Glen O. Njus

First Metatarsal-Phalangeal Joint Arthrodesis: A Biomechanical Assessment of Stability:

Background: First metatarsal phalangeal joint (MTP) arthrodesis is a commonly performed procedure for the treatment of hallux rigidus, severe and recurrent bunion deformities, rheumatoid arthritis and other less common disorders of the joint. There are different techniques of fixation of the joint to promote arthrodesis including oblique lag screw fixation, lag screw and dorsal plate fixation, crossed Kirschner wires, dorsal plate fixation alone and various types of external fixation. Ideally the fixation method should be reproducible, lead to a high rate of fusion, and have a low incidence of complications. Methods: In the present study, we compared the strength of fixation of five commonly utilized techniques of first MTP joint arthrodesis. These were: Surface excision with machined conical reaming and fixation with a 3.5 mm cortical interfragmentary lag screw. Surface excision with machined conical reaming and fixation with crossed 0.062 Kirschner wires. Surface excision with machined conical reaming and fixation with a 3.5 mm cortical lag screw and a four hole dorsal miniplate secured with 3.5 mm cortical screws. Surface excision with machined conical reaming and fixation with a four hole dorsal miniplate secured with 3.5 mm cortical screws and no lag screw. Planar surface excision and fixation with a single oblique 3.5 mm interfragmentary cortical lag screw. Testing was done on an Instron materials testing device loading the first MTP joint in dorsiflexion. Liquid metal strain gauges were placed over the joint and micromotion was detected with varying loads and cycles. Results: The most stable technique was the combination of machined conical reaming and an oblique interfragmentary lag screw and dorsal plate. This was greater than two times stronger than an oblique lag screw alone. Dorsal plate alone and Kirschner wire fixation were the weakest techniques. Conclusions: First MTP fusion is a commonly performed procedure for the treatment of a variety of disorders of the first MTP joint. The most stable technique for obtaining fusion in this study was the combination of an oblique lag screw and a dorsal plate. This should lead to higher rates of arthrodesis.

Distal Femoral Fixation : A Biomechanical Comparison of Trigen Retrograde Intramedullary (I.M.) Nail, Dynamic Condylar Screw (DCS), and Locking Compression Plate (LCP) Condylar Plate

BACKGROUND The purpose of this study was to establish if there are biomechanical differences between implants in stiffness of construct, microdisplacement, and fatigue failure in a supracondylar femoral fracture model. METHODS A retrograde intramedullary (i.m.) nail, dynamic condylar screw (DCS), and locked condylar plate (LCP) were tested using 33-cm long synthetic femurs. A standardized supracondylar medial segmental defect was created in the distal femur bone models. A gap away from the distal joint axis and parallel to the knee axis was created for axial testing of the specimens (Arbeitsgemeinschaft fur Osteosynthesefragen [AO] type 33-A) and a T-fracture (33-C) was created for the fatigue testing of the specimens. Peak displacements were measured, and analysis was done to determine construct stiffness and gap micromotion in axial loading. Cyclic loading was performed for fatigue testing. RESULTS It was observed that there were statistically significant differences in micromotion across the fracture gap and overall stiffness of various implant constructs. The stiffness of the i.m. nail, DCS, and LCP were 1,106, 750, and 625 N/mm, respectively. The average total micromotion across the fracture gap for the i.m. nail, DCS, and LCP were 1.96, 10.55, and 17.74 mm, respectively. In fatigue testing, the i.m. nail distal screws failed at 9,000 cycles, the DCS did not fail (80,000 cycles completed), and the LCP failed at 19,000 and 23,500 cycles. CONCLUSIONS When considering micromotion and construct stiffness, the i.m. nail had statistically significant higher stiffness and significantly lower micromotion across the fracture gap with axial compression. Hence, the i.m. nail tested had the greatest stability for type 33-A fractures. However, the nail demonstrated the least amount of resistance to fatigue failure with type 33-C fractures, whereas the DCS did not fail with testing in any pattern.

Material and structural characterization of human saphenous vein

Saphenous vein patch rupture after carotid endarterectomy is an infrequent but devastating complication. This study was undertaken to evaluate the material and structural properties of fresh human saphenous veins to understand the causes of this complication. Segments of saphenous veins were obtained from 22 patients from vein harvested during coronary artery bypass surgery. Ninety-three specimens, oriented in both circumferential (n = 45) and longitudinal (n = 48) directions, were prepared from the available vein segments for testing. Specimens were mounted on specially designed grips and then subjected to uniaxial tension testing. For each specimen the following material and structural parameters were determined: vessel diameter, tensile stiffness, failure and ultimate forces, and tensile modulus, failure stress, and strain. The physical properties of specimens evaluated in longitudinal orientations and thus limit the inherent strength of the vein. The physical properties of circumferentially tested vein specimens were negatively correlated to age, female gender, diabetes, and hypertension. The data obtained in this investigation suggest that age, hypertension, as well as diabetes and gender may adversely influence the circumferential tensile strength of human saphenous veins used as patch grafts.

Tibiotalocalcaneal Arthrodesis: A Biomechanical Assessment of Stability

Background: Combined ankle and subtalar (tibiotalocalcaneal) arthrodesis is a procedure that can be used to successfully treat disabling foot and ankle arthropathy and is a reasonable salvage alternative to amputation for the treatment of nonbraceable neuropathic, diabetic, degenerative, or rheumatoid joints. Although many methods of tibiotalocalcaneal (TTC) arthrodesis have been described in the literature, the most popular current methods involve the use of crossed cancellous bone screws, plates, or a locked retrograde intramedullary rod. Fusion in these patients can be difficult, with significant complications including infection, malunion, and nonunion. A persistent nonunion can lead to failure of the hardware and recurrent deformity. Methods: We biomechanically tested the stability and micromotion in four methods of TTC arthrodesis using liquid metal strain gauges and Instron (Norwood, MA) material testing systems. Anatomically identical synthetic bones with properties very similar to human bone were instrumented and tested. Four instrumentation techniques were tested: 1) three crossed 6.5-mm cancellous screws, 2) two crossed 6.5-mm cancellous screws, 3) locked retrograde intramedullary rod, and 4) locked retrograde intramedullary rod augmented with a single anteromedial bone staple. Six separate specimens for each technique were tested. Results: The three crossed cancellous screw technique provided the greatest stability with respect to micromotion (p < 0.05). The addition of a tibiotalar staple to the locked intramedullary rod conferred stability nearly equal to that of the three crossed cancellous screw fixation (p < 0.05). The locked intramedullary rod group and the two crossed cancellous screw group allowed significant micromotion at the arthrodesis sites, which was a full order of magnitude higher (p < 0.05) than in the three crossed cancellous screw group and the staple augmented intramedullary rod group. Conclusions: Biomechanically, a staple augmented locked intramedullary rod for TTC arthrodesis confers excellent stability nearly equal to the three crossed cancellous screw technique for TTC arthrodesis.

Processing Methodologies for Polycaprolactone-Hydroxyapatite Composites: A Review

ABSTRACT Biodegradable implants have shown great promise for the repair of bone defects and have been commonly used as bone substitutes, which traditionally would be treated using metallic implants. The need for a second surgery exacerbated by the stress shielding effect caused by an implant has led researchers to consider more effective, synthetic biodegradable graft substitutes. The hierarchical structures commonly designed are inspired by nature in human bones, which consist of minerals such as hydroxyapatite, a form of calcium phosphate and protein fiber. The bone graft bio-substitutes should possess a combination of properties for the purpose of facilitating cell growth and adhesion, a high degree of porosity, which would facilitate the transfer of nutrients and excretion of the waste products, and the scaffold should have high tensile strength and high toughness in order to be consistent with human tissues. Blending of polycaprolactone and hydroxyapatite has demonstrated great potential as bone substitutes. It is essential to identify a standardized processing methodology for the composite, which would result in optimum mechanical property for the biocomposite. In this study, biocomposites made of polycaprolactone (PCL) and hydroxyapatite (HAP) are reviewed for their applications in bone tissue engineering. The processing methodologies are discussed for the purpose of obtaining the porosity and pore size required in an ideal tissue scaffold. The properties of the composite can be varied based on the change in pore size, porosity, and processing methodology. This paper reviews and evaluates the methods to produce the hydroxyapatite-polycaprolactone scaffolds.

The effect of pilot hole size on the insertion torque and pullout strength of self-tapping cortical bone screws in osteoporotic bone.

BACKGROUND All surgical screws can experience failure if the torsional, tensile, and flexion loads exerted on the screws are excessively high. The use of self-tapping screws (STS) results in higher insertion torques (IT) as these screws cut their own threads in the pilot hole drilled in the bone. In this study, the torque for inserting the STS into an osteoporotic bone block for different pilot hole sizes (PHS) was measured and the pullout strength (PS) for extraction of the screws was determined for different depths of insertion, 0 mm, 1 mm, and 2 mm beyond the far cortex. METHODS Seventy-two Synthes stainless steel STS (40 mm length and 3.5 mm diameter) were inserted into pilot holes of sizes 2.55 (A: 73% OD), 2.50 (B: 71.5%), 2.45 (C: 70%), and 2.8 mm (D: 80%). Using a digital torque screwdriver, screws were inserted to 0 mm, 1 mm or 2 mm past the far cortex. Pullout tests were conducted with an Instron materials testing system. Analysis of variance and Student-Neuman-Keuls tests were performed to determine the effect of DOI and PHS on the loading energy, PS, and IT. RESULTS Results demonstrated that IT of the screws inserted into pilot holes A, B, and C were higher than those in D. It was also observed that PS and loading energy for 1 mm and 2 mm penetration past the far cortex were higher than those for 0 mm regardless of PHS. This study also found that an increase in PHS to 2.8 mm will reduce IT but will also reduce the PS relative to a PHS of 2.5 mm, the current standard for 3.5 mm screws. CONCLUSIONS The results of previously published studies regarding the effect of pilot hole size on PS in healthy cortical bone cannot be applied to the osteoporotic environment. The findings presented in this research support using PHS no larger than 71.5% of the screw outer diameter (i.e., pilot hole size of 2.5 mm for 3.5 mm screws) and inserting screws at least 2 mm beyond the far cortex to maximize PS and minimize iatrogenic damage in osteoporotic bone.

Pullout Strength and Load to Failure Properties of Self-Tapping Cortical Screws in Synthetic and Cadaveric Environments Representative of Healthy and Osteoporotic Bone

BACKGROUND The parameters of self-tapping screw (STS) performance in normal and osteoporotic bone have been defined in representative environments, but the question remains as to the clinical application of such findings. The goal of this study was to analyze the biomechanical performance of STSs in cadaveric and synthetic environments representative of healthy and osteoporotic bone. METHODS Ninety-six Synthes STSs were inserted into cadaveric and synthetic models representative of osteoporotic and healthy bone. Screws were inserted to depths of 1 mm short of the far cortex, flush and 1 mm and 2 mm beyond the far cortex. Screws were tested with an Instron 8511 material testing system utilizing axial pullout forces. A SAS procedure was used to conduct analysis of variance for unbalanced datasets. RESULTS Substantial differences were appreciated with respect to screw performance between osteoporotic and healthy bone specimens. Although a similar pattern of increased pullout strength and loading energy with increasing depth of insertion was demonstrated, absolute values were lower in osteoporotic specimens. Although performance trends were similar in cadaveric and synthetic testing models for both osteoporotic and healthy bone, values obtained during testing were different. Incomplete insertion of STSs resulted in a 21.5% and 37% reduction of biomechanical properties in osteoporotic and normal bone, respectively. CONCLUSIONS These results indicate that previously published findings on the performance of STSs in synthetic models cannot reasonably be applied to the clinical realm. Although trends may be similar, screw performance in synthetic, as compared with cadaveric, models is markedly different.

Triple Arthrodesis: A Biomechanical Evaluation of Screw Versus Staple Fixation:

The subtalar, talonavicular, and calcaneocuboid joints were internally fixed to simulate triple arthrodesis was performed on eight matched pairs of human cadaver feet. Feet were randomly assigned such that one specimen in each pair was internally fixed with cobalt-chrome staples and one specimen with stainless steel screws. Liquid metal strain gauges were placed in a perpendicular fashion across the three joints of each specimen. Each foot was then secured to the Shore Western Materials Test System where a series of 10 increasing eversion stresses across the foot was created. Displacement was measured at each joint with every increase in eversion stress. Our results show that there is no statistical difference in fixation strength between screws and staples at the talonavicular, calcaneocuboid, or subtalar joints (P = 0.862). Although many studies determining the strength to failure of different implants have been performed, shear stress and micromotion at the joint surfaces have not been evaluated, to our knowledge, and no single implant in this study has shown superior immobilization characteristics to recommend its use as the implant of choice in triple arthrodesis.

Finite Element Analysis as a Tool for Parametric Prosthetic Foot Design and Evaluation. Technique Development in the Solid Ankle Cushioned Heel (SACH) Foot

In this study, we developed an approach for prosthetic foot design incorporating motion analysis, mechanical testing and computer analysis. Using computer modeling and finite element analysis, a three-dimensional (3D), numerical foot model of the solid ankle cushioned heel (SACH) foot was constructed and analyzed based upon loading conditions obtained from the gait analysis of an amputee and validated experimentally using mechanical testing. The model was then used to address effects of viscoelastic heel performance numerically. This is just one example of the type of parametric analysis and design enabled by this approach. More importantly, by incorporating the unique gait characteristics of the amputee, these parametric analyses may lead to prosthetic feet more appropriately representing a particular users needs, comfort and activity level.

Analysis of Cryotherapy Penetration: A Comparison of the Plaster Cast, Synthetic Cast, Ace® Wrap Dressing, and Robert-Jones Dressing

Four groups were studied to determine the cooling effect at skin level of ice application over common ankle bandages and casts. The plaster cast, synthetic cast, and Ace® wrap dressing groups had 18 participants and the Robert-Jones dressing group had four participants. In a standardized fashion, ice bags were applied over each bandage or cast and skin temperatures were measured with a thermocouple over a 90-min period. The average final skin temperature was 16.5° in the plaster cast, 18.8° in the synthetic cast, 21.2° in the Ace wrap dressing, and 28.7° in the Robert-Jones dressing. The temperature curves showed that ice application effectively decreased skin temperatures through the plaster cast, synthetic cast, and Ace wrap dressing. The cooler temperature did not adequately penetrate the Robert-Jones dressing.