Comparison of the deformation response of synthetic and cadaveric femora during simulated one-legged stance

Additional fixation of medial plate over the unstable lateral locked plating of distal femur fractures: A biomechanical study

INTRODUCTION

Lateral locked plating is a standard treatment option for distal femur fractures. However, the unstable conditions after lateral locked plating are increasing. The objective of this study was to investigate the biomechanical strength of additional medial plate fixation over the unstable lateral locked plating of distal femur fractures.

MATERIALS AND METHODS

A distal femur fracture model (AO/OTA 33-A3) was created with osteotomies in the composite femur. Three study groups consisting of 6 specimens each were created for single-side lateral locked plating with 6 distal locking screws (LP-6), single-side lateral locked plating with 4 distal locking screws (LP-4), and additional medial locked plating on LP-4 construct (DP-4). A compressive axial load (10 mm/min) was applied in the failure test. Mode of failure, load to failure, and ultimate displacement were documented.

RESULTS

All single-side lateral locked plating (LP-4 and LP-6) showed plate bending at the fracture gap, while none of the DP-4 showed plate bending at the fracture gap. Load to failure of DP-4 (mean 5522 N) was 17.1% greater than that of LP-6 (mean 4713.3 N, p < 0.05) and 29.2% greater than that of LP-4 (mean 4273.2 N, p < 0.05). Ultimate displacement of DP-4 (mean 5.6 mm) was significantly lower than that of LP-6 (mean 8.8 mm, p < 0.05) and LP-4 (mean 9.1 mm, p < 0.05).

CONCLUSIONS

Additional fixation of medial plate significantly increased the fracture stability in distal femur fractures fixed with the lateral locked plating. Especially in the clinical situations where sufficient stability cannot be provided at the distal segment, the medial plate may be considered as a useful biomechanical solution to obtain adequate stability for fracture healing.

Retrograde Stainless Steel Flexible Nails Have Superior Resistance to Bending in Distal Third Femoral Shaft Fractures

BACKGROUND

It has been shown that retrograde titanium flexible intramedullary nails (Ti FIN) provide superior resistance to bending compared to antegrade Ti FIN in distal femur fractures. The purpose of this study was to compare resistance to torsional and bending forces of stainless steel (SS) FIN, with or without a locking screw, and Ti FIN in distal third femoral shaft fractures. We hypothesize that locked retrograde SS FIN will demonstrate greater resistance to both bending and torsional forces.

METHODS

Thirty adolescent synthetic femur models were used to simulate transverse distal femoral fractures at either 60 mm or 90 mm proximal to the distal femoral physis. The femurs were instrumented with antegrade Ti FIN, antegrade SS FIN, retrograde Ti FIN, retrograde SS FIN, or retrograde locked SS FIN. Three models for each construct at both osteotomy levels were tested. Models were analyzed to determine maximum resistance to bending and torsion.

RESULTS

In fractures 60 mm from the physis, retrograde SS FIN demonstrated statistically superior resistance to bending when compared with both antegrade and retrograde Ti FIN (P=0.001 and 0.008, respectively) and antegrade SS FIN (P=0.0001). Locked SS constructs showed a trend towards greater resistance to bending forces when compared with unlocked constructs (P>0.05). No significant difference was seen in resistance to bending when fractures were 90 mm proximal to the distal femoral physis between the five groups. No significant differences were observed in resistance to torsion in either the proximal or distal fracture models, regardless of construct type.

CONCLUSIONS

Retrograde SS FIN confer significantly greater resistance to bending forces for fractures 60 mm proximal to the distal femoral physis compared with Ti FIN or antegrade entry SS FIN. In fractures 90 mm from the physis, no differences were noted in our model. Our results support the use of retrograde SS nails in the pediatric patient with distal femoral shaft fractures.

LEVEL OF EVIDENCE

Level II-comparative biomechanical study.

Biomechanical analysis comparing titanium elastic nails with locked plating in two simulated pediatric femur fracture models

BACKGROUND

Increasing attention is being paid to the influences that the body habitus and weight of the pediatric patient impose upon the fixation methods for femur fractures. Of the widely accepted treatment options, little biomechanical or clinical data exist comparing flexible intramedullary nailing and locked plating. The aim of this study was to compare the mechanical stability of unstable pediatric diaphyseal femur fractures fixed with titanium flexible intramedullary nails or a titanium locking plate using a synthetic femur model.

METHODS

Fracture stabilization was carried out with either 4.0-mm titanium elastic nails or 16-hole 4.5-mm narrow titanium locking compression plates. Axial and rotational testing of each specimen was performed. The axial loading rate was 0.20 mm/s. The torsional loading rate was 0.1 degrees rotation per second. The axial compressive modulus was defined as the compressive stress divided by the compressive strain. The rotational stiffness was defined as the torque moment applied to the femoral head divided by the resulting rotational displacement (in radians). The yield point or load to failure of the simulated fracture constructs was recorded for each specimen.

RESULTS

The modulus for comminuted fractures measured during the application of axial compression was 0.657 GPa for plate constructs and 0.326 GPa for elastic nail constructs (P=0.021). The modulus for oblique fractures during axial loading treated with plate fixation or titanium elastic nails was 1.63 and 0.466 GPa, respectively (P<0.0001). The yield point for comminuted fractures occurred at an axial load of 2304.7 N (SD ± 315.77) for plate constructs and 383.6 N (SD ± 139.2) for elastic nail constructs (P<0.001). For oblique fractures, the yield load occurred at 3111.9 N (SD ± 821.9) for plate constructs and at 1367.0 N (SD ± 98.9) for elastic nail constructs (P<0.0001).

CONCLUSIONS

Locked plating provides a biomechanically more stable construct than elastic intramedullary nailing. Its use as part of the technique of indirect reduction and submuscular plating remain a viable alternative in the treatment of length-unstable pediatric femur fracture patterns.

CLINICAL RELEVANCE

: Provide biomechanical evidence supporting the use of plating techniques in the pediatric femur fracture population.

Finite element analysis of different repair methods of Vancouver B1 periprosthetic fractures after total hip arthroplasty

OBJECTIVE

To use finite element analysis to study the stability of different fixation methods used to repair Vancouver type B1 periprosthetic fractures occurring after total hip arthroplasty (THA).

METHODS

An artificial femur was used as the basis for the solid model; U2 series femoral stem (United Orthopedic Corporation, Hsinchu, Taiwan) was used for modelling of the prosthesis; and the modelling of the cable plate, wires and screws was based on information given in the manufacturer's catalogue (Zimmer, Warsaw, IN, USA). The analysis model was constructed using the ANSYS software, and all material settings were based on literature values. A six-orifice cable plate, unicortical screws (20mm long and 4.5mm in diameter) and bicortical screws (50mm long and 4.5mm in diameter) were constructed. Four analysis models were defined. The basic model had a plate and three cable wires above the fracture line and two bicortical screws below the fracture line. In the second model, two unicortical screws were added above the fracture line. In the third model, three wires were added below the fracture line. In the fourth model, both the proximal screws of the second model and the distal wires of the third model were added to the basic model. To ensure that the numerical values produced by analysis reached convergence, mesh convergence was tested.

