Cyclic Loading of Periprosthetic Fracture Fixation Constructs

The influence of composite bone plates in Vancouver femur B1 fracture fixation after post-operative, and healed bone stages: A finite element study

Conventional stainless steel or titanium plates are used for bone fracture fixation to provide support at fracture location. Plates with high elastic modulus reduce the transfer of compressive load at the fracture location (due to stress shielding), causing failure. The objective of the study is to find for composite bone plates with different types of fibers and varied fiber orientations for post-operative (PO) and healed bone (HB) conditions which can reduce the stress shielding. Femur fracture fixation was constructed with 12 holes narrow type with metal and composite bone plates. The fracture gap was constructed with soft bone region for post-operative (PO) condition and harder bone for healed bone (HB). Composite bone plates with different configurations (fiber directions) and types (thickness and width) were analyzed to study the stress distribution and movement in the fracture location. The models were analyzed and the stresses in plate and callus, movement and strain in axial and shear direction in both metal and composite bone plates were studied. The metal and composite plates (carbon fiber/epoxy, fiberglass/epoxy, and flax/epoxy) used for most common Vancouver type B1 fracture to observe the biomechanical behavior of different models in PO and HB condition. The FE simulation on different configurations and types of composite plates provide in-depth idea about choosing the suitable composite bone plate. There are variations in behavior for varying types and configurations, but the performance of most of the plates are either better or similar to that of metal plate, except the plates with higher width.

Surgical and Pharmacological Management of Periprosthetic Atypical Femoral Fractures: A Narrative Literature Review

Introduction

An increasing number of patients is annually undergoing total hip arthroplasty (THA), and a significant proportion of these patients are elderly and consequently at a higher risk of complications because of age, osteoporosis, and medical comorbidities. Periprosthetic femoral fractures (PFFs) are one of the worst complications of THA associated with high rates of unfavorable prognosis. Besides, in the last decade, a new independent disease entity called “atypical femoral fracture” (AFF) has been identified and defined by the American Society for Bone and Mineral Research (ASBMR) task force. Some PFFs present clinical history and radiographic aspect consistent with an AFF, meeting the ASBMR criteria for the diagnosis of AFF except that PFFs by themselves are an exclusion criterion for AFF. However, there is an increasing number of published studies suggesting that periprosthetic atypical femoral fractures (PAFFs) exist and should not be excluded by definition.

Significance

Nowadays, although there is an increasing interest in PAFFs, there are still very few studies published on the topic and a lack of consensus regarding their treatment. This narrative literature review aims to introduce this new emerging topic to a wider readership describing the characteristics of PAFFs and the state-of-the-art in their management.

Conclusions

Many authors agree that PAFFs should be considered as a subgroup of PFFs that have atypical characteristics; they also show a significant correlation with prolonged bisphosphonate use. A correct diagnosis is paramount for proper treatment of the disease that requires both surgical and medical actions to be taken.

A biomechanical study on the laminate stacking sequence in composite bone plates for vancouver femur B1 fracture fixation

BACKGROUND AND OBJECTIVES

Composite bone plates are proposed for fracture fixation in periprosthetic femoral fracture. Metallic plates, having high stiffness compared to bone lead to stress shielding, reduce the compression force in the fracture site, affectthe healing process. Reduction of stiffness in the axial direction due to above reason without lowering the stiffness in transverse to avoid much of shear strain and thus avoiding instability at the fracture site leads to selective stress shielding. This can only be achieved through meticulously designed fiber reinforced composite. In the present work varied fiber orientations in the stacked laminates with varied fiber types are employed in a post-operative femur fixation for the in-silico analyses of their effectiveness using finite element analysis.

METHODS

In this study a Total Hip Arthroplasty (THA) model is constructed with composite bone plates. Three-dimensional narrow type metal plate is modeled with 12 holes and length of 194 mm. Three different types of composite bone plates are modeled with 12 holes of different size for the analysis i.e. Type 1 (5.6 mm thickness and 16 mm width), Type 2 (6 mm thickness and 16 mm width) and Type 3(6 mm thickness and 18 mm width). Anatomical 3D FE models of THA with composite bone plates are constructed to find out the interfacial stresses and strains. The finite element software ANSYS is used to perform the analysis.

RESULTS

A three-dimensional FE model of immediately post-operative femur fixation is developed and studied the maximum stress distribution, strain and movement in axial/shear direction in the metal and composite bone plate near to the fracture site. In the present study, the metal and composite plate (carbon/epoxy, glass/epoxy and flax/epoxy) used for most common Vancouver type B1 fracture to observe the biomechanical behavior of different models in IPO condition using FEA.

CONCLUSIONS

Optimizing the fiber orientations of composite bone plates of Total Hip Arthroplasty (THA) model by controlling the biomechanical stresses could be a favorable approach. The finite element analysis approach gives a viable solution to design the composite bone plate and for designing future models that preserves the biomechanical function of THA with composite bone plate.

Comparison of femur strain under different loading scenarios: Experimental testing

Bone fracture, formation and adaptation are related to mechanical strains in bone. Assessing bone stiffness and strain distribution under different loading conditions may help predict diseases and improve surgical results by determining the best conditions for long-term functioning of bone-implant systems. In this study, an experimentally wide range of loading conditions (56) was used to cover the directional range spanned by the hip joint force. Loads for different stance configurations were applied to composite femurs and assessed in a material testing machine. The experimental analysis provides a better understanding of the influence of the bone inclination angle in the frontal and sagittal planes on strain distribution and stiffness. The results show that the surface strain magnitude and stiffness vary significantly under different loading conditions. For the axial compression, maximal bending is observed at the mid-shaft, and bone stiffness is also maximal. The increased inclination leads to decreased stiffness and increased magnitude of maximum strain at the distal end of the femur. For comparative analysis of results, a three-dimensional, finite element model of the femur was used. To validate the finite element model, strain gauges and digital image correlation system were employed. During validation of the model, regression analysis indicated robust agreement between the measured and predicted strains, with high correlation coefficient and low root-mean-square error of the estimate. The results of stiffnesses obtained from multi-loading conditions experiments were qualitatively compared with results obtained from a finite element analysis of the validated model of femur with the same multi-loading conditions. When the obtained numerical results are qualitatively compared with experimental ones, similarities can be noted. The developed finite element model of femur may be used as a promising tool to estimate proximal femur strength and identify the best conditions for long-term functioning of the bone-implant system in future study.

Dual Motor Drill Continuously Measures Drilling Energy to Calculate Bone Density and Screw Pull-out Force in Real Time

INTRODUCTION

Low bone density complicates the surgical management of fractures. Screw stripping in osteoporotic bone leads to decreased fixation strength and weakening of the fixation construct. If low density could be detected during drilling, augmentation may be performed to prevent screw stripping. Furthermore, continuous monitoring of the drill bit depth and bone density can allow detection of the far cortex where density suddenly increases, providing immediate and accurate screw length measurement and reducing the risk of overpenetration or plunge in osteoporotic bone. Therefore, a dual motor drill was created to calculate bone density and pull-out force in real time. The purpose of this study was to determine whether real-time monitoring of drill bit torque and depth could be used to estimate bone density and pull-out force. We hypothesized that the calculated drilling energy could be used to determine density and would correlate with pull-out force.

METHODS

Drilling and screw insertion were performed using a validated composite unicortical bone model. Screws of 5-, 10-, and 20-mm length were placed into blocks of known densities (10, 20, 30, and 40 pounds per cubic foot). During creation of holes by the dual motor drill, drilling energy was recorded and used to calculate density. Calculated bone density was then compared with the known density of the block. The drill bit was exchanged for a screwdriver, and screw insertion energy was recorded in a similar fashion during screw placement. Screws were then subjected to maximal axial pull-out force testing with a material testing device. Recorded drilling energy and screw insertion energy were then correlated with the measured pull-out force.

RESULTS

Calculated bone density correlated very strongly with the known control density, confirming the accuracy of density calculations in real time. Drilling energy and screw insertion energy correlated very strongly with the measured pull-out force by destructive testing confirming ultimate pull-out force could be quantified during drilling or placement of a screw.

