Posterior wall acetabular fracture fixation: A mechanical analysis of fixation methods.

Stable, anatomical fixation of acetabular fractures gives the best chance of successful outcome, while penetration of the acetabular articular surface with screws is associated with poor outcomes. Spring plates are an alternative to interfragmentary lag screws when penetration is a concern. A mechanical study comparing fracture stability and construct stiffness of three fixation methods for posterior wall acetabular fractures with transverse comminutions was performed. The three fixation methods tested were a posterior wall rim plate, a posterior wall buttress plate with separate lag screws and a posterior wall plate with two spring plates. Nine samples were tested, three for each fixation method. Two-dimensional motion analysis was used to measure fracture fragment displacement and construct stiffness. After two 6000 cycle-loading protocols, to a maximum 1.5 kN, the mean fracture displacement was 0.154 mm for the rim plate model, 0.326 mm for the buttress plate and 0.254 mm for the spring plate model. Mean maximum displacement was significantly less for the rim plate fixation than the buttress plate (p = 0.015) and spring plate fixation (p = 0.02). The rim plate was the stiffest construct 10,962 N/mm, followed by the spring plate model 5637 N/mm and the buttress plate model 4882 N/mm. Based on data obtained in this study, where possible a rim plate with interfragmentary lag screws should be used for isolated posterior wall fractures as this is the stiffest and most stable construct. When this method is not possible, spring plate fixation is a safe and a superior alternative to a posterior buttress plate method.

Elevated total plasma-adiponectin is stable over time in young women with bulimia nervosa.

BACKGROUND: Bulimia nervosa (BN) is characterized by dysregulated eating behaviour and present data suggest adipokines may regulate food intake. We investigated a possible association between BN and adipokine levels and hypothesized that plasma (P)-adiponectin would be elevated and P-leptin and P-leptin-adiponectin-ratio would be reduced in women with BN. METHODS: The study was designed as a cross-sectional study with a longitudinal arm for patients with BN. Plasma-adiponectin and leptin was measured in 148 female patients seeking psychiatric ambulatory care and 45 female controls. Fifteen patients were diagnosed with BN and the remaining with other affective and anxiety disorders. P-adiponectin and P-leptin levels were compared between patients with BN, patients without BN and controls. At follow-up 1-2years later, adipokines were reassessed in patients with BN and the Eating Disorder Examination Questionnaire was used to assess symptom severity. RESULTS: P-adiponectin was elevated in patients with BN at baseline and at follow-up when compared to patients without BN and controls (P<0.004 and <0.008 respectively). The difference remained significant after controlling for body mass index. P-adiponectin was correlated to symptom severity at follow-up in patients with BN without morbid obesity (ρ=0.72, P<0.04). P-leptin-adiponectin-ratio was significantly lower in patients with BN compared to controls (P<0.04) and P-leptin non-significantly lower. CONCLUSIONS: Findings indicate a stable elevation of P-adiponectin in women with BN. P-adiponectin at follow-up correlates to eating disorder symptom severity in patients without morbid obesity, indicating that P-adiponectin should be further investigated as a possible potential prognostic biomarker for BN.

Analysis of failed Van Straten LPM proximal interphalangeal prostheses.

The two-piece Van Straten Leuwen Poeschmann Metal (LPM) prosthesis was intended for the proximal interphalangeal joints. However, revision rates of 29% after 19 months were reported, as well as massive osteolysis. Five failed LPM titanium-niobium coated cobalt chromium components were obtained, three distal and two proximal, and subjected to a forensic retrieval analysis. Components were analyzed using a Talysurf contacting profilometer, ZYGO noncontacting profilometer, and environmental-scanning electron microscope. All components were heavily worn. In some regions the titanium-niobium coating had been scratched and penetrated. Elsewhere this coating had been removed where there was minimal scratching, which may have been due to corrosion between the coating and substrate. The osteolysis reported clinically was likely to be linked to the wear debris from the failed titanium-niobium coating and substrate.

A novel closed-loop electromechanical stimulator to enhance osseointegration with immediate loading of dental implant restorations.

