Finite element analysis of four different implants inserted in different positions to stabilize an idealized trochanteric femoral fracture

Perioperative peri-implant fracture after osteosynthesis for geriatric femoral pertrochanteric fracture with the linear compression integrated screw intramedullary nail system (INTERTAN™): a retrospective study

BACKGROUND

Osteosynthesis for geriatric femoral pertrochanteric fractures using the linear compression integrated screw intramedullary nail system (INTERTAN™) has become popular. Nonetheless, cases of perioperative peri-implant fractures have been reported following this surgical technique. The factors responsible for this complication remain unclear. Therefore, we investigated perioperative peri-implant fracture risk factors and incidence, as well as overall outcomes, using the INTERTAN™ system for geriatric femoral pertrochanteric fractures.

METHODS

We retrospectively reviewed 98 consecutive patients with geriatric femoral pertrochanteric fractures after INTERTAN™ fixation, with at least a 12-month follow-up period between May 2020 and April 2022 at a single medical institute. The patients' demographic characteristics, fracture pattern, quality of reduction, quality of fixation, nail length, morphology of the femur, and perioperative complications were recorded and analyzed.

RESULTS

Among the 98 patients, 92 achieved union during follow-up. Twelve perioperative peri-implant fractures (12.2%) were recorded, all of which occurred during or within 1 month of osteosynthesis. Except for one patient who underwent re-osteosynthesis, the others underwent nonoperative treatment, and all achieved union. Multiple regression analysis revealed morphology of the femur with low-lesser trochanter width (odds ratio (OR) 0.532, 95% confidence interval (CI) 0.33-0.86, p = 0.01) to be the only factor contributing to perioperative peri-implant fractures. When the Youden index was used, the optimal cut-off value was 20.2 mm of low-lesser trochanter width. Low-lesser trochanter width < 20.2 mm was found to be a potential factor causing perioperative peri-implant fractures (OR 17.81, 95% CI 1.67-19.76, p = 0.017).

CONCLUSIONS

Morphology of the femur with a low-lesser trochanter width smaller than 20.2 mm was found to be the only potential contributor to perioperative peri-implant fractures when using INTERTAN™ for geriatric femoral pertrochanteric fractures. Care should be taken during osteosynthesis, focusing not only on the fracture site but also on the femoral cortex around the implant. Although perioperative peri-implant fractures were observed within one month following osteosynthesis, the majority of these cases were effectively treated without surgical intervention.

Computer aided analysis of biomechanical performance of schanz screw with different additive manufacturing materials used in pertrochanteric fixator on an intertrochanteric femoral fracture (corrosion resistance approach)

This study examines the use of computer-aided analysis to evaluate the biomechanical performance of Schanz screws made from different additive manufacturing materials (Ti6Al4V, 316 L, Inconel 625, and Inconel 718) in a pertrochanteric fixator for the treatment of intertrochanteric femoral fractures. Intertrochanteric fractures (ITFs) are severe traumas often seen in the elderly population and can lead to serious consequences. The primary objective of ITF surgery is to provide stability and allow for early ambulation and rehabilitation. The Pertrochanteric Fixator is a surgical implant used to treat hip fractures near the greater trochanter, and is attached to the femur with screws. The procedure is performed under general anesthesia and typically takes 1-2 h. Possible complications include infection, nerve injury, and hardware failure. The aim of this study is to evaluate the biomechanical performance of Schanz screw using computer-aided analysis, comparing the effects of various additive manufacturing materials including Ti6Al4V, 316 L, Inconel 625 and Inconel 718 in a pertrochanteric fixator for intertrochanteric femoral fractures. Additionally, this study will also consider the corrosion resistance of these materials to ensure long-term durability and effectiveness in a clinical setting. The stress values mentioned for the implant materials are as follows. Ti6Al4V: 153.33 MPa, 316 L: 180.98 MPa, Inconel 625: 158.94 MPa, Inconel 718: 148.91 MPa. Higher stress values indicate a greater load transfer to the bone, which can potentially lead to stress shielding. Stress shielding occurs when an implant bears a significant portion of the load that should be transferred to the bone. This reduced stress at the fracture site can prevent the healing process, as bones require adequate stress levels for optimal remodeling and regeneration.

Comparison of Intramedullary Nails in the Treatment of Trochanteric and Subtrochanteric Fractures: An Observational Study of 13,232 Fractures in the Norwegian Hip Fracture Register

BACKGROUND

Intramedullary nails are commonly used in the treatment of trochanteric and subtrochanteric fractures. We aimed to compare intramedullary nails in widespread use in Norway on the basis of reoperation risk.

METHODS

We assessed data from 13,232 trochanteric or subtrochanteric fractures treated with an intramedullary nail and registered in the Norwegian Hip Fracture Register between 2007 and 2019. The primary outcome measure was the risk of reoperation for various types of short and long intramedullary nails. Secondly, we compared risk of reoperation for the selected nails with respect to fracture type (AO/OTA type A1, A2, A3, and subtrochanteric fractures). Cox regression analysis adjusted for sex, age, and American Society of Anesthesiologists class was used to estimate hazard rate ratios (HRRs) for reoperation.

RESULTS

The mean patient age was 82.9 years, and 72.8% of the nails were used in the treatment of female patients. We included 8,283 short and 4,949 long nails. A1 fractures accounted for 29.8%, A2 for 40.6%, A3 for 7.2%, and subtrochanteric fractures for 22.4%. When comparing short nails regardless of fracture type, the TRIGEN INTERTAN had an increased risk of reoperation at 1 year (HRR, 1.31 [95% confidence interval (CI), 1.03 to 1.66]; p = 0.028) and 3 years (HRR, 1.31 [95% CI, 1.07 to 1.61]; p = 0.011) postoperatively compared with the Gamma3. For individual fracture types, we found no significant differences in reoperation risk between the various types of short nails. When comparing long nails, the TRIGEN TAN/FAN had an increased risk of reoperation at 1 year (HRR, 3.05 [95% CI, 2.10 to 4.42]; p < 0.001) and 3 years (HRR, 2.54 [95% CI, 1.82 to 3.54]; p < 0.001) postoperatively compared with the long Gamma3.

CONCLUSIONS

This study may indicate a slightly increased risk of reoperation for the short TRIGEN INTERTAN compared with other short nails in widespread use in Norway. In analyses of long nails, the TRIGEN TAN/FAN nail was associated with a higher risk of reoperation in the treatment of trochanteric and subtrochanteric fractures.