DISCUSSION

Our results confirmed that a dual motor drill can accurately and immediately allow determination of bone density and screw pull-out force before placing a screw. This knowledge could allow a surgeon to perform augmentation or alter surgical technique to prevent screw stripping and loss of fixation as well as detect the far cortex and prevent overpenetration in osteoporotic bone.

The Proximal and Distal Femoral Canal Geometry Influences Cementless Stem Anchorage and Revision Hip and Knee Implant Stability

Although cementless revision arthroplasty of the hip has become the gold standard, revision arthroplasty of the distal femur is controversial. This study evaluated the anchoring principles of different femoral revision stem designs in extended bone defect situations, taking into account the anatomical conditions of the proximal and distal femur, and the resulting primary stability. Cementless press-fit stems of 4 different designs were implanted in synthetic femurs. The specimens were analyzed by computed tomography and were tested considering axial/torsional stiffness and migration resistance. Different stem designs anchored in different femoral canal geometries achieved comparable primary stability. Despite considerably different anchorage lengths, no difference in migration behavior or stiffness was found. Both in the distal femur and in the proximal femur, the conical stems showed a combination of conical and 3-point anchorage. Regarding the cylindrical stem tested, a much shorter anchorage length was sufficient in the distal femur to achieve comparable primary stability. In the investigated osseous defect model, the stem design (conical vs cylindrical), not the geometry of the femoral canal (proximal vs distal), was decisive regarding the circumferential anchorage length. For the conical stems, it can be postulated that there are reserves available for achieving a conical-circular fixation as a result of the large contact length. For the cylindrical stems, only a small reserve for a stable anchorage can be assumed. [Orthopedics. 2018; 41(3):e369-e375.].

An international, cross-sectional survey of the management of Vancouver type B1 periprosthetic femoral fractures around total hip arthroplasties

INTRODUCTION

The incidence of periprosthetic femoral fractures around total hip arthroplasties is increasing. Fractures around a stable implant stem (Vancouver type B1) are among the most common of these fractures. Various fixation strategies for Vancouver type B1 periprosthetic fractures have been reported in the literature; however, little high-level evidence exists. This study was designed to determine the current management strategies and opinions among orthopaedic surgeons treating Vancouver type B1 periprosthetic femoral fractures, and to evaluate the need for a large prospective randomized controlled trial for the management of these injuries.

METHODS

Orthopaedic surgeon members of the Orthopaedic Trauma Association (OTA), the Canadian Orthopaedic Association (COA), and the Hip Society were invited to participate in a 51-item web-based survey surrounding the management of periprosthetic femoral fractures around total hip replacements, as well as the perceived need for future research in this area. Responses were summarized using proportions, and further stratified by practice type, case volume, surgeon age, and fellowship training.

RESULTS

For Vancouver type B1 fractures, open reduction and internal fixation (ORIF) with locked plating was favoured slightly over ORIF with cable plating ± cortical strut allograft (51.1% versus 45.5%). When compared to cable plating with cortical strut allograft, respondents believed that isolated locked plating resulted in lower nonunion and reoperation rates, but similar infection and malunion rates. Subgroup analyses revealed that practice type, surgeon age, case volume, and fellowship training influenced surgeons' management of periprosthetic femoral fractures and beliefs regarding complications. There is high demand for a large prospective randomized controlled trial for Vancouver type B1 fracture fixation.

CONCLUSIONS

Consensus surrounding the management of Vancouver type B1 periprosthetic femoral fractures is lacking, and there is a perceived need among orthopaedic surgeons for a large prospective randomized controlled trial in order to define the optimal management of these injuries.

Is immediate weight bearing safe for periprosthetic distal femur fractures treated by locked plating? A feasibility study in 52 consecutive patients

Background

Periprosthetic distal femur fractures associated with total knee replacement are increasing in incidence. We hypothesized that a standardized management protocol would result in few implant failures and a low rate of postoperative complications.

Methods

Retrospective observational cohort study at an urban level 1 trauma center and academic level 2 trauma center. Consecutive patients with periprosthetic distal femur fractures and stable total knee arthroplasty were included between January 1, 2011 and December 31, 2014. Patients were managed by a standardized protocol of co-management by a hospitalist service, fracture fixation within 24 h of admission by less-invasive locked bridge plating, and immediate unrestricted postoperative weight bearing. The primary outcome measure was the rate of postoperative complications. Secondary outcome measures included time to surgery, intraoperative blood loss, duration of surgery, length of hospital stay, time to full weight bearing, and time to radiographic fracture healing.

Results

Fifty four fractures were treated in 52 patients. There were three implant failures, one deep infection, one nonunion and two patients with symptomatic malunion. One patient had knee pain due to patellar component instability associated with valgus alignment. There were ten thromboembolic complications despite consistent anticoagulation. Two patients died within 12 months of injury. Thirty-eight patients had returned to their pre-injury ambulation status at 1 year follow-up.

Conclusion

A standardized approach of less-invasive locked plating fixation and immediate unrestricted weight bearing appears safe and feasible in the management of this vulnerable patient cohort.

Trial registration number

This is a retrospective observational study without a Trial registration number.

Enhancing fixation strength in periprosthetic femur fractures by orthogonal plating-A biomechanical study

Orthogonal plate osteosynthesis enhances fixation stability in periprosthetic femur fractures. Another option are locking attachment plates (LAP) allowing bicortical locking screw placement lateral to the prosthesis stem. Stability of lateral plate osteosynthesis with two LAP (2LAP) was compared to anterolateral orthogonal plate osteosynthesis (OP) with one LAP in a periprosthetic femur fracture model. In six pairs of fresh frozen human femora with cemented Charnley hip prosthesis, a transverse osteotomy was set distal to the tip of the prosthesis simulating a Vancouver type B1 fracture. Each pair was instrumented using a plate tensioner with either one lateral plate and two LAP, or two orthogonal anterolateral plates and one LAP. Stiffness was determined in a four-point-bending test prior to cyclic testing (2Hz) with physiologic profile and progressively increasing load up to catastrophic construct failure. Paired t-test and Wilcoxon-signed-rank test were used for statistical evaluation at a level of significance p = 0.05. The OP construct exhibited a significantly higher number of cycles and load to failure (39,627 cycles ± 4,056; 4,463 N ± 906) compared to the 2LAP construct (32,927 cycles ± 3,487; 3,793 N ± 849), p < 0.01. Mediolateral bending and torsional stiffness of the OP (1610 N/mm ± 249; 16.9 Nm/mm ± 6.3) were significantly higher compared to 2 LAP (1077 N/mm ± 189; 12.1 Nm/mm ± 3.9), p = 0.03 for both comparisons. Orthogonal plate osteosynthesis is a valuable option in periprosthetic fracture surgery, offering increased stability compared to a single lateral plate fixed with two LAP.

Biomechanical Concepts for Fracture Fixation

Application of the correct fixation construct is critical for fracture healing and long-term stability; however, it is a complex issue with numerous significant factors. This review describes a number of common fracture types and evaluates their currently available fracture fixation constructs. In the setting of complex elbow instability, stable fixation or radial head replacement with an appropriately sized implant in conjunction with ligamentous repair is required to restore stability. For unstable sacral fractures with vertical or multiplanar instabilities, "standard" iliosacral screw fixation is not sufficient. Periprosthetic femur fractures, in particular Vancouver B1 fractures, have increased stability when using 90/90 fixation versus a single locking plate. Far cortical locking combines the concept of dynamization with locked plating to achieve superior healing of a distal femur fracture. Finally, there is no ideal construct for syndesmotic fracture stabilization; however, these fractures should be fixed using a device that allows for sufficient motion in the syndesmosis. In general, orthopaedic surgeons should select a fracture fixation construct that restores stability and promotes healing at the fracture site, while reducing the potential for fixation failure.

