Biomechanical femoral neck fracture experiments—A narrative review

doi:10.1016/j.injury.2012.03.032

Injury

Volume 43, Issue 10, October 2012, Pages 1633-1639

https://doi.org/10.1016/j.injury.2012.03.032 Get rights and content

Abstract

Introduction

Orthopaedic implants can be introduced in clinical practice if equivalency to an already approved implant can be demonstrated. A preclinical laboratory test can in theory provide the required evidence. Due to the lack of consensus on the optimum design of biomechanical experiments, setups vary considerably. This review aims to make femoral neck fracture models more accessible for evaluation to orthopaedic surgeons without any particular background in biomechanics. Additionally, the clinical relevance of the different setups is discussed.

Methods

This is a narrative review based on a non-systematic search in PubMed, Scopus and Cochrane.

Summary

Biomechanical femoral neck fracture experiments should aim at optimizing the recreation of the in vivo situation. The bone quality of the experimental femurs should resemble the hip fracture population, hence cadaveric bones should be preferred to the available synthetic replica. The fracture geometry must be carefully selected to avoid bias. The load applied to the specimen should result in forces within the range of in vivo measured values and the magnitude should be related to the actual weight of the donor.

A well designed biomechanical experiment can prevent harmful devices from being introduced in clinical practice, however, positive results can never exclude the necessity of subsequent clinical studies.

Introduction

The annual worldwide incidence of hip fractures exceeds 1.7 million.¹ Femoral neck fractures account for 60% of these fractures and mainly occur in the elderly population.² With rare exceptions, all femoral neck fractures are treated surgically with either internal fixation or arthroplasty. Today an increasing proportion receives prosthetic replacement.² Nevertheless, a significant number of patients still have their proximal fragment fixed using various fixation techniques, and improvement of this treatment modality is therefore of clinical interest.

Clinical outcome after femoral neck fractures (FNF) in patients selected for internal fixation (IF) can be improved by better preoperative selection, optimizing surgical procedures of existing devices and by introducing improved implants and techniques.

Novel orthopaedic devices with similar design as already approved implants may not require level III classification by the U.S Food and Drug Administration (FDA) if equivalency to already approved implants has been demonstrated.³ New fracture fixation designs are often based on implants already established and are hence likely to fall into this category. Despite obvious limitations, biomechanical laboratory experiments can thus provide sufficient evidence for releasing a novel design.³

Evaluation of biomechanical femoral neck fracture experiments is troubled by the variety of the experimental setups used. A basic understanding of the most common setups can prove useful when the clinician evaluates the results and later decides whether to introduce a new device to his practice or not. Biomechanical experiments can also throw light on clinically relevant aspects concerning existing implants.

The most obvious shortcoming of biomechanical laboratory studies is their limitation in describing in vivo bone response to mechanical stimuli. Direct investigation of avascular necrosis, fracture healing, stress shielding and late implant loosening due to local bone necrosis⁴ requires response from live bone. Consequently, short-term failures such as early loosening, implant cut-outs and implant breakage can be demonstrated in laboratory experiments, while evaluation of most long-term outcomes cannot. Computerized models, animal experiments and examination of human samples harvested at surgery or postmortem may supply complementary information.

This paper does not fully describe the complexity of hip biomechanics. The aim is to provide background information necessary to comprehend biomechanical femoral neck fracture models and evaluate their results. By doing so, we hope to make interpretation of laboratory femoral neck fracture-research more accessible to clinicians with no particular background in biomechanics. Important factors like loading conditions, fracture morphology and clinically relevant endpoints are reviewed. The strengths and weaknesses of the different models are discussed.