LEVEL OF EVIDENCE

Therapeutic Level III . See Instructions for Authors for a complete description of levels of evidence.

Towards the simulation of bone-implant systems with a stratified material model

BACKGROUND

The clinical performance of medical devices is becoming increasingly important for the requirements of modern development processes and the associated regulations. However, the evidence for this performance can often only be obtained very late in the development process via clinical trials or studies.

OBJECTIVE

The purpose of the presented work is to show that the simulation of bone-implant-system has advanced in various aspects, including cloud-based execution, Virtual Clinical Trials and material modeling towards a point where and widespread utilization in healthcare for procedure planning and enhancing practices seems feasible. But this will only hold true if the virtual cohort data build from clinical Computer Tomography data are collected and analysed with care.

METHODS

An overview of the principal steps necessary to perfor Finite Element Method-based structural mechanical simulations of bone-implant systems based on clinical imaging data is presented. Since these data form the baseline for virtual cohort construction, we present an enhancement method to make them more accurate and reliable.

RESULTS

The findings of our work comprise the initial step towards a virtual cohort for the evaluation of proximal femur implants. In addition, results of our proposed enhancement methodology for clinical Computer Tomography data that demonstrate the necessity for the usage of multiple image reconstructions are presented.

CONCLUSION

Simulation methodologies and pipelines nowadays are mature and have turnaround times that allow for a day-to-day use. However, small changes in the imaging and the pre-processing of data can have a significant impact on the obtaind results. Consequently, first steps towards virtual clinical trials, like collecting bone samples, are done, but the reliability of the input data remains subject to further research and development.

Fractures after cephalomedullary nailing of the femur : Systematization of surgical fixation based on the analysis of a single-center retrospective cohort

PURPOSES

Femoral implant related fractures (IRF) are a growing pathology in an increasingly elderly and frail population. A series of IRF after cephalomedullary nail (CMN) fixation of a femoral fracture is analyzed and an algorithm described to guide the management of such fractures.

METHODS

All eligible patients operated on for IRF fixation after CMN were reviewed regarding their demographics, comorbidities, injury pattern, and treatment. Primary outcomes were mortality and local complications. Secondary outcomes were time to consolidation, time to weight-bearing initiation, length of hospitalization, and discharge destination.

RESULTS

The incidence of IRF requiring fixation was 1.3% after 3401 CMN implantation procedures. Elderly women with comorbidities and plate fixation predominated. One-year mortality was 18.6%, being higher for patients presenting with infection and those unable to walk at the end of follow-up. Local complications occurred in 25.6%. Median time to weight-bearing was 9.1 weeks, but longer for patients with plate fixation or complications. Patients presenting with an infection and those discharged to nursing facilities had more comorbidity.

CONCLUSIONS

Following an algorithm presented here, patients were treated either with nail exchange or lateral locking plate fixation, permitting straightforward evaluations and acceptable results in a very high-risk population.

Investigation of the process intergrowth of bone tissue into the hole in titanium implants (Experimental research)

INTRODUCTION

Despite the use of modern implants, complications such as nonunion and avascular necrosis of the femoral head are observed in femoral neck fractures (FNF). We have created a new perforated I-beam implant for FNF osteosynthesis and developed a new osteosynthesis philosophy based not only on the mechanical and biomechanical interaction of the bone-implant system, but also on the interaction of the biological properties of the bone and the implant. The purpose of the work is to study the interaction of the biological process of the bone - its regeneration (germination) of bone tissue into the holes of the implant.

MATERIALS AND METHODS

The experiment was carried out on fourteen Chinchilla rabbits in accordance with all international standards. A perforated implant specially made of titanium (ChM, Poland) was surgically implanted into the proximal femur. The implant measurements were as follows: length - 6 mm, width - 3 mm, thickness - 2 mm, 2 holes with a diameter of 2 mm. The 14 rabbits were divided into 7 groups. After 1, 2, 3, 4, 5, 10 and 12 weeks the animals were withdrawn from the experiment according to the standard rules in sequential order. The preparations were placed in a formalin solution and sent to the pathomorphology laboratory (CITO, Russia) for histological studies.

RESULTS

Weekly histopathological studies revealed a gradual transition from the organization of a hematoma to the formation of mature bone tissue in the holes of the implants. The titanium implant is bioinert and did not cause any visible reactions from the bone tissue. Simultaneous integration of vascular proliferation and newly formed bone tissue into the implant holes were revealed. On 10-12-week preparations, the formation of trabecular structures of mature bone tissue was revealed in the holes of the implants and elements of adipose and bone marrow tissue were observed. Macroscopic examination of 4-5-week preparations showed almost complete filling of the holes with bone tissue. On 10-12-week preparations, the bone tissue in the holes of the implants did not differ from the bone tissue surrounding the implant. The processes of formation of mature bone tissue in the holes of the implants were similar to the processes of physiological bone healing (regeneration) at the fracture site.

CONCLUSIONS

The obtained results show the following: 1.The titanium implant is bioinert and does not cause any visible reactions from the bone tissue; 2. There is a gradual process of formation of new vessels, and then the formation of new bone tissue in the holes of the implant instead of the one damaged during implantation. Thus, the results of this experiment indirectly confirm our assumption that a perforated implant for FNF osteosynthesis will participate not only in the mechanical and biomechanical interaction of the bone-implant system, but will also include the 3rd element in this system - the biological properties of the bone itself. We assume that these properties of the new implant will increase blood flow in the femoral neck and partially replenish the volume of bone tissue destroyed during osteosynthesis which does not occur with FNF osteosynthesis by any of the known implants.

Trochanteric Nails for the Reduction of Intertrochanteric Fractures: A Biomechanical Analysis Based on Finite Element Analysis and DIC System

Purpose

Intertrochanteric fractures are common among femoral fractures in the elderly population. The trochanteric nail is a standard internal fixator used in treating femoral intertrochanteric fractures. The technique of femoral fracture reduction affects the postoperative outcome. Here, we applied finite element analysis (FEA) to study mechanical effects of different reduction approaches using the trochanteric nail in treating both stable and unstable intertrochanteric fractures.

Methods

We combined FEA and in vitro experiments using a digital imaging correlation (DIC) technique to study effects of different alignment conditions after treating 4 cases of intertrochanteric fractures using the trochanteric nail system. A downward force of 2250 N was applied to the femoral head, and the distal end of the femur was fixed. The observed indicators were the femur displacement, together with the stress on the femur and trochanteric nail system. In addition, the displacement distribution was analyzed using DIC.