RESULTS

Adding two proximal unicortical screws to the basic Ogden construct (plate, proximal wires and distal screws) lessened displacement of the fracture and decreased the von Mises stress on the repair. Adding three distal wires to the basic construct had no noticeable effect.

CONCLUSION

Better fixation power is achieved by using both proximal and distal screws (the locking-plate concept) in treating Vancouver type B1 periprosthetic fracture after THA.

Stiffness of knee-spanning external fixation systems for traumatic knee dislocations: a biomechanical study

OBJECTIVE

The purpose of this study was to compare the relative stiffness of four common external fixation (XF) configurations used to span and stabilize the knee after knee dislocation.

METHODS

Synthetic composite femora and tibiae connected with cords were used to simulate a knee. Four configurations of external fixation were tested: anterior femoral pins with monotube (XF1), anterolateral femoral pins with monotube (XF2), anterolateral femoral pins with two connecting rods (XF3), and hinged ring fixator (XF4). Six specimens of each configuration were loaded nondestructively in varus/valgus, anterior-to-posterior shear, flexion/extension, axial compression, internal/external torsion, and failure in varus.

RESULTS

XF2 was stiffer than XF1 in varus, valgus, and axial loading (P < 0.01) demonstrating that anterolateral pins provided greater stiffness than anterior femoral pins. XF3 was stiffer than XF2 in varus, valgus, and anterior-to-posterior shear (P < 0.002), indicating that two connecting rods provided greater stiffness than the monotube. XF4 was similar to the other configurations in anterior-to-posterior shear and torsion, indicating the hinged frame provided adequate stability. The average load to failure in varus mode was 250 N-m, which was far beyond the nondestructive loading of all specimens. There was no statistical difference between the different constructs in load to failure.

CONCLUSIONS

The stiffest construct for external fixation of a knee dislocation was achieved when pins were placed anterior lateral on the femur and two connecting rods were used. A stiffer construct may provide a better clinical outcome and we therefore recommend this frame configuration.

A biomechanical comparison of trochanteric nail proximal screw configurations in a subtrochanteric fracture model

OBJECTIVES

Historically, because of the magnitude of muscle forces exerted locally, as well as the commonly associated comminution, subtrochanteric fractures have been difficult to treat. Tencer et al found intramedullary nail fixation to be superior to lateral plate constructs in axial compression and combined bending. In addition, reconstruction-type intramedullary nails of more recent design have been shown to provide strength and stiffness superior to that supplied by the earlier antegrade intramedullary implants. A relatively new reconstruction nail, the DePuy VersaNail Troch Entry Nail (DePuy Orthopaedics, Inc., Warsaw, IN, USA), is unique in that it allows for two different proximal two-screw configurations: (1) the common parallel cephalomedullary arrangement and (2) a novel crossed-screw pattern. Our hypothesis was that the crossed-screw configuration would be as strong in axial loading as the cephalomedullary screw configuration.

METHODS

Twenty composite femurs were instrumented using the DePuy VersaNail Troch Entry Nail in a subtrochanteric fracture model: 10 with the crossed proximal screw configuration and 10 with the traditional parallel screw configuration. These constructs were first loaded axially to calculate their stiffness and then axially loaded to failure.

RESULTS

One specimen was rendered unusable for all calculations. Therefore, 19 constructs were evaluated: 9 parallel screw constructs, 10 crossed-screw constructs. The crossed-screw construct had a significantly higher stiffness than the parallel screw construct (347 +/- 73 N/mm and 261 +/- 42 N/mm, respectively; P = 0.01) and a significantly higher axial load to failure (2848 +/- 391 N vs. 2300 +/- 444 N; P = 0.01).

CONCLUSIONS

This study shows that axial failure loads of the crossed-screw configuration were greater than those of the parallel screw configuration. Clinically, this provides the surgeon more options for stabilizing a subtrochanteric femur fracture. This decision may be made intra-operatively if necessary, facilitating fracture fixation and providing a stable construct.

Biomechanical comparison of proximal locking plates and blade plates for the treatment of comminuted subtrochanteric femoral fractures

OBJECTIVES

The 95 degrees angled blade plate is an accepted standard for plating subtrochanteric femoral fractures but can be technically demanding and often requires extensive soft tissue exposure. Proximal femoral locking plates (PFLPs) have been developed for subtrochanteric and pertrochanteric fractures and are potentially easier to apply with less soft tissue dissection. Clinical experience has raised concerns regarding the strength of the PFLP. The purpose of our study was to compare the relative stability of two designs of PFLP with the 95 degrees angled blade plate under loads simulating the first 3 months of progressive weight bearing after fracture fixation.

METHODS

A comminuted subtrochanteric femoral fracture model was created with a 2-cm gap below the lesser trochanter in 15 synthetic femora. Fracture fixation of three plates (95 degrees angled blade plate [blade plate], the original version of the PFLP [O-PFLP], and the newest version of the PFLP [N-PFLP]), all manufactured by Synthes, Inc., Paoli, PA, was tested under progressive cyclic loading to reproduce progressive weight bearing during 3 months after fracture fixation. The force and number of cycles to reach 5 mm of displacement of the femoral head or failure of the implant were compared for each implant.

RESULTS

N-PFLPs were significantly stiffer than blade plates and O-PFLPs (P = 0.01) and had a trend toward withstanding more cycles before failure (P = 0.06). All five O-PFLPs demonstrated catastrophic fatigue failure before completion of the protocol. One each of the blade plates and the N-PFLPs failed to complete the protocol (P = 0.04).

CONCLUSIONS

In the model studied, N-PFLPs were shown to have biomechanical properties that were at least equivalent to those of the blade plate. The fatigue failures of O-PFLPs mirrored our clinical experience. Use of the N-PFLP might be a viable alternative fixation method for comminuted subtrochanteric femoral fractures that currently are treated with blade plates.

Biomechanical analysis of retrograde intramedullary nail fixation in distal femoral fractures

This study employed both mechanical testing and finite element analysis to compare the stiffness variations among different intramedullary nail constructs used in the treatment of distal femoral fractures. Compressive and torsional experiments were conducted on a transversely, as well as an obliquely fractured sawbone femur restored with the retrograde intramedullary nail. Corresponding finite element models were established to evaluate the stress distributions around screw holes. The results showed that a perifracture screw could increase stiffness by 40% for the obliquely fractured femur, but that it played an insignificant role in stiffness improvement for the transverse fracture groups. Moreover, compared to proximal-screw fixation, distal-screw fixation could improve construct stiffness by 20%. The absence of one of the two distal screws would increase the screw-hole stress by 70%. Therefore, the distal screw around the metaphyseal region has a more important stabilizing effect in the femur-nail construct than does the proximal screw. A twisting stress pattern occurs on the unused screw holes of the metaphyseal region and induces a higher risk for fatigue fracture. The locking screw at the fracture site would be most effective only if it passed through the fracture gap to integrate the separated femoral pieces.

Biomechanical comparison of a 2 and 3 proximal screw-configured antegrade piriformis intramedullary nail with a trochanteric reconstruction nail in an unstable subtrochanteric fracture model

OBJECTIVE

The objective of this study was to test the stiffness and ultimate load to failure of new intramedullary (IM) nail proximal screw configurations as compared to a trochanteric reconstruction nail.

