Fatigue performance of composite analogue femur constructs under high activity loading

Biomechanical properties of artificial bones made by Sawbones: A review

Biomedical engineers and physicists frequently use human or animal bone for orthopaedic biomechanics research because they are excellent approximations of living bone. But, there are drawbacks to biological bone, like degradation over time, ethical concerns, high financial costs, inter-specimen variability, storage requirements, supplier sourcing, transportation rules, etc. Consequently, since the late 1980s, the Sawbones® company has been one of the world's largest suppliers of artificial bones for biomechanical testing that counteract many disadvantages of biological bone. There have been many published reports using these bone analogs for research on joint replacement, bone fracture fixation, spine surgery, etc. But, there exists no prior review paper on these artificial bones that gives a comprehensive and in-depth look at the numerical data of interest to biomedical engineers and physicists. Thus, this paper critically reviews 25 years of English-language studies on the biomechanical properties of these artificial bones that (a) characterized unknown or unreported values, (b) validated them against biological bone, and/or (c) optimized different design parameters. This survey of data, advantages, disadvantages, and knowledge gaps will hopefully be useful to biomedical engineers and physicists in developing mechanical testing protocols and computational finite element models.

Novel slide compression anatomic plates of the femoral neck for treating unstable femoral neck fracture: A biomechanical study

To compare the biomechanical stability of slide compression anatomic plates of the femoral neck, cannulated compression screws and dynamic hip screws with derotation screws for stabilizing unstable femoral neck fractures (Pauwels angle = 70°). Pauwels III femoral neck fractures were created on 45 Sawbones femurs and randomly assigned to three implant groups (1:1:1). The biomechanical stability of all Sawbones in each treatment group was evaluated with three tests. First, in the static loading test, the load-displacement curve, vertical stiffness (load/vertical displacement [N/mm]) and 5 mm failure load were recorded. Second, in the incremental cyclic loading test (700, 1000, and 1400 N), the cyclic-displacement curve and the displacement of the fragments were recorded. Third, in the torsion test, the torsional rigidity, maximum torque, and torsional angle corresponding to the maximum torque were recorded. The static compression test showed that slide compression anatomic place-femoral neck (SCAP-FN) had the largest vertical stiffness (275 ± 11 N/mm, p < 0.01) and 5 mm failure load (1232 ± 156, p < 0.01). The cyclic loading test showed that SCAP-FN had the lowest change in displacement after 30000 cycles of loading. The torsional stiffness and the maximum torque followed the order SCAP-FN > dynamic hip screw systems (DHS) + derotational screw (DS) > CCS, and the torsional angle corresponding to the maximum torque followed the order SCAP-FN < DHS + DS < CCS. The SCAP-FN construct provides stiffness and stability compared with other standard fixation techniques (3CS and DHS + DS). The fixation strategy of SCAP-FN might be sufficient for clinical use, indicating studies in the human body are warranted.

A New System for Periprosthetic Fracture Stabilization-A Biomechanical Comparison

In recent years, an increase in periprosthetic femur fractures has become apparent due to the increased number of hip replacements. In the case of Vancouver type B1 fractures, locking plate systems offer safe procedures. This study compared the distal lateral femur plate (LOQTEQ^®, aap Implantate AG) with a standard L.I.S.S. LCP^® (DePuy Synthes) regarding their biomechanical properties in fixation of periprosthetic femur fractures after hip arthroplasty. We hypothesized that the new LOQTEQ system has superior stability and durability in comparison. Eighteen artificial left femurs were randomized in two groups (Group A: LOQTEQ^®; Group B: L.I.S.S. LCP^®) and tested until failure. Failure was defined as 10° varus deformity and catastrophic implant failure (loosening, breakage, progressive bending). Axial stiffness, loads of failure, cycles of failure, modes of failure were recorded. The axial stiffness in Group A with 73.4 N/mm (SD +/− 3.0) was significantly higher (p = 0.001) than in Group B (40.7 N/mm (SD +/− 2.8)). Group A resists more cycles than Group B until 10° varus deformity. Catastrophic failure mode was plate breakage in Group A and bending in Group B. In conclusion, LOQTEQ^® provides higher primary stability and tends to have higher durability.

The effect of surgeon-controlled variables on construct stiffness in lateral locked plating of distal femoral fractures

Background

Nonunion following treatment of supracondylar femur fractures with lateral locked plates (LLP) has been reported to be as high as 21 %. Implant related and surgeon-controlled variables have been postulated to contribute to nonunion by modulating fracture-fixation construct stiffness. The purpose of this study is to evaluate the effect of surgeon-controlled factors on stiffness when treating supracondylar femur fractures with LLPs:

Does plate length affect construct stiffness given the same plate material, fracture working length and type of screws?

Does screw type (bicortical locking versus bicortical nonlocking or unicortical locking) and number of screws affect construct stiffness given the same material, fracture working length, and plate length?

Does fracture working length affect construct stiffness given the same plate material, length and type of screws?

Does plate material (titanium versus stainless steel) affect construct stiffness given the same fracture working length, plate length, type and number of screws?

Methods

Mechanical study of simulated supracondylar femur fractures treated with LLPs of varying lengths, screw types, fractureworking lenghts, and plate/screw material. Overall construct stiffness was evaluated using an Instron hydraulic testing apparatus.

Results

Stiffness was 15 % higher comparing 13-hole to the 5-hole plates (995 N/mm849N vs. /mm, p = 0.003). The use of bicortical nonlocking screws decreased overall construct stiffness by 18 % compared to bicortical locking screws (808 N/mm vs. 995 N/mm, p = 0.0001). The type of screw (unicortical locking vs. bicortical locking) and the number of screws in the diaphysis (3 vs. 10) did not appear to significantly influence construct stiffness (p = 0.76, p = 0.24). Similarly, fracture working length (5.4 cm vs. 9.4 cm, p = 0.24), and implant type (titanium vs. stainless steel, p = 0.12) did also not appear to effect stiffness.