Results

In the case of space reduction, the force was transmitted by the trochanteric nail system, resulting in greater stress imposed on the femur or the trochanteric nail system. In the case of closed reduction, the stress was much smaller. In the case of unstable fracture reduction, closed reduction was associated with a smaller contact area at the fracture site, resulting in greater stress on both trochanter and the trochanteric nail system.

Conclusion

When the trochanteric nail system was used for fixation, the fracture site was well aligned, reducing the stress on the femur or the trochanteric nail.

Modified Short Proximal Femoral Nail for Intertrochanteric Fractures of Femur in Indian Patients - our Experience

Introduction

Proximal femoral nail (PFN) is a commonly used implant for intertrochanteric fractures which is designed according to western femoral measurements. However, anthropometry of proximal femur in Indian and in general, Asian, are smaller. So a modified short PFN with smaller dimensions was developed. This study analyses the radiological and functional outcome of treatment of intertrochanteric fractures with modified short PFN.

Materials and Methods

A retrospective study analysed 120 adult patients operated between 2014-2017 using modified short PFN for intertrochanteric fractures, having a minimum follow-up of 12 months. Clinical and radiological parameters including tip-apex distance (TAD), position of tip of lag screw in femoral head, lateral slide of lag screw as well as length of anti-rotation screw were measured. Final functional outcome was assessed using Barthel's index and Kyle's criteria.

Results

Good reduction was achieved in 90.83% cases and 79.16% had ideal placement of lag screw in femoral head. Intra-operative difficulties were encountered in 13.33% (n=16). Mean TAD AP (anteroposterior) was 11.8mm, TAD LAT (lateral) was 11.0mm and mean TAD TOT was 22.8mm. Overall mean lateral slide was 3.20mm and it was more in unstable fracture. We had five mechanical failures, one patient with screw breakage without loss of reduction and two peri-implant fractures after union. 81.66% returned to pre-injury levels of activity with 88.33% good to excellent outcome as per Kyle's criteria.

Conclusion

Although, not devoid of complications, modified short PFN results in good functional recovery of patients with intertrochanteric fractures of femur.

Should early weightbearing be allowed after intramedullary fixation of trochanteric femur fractures? A finite element study

BACKGROUND

This study aims to investigate the effects of early weightbearing after intramedullary fixation of trochanteric fractures.

METHODS

Femurs with different types of trochanteric fractures were modeled according to AO/OTA classification. Fractures were ideally reduced with one mm gap between fragments and fixed with intramedullary nails. Forces were applied simulating single- (Body weight: 60 kg, joint reaction force: 1999.2 N, abductor muscle force:1558.8 N) and double-leg standing positions (Joint reaction force: 196 N). In another model, a 500 Nm rotational force was applied as a simulation of a fall.

RESULTS

A higher level of stress was determined at the calcar femorale, the fracture site, the holes for the lag screws, and the hole for the proximal locking screw on the nail, the threadless parts of the lag screws, and the mid-portion of the nail. During the single-leg stance, up to 3 mm displacement was observed with the reverse oblique type of fractures. In the simulation of the fall, 1.5 mm displacement occurred at the fracture site. No displacement was measured at stabile and type 31A2 fracture models. In addition, higher levels of stress were measured at the body of the nail (up to 133 MPa), proximal screws (up to 133 MPa) and at the bone distal to the nail (up to 84.3 MPa), but all values were under the limit of the yield stress of the bone and the titanium.

CONCLUSION

Full weightbearing after intramedullary fixation of trochanteric femur fractures may be allowed except in obese patients and patients with 31A3 type fractures according to the AO/OTA classification. The use of support is recommended in order to prevent complications. Implant removal can be discussed with patients after fracture union in order to prevent possible periprosthetic fractures.

Parametric Finite Element Analysis of Intramedullary Nail Fixation of Proximal Femur Fractures

Proximal femur fracture fixation with intramedullary nailing relies on stability at the fracture site and integrity of the fixation construct to achieve union. The biomechanics that dictate fracture site stability and implant stress depend on fracture type as well as implant features such as nail length, nail diameter, presence of distal fixation screws, and material composition of the implant. When deciding how to fix a fracture, surgeons have choices in these implant-related design variables. This study models all combinations of a range of implant variables for nine standard AO/OTA proximal femur fractures using finite element analysis. Under simulated maximum load during gait, the maximum stress in the implant and screws as well as interfragmentary motions at the fracture site in the axial and shear directions were computed. The results were separated by fracture type to show the influence of each design variable on measured biomechanical outcomes. Filling the reamed canal with the largest fitting nail diameter reduced axial and shear interfragmentary motion for all fracture types. Nail length was less predictive of shear interfragmentary motion for most simulated fracture types than other construct variables. Furthermore, gapping at the fracture site predisposed the construct to higher implant stresses and larger interfragmentary motions. Clinical significance: Biomechanical outcomes from this computational study can aid in surgical decision-making for optimizing hip fracture fixation with IM nailing. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2358-2366, 2019.

Efficacy and efficiency of multivariate linear regression for rapid prediction of femoral strain fields during activity

Multivariate Linear Regression-based (MLR) surrogate models were explored to reduce the computational cost of predicting femoral strains during normal activity in comparison with finite element analysis. The musculoskeletal model of one individual, the finite-element model of the right femur, and experimental force and motion data for normal walking, fast walking, stair ascent, stair descent, and rising from a chair were obtained from a previous study. Equivalent Von Mises strain was calculated for 1000 frames uniformly distributed across activities. MLR surrogate models were generated using training sets of 50, 100, 200 and 300 samples. The finite-element and MLR analyses were compared using linear regression. The Root Mean Square Error (RMSE) and the 95th percentile of the strain error distribution were used as indicators of average and peak error. The MLR model trained using 200 samples (RMSE < 108 µε; peak error < 228 µε) was used as a reference. The finite-element method required 66 s per frame on a standard desktop computer. The MLR model required 0.1 s per frame plus 1848 s of training time. RMSE ranged from 1.2% to 1.3% while peak error ranged from 2.2% to 3.6% of the maximum micro-strain (5020 µε). Performance within an activity was lower during early and late stance, with RMSE of 4.1% and peak error of 8.6% of the maximum computed micro-strain. These results show that MLR surrogate models may be used to rapidly and accurately estimate strain fields in long bones during daily physical activity.

[A clinical study on the relationship of the tail femur distance and the lag screw migration or cutting-out after the third generation of Gamma nail fixation of intertrochanteric fracture]

Objective

To confirm the association between tail femur distance (TFD) and lag screw migration or cutting-out in the treatment of intertrochanteric fracture with the third generation of Gamma nail (TGN).