METHODS

Twenty-one synthetic composite femurs were mounted on a Material Testing System and tested in axial compression 5 times. The femurs had an 1 of 2 IM nail types inserted with 1 of 3 proximal screw configurations: a 3-screw configuration with 2 transverse screws and a screw angled into the femoral neck; a 2-screw design with a single transverse screw and a single screw angled into the femoral neck; 2 parallel screws angled into the femoral neck. There were 7 specimens in each group. An unstable fracture (OTA/AO 32-C3.2) was created. and the stiffness of these constructs was tested in compression 5 times. Each construct was then loaded to failure in compression.

RESULTS

The 3-screw construct provided more axial stiffness (214 N/mm +/- 75) than either the 2-screw construct (123 N/mm +/- 32) or the trochanteric reconstruction nail (127 N/mm +/- 21) (P = 0.017 and 0.035 for 3-screw vs. 2-screw and recon respectively, P = 0.45 for 2-screw vs. recon). Load-to-failure testing demonstrated similarity among the different screw configurations (3-screw = 2230 N +/- 265, 2-screw = 2283 N +/- 260, Reconstruction nail = 2121 N +/- 156) (P = 1.0 all groups).

CONCLUSIONS

The proximal 3-screw configuration provided more stiffness than either the 2-screw configuration or trochanteric reconstruction nail. The 2-screw configuration performed equally to a standard trochanteric reconstruction nail in stiffness testing. The ultimate loads to failure for the 3 tested constructs were not significantly different.

Biomechanical evaluation of retrograde intramedullary stabilization for femoral fractures: the effect of fracture level

BACKGROUND

Retrograde stabilization of mid-diaphysis adolescent femur fractures has shown excellent biomechanical stability. However, it is unclear whether adequate stability is maintained for distal femur fractures using the retrograde approach compared with the clinically recommended antegrade approach. The purpose of this study was to evaluate the biomechanical stability of retrograde and antegrade nailing for mid-diaphyseal and distal diaphysis femoral fractures.

METHODS

Twenty adolescent-sized synthetic femurs were randomly assigned to fracture location and surgical approach groups. Comminuted fractures were simulated at the mid-diaphysial level and 4 cm proximal to the distal physis. The retrograde approach used 2 c-shaped 3.5-mm titanium nails. The antegrade used c and s 3.5-mm nail configurations. Both techniques achieved maximum nail divergence at the level of the fracture. Biomechanical testing was conducted to determine differences in torsional range of motion (degrees)and failure load (N) at 5 mm. These data were analyzed with a 2-way analysis of variance (p < 0.05).

RESULTS

In torsion, there were no differences related to surgical approach or fracture level. For axial compression to 5 mm, the antegrade approach required significantly greater force to achieve 5 mm of compression compared with the retrograde approach. The mid-diaphyseal fracture required significantly greater force to achieve 5 mm of compression compared with the distal diaphysis group.

CONCLUSIONS

For maximum stabilization of a distal femur fracture, c- and s-shaped nails placed in the antegrade position is suggested.

CLINICAL RELEVANCE

Surgical decision making regarding the use of either the antegrade or retrograde approach will be influenced by both the stability provided (antegrade) and the ease of insertion (retrograde).

Fixation strength comparison of onlay and inset patellar implants

Patellar implant fixation continues to be one of the most troublesome areas in total knee arthroplasty (TKA). It has been reported that patellofemoral complications in TKA are responsible for almost half of all re-operations. The literature review revealed the rate of primary all-polyethylene patellar implant loosening ranging 1%-4.2% [Berend ME, Ritter MA, Keating EM, Faris PM, Crites BM. The failure of all-polyethylene patellar components in total knee replacement. Clin Orthop 2001;388:105-11, Chew JT, Stewart NJ, Hanssen AD, Luo ZP, Rand JA, An KN. Differences in patellar tracking and knee kinematics among three different total knee designs. Clin Orthop 1997;345:87-98, Barrack RL, Wolfe MW, Waldman DA, et al. Patellar resurfacing in total knee arthroplasty: a five to seven year follow-up of prospective, randomized, double-blind study. Proceedings of Sixty-Seventh Annual Meeting of the American Academy of Orthopaedic Surgeons 2000. p. 547]. The loosening rates for metal-backed or following patellar component revisions were considerably higher [Chew JT, Stewart NJ, Hanssen AD, Luo ZP, Rand JA, An KN. Differences in patellar tracking and knee kinematics among three different total knee designs. Clin Orthop 1997;345:87-98, Jordan LR, Sorrells RB, Jordan LC, Olivo JL. The long-term results of a metal-backed mobile bearing patella. Clin Orthop 2005;436:111-8, Berger RA, Lyon, JH, Jacobs JJ, Barden RM, Berkson EM, Sheinkop MB, et al. Problems with cementless total knee arthroplasty at 11 years followup. Clin Orthop 2001;392:196-207, Ritter MA, Pierce MJ, Zhou H, Meding JB, Faris PM, Keating EM. Patellar complications (total knee arthroplasty). Effect of lateral release and thickness. Clin Orthop 1999;367:149-57] Onlay and inset patellar components with variable fixation surface geometry are currently available for clinical use. The purpose of this study was to quantify the shear disassociation strength for both onlay and inset patellar fixation techniques. The variation in host material was minimized by the use of synthetic patellae, which has been previously validated in implant fixation studies. The testing revealed that inset patellar fixation resistance to shear disassociation was 25% higher than onlay patellae (p=0.0002).

Fatigue performance of composite analogue femur constructs under high activity loading

Synthetic mechanical analogue bone models are valuable tools for consistent analysis of implant performance in both equilibrium and fatigue biomechanical testing. Use of these models has previously been limited by the poor fatigue performance when tested under realistic service loads. An objective was to determine whether a new analogue bone model (Fourth-Generation) using enhanced analogue cortical bone provides significantly improved resistance to high load fracture and fatigue as compared to the current (Third-Generation) bone models in clinically relevant in situ type testing of total hip implants. Six Third-Generation and six Fourth-Generation mechanical analogue proximal femur models were implanted with a cemented mock hip arthroplasty. Each specimen was loaded at 5 Hz in simulated one-legged stance under load control with a maximum compressive load of 2670 N and load ratio of 0.1. Average complete structural failure in Third-Generation femurs occurred at 3.16 million cycles; all specimens exhibited substantial displacement and crazing at well below 3 million cycles. In contrast, all Fourth-Generation femurs sustained 10 million cycles without complete structural failure and showed little change in actuator deflection. The Fourth-Generation femur model performance was sufficient to allow the model to be used in biomechanically relevant load bearing levels with an intramedullary device without model compromise that would affect test results.