Discussion

Using shorter plates and using bicortical nonlocking screws (vs. bicortical locking screws) reduced overall construct stiffness. Using more screws, using unicortical locking screws, increasing fracture working length and varying plate material (titanium vs. stainless steel) does not appear to significantly alter construct stiffness. Surgeons can adjust plate length and screw types to affect overall fracture-fixation construct stiffness; however, the optimal stiffness to promote healing remains unknown.

How to Build a Patient-Specific Hybrid Simulator for Orthopaedic Open Surgery: Benefits and Limits of Mixed-Reality Using the Microsoft HoloLens

Orthopaedic simulators are popular in innovative surgical training programs, where trainees gain procedural experience in a safe and controlled environment. Recent studies suggest that an ideal simulator should combine haptic, visual, and audio technology to create an immersive training environment. This article explores the potentialities of mixed-reality using the HoloLens to develop a hybrid training system for orthopaedic open surgery. Hip arthroplasty, one of the most common orthopaedic procedures, was chosen as a benchmark to evaluate the proposed system. Patient-specific anatomical 3D models were extracted from a patient computed tomography to implement the virtual content and to fabricate the physical components of the simulator. Rapid prototyping was used to create synthetic bones. The Vuforia SDK was utilized to register virtual and physical contents. The Unity3D game engine was employed to develop the software allowing interactions with the virtual content using head movements, gestures, and voice commands. Quantitative tests were performed to estimate the accuracy of the system by evaluating the perceived position of augmented reality targets. Mean and maximum errors matched the requirements of the target application. Qualitative tests were carried out to evaluate workload and usability of the HoloLens for our orthopaedic simulator, considering visual and audio perception and interaction and ergonomics issues. The perceived overall workload was low, and the self-assessed performance was considered satisfactory. Visual and audio perception and gesture and voice interactions obtained a positive feedback. Postural discomfort and visual fatigue obtained a nonnegative evaluation for a simulation session of 40 minutes. These results encourage using mixed-reality to implement a hybrid simulator for orthopaedic open surgery. An optimal design of the simulation tasks and equipment setup is required to minimize the user discomfort. Future works will include Face Validity, Content Validity, and Construct Validity to complete the assessment of the hip arthroplasty simulator.

Effect of the length of the superficial plate on bending stiffness, bending strength and strain distribution in stacked 2.0–2.7 veterinary cuttable plate constructs

Objectives: Use of stacked veterinary cut-table plates (VCP) increases the construct stiffness, but it also increases the stress protection and concentrates the stress at the extremities of the implants. We hypothesized that by shortening the superficial plate, it would not reduce the stiffness of the construct, but that it would reduce the stress concentration at the plate ends.

Methods: A 3 mm fracture gap model was created with copolymer acetal rods, stacked 2.0–2.7 VCP and 2.7 screws. The constructs consisted of an 11-hole VCP bottom plate and a 5-, 7-, 9- or 11-hole VCP superficial plate. Five of each construct were randomly tested for failure in four-point bending and axial loading. Stiffness, load at yield, and area under the curve until contact (AUC) were measured. Strains were recorded during elastic deformation for each configuration.

Results: During both testing methods, stiffness, load at yield and AUC progressively decreased when decreasing the length of the superficial plate. No statistically significant differences were obtained for load at yield in four-point bending and AUC in axial loading. The strain within the implant over the gap increased as the length of the superficial plate decreased.

Clinical significance: Shortening the superficial plate reduces the stiffness and strength of the construct, and decreases stress concentration at the implants ends. As the cross section of the implant covering the gap remained constant, friction between the plates may play a role in the mechanical properties of stacked VCP.

A Validated Open-Source Multisolver Fourth-Generation Composite Femur Model

Synthetic biomechanical test specimens are frequently used for preclinical evaluation of implant performance, often in combination with numerical modeling, such as finite-element (FE) analysis. Commercial and freely available FE packages are widely used with three FE packages in particular gaining popularity: abaqus (Dassault Systèmes, Johnston, RI), ansys (ANSYS, Inc., Canonsburg, PA), and febio (University of Utah, Salt Lake City, UT). To the best of our knowledge, no study has yet made a comparison of these three commonly used solvers. Additionally, despite the femur being the most extensively studied bone in the body, no freely available validated model exists. The primary aim of the study was primarily to conduct a comparison of mesh convergence and strain prediction between the three solvers (abaqus, ansys, and febio) and to provide validated open-source models of a fourth-generation composite femur for use with all the three FE packages. Second, we evaluated the geometric variability around the femoral neck region of the composite femurs. Experimental testing was conducted using fourth-generation Sawbones® composite femurs instrumented with strain gauges at four locations. A generic FE model and four specimen-specific FE models were created from CT scans. The study found that the three solvers produced excellent agreement, with strain predictions being within an average of 3.0% for all the solvers (r2 > 0.99) and 1.4% for the two commercial codes. The average of the root mean squared error against the experimental results was 134.5% (r2 = 0.29) for the generic model and 13.8% (r2 = 0.96) for the specimen-specific models. It was found that composite femurs had variations in cortical thickness around the neck of the femur of up to 48.4%. For the first time, an experimentally validated, finite-element model of the femur is presented for use in three solvers. This model is freely available online along with all the supporting validation data.