Tangential Bicortical Locked Fixation Improves Stability in Vancouver B1 Periprosthetic Femur Fractures: A Biomechanical Study

OBJECTIVES

The biomechanical difficulty in fixation of a Vancouver B1 periprosthetic fracture is purchase of the proximal femoral segment in the presence of the hip stem. Several newer technologies provide the ability to place bicortical locking screws tangential to the hip stem with much longer lengths of screw purchase compared with unicortical screws. This biomechanical study compares the stability of 2 of these newer constructs to previous methods.

METHODS

Thirty composite synthetic femurs were prepared with cemented hip stems. The distal femur segment was osteotomized, and plates were fixed proximally with either (1) cerclage cables, (2) locked unicortical screws, (3) a composite of locked screws and cables, or tangentially directed bicortical locking screws using either (4) a stainless steel locking compression plate system with a Locking Attachment Plate (Synthes) or (5) a titanium alloy Non-Contact Bridging system (Zimmer). Specimens were tested to failure in either axial or torsional quasistatic loading modes (n = 3) after 20 moderate load preconditioning cycles. Stiffness, maximum force, and failure mechanism were determined.

RESULTS

Bicortical constructs resisted higher (by an average of at least 27%) maximum forces than the other 3 constructs in torsional loading (P < 0.05). Cables constructs exhibited lower maximum force than all other constructs, in both axial and torsional loading. The bicortical titanium construct was stiffer than the bicortical stainless steel construct in axial loading.

CONCLUSIONS

Proximal fixation stability is likely improved with the use of bicortical locking screws as compared with traditional unicortical screws and cable techniques. In this study with a limited sample size, we found the addition of cerclage cables to unicortical screws may not offer much improvement in biomechanical stability of unstable B1 fractures.

Intraoperative Periprosthetic Femur Fracture: A Biomechanical Analysis of Cerclage Fixation

Intraoperative periprosthetic femur fracture is a known complication of total hip arthroplasty (THA) and a variety of cerclage systems are available to manage these fractures. The purpose of this study was to examine the in situ biomechanical response of cerclage systems for fixation of periprosthetic femur fractures that occur during cementless THA. We compared cobalt chrome (CoCr) cables, synthetic cables, monofilament wires and hose clamps under axial compressive and torsional loading. Metallic constructs with a positive locking system performed the best, supporting the highest loads with minimal implant subsidence (both axial and angular) after loading. Overall, the CoCr cable and hose clamp had the highest construct stiffness and least reduction in stiffness with increased loading. They were not demonstrably different from each other.

Plate failure following plate osteosynthesis in periprosthetic femoral fractures

BACKGROUND

Increasing numbers of total knee and hip arthroplasties result in a growing number of periprosthetic femoral fractures (PPFF). PPFF with a stable stem component are treated commonly with plate osteosynthesis. Therefore plate failure is seen as a major complication. The aim of this retrospective study was to investigate the patients' outcome after plate failure.

METHODS

The database of a Level 1 trauma center was searched for all patients treated for a PPFF with plate osteosynthesis. Patients with plate failure were investigated specifically. Standard demographic data, details on initial arthroplasty, trauma, and treatment were recorded for all patients. All fractures were classified and their outcome reviewed.

RESULTS

Seven (8.8%) out of 80 patients treated with plate osteosynthesis following PPFF met our inclusion criterion being plate failure. All these patients were female, with an average age at primary surgery of 74 ± 13 years and a mean follow-up of 885 days (range, 264-2549). Four patients suffered a PPFF after total hip arthroplasty (THA) (2 Vancouver Type B1 and 2 Type C) and three after total knee arthroplasty (TKA) (Lewis-Rorabeck Type II). Following plate failure, four patients healed uneventfully and three patients experienced complications such as pseudarthrosis, screw loosening, and further plate failure.

CONCLUSION

In patients with poor bone quality, bone graft, bone cement, and bone biologics have to be considered in revision surgery. Furthermore, long-stem revision and tumor prosthesis are an additional solution.

Biomechanical evaluation of fracture fixation constructs using a variable-angle locked periprosthetic femur plate system

BACKGROUND

In the United States there are more than 230,000 total hip replacements annually, and periprosthetic femoral fractures occur in 0.1-4.5% of those patients. The majority of these fractures occur at the tip of the stem (Vancouver type B1). The purpose of this study was to compare the biomechanically stability and strength of three fixation constructs and identify the most desirable construct.

METHODS

Fifteen medium adult synthetic femurs were implanted with a hip prosthesis and were osteotomized in an oblique plane at the level of the implant tip to simulate a Vancouver type B1 periprosthetic fracture. Fractures were fixed with a non-contact bridging periprosthetic proximal femur plate (Zimmer Inc., Warsaw, IN). Three proximal fixation methods were used: Group 1, bicortical screws; Group 2, unicortical screws and one cerclage cable; and Group 3, three cerclage cables. Distally, all groups had bicortical screws. Biomechanical testing was performed using an axial-torsional testing machine in three different loading modalities (axial compression, lateral bending, and torsional/sagittal bending), next in axial cyclic loading to 10,000 cycles, again in the three loading modalities, and finally to failure in torsional/sagittal bending.

RESULTS

Group 1 had significantly greater load to failure and was significantly stiffer in torsional/sagittal bending than Groups 2 and 3. After cyclic loading, Group 2 had significantly greater axial stiffness than Groups 1 and 3. There was no difference between the three groups in lateral bending stiffness. The average energy absorbed during cyclic loading was significantly lower in Group 2 than in Groups 1 and 3.

CONCLUSIONS

Bicortical screw placement achieved the highest load to failure and the highest torsional/sagittal bending stiffness. Additional unicortical screws improved axial stiffness when using cable fixation. Lateral bending was not influenced by differences in proximal fixation.

CLINICAL RELEVANCE

To treat periprosthetic fractures, bicortical screw placement should be attempted to maximize load to failure and torsional/sagittal bending stiffness.

Superior outcome of strut allograft-augmented plate fixation for the treatment of periprosthetic fractures around a stable femoral stem

PURPOSE

This study was designed to compare the outcome of two surgical approaches for treating femoral periprosthetic fractures around a stable femoral stem. The hypothesis was that plate fixation alone might be associated with a higher complication rate due to insufficient mechanical stability. We also considered that the addition of a strut allograft would contribute to fracture healing by means of osteoconduction.

METHODS

We retrospectively assessed the outcome of 21 patients who sustained periprosthetic fractures around a total hip replacement system (Vancouver type B1 and type C fractures) and who were treated in our department (January 2006 and August 2011) either by plate fixation alone or by plate fixation and a strut allograft. The mean postoperative follow-up was 23 months (range 9-69 months). Eleven patients were treated by plate fixation alone (Plate Group), and 10 patients were treated by plate fixation and a deep frozen cortical strut allograft (AG Group). Functional outcome was rated by the Harris Hip scoring system. Postoperative radiographs were assessed for evidence of fracture union. Surgical failure was defined as any complication requiring surgical revision.

RESULTS

The 21 patients included 17 females and 4 males. The average age was 79 years (range, 73-88) for the Plate Group and 82 years (range, 53-94) for the AG Group, and the average time to fracture union was 12 weeks (range, 2.5-6 months) and 12.95 weeks (range, 1.5-3) respectively. The overall failure rate was significantly higher in the Plate Group: 5 of them required revision surgery compared to none in the AG Group (p=0.014).

CONCLUSION

The results of this analysis indicate that a strut allograft augmentation approach to Vancouver type B1 and type C periprosthetic fractures results in a better outcome than plate fixation alone by apparently adding mechanical stability and enhancing the biological healing process.

The effect of fracture stability on the performance of locking plate fixation in periprosthetic femoral fractures

Periprosthetic femoral fracture (PFF) fixation failures are still occurring. The effect of fracture stability and loading on PFF fixation has not been investigated and this is crucial for optimum management of PFF. Models of stable and unstable PPFs were developed and used to quantify the effect of fracture stability and loading in a single locking plate fixation. Stress on the plate was higher in the unstable compared to the stable fixation. In the case of unstable fractures, it is possible for a single locking plate fixation to provide the required mechanical environment for callus formation without significant risk of plate fracture, provided partial weight bearing is followed. In cases where partial weight bearing is unlikely, additional biological fixation could be considered.