Finite Element and Experimental Cortex Strains of the Intact and Implanted Tibia

Finite Element (FE) models for the simulation of intact and implanted bone find their main purpose in accurately reproducing the associated mechanical behavior. FE models can be used for preclinical testing of joint replacement implants, where some biomechanical aspects are difficult, if not possible, to simulate and investigate in vitro. To predict mechanical failure or damage, the models should accurately predict stresses and strains. Commercially available synthetic femur models have been extensively used to validate finite element models, but despite the vast literature available on the characteristics of synthetic tibia, numerical and experimental validation of the intact and implant assemblies of tibia are very limited or lacking. In the current study, four FE models of synthetic tibia, intact and reconstructed, were compared against experimental bone strain data, and an overall agreement within 10% between experimental and FE strains was obtained. Finite element and experimental (strain gauge) models of intact and implanted synthetic tibia were validated based on the comparison of cortex bone strains. The study also includes the analysis carried out on standard tibial components with cemented and noncemented stems of the P.F.C Sigma Modular Knee System. The overall agreement within 10% previously established was achieved, indicating that FE models could be successfully validated. The obtained results include a statistical analysis where the root-mean-square-error values were always <10%. FE models can successfully reproduce bone strains under most relevant acting loads upon the condylar surface of the tibia. Moreover, FE models, once properly validated, can be used for preclinical testing of tibial knee replacement, including misalignment of the implants in the proximal tibia after surgery, simulation of long-term failure according to the damage accumulation failure scenario, and other related biomechanical aspects.

Application of Motor Learning Principles to Complex Surgical Tasks: Searching for the Optimal Practice Schedule

Practice of complex tasks can be scheduled in several ways: as whole-task practice or as practice of the individual skills composing the task in either a blocked or a random order. The authors used those 3 schedules to study 18 participants' learning of an orthopedic surgical task. They assessed learning by obtaining expert evaluation of performance and objective kinematic measures before, immediately after, and 1 week after practice (transfer test). During acquisition, the blocked group showed superior performance for simple skills but not for more complex skills. For the expert-based measures of performance, all groups improved from pretest to posttest and remained constant from posttest to transfer. Measures of the final product showed that the whole-practice group's outcomes were significantly better than those of the random group on transfer. All groups showed better efficiency of motions in the posttest than in the pretest. Those measures were also poorer on the transfer test than on the posttest. The present evidence does not support the contextual interference effect--hypothetically, because of the inherent cognitive effort effect associated with some of the component skills. The authors recommend that surgical tasks composed of several discrete skills be practiced as a whole. The results of this study demonstrate the importance of critically appraising basic theories in applied environments.

Experimental validation of intact and implanted distal femur finite element models

Four finite element (FE) models of intact and distal femur of knee replacements were validated relative to measured bone strains. FE models of linear tetrahedrons were used. Femoral replacements with cemented stemless, cemented and noncemented femoral stems of the PFC Sigma Modular Knee System were analyzed. Bone strains were recorded at ten locations on the cortex. The magnitude of the FE bone strains corresponded to the mean measured strains, with an overall agreement of 10%. Linear regression between the FE and mean experimental strains produced slopes between 0.94 and 1.06 and R(2) values between 0.92 and 0.99. RSME values were less than 12%. The FE models were able to adequately replicate the mechanical behavior of distal femur reconstructions.

Biomechanical comparison of flexible stainless steel and titanium nails with external fixation using a femur fracture model

There are several options available for surgical stabilization of pediatric femoral shaft fractures. The purpose of this study was to compare the stability afforded by Ender stainless steel nails, titanium elastic nails, and one-plane unilateral external fixators for the fixation using a synthetic adolescent midshaft femur fracture model. The anterior-posterior (sagittal plane) bending, lateral (coronal plane) bending, torsional, and axial stiffness values were calculated using 6 different fixation configurations. These included pairs of 3.5-mm-diameter Ender nails with and without distal locking, 3.5- and 4.0-mm-diameter titanium elastic nails as well as single- and double-stacked monolateral external fixators. Eight synthetic femur models, 4 each with simulated transverse and comminuted fracture patterns, were sequentially tested for stability afforded by the various fracture fixation configurations. External fixation exhibited significantly greater control of anterior-posterior angulation compared with all flexible-nailing systems. Although Ender nails were slightly superior to titanium nails in control of sagittal plane angulation, this was not statistically significant. Compared with the external fixation constructs, all 4 flexible nail constructs demonstrated higher torsional stability. For prevention of axial shortening, all fixation methods were similar for the transverse fracture pattern, whereas external fixation was superior to flexible nails in the comminuted fracture model. No significant benefit was demonstrated with double stacking of external fixators. These findings may help guide clinicians choose the optimal fixation method for treatment of pediatric femoral shaft fractures.

The influence of practice schedules in the learning of a complex bone-plating surgical task

BACKGROUND

Practicing surgical tasks on bench models can be arranged in 3 ways: as the entire task, or as individual skills practiced separately in blocked or random order. The issue of the optimal practice schedule for the acquisition of surgical tasks is critical for enhancing training programs.

METHOD

An orthopedic bone-plating task was practiced as a whole, or in parts in either a random or a blocked order. Learning was assessed on global ratings, checklists, and final product analysis before, immediately after, and an hour after practice.

RESULTS

Checklists, and final product analysis, but not the global ratings showed that practicing the entire task resulted in the most learning, followed by the random practice. Practice of the skills in a blocked order yielded the least amount of learning.

CONCLUSIONS

It is recommended that surgical tasks composed of several discrete skills should be practiced as a whole. However, if part practice is necessary, these skills should be arranged in random order to optimize learning.

Biomechanical comparison of stainless steel and titanium nails for fixation of simulated femoral fractures

Flexible intramedullary nails are commonly used to treat femoral fractures in children. This study evaluated the biomechanical differences between stainless steel and titanium nails when securing transverse and comminuted fractures in a synthetic femur model. Retrograde flexible stainless steel and titanium nails placed in a divergent "C" pattern were mechanically tested, and axial rotation and compression stiffness were analyzed with a two-way ANOVA. Rotational stability was significantly greater for titanium nails than stainless steel nails for both fracture patterns. Axial compression stiffness was significantly greater for titanium nails than stainless steel nails for both fracture patterns. There was no statistical difference between materials for axial "failure" load that produced 5 mm of shortening. Titanium intramedullary nails were more stable than stainless steel nails in torsion and axial compression. Both materials stabilized simulated fractures at levels beyond physiologic non-weight-bearing loads without permanent deformation.

Subtrochanteric fixation stability depends on discrete fracture surface points

Fixation of subtrochanteric femur fractures may present complications, including malunion, delayed union, or nonunion, and is thought to be related to early fracture stability. To examine the initial stability of subtrochanteric fracture fixation, we investigated construct stiffness, interfragmentary gaps, and overall and point-wise interfragmentary motion (ie, axial and shear displacements) in synthetic composite femurs fixed with a cephalomedullary nail or condylar blade plate. Simulated stable and unstable subtrochanteric femur fractures were created in composite femurs, anatomically reduced, fixed with either a long Gamma nail or a blade plate, and subjected to combined axial, bending, and torsional loading. The long Gamma nail group consistently showed greater displacement magnitudes than the blade plate group; these differences included axial and shear displacement magnitudes in the stable fracture group and shear displacement magnitudes in the unstable fracture group. Overall differences in fixation stability were dependent on discrete points around the periphery of the contiguous fracture surfaces, especially in the unstable fracture group. These differences in interfragmentary motion patterns between implant constructs were detected despite the lack of difference in combined axial, bending, and torsional construct stiffness or initial interfragmentary gap.