Biomechanical Comparison of Fixation Devices for First Metatarsocuneiform Joint Arthrodesis

Common surgical treatment of first tarsal-metatarsal arthritis is by first metatarsocuneiform joint arthrodesis. While crossed-screw and locking plate fixation are the most widely used methods, a novel construct was designed to alleviate soft tissue irritation while still providing stable fixation. Using anatomic first metatarsal and medial cuneiform composites, we compared 3 arthrodesis implants (crossed-screw, dorsal locking plate, and IO Fix) under 2 cyclic bending loading scenarios (cantilever and 4-point bending). Additionally, the optimal orientation (plantar-dorsal or dorsal-plantar) of the IO Fix construct was determined. Failure load, diastasis, joint space angle, and axial and angular stiffness were determined. Both crossed-screw fixation and the IO Fix constructs experienced significantly higher failure loads than the dorsal locking plate during both loading scenarios. Additionally, they had lower plantar diastasis and joint space angle at failure than the plate. Moreover, the plantar-dorsal IO Fix construct was significantly stiffer than the crossed-screw during cantilever bending. Finally, the plantar-dorsal orientation of the IO Fix device had higher failure load and lower diastasis and angle at failure than in the dorsal-plantar orientation. The results suggest that the IO Fix system can reduce motion at the interfragmentary site and ensure compression for healing comparable to that of the crossed-screw fixation.

LEVELS OF EVIDENCE

Level V: Bench testing.

The effect of different torque wrenches on rotational stiffness in compressive femoral nails: a biomechanical study

PURPOSE

Rotation instability and locking screws failure are common problems. We aimed to determine optimal torque wrench offering maximum rotational stiffness without locking screw failure.

METHODS

We used 10 conventional compression nails, 10 novel compression nails and 10 interlocking nails with 30 composite femurs. We examined rotation stiffness and fracture site compression value by load cell with 3, 6 and 8 Nm torque wrenches using torsion apparatus with a maximum torque moment of 5 Nm in both directions. Rotational stiffness of composite femur-nail constructs was calculated.

RESULTS

Rotational stiffness of composite femur-compression nail constructs compressed by 6 Nm torque wrench was 3.27 ± 1.81 Nm/angle (fracture site compression: 1588 N) and 60% more than that compressed with 3 Nm torque wrench (advised previously) with 2.04 ± 0.81 Nm/angle (inter fragmentary compression: 818 N) (P = 0.000). Rotational stiffness of composite-femur-compression nail constructs compressed by 3 Nm torque wrench was 2.04 ± 0.81 Nm/angle (fracture site compression: 818 N) and 277% more than that of interlocking nail with 0.54 ± 0.08 Nm/angle (fracture site compression: 0 N) (P = 0.000).

CONCLUSION

Rotational stiffness and fracture site compression value produced by 3 Nm torque wrench was not satisfactory. To obtain maximum rotational stiffness and fracture site compression value without locking screw failure, 6 Nm torque wrench in compression nails and 8 Nm torque wrench in novel compression nails should be used.

Biomechanical Comparison of Cadaveric and Commercially Available Synthetic Osteoporotic Bone Analogues in a Locked Plate Fracture Model Under Torsional Loading

OBJECTIVES

Biomechanical studies of osteoporotic bone have used synthetic models rather than cadaveric samples because of decreased variability, increased availability, and overall ease of the use of synthetic models. We compared the torsional mechanical properties of cadaveric osteoporotic bone with those of currently available synthetic osteoporotic bone analogues.

METHODS

We tested 12 osteoporotic cadaveric humeri and 6 specimens each of 6 types of synthetic analogues. A 5-mm fracture gap model and posterior plating technique with 4.5-mm narrow 10-hole locking compression plate were used. Torque was applied to a peak of ±10 N·m for 1000 cycles at 0.3 Hz. Data were continuously collected during cyclical and ramped loading with a servohydraulic materials testing system.

RESULTS

Cadaveric bone had a 17% failure rate before completing 1000 cycles. Three osteoporotic bone models had 100% failure (P < 0.05), 2 had 17% failure, and 1 had 0% failure before 1000 cycles. Significant differences in the stiffness of the 3 types of synthetic bone models that survived cyclic loading were noted compared with the cadaveric bone model (P < 0.05). Osteoporotic bone analogues had torsional mechanical properties different from those of osteoporotic cadaveric specimens.

CONCLUSIONS

The differences between osteoporotic cadaveric humeri and synthetic osteoporotic bone analogues ranged from profound with complete catastrophic failure after a few cycles to subtler differences in stiffness and strain hardening. These findings suggest that different bone analogue models vary substantially in their torsional mechanical properties and might not be appropriate substitutes for cadaveric bone in biomechanical studies of osteoporotic bone.

Primary stability of implants placed at different angulations in artificial bone

INTRODUCTION

The aim of this study was to evaluate the primary stability (PS) of titanium implants with a progressive thread design and more thread stability in the apical threads placed in artificial bone materials.

MATERIALS AND METHODS

A total of 120 implants were placed in commercially available polyurethane composite bone blocks. The angulations that were chosen to place the implants in bone types II and IV were 0, 10, and 20 degrees, respectively. The implant dimensions were 11 mm in length and 3.5 mm in diameter. Two clinicians placed all implants, and an independent examiner evaluated the PS using the Osstell (ISQ) and Periotest devices. The χ test was used to evaluate the statistical differences between the PS at different angulations.

RESULTS

This study showed that there was a statistically significant difference (P = 0.02) of the PS values, when measured using the Periotest values, among all 3 angulations in both bone qualities. Tilted implants with 10 degrees, angulation had a better stability than conventionally placed implants.

CONCLUSIONS

The PS of dental implants is higher for implants placed in type II when compared with type IV artificial bone. A higher stability was found for implants placed with 10-degree angulations.

A novel nail providing more biomechanical rotational and axial stability than conventional interlocking nail in femur complex fracture model

PURPOSE

Inter-fragmentary rotational and axial instabilities are major challenges in nailing of complex or comminuted fractures. We aimed to compare the inter-fragmentary rotational and axial stability of novel anti-rotation interlocking nail and the conventional interlocking nail in complex or comminuted femur shaft fractures.