The degree of osseomechanical integration of dental implants is acutely sensitive to their mechanical environment. Bone, both as a tissue and structure, adapts its mass and architecture in response to loading conditions. Therefore, application of predefined controlled loads may be considered as a treatment option to promote early maturation of bone/implant interface prior to or in conjunction with crown/prosthesis attachment. Although many studies have established that the magnitude, rate of the applied strain, and frequency have significant effects on the osteogenic response, the actual specific relationships between strain parameters and frequency have not yet been fully defined. The purpose of this study was to develop a stimulator to apply defined mechanical stimuli to individual dental implants in vivo immediately after implantation, exploring the hypothesis that immediate controlled loading could enhance implant integration. An electromechanical device was developed, based on load values obtained using a two-dimensional finite element analysis of the bone/implant interface generating 1000 to 4000 pe and operated at 30 and 3 Hz respectively. The device was then tested in a cadaveric pig mandible, and periosteal bone surface strains were recorded for potential future comparison with a three-dimensional finite element model to determine loading regimens to optimize interface strains and iterate the device for clinical use.

Numerical and experimental simulation of the effect of long bone fracture healing stages on ultrasound transmission across an idealized fracture.

The effect of various stages of fracture healing on the amplitude of 200 kHz ultrasonic waves propagating along cortical bone plates and across an idealized fracture has been modeled numerically and experimentally. A simple, water-filled, transverse fracture was used to simulate the inflammatory stage. Next, a symmetric external callus was added to represent the repair stage, while a callus of reducing size was used to simulate the remodeling stage. The variation in the first arrival signal amplitude across the fracture site was calculated and compared with data for an intact plate in order to calculate the fracture transmission loss (FTL) in decibels. The inclusion of the callus reduced the fracture loss. The most significant changes were calculated to occur from the initial inflammatory phase to the formation of a callus (with the FTL reducing from 6.3 to between 5.5 and 3.5 dB, depending on the properties of the callus) and in the remodeling phase where, after a 50% reduction in the size of the callus, the FTL reduced to between 2.0 and 1.3 dB. Qualitatively, the experimental results follow the model predictions. The change in signal amplitude with callus geometry and elastic properties could potentially be used to monitor the healing process.

Development of degrees in medical engineering at the University of Bath.

Because of increasing demands on the health care sector resulting from an ageing population and increasing quality of life expectations, the health care sector in general is growing and the requirement for trained engineers to work in this sector is likely to increase. Medical engineering is a relatively new area of university study, in which engineering principles are applied to medical systems, devices, and products. This paper outlines the initial design and development of a specialized degree course in Medical Engineering at the University of Bath and reports the subsequent experience with this as the degree has evolved.

Quantification of fracture healing from radiographs using the maximum callus index.

UNLABELLED: Callus formation and growth are an essential part of secondary fracture healing. Callus growth can be observed radiographically and measured using the "Callus Index," which is defined as the maximum diameter of the callus divided by the diameter of the bone. We compared three groups of patients with tibial fractures treated by external fixation, intramedullary nailing, and casting to assess the validity of using serial measurements of callus index as a measure of fracture healing. When callus index was plotted against time for each patient, the point at which the fracture began to remodel, indicated by the highest point of the curve, was observed as a consistent feature regardless of fixation method. This occurred on average at 2(1/2) weeks after plaster cast removal (14 weeks post injury), 5 weeks after external fixator removal (22 weeks post injury), and 27 weeks post injury for the intramedullary nailed fractures. Because remodeling only occurs once the fracture is stable, a peak in callus index is a reliable sign that the fracture has united. Serial measurements of callus index would therefore appear to offer a simple method of quantifying secondary fracture healing regardless of the treatment method used. LEVEL OF EVIDENCE: Level III, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.

Monitoring the mechanical properties of healing bone.

Fracture healing is normally assessed through an interpretation of radiographs, clinical evaluation, including pain on weight bearing, and a manual assessment of the mobility of the fracture. These assessments are subjective and their accuracy in determining when a fracture has healed has been questioned. Viewed in mechanical terms, fracture healing represents a steady increase in strength and stiffness of a broken bone and it is only when these values are sufficiently high to support unrestricted weight bearing that a fracture can be said to be healed. Information on the rate of increase of the mechanical properties of a healing bone is therefore valuable in determining both the rate at which a fracture will heal and in helping to define an objective and measurable endpoint of healing. A number of techniques have been developed to quantify bone healing in mechanical terms and these are described and discussed in detail. Clinical studies, in which measurements of fracture stiffness have been used to identify a quantifiable end point of healing, compare different treatment methods, predictably determine whether a fracture will heal, and identify factors which most influence healing, are reviewed and discussed.

Determination of pelvic ring stability: a new technique using a composite hemi-pelvis.