Biomechanical analysis of antegrade and retrograde flexible intramedullary nail fixation of pediatric femoral fractures using a synthetic bone model

Biomechanical testing was performed to evaluate the stability of simulated transverse and comminuted femoral fractures after retrograde and antegrade flexible titanium intramedullary nail fixation. Ten synthetic adolescent-sized femoral bone models were used. Five underwent retrograde fixation with two C-shaped nails inserted from medial and lateral entry portals. The other five underwent antegrade fixation using one C- and one S-shaped nail through lateral entry holes just inferior to the greater trochanter. Retrograde nail fixation demonstrated significantly less axial range of motion and greater torsional stiffness than antegrade fixation in both transverse and comminuted fracture patterns. However, there appeared to be a biomechanical trend of greater resistance to shortening for antegrade nails.

Experimental investigation of bone remodelling using composite femurs

OBJECTIVE

To determine the load transfer patterns of femurs in the intact, immediate post-operative and long-term (remodelled) post-operative implanted conditions for Lubinus SPII and Müller-Curved cemented hip prostheses, and to examine to what extent remodelling may influence the long-term outcome.

DESIGN

Experimental and finite element (FE) methods were applied to composite femurs under loads representing the heel-strike phase of gait, determining cortical bone and cement strains for the different femur conditions.

BACKGROUND

The authors previously developed protocols to measure bone and cement strains, and to produce remodelled femur specimens, yet these have not been applied together to compare strain patterns of different femur conditions. The Lubinus SPII is clinically more successful than the Müller-Curved stem, with failure mainly due to aseptic loosening.

METHODS

Cortical bone strains were determined in intact femurs. Six femurs each were implanted with the two stem types and cortical bone and cement strains were measured. Bone remodelling was recreated using a validated CAD-CAM procedure to remove a layer of proximal cortical bone, replicating a typical scenario found in stable clinical retrievals. Strains were remeasured. FE methods were used to compliment the experiments.

RESULTS

Stress shielding was reduced with remodelling, though bone strains did not return to their intact values, particularly around the calcar. Cement strains increased with remodelling. Differences occurred between the two stems; the Müller-Curved produced a more severe strain transition.

CONCLUSIONS

Procedures were successfully combined together to investigate in vitro the performance of two cemented stems, in immediate and long-term post-operative conditions. The increase of cement strains with remodelling is a potential indicator for in vivo cement failure.

RELEVANCE

The consequences of femoral bone remodelling on the long-term success of joint replacements are not well understood, where remodelling may lead to increased bone and cement stresses.

A proximal femoral implant preserves physiological bone deformation: a biomechanical investigation in cadaveric bones

The aim of this study was to compare the perturbances in bone deformation patterns of the proximal femur due to a conventional cemented femoral stem and a novel uncemented implant designed on the principles of osseointegration. Five matched pairs of fresh frozen human femora were mechanically tested. Bone deformation patterns, measured with a video digitizing system under 1.5 kN joint force, showed that the cemented Spectron femoral implant caused significant alterations to the proximal femoral deformation pattern, whereas the Gothenburg osseointegrated titanium femoral implant did not significantly alter the bone behaviour (p < 0.05). Vertical micromotions measured under 1 kN after 1000 cycles were within the threshold of movement tolerable for bone ingrowth (21 microm for the Gothenburg system and 26 microm for the cemented implant).

Second generation intramedullary nailing of subtrochanteric femur fractures: a biomechanical study of fracture site motion

OBJECTIVES

To compare fracture site motion between different second-generation intramedullary nails used to fix subtrochanteric fractures of the proximal femur with and without femoral neck fractures.

DESIGN

Nondestructive mechanical testing of four types of femoral intramedullary nails was undertaken to evaluate fracture site motion using a model that simulated single-leg and double-leg stance.

METHODS

Three types of reconstruction nails (the Russell-Taylor Delta [Smith & Nephew, Memphis, TN], the Uniflex [Biomet, Warsaw, IN], Alta CFX [Howmedica-Osteonics, Rutherford, NJ]) and the Long Gamma nail (Howmedica-Osteonics, Rutherford, NJ), each measuring 11 x 380 mm, were inserted in fiberglass composite femurs. Four fracture patterns were studied (transverse subtrochanteric, subtrochanteric with posteromedial wedge comminution, subtrochanteric with one-centimeter gap, and a one-centimeter gap with a subcapital neck fracture). Single-and double-leg stance loading was simulated using a servohydraulic load frame (MTS, Eden Prairie, MN). Two-way analysis of variance and post hoc t tests were used to determine any statistically significant differences between groups.

RESULTS

In single-leg stance there were significant differences in coronal plane rotation, shear, and axial translation across the subtrochanteric fracture site between the different nail types and the different fracture patterns (p < 0.001). In double-leg stance there were significant differences in coronal plane rotation and femoral head vertical motion between the different nail types and the different fracture patterns (p < 0.001), and there were significant differences in shear and axial translation between the different fracture patterns (p < 0.001) but not the different nail types (p > 0.05).

CONCLUSIONS

For simple, well-reduced fractures the choice of implant is not critical. As fracture severity increased (comminution, gap, and combined neck fracture), the choice of implant, particularly with reference to proximal nail dimensions and implant materials, was a significant factor in reducing fracture site motion. Therefore, our laboratory data suggest that when subtrochanteric fractures are unstable (e.g., comminution, segmental bone loss) and early weight bearing is desirable, the choice of implant is critical and should be restricted to implants that allow minimal fracture site motion (Long Gamma and Russell-Taylor).

An implantable strain measurement system designed to detect spine fusion: preliminary results from a biomechanical in vivo study

STUDY DESIGN

The strain distribution on the thoracic vertebrae during anteroposterior bending and torsion was examined for use with an implantable strain gauge system and miniature radio transmitter, which also were evaluated.

OBJECTIVES

To identify strain gauge placement sites by testing cadaver spines in vivo, and to evaluate an implantable gauge bonding technique and subminiature radio transmitter for accurate strain monitoring.

SUMMARY OF BACKGROUND DATA

Fusion is determined currently through the use of radiographic techniques. Discrepancies exist between radiographic evidence and more direct measurements of fusion such as operative exploration4,5,12 and biomechanical or histologic measurements.12,15 To facilitate the return of patients to full unrestricted activity, it would be useful to develop a technique for accurate in vivo determination of fusion.

METHODS

Three cadaver spines were tested during anteroposterior bending and torsional loading in the control, instrumented, and instrumented plus polymethylmethacrylate states. The spines were instrumented with an ISOLA(R) (Acromed Corporation, Cleveland, Ohio) construct, and a simulated fusion was achieved through the application of polymethylmethacrylate. Strain gauges were attached in uniaxial, biaxial, and rosette configurations. The principal strains were calculated. Calcium phosphate ceramic-coated gauges were implanted in patients and recovered after up to 15 months in vivo. A radio transmitter was developed and tested for use in patients.

RESULTS

The largest and most consistent strain changes after simulated fusion were recorded during torsional loading on the laminae of a vertebra directly underneath a hook. Calcium phosphate ceramic-coated strain gauges showed excellent bone bonding to the lamina when fusion occurred. Radio telemetry accurately tracked strain magnitudes and strain rates expected in patients.