Section snippets

Basic biomechanics of the hip

Intuitively, weight-bearing during, e.g. walking, compresses the length axis of the femur. The spatial position of the femoral head is located medial and anterior with respect to the anatomical axis of the diaphysis. Therefore weight-bearing also causes an additional bending of the femur. Surrounding soft tissues tend to minimize this bending. Nevertheless, tension on the lateral aspect is still present in vivo.⁵ In the stance phase of gait the femoral head is loaded with the femur condyles

Stiffness and load-to-failure

A static compression test can reveal the stiffness of a bone-implant construct. The stiffness can be compared to that of intact bone or to a comparable construct. Compression will cause head deflection. This provides data for a load/deflection curve, and stiffness can be calculated from the elastic part of the slope. Further compression will reveal yield load with damage accumulation/micro-fracturing culminating in plastic deformation and eventually failure.³⁷ Stiffness and load-to-failure are

Discussion

A variety of biomechanical setups are used when investigating femoral neck fractures. This opens up the possibility to thoroughly examine this common fracture, but also hampers direct comparison of study results and makes assessment of clinical relevancy difficult. High quality preclinical investigation is essential to secure the patient's welfare, and awareness of the limitations of such studies is essential.

Clinically, failure of internal fixation in femoral neck fractures rarely occurs due

Conclusions

In our opinion, a well-designed biomechanical FNF model for investigation of internal fixation devices is one that closely mirrors the in vivo situation. Human cadaveric femurs should be preferred instead of synthetic replicates, and the age of the donor and quality of bone should resemble the actual patient population. Standardized fractures should mimic fractures as they present themselves in vivo to avoid bias. The constraining of the femur should mirror the physiological situation as good

Conflict of interest statement

All authors state no conflict of interest.

Acknowledgements

We want to acknowledge Prof. Finsen for highly appreciated help in improving the language. The work has been financed by the Department of Orthopaedics, St. Olavs Hospital.

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Intertrochanteric fractures with a posteromedial intermediate fragment are unstable because of the loss of medial support. Additional fixation with a cerclage is used in subtrochanteric fractures, but not in intertrochanteric fractures. The aim of this biomechanical study is to evaluate whether cerclage fixation improves stability of intertrochanteric fractures.
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There were no statistically significant differences between force to failure (group 1: 681 N; group 2: 846 N; group 3: 699 N; group 4: 806 N; ANOVA p = 0.23) and stiffness (group 1: 769 N/mm; group 2: 819 N/mm; group 3: 815 N/mm; group 4: 810 N/mm; ANOVA p = 0.84) between groups. There were significant differences in the widening of the lag screw canal (group 1: 2.16 mm; group 2: 4.5 mm; group 3: 3 mm; group 4: 2.5 mm; ANOVA p = 0.017). In individual comparison, the differences were significant only between the anatomical reduction group and the non-anatomical reduction (p = 0.04) and the no cable group (p = 0.02).
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Limited studies are performed on experimental investigations of biomechanical properties at different displacement rate in compressive stance configuration of femur bone. Besides, an investigation has been performed using finite element analysis in available literature [20,38,39]. From the literature, it has been observed that very limited studies have been done on mechanical properties of femur bone.
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ReviewBiomechanical femoral neck fracture experiments—A narrative review

Abstract

Introduction

Methods

Summary

Introduction

Section snippets

Basic biomechanics of the hip

Stiffness and load-to-failure

Discussion

Conclusions

Conflict of interest statement

Acknowledgements

J Biomech

J Biomech

J Biomech

J Biomech

Clin Biomech (Bristol, Avon)

Injury

Injury

Injury

Injury

Clin Biomech (Bristol, Avon)

Injury

J Biomech

Bone

J Biomech

J Biomech

Burden of major musculoskeletal conditions

Bull World Health Organ

The Norwegian hip fracture register 2010

The evidence-based approach in bringing new orthopaedic devices to market

J Bone Joint Surg Am

Early temporary porosis of bone induced by internal fixation implants. A reaction to necrosis, not to stress protection?

Clin Orthop Relat Res

In vivo measurements show tensile axial strain in the proximal lateral aspect of the human femur

J Orthop Res

Telemetric force measurements across the hip after total arthroplasty

J Bone Joint Surg Am

The effect of cane use on hip contact force

Clin Orthop Relat Res

Postoperative weight-bearing after a fracture of the femoral neck or an intertrochanteric fracture

J Bone Joint Surg Am

Review
Biomechanical femoral neck fracture experiments—A narrative review