Methods

The clinical data of 124 cases of intertrochanteric fracture treated with TGN internal fixation and followed up more than 18 months between January 2012 and December 2015 were reviewed and analyzed. There were 52 males and 72 females, with an age of 46-93 years (mean, 78.5 years). According to AO/Association for the Study of Internal Fixation (AO/ASIF) classification, 43 cases were type 31-A1, 69 cases were type 31-A2, and 12 cases were type 31-A3. The time from injury to operation was 1-10 days (mean, 2.9 days). According to the fracture healing of the patients, the patients were divided into the healing group and failure group. The age, gender, height, bone mineral density (BMD), fracture AO/ASIF classification, the time from injury to operation, and the TFD value at 1 day after operation were recorded and compared. The risk factors for the migration or cutting-out of lag screw were analyzed by logistic regression.

Results

There were 111 cases in healing group, the healing time was 80-110 days (mean, 95.5 days). There were 13 cases in failure group, including 2 cases of lag screw cutting-out and 11 cases of significant migration. Except for the TFD value at 1 day after operation in failure group was significantly higher than that in the healing group( t=5.14, P=0.00), there was no significant difference in gender, age, height, BMD, fracture of AO/ASIF classification, and the time from injury to operation ( P>0.05) between 2 groups. logistic regression analysis showed that TFD value was a risk factor for the migration or cutting-out of lag screw (B=1.22, standardized coefficient=0.32, Wald χ ²=14.66, P=0.00, OR=3.37).

Conclusion

The patients with higher TFD value had higher risk of postoperative lag screw migration or cutting-out. This result indicates that the appropriate length of the lag screw is helpful to reduce TFD value and prevent postoperative lag screw migration or cutting-out.

Is calcar referenced tip-apex distance a better predicting factor for cutting out in biaxial cephalomedullary nails than tip-apex distance?

PURPOSE

To test the significance of calcar referenced tip-apex distance (CalTAD) and the length of anti-rotation screw (AR screw) as predictors for failure after biaxial cephalomedullary (CM) nailing of intertrochanteric fractures.

METHODS

We retrospectively reviewed 190 consecutive fractures that had undergone biaxial CM nailing. Of these, 67 met the inclusion criteria of a non-pathological fracture with a minimum of 90 days radiological follow-up (mean 458 days; 91 days to 4.9 years).

RESULTS

The overall failure rate was 15% (10 of 67). Failure was associated with a higher CalTAD in most of the patients (13%, p < 0.001). A higher tip-apex distance (TAD) was not significantly associated with failure ( p = 0.132), when the CalTAD was less than 25 mm. When the AR screw length exceeded a line connecting the tip of the nail and the lag screw, screw cutout occurred only in one patient ( p = 0.095).

CONCLUSION

Our data provide the first reported clinical evidence that the CalTAD is a better predictor of cutout in biaxial CM nailing than TAD measurement. However, the length of anti-rotational element does not seem to be an independent predictor of CM nailing failure.

Impact of tip-apex distance and femoral head lag screw position on treatment outcomes of unstable intertrochanteric fractures using cephalomedullary nails

Background

Cephalomedullary nails are frequently used in unstable intertrochanteric fractures. The implant position is an important factor for surgical success. Thus, in the present study, finite element analysis methods were used to investigate the biomechanical behavior of five different cephalomedullary nail positions in unstable intertrochanteric fractures.

Methods

Five different cephalomedullary nail implant positions were investigated. The observed indicators were the maximum displacement of the lag screw, the stress on the intertrochanteric fracture with involvement of the posteromedial cortex, and the tip–apex distance.

Results

The smallest lag screw displacement was achieved when the implant was closer to the inferior femoral head. Lower stress was placed on the posteromedial cortex when the implant was positioned closer to the inferior femoral head. However, the tip–apex distance increased when the lag screw was positioned more inferiorly.

Conclusions

The results of this study suggest that positioning the lag screw closer to the inferior aspect of the femoral head can reduce stress on the posteromedial cortex and deformation of the implant in unstable intertrochanteric fractures. These findings provide a biomechanical basis for selection of the cephalomedullary nail implantation site.

Level of evidence

III.

The nail-shaft-axis of the of proximal femoral nail antirotation (PFNA) is an important prognostic factor in the operative treatment of intertrochanteric fractures

INTRODUCTION

Due to the world's aging population, intertrochanteric fractures are frequent. Considering age and comorbidities of most of these patients, it is indispensable to enable early postoperative mobilization of these patients. Intramedullary osteosynthesis with PFN-A is widely used and, in general, considered safe and reliable for the operative treatment of intertrochanteric fractures. However, implant -related complications are reported in 6-21% of all cases. In this study, we are analyzing complication rates and risk factors for implant-related complications.

MATERIALS AND METHODS

All intertrochanteric fractures admitted to our hospital and treated with PFN-A between January 2012 and January 2016 were analysed retrospectively. Radiological analyses of the CCD and the lateral offset on the uninjured side was compared to the CCD initially postoperatively and during follow-up on the operated side. Furthermore, we analysed the tip-apex distance (TAD), blade position in the femoral head and introduced the nail-shaft axis as a new parameter.

RESULTS

Within 101 intertrochanteric fractures included, 16 implant-related complications were encountered (15.84%). Analyses of risk factors for secondary varus displacement of greater than 10° within the follow-up and thus risk for cut-out in the osteosynthesis revealed that etasblished risk factors such as a TAD > 25 mm, reduction in varus and an improper position of the blade were also significant risk factors in our cohort for failure of the osteosynthesis. Moreover, we introduced the nail-shaft axis a new potential risk factor and could show that a too medial or too lateral nail-shaft axis is also significantly associated with secondary varus displacement.

CONCLUSION

When treating introchanteric fractures with PFNA reduction in neutral or even slight valgus, aiming for a TAD < 25 mm and a correct position of the blade within the femoral head reduced the risk for secondary varus displacement significantly. Furthermore, we suggest to aim for a central nail-shaft axis.

Dual head screw hip nailing for trochanteric fractures

INTRODUCTION

There are limited information and inconclusive results for dual head screw intramedullary hip nails for trochanteric fractures. Therefore, we performed a prospective study to evaluate the healing of fractures, and survival, function, and complications of patients operated with this implant.