CONCLUSIONS

The consistency obtained in torsional loading indicates that this type of loading will provide the most useful data from patients in vivo. Excellent bonebonding and accurate strain transmission using a long-term strain measurement system and miniature radio transmitter indicate that strains collected from patients with this system will be accurate.

A CAD-CAM methodology to produce bone-remodelled composite femurs for preclinical investigations

Femoral bone remodelling, following total hip arthroplasty, is a clinically observed phenomenon attributed to the changed stress environment of the postoperative implanted hip. While this process cannot be avoided, there is concern as to its consequences on the long-term survival of hip joint replacements. Previous methods of studying remodelling, such as clinical or animal-based studies, or finite element analyses, have their limitations. The aim of this study is to develop experimental specimens incorporating bone resorption features typical of clinically successful implants. This work describes the use of computer aided design/manufacturing methods (CAD-CAM) to produce these specimens, based on modifying commercially available composite femurs. The procedures are investigated and verified for two different designs of cemented prostheses (Lubinus SPII and Muller Curved). Quantitative clinical data is used to define the remodelled geometry of a CAD model of the femur for each stem design. Composite femur specimens are machined using a three-axis milling machine, where each specimen can be accurately positioned using a custom-designed jig and a digitizer system. The accuracy of the process is assessed by analysing the deviation of the digitized premachined and postmachined surfaces of each specimen in relation to the CAD model. The results demonstrate that the procedure can be used for developing in vitro specimens with bone resorption features. These specimens are proposed as a useful tool for performing preclinical trials, such as load transfer or longevity/stability testing, with the advantage of modelling a long-term clinical situation, rather than solely analysing implanted femurs in an immediate postoperative state.

Mechanical comparison of a distal femoral side plate and a retrograde intramedullary nail

OBJECTIVE

To compare quantitatively the axial and torsional stiffness of a retrograde intramedullary nail and a fixed angle screw side plate in treating a supracondylar femur fracture in osteopenic femora. To determine the modes of failure of an intramedullary nail and a side plate under axial loading.

DESIGN

Matched pair cadaveric study.

SETTING

Orthopaedic biomechanics laboratory.

PATIENTS AND OTHER PARTICIPANTS

Eleven matched pairs of preserved human femora were selected. The cadaveric specimens were harvested from relatively elderly donors with an average age of 75.6 years, which represents the principal population at risk for poor fracture fixation.

INTERVENTION

The eleven matched pairs were osteotomized to simulate segmental structural defects in the supracondylar region. One femur of each matched pair was fixed with an intramedullary nail, and the contralateral femur was fixed with a side plate.

MAIN OUTCOME MEASURES

Axial and torsional stiffness values. Axial modes of failure.

RESULTS

The intramedullary nail axial stiffness was 14 percent (p = 0.04) less and torsional stiffness was 17 percent (p = 0.05) less than that provided by the side plate. The axial failure of the intramedullary nail occurred distally, allowing the hardware to protrude into the articular space. The side plate also failed distally by displacing the condylar screw into a varus angulation.

CONCLUSION

The mechanical advantages favor the use of the side plate if fixation stiffness is essential. The axial mode of failure occurs distally for both fixation devices.

Femoral surface strain in intact composite femurs: a custom computer analysis of the photoelastic coating technique

Understanding how forces are distributed through the proximal femur has many clinical applications for surgeons, researchers, and prosthetic designers. A new system for two-dimensional analysis of femoral surface strain was developed and applied to intact composite femurs. The photoelastic coating method was used to resolve the surface strain under axial loading, and strain analysis was performed using digital imaging of the strain patterns and original computer programs. The technique provides qualitative and quantitative data that describes overall femoral surface strains more completely than previous point analysis and strain gauge techniques. Results from repeated testing found the photoelastic process, computer imaging and computer analysis of strain areas to be statistically repeatable.

Biomechanical study of the resurfacing hip arthroplasty: finite element analysis of the femoral component

Finite element analysis was performed using 3-dimensional models to examine the biomechanical characteristics of the femoral component in resurfacing hip arthroplasty. Stress concentration was observed in the cortical bone adjacent to the rim of the prosthesis. Stress shielding was observed in the anterosuperior regions on the cancellous bone cross-sections near the cup rim. These biomechanical characteristics may lead to complications such as femoral neck fractures in patients with osteopenic bone and long-term loosening.

Design and validation of a machine for reproducible precision insertion of femoral hip prostheses for preclinical testing

Preclinical testing of orthopaedic implants is becoming increasingly important to eliminate inferior designs before animal experiments or clinical trials are begun. Preclinical tests can include both laboratory bench tests and computational modeling. One problem with bench tests is that variability in prosthesis insertion can significantly influence the failure rate; this makes comparison of prostheses more difficult. To solve this problem an insertion method is required that is both accurate and reproducible. In this work, a general approach to the insertion of hip prostheses into femoral bones is proposed based on physically replicating an insertion path determined using computer animation. As a first step, the seated prosthesis position is determined from templates and femur radiographs. Three-dimensional images of the prosthesis and bone are then imported into computer animation software and an insertion path in the coronal plane is determined. The insertion path is used to determine the profile of a cam. By attaching the prosthesis to a carriage, which is pneumatically moved along this cam, the required insertion motion of the prosthesis in the coronal plane can be achieved. This paper describes the design and validation of the insertion machine. For the validation study, a nonsymmetric hip prosthesis design (Lubinus SPII, Waldemar Link, Germany) is used. It is shown that the insertion machine has sufficient accuracy and reproducibility for preclinical mechanical testing.

An experimental method for the application of lateral muscle loading and its effect on femoral strain distributions

Experimental models that have been used to evaluate hip loading and the effect of hip implants on bone often use only a head load and abductor load. Anatomic considerations and in vivo measurements have lead several investigators to suggest that these models are inaccurate because they do not incorporate the loads imposed by additional muscles. The aim of this study was to evaluate the strains in the proximal and mid diaphysis of the femur for five hip loading models, one with a head load and abductor load only and four which incorporated lateral muscle loads as well. Head load to body weight load ratios were used to evaluate the physiologic accuracy of these models and strains were compared to determine the extent of strain changes as a function of model complexity. All models which incorporated additional lateral muscle loads more accurately simulated head load to body-weight load ratios than the simple abductor-only model. The model which incorporated a coupled vastus lateralis and iliotibial band load in addition to the abductor load provided the simplest configuration with a reasonable body-weight to head-load ratio.

Mechanical validation of whole bone composite tibia models

Composite synthetic models of the human tibia have recently become commercially available as substitutes for cadaveric specimens. Their use is justified by the advantages they offer as a substitute for real tibias. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human specimens for different loading conditions. The stiffness of the tibias was measured with a torsional load applied along the long axis, and with a bending load applied both in the latero-medial and in the antero-posterior direction. The bending stiffness of the composite tibias matched well with that of the cadaveric specimens. This was not true for the torsional stiffness. In fact, the composite tibias were much stiffer than the cadaveric specimens, possibly due to the structure of the reinforcement material. The inter-specimen variability for the composite tibias was much lower than that for the cadaveric specimens. Thus, it seems that the composite tibias are suitable to replace cadaveric specimens for certain types of test, whereas they might be unsuitable for others, depending on the loading regimen.