METHODS

Twenty composite femurs were divided into two groups, 30 mm was resected from the mid-portion of all composite femurs. The inter-fragmentary rotational and axial stabilities were assessed.

RESULTS

Between 10-N m external and 6-N m internal rotation torques, mean maximum inter-fragmentary rotational arc motion in the novel nails was 1.63 mm and 291 % less than that of the conventional nails (6.38 mm, P = 0.000). Between 150 N distraction and 2300 N compression, mean axial motion in the novel nails was 0.8 mm and 257 % less than that of the conventional nails (2.86 mm, p = 0.000).

CONCLUSION

An anti-rotational novel nail is superior to the conventional interlocking nail in terms of maximum inter-fragmentary rotational and axial stabilities in complex and comminuted femur shaft fractures.

Maximum load to failure and tensile displacement of an all-suture glenoid anchor compared with a screw-in glenoid anchor

PURPOSE

The purpose of this study was to evaluate the biomechanical behavior of an all-suture glenoid anchor in comparison with a more conventional screw-in glenoid anchor, with regard to maximum load to failure and tensile displacement.

METHODS

All mechanical testing was performed using an Instron ElectroPuls E1000 mechanical machine, with a 10 N pre-load and displacement rate of 10 mm/min. Force-displacement curves were generated, with calculation of maximum load, maximum displacement, displacement at 50 N and stiffness. Pretesting of handset Y-Knots in bone analog models revealed low force displacement below 60 N of force. Subsequently, three groups of anchors were tested for pull out strength in bovine bone and cadaver glenoid bone: a bioabsorbable screw-in anchor (Bio Mini-Revo, ConMed Linvatec), a handset all-suture anchor (Y-Knot, ConMed Linvatec) and a 60 N pre-tensioned all-suture anchor (Y-Knot). A total of 8 anchors from each group was tested in proximal tibia of bovine bone and human glenoids (age range 50-90).

RESULTS

In bovine bone, the Bio Mini-Revo displayed greater maximum load to failure (206 ± 77 N) than both the handset (140 ± 51 N; P = 0.01) and the pre-tensioned Y-Knot (135 ± 46 N; P = 0.001); no significant difference was seen between the three anchor groups in glenoid bone. Compared to the screw-in anchors, the handset all-suture anchor displayed inferior fixation, early displacement and greater laxity in the bovine bone and cadaveric bone (P < 0.05). Pre-tensioning the all-suture anchor to 60 N eliminated this behavior in all bone models.

CONCLUSIONS

Handset Y-Knots display low force anchor displacement, which is likely due to slippage in the pilot hole. Pre-tensioning the Y-Knot to 60 N eliminates this behavior.

LEVEL OF EVIDENCE

I.

Posterior glenoid wear in total shoulder arthroplasty: eccentric anterior reaming is superior to posterior augment

BACKGROUND

Uncorrected glenoid retroversion during total shoulder arthroplasty may lead to an increased likelihood of glenoid prosthetic loosening. Augmented glenoid components seek to correct retroversion to address posterior glenoid bone loss, but few biomechanical studies have evaluated their performance.

QUESTIONS/PURPOSES

We compared the use of augmented glenoid components with eccentric reaming with standard glenoid components in a posterior glenoid wear model. The primary outcome for biomechanical stability in this model was assessed by (1) implant edge displacement in superior and inferior edge loading at intervals up to 100,000 cycles, with secondary outcomes including (2) implant edge load during superior and inferior translation at intervals up to 100,000 cycles, and (3) incidence of glenoid fracture during implant preparation and after cyclic loading.

METHODS

A 12°-posterior glenoid defect was created in 12 composite scapulae, and the specimens were divided in two equal groups. In the posterior augment group, glenoid version was corrected to 8° and an 8°-augmented polyethylene glenoid component was placed. In the eccentric reaming group, anterior glenoid reaming was performed to neutral version and a standard polyethylene glenoid component was placed. Specimens were cyclically loaded in the superoinferior direction to 100,000 cycles. Superior and inferior glenoid edge displacements were recorded.

RESULTS

Surviving specimens in the posterior augment group showed greater displacement than the eccentric reaming group of superior (1.01 ± 0.02 [95% CI, 0.89-1.13] versus 0.83 ± 0.10 [95% CI, 0.72-0.94 mm]; mean difference, 0.18 mm; p = 0.025) and inferior markers (1.36 ± 0.05 [95% CI, 1.24-1.48] versus 1.20 ± 0.09 [95% CI, 1.09-1.32 mm]; mean difference, 0.16 mm; p = 0.038) during superior edge loading and greater displacement of the superior marker during inferior edge loading (1.44 ± 0.06 [95% CI, 1.28-1.59] versus 1.16 ± 0.11 [95% CI, 1.02-1.30 mm]; mean difference, 0.28 mm; p = 0.009) at 100,000 cycles. No difference was seen with the inferior marker during inferior edge loading (0.93 ± 0.15 [95% CI, 0.56-1.29] versus 0.78 ± 0.06 [95% CI, 0.70-0.85 mm]; mean difference, 0.15 mm; p = 0.079). No differences in implant edge load were seen during superior and inferior loading. There were no instances of glenoid vault fracture in either group during implant preparation; however, a greater number of specimens in the eccentric reaming group were able to achieve the final 100,000 time without catastrophic fracture than those in the posterior augment group.

CONCLUSIONS

When addressing posterior glenoid wear in surrogate scapula models, use of angle-backed augmented glenoid components results in accelerated implant loosening compared with neutral-version glenoid after eccentric reaming, as shown by increased implant edge displacement at analogous times.

CLINICAL RELEVANCE

Angle-backed components may introduce shear stress and potentially compromise stability. Additional in vitro and comparative long-term clinical followup studies are needed to further evaluate this component design.