Traumatic disruption of the pelvic ring caused by high-energy impact is associated with significant mortality and morbidity. A variety of fixation techniques have been developed for treating these injuries with the main aims of restoring the stability of the pelvic ring and promoting recovery of normal function. The stability of fixation of these techniques is often analysed by cadaveric studies, which can introduce high variability into the results and have small sample numbers. This study presents a new method that uses a composite hemi-pelvis to enable stabilization of the pelvic ring to be accurately determined. The pelvis was loaded cyclically to simulate walking conditions and to assess the stability of the fixation, which was measured using a displacement transducer that monitors the motion of the pelvis in six degrees of freedom. The motions measured showed comparable results with previously published cadaveric studies. This reproducible method of testing with a hemi-pelvis composite model and rig allows valid analysis of pubic symphysis implants, obviating the difficulties of performing a cadaveric study.

Bone vibration measurement using ultrasound: application to detection of hip prosthesis loosening.

Hip prosthesis loosening can be determined in vivo using a vibration-based technique called vibrometry. In this technique, a low frequency (<1000Hz) sinusoidal vibration is applied to the femoral condyles and the resulting vibration is measured at the greater trochanter. If the prosthesis is securely fixed, the output vibration signal matches that of the input vibration, whereas if the prosthesis is loose, the output vibration signal is distorted and shows the marked presence of harmonics of the input frequency. One of the main problems with this application of this technique is in measuring the output vibration where significant amounts of soft tissue cover the measurement site. In order to circumvent this problem, an ultrasound probe, normally used for the measurement of blood flow, has been used to measure the output vibration. This has been evaluated by comparing the results obtained from the ultrasound probe with those from a conventional accelerometer in models representing a tight and loose hip prosthesis under simulated clinical conditions. The ultrasound probe was able to consistently detect the output vibration, for both the loose and secure prostheses. Under the test conditions used (which attempted to simulate a large thickness of soft tissue), the ultrasound probe was able to produce a greatly enhanced output vibration signal compared to the accelerometer. This suggests that the use of an ultrasound probe to detect mechanically induced vibration through significant amounts of soft tissue appears to be viable and could lead to enhanced detection of prosthesis loosening using this technique.

Weight bearing after tibial fracture as a guide to healing.

BACKGROUND: Judging when it is safe to remove an external fixator or plaster cast requires clinical and radiological assessment, both of which are subjective. Weight bearing has been shown to increase with time post-fracture and we hypothesised that it could be used as an objective measure of fracture healing. METHODS: Ground reaction force (and hence weight bearing) and fracture stiffness were measured serially in a group of 12 patients with tibial fractures treated by external fixation. Ground reaction force was measured for both fractured and non-fractured limbs using a force plate and the fracture stiffness was measured using the Orthometer, a commercially produced device for measuring the stiffness of fractures treated by external fixation. FINDINGS: In 10 patients who made good recoveries, prior to fixator removal, weight bearing though the injured leg was seen to approach 90% of that through the uninjured leg and the fracture stiffness exceeded 15 Nm/deg. Two patients with delayed union achieved weight bearing of 40% of normal and a fracture stiffness of less than 5 Nm/deg at 20 weeks. INTERPRETATION: Weight bearing correlates reasonably well with fracture stiffness. It is quicker and easier to measure than fracture stiffness and potentially has relevance to other fracture fixation methods.

Precision of wear measurement using the shadowgraph technique.

The shadowgraph technique is a relatively easy-to-use and inexpensive method of wear measurement from explanted acetabular cups. In this technique, from a mould of the internal surface of the cup, measurements of linear wear and wear angle can be obtained, from which volumetric wear can be calculated. In this study the measurement precision of this technique was assessed and the influence of different observers and multiple moulds was also determined. It was found that linear wear (coefficient of variation (CV) = 1.49 per cent) can be more precisely determined than wear angle (CV = 8.18 per cent) and that both the observer and the mould can significantly influence the results obtained, although the influence of the mould is considerably less than that of the observer.

Modelling the effects of different fracture geometries and healing stages on ultrasound signal loss across a long bone fracture.

The effect on the signal amplitude of ultrasonic waves propagating along cortical bone plates was modelled using a 2D Finite Difference code. Different healing stages, represented by modified fracture geometries were introduced to the plate model. A simple transverse and oblique fracture filled with water was introduced to simulate the inflammatory stage. Subsequently, a symmetric external callus surrounding a transverse fracture was modelled to represent an advanced stage of healing. In comparison to the baseline (intact plate) data, a large net loss in signal amplitude was produced for the simple transverse and oblique cases. Changing the geometry to an external callus with different mechanical properties caused the net loss in signal amplitude to reduce significantly. This relative change in signal amplitude as the geometry and mechanical properties of the fracture site change could potentially be used to monitor the healing process.

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.

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.

Shear bond strength and residual adhesive after orthodontic bracket debonding.