"Biological" internal fixation of long bone fractures: a biomechanical study of a "noncontact" plate system

Based on existing knowledge of noncontact plates, an experimental prototype of a nonperiosteal contact internal fixation implant ("noncontact internal fixator") has been designed. The construct rigidity of osteotomised synthetic composite femora, fixed with the noncontact fixator and a reamed, statically-locked intramedullary nail were compared in axial compression, two plane bending and torsion in four types of diaphyseal fractures. With the exception of axial loading in the presence of extensive comminution, the fixation stability provided by the noncontact fixator is significantly higher than that of the tested intramedullary nail. Any degree of cortical contact between the two main fragments is important for the stability of this nonperiosteal contact fixation system under axial load. Appropriately-designed "internal fixators" could provide not only a number of biological and technical advantages, but also fixation stability comparable and in certain aspects superior to that of other fixation methods.

A mechanical comparison of subtrochanteric femur fracture fixation

OBJECTIVE

To determine whether the mechanical properties of first-generation interlocking femoral nails are different from those of second-generation interlocking femoral nails in a subtrochanteric femur fracture model.

DESIGN

Randomized laboratory investigation using a synthetic subtrochanteric femur fracture model.

SETTING

Simulated stable and unstable fractures were created at three levels in the subtrochanteric region of synthetic femora. Instrumented specimens were tested elastically in a biomaterials testing system.

INTERVENTION

Synthetic femora were instrumented with either a statically locked first-generation femoral nail or a statically locked second-generation femoral nail.

MAIN OUTCOME MEASUREMENTS

Elastic stiffness for both the stable and unstable fracture groups was measured in both compression and torsion. Unstable fracture specimens were tested to failure in compression, and load to failure was measured.

RESULTS

Throughout the subtrochanteric region, second-generation femoral nail constructs were consistently stiffer in compression and torsion than were statically locked first-generation femoral nail constructs. In general, second-generation constructs also withstood larger loads to failure in the unstable fracture model.

CONCLUSIONS

Second-generation nails provided significantly enhanced mechanical stiffness compared with first-generation femoral nails when used to treat both stable and unstable subtrochanteric femur fractures. Although these results were obtained by using a well-controlled, mechanically consistent model, clinical validation of an increased incidence of fracture unions or of decreased time to union is required before we can recommend that second-generation nails be used routinely to treat subtrochantenic femur fractures.

Cement-mantle thickness affects cement strains in total hip replacement

Bone cement is commonly used to affix femoral implants to the bone during total hip reconstruction. Previous studies suggest that the expected life of a cemented femoral implant may depend on the thickness of the cement mantle surrounding the implant and the implant geometry. The purpose of this study was to determine whether different cement-mantle thicknesses and femoral stem sizes affected strain patterns in the bone cement around cemented femoral stems. Two different sizes of cobalt-chromium stems were cemented into composite femora with varying cement-mantle thickness. Strain gages were embedded in the cement mantle and the implanted stems were loaded axially and under conditions simulating walking and standing. An increase in stem size with the same cement-mantle thickness (approximately 2.2 mm) caused a 65% decrease in proximal medial cement strains. Increasing cement mantle thickness from 2.4 to 3.7 mm caused substantial strain reductions in the distal cement (40-49%). We conclude that increased cement-mantle thickness around femoral stems may increase the fatigue life of a bone-implant system by reducing peak strains within the cement.

Modified transverse locking nail fixation of proximal femoral fractures

It was hypothesized that transverse locking screws of intramedullary nails, seated above the lesser trochanter, provide equal strength to that of reconstruction nails, and that screws placed through the medial cortex of the femoral neck do not have adverse biomechanical effects during physiologic loading. Synthetic femurs (n = 10) and paired anatomic specimen femurs (n = 14) were tested intact and with an intramedullary device in place. Intact specimens were loaded nondestructively, then a segmental subtrochanteric defect was created and either a high seated transverse locking nail or a reconstruction nail was inserted and statistically locked. Axial and torsional stiffness were determined followed by axial failure testing. Mechanical parameters evaluated were stiffness, displacement, and energy. The implanted specimens did not show any statistically significant difference between transverse or reconstruction screw constructs with any of the measured parameters (stiffness, displacement, and energy). Failure tests in implanted specimens also did not show any statistically significant difference in yield load, yield displacement, or energy to failure between implant constructs. All anatomic specimens failed, with fractures of the proximal fragment involving medial and lateral cortices. Synthetic specimens did not fracture but showed failure with implant deformation at the level of the skeletal defect. The use of high seated transverse locking nails for complex proximal femoral fractures is a viable option and has comparable in vitro mechanical performance with reconstruction nails. Although not shown to be a problem in the present study, clinical evaluation of screws through the medial femoral neck cortex is required.

Effect of superior and superolateral relocations of the hip center on hip joint forces. An experimental and analytical analysis

With the extensive use of uncemented acetabular components in total hip arthroplasty, relocation of the hip center has become increasingly necessary to avoid bulk grafts and to promote contact between the porous prosthetic surface and bone. Compared with the anatomic hip center, superolateral relocation theoretically results in higher hip joint forces and has been shown in cemented acetabular components to result in an increased clinical failure rate. This study experimentally and analytically compared the hip joint forces in normal, superior, and superolateral hip center locations during both single-leg stance and stairclimbing, performing this comparison over a wide range of hip joint applied flexion moments. An advanced loading fixture was designed to allow any applied moment to be set independently of femoral position, incorporating all three major muscle groups active in stairclimbing position: extensors, abductors, and adductors. For all positions and moments tested, it was found that superolateral relocation caused significant increases in the total hip joint force, but did not affect the nonsagittal force component. Also, superior-only hip center relocation did not significantly affect the total joint force magnitudes or directions. The force increase on hip center lateralization can be attributed to a corresponding increase in the adduction moment. Results from the static analytical model developed supported these findings. The results of this study suggest that superolateral hip center relocation should be avoided and that superior-only relocation may be mechanically acceptable within the confines of the osseous anatomy of the acetabulum.

Numerical and experimental stress analysis of a polymeric composite hip joint prosthesis

A comparative stress analysis of a polymeric composite hip joint replacement was performed. A prototype short carbon-fiber reinforced PEEK (CF/PEEK) prosthesis was manufactured by injection molding. Finite element (FE) analysis was conducted on intact femurs and femurs fitted with the CF/ PEEK and the titanium prostheses under various loading conditions. FE models were validated by experimental strain gauge measurements by using synthetic femurs. There was a good agreement between the two methods except in the hoop strain of the femur in the calcar region because of the assumption of the isotropic material properties. The stem stresses were lower for the CF/PEEK prosthesis than for the titanium prosthesis. The maximum stress was in the spigot of the CF/PEEK prosthesis, but in the middle third of the stem of the titanium prosthesis. Stress generated in the cement was almost equal for both prostheses although more load was transferred, via cement, to the femur with the CF/ PEEK prosthesis because the load transfer took place over a larger area. An out-of-plane component of the joint load causes higher prosthesis and cement stresses.