Improving stability of elastic stable intramedullary nailing in a transverse midshaft femur fracture model: biomechanical analysis of using end caps or a third nail

BACKGROUND

Elastic stable intramedullary nailing (ESIN) is accepted widely for treatment of diaphyseal femur fractures in children. However, complication rates of 10 to 50 % are described due to shortening or axial deviation, especially in older or heavier children. Biomechanical in vitro testing was performed to determine whether two modified osteosyntheses with end caps or a third nail could significantly improve the stability in comparison to classical elastic stable intramedullary nailing in a transverse femur fracture model.

METHODS

We performed biomechanical testing in 24 synthetic adolescent femoral bone models (Sawbones®) with a transverse midshaft (diaphyseal) fracture. First, in all models, two nails were inserted in a C-shaped manner (2 × 3.5 mm steel nails, prebent), then eight osteosyntheses were modified by using end caps and another eight by adding a third nail from the antero-lateral (2.5-mm steel, not prebent). Testing was performed in four-point bending, torsion, and shifting under physiological 9° compression.

RESULTS

The third nail from the lateral showed a significant positive influence on the stiffness in all four-point bendings as well as in internal rotation comparing to the classical 2C configuration: mean values were significantly higher anterior-posterior (1.04 vs. 0.52 Nm/mm, p < 0.001), posterior-anterior (0.85 vs. 0.43 Nm/mm, p < 0.001), lateral-medial (1.26 vs. 0.70 Nm/mm, p < 0.001), and medial-lateral (1.16 vs. 0.76 Nm/mm, p < 0.001) and during internal rotation (0.16 vs. 0.11 Nm/°, p < 0.001). The modification with end caps did not improve the stiffness in any direction.

CONCLUSIONS

The configuration with a third nail provided a significantly higher stiffness than the classical 2C configuration as well as the modification with end caps in this biomechanical model. This supports the ongoing transfer of the additional third nail into clinical practice to reduce the axial deviation occurring in clinical practice.

Biomechanical fatigue analysis of an advanced new carbon fiber/flax/epoxy plate for bone fracture repair using conventional fatigue tests and thermography

The current study is part of an ongoing research program to develop an advanced new carbon fiber/flax/epoxy (CF/flax/epoxy) hybrid composite with a "sandwich structure" as a substitute for metallic materials for orthopedic long bone fracture plate applications. The purpose of this study was to assess the fatigue properties of this composite, since cyclic loading is one of the main types of loads carried by a femur fracture plate during normal daily activities. Conventional fatigue testing, thermographic analysis, and scanning electron microscopy (SEM) were used to analyze the damage progress that occurred during fatigue loading. Fatigue strength obtained using thermography analysis (51% of ultimate tensile strength) was confirmed using the conventional fatigue test (50-55% of ultimate tensile strength). The dynamic modulus (E(⁎)) was found to stay almost constant at 47GPa versus the number of cycles, which can be related to the contribution of both flax/epoxy and CF/epoxy laminae to the stiffness of the composite. SEM images showed solid bonding at the CF/epoxy and flax/epoxy laminae, with a crack density of only 0.48% for the plate loaded for 2 million cycles. The current composite plate showed much higher fatigue strength than the main loads experienced by a typical patient during cyclic activities; thus, it may be a potential candidate for bone fracture plate applications. Moreover, the fatigue strength from thermographic analysis was the same as that obtained by the conventional fatigue tests, thus demonstrating its potential use as an alternate tool to rapidly evaluate fatigue strength of composite biomaterials.

Composite bone models in orthopaedic surgery research and education

Composite bone models are increasingly used in orthopaedic biomechanics research and surgical education-applications that traditionally relied on cadavers. Cadaver bones are suboptimal for many reasons, including issues of cost, availability, preservation, and inconsistency between specimens. Further, cadaver samples disproportionately represent the elderly, whose bone quality may not be representative of the greater orthopaedic population. The current fourth-generation composite bone models provide an accurate reproduction of the biomechanical properties of human bone when placed under bending, axial, and torsional loads. The combination of glass fiber and epoxy resin components into a single phase has enabled manufacturing by injection molding. The high level of anatomic fidelity of the cadaver-based molds and negligible shrinkage properties of the epoxy resin results in a process that allows for excellent definition of anatomic detail in the cortical wall and optimized consistency of features between models. Recent biomechanical studies of composites have validated their use as a suitable substitute for cadaver specimens.

Modification of elastic stable intramedullary nailing with a 3rd nail in a femoral spiral fracture model - results of biomechanical testing and a prospective clinical study

Background

Elastic stable intramedullary nailing (ESIN) is the standard treatment for displaced diaphyseal femoral fractures in children. However, high complication rates (10-50%) are reported in complex fractures. This biomechanical study compares the stiffness with a 3rd nail implanted to that in the classical 2C-shaped configuration and presents the application into clinical practice.

Methods

For each of the 3 configurations of ESIN-osteosynthesis with titanium nails eight composite femoral grafts (Sawbones®) with an identical spiral fracture were used: 2C configuration (2C-shaped nails, 2 × 3.5 mm), 3CM configuration (3rd nail from medial) and 3CL configuration (3rd nail from lateral). Each group underwent biomechanical testing in 4-point bending, internal/external rotation and axial compression.

Results

2C and 3CM configurations showed no significant differences in this spiroid type fracture model. 3CL had a significantly higher stiffness during anterior-posterior bending, internal rotation and 9° compression than 2C, and was stiffer in the lateral-medial direction than 3CM. The 3CL was less stable during p-a bending and external rotation than both the others. As biomechanical testing showed a higher stability for the 3CL configuration in two (a-p corresponding to recurvation and 9° compression to shortening) of three directions associated with the most important clinical problems, we added a 3rd nail in ESIN-osteosynthesis for femoral fractures. 11 boys and 6 girls (2.5-15 years) were treated with modified ESIN of whom 12 were ‘3CL’; due to the individual character of the fractures 4 patients were treated with ‘3CM’ (third nail from medial) and as an exception 1 adolescent with 4 nails and one boy with plate osteosynthesis. No additional stabilizations or re-operations were necessary. All patients achieved full points in the Harris-Score at follow-up; no limb length discrepancy occurred.