Biomechanical analysis using infrared thermography of a traditional metal plate versus a carbon fibre/epoxy plate for Vancouver B1 femur fractures

Traditional high-stiffness metal plates for Vancouver B1 femur shaft fractures below the tip of a hip implant can cause stress shielding, bone resorption, and implant loosening. This is the first study to compare the biomechanics of a traditional metal plate versus a low-stiffness carbon fibre/epoxy composite plate for this injury. A total hip replacement was implanted in two previously validated intact artificial femurs. Femurs were fitted with either a metal or composite plate and had a 5 mm fracture gap created to simulate a Vancouver B1 shaft fracture. Femurs were cyclically loaded using 5 Hz at 7° of adduction with an average axial load of 800 N (range = 400–1200 N). Overall mechanical stiffnesses and femur and plate thermographic stresses were obtained. Femur/metal plate stiffness (698 N/mm) was only 12% higher than femur/composite plate stiffness (625 N/mm). The femur with the composite plate had 22.7% higher combined average stress compared to the femur with the metal plate, having specific differences of 29.5% (anterior view), 33.9% (posterior view), 1.0% (medial view), and 26.4% (lateral view). The composite plate itself had an average 21.1% reduction in stress compared to the metal plate. The composite plate reduced stress shielding, yet provided adequate stiffness.

Biomechanical comparison of 2 different locking plate fixation methods in vancouver b1 periprosthetic femur fractures

Locking plates are commonly used to treat fractures around a well-fixed femoral component. The optimal construct should provide sufficient fixation while minimizing soft-tissue dissection. The purpose of the current study was to determine whether plate length, working length, or bone mineral density affects survival of locking plate fixation for Vancouver type B1 periprosthetic hip fractures. A transverse osteotomy was created just distal to cemented femoral prostheses in 9 pairs of cadaveric femurs. Fractures were stabilized with long (20-hole) or short (12-hole) locking plates that were secured proximally with cables and screws and distally with screws only. Specimens were then cycled 10 000 times at 2500 N of axial force and 15 Nm of torque to simulate full weightbearing. A motion capture system was used to record fracture displacement during cycling. Failure occurred in 5 long and 3 short plates, with no significant differences found in the number of cycles to failure. For the specimens that survived, there were no significant differences found between long and short plates for displacement or rotation observed at the fracture site. A shorter working length was not associated with increased failure rate. Lower bone mineral density was significantly associated with failure (P = .02). We concluded that long locked plates do not appear to offer a biomechanical advantage over short locking plates in terms of fixation survival, and that bone mineral density was a better predictor of failure than was the fixation construct type.

The biomechanical effect of notch size, notch location, and femur orientation on hip resurfacing failure

For hip resurfacing, this is the first biomechanical study to assess anterior and posterior femoral neck notching and femur flexion and extension. Forty-seven artificial femurs were implanted with the Birmingham hip resurfacing (BHR) using a range of notch sizes (0, 2, and 5 mm), notch locations (superior, anterior, and posterior), and femur orientations (neutral stance, flexion, and extension). Implant preparation was done using imageless computer navigation, and mechanical tests measured stiffness and strength. For notch size and location, in neutral stance the unnotched group had 1.9 times greater strength than the 5-mm superior notch group (4539 N versus 2423 N, p=0.047), and the 5-mm anterior notch group had 1.6 times greater strength than the 5-mm superior notch group, yielding a borderline statistical difference (3988 N versus 2423 N, p = 0.056). For femur orientation, in the presence of a 5-mm anterior notch, femurs in neutral stance had 2.2 times greater stiffness than femurs in 25° flexion (1542 N/mm versus 696 N/mm, p = 0.000). Similarly, in the presence of a 5-mm posterior notch, femurs in neutral stance had 2.8 times greater stiffness than femurs in 25° extension (1637 N/mm versus 575 N/mm, p = 0.000). No other statistical differences were noted. All femurs failed through the neck. The results have implications for BHR surgical techniques and recommended patient activities.

Biomechanical properties of an advanced new carbon/flax/epoxy composite material for bone plate applications

This work is part of an ongoing program to develop a new carbon fiber/flax/epoxy (CF/flax/epoxy) hybrid composite material for use as an orthopaedic long bone fracture plate, instead of a metal plate. The purpose of this study was to evaluate the mechanical properties of this new novel composite material. The composite material had a "sandwich structure", in which two thin sheets of CF/epoxy were attached to each outer surface of the flax/epoxy core, which resulted in a unique structure compared to other composite plates for bone plate applications. Mechanical properties were determined using tension, three-point bending, and Rockwell hardness tests. Also, scanning electron microscopy (SEM) was used to characterize the failure mechanism of specimens in tension and three-point bending tests. The results of mechanical tests revealed a considerably high ultimate strength in both tension (399.8MPa) and flexural loading (510.6MPa), with a higher elastic modulus in bending tests (57.4GPa) compared to tension tests (41.7GPa). The composite material experienced brittle catastrophic failure in both tension and bending tests. The SEM images, consistent with brittle failure, showed mostly fiber breakage and fiber pull-out at the fractured surfaces with perfect bonding at carbon fibers and flax plies. Compared to clinically-used orthopaedic metal plates, current CF/flax/epoxy results were closer to human cortical bone, making the material a potential candidate for use in long bone fracture fixation.

A biomechanical comparison of four different cementless press-fit stems used in revision surgery for total knee replacements

Few biomechanical studies exist on femoral cementless press-fit stems for revision total knee replacement (TKR) surgeries. The aim of this study was to compare the mechanical quality of the femur-stem interface for a series of commercially available press-fit stems, because this interface may be a 'weak link' which could fail earlier than the femur-TKR bond itself. Also, the femur-stem interface may become particularly critical if distal femur bone degeneration, which may necessitate or follow revision TKR, ever weakens the femur-TKR bond itself. The authors implanted five synthetic femurs each with a Sigma Short Stem (SSS), Sigma Long Stem (SLS), Genesis II Short Stem (GSS), or Genesis II Long Stem (GLS). Axial stiffness, lateral stiffness, 'offset load' torsional stiffness, and 'offset load' torsional strength were measured with a mechanical testing system using displacement control. Axial (range = 1047-1461 N/mm, p = 0.106), lateral (range = 415-462 N/mm, p = 0.297), and torsional (range = 115-139 N/mm, p > 0.055) stiffnesses were not different between groups. The SSS had higher torsional strength (863 N) than the other stems (range = 167-197 N, p < 0.001). Torsional failure occurred by femoral 'spin' around the stem's long axis. There was poor linear correlation between the femur-stem interface area versus axial stiffness (R = 0.38) and torsional stiffness (R = 0.38), and there was a moderate linear correlation versus torsional strength (R = 0.55). Yet, there was a high inverse linear correlation between interfacial surface area versus lateral stiffness (R = 0.79), although this did not result in a statistical difference between stem groups (p = 0.297). These press-fit stems provide equivalent stability, except that the SSS has greater torsional strength.

Angulated locking plate in periprosthetic proximal femur fractures: biomechanical testing of a new prototype plate

INTRODUCTION

To improve proximal plate fixation of periprosthetic femur fractures, a prototype locking plate with proximal posterior angulated screw positioning was developed and biomechanically tested.

METHODS

Twelve fresh frozen, bone mineral density matched human femora, instrumented with cemented hip endoprosthesis were osteotomized simulating a Vancouver B1 fracture. Specimens were fixed proximally with monocortical (LCP) or angulated bicortical (A-LCP) head-locking screws. Biomechanical testing comprised quasi-static axial bending and torsion and cyclic axial loading until catastrophic failure with motion tracking.