OBJECTIVE: To compare the shear bond strength and determine the area of residual adhesive on teeth after the debonding of brackets bonded with two types of orthodontic adhesives. These were a resin-modified glass ionomer cement (RMGIC; Fuji ORTHO LC, GC Corporation, Tokyo, Japan) and a resin applied as a precoated bracket (APC bracket, 3M Unitek GmbH, Seefeld, Germany). MATERIALS AND METHODS: A total of 60 premolar teeth were randomly divided into two groups, and brackets were bonded according to the manufacturers' instructions. In group 1, the teeth were conditioned using 10% polyacrylic acid, and the brackets were bonded using Fuji Ortho LC in wet condition. In group 2, the teeth were etched using 37% phosphoric acid, and the APC brackets were bonded. Bond strength was measured using a testing instrument (2000S, Lloyds Instruments, Fareham, England) at a crosshead speed of 1 mm/min, and the residual adhesive was quantified using a three-dimensional laser scanning instrument. RESULTS: The Mann-Whitney test showed that the median bond strength of group 1 was significantly lower than that of group 2 (P < .001). A Pearson chi-square test of the Adhesive Remnant Index (ARI) revealed a significant difference among the groups tested. All the adhesives in group 1 failed at the enamel/adhesive interface (100%), whereas group 2 exhibited cohesive failure of the adhesive (90%). CONCLUSIONS: The bond strength values obtained with the RMGIC were above the minimum values suggested in the literature to achieve a clinically effective adhesion in orthodontics.

A novel approach to bite force measurements in a porcine model in vivo.

A novel device for a bite force measurement system in a porcine model is described. A single polyethylene layer was vacuum-formed into a splint, and a force sensor was fitted on to the splint occlusal surface and seated with a silicone layer. This design enabled the measurement of bite forces on selected teeth in a large animal model with either natural dentition or single implant crowns and could be used in assessing information on biomechanical adaptation of the bone-implant interface to masticatory loads. Preliminary recordings of force values obtained during mastication in the premolar region (200-560 N) and various limiting factors of the technical design are discussed.

A radiological and laboratory assessment of polyethylene wear in failed total hip replacement.

This study assessed the reliability of radiological assessment of polyethylene wear in loose total hip replacements (THRs) by comparing it with a laboratory assessment using the shadowgraph technique. It was performed in Avon Orthopaedic Centre, Southmead Hospital, from September 1997 to June 1998. True linear wear depth measured using the shadowgraph technique was 42% greater than that determined from the radiological femoral head eccentricity in standard non-weightbearing plain x-rays. There was an inverse statistically significant correlation between cup inclination angle and the volumetric wear rate. The linear wear rate, as determined with the shadowgraph technique, was greater in retrieved cups during the first six years postoperatively, but reduced to a lower level in cups retrieved after that time of service.

The compatibility of ceramic bone graft substitutes as allograft extenders for use in impaction grafting of the femur.

This study investigates the use of porous biphasic ceramics as graft extenders in impaction grafting of the femur during revision hip surgery. Impaction grafting of the femur was performed in four groups of sheep. Group one received pure allograft, group two 50% allograft and 50% BoneSave, group three 50% allograft and 50% BoneSave type 2 and group four 10% allograft and 90% BoneSave as the graft material. Function was assessed using an index of pre- and post-operative peak vertical ground reaction force ratios. Changes in bone mineral density were measured by dual energy X ray absorptiometry (DEXA) scanning. Loosening and subsidence were assessed radiographically and by histological examination of the explanted specimens. There was no statistically significant difference between the four groups after 18 months of unrestricted functional loading for all outcome measures.

Glass ionomer as an expander of allograft in revision arthroplasty of the hip.

The use of glass ionomer as a bone graft expander was investigated in an in vivo model of revision hip arthroplasty. Bone grafts of pure allograft and allograft + glass ionomer particles in a 50:50 by weight mixture were implanted in an ovine hemi-arthroplasty model. Post-operative assessments of locomotor function, radiographic appearance and quantitative changes in mineralisation around the graft were made at 2, 4 and 6 months. Post-mortem assessments of radiographic and histologic appearance of the grafts were made at 6 months. No significant differences were noted in any of the measured or assessed parameters between the two graft types. The glass ionomer particles seemed to be well tolerated within the matrix of new bone, smaller sized particles appearing to be better incorporated than larger ones. The use of particles of glass ionomer as a bone graft expander, in this in vivo model of revision hip arthroplasty, would therefore appear to offer no detriment in performance over pure allograft in the short to medium term.