Mechanical validation of whole bone composite femur models

Composite synthetic models of the human femur have recently become commercially available as substitutes for cadaveric specimens. Their quick diffusion was justified by the advantages they offer as a substitute for real femurs. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human fresh-frozen and dried-rehydrated specimens for different loading conditions. First, the viscoelastic behaviour of the models was investigated under simulated single leg stance loading, showing that the little time dependent phenomena observed tend to extinguish within a few minutes of the load application. The behaviour under axial loading was then studied by comparing the vertical displacement of the head as well as the axial strains, by application of a parametric descriptive model of the strain distribution. Finally, a four point bending test and a torsional test were performed to characterize the whole bone stiffness of the femur. In all these tests, the composite femurs were shown to fall well within the range for cadaveric specimens, with no significant differences being detected between the synthetic femurs and the two groups of cadaveric femurs. Moreover, the interfemur variability for the composite femurs was 20-200 times lower than that for the cadaveric specimens, thus allowing smaller differences to be characterized as significant using the same simple size, if the composite femurs are employed.

A comparative biomechanical evaluation of a noncontacting plate and currently used devices for tibial fixation

The appearance of porous bone under fixation plates during fracture healing, attributed to disturbance of blood supply by the plate, has led to new plate designs with reduced plate to bone contact. The fixation stability afforded by these devices, in comparison to implants commonly used for fixation, is not well known. Therefore, the construct stiffnesses of osteotomized synthetic tibiae fixed with dynamic compression plates, external fixators, or two configurations of noncontact plates were compared in axial compression, bending, and torsion with and without cortical contact at the osteotomy site. The results of this study show that (1) the noncontact plated constructs achieve comparable fixation rigidity to constructs using dynamic compression plates or external fixators, if applied at a distance no greater than 5 mm from the surface of the tibia, and (2) the fixation rigidity of the noncontact plate decreases as the distance between plate and bone surface increases.

Tibiofemoral contact stress and stress distribution evaluation of total knee arthroplasties

The Fuji film (Itochu, Los Angeles, CA) area analysis technique demonstrates that a more accurate assessment of tibiofemoral contact stresses is possible when the film is used at 37 degrees C and at the upper end of its sensitivity range (in this case, a 2,000-N load). An AMK with a regular and Hylamer-M insert (DePuy, Warsaw, IN), an MG II (Zimmer, Warsaw, IN), an Omnifit (Osteonics, Allendale, NJ), an Ortholoc III (Dow Corning Wright, Midland, MI), a PCA II (Howmedica, Rutherford, NJ), and a PFC (Johnson & Johnson Orthopaedics, Raynham, MA) had average contact stresses that varied only 12% at 60 degrees flexion. At 0 degrees, 15 degrees and 60 degrees flexion, stresses ranged from 13 to 25 MPa. Contact area distribution ratios, which were smaller at 37 degrees C than at 24 degrees C, provide a quantitative means of grouping implants according to the shape of the tibiofemoral contact area. The Omnifit, MG II, PCA II, and PFC had small ratios (symmetric areas). The AMK and Ortholoc III had large ratios (asymmetric contact areas). If the impression is reflective of wear, it would be expected to be focal in knees with small ratios and contact areas, and uniform in knees with large ratios and contact areas, whereas large ratios and small areas would imply a linear wear pattern. Calibrated electrical resistance contact stress measurements indicated that the Fuji film measurements underestimated the magnitude of contact stresses. They also provided a means of quantifying the rate of area increase during initial loading of the knees, with the highest area increase noted for the knee with the roughest insert (Ortholoc III) and the lowest area increase for the knee with the smoothest insert (PCA II).

Influence of thigh muscles on the axial strains in a proximal femur during early stance in gait

This work is focused on the in vitro simulation of the loads occurring in the femur during early stance in gait, for hip prosthesis stress shielding test purposes. Ten thigh muscles (the three gluteal muscles, the three vasti, rectus femoris, adductor longus and magnus, biceps femoris), simulated by nylon straps, were tested in order to establish their influence on the strains in the proximal femur. Axial and hoop strains were recorded from 16 strain gauges for the effect of each muscle and compared to the strains recorded as a result of the hip joint reaction force only (i.e. without muscle simulation). It appears that the three glutei are the principal muscles in determining the vertical strains, however the rectus femoris, biceps femoris and the adductors were also seen to significantly affect the strain pattern. The inadequacy of increasing the adduction angle and applying the resultant force at the hip joint to simulate the abductors was also confirmed.

Initial stability of uncemented hip stems: an in-vitro protocol to measure torsional interface motion

The difficulty in quantitatively assessing the inherent variables of surgical stem insertion and interfemur differences continues to be a problem in experimental methodologies which assess hip stem stability. An in-vitro torsional stability protocol was developed which limited the mechanical testing variability and provided a reproducible micromotion measurement of an uncemented stem in synthetic composite femurs. Using a controlled mechanical stem insertion resulted in less interfemur variability within each group with the coefficient of variation being reduced from 35% overall to less than 20%. Femurs with shallow stem insertion depths had significantly larger micromotion than femurs having deep stem insertion depths. The sensitivity of the experimental protocol and the synthetic composite femurs to the varied functional behaviour of three different stem designs was demonstrated. The stem with a hollowed anterior-to-posterior proximal section experienced significantly more motion than the two stems with full proximal sections, reinforcing the need for proximal contact to ensure minimal micromotion in torsional loading.

Evaluation of experimental and finite element models of synthetic and cadaveric femora for pre-clinical design-analysis

The aim of this study was to determine the validity with which the finite element method could model synthetic bone and thereby determine the appropriateness of such femur analogues for application in pre-clinical tests. The performance of these synthetic femora was compared with cadaveric bone when employing the same geometric and material definition protocols. A four-point bend loading configuration was selected for this analysis. Four synthetic femurs and an embalmed cadaveric bone were tested experimentally to determine the structural bending stiffness (k) for the diaphysis of these bones. A finite element (FE) model was generated and an analysis performed for each bone type to estimate the Young's modulus (E) required to obtain a model stiffness equivalent to that obtained experimentally. The estimated material elastic modulus in the FE model for the synthetic femur was found to be very similar to available data for this bone analogue. The estimated cadaveric bone modulus however was found to differ significantly from documented values for cortical bone. A theoretical analysis demonstrated the great sensitivity of the estimated modulus value to the accuracy of the geometric definition. The very low variability found in the experimental test on the synthetic bones together with their more regular geometry and the possibility of achieving greater accuracy in geometric definition was shown to enable the production of a valid FE model of this bone for an isotropic homogeneous material description. Conversely, the greater irregularity of geometry, together with the less obvious differentiation between the cortical and cancellous bone in the cadaveric specimen makes accurate geometric description of this bone very difficult. This fact, together with the uncertainty concerning the quality of the cadaveric bone and its viscoelastic response during mechanical testing, makes reproduction of its behaviour in a FE model a much more demanding task. It is suggested that this greater capability of reproducing the experimental behaviour of the synthetic bone makes them a very useful model for both experimental and numerical studies which involve in-vitro pre-clinical testing of implant design and stem-bone behaviour.