METHODS

We prospectively studied 79 patients (61 women and 18 men; mean age: 84.7 years; range: 65-96 years) with a low-energy trochanteric fracture, treated with a dual head screw intramedullary hip nail from 2013 to 2016. The mean follow-up was 2.1 years (range: 1-3 years); seven patients were lost to follow up. This left 72 patients for further analysis. We evaluated the healing of fractures, and survival, function, and complications of patients.

RESULTS

Fracture healing was evident in 70 patients (97.2%) at 2-3 months postoperatively. One patient experienced cut-out and z-effect phenomenon of the head screws. Another patient experienced a periprosthetic femoral diaphysis fracture at the distal tip of the nail. A third patient experienced an acute postoperative superficial skin infection that was treated successfully with wound dressing changes and a course of antibiotics. Sixteen patients (22.2%) deceased within 12 months postoperatively. In the remaining patients, the Harris Hip Score (HHS) at 12 months postoperatively was excellent in 16 (28.6%), good in 23 (41.1%), fair in 10 (17.8%), and poor in 7 patients (12.5%). The function declined after the patients' fracture. Fair and poor results were related to age > 85 years, poor pre-fracture level of function, and AO/OTA-31-A3 fracture types.

CONCLUSION

The dual head screw intramedullary hip nail is associated with high healing and low complication rates for intertrochanteric fractures. The function of the patients is good or excellent in most cases; however, it declines, especially for those patients with age > 85 years, poor pre-fracture level of function, and AO/OTA-31-A3 fracture types.

Biomechanical effect of different femoral neck blade position on the fixation of intertrochanteric fracture: a finite element analysis

Medial migration or cutout of the neck helical blade has commonly occurred in the treatment of trochanteric fracture of the femur. The position of the helical blade within the femoral head is one of the influencing factors that cause the blade to perforate the intact joint surface; however, the ideal placement of the helical blade is not currently known. A finite element model of a femur/nail construct was utilized to analyze five possible blade positions in the femoral head. Normal strain at the fracture surface, the minimum principal strain in the cancellous bone, and the von Mises stress in the implant itself were calculated and compared between different blade positions. The results showed that a large area of normal compressive strain at the fracture surface was observed in the inferior and posterior blade positions. The volume of cancellous bone strained to yielding in the femoral head and neck was lower for the inferior and posterior positions, whereas it was the highest for the superior position. The inferior and posterior positions had lower von Mises stress in the implant itself. The inferior and posterior positions may be the ideal position for the intramedullary nail with a helical neck blade.

3D CAD/reverse engineering technique for assessment of Thai morphology: Proximal femur and acetabulum

PURPOSE

To assess morphological parameters of proximal femur and acetabulum in Thai population with three-dimensional measurement technique, and to analysis of collateral side symmetric, gender difference, and correlation between morphometric parameters.

METHODS

Investigation was performed in 240 femurs. All three-dimensional femur models were acquitted from 64-slice spiral CT scanner. Morphometric parameters under consideration included acetabular diameter, femoral head diameter, shaft isthmus location, intramedullary canal diameter, diaphyseal diameter, femoral head height, femoral neck isthmus, femoral neck length, neck shaft angle, bow angle, and anteversion angle. All parameters were measured based on functions and least-square regression function in CAD software. Obtained measured data were then used for analysis of collateral side symmetric, gender difference, correlation between morphometric parameters, and compared with other populations.

RESULTS

Female had a smaller dimension compared with male in most of the parameters. No significant difference was observed between left and right femurs. High correlation pairs of morphometric parameters included femoral head diameter-acetabular diameter, femoral head diameter-neck isthmus diameter, femoral head diameter-diaphyseal diameter at shaft isthmus level, acetabular diameter-neck isthmus diameter, neck isthmus diameter-diaphyseal diameter at shaft isthmus level, and acetabular diameter-diaphyseal diameter at shaft isthmus level. Some morphometric parameters of Thai are smaller than other Caucasian, and some Asian nation, i.e. femoral head diameter, femoral neck length, and femoral head height.

CONCLUSIONS

This study provides essential morphometric data for various orthopedic implant designs relating to proximal femur region.

New Proximal Femoral Compaction Blade Provides Strong Antirotation Stability of the Femoral Head

This study investigated the mechanical properties of a new rectangular compaction blade and compared this blade with other types of nail. Three types of nail were tested: the Magnum lag screw (Robert Reid Inc, Tokyo, Japan), proximal femoral nail, and Magnum Fid blade (Robert Reid Inc). The nails were inserted into solid rigid polyurethane foam, and the torsional moments were loaded with an Instron testing machine (Instron, Kanagawa, Japan). The force curve was recorded, and the average maximum torque was calculated from this curve. A simulation study was performed with finite element models to determine the mechanism underlying differences in rotational stability. Mechanical testing showed that the new compaction blade had stronger resistance against rotational force than the helical blade and lag screw implants. Finite element analysis also showed that the new compaction blade had stronger resistance to migration of the polyurethane foam cylinder than the other implant types. In addition, the new compaction blade had strong rotational stability. This implant should be useful for the treatment of unstable trochanteric fracture in patients with osteoporosis. [Orthopedics. 2017; 40(3):e491-e494.].

Intertrochanteric fracture: Association between the coronal position of the lag screw and stress distribution

BACKGROUND/PURPOSE

The best position of the lag screw in the femoral head for the fixation of intertrochanteric fracture is controversial. Traditional view suggests that it should be positioned in the central axis of the femoral neck with a tip-apex distance (TAD) of <25 mm, but the mechanical properties have not been reported yet. Herein, we aimed to investigate internal fixation with the lag screw placed in different positions on the femoral coronal plane by performing a finite element analysis and to identify a reasonable lag screw position after the internal stress distributions at the femoral head.

METHODS

A three-dimensional finite element model of a healthy male's femur was set up, on which the intertrochanteric fracture model with proximal femoral nail antirotation (PFN-A) was based. Nine modalities of the model were established in accordance with different lag screw positions in the femoral head. Three-dimensional finite element calculations were conducted, and the distribution trends of characteristic high-stress concentration points were observed.

RESULTS

The area of high-stress concentration was distributed from the top of the femoral head to the medial cortex of the trochanteric region. Four characteristic high-stress concentration points were observed, and the following trends indicated that the lower the position of the lag screw, the greater its length.

CONCLUSIONS

A longer and lower lag screw may make the fixation sustain greater stress, reduce bone tissue stress correspondingly in intertrochanteric fractures fixated with PFN-A, and sustain greater stress and more cyclic load at the same bone density.