Conclusion

The 3CL configuration provided a significantly higher stiffness than 2C and 3CM configurations in this biomechanical model. These results were successfully transmitted into clinical practice. All children, treated by 3CL or 3CM according to the individual character of each fracture, needed no additional stabilization and had no Re-Do operations. As a consequence, at our hospital all children with femoral diaphyseal fractures with open physis are treated with this modified ESIN-technique.

Feasibility of a braided composite for orthopedic bone cast

A tubular braided composite bone cast for improving the efficiency and quality of bone fracture treatment is investigated. Finite element analysis was used to evaluate stress concentrations in fracture sites supported with plate and tubular casts. The stress in a plated bone is 768 % of that in a whole bone at the same location, while it is only 47 % in a bone with a tubular cast. Three unbroken synthetic humeri were mechanically tested using an in-vitro long bone testing procedure developed in-house to find their stiffness at 20° and 60° abduction; these were found to be 116.8 ± 1.5 N/mm and 20.63 ± 0.02 N/mm, respectively. A 2 cm gap osteotomy was cut through the diaphysis in each bone. The bones were casted with a Kevlar/Cold cure composite, with calculated braid angles and thicknesses that Closely matched bone propoerties. The stiffness tests were repeated, and the results were within 10 % of the unbroken bone. This novel method of bone casting is promising if other clinical challenges can be minimized.

Locking buttons increase fatigue life of locking plates in a segmental bone defect model

BACKGROUND

Durability of plate fixation is important in delayed union. Although locking plates result in stronger constructs, it is not known if locking affects the fatigue life of a plate. Two locking screws on either side of the nonunion could decrease working length and increase strain in the plate. However, the reinforcing effect of the locking head on the plate may compensate, so that it is unclear whether locking reduces fatigue life.

QUESTIONS/PURPOSES

We determined whether locking screws, compression screws, and locking buttons reduce or increase the fatigue life of a plate.

METHODS

We tested fatigue life of four constructs using an eight-hole locking plate in a segmental defect model: (1) all locking screws (Locked; n = 5); (2) all compression screws (Unlocked; n = 5); (3) six compression screws with two locking buttons in the central holes (Button; n = 6); and (4) six compression screws with two open central holes (Open; n = 6).

RESULTS

The Button group had the longest fatigue life (1.3 million cycles). There was no difference between the Locked and Unlocked groups. All of the constructs failed by fracture of the plates through a screw hole adjacent to the defect.

CONCLUSIONS

Locking screws did not improve fatigue life, however a locking button increased the fatigue life of a locking plate in a segmental bone defect model.

CLINICAL RELEVANCE

Locking buttons in holes adjacent to a defect may improve durability, which is important when delayed union is a possibility.

Biomechanical analysis of a volar variable-angle locking plate: the effect of capturing a distal radial styloid fragment

PURPOSE

Variable-angle volar locked constructs for distal radius fractures are a recent treatment addition. This study sought to biomechanically evaluate a variable-angle volar locking plate as compared with a fixed-angle construct.

METHODS

We created 2 different AO-C3 osteotomies in fourth-generation synthetic composite distal radiuses and labeled them proximal and distal. The distal osteotomy consisted of a smaller radial styloid fragment. We then fixed both sets of specimens with either a fixed-angle or variable-angle volar locking construct. We tested samples in axial compression with regard to cyclical loading and load to failure. Articular stepoff, stiffness, and load to failure data were then analyzed.

RESULTS

Neither the proximal nor the distal osteotomy groups showed articular failure after cyclic loading, significant loss of stiffness over cycling, or superior stiffness compared with the other. After load to failure in the proximal osteotomy, 1 of 8 fixed-angle and none of 8 variable-angle constructs had articular failure, whereas in the distal osteotomy, all 8 fixed-angle and none of 8 variable-angle constructs had articular failure.

CONCLUSIONS

Variable-angle and fixed-angle volar locked fixation of unstable intra-articular distal radius fractures in fourth-generation composite radii provide mechanically sound constructs with high load to failure values and no loss of stiffness over testing. The variable-angle construct exhibited excellent resistance to articular stepoff at load to failure and no loss of stiffness throughout cyclic loading, and did not exhibit significantly less overall stiffness compared with fixed-angle constructs. The variable-angle fixation exhibited a distinct mechanical advantage over fixed-angle fixation in the setting of a smaller radial styloid fragment.

CLINICAL RELEVANCE

Variable-angle constructs could be expected to hold up to standard loads in the postoperative period as well as traditional fixed-angle devices. The additional cost associated with variable-angle constructs may be warranted when treating distal radius fractures with radial styloid fragments, owing to the fragment-specific fixation allowed by customized screw placement.

Increasing stability by pre-bending the nails in elastic stable intramedullary nailing: a biomechanical analysis of a synthetic femoral spiral fracture model

Elastic stable intramedullary nailing (ESIN) is generally acknowledged to be the treatment of choice for displaced diaphyseal femoral fractures in children over the age of three years, although complication rates of up to 50% are described. Pre-bending the nails is recommended, but there are no published data to support this. Using synthetic bones and a standardised simulated fracture, we performed biomechanical testing to determine the influence on the stability of the fracture of pre-bending the nails before implantation. Standard ESIN was performed on 24 synthetic femoral models with a spiral fracture. In eight cases the nails were inserted without any pre-bending, in a further eight cases they were pre-bent to 30° and in the last group of eight cases they were pre-bent to 60°. Mechanical testing revealed that pre-bending to 60° produced a significant increase in the stiffness or stability of the fracture. Pre-bending to 60° showed a significant positive influence on the stiffness compared with unbent nails. Pre-bending to 30° improved stiffness only slightly. These findings validate the recommendations for pre-bending, but the degree of pre-bend should exceed 30°. Adopting higher degrees of pre-bending should improve stability in spiral fractures and reduce the complications of varus deformity and shortening.

