of the bone cement porosity on the optimum volume of PMMA, for various combinations of the frontal and transverse angles of the fall on the greater trochanter.

Prophylactic augmentation of the osteoporotic proximal femur-mission impossible?

The high incidence of secondary hip fractures and the associated markedly increased mortality call for preventive actions that could help to avoid these injuries. By providing immediate strengthening and not relying on patient compliance, internal prophylactic augmentation of the osteoporotic proximal femur may overcome the main limitations of systemic bone drugs and wearable protective pads. However, such a method would have to provide sufficient and reliable strengthening effect with minimal risks and side effects to justify the need of an invasive treatment. The requirements for an internal reinforcement approach are thus strict and include mechanical, biological, clinical, ethical and financial criteria. Here we first attempt to describe the properties of an ideal augmentation method. Previously published methodologies and techniques developed at our research institute, including approaches using cements, metals, other materials or combined approaches, are then reviewed and evaluated according to these aspects. We conclude that none of the discussed methodologies appears to be able to deliver a sufficiently high gain-versus-risk ratio that could justify the clinical application and thus augmentation of the osteoporotic proximal femur remains a challenge. Finally, we provide suggestions for the development and evaluation of future strategies.

Unmet needs and current and future approaches for osteoporotic patients at high risk of hip fracture

This review provides a critical analysis of currently available approaches to increase bone mass, structure and strength through drug therapy and of possible direct intra-osseous interventions for the management of patients at imminent risk of hip fracture.

PURPOSE

Osteoporotic hip fractures represent a particularly high burden in morbidity-, mortality- and health care-related costs. There are challenges and unmet needs in the early prevention of hip fractures, opening the perspective of new developments for the management of osteoporotic patients at imminent and/or at very high risk of hip fracture. Amongst them, preventive surgical intervention needs to be considered.

METHODS

A European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO)/International Osteoporosis Foundation (IOF) working group reviewed the presently available intervention modalities including preventive surgical options for hip fragility. This paper represents a summary of the discussions.

RESULTS

Prevention of hip fracture is currently based on regular physical activity; prevention of falls; correction of nutritional deficiencies, including vitamin D repletion; and pharmacological intervention. However, efficacy of these various measures to reduce hip fractures is at most 50% and may need months or years before becoming effective. To face the challenges of early prevention of hip fractures for osteoporotic patients at imminent and/or at very high risk of hip fracture, preventive surgical intervention needs further investigation.

CONCLUSION

Preventive surgical intervention needs to be appraised for osteoporotic patients at imminent and/or at very high risk of hip fracture.