The interplay of stiffness and force anisotropies drives embryo elongation

The morphogenesis of tissues, like the deformation of an object, results from the interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognized, whereas the importance of tissue material properties, in particular stiffness, has received much less attention. Using Caenorhabditis elegans as a model, we examine how both aspects contribute to embryonic elongation. Measuring the opening shape of the epidermal actin cortex after laser nano-ablation, we assess the spatiotemporal changes of actomyosin-dependent force and stiffness along the antero-posterior and dorso-ventral axis. Experimental data and analytical modelling show that myosin-II-dependent force anisotropy within the lateral epidermis, and stiffness anisotropy within the fiber-reinforced dorso-ventral epidermis are critical in driving embryonic elongation. Together, our results establish a quantitative link between cortical tension, material properties and morphogenesis of an entire embryo.

DOI:http://dx.doi.org/10.7554/eLife.23866.001

Animals come in all shapes and size, from ants to elephants. In all cases, the tissues and organs in the animal’s body acquire their shape as the animal develops. Cells in developing tissues squeeze themselves or push and pull on one another, and the resulting forces generate the final shape. This process is called morphogenesis and it is often studied in a worm called Caenorhabditis elegans. This worm’s simplicity makes it easy to work with in the laboratory. Yet processes that occur in C. elegans also take place in other animals, including humans, and so the discoveries made using this worm can have far-reaching implications.

As they develop, the embryos of C. elegans transform from a bean-shaped cluster of cells into the characteristic long shape of a worm, with the head at one end and the tail at the other. The force required to power this elongation is provided by the outer layer of cells of the embryo, known as the epidermis. In these cells, motor-like proteins called myosins pull against a mesh-like scaffold within the cell called the actin cytoskeleton; this pulling is thought to squeeze the embryo all around and cause it to grow longer.

Six strips of cells, running from the head to the tail, make up the epidermis of a C. elegans embryo. Myosin is mostly active in two strips of cells that run along the two sides of the embryo. In the strips above and below these strips (in other words, those on the upper and lower sides of the worm), the myosins are much less active. However, it is not fully understood how this distribution of myosin causes worms to elongate only along the head-to-tail axis.

Vuong-Brender et al. have now mapped the forces exerted in the cells of the worm’s epidermis. The experiments show that, in the strips of cells on the sides of the embryo, myosin’s activity causes the epidermis to constrict around the embryo, akin to a boa constrictor tightening around its prey. At the same time, the actin filaments in the other strips form rigid bundles oriented along the circumference that stiffen the cells in these strips. This prevents the constriction from causing the embryo to inflate at the top and bottom strips. As such, the only direction the embryo can expand is along the axis that runs from its head to its tail.

Together, these findings suggest that a combination of oriented force and stiffness ensure that the embryo only elongates along the head-to-tail axis. The next step is to understand how this orientation and the coordination between cells are controlled at the molecular level.

DOI:http://dx.doi.org/10.7554/eLife.23866.002

The importance of intramedullary hip nail positioning during implantation for stable pertrochanteric fractures: biomechanical analysis

PURPOSE

Proximal femoral fractures are among the most commonly sustained fractures. The current treatment of stable proximal femoral fractures located in trochanteric region primarily involves the use of two systems: extramedullary dynamic hip screws and intramedullary hip nails. Given that these fractures are mainly found in the elderly population, the necessity of a repeat, due to failure of the first, may jeopardize the patient's life. Decisive factors contributing to the healing of a fracture (or the failure thereof) include fracture pattern, technical implementation of the operation (i.e., position of the implant), implant's properties and its changes in relation to the surrounding bone tissue during loading. Each screw insertion variant results in damage to various load-bearing bone structures, which can be expected to influence healing quality and stability of newly formed bone.

METHOD

With the aid of a numerical model and finite element methods, the authors analyzed several different positions of IMHN/PFH-nails in the proximal femur, with the objective of determining positions with an increased risk of failure.

RESULTS AND CONCLUSION

In model situations, it has been shown that in stable fractures results do not depend on absolutely precise positioning and small deflections in the nails and neck screws positions do not significantly increase the risk of failure for the entire fixation. Damage to load-bearing structures relative to various implant placements does not impact the resultant overall fixation stability. Therefore, it is not necessary to re-introduce implants in the ideal position, which can lead to reduced patient radiation doses during surgery.

A Biomechanical Comparison of Two Intramedullary Implants for Subtrochanteric Fracture in Two Healing Stages: A Finite Element Analysis

Background. The biomechanical effect of two implants, namely, proximal femoral nail antirotation for Asia (PFNA-II) and Expert Asian Femoral Nail (A2FN), for treating subtrochanteric fracture during healing stages, is still unclear. Methods. A 3D finite element model of an intact femur was constructed and validated. The fractured and postoperative models were accordingly produced. The postoperative models were loaded with the peak joint forces during gait for the soft and hard callus stages. The effects of stress distribution on the implants, femoral head and callus, and the deformation of the proximal femur were examined. Results. Both implants showed similar biomechanical effect in two healing stages. As the healing duration increased, the von Mises stress of two implants and the tensile stress of the femoral head decreased, whereas the compressive stress of the femoral head increased. However, the PFNA-II operation resulted in higher stress on the implant, lower stress on the proximal femur, and lower compressive stress and higher tensile stress on the callus than A2FN operation. Conclusions. The A2FN implant may provide a biomechanically superior construct for subtrochanteric fracture healing. However, the upper screw of the A2FN implant may be more likely to be loose in the healing process.

The effect of two different trochanteric nail lag-screw designs on fixation stability of four-part intertrochanteric fractures: a clinical and biomechanical study

OBJECTIVES

To compare lag-screw sliding characteristics and fixation stability of two cephalomedullary nails (CMN) with different lag-screw designs (solid and telescopic), we conducted a biomechanical study and an analysis of clinical results.

METHODS

Six pairs of cadaver femurs with simulated intertrochanteric fractures were randomly assigned to one of two CMN fixations. Femur constructs were statically then cyclically loaded on an MTS machine. Lag-screw sliding and inferior and lateral femoral head displacements were measured, following which failure strength of the construct was determined. Forty-five patients with intertrochanteric fractures treated with these CMN were identified. Medical records and radiographs were reviewed and analyzed using Fisher's exact test and Student's t test to determine lag-screw sliding.