Biomechanical analysis of a synthetic femoral spiral fracture model: Do end caps improve retrograde flexible intramedullary nail fixation?

Background

Elastic Stable intramedullary Nailing (ESIN) of dislocated diaphyseal femur fractures has become an accepted method for the treatment in children and adolescents with open physis. Studies focused on complications of this technique showed problems regarding stability, usually in complex fracture types such as spiral fractures and in older children weighing > 40 kg. Biomechanical in vitro testing was performed to evaluate the stability of simulated spiral femoral fractures after retrograde flexible titanium intramedullary nail fixation with and without End caps.

Methods

Eight synthetic adolescent-size femoral bone models (Sawbones^® with a medullar canal of 10 mm and a spiral fracture of 100 mm length identically sawn by the manufacturer) were used for each group. Both groups underwent retrograde fixation with two 3.5 mm Titanium C-shaped nails inserted from medial and lateral entry portals. In the End Cap group the ends of the nails of the eight specimens were covered with End Caps (Synthes Company, Oberdorf, Switzerland) at the distal entry.

Results

Beside posterior-anterior stress (4.11 Nm/mm vs. 1.78 Nm/mm, p < 0.001), the use of End Caps demonstrated no higher stability in 4-point bending compared to the group without End Caps (anterior-posterior bending 0.27 Nm/mm vs. 0.77 Nm/mm, p < 0.001; medial-lateral bending 0.8 Nm/mm vs. 1.10 Nm/mm, p < 0.01; lateral-medial bending 0.53 Nm/mm vs. 0.86 Nm/mm, p < 0.001) as well as during internal rotation (0.11 Nm/° vs. 0.14 Nm/°, p < 0.05). During compression in 9°- position and external rotation there was no statistical significant difference (0.37 Nm/° vs. 0.32 Nm/°, p = 0.13 and 1.29 mm vs. 2.18 mm, p = 0.20, respectively) compared to the "classic" 2-C-shaped osteosynthesis without End Caps.

Conclusion

In this biomechanical study the use of End Caps did not improve the stability of the intramedullary flexible nail osteosynthesis.

Biomechanical analysis of a synthetic femur spiral fracture model: Influence of different materials on the stiffness in flexible intramedullary nailing

BACKGROUND

Flexible intramedullary nail fixation of dislocated diaphyseal femur fractures has gained wide acceptance for children and adolescents with open physes. Studies with a special emphasis on complications reveal frequent problems regarding stability, usually in complex fracture types such as spiral fractures and in older children weighing >40kg. This biomechanical study analyses how much the material of the nails influences the stiffness in a synthetic bone model.

METHODS

Twenty-four composite grafts (Sawbones®, 4th generation, medullar canal of 10mm) with an identical spiral fracture were used in three configurations of eight grafts. Elastic stable intramedullary nailing was performed in a retrograde C-shaped manner with two nails of equal size (2×3.5mm). Close contact of the fragments could be achieved. We compared Group A (steel nails) with Group B and C (two types of titanium nails). All specimens underwent 4-point bending, torsion and axial compression in the 0° and 9° positions, and the results were analysed.

FINDINGS

Group A (steel nails) revealed a significantly higher stiffness in all directions than Group B. Apart from compression in the 9° position this steel nail fixation showed significant higher stiffness than titanium nails of Group C. Comparing Group B and C did not show an systematic difference.

INTERPRETATION

In this biomechanical study with composite artificial bones the use of steel Nails demonstrated the highest stiffness in our model when compared to two different titanium nail configurations. Apart from in cases of known allergy or planned MRI-examinations our results and data from the literature question the use of titanium nails.

Anatomical plate configuration affects mechanical performance in distal humerus fractures

BACKGROUND

Because of strong loads acting in the elbow joint, intraarticular fractures with a methaphyseal comminuted fracture site at the distal humerus demand a lot from the osteosynthetic care. Ambiguities arise concerning to the anatomic position of the implants and the resulting mechanic performance. The aim of this biomechanical study was to compare the performance of different anatomical plate configurations for fixation of comminuted distal humerus fractures within one system.

METHODS

In an artificial bone model two perpendicular and one parallel plating configuration of a dedicated elbow plating system were compared with respect to system rigidity (flexion and extension) and dynamic median fatigue limit (extension). The flexion tests were conducted under 75° and the extension tests under 5°. Furthermore, the relative displacements were recorded. As a fracture model an AO C 2.3-fracture on an artificial bone (4th Gen. Sawbone) was simulated via double osteotomy in sagittal and transversal plane.

FINDINGS

Large differences in mechanical performance were observed between flexion and extension loading modes. In extension the parallel configuration with lateral and medial plates achieved the highest bending stiffness and median fatigue limit. In flexion the highest bending stiffness was reached by the construct with a medial and a postero-lateral plate. Failure of the implant system predominantly occurred at the screw-bone interface or by fatigue of the plate around the screw holes.

INTERPRETATION

All three plate configurations provided sufficient mechanical stability to allow early postoperative rehabilitation with a reduced loading protocol. Although the individual fracture pattern determines the choice of plate configuration, the parallel configuration with lateral and medial plates revealed biomechanical advantages in extension only.

