Anteroposterior Bending Research Articles

A biomechanical strength comparison of external fixators.

The purpose of this study was to biomechanically test the current commercially available uniplanar, half-pin external fixators, comparing stiffness, weight, and cost. The Hammer, HexFix, Hoffmann, Monotube Blue, Monotube Red, Torus, TraumaFix, and Ultra-X were tested using previously published methods. The Instron 4500 was used to assess the strength characteristics in axial, torsional, anteroposterior, and lateral bending of each device. Weight was based on the unassembled fixator construct. Cost was determined from the purchase price of each individual fixator. The results of this study revealed that the Torus was the stiffest external fixator tested in torsion. The Monotube Red was the stiffest in axial loading, anteroposterior bending, and lateral bending. The Hammer and Hoffmann external fixators were the heaviest constructs. The Torus and HexFix were the most expensive. Many factors, including stiffness, weight, cost, ease of application, fracture characteristics, and personal preference, go into deciding which external fixator to use. The data presented compare stiffness characteristics of several fixators under standardized loading conditions. These data indicate that the Torus and Monotube Red provide the greatest stiffness when comparing all modes of failure.

Patterns of strain in the macaque ulna during functional activity.

In vivo bone strain experiments were performed on the ulnae of three female rhesus macaques to test how the bone deforms during locomotion. The null hypothesis was that, in an animal moving its limbs predominantly in sagittal planes, the ulna experiences anteroposterior bending. Three rosette strain gauges were attached around the circumference of the bone slightly distal to midshaft. They permit a complete characterization of the ulna's loading environment. Strains were recorded during walking and galloping activities. Principal strains and strain directions relative to the long axis of the bone were calculated for each gauge site. In all three animals, the lateral cortex experienced higher tensile than compressive principal strains during the stance phase of walking. Compressive strains predominated at the medial cortex of two animals (the gauge on this cortex of the third animal did not function). The posterior cortex was subject to lower strains; the nature of the strain was highly dependent on precise gauge position. The greater principal strains were aligned closely with the long axis of the bone in two animals, whereas they deviated up to 45 degrees from the long axis in the third animal. A gait change from walk to gallop was recorded for one animal. It was not accompanied by an incremental change in strain magnitudes. Strains are at the low end of the range of strain magnitudes recorded for walking gaits of nonprimate mammals. The measured distribution of strains in the rhesus monkey ulna indicates that mediolateral bending, rather than anteroposterior bending, is the predominant loading regime, with the neutral axis of bending running from anterior and slightly medial to posterior and slightly lateral. A variable degree of torsion was superimposed over this bending regime. Ulnar mediolateral bending is apparently caused by a ground reaction force vector that passes medial to the forearm. The macaque ulna is not reinforced in the plane of bending. The lack of buttressing in the loaded plane and the somewhat counterintuitive bending direction recommend caution with regard to conventional interpretations of long bone cross-sectional geometry.

The use of direct measurement of ankle kinetics in the calculation of energy storage and return in a dynamic elastic response prosthesis

The finite element analysis (FEA) has been identified as a useful tool for the stress and strain behaviour determination in lower limb prosthetics. The residual limb and prosthetic socket interface was the main subject of interest in previous studies. This paper focuses on the finite element analysis for the evaluation of structural behaviour of the Sure-flex™ prosthetic foot and other load-bearing components. A prosthetic socket was not included in the FEA. An approach for the finite element modelling including foot analysis, reverse engineering and material property testing was used. The foot analysis incorporated ground reaction forces measurement, motion analysis and strain gauge analysis. For the material model determination, non-destructive laboratory testing and its FE simulation was used. A new, realistic way of load application is presented along with a detailed investigation of stress distribution in the load-bearing components of the prosthesis. A novel approach for numerical and experimental agreement determination was introduced. This showed differences in the strain on the pylon between the experimental and the numerical model within 30% for the anteroposterior bending and up to 25% for the compression. The highest von Mises stresses were found on the foot–pylon connecting component at toe off. Peak stress of 216 MPa occurred on the posterior adjusting screw and maximum stress of 156 MPa was found at the neck of the male pyramid.

Distal interphalangeal joint arthrodesis comparing tensionband wire and herbert screw: A biomechanical and dimensional analysis

Thirty cadaveric distal interphalangeal joints (15 male and 15 female joints) were prepared with either a Herbert screw or a tension-band wire technique to simulate an arthrodesis. To elucidate mechanical differences between these constructs, the strength of the specimens was determined for three-point anteroposterior and lateral bending and for axial torsion. The Herbert screw demonstrated significantly greater anteroposterior bending strength and greater torsional rigidity when compared to the tension-band wire technique. For dimensional analysis, the height and width of each distal phalanx was measured prior to fixation, 4 mm from the distal tip of the bone (the region that must accommodate the large-diameter threads of the Herbert screw). Results indicated that the mean height of the distal phalanx (3.55 mm) is smaller than the diameter of the screw (3.90 mm). Fracture or thread penetration at the tip of the distal phalanx during screw placement occurred in 25 of the specimens overall and in all the female phalanges, often resulting in stretching or violation of the nail bed. Despite fracture or screw penetration, the Herbert screw appears to offer additional strength that may be clinically important for joint arthrodesis.

Effects of erect bipedal standing on the cross-sectional geometry of the rat femur.

Effects of erect bipedal standing exercise on the skeletal morphology were investigated in seventeen growing male rats divided into control and exercise group. Using the newly devised 'bipedal training box', in which rats achieved a fully upright stance through positively reinforced operant conditioning, the exercise group was burdened with the bipedal standing exercise from 64 days to 140 days of age, totally in 136-138 sessions. At the age of 140 days, the left femur was dissected out, ten serial cross sections of the femoral diaphysis were cut from proximal to distal and cross-sectional properties were calculated from the photographs of the sections. The bipedal standing exercise had the following effects on the femoral diaphysis; an increase in the cross-sectional area, area moments of inertia in the proximal half of the shaft, i.e. the strength of the femoral diaphysis increased against axial compressive, antero-posterior bending and medio-lateral bending, respectively; an increase in the polar moment of inertia and an external rotation of the principal axis in the vicinity of mid-shaft, i.e. the strength against the torsional load increased and the direction to resist the maximum bending load more or less approached the antero-posterior direction. These observations were discussed in comparison with the effects of quadrupedal running exercise on the femoral cross section previously observed by us.

Structural stiffness of the Hoffmann simple anterior tibial external fixation frame.

Tibial external fixation frames were constructed on aluminum tube simulating tibia bone. A 20-mm gap was left at the fracture site in order to measure the structural stiffness of the frame rather than the aluminum tube. The performance of the frames were experimentally evaluated and quantified using tests which simulated the loading conditions encountered in normal walking. These included axial compression, anteroposterior (AP) bending, lateral bending and torsional loading of the frame. The parameters studied were (a) number of fixation pins, (b) number of connecting rods and (c) location of clamps on the pins. Four constants were evaluated from these tests using various structural configurations of the frames; these resulted in four stiffness coefficients in compression, AP bending, lateral bending and torsion. Stiffnesses of various frames with different geometric configurations were compared by comparing their appropriate stiffness coefficients. Such comparison can set forth a quantitative guideline in selecting a suitable frame configuration for the type of injury and condition of fracture pattern. This type of quantitative analysis can also be useful in modifying the frame during the postoperative bone healing process.

Sexual dimorphism in human lower limb bone structure: relationship to subsistence strategy and sexual division of labor

Cross-sectional geometric properties of the human femur and tibia are compared in males and females in a number of recent and archaeological population samples extending back to the Middle Paleolithic. There is a consistent decline in sexual dimorphism from hunting-gathering to agricultural to industrial subsistence strategy levels in properties which measure relative anteroposterior bending strength of the femur and tibia in the region about the knee. This trend parallels and is indicative of reductions in the sexual division of labor, in particular differences in the relative mobility of males and females. Sexual dimorphism in mediolateral bending strength near the hip shows no consistent temporal trend, probably reflecting relatively constant sex differences in pelvic structure related to the requirements of childbirth. Upper and Middle Paleolithic samples are indistinguishable in terms of sexual dimorphism from modern huntergatherers, suggesting a similar sexual division of labor. The results illustrate the utility of cross-sectional geometric parameters of long bone diaphyses in reconstructing behavioral differences within and between past populations. Some variations in the accuracy of sexing techniques based on diaphyseal measurements of the lower limb long bones may also be explained by these behavioral and structural factors.

Biomechanical analysis of internal fixation techniques for proximal interphalangeal joint arthrodesis

Although numerous fixation techniques have been developed for performing small joint arthrodesis, no previous study of the biomechanical properties of these constructs has been published. The strength of specimens of arthrodesis of the proximal interphalangeal joint performed with cadaver material was determined for three-point anteroposterior bending, axial torsion, and lateral cantilever bending stress. Crossed Kirschner wires, intraosseous wiring, and figure of eight tension-band wiring were studied. Figure of eight tension-band wiring demonstrates superior strength in anteroposterior bending and torsion. For lateral bending stress, no significant difference exists among the techniques studied.

Comparison of the effects of compression plates and external fixators on early bone-healing.

We used two mechanically dissimilar devices, compression bone-plates and unilateral-frame external fixators, in a standard canine osteotomy model; both methods were highly successful in achieving mature bone union. Bone union was studied by histological, physiological, and biomechanical means. At 120 days after injury, union was biomechanically less mature on the external fixator side. These tibiae had less intracortical new-bone formation (p less than 0.01), more bone resorption (p less than 0.045), and more bone porosity (p less than 0.04) when compared with paired tibiae that had been treated with compression plates. This higher level of bone turnover on the external fixator side was accompanied by an increase in blood flow (measured by clearance of 85Sr) (p less than 0.04). At the osteotomy site, pre-experimental or unlabeled bone and porosity were greater on the external fixator side and endosteal new-bone formation was greater on the plated side. Since the in vitro stiffness of the external fixator was less in all modes tested (compression, distraction, torsion, and anteroposterior bending) except lateral bending, it may be that the rigidity of the fixation is an important factor in early bone-remodeling of a healing osteotomy.

Stability of External Fixators Used for Knee Arthrodesis after Failed Knee Arthroplasty

Secondary arthrodesis as treatment for failed knee arthroplasty is being used increasingly. The usual method is sustained rigid external fixation. The fusion rate is low in cases with considerable metaphyseal bone loss and poor bone quality. Currently, the Hoffmann- Vidal fixator or comparable designs are preferred to the less stable Charnley single frame fixator. The standard Hoffmann- Vidal device uses only transverse percutaneous fixating pins and gives an anteroposterior bending stiffness that is considerably lower than the lateral bending stiffness. In the present study, external fixators were tested for stability in a material testing machine under standardized conditions using synthetic bones. The anteroposterior bending stiffness was significantly improved when sagittal pins connected to a ventral compression rod were added to a Hoffmann- Vidal fixator. Stiffness was still more improved when sagittal pins were connected to ventral frames as in a modification of the Hoffmann- Vidal or the similar Ace-Fischer fixator.

Comparison of Mechanical Performance in Four Types of External Fixators

The Hoffmann-Vidal triangular apparatus for fracture fixation of the femur, the Volkov-Oganesian device with Steinmann pins in tension, the Kronner device with a plastic elliptic frame, and the Roger-Anderson apparatus (both standard and compression-distraction designs) were studied based on loading and analytic methods established previously. The results were compared with those for the standard Hoffmann-Vidal quadrilateral configuration. The Kronner device, with five connecting bars, was the stiffest under most loading modes, closely followed by the Hoffmann-Vidal quadrilateral apparatus. The standard Roger-Anderson device demonstrated average rigidity; its new design with compression-distraction capability was relatively weak. All devices showed an apparent weakness in antero-posterior bending. The Volkov-Oganesian design demonstrated low overall stiffness but was the most effective apparatus in resisting anteroposterior bending. External fixators provide effective compression at the fracture site, and the amount of compression is proportional to the stiffness of the apparatus.

Biomechanics: Research and development at IMA

s 283 BIOMECHANICS: RESEARCH AND DEVELOPMENT AT IMA L. C. NAVA and P. A. A. LAURA (Instituto de Mecanica Aplicada, Base Naval Puerto Belgrano, 8 111, Argentina) From a modern engineering viewpoint one must consider crutches or canes as dynamic mechanical systems which act in an active manner in order to complement the disabled human body. The present paper describes some of the recent developments in this area accomplished at IMA and the resulting experiences: retractile, anti-shock, non slipping crutches; variable length, non slipping canes; angleadjustable fore-arm crutches, etc. The paper deals also with elements specially developed for disabled children, for instance, a crutch-toy which consists in a crutch which can be used as a ‘rifle’ by the child. BENDING STIFFNESS OF UNILATERAL AND BILATERAL EXTERNAL FIXATOR FRAMES F. BEHRENS, W. JOHNSON, T. KOCH and N. KOVACEVIC (University of Minnesota and Metropolitan Medical Center) Although external fixators are now widely used in the immobilization of severe open and infected fractures, there exists little practical information about their mechanical properties. In order to gain a better insight into the ability of these devices to resist anteroposterior and transverse bending moments, which are of particular importance in the early treatment period, different configurations of two commercially available devices are mounted on model bone and then tested in sagittal and transverse bending using a universal testing machine. The following conclusions were reached: Small increases in pin and rod diameters allowed for the construction of simpler frames which do not interfere with vital anatomical structures, and can be more easily adapted to the clinical and mechanical demands of a particular injury. Mechanically, unilateral frames applied anteriorly were most effective, particularly when the pins in each main bony fragment were maximally spread and the longitudinal rod was applied as close as possible to the bony surface. SINGLE-PLANE X-RAY PHOTOGRAMMETRY FOR DETECTING TOTAL JOINT LOOSENING F. G. LIPPERT, R. M. HARRINGTON (Department of Orthopaedics, Seattle VAMC) S. A. VERESS and T. EL-GARF (Civil Engineering, University of Washington, Seattle, WA 91915 U.S.A.) X-ray photogrammetry is an accurate method of measuring displacements in the skeletal system. For clinical use 0.9 mm spherical stainless steel markers are implanted in the bones to serve as accurately observable landmarks. Two different systems for obtaining three-dimensional displacements have been developed and tested to determine their accuracy (Lippert, 1982). The primary use of these systems has been to measure the displacements of total hip and total knee implants relative to the surrounding bone. Both systems measure relative motions with a root mean square error of 0.1 mm or less in model testing and 0.2 mm in human subjects. In order to develop a screening test which can be more widely adapted for use in hospital radiology departments, a single-plane (two-dimensional) X-ray photogrammetry method is being tested. A single-plane measurement could detect such motions as pistoning and toggling of the femoral component of total hip implants, or loosening in them. The orthopaedist will be able to choose measurements in one or more planes based on clinical examination. ORIGIN OF KNEE MOMENTS DURING AMBULATION AND THEIR MODIFICATION BY ANKLE-FOOT ORTHOSES J. F. LEHMANN, M. J. Ko, and B. J. DELATEUR, (University of Washington, Seattle, WA 98195, U.S.A.) This study examines the origin of knee moments during normal gait, the changes caused by wearing an ankle-foot orthosis and the clinical importance of knee moments. In normal subjects, force plate data and kinematic data from film of the subject were used to derive knee moments during normal walking and with an ankle-foot orthosis adjusted to allow only 5” dorsiflexion or 5’ plantarflexion. The knee moment components, due to the fore-aft shear and vertical forces, tend to counterbalance each other, minimizing the magnitude of the total knee moment, which demonstrated a flexion peak at toe-strike, changing to

Mechanisms of femoral fracture

Mechanisms of femoral fracture of the condyles and shaft were experimentally investigated through controlled knee impact of denuded femurs in six human cadavers. High-speed movies recorded knee joint compression, femoral displacements and deformation, and fracture initiation. Fracture initiated at 10.6 ± 2.7 kN knee load after 1.3 ± 0.1 cm of knee joint compression for a 10.1 kg rigid impact at 13.2 ± 1.4 m/s. Interestingly, fracture occurred 0.5 ms–1.5 ms after the peak in applied knee load of 18.3 ± 6.9 kN, probably because a significant portion of the load is developed by inertial accelerations displacing the femur and coupled masses. Axial strain measurements at the femoral midshaft showed increasing anteroposterior bending and compressional deformations until the initiation of observed fracture. The kinematics of the observed fracture and the midshaft deformational strains indicate that fracture is predominantly due to tensile strain from anteroposterior bending of the femoral shaft or patellar wedging of the condyles.