RESULTS

Axial bending and torsional stiffness of the A-LCP construct were (1,633 N/mm ± 548 standard deviation (SD); 0.75 Nm/deg ± 0.23 SD) at the beginning and (1,368 N/mm ± 650 SD; 0.67 Nm/deg ± 0.25 SD) after 10,000 cycles compared to the LCP construct (1,402 N/mm ± 272 SD; 0.54 Nm/deg ± 0.19 SD) at the beginning and (1,029 N/mm ± 387 SD; 0.45 Nm/deg ± 0.15) after 10,000 cycles. Relative movements for medial bending and axial translation differed significantly between the constructs after 5,000 cycles (A-LCP 2.09° ± 0.57 SD; LCP 5.02° ± 4.04 SD; p = 0.02; A-LCP 1.25 mm ± 0.33 SD; LCP 2.81 mm ± 2.32 SD; p = 0.02) and after 15,000 cycles (A-LCP 2.96° ± 0.70; LCP 6.52° ± 2.31; p = 0.01; A-LCP 1.68 mm ± 0.32; LCP 3.14 mm ± 0.68; p = 0.01). Cycles to failure (criterion 2 mm axial translation) differed significantly between A-LCP (15,500 ± 2,828 SD) and LCP construct (5,417 ± 7,236 SD), p = 0.03.

CONCLUSION

Bicortical angulated screw positioning showed less interfragmentary osteotomy movement and improves osteosynthesis in periprosthetic fractures.

A preliminary biomechanical study of cyclic preconditioning effects on canine cadaveric whole femurs

Biomechanical preconditioning of biological specimens by cyclic loading is routinely done presumably to stabilize properties prior to the main phase of a study. However, no prior studies have actually measured these effects for whole bone of any kind. The aim of this study, therefore, was to quantify these effects for whole bones. Fourteen matched pairs of fresh-frozen intact cadaveric canine femurs were sinusoidally loaded in 4-point bending from 50 N to 300 N at 1 Hz for 25 cycles. All femurs were tested in both anteroposterior (AP) and mediolateral (ML) bending planes. Bending stiffness (i.e., slope of the force-vs-displacement curve) and linearity R(2) (i.e., coefficient of determination) of each loading cycle were measured and compared statistically to determine the effect of limb side, cycle number, and bending plane. Stiffnesses rose from 809.7 to 867.7 N/mm (AP, left), 847.3 to 915.6 N/mm (AP, right), 829.2 to 892.5 N/mm (AP, combined), 538.7 to 580.4 N/mm (ML, left), 568.9 to 613.8 N/mm (ML, right), and 553.8 to 597.1 N/mm (ML, combined). Linearity R(2) rose from 0.96 to 0.99 (AP, left), 0.97 to 0.99 (AP, right), 0.96 to 0.99 (AP, combined), 0.95 to 0.98 (ML, left), 0.94 to 0.98 (ML, right), and 0.95 to 0.98 (ML, combined). Stiffness and linearity R(2) versus cycle number were well-described by exponential curves whose values leveled off, respectively, starting at 12 and 5 cycles. For stiffness, there were no statistical differences for left versus right femurs (p = 0.166), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p < 0.0001). Similarly, for linearity, no statistical differences were noted due to limb side (p = 0.533), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p = 0.006). A minimum of 12 preconditioning cycles was needed to fully stabilize both the stiffness and linearity of the canine femurs. This is the first study to measure the effects of mechanical preconditioning on whole bones, having some practical implications on research practices.

The biomechanics of three different fracture fixation implants for distal femur repair in the presence of a tumor-like defect

The femur is the most common long bone involved in metastatic disease. There is consensus about treating diaphyseal and epiphyseal metastatic lesions. However, the choice of device for optimal fixation for distal femur metaphyseal metastatic lesion remains unclear. This study compared the mechanical stiffness and strength of three different fixation methods. In 15 synthetic femurs, a spherical tumor-like defect was created in the lateral metaphyseal region, occupying 50% of the circumference of the bone. The defect was filled with bone cement and fixed with one of three methods: Group 1 (retrograde nail), Group 2 (lateral locking plate), and Group 3 (lateral nonlocking periarticular plate). Constructs were tested for mechanical stiffness and strength. There were no differences between groups for axial stiffness (Group 1, 1280 ± 112 N/mm; Group 2, 1422 ± 117 N/mm; and Group 3, 1403 ± 122 N/mm; p = 0.157) and offset torsional strength (Group 1, 1696 ± 628 N; Group 2, 1771 ± 290 N; and Group 3, 1599 ± 253 N; p = 0.816). In the coronal plane, Group 2 (296 ± 17 N/mm) had a higher stiffness than Group 1 (263 ± 17 N/mm; p = 0.018). In the sagittal plane, Group 1 (315 ± 9 N/mm) had a higher stiffness than Group 3 (285 ± 19 N/mm; p = 0.028). For offset torsional stiffness, Group 1 (256 ± 23 N/mm) had a higher value than Group 3 (218 ± 16 N/mm; p = 0.038). Group 1 had equivalent performance to both plating groups in two test modes, and it was superior to Group 3 in two other test modes. Since a retrograde nail (i.e. Group 1) would require less soft-tissue stripping in a clinical context, it may be the optimal choice for tumor-like defects in the distal femur.

The locking attachment plate for proximal fixation of periprosthetic femur fractures--a biomechanical comparison of two techniques

PURPOSE

Mechanical properties of a locking attachment plate construct (LAP-LCP), allowing bicortical screw placement laterally to the prosthesis stem, are compared to a cerclage-LCP construct.

METHODS

Eight right synthetic femora with implanted uncemented hip endoprosthesis were cut distally and fixed with LCP, monocortical locking screws and either LAP (n = 4) or cerclage (n = 4). Cyclic testing was performed with monotonically increasing sinusoidal load until failure. Relative movements at the plate-femur interface were registered by motion tracking. Statistical differences were detected by unpaired t-test and general linear model repeated measures.

RESULTS

Stiffness of the LAP-LCP was significantly higher at the beginning (875.4 N/mm ± 29.8) and after 5000 cycles (1213.0 N/mm ± 101.1) compared to the cerclage-LCP (644.96 N/mm ± 50.1 and 851.9 N/mm ± 81.9), with p = 0.013. Relative movements for AP-bending (B) and axial translation (T) of the LAP-LCP at the beginning (0.07° ± 0.02, 0.20 mm ± 0.08), after 500 cycles (0.16° ± 0.10, 0.26 mm ± 0.07) and after 5000 cycles (0.26° ± 0.11, 0.31 mm ± 0.07) differed significantly from the cerclage-LCP (beg.: 0.26° ± 0.04, 0.28 mm ± 0.05; 500 cyc: 0.47° ± 0.03, 0.53 mm ± 0.07; 5000 cyc.: 0.63° ± 0.18, 0.79 mm ± 0.13), with B: p = 0.02, T: p = 0.04. Relative movements for medial bending were not significantly different between the two constructs. Cycles to failure (criterion 1 mm axial translation) differed significantly between LAP-LCP (19,519 ± 1,758) and cerclage-LCP (11,265 ± 2,472), with p = 0.035.

CONCLUSIONS

Biomechanically, the LAP-LCP construct improves proximal fixation of periprosthetic fractures compared to the cerclage-LCP construct.

Influence of fragment volume on stability of 3-part intertrochanteric fracture of the femur: a biomechanical study

Complex unstable fracture can complicate the treatment outcome of intertrochanteric fracture of the femur, and fixation failure after surgery is a significant problem in elderly patients. This study aimed to evaluate the effect of fracture geometry on the stability of 3-part intertrochanteric fracture by assessing the fragment size. Four categories (group I: large greater trochanter, small lesser trochanter; group II: large greater trochanter, large lesser trochanter; group III: small greater trochanter, small lesser trochanter; and group IV: small greater trochanter, large lesser trochanter) of a 3-part intertrochanteric fracture model were designed. Three-dimensional computer tomography scanning was performed to measure the volume of each fragment. After fixation with a dynamic hip screw, a cyclic loading study was conducted using a servohydraulic machine. There was a significant difference in fatigue failure between each group. After all specimens had endured 10,000 cycles with a range of loads (100-1,000 N), the mean number of cycles until fixation failure with a load range of 200-2,000 N was 1,467.67 ± 199.92 in group I; 579 ± 93.48, group II; 398.17 ± 37.92, group III; and 268.67 ± 19.92, group IV. Fixation strength was approximately 5 times greater in group I than in group IV. In 3-part intertrochanteric fracture, the sizes of the greater and lesser trochanteric fragments are important factors for determining stability after dynamic compression screw fixation. This study supports our hypothesis that the volumetric ratio of ∆lesser trochanter/∆greater trochanter can be used to predict stability of intertrochanteric femoral fracture.