Biomechanical comparison of 3 possible fixation strategies to resist femoral neck shortening after fracture

In light of recent reports that patients with femoral neck shortening following fracture fixation are dissatisfied with their outcomes, the objective of this study was to compare the compressive strength, or resistance to shortening, of 3 possible strategies for stabilization of the femoral neck that should resist shortening. The proximal portion of 21 synthetic composite femurs were prepared to isolate the femoral neck for study. A 4-mm segment of the femoral neck was removed to simulate a transcervical comminuted fracture that would be expected to shorten under standard treatment conditions. These simulated fractures were fixed by 1 of 3 methods: a 3-screw configuration using parallel partially threaded screws augmented with an injectable hydroxyapatite bone substitute in the fracture site; a 3-screw configuration using parallel fully threaded screws; or a nonparallel 3-screw configuration using partially threaded screws. The specimens were tested in compression along the axis of the femoral neck, and the mean stiffness and load to failure values were calculated.The hydroxyapatite bone substitute-augmented partially threaded screw fixation construct resulted in the highest stiffness (1928+/-135 N/mm) and load to failure (6529+/-674 N), followed by the fully threaded screw construct (1210+/-166 N/mm and 3987+/-419 N, respectively), and finally the nonparallel construct (518+/-176 N/mm and 592+/-295 N, respectively) (P<.001 for all groups). This study supports further evaluation of hydroxyapatite bone substitute augmentation at the fracture site to prevent femoral neck shortening in femoral neck fractures receiving internal fixation.

Type of hip fracture determines load share in intramedullary osteosynthesis

The choice of the appropriate implant continues to be critical for fixation of unstable hip fractures. Therefore, the goal of this study was to develop a numerical model to investigate the mechanical performance of hip fracture osteosynthesis. We hypothesized that decreasing fracture stability results in increasing load share of the implant and therefore higher stress within the implant. We also investigated the relationship of interfragmentary movement to the fracture stability. A finite element model was developed for a cephalomedullary nail within a synthetic femur and simulated a pertrochanteric fracture, a lateral neck fracture, and a subtrochanteric fracture. The femur was loaded with a hip force and was constrained physiologically. The FE model was validated by mechanical experiments. All three fractures resulted in similar values for stiffness (462-528 N/mm). The subtrochanteric fracture resulted in the highest local stress (665 MPa), and the pertrochanteric fracture resulted in a lower stress (621 MPa) with even lower values for the lateral neck fracture (480 MPa). Thus, intramedullary implants can stabilize unstable hip fractures with almost the same amount of stiffness as seen in stable fractures, but they have to bear a higher load share, resulting in higher stresses in the implant.

Stiffness modulation of locking plate constructs using near cortical slotted holes: a preliminary study

OBJECTIVES

Axial stiffness is a critical mechanical parameter in fracture plating. Standard locked plates allow minimal opportunities for stiffness alteration, and current methods are arbitrary and may lead to stiffness mismatch between the implant and bone. Milling the near cortex into a slot allows for an increase in translation of the screw shaft at the near cortex. The purpose of this proof of concept study was to determine the effects of slots on stiffness and their ability to maintain fixation of locking plates under cyclic loading.

METHODS

Using segments of fourth-generation synthetic diaphyseal bone, a simulated fracture with a gap was created and locked plates were applied with 4 bicortical locked screws in each fragment. On one fragment, the 4 near cortex holes were sequentially milled to 5 x 6-mm slots. Axial and torsional stiffnesses were determined for constructs with 0 through 4 slots. Specimens with 4 slots then underwent axial cyclic loading to determine the change in stiffness and loss of fixation. Extraction torque was measured for all screws to assess for screw loosening with cycling.

RESULTS

In constructs with 4 slots, axial stiffness decreased by 73% (P < 0.05) relative to the 0-slot constructs. Torsional stiffness of the 3- and 4-slot specimens decreased by 20% (SD, 13%; P < 0.05) and 17% (SD, 13%; P < 0.05), respectively, compared with the 0-slot specimens. With cyclic loading, no failures occurred in any specimen. No change in stiffness had occurred by the end of cycling (106% of initial stiffness; SD, 4%; P = 0.96). No screw loosening occurred during cyclic loading.

CONCLUSIONS

Purposeful stiffness modulation in fracture fixation is critical to facilitate uneventful fracture healing. Converting near cortical holes to slots allowed selective axial stiffness adjustment without sacrificing fixation stability under cyclic loading. With further refinement, this simple modification of standard implant application may allow the surgeon to decrease the modulus mismatch between plating constructs and bone to decrease the risk of fixation failure.

Structural properties of a novel design of composite analogue humeri models

BACKGROUND

Mechanical analogue composite bone models have been used as cadaveric bone substitutes in a wide variety of biomechanical tests. The objective of this study was to compare the structural properties of two types (Third- and Fourth-Generation) of commercially available composite analogue humeri.

METHODS

Eighteen of each generation composite analogue humeri were evaluated for flexural rigidity, torsional rigidity, and failure strength. Three tests were performed: medial-lateral four-point bending, anterior-posterior four-point bending, and external rotational torque.

RESULTS

The Fourth-Generation analogue humeri performed more closely to the biological average with respect to failure strength, flexural rigidity, and torsional stiffness when compared to the Third-Generation humeri. Both the Third- and Fourth-Generation analogues were within the range of published human bone values. There was a statistically significant difference in strength in all modes of testing between the Fourth-Generation humeri and the Third-Generation humeri.

CONCLUSION

These composite analogue humeri are ideal for standardization in biomechanical analyses. The advantage of these humeri is that their variability is significantly lower than that of cadaveric specimens for all loading regimens. The widely varying results observed when comparing composite analogue humeri to cadaveric humeri might be derived from the use of different ranges of applied load, varied test methodologies, and diverse methods of computing the stiffness. Mechanical validation of whole Fourth-Generation humeri bone models would be an appropriate follow-up to this study with a direct comparison to cadaveric humeri.

CLINICAL RELEVANCE

This study validated and advanced our overall understanding of the capacity of composite analogue humeri to model the structural properties of human bone.