RESULTS

No difference was seen with cycling in inferior femoral head displacement between the two screw designs. The solid screw had an average inferior head displacement of 1.75 mm compared with 1.59 mm for the telescoping screw (p = 0.772). The solid lag screws slid an average of 2.79 mm lateral from the nail, whereas the telescoping screws slid an average of 0.27 mm (p = 0.003). In our clinical review, the average lateral sliding of the telescoping screw was 0.5 mm and of the solid screw was 3.7 mm (p < 0.001). Despite differences in lateral sliding, there were no reoperations for prominent or painful hardware in either group.

CONCLUSIONS

Both designs are acceptable devices for stabilization of intertrochanteric fractures. Clinical and biomechanical data demonstrate greater lateral sliding in the solid lag-screw group, making for greater potential for lateral-sided hip pain in CMNs with solid lag screws as opposed to telescoping lag screws.

Intramedullary nailing of trochanteric fractures: central or caudal positioning of the load carrier? A biomechanical comparative study on cadaver bones

BACKGROUND

Current recommendations with regard to central or caudal positioning of the femur head carrier in the management of trochanteric fractures are contradictory.

METHODS

A standardised pertrochanteric osteotomy was stabilised in 15 pairs of cadaver femurs by means of intramedullary osteosynthesis (5xPFN-A-Synthes, 5xIntertan-Smith&Nephew, 5xTargon-PF-Aesculap). For each pair randomised central (group A) or caudal (group B) implantation of the femoral neck component was performed. Subsequently, the constructs were axially loaded to 2100N. In the absence of cut out after 20,000 cycles, load was increased to a maximum force of 3100N. Angular displacement was recorded based on ultrasound. Migration of the load carrier in the femoral head was monitored radiologically.

FINDINGS DISPLACEMENT

No significant difference between groups (p>0.15) was found for the first 50 load cycles. A significantly greater degree of varus deformity was observed in group A (p=0.049) after 2000 load cycles and became more apparent as the number of load cycles increased (after 6000 cycles p=0.039, after 20,000 cycles p=0.034, after 22,000 cycles p=0.016). Angular displacement in the other two planes did not differ significantly across groups. CUT OUT: Migration of the load carrier in the femoral head was not significantly different for the two groups. Overall cut out occurred in 9 constructs, 3 in group A and 6 in group B. The difference in cut-out rate was not significant (p=0.213, chi-squared test).

CONCLUSION

Biomechanical superiority can be shown for caudal positioning of the femoral neck load carrier in terms of reduced varus deformity. The incidence of cut out is however unaffected by the position of the load carrier.

How to prevent cut-out and cut-through in biaxial proximal femoral nails: is there anything beyond lag screw positioning and tip-apex distance?

PURPOSE

Hip perforation is a major complication in proximal femoral nailing. For biaxial nails, knowledge of their biomechanics is limited. Besides re-evaluation of accepted risk factors like the tip-apex distance (TAD), we analysed the influence of anti-rotational pin length.

METHODS

We compared 22 hip perforation cases to 50 randomly chosen controls. TAD, lag-screw position, angle between lag-screw and femoral neck axis, lag-screw gliding capacity, displacement and anti-rotational pin length were investigated.

RESULTS

Hip perforation was associated with a higher angle of deviation between lag-screw and femoral neck axis (p = 0.001), a lower telescoping capacity of the lag screw (p = 0.02), and higher TAD (p = 0.048). If the anti-rotational pin exceeded a line connecting the tip of the nail and the lag screw (NS line), hip perforation incidence was increased (p = 0.009). Inadequate pin length resulted in an odds ratio of 10.8 for hip perforation (p = 0.001).

CONCLUSIONS

In biaxial nails anti-rotational element positioning is underestimated, however, crucial.

The importance of lag screw position for the stabilization of trochanteric fractures with a sliding hip screw: a subject-specific finite element study

Using finite element analysis, we compared the biomechanical performance of a CT scan-based three-part trochanteric fracture model (31-A2 in the AO classification) stabilized with a sliding hip screw for nine different positions of the lag screw (3 × 3 arrangement, from anterior to posterior and from inferior to superior). Our results showed that the volume of bone susceptible to yielding in the head and neck region is the lowest for inferior positions and increases as the lag screw is moved superiorly. Overall, for this specific subject, the models less likely to lead to cut-out are the ones corresponding to inferior middle and inferior posterior positions of the lag screw. In our study, the tip-apex distance (TAD) was anti-correlated with the risk of cut-out, as quantified by the volume of bone susceptible to yielding, which suggests that a TAD >25 mm cannot be considered to be an accurate predictor of lag screw cut-out. Further clinical studies investigating lag screw cut-out should attempt to find more reliable predictors of cut-out that should better reflect the biomechanics and subject-specificity of the femoral head.

Biomechanical femoral neck fracture experiments--a narrative review

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.

Occurrence of secondary fracture around intramedullary nails used for trochanteric hip fractures: a systematic review of 13,568 patients

INTRODUCTION

A sliding hip screw (SHS) is currently the treatment of choice for trochanteric hip fractures, largely due to the low incidence of complications. An alternative treatment is the use of intramedullary proximal femoral nails. Unfortunately these implants have been associated with a risk of later fracture around the implant. The aim of this study was to see if any improvements have been made to the current intramedullary nails, to reduce the incidence of secondary fracture around the distal tip of the nail.

METHODS

We analysed data related to 13,568 patients from 89 studies, focusing on the incidence of post operative secondary femoral shaft fracture following the use of intramedullary nails in the fixation of trochanteric hip fractures.

RESULTS

The overall reported incidence of secondary fracture around the nail was 1.7%. The incidence of fracture has reduced in the 3rd generation Gamma nails when compared to the older Gamma nail (1.7% versus 2.6%, p value 0.03). However, the incidence of secondary fracture in the 3rd generation Gamma nails is still significantly higher than the other brands of short nail (1.7% versus 0.7%, p value 0.0005). Long nails had a slight tendency towards a lower risk of fracture although the difference was not statistically significant (1.1% versus 1.7%, p value 0.28). There was a significantly lower risk of fracture for those nails with a biaxial fixation as opposed to uniaxial fixation (0.6% versus 1.9%, p value <0.0001).

CONCLUSION

Secondary fracture around a proximal femoral nail is one of the most significant of fracture healing complications, and this study suggests that continuing design changes to this method of fixation has reduced the risk of this complication occurring.

Radiological evaluation of intertrochanteric fracture fixation by the proximal femoral nail

BACKGROUND

Successful treatment of intertrochanteric femoral fractures was reportedly influenced by the position of the fixation devices, by reduction quality and by fracture type.

METHODS

The records of 227 patients with intertrochanteric fractures treated by intramedullary hip screws were analysed retrospectively. The angle and distance from the femur head apex were transformed into Cartesian coordinates. Comparisons were performed between patients with no mechanical failure (207 patients, 90.7%), with cutouts (15 patients, 6.6%) and with secondary loss of reduction (5 patients, 2.2%).

RESULTS

The standard tip apex distance (TAD) measurement above 25 mm did not predict failure (p=0.62). Mechanical failure rates increased from 4.8% to 34.4% when the centre of lag screw was not in the second quarter of the head-neck interface line (the so-called "safe zone") (p=0.001). Lag screw insertion lower or higher than 11 mm of the head apex line were associated with failure rates of 5.5% and 18.6%, respectively (p=0.004). Multivariate logistic regression showed that lag screw insertion not within the "safe-zone" was associated an Odds Ratio of 13.4 (95% CI 2.24-81) for mechanical failure (p=0.004).

CONCLUSIONS

The TAD scale focuses on length measurement and lacks the vector properties of multidirectional measurements. Vector analysis revealed that the caudal-cranial correct lag screw position is the most important factor in preventing mechanical failure.

Effects of muscle-equivalent forces on the biomechanical behavior of proximal femur fracture models: a pilot study on artificial bones

Introduction:

There has been extensive analysis of the influence of muscle forces and their effects on the biomechanical behavior of the proximal femur. Nevertheless, these forces have only been taken into account in a handful of biomechanical studies in the field of traumatology. The aim of this study was to analyze the biomechanical behavior of two typical fracture models of the proximal femur based on muscle-equivalent forces.

Method:

Plate osteosynthesis was performed on two groups of artificial femora to stabilize either a trochanteric osteotomy ( n = 5) or a femur shaft osteotomy ( n = 5). After fixation axial loading was applied to the constructs first without muscle-equivalent forces and then with the addition of these forces (abductor groups and vastus lateralis). Displacement at the osteotomy site and the stiffness of the whole construct were measured during loading.

Results:

Comparison of the two loading modes revealed no significant differences for displacement or stiffness for the trochanteric fractures. For the femur shaft fractures, a significant difference was found for displacement ( p = 0.023) and stiffness ( p = 0.003) with or without muscle-equivalent forces.

Conclusion:

The loading protocol for implant testing on femur shaft fractures should include muscle-equivalent forces. For trochanteric fractures, consideration of muscle forces is not entirely necessary since they will have little effect on the results, for example, when comparing implants.

PROSPECTIVE ASSESSMENT OF THE CLINICAL, RADIOGRAPHIC AND FUNCTIONAL EVOLUTION OF TREATMENT FOR UNSTABLE TROCHANTERIC FRACTURES OF THE FEMUR USING A CEPHALOMEDULLARY NAIL

To assess the clinical, radiological and functional evolution of osteosynthesis using a cephalomedullary nail, in unstable trochanteric fractures of the femur, over a one-year postoperative follow-up. Methods: Fourteen men and 23 women of mean age 77.7 years were evaluated. Twenty-seven of them had fractures classified as AO/ASIF 31A2 and ten as 31A3. The patients were evaluated clinically, radiologically and functionally one week, two weeks, one month, two months, six months and one year after the operation. Results: The clinical complications comprised five cases of death, one case of calcaneal ulcer, one case of acute arterial obstruction and two cases of deep vein thrombosis. The radiographic evaluation showed that the mean cervicodiaphyseal angle in the immediate postoperative period was 132.5°. The mean tip-apex index was 22.8 mm. After one year, the mean cervicodiaphyseal angle was 131.7°. Fracture consolidation was seen in all the patients six months after the operation, except in one case that presented cut-out. There were no cases of fracture below the implant. The functional evaluation using the Harris score after one year showed a mean of 69.3 points. The evaluation of walking progress showed that after one year, 40.6% of the patients had the same ability to walk that they had before the fracture. The visual analogue pain scale showed that a significant decrease in pain complaints occurred, going from 5.19 in the first week to 2.25 after 1 year. Conclusion: Osteosynthesis using a cephalomedullary nail resulted in low rates of clinical and mechanical complications and adequate functional outcomes.

Periprosthetic fracture fixation of the femur following total hip arthroplasty: a review of biomechanical testing

BACKGROUND

periprosthetic femoral fracture can occur following total hip arthroplasty. Fixation of these fractures are challenging due to the combination of fractured bone with an existing prosthesis. There are several clinical studies reporting the failure of fixation methods used for these fractures, highlighting the importance of further biomechanical studies in this area.

METHODS

the current literature on biomechanical models of periprosthetic femoral fracture fixation is reviewed. The methodologies involved in the experimental and computational studies of this fixation are described and compared.

FINDINGS

areas which require further investigation are highlighted and the potential use of finite element analysis as a computational tool to test the current fixation methods is addressed.

INTERPRETATION

biomechanical models have huge potential to assess the effectiveness of different fixation methods. Experimental in vitro models have been used to mimic periprosthetic femoral fracture fixation however, the numbers of measurements that are possible in these studies are relatively limited due to the cost and data acquisition constraints. Computer modelling and in particular finite element analysis is a complimentary method that could be used to examine existing protocols for the treatment of periprosthetic femoral fracture and, potentially, find optimum fixation methods for specific fracture types.

A Biomechanical Evaluation of Orthopaedic Implants for Hip Fractures by Finite Element Analysis and In-Vitro Tests

The aim of this study was to test the hypothesis that a reinforced gamma nail for the fixation of subtrochanteric fractures would experience less stress during loading compared with a common gamma nail. The issue of whether the use of the stronger implant would result in more stress shielding in the surrounding bone was also addressed.

A finite element analysis (FEA) of a synthetic bone was employed to calculate the stress distribution in implant and bone for two fracture types (AO 31-A3.1 and AO 31-A3.3). The FEA was validated by mechanical tests on six synthetic femurs. To test the hypothesis in vitro, mechanical tests on six pairs of fresh-frozen human femurs were conducted. The femurs were supplied with a common or a reinforced gamma nail in a cross-over study design. Strains were measured on the nail shaft to quantify the loading of the nail.

The FEA resulted in 3—51 per cent lower stresses for the reinforced gamma nail. No increase in stress shielding could be observed. In the in-vitro tests, the reinforced gamma nail experienced less strain during loading ( p < 0.016). The study demonstrated the benefit of a reinforced gamma nail in subtrochanteric fractures. It experienced less stress but did not result in more stress shielding.