Biomechanical stress maps of an artificial femur obtained using a new infrared thermography technique validated by strain gages

Femurs are the heaviest, longest, and strongest long bones in the human body and are routinely subjected to cyclic forces. Strain gages are commonly employed to experimentally validate finite element models of the femur in order to generate 3D stresses, yet there is little information on a relatively new infrared (IR) thermography technique now available for biomechanics applications. In this study, IR thermography validated with strain gages was used to measure the principal stresses in the artificial femur model from Sawbones (Vashon, WA, USA) increasingly being used for biomechanical research. The femur was instrumented with rosette strain gages and mechanically tested using average axial cyclic forces of 1500 N, 1800 N, and 2100 N, representing 3 times body weight for a 50 kg, 60 kg, and 70 kg person. The femur was oriented at 7° of adduction to simulate the single-legged stance phase of walking. Stress maps were also obtained using an IR thermography camera. Results showed good agreement of IR thermography vs. strain gage data with a correlation of R(2)=0.99 and a slope=1.08 for the straight line of best fit. IR thermography detected the highest principal stresses on the superior-posterior side of the neck, which yielded compressive values of -91.2 MPa (at 1500 N), -96.0 MPa (at 1800 N), and -103.5 MPa (at 2100 N). There was excellent correlation between IR thermography principal stress vs. axial cyclic force at 6 locations on the femur on the lateral (R(2)=0.89-0.99), anterior (R(2)=0.87-0.99), and posterior (R(2)=0.81-0.99) sides. This study shows IR thermography's potential for future biomechanical applications.

A preliminary biomechanical assessment of a polymer composite hip implant using an infrared thermography technique validated by strain gage measurements

With the resurgence of composite materials in orthopaedic applications, a rigorous assessment of stress is needed to predict any failure of bone-implant systems. For current biomechanics research, strain gage measurements are employed to experimentally validate finite element models, which then characterize stress in the bone and implant. Our preliminary study experimentally validates a relatively new nondestructive testing technique for orthopaedic implants. Lock-in infrared (IR) thermography validated with strain gage measurements was used to investigate the stress and strain patterns in a novel composite hip implant made of carbon fiber reinforced polyamide 12 (CF/PA12). The hip implant was instrumented with strain gages and mechanically tested using average axial cyclic forces of 840 N, 1500 N, and 2100 N with the implant at an adduction angle of 15 deg to simulate the single-legged stance phase of walking gait. Three-dimensional surface stress maps were also obtained using an IR thermography camera. Results showed almost perfect agreement of IR thermography versus strain gage data with a Pearson correlation of R(2) = 0.96 and a slope = 1.01 for the line of best fit. IR thermography detected hip implant peak stresses on the inferior-medial side just distal to the neck region of 31.14 MPa (at 840 N), 72.16 MPa (at 1500 N), and 119.86 MPa (at 2100 N). There was strong correlation between IR thermography-measured stresses and force application level at key locations on the implant along the medial (R(2) = 0.99) and lateral (R(2) = 0.83 to 0.99) surface, as well as at the peak stress point (R(2) = 0.81 to 0.97). This is the first study to experimentally validate and demonstrate the use of lock-in IR thermography to obtain three-dimensional stress fields of an orthopaedic device manufactured from a composite material.

The biomechanical effect of changes in cancellous bone density on synthetic femur behaviour

Biomechanical researchers increasingly use commercially available and experimentally validated synthetic femurs to mimic human femurs. However, the choice of cancellous bone density for these artificial femurs appears to be done arbitrarily. The aim of the work reported in this paper was to examine the effect of synthetic cancellous bone density on the mechanical behaviour of synthetic femurs. Thirty left, large, fourth-generation composite femurs were mounted onto an Instron material testing system. The femurs were divided evenly into five groups each containing six femurs, each group representing a different synthetic cancellous bone density: 0.08, 0.16, 0.24, 0.32, and 0.48 g/cm3. Femurs were tested non-destructively to obtain axial, lateral, and torsional stiffness, followed by destructive tests to measure axial failure load, displacement, and energy. Experimental results yielded the following ranges and the coefficient of determination for a linear regression ( R2) with cancellous bone density: axial stiffness (range 2116.5–2530.6 N/mm; R2 = 0.94), lateral stiffness (range 204.3–227.8 N/mm; R2 = 0.08), torsional stiffness (range 259.9–281.5 N/mm; R2 = 0.91), failure load (range 5527.6–11 109.3 N; R2 = 0.92), failure displacement (range 2.97–6.49 mm; R2 = 0.85), and failure energy (range 8.79–42.81 J; R2 = 0.91). These synthetic femurs showed no density effect on lateral stiffness and only a moderate influence on axial and torsional stiffness; however, there was a strong density effect on axial failure load, displacement, and energy. Because these synthetic femurs have previously been experimentally validated against human femurs, these trends may be generalized to the clinical situation. This is the first study in the literature to perform such an assessment.

The biomechanics of plate repair of periprosthetic femur fractures near the tip of a total hip implant: the effect of cable-screw position

Optimal surgical positioning of cable-screw pairs in repairing periprosthetic femur fractures near the tip of a total hip implant still remains unclear. No studies in the literature to date have developed a fully three-dimensional finite element (FE) model that has been validated experimentally to assess these injury patterns. The aim of the present study was to evaluate the biomechanical performance of three different implant–bone constructs for the fixation of periprosthetic femoral shaft fractures following total hip arthroplasty. Experimentally, three bone–plate repair configurations were applied to the periprosthetic synthetic femur fractured with a 5 mm gap near the tip of a total hip implant. Constructs A, B, and C, respectively, had successively larger distances between the most proximal and the most distal cable-screw pairs used to affix the plate. Specimens were oriented in 15° adduction, subjected to 1000 N of axial force to simulate the single-legged stance phase of walking, and instrumented with strain gauges. Computationally, a linearly elastic and isotropic three-dimensional FE model was developed to mimic the experimental setup. Results showed excellent agreement between experimental versus FE analysis strains, yielding a Pearson linearity coefficient, R2, of 0.90 and a slope for the line of best data fit of 0.96. FE axial stiffnesses were 601 N/mm (Construct A), 849 N/mm (Construct B), and 1359 N/mm (Construct C). FE surface stress maps for cortical bone showed maximum von Mises values of 74 MPa (Construct A), 102 MPa (Construct B), and 57 MPa (Construct C). FE stress maps for the metallic components showed minimum von Mises values for Construct C, namely screw (716 MPa), cable (445 MPa), plate (548 MPa), and hip implant (154 MPa). In the case of good bone stock, as modelled by the present synthetic femur model, optimal fixation can be achieved with Construct C.

Salvage procedures for trochanteric femoral fractures after internal fixation failure: Biomechanical comparison of a plate fixator and the dynamic condylar screw

The aim of this study was the biomechanical evaluation of the reversed less invasive stabilization system (LISS) internal fixation as a joint-preserving salvage procedure for trochanteric fractures. Five LISS plates and five dynamic condylar screws (DCS) were tested using synthetic femora (Sawbones®) with an osteotomy model similar to a type-A2.3 pertrochanteric fracture. The constructs were subjected to axial loading up to 1000 N for five cycles. Then, the force was continuously increased until fixation failure. For the evaluation of the biomechanical behaviour, the stiffness levels were recorded and the osteotomy gap displacement was mapped three-dimensionally. The average stiffness for the constructs with LISS plates was 412 N/mm (with a standard deviation (SD) of 103N/mm) and 572 N/mm (SD of 116 N/mm) for the DCS constructs ( p = 0.051). Local displacement at the osteotomy gap did not yield any significant differences. The LISS constructs failed at a mean axial compression of 2103 N (SD of 519 N) and the DCS constructs at a mean of 2572 N (SD of 372 N) ( p = 0.14). It is concluded that the LISS plate offers a reliable fixation alternative for salvage procedures.

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.

Biomechanical Measurements of Torsion-Tension Coupling in Human Cadaveric Femurs

The mechanical behavior of human femurs has been described in the literature with regard to torsion and tension but only as independent measurements. However, in this study, human femurs were subjected to torsion to determine if a simultaneous axial tensile load was generated. Fresh frozen human femurs (n=25) were harvested and stripped of soft tissue. Each femur was mounted rigidly in a specially designed test jig and remained at a fixed axial length during all experiments. Femurs were subjected to external and internal rotation applied at a constant angulation rate of 0.1 deg/s to a maximum torque of 12 N m. Applied torque and generated axial tension were monitored simultaneously. Outcome measurements were extracted from torsion-versus-tension graphs. There was a strong relationship between applied torsion and the resulting tension for external rotation tests (torsion/tension ratio=551.7±283.8 mm, R2=0.83±0.20, n=25), internal rotation tests (torsion/tension ratio=495.3±233.1 mm, R2=0.87±0.17, n=24), left femurs (torsion/tension ratio=542.2±262.4 mm, R2=0.88±0.13, n=24), and right femurs (torsion/tension ratio=506.7±260.0 mm, R2=0.82±0.22, n=25). No statistically significant differences were found for external versus internal rotation groups or for left versus right femurs when comparing torsion/tension ratios (p=0.85) or R2 values (p=0.54). A strongly coupled linear relationship between torsion and tension for human femurs was exhibited. This suggests an interplay between these two factors during activities of daily living and injury processes.

Treatment of periprosthetic femoral fractures with two different minimal invasive angle-stable plates: Biomechanical comparison studies on cadaveric bones

INTRODUCTION

The introduction of fixed-angle plate osteosynthesis techniques has provided us a further means to treat periprosthetic femoral fractures. The goal of this experimental study is to evaluate the biomechanical properties and stability of treated periprosthetic fractures when using two different plate systems, which vary in the locking mechanism and the screw placement (monocortical or bicortical) with respect to the prosthesis stem.

MATERIALS AND METHODS

Using five pairs of formalin-fixed femora, a Vancouver B1 periprosthetic fracture was treated either with a 13-hole LISS(®) titanium plate using four monocortical periprosthetic screws or with a non-contact bridging plate (NCB) DF(®) plate using bicortical angle-stable blocked screws positioned ventrally or dorsally to the prosthesis stem. Bones were loaded under axial and cyclic compression with a progressively increased load until failure. Displacement at the osteotomy gap was measured during loading using an ultra-sound measuring system.

RESULTS

The mean displacement in the region of the fracture gap was not significantly different at any time during the experiments for the two models. The mean force resulting in subsequent model failure was similar in both models; the failure morphology varied slightly between the models, however. Four of the five LISS(®) models exhibited either a tear-out of the monocortical screws or a decortication from the bony shaft of the cortical lamella surrounding the screws. On the other side, two of the NCB models showed macroscopically visible fissures along the osteosynthesis plates at the height of the osteotomy gap, and were hence considered implant failures. Only one NCB model showed tear-out of the bicortically placed screws.

CONCLUSION

Bicortical screw placement provides more stable anchoring when compared to monocortical screw fixation. However, in relation to the amount of motion at the osteotomy gap and to failure loads, stabilisation of periprosthetic femoral fractures can be equally well achieved using either the LISS(®) plate with periprosthetic monocortical screws or the NCB plate with poly-axially placed bicortical screws.

The effect of fixation technique on the stiffness of comminuted Vancouver B1 periprosthetic femur fractures

The purpose of this study was to evaluate the stiffness of 3 different constructs for the fixation of comminuted Vancouver B1 periprosthetic femoral shaft fractures: a single lateral locking plate, a single lateral locking plate plus an anterior strut allograft, and a lateral locking plate plus an anterior locking plate. The axial stiffness, lateral bending stiffness, and torsional stiffness of 10 synthetic periprosthetic femur fracture models were tested. Differences in stiffness between constructs were determined with a 1-way repeated-measures analysis of variance. Fixation technique was found to have a significant effect for all loading modalities (P < .0001). A lateral locked plate plus an anterior locked plate was significantly stiffer than the allograft that in turn was significantly stiffer than the single plate (P < .0001).

The biomechanical analysis of three plating fixation systems for periprosthetic femoral fracture near the tip of a total hip arthroplasty

Background

A variety of techniques are available for fixation of femoral shaft fractures following total hip arthroplasty. The optimal surgical repair method still remains a point of controversy in the literature. However, few studies have quantified the performance of such repair constructs. This study biomechanically examined 3 different screw-plate and cable-plate systems for fixation of periprosthetic femoral fractures near the tip of a total hip arthroplasty.

Methods

Twelve pairs of human cadaveric femurs were utilized. Each left femur was prepared for the cemented insertion of the femoral component of a total hip implant. Femoral fractures were created in the femurs and subsequently repaired with Construct A (Zimmer Cable Ready System), Construct B (AO Cable-Plate System), or Construct C (Dall-Miles Cable Grip System). Right femora served as matched intact controls. Axial, torsional, and four-point bending tests were performed to obtain stiffness values.

Results

All repair systems showed 3.08 to 5.33 times greater axial stiffness over intact control specimens. Four-point normalized bending (0.69 to 0.85) and normalized torsional (0.55 to 0.69) stiffnesses were lower than intact controls for most comparisons. Screw-plates provided either greater or equal stiffness compared to cable-plates in almost all cases. There were no statistical differences between plating systems A, B, or C when compared to each other (p > 0.05).

Conclusions

Screw-plate systems provide more optimal mechanical stability than cable-plate systems for periprosthetic femur fractures near the tip of a total hip arthroplasty.

A biomechanical assessment of modular and monoblock revision hip implants using FE analysis and strain gage measurements

Background

The bone loss associated with revision surgery or pathology has been the impetus for developing modular revision total hip prostheses. Few studies have assessed these modular implants quantitatively from a mechanical standpoint.

Methods

Three-dimensional finite element (FE) models were developed to mimic a hip implant alone (Construct A) and a hip implant-femur configuration (Construct B). Bonded contact was assumed for all interfaces to simulate long-term bony ongrowth and stability. The hip implants modeled were a Modular stem having two interlocking parts (Zimmer Modular Revision Hip System, Zimmer, Warsaw, IN, USA) and a Monoblock stem made from a single piece of material (Stryker Restoration HA Hip System, Stryker, Mahwah, NJ, USA). Axial loads of 700 and 2000 N were applied to Construct A and 2000 N to Construct B models. Stiffness, strain, and stress were computed. Mechanical tests using axial loads were used for Construct A to validate the FE model. Strain gages were placed along the medial and lateral side of the hip implants at 8 locations to measure axial strain distribution.

Results

There was approximately a 3% average difference between FE and experimental strains for Construct A at all locations for the Modular implant and in the proximal region for the Monoblock implant. FE results for Construct B showed that both implants carried the majority (Modular, 76%; Monoblock, 66%) of the 2000 N load relative to the femur. FE analysis and experiments demonstrated that the Modular implant was 3 to 4.5 times mechanically stiffer than the Monoblock due primarily to geometric differences.

Conclusions

This study provides mechanical characteristics of revision hip implants at sub-clinical axial loads as an initial predictor of potential failure.

The effect of cortex thickness on intact femur biomechanics: A comparison of finite element analysis with synthetic femurs

Biomechanical studies on femur fracture fixation with orthopaedic implants are numerous in the literature. However, few studies have compared the mechanical stability of these repair constructs in osteoporotic versus normal bone. The present aim was to examine how changes in cortical wall thickness of intact femurs affect biomechanical characteristics. A three-dimensional, linear, isotropic finite element (FE) model of an intact femur was developed in order to predict the effect of bicortical wall thickness, t, relative to the femur's mid-diaphyseal outer diameter, D, over a cortex thickness ratio range of 0 ≤ t/ D ≤ 1. The FE model was subjected to loads to obtain axial, lateral, and torsional stiffness. Ten commercially available synthetic femurs were then used to mimic ‘osteoporotic’ bone with t/ D = 0.33, while ten synthetic left femurs were used to simulate ‘normal’ bone with t/ D = 0.66. Axial, lateral, and torsional stiffness were measured for all femurs. There was excellent agreement between FE analysis and experimental stiffness data for all loading modes with an aggregate average percentage difference of 8 per cent. The FE results for mechanical stiffness versus cortical thickness ratio (0 ≤ t/ D ≤ 1) demonstrated exponential trends with the following stiffness ranges: axial stiffness (0 to 2343 N/mm), lateral stiffness (0 to 62 N/mm), and torsional stiffness (0 to 198 N/mm). This is the first study to characterize mechanical stiffness over a wide range of cortical thickness values. These results may have some clinical implications with respect to appropriately differentiating between older and younger human long bones from a mechanical standpoint.

The effect of load application rate on the biomechanics of synthetic femurs

Biomechanical investigations are increasingly using commercially available synthetic femurs as surrogates for human cadaveric femurs. However, the rate of force application in testing these artificial femurs appears to be chosen arbitrarily without much consideration to their visco-elastic time-dependent nature. The aim of this study, therefore, was to examine the effect of loading rate on the mechanical behaviour of synthetic femurs. Ten left, medium, fourth-generation composite femurs (Model 3403, Pacific Research Laboratories, Vashon, WA, USA) were fixed distally into cement-filled steel cubic chambers for mounting into a mechanical tester. In randomized order, each of the ten femurs was loaded at rates of 1, 2.5, 5, 7.5, 10, 20, 30, 40, 50, and 60 mm/min to obtain axial, lateral, and torsional stiffness. Axial stiffness showed an aggregate average value of 1742.7 ± 174.7 N/mm with a high linear correlation with loading rate ( R2 = 0.80). Lateral stiffness yielded an aggregate average value of 56.9 ± 10.2 N/mm and was linearly correlated with loading rate ( R2 = 0.85). Torsional stiffness demonstrated an aggregate average value of 176.9 ± 14.5 N/mm with a strong linear correlation with loading rate ( R2 = 0.59). Despite the high correlations between stiffness and speed, practically this resulted in an overall average difference between the lowest and highest stiffness of only 4 per cent. Moreover, no statistical comparisons between loading rates for axial, lateral, or torsional test modes showed differences ( p ≤ 0.843). Future biomechanical investigators utilizing these synthetic femurs need not be concerned with loading rate effects over the range tested presently. This is the first study in the literature to perform such an assessment.

The effect of the screw pull-out rate on cortical screw purchase in unreamed and reamed synthetic long bones

Orthopaedic fracture fixation constructs are typically mounted on to human long bones using cortical screws. Biomechanical studies are increasingly employing commercially available synthetic bones. The aim of this investigation was to examine the effect of the screw pull-out rate and canal reaming on the cortical bone screw purchase strength in synthetic bone. Cylinders made of synthetic material were used to simulate unreamed (foam-filled) and reamed (hollow) human long bone with an outer diameter of 35 mm and a cortex wall thickness of 4 mm. The unreamed and reamed cylinders each had 56 sites along their lengths into which orthopaedic cortical bone screws (major diameter, 3.5 mm) were inserted to engage both cortices. The 16 test groups ( n = 7 screw sites per group) had screws extracted at rates of 1 mm/min, 5 mm/min, 10 mm/min, 20 mm/min, 30 mm/min, 40 mm/min, 50 mm/min, and 60 mm/min. The failure force and failure stress increased and were highly linearly correlated with pull-out rate for reamed ( R2 = 0.60 and 0.60), but not for unreamed ( R2 = 0.00 and 0.00) specimens. The failure displacement and failure energy were relatively unchanged with pull-out rate, yielding low coefficients for unreamed ( R2 = 0.25 and 0.00) and reamed ( R2 = 0.27 and 0.00) groups. Unreamed versus reamed specimens were statistically different for failure force ( p = 0.000) and stress ( p = 0.000), but not for failure displacement ( p = 0.297) and energy (0.054< p<1.000). This is the first study to perform an extensive investigation of the screw pull-out rate in unreamed and reamed synthetic long bone.

The biomechanics of the T2 femoral nailing system: A comparison of synthetic femurs with finite element analysis

Intramedullary nails are commonly used to repair femoral fractures. Fractures in normal healthy bone often occur in the young during motor vehicle accidents. Although clinically beneficial, bone refracture and implant failure persist. Large variations in human femur quality and geometry have motivated recent experimental use of synthetic femurs that mimic human tissue and the development of increasingly sophisticated theoretical models. Four synthetic femurs were fitted with a T2 femoral nailing system (Stryker, Mahwah, New Jersey, USA). The femurs were not fractured in order to simulate post-operative perfect union. Six configurations were created: retrograde nail with standard locking (RS), retrograde nail with advanced locking ‘off’ (RA-off), retrograde nail with advanced locking ‘on’ (RA-on), antegrade nail with standard locking (AS), antegrade nail with advanced locking ‘off’ (AA-off), and antegrade nail with advanced locking ‘on’ (AA-on). Strain gauges were placed on the medial side of femurs. A 580 N axial load was applied, and the stiffness was measured. Strains were recorded and compared with results from a three-dimensional finite element (FE) model. Experimental axial stiffnesses for RA-off (771.3 N/mm) and RA-on (681.7 N/mm) were similar to intact human cadaveric femurs from previous literature (757 ± 264 N/mm). Conversely, experimental axial stiffnesses for AS (1168.8 N/mm), AA-off (1135.3 N/mm), AA-on (1152.1 N/mm), and RS (1294.0 N/mm) were similar to intact synthetic femurs from previous literature (1290 ± 30 N/mm). There was better agreement between experimental and FE analysis strains for RS (average percentage difference, 11.6 per cent), RA-on (average percentage difference, 11.1 per cent), AA-off (average percentage difference, 13.4 per cent), and AA-on (average percentage difference, 16.0 per cent), than for RA-off (average percentage difference, 33.5 per cent) and AS (average percentage difference, 32.6 per cent). FE analysis was more predictive of strains in the proximal and middle sections of the femur—nail construct than the distal. The results mimicked post-operative clinical stability at low static axial loads once fracture healing begins to occur.

Cancellous bone screw purchase: A comparison of synthetic femurs, human femurs, and finite element analysis

Biomechanical assessments of orthopaedic fracture fixation constructs are increasingly using commercially available analogues such as the fourth-generation composite femur (4GCF). The aim of this study was to compare cancellous screw purchase directly between these surrogates and human femurs, which has not been done previously. Synthetic and human femurs each had one orthopaedic cancellous screw (major diameter, 6.5 mm) inserted along the femoral neck axis and into the spongy bone of the femoral head to a depth of 30 mm. Screws were removed to obtain pull-out force, shear stress, and energy values. The three experimental study groups ( n = 6 femurs each) were the 4GCF with a ‘solid’ cancellous matrix, the 4GCF with a ‘cellular’ cancellous matrix, and human femurs. Moreover, a finite element model was developed on the basis of the material properties and anatomical geometry of the two synthetic femurs in order to assess cancellous screw purchase. The results for force, shear stress, and energy respectively were as follows: 4GCF solid femurs, 926.47 ± 66.76 N, 2.84 ± 0.20 MPa, and 0.57 ± 0.04 J; 4GCF cellular femurs, 1409.64 ± 133.36 N, 4.31 ± 0.41 MPa, and 0.99 ± 0.13 J; human femurs, 1523.29 ± 1380.15 N, 4.66 ± 4.22 MPa, and 2.78 ± 3.61 J. No statistical differences were noted when comparing the three experimental groups for pull-out force ( p = 0.413), shear stress ( p = 0.412), or energy ( p = 0.185). The 4GCF with either a ‘solid’ or ‘cellular’ cancellous matrix is a good biomechanical analogue to the human femur at the screw thread—bone interface. This is the first study to perform a three-way investigation of cancellous screw purchase using 4GCFs, human femurs, and finite element analysis.