Sabina Gheduzzi

Mechanical characteristics of antibiotic-laden bone cement.

We studied the mechanical characteristics of cement-antibiotic combinations in vitro. Palacos R was tested without antibiotics, with gentamicin alone and with gentamicin plus vancomycin or flucloxacillin. Palacos LV was studied only with gentamicin added. CMW I was studied with gentamicin added, with gentamicin plus vancomycin, and with gentamicin plus flucloxacillin. We performed four-point bending tests on beams of cement to establish bending strength and modulus, and compared the values to ISO standards. Density was also assessed. Palacos R was the strongest of the cements (bending strength 80 MPa). Palacos formulations (apart from Palacos LV) had a higher density and bending modulus than CMW 1. Statistical comparison of various cements with plain Palacos R showed lower density in 4 of the mixtures, and lower bending strength and modulus in 6 of the mixtures. Palacos R/gentamicin plus vancomycin and CMW 1/gentamicin plus vancomycin had bending strength slightly above minimum ISO standards, suggesting that the addition of vancomycin during cementmixing may compromise the outcome in revision surgery for sepsis.

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.

Ultrasound mimics the effect of mechanical loading on bone formation in vivo on rat ulnae.

While the effect of ultrasound as an extreme example of low-magnitude high-frequency stimulation has been explored in the response of bone to injury, little is known about its effect on normal bone. This experiment was designed to test the hypothesis that ultrasound exerts a similar influence on bone as mechanical stimulation at a physiological level. Three groups of female Wistar rats were anaesthetised (6 per group). In one group, the left ulna was loaded cyclically in vivo 40 times, repeated on a further 5 occasions on alternate days. In a second group, transcutaneous low-intensity pulsed ultrasound stimulation was applied to the left ulnae for the same duration as the period of loading. In a third group, loading and ultrasound stimulation were applied concurrently. The right ulna served as non-loaded control in each animal. At the end of the experiment after 14 days, both ulnae were removed. Induced bone formation was assessed by measuring the proportion of medial periosteal bone surface with double label (dLS/BS, %) and by calculation of mineral apposition rate (MAR) from the inter-label distance. All three treatments induced a significant periosteal response, increasing dLS/BS values from <10% in control limbs to >80% in treated limbs. Increases in MAR of experimental ulnae versus contralateral control ulnae were 2.9 (+/-0.9), 8.6 (+/-2.4) and 8.7 microm (+/-3.2) for the ultrasound only, ultrasound and load, and load only groups, respectively. The effects of loading plus ultrasound were not significantly different from ultrasound alone. These data suggest that ultrasound is able to induce changes in bone that share at least some features with mechanical loading.

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.

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.

The development of a dynamic, six-axis spine simulator

BACKGROUND CONTEXT Although a great deal of research has been completed to characterize the stiffness of spinal specimens, there remains a limited understanding of the spine in 6 df and there is a lack of data from dynamic testing in six axes. PURPOSE This study details the development and validation of a dynamic six-axis spine simulator. STUDY DESIGN Biomechanical study. METHODS A synthetic spinal specimen was used for the purpose of tuning the simulator, completing positional accuracy tests, and measuring frequency response under physiological conditions. The spine simulator was used to complete stiffness matrix tests of an L3-L4 lumbar porcine functional spinal unit. Five testing frequencies were used, ranging from quasistatic (0.00575 Hz) to dynamic (0.5 Hz). Tests were performed without an axial preload and with an axial preload of 500 N. RESULTS The validation tests demonstrated that the simulator is capable of producing accurate positioning under loading at frequencies up to 0.5 Hz using both sine and triangle waveforms. The porcine stiffness matrix tests demonstrated that the stiffness matrix is not symmetrical about the principal stiffness diagonal. It was also shown that while an increase in test frequency generally increased the principal stiffness terms, axial preload had a much greater effect. CONCLUSIONS The spine simulator is capable of characterizing the dynamic biomechanics of the spine in six axes and provides a means to better understand the complex behavior of the spine under physiological conditions.

Antibiotic elution from bone cement: A study of common cement-antibiotic combinations

The in vitro antibiotic elution characteristics (including the effects of cement fracture) of the following cements were studied: 1) CMW 1 with gentamicin, 2) Palacos R with gentamicin, 3) Palacos LV with gentamicin, 4) CMW 1 with gentamicin and vancomycin, 5) Palacos R with gentamicin and vancomycin, 6) CMW 1 with gentamicin and flucloxacillin, and 7) Palacos R with gentamicin and flucloxacillin. Elution of both gentamicin and vancomycin was satisfactory in all cases. There tended to be a peak of antibiotic release on cement fracture, suggesting sequestration of active antibiotic within deeper layers of the cement. Palacos LV exhibited the best antibiotic elution characteristics but with the highest post-fracture peak. Palacos R was superior to CMW 1. Flucloxacillin was present only until day 4. Adulteration of proprietary Palacos R/gentamicin with flucloxacillin produced prolonged high elution of gentamicin, possibly due to porosity. Flucloxacilloic acid (microbiologically inactive) was present from day 4 onwards after flucloxacillin was added to cement. These findings suggest that flucloxacillin is not a suitable additive to bone cement in revision surgery. (Hip International 2002; 1: 23-7).

The dynamic, six-axis stiffness matrix testing of porcine spinal specimens

BACKGROUND CONTEXT Complex testing protocols are required to fully understand the biomechanics of the spine. There remains limited data concerning the mechanical properties of spinal specimens under dynamic loading conditions in six axes. PURPOSE To provide new data on the mechanical properties of functional spinal unit (FSU) and isolated disc (ISD) spinal specimens in 6 df. STUDY DESIGN Dynamic, six-axis stiffness matrix testing of porcine lumbar spinal specimens. METHODS The stiffness matrix testing of lumbar porcine FSU (n=6) and ISD (n=6) specimens was completed in a custom six-axis spine simulator using triangle wave cycles at a frequency of 0.1 Hz. Specimens were first tested without an axial preload, then with an axial preload of 500 N, with equilibration times of both 30 and 60 minutes. RESULTS The stiffness matrices were not symmetrical about the principal stiffness terms. The facets increased all the principal stiffness terms with the exception of axial compression-extension. Significant differences were detected in 15 stiffness terms because of the application of an axial preload in the ISD specimens, including an increase in all principal stiffness terms. There were limited differences in stiffness because of equilibration time of 30 and 60 minutes. CONCLUSIONS The assumption of stiffness matrix symmetry used in many previous studies is not valid. The biomechanical testing of spinal specimens should be completed in 6 df, at physiologic loading rates, and incorporate the application of an axial preload. The present study has provided new data on the mechanical properties of spinal specimens and demonstrates that the dynamic stiffness matrix method provides a means to more fully understand the natural spine and quantitatively assess spinal instrumentation.

A biomechanical evaluation of hinged total knee replacement prostheses

The number of total knee replacements being performed worldwide is undergoing an unprecedented increase. Hinged total knee replacements, used in complex salvage and revision procedures, currently account for a small but growing proportion of prostheses implanted. Modern hinged prostheses share the same basic configuration, allowing flexion–extension and tibial rotation. One aspect on which designs differ is the anteroposterior location of the hinge. A more posterior hinge is designed to increase the patellar tendon moment arm, reducing the quadriceps force required for a given activity and benefiting the patient. Five commonly used total knee replacements were evaluated in terms of quadriceps force and patellar tendon moment arm using a laboratory-based rig. Significant differences were identified between the five prostheses in quadriceps force and patellar tendon moment arm. Analysis of the correlation between these two parameters indicates that while patellar tendon moment arm influences quadriceps force, it is not the only factor. Also important is the lever function of the patella, and it is suggested here that the non-physiological nature of the prosthetic patellofemoral geometry may result in unnatural joint function. Thus, a thorough understanding of the resulting kinematic function of hinged total knee replacements is becoming increasingly important in complex revision total knee replacement to meet rising patient expectations and functional demands.

Intradiscal pressure changes with dynamic pedicle screw systems.

Study Design In vitro study using porcine spines instrumented with pedicle screw and rod fixation. Objectives To determine the intradiscal pressure (IDP) changes with the use of dynamic and rigid pedicle screw systems in simulated spinal fusion. Summary of Background Data The intervertebral discs are prone to injury under conditions of altered IDP. The effects of instrumentation with dynamic pedicle screw systems on IDP have not been clearly delineated. Methods A 2-level posterior instrumentation was applied to fresh porcine spinal segments (n=16). Dynamic and rigid pedicle screw constructs along with uninstrumented (n=6) spinal segments as controls were tested. The spinal segments were subjected to 24,000 cycles of flexion compression loading at 5 Hz. IDP within the instrumented (L2-L3 and L3-L4) and adjacent (L1-L2 and L4-L5) discs were measured using a pressure transducer needle. Results were recorded at 6000 cycle intervals. Results Instrumentation increased IDP. Within the instrumented levels, the greatest increase in IDP was found at the L2-L3 disc. Here, after 24,000 loading cycles, IDP for spines instrumented with mobile screws was 6.8 times higher than that of uninstrumented spines whereas for rigid screws the factor was 9.1. For the L3-L4 cases, the presence of instrumentation increased IDP by factors of 1.7 and 2.7 for mobile and rigid screws, respectively. In the uninstrumented levels, IDP at L1-L2 and L4-L5 was lower with mobile screws. These were statistically significant at for L1-L2 (24,000 cycles, P=0.008) and L4-L5 level (12,000, 18,000, and 24,000 cycles, P<0.04 in all cases). Conclusions Of the 2 types, mobile screws produced the least increase in IDP. This feature might be beneficial for the fusion process while at the same time prevent secondary pathology such as premature disc degeneration and facet joint pathology due to excessive disc pressures.