Effect of trunnion roughness and length on the modular taper junction strength under typical intraoperative assembly forces.

Modular hip implants are at risk of fretting-induced postoperative complications most likely initiated by micromotion between adjacent implant components. A stable fixation between ball head and stem-neck taper is critical to avoid excessive interface motions. Therefore, the aim of this study was to identify the effect of trunnion roughness and length on the modular taper strength under typical intraoperative assembly forces. Custom-made Titanium trunnions (standard/mini taper, smooth/grooved surface finish) were assembled with modular Cobalt-chromium heads by impaction with peak forces ranging from 2kN to 6kN. After each assembly process these were disassembled with a materials testing machine to detect the pull-off force as a measure for the taper strength. As expected, the pull-off forces increased with rising peak assembly force (p < 0.001). For low and moderate assembly forces, smooth standard tapers offered higher pull-off forces compared to grooved tapers (p < 0.038). In the case of an assembly force of 2kN, mini tapers showed a higher taper strength than standard ones (p=0.037). The results of this study showed that smooth tapers provided a higher strength for taper junctions. This higher taper strength may reduce the risk of fretting-related complications especially in the most common range of intraoperative assembly forces.

A biomechanical study of the effects of simulated ulnar deviation on silicone finger joint implant failure.

Silicone finger arthroplasties are used widely, especially for metacarpophalangeal joint replacement in patients with inflammatory arthritis. Implant failure is well recognized. The rates of failure in vivo differ substantially from experience in vivo. One cause of failure is felt to be post-operative ulnar deviation. The aim of our study was to test the effect of ulnar deviation testing on silicone finger implants. We tested 12 implants in three groups of four implants. The implants were submerged in a bath of Ringer's solution at 370 °C throughout the experiment and tested in a rig held in 0°, 10° and 20° deviation. The rig was cycled at 1.5 Hz from 0°-90°. The implants were inspected every 500,000 cycles until a total of 4 million cycles. There was consistently increased wear and supination plastic deformity in going from 0°-20° deviation. This study confirms the adverse effects of ulnar deviation on silicone finger implant wear. It is likely that this combines with lateral pinch forces and sharp bone edges to cause catastrophic silicone implant failure. LEVEL OF EVIDENCE: III.

In vitro method for assessing the biomechanics of the patellofemoral joint following total knee arthroplasty.

The patellofemoral joint is a common site of pain and failure following total knee arthroplasty. A contributory factor may be adverse patellofemoral biomechanics. Cadaveric investigations are commonly used to assess the biomechanics of the joint, but are associated with high inter-specimen variability and often cannot be carried out at physiological levels of loading. This study aimed to evaluate the suitability of a novel knee simulator for investigating patellofemoral joint biomechanics. This simulator specifically facilitated the extended assessment of patellofemoral joint biomechanics under physiological levels of loading. The simulator allowed the knee to move in 6 degrees of freedom under quadriceps actuation and included a simulation of the action of the hamstrings. Prostheses were implanted on synthetic bones and key soft tissues were modelled with a synthetic analogue. In order to evaluate the physiological relevance and repeatability of the simulator, measurements were made of the quadriceps force and the force, contact area and pressure within the patellofemoral joint using load cells, pressure-sensitive film, and a flexible pressure sensor. The results were in agreement with those previously reported in the literature, confirming that the simulator is able to provide a realistic physiological loading situation. Under physiological loading, average standard deviations of force and area measurements were substantially lower and comparable to those reported in previous cadaveric studies, respectively. The simulator replicates the physiological environment and has been demonstrated to allow the initial investigation of factors affecting patellofemoral biomechanics following total knee arthroplasty.

Low torque levels can initiate a removal of the passivation layer and cause fretting in modular hip stems.

Taper connections of modular hip prostheses are at risk of fretting and corrosion, which can result in reduced implant survival. The purpose of this study was to identify the minimum torque required to initiate a removal of the passivation layer at the taper interface as a function of assembly force and axial load. Titanium stems and cobalt-chromium heads were assembled with peak impaction forces of 4.5 kN or 6.0 kN and then mounted on a materials testing machine whilst immersed in Ringer's solution. The stems were subjected to a static axial load (1 kN or 3 kN) along the taper axis. After a period of equilibration, a torque ramp from 0 to 15 Nm was manually applied and the galvanic potential was continuously recorded. Prostheses assembled with a force of 6 kN required a significantly higher torque to start a removal of the passivation layer compared to those assembled with 4.5 kN (7.23±0.55 Nm vs. 3.92±0.97 Nm, p=0.029). No influence of the axial load on the fretting behaviour was found (p=0.486). The torque levels, which were demonstrated to initiate surface damage under either assembly force, can be readily reached during activities of daily living. The damage will be intensified in situations of large weight and high activity of the patient or malpositioning of the prosthesis.

Biomechanical study of the efficacy of a new design of wrist guard.

BACKGROUND: Upper extremity injuries are frequent in the elderly or those undertaking extreme sporting activities. Commercially available wrist guards reduce the frequency of wrist fractures but are not widely used as they greatly restrict movement. METHODS: A new wrist guard was developed which provided protection to the "impact area" but does not restrict wrist or digital movement. A human hand model and a biomechanical test rig, which allowed the simulation of an adult fall from height, were developed. The ability of the new guard, which was tested with different levels of padding, to reduce peak impact forces and absorb energy on impact was measured and compared to a commercially available wrist guard. FINDINGS: The use of any guard reduced peak impact forces by a minimum of 31.8%. The new guard, despite a substantially reduced impact surface area, demonstrated the same reductions in peak force (48%) and ability to absorb energy on impact as the standard guard when fitted with comparable levels of padding. INTERPRETATION: These results indicate that the new guard, which allows movement of the wrist and digits, demonstrates the same ability to reduce impact forces and absorb energy as a commercially available guard despite its substantially reduced impact area. Such a guard may provide a better compromise between joint flexibility and protection than the status quo.

Biomechanical comparison of a novel castless arthrodesis plate with T-plate and cross pin techniques for canine partial carpal arthrodesis.

OBJECTIVES: To describe a novel canine castless partial carpal arthrodesis plate (par-CA) and its ex vivo biomechanical comparison with T-plate and cross pinning techniques for canine partial carpal arthrodesis. METHODS: The three implant systems were applied to three cohorts of six forelimbs from Greyhounds euthanatized for reasons unrelated to the study. Intercarpal and carpometacarpal palmar fibrocartilage and ligaments were sectioned. Potentiometers were applied between the radial carpal and third metacarpal bones to measure micromotion, and limbs were loaded at 30% of bodyweight at 1 Hertz for 10,000 cycles on a servo-hydraulic universal testing machine. Following assessment of micromotion, limbs were loaded to failure at 20 mm/s and ultimate strength, ultimate displacement, and stiffness were measured. RESULTS: The T-plate (p <0.01) and par-CA (p <0.01) had reduced micromotion relative to the cross pin constructs but there was no significant difference between the control, T-plate and par-CA constructs. There was no significant difference in ultimate strength between constructs. Ultimate displacement was reduced in the plated constructs. Stiffness did not differ between constructs. CLINICAL SIGNIFICANCE: The novel par-CA construct was biomechanically similar to the T-plate and both were superior to cross pins in resisting micromotion. There was no difference in load at failure between constructs. The par-CA plate permits radial and ulnar carpal bone compression, a more distal location of the plate to limit impingement, and placement of screws in two metacarpal bones; features which may offer clinical benefits over T-plate fixation.

Optimal configuration of the spiral linking technique for tendon repair. Biomechanical evaluation.

A previous study described a new spiral linking technique for tendon repairs and demonstrated that it was strong enough to be used in clinical practice as an alternative to the Pulvertaft tendon weave repair. However the repairs were less stiff, needed slightly more tendon length for the same repair and were a little bulkier. In this study two variables have been changed with a view to improving the spiral technique. At first the number of spirals was reduced consecutively, keeping the same number of standard mattress sutures. Once the optimal number of spirals had been identified, repairs with different numbers of sutures were tested using an alternative cross-stitch technique. The spiral repair technique using two spirals linked with six sutures was at least as strong and stiff as a four-weave Pulvertaft technique and was also easier to do.

A computer model of the artificially ventilated human respiratory system in adult intensive care.

A multi-technique approach to modelling artificially ventilated patients on the adult general intensive care unit (ICU) is proposed. Compartmental modelling techniques were used to describe the mechanical ventilator and the flexible hoses that connect it to the patient. 3D CFD techniques were used to model flow in the major airways and a Windkessel style balloon model was used to model the mechanical properties of the lungs. A multi-compartment model of the lung based on bifurcating tree structures representing the conducting airways and pulmonary circulation allowed lung disease to be modelled in terms of altered V/Q ratios within a lognormal distribution of values and it is from these that gas exchange was determined. A compartmental modelling tool, Bathfp, was used to integrate the different modelling techniques into a single model. The values of key parameters in the model could be obtained from measurements on patients in an ICU whilst a sensitivity analysis showed that the model was insensitive to the value of other parameters within it. Measured and modelled values for arterial blood gases and airflow parameters are compared for 46 ventilator settings obtained from 6 ventilator dependent patients. The results show correlation coefficients of 0.88 and 0.85 for the arterial partial pressures of the O(2) and CO(2), respectively (p<0.01) and of 0.99 and 0.96 for upper airway pressure and tidal volume, respectively (p<0.01). The difference between measured and modelled values was large in physiological terms, suggesting that some optimisation of the model is required.

Mechanical properties of three different compositions of calcium phosphate bioceramic following immersion in Ringer's solution and distilled water.

Dissolution tests were carried out to compare the mechanical properties of calcium phosphate based bioceramics with different compositions, before and after ageing for various time periods in Ringer's solution (pH 7.2) or distilled water (pH 7.2 and 4.0) at 37 degrees C. The results indicate that the sample composition seems to have more of an effect on the mechanical properties than does the storage environment. No obvious decrease in mechanical properties was found after samples had been aged in the various solutions during the different time periods. This indicates that these samples could be of significant clinical interest as their good structural properties were retained.

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.

Influence of setting liquid composition and liquid-to-powder ratio on properties of a Mg-substituted calcium phosphate cement.

The influence of four variables on various properties of a Mg-substituted calcium phosphate cement (CPC) was investigated. The variables were the heat treatment temperature of the precipitated powders, the composition of the setting liquid, the liquid-to-powder ratio (LPR), and the time over which hardened specimens were cured in air. The properties analysed were the phase composition of the starting powder, the initial setting time, the evolution of the storage shear modulus (G') and the loss shear modulus (G'') with the cement paste curing time (t), and the compressive strength. The presence of alpha-TCP in CPC facilitated the setting and hardening properties due to its progressive dissolution and the formation of brushite crystals. As far as the liquid composition is concerned, in cases where citric acid was used, adding a rheology modifier (10 wt.% polyethylene glycol or 0.5 wt.% hydroxyl propylmethylcellulose) to the acid led to an increase in the initial setting time, while an increase in the acid concentration led to a decrease in the initial setting time. The initial setting time showed to be very sensitive towards the LPR. The evolution of G' and G'' with curing time reflected the internal structural changes of cement pastes during the setting process. The compressive strength of the wet-hardened cement specimens with and without Mg increased with curing time increasing, being slightly higher in the case of Mg-substituted CPC. The results suggest that Mg-substituted CPC holds a promise for uses in orthopaedic and trauma surgery such as for filling bone defects.

Quantification of lung injury using ventilation and perfusion distributions obtained from gamma scintigraphy.

This paper explores the potential of isotope V/Q lung scans to quantify lung disease. Areas of restricted perfusion in subjects with a pulmonary embolus (PE) were identified in 3D reconstructions of V/Q images achieved using anatomical data from the Visible Human Project. From these, the extent of lung damage was quantified. Significant differences in the values of both LogSD V and LogSD Q (p > 0.05) obtained from plots of V and Q against Log(V/Q) were found between normal subjects and subjects with a PE, but no correlation was found between either of these parameters and the degree of lung damage in subjects with a PE (p > 0.05). Whilst V/Q values were log normally distributed, the V/Q distributions from the subjects with a PE failed to show the bimodal distribution predicted from theoretical considerations and MIGET measurements previously reported. There was a statistically significant difference in the mean and standard deviation values of the V/Q distributions between normal subject and subjects with a PE (p < 0.05) but not in the median values (p > 0.05). There was no correlation between the mean, median and standard deviation of the distributions from the subjects with a PE and the percentage of damage present (p > 0.05).

In-vitro study of medial strain distribution in the femur during impaction grafting.

Impaction bone grafting (IBG) is widely used for revision hip surgery to compensate for bone stock loss. It is performed by impacting morsellized allograft into the femoral canal and acetabulum prior to cementing new total hip components. Per- and post-operative femoral fractures and post-operative implant subsidence are major complications in IBG. The aim of this study was to investigate the strain distribution on the medial side of the femur during impaction grafting and the subsequent stability of the stem under uniaxial cyclic loading. The Exeter IBG technique was used in conjunction with Howmedica X-change instrumentation. Sawbones composite femora were used. An impactometer, which provides a known impaction energy and momentum, was used to standardize the impaction process. Three drop heights, 130, 260, and 390 mm, were used for proximal impaction. In-vitro medial hoop strains and the number of impacts were recorded. A drop height of 260 mm was found to provide sufficient energy for impaction without introducing excessive strains to achieve implant stability. Furthermore, a feasibility study was performed on the use of a proximal impaction cap (PIC) to restrain extrusion of the graft during impaction. Although no significant difference in impaction strains or stem stability in uniaxial cylic loading was found by using a PIC, it is postulated that the design of a proximal impactor could be improved to assist with proximal stem alignment and graft containment.

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.

The development of a model for in vitro testing of femoral stems in impaction bone grafting.

Comprehension of the biomechanical behaviour of orthopaedic implants is essential. This paper describes the development of an in vitro model to investigate the behaviour of femoral implants in the revision setting. The development of a femoral model and a bone graft substitute is described. The properties of human, bovine, ovine morselized bone graft, and a graft substitute were compared. On measuring hoop strain after impaction bone grafting there was no significant difference between the ovine bone graft and graft substitute with the size 1 Exeter stem. The results suggest that this bone graft substitute is a viable alternative for in vitro testing. The authors recommend the use of the graft substitute and the femoral model to predict femoral stem biomechanics.

Mechanical characterization of dense calcium phosphate bioceramics with interconnected porosity.

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramics were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. The samples had approximately 5-10% interconnected porosity and controlled pore sizes appropriate to allow bone ingrowth, combined with good mechanical properties. A range of polyurethane foams with 20, 30 and 45 pores per inch (ppi) were used as templates to produce samples for testing. The foams were inpregnated with solid loadings in the range of 60-140 wt%. The results indicated that the average apparent density of the HA/TCP samples was 2.48 g/cm(3), the four-point bending strength averaged 16.98 MPa, the work of fracture averaged 15.46 J/m(2) and the average compressive strength was 105.56 MPa. A range of mechanical properties resulted from the various combinations of different grades of PU foam and the solid loading of slips. The results indicated that it is possible to manufacture open pore HA/TCP bioceramics, with compressive strengths comparable to human bone, which could be of significant clinical interest.

Fabrication of porous bioceramics with porosity gradients similar to the bimodal structure of cortical and cancellous bone.

The aim of this study was to fabricate porous implant materials with graded pore structures similar to the bimodal structure of cortical and cancellous bone. Porous hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramics with interconnected porosity and controlled pore sizes required to simulate natural bone tissue morphology were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. Functionally gradient materials (FGMs) with porosity gradients were made by joining different pore per inch (ppi) foams together by either stitching or pressfitting to form templates. Post production, no defects could be seen at the interface between the two different porosity sections. The macropore sizes of the HA/TCP bioceramics were larger than 100 mum which is appropriate for bone ingrowth. A sample with a graded porous structure which is close to the human bone morphology was also developed. The two component structures were conspicuously different but joined together firmly. Four point bend testing of FGM samples showed them to have similar mechanical properties to homogeneous ceramics based on foam templates with uniform pore sizes, with no evidence of interfacial weakness. Many potential biomedical applications could be developed utilising graded porous structures. The ease of processing will make it possible to fabricate a range of complex shapes for different applications.

Fabrication and mechanical testing of porous calcium phosphate bioceramic granules.

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) granules were fabricated by a novel technique of vacuum impregnation of reticulated polyurethane (PU) foams with ceramic slip. The resultant granules had 5-10% interconnected porosity with controlled pore sizes necessary to allow bone ingrowth combined with good mechanical properties. Using PU foams with a different number of pores per inch (ppi), porous HA/TCP granules in the size range of 2-8 mm were successfully manufactured. Dieplunger tests were used to compare the compression and relaxation properties of the granules with those of a commercially available bone graft product, BoneSave. The results of the die-plunger testing showed that the experimental granules were stiffer than the BoneSave materials and had less of a tendency to crumble to powder after testing. This therefore suggests that these experimental granules would be useful for impaction grafting and space filling applications.

An in vitro biomechanical study comparing the spiral linking technique against the pulvertaft weave for tendon repair.

A new spiral linking technique for tendon repair in which one end of the tendon is spiralled around the other end has been developed. Using pig trotter extensor tendons, the Pulvertaft weave technique was compared with this new technique. Twenty-five repairs using each technique were tested by tensile loading with an Instron testing machine. The spiral linking technique matched the strength of Pulvertaft method: the mean peak loads were 102 and 105 N, respectively. The Pulvertaