Biomedical subjects

A W Miles

Publications and source records attributed to A W Miles.

At least 19 recordsLinked to original sources

PubMed2006

An in vitro study of ultrasound signal loss across simple fractures in cortical bone mimics and bovine cortical bone samples.

Measurements have been performed on Sawbones and bovine cortical bone samples at 200 kHz using an axial transmission technique to investigate the factors that determine how ultrasonic waves propagate across a simulated fracture. The peak amplitude of the first arrival signal (FAS) was studied. Results taken from intact specimens were compared with those produced when a simple transverse fracture was introduced. These fracture simulation experiments were found to be consistent with Finite Difference modelling of the experimental conditions. The peak amplitude showed a characteristic variation across the fracture caused by interference between reradiated and scattered/diffracted waves at the fracture site and a net Fracture Transmission Loss (FTL). For small fracture gaps, the change in amplitude was sensitive to the presence of the fracture. This sensitivity suggests that this parameter could be a good quantitative indicator for the fracture healing process assuming the relative change in this parameter brought about by healing is measurable.

Animals↗

PubMed2006

Ultrasonic propagation in cortical bone mimics.

Understanding the velocity and attenuation characteristics of ultrasonic waves in cortical bone and bone mimics is important for studies of osteoporosis and fractures. Three complementary approaches have been used to help understand the ultrasound propagation in cortical bone and bone mimics immersed in water, which is used to simulate the surrounding tissue in vivo. The approaches used were Lamb wave propagation analysis, experimental measurement and two-dimensional (2D) finite difference modelling. First, the water loading effects on the free plate Lamb modes in acrylic and human cortical bone plates were examined. This theoretical study revealed that both the S0 and S1 mode velocity curves are significantly changed in acrylic: mode jumping occurs between the S0 and S1 dispersion curves. However, in human cortical bone plates, only the S1 mode curve is significantly altered by water loading, with the S0 mode exhibiting a small deviation from the unloaded curve. The Lamb wave theory predictions for velocity and attenuation were then tested experimentally on acrylic plates using an axial transmission technique. Finally, 2D finite difference numerical simulations of the experimental measurements were performed. The predictions from Lamb wave theory do not correspond to the measured and simulated first arrival signal (FAS) velocity and attenuation results for acrylic and human cortical bone plates obtained using the axial transmission technique, except in very thin plates.

Acrylates↗

PubMed2006

A comparison of the viscoelastic properties of bone grafts.

BACKGROUND: The expansion in joint arthroplasty surgery in the 1970s has resulted in a large group of patients who require revision arthroplasty for aseptic loosening. Impaction bone grafting to deal with bone stock loss has become an increasingly popular procedure in revision hip surgery. The results of this revision surgery are variable and very much dependent on the grafting techniques adopted at the time of surgery. In vitro testing of impaction bone grafting methods can constitute an important tool to improve long-term clinical results. The increasing clinical demand for human allograft limits its availability for use in in vitro laboratory studies therefore suitable experimental alternatives are required. METHODS: Human, porcine and ovine cancellous bone grafts were morsellised and prepared for in vitro laboratory testing following a standard operative technique. Each graft type was compressed using a die plunger test. After compression to a predetermined level the graft was left to relax for 120s. A comparison of the compression moduli and of the relaxation characteristics for each graft preparation was performed. In addition, the effects of washing the graft and of cartilage removal from the graft mixture were investigated. RESULTS: This study has demonstrated that there is no statistical difference in the compression modulus or relaxation percentage between human and ovine graft preparations. The effect of removing cartilage and washing the graft mixtures were inconsistent with regard to alterations in the viscoelastic properties of the grafts. INTERPRETATION: On the basis of the experiments performed we recommend the use of ovine bone graft as a suitable substitute for human allograft for in vitro testing of impaction bone grafting methods. The properties of ovine graft were similar for both compression moduli and relaxation properties to human allograft.

Animals↗

PubMed2005

Development of an in-vivo method of wrist joint motion analysis.

BACKGROUND: A clinically applicable method of plotting wrist joint motion in three-dimensions has not been described. Computer modelling has been used to improve joint arthroplasty elsewhere in the body. We aimed to develop a method of measuring, and modelling, wrist joint motion that could potentially be used to improve the kinematic performance of wrist arthroplasty designs. METHODS: An electromagnetic system was used to record wrist motion in three-dimensions. A small pilot study attempted to assess repeatability. A larger group of volunteers with normal wrists was also studied. An iterative computer model, using a two-axis hinge, was developed. One output from this model, the offset of the two axes of motion, is presented as an example of the possible applications of this method of analysis. FINDINGS: For any one individual, in the pilot study, the offset of the axes calculated was relatively reproducible. Between individuals the difference in the offset of the axes was more marked. In 99 normal sets of data the mean axis offset was 6.8mm (range 28 mm to -21 mm) A positive value represented the radio-ulnar deviation axis placed distal to the flexion-extension axis. INTERPRETATION: The three-dimensional motion plots generated using this method could be used clinically to follow disease progression or recovery following surgery. The computer modelling method described has potential applications, if further refined, to wrist joint arthroplasty design.

Algorithms↗

PubMed2005

Compression forces generated by Mini bone screws--a comparative study done on bone model.

The compressive forces generated by the AO/ASIF 3.0 mm cannulated cancellous and 2.0 mm cortical screws, Mini-Acutrak and Herbert/Whipple small bone cannulated screws were measured in the laboratory with the use of simulated cancellous bone and a load cell washer as a means of quantifying their fixation capabilities. The Herbert/Whipple screw and the Mini-Acutrak screw were found to have nearly identical compression capabilities and provided more compression than the cortical screw. The AO/ASIF cannulated screw when used with a support screw demonstrated a compressive capacity twice that of the 2.0 mm cortical screw and higher than the headless Mini-Acutrak and Herbert/Whipple screws. The Mini-Acutrak screw produced about 70% of compression of the cancellous screw in spite of having a diameter almost half that of the cancellous screw. The Herbert/Whipple screw in spite of its larger size compared to the Mini-Acutrak produced almost the same amount of compression.

Biomechanical Phenomena↗

PubMed2005

An in-vitro investigation into the cement pressurization achieved during insertion of four different femoral stems.

Adequate cement pressurization during stem insertion improves the interdigitation of cement into bone. This increases the strength of the cement-bone interface, thus contributing to the reduction of the incidence of aseptic loosening, the commonest cause of revision surgery. This in-vitro study compared the cement pressurization achieved during insertion of four different stems of equivalent sizes: the Elite Plus (DePuy, UK), C-Stem (DePuy, UK), Exeter (Stryker, USA), and CPS-Plus (Plus Orthopedics, Switzerland). The maximum pressures attained at the time of stem insertion were recorded at proximal, mid and distal stem levels. The Elite Plus generated significantly higher distal pressures than the other stems. The CPS-Plus generated significantly greater proximal cement pressures than the Elite Plus, C-Stem, and Exeter prostheses. The triple taper of the C-Stem increased the cement pressurization medial to the stem. The stem shape and the presence or absence of a proximal stem centralizer affect cement pressurization. The presence of a proximal stem centralizer, a large stem volume, and a lateral-medial taper are all factors associated with increased cement pressurization during stem insertion.

Adhesiveness↗

PubMed2004

A kinematic model of the wrist based on maximization of joint contact area.

The wrist is a complex joint and the factors governing its behaviour are poorly understood. A hypothesis that the movement of the carpal bones could be predicted using a minimum energy principle was tested. Carpal bones were dissected from a cadaveric forearm and their shapes were laser-digitized to obtain three-dimensional computer models. A computer program was created to measure contact area between neighbouring articular surfaces and to maximize this quantity by adjusting the six degrees of freedom of the bone models. This procedure was performed for 1.0 degree increments of rotation applied to the capitate bone up to 20 degrees of ulnar and 10 degrees of radial deviation. The model correctly predicted certain aspects of the complex behaviour of the carpal bones. The results for the scaphoid in particular displayed characteristics in common with known behaviour of this bone. During 20 degrees of unlar deviation and 10 degrees of radial deviation, the bone demonstrated 11.3 degrees of extension and 9.4 degrees of flexion respectively. The novelty of the study lay in the fact that the model did not rely upon ligamentous constraints. The results are encouraging, considering the only information used by the algorithm was the shape of the articular surfaces.

Biomechanical Phenomena↗

PubMed2004

A method for measuring vertical forces applied to the upper limb during sit-to-stand.

The aim of this study was to develop a basic measurement system to estimate the vertical loading of the upper limb during the sit-to-stand activity, with a view to increasing the understanding of the loading of the wrist in daily living activities. A chair was adapted and instrumented with strain gauges and position sensors so that the force applied through the upper limbs to the arms of the chair could be calculated. Four aspects of the chair's geometry could be varied. A force plate was positioned on the floor between the legs of the chair to record the corresponding foot loading. Twenty normal subjects (22-56 years, mean 32.7 years) participated in a pilot study in which loading through the upper and lower limbs was recorded for a range of chair geometries. The vertical force transmitted through each upper limb was typically 20-30 per cent of bodyweight. The vertical upper limb load averaged across all subjects showed a small reduction when either the seat height or the height of the chair arms was increased.

Adult↗

PubMed2002

Subsidence in impaction grafting: the effect of adding a ceramic bone graft extender to bone.

The incidence of revision total hip arthroplasty is increasing dramatically and the associated demand for allograft bone is likely to exceed the available supply. In addition, allograft presents potential problems with regard to infection, antigenicity, availability, reproducibility and cost. It is therefore desirable to develop an alternative to allograft. This study investigated BoneSave, a porous tricalcium phosphate-hydroxyapatite ceramic for use in impaction grafting of the femur at revision total hip arthroplasty. The findings of an in vitro mechanical study comparing the initial stability of pure allograft, a volume mixture of 50 per cent allograft and 50 per cent BoneSave, and a volume mixture of 10 per cent allograft and 90 per cent BoneSave are reported. The BoneSave-allograft mixtures exhibit both much greater mechanical stability and reproducibility than the pure allograft (p < 0.05) at all tested loads (200-800 N). At high peak loads the high volume (90%, v/v) BoneSave mix also provided higher mechanical stability than the medium volume (50 per cent BoneSave-50 per cent allograft) mix (p < 0.05). These results demonstrate thatfrom a mechanical standpoint the tested ceramic provides adequate initial stability to be used as a bone graft extender with allograft in impaction grafting of the femur.

Animals↗

PubMed2001

Initial rotational stability of distal tibial fractures nailed without proximal locking: the importance of fracture type and degree of cortical contact.

Although distally locked (dynamic) nailing is generally recommended for fractures below the isthmus of the tibia in the presence of adequate proximal nail-bone contact, rotational stability in the above situation appears to be a major concern and can increase the risk of malunion. However, there is no published experimental evidence to quantify this mechanical parameter or to relate factors such as the fracture pattern with the final clinical outcome. This in-vitro biomechanical experimental study was set out to measure the initial rotational stability of dynamically nailed fractures of the distal tibial diaphysis. Using a composite tibial model, three non-comminuted types (spiral, oblique and transverse) and various comminuted patterns (comminution, 0-85%) of dynamically nailed fractures of the distal tibial diaphysis were tested. Using a special rig to simultaneously apply axial and torsional loading measurements of torsional stiffness and the previously described "spring-back angle" were carried out. Our results showed that in terms of torsional stiffness and the "spring-back" angle oblique fractures are the most stable followed by transverse and spiral fractures. Furthermore, when testing of the above parameters against the degree of comminution was carried out, a significant reduction of rotational stability was evident with comminution of 50% or above. It is concluded that oblique fractures of the distal tibial third that can be reduced with at least 50% cortical apposition present the optimal rotational stability following dynamic nailing.

Biomechanical Phenomena↗

PubMed2000

Effect of mixing technique on the properties of acrylic bone-cement: a comparison of syringe and bowl mixing systems.

Syringe mixing systems have been introduced, but few data exist regarding the mechanical performance of cement they produce. We compared the properties of polymethyl methacrylate cement produced by these systems with that produced by a multiaxial bowl. Mixtures of cement were prepared using the Optivac, Cemvac, and Summit syringes and the Summit bowl. The mixtures were cured in molds to create casts that were radiographed and analyzed for void content, then cut into strips, weighed, measured, and tested to failure in 4-point bending. Syringe-mixed cement was of greater density, bending modulus, and bending strength than bowl-mixed cement (Mann-Whitney, P < .01) and contained fewer microvoids and macrovoids (Mann-Whitney, P < .01). No significant differences between the syringes were found for these variables (Kruskall-Wallis, P > .05).

Bone Cements↗

PubMed1999

Axial preload in external fixator half-pins: a preliminary mechanical study of an experimental bone anchorage system.

OBJECTIVE: To demonstrate the effect of axial preload achieved with an experimental external fixator pin prototype on the initial stability of the pin-bone interface. DESIGN: An in vitro mechanical study comparing pin stability of an axially-preloaded experimental pin and a radially-preloaded conventional external fixation pin. BACKGROUND: The most common problem in clinical practice of external fixation is pin site failure. An experimental external fixator half-pin has been designed introducing axial preload as an alternative method of achieving a stable pin-bone interface. METHODS: Standardized measurements of pin insertion and removal torque comparing the experimental pin prototype and a conventional external fixator pin in a synthetic composite material and ovine tibial cortical bone. RESULTS: There is a positive correlation between axial preload and torque resistance of the coaxial experimental half-pin as well as significantly increased torque resistance of the axially-preloaded experimental pin over the conventional pin tested. In addition, significantly lower insertion torque of the experimental pin was observed with the applied insertion technique. CONCLUSIONS: 'Axial preload' achieved with appropriately-designed half pins might represent an effective biology-sparing method of increasing the mechanical stability of the pin-bone interface in unilateral external fixator frames. RELEVANCE: Several aspects of the current external fixator pin design and insertion technique have been implicated for the significant incidence of failure at the pin-bone interface manifested as pin loosening, pin track infection or even osteomyelitis. An axially-preloaded bone fastener system may be mechanically and biologically superior and therefore clinically advantageous.

Animals↗

PubMed1999

The influence of the time-dependent properties of bone cement on stress in the femoral cement mantle of total hip arthroplasty.

An empirically determined formula for the creep behavior of bone cement was incorporated into a validated computer model of a cemented femoral total hip arthroplasty component. The stress patterns in the cement mantle were observed over a period of one week, in one instance where the stem-cement interface was rigidly bonded, and in a second, where it was allowed to slip. Principal stresses and maximum shear stresses were shown to decrease rapidly after loading in both situations, suggesting that the stresses generated were not high enough to cause immediate failure, although they may be significant in the long term.

Journal Article↗

PubMed1998

"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.

Biomechanical Phenomena↗

PubMed1997

An investigation of the stress distribution generated in articular cartilage by crystal aggregates of varying material properties.

Several joint diseases are associated with the deposition of crystals within the articular cartilage. A variety of crystal aggregates have previously been identified throughout the thickness of the cartilage. A linear elastic finite element model representing instantaneous, or short-term, loading conditions has been developed of a large crystal aggregate surrounded by articular cartilage. The material properties of the aggregate and the cartilage were varied and the resultant shear stress and equivalent strain distribution in the surrounding cartilage studied in order to provide some indication of the relative potential of various types of crystal aggregate to cause damage to the articular cartilage. Results indicated that aggregates with a Young's modulus either much less, or much greater, than that of the surrounding cartilage generated the maximum shear stress and equivalent strain concentrations at the interface between the aggregate and the cartilage. Also, that highly compressible aggregates, with a very low Poisson's ratio, generated higher shear stress and equivalent strain concentrations in the surrounding cartilage than aggregates of a more incompressible nature. Under conditions of short-term loading these results suggest that crystal aggregates present within the cartilage layer will increase the shear stress and equivalent strain concentrations in the surrounding cartilage, and therefore have the potential to cause damage to the cartilage.

Biomechanical Phenomena↗

PubMed1997

An experimental study of the failure modes of the Gamma Locking Nail and AO Dynamic Hip Screw under static loading: a cadaveric study.

The sliding compression screw is widely regarded as the optimum treatment for intertrochanteric fractures of the femur, allowing bone fragments to impact until a bony support has been established across the fracture site. This study carried out biomechanical, cadaveric tests to establish the influence of direct static loading situations on the modes of failure of the Gamma Nail compared with the Dynamic Hip Screw (DHS). Clinical studies report DHS failures of lag screws cutting-out, bending of the lag screws and cortical screws pulling out causing plate loosening. Gamma Nail failures include lag screw cut-out or fractures of the femoral shaft around the distal locking screws or nail tip. In this study each failure mode has been isolated, to establish the loads to failure under various fracture configurations. The biomechanical results indicated that the intramedullary Gamma Locking Nail can be recommended over a standard DHS in cases of subtrochanteric fracture or conditions of very poor bone quality.

Aged↗

PubMed1997

The influence of the stem-cement interface in total hip replacement--a comparison of experimental and finite element approaches.

Experimental and finite element investigations were carried out on axisymmetric models of the femoral component of a total hip replacement. In one instance, the interface between the stem and the surrounding bone cement was assumed to be rigidly bonded; in a second, it was allowed to slip. For the latter case, a friction coefficient of 0.2 was determined experimentally. The predictions of the finite element models demonstrated excellent agreement with the results from the experimental tests at all sites where comparisons were made, thus validating these models. The effect of stem-cement slip was shown to reduce the maximum shear stress in the cement mantle by approximately 30 per cent.

Biomechanical Phenomena↗

PubMed1997

Comparative dynamic evaluation of the sliding/characteristics of the Gamma nail: a biomechanical analysis.

This study was directed at establishing the influence of dynamic loading situations on the sliding characteristics of the Gamma locking nail (Howmedica). It was postulated that the changing regions of load contact area that occur within the sliding mechanism during flexion and extension would favourably modify the jamming behaviour observed under static test conditions. The forces required to initiate sliding of the lag screw in the intramedullary device were investigated under a range of conditions representing clinical situations. The optimum sliding performance of the Gamma nail was shown to occur during test conditions of dynamic loading, when the rate of application of the load was rapid and the cyclic angle of flexion was greatest. The size and weight of the patient also influence the sliding characteristics, as a shorter lag screw length protruding from the barrel and a reduced vertical static load (body weight) yielded a lower axial sliding force. The results under dynamic loading conditions suggest that static testing of sliding hip screws does not accurately represent their clinical performance.

Biomechanical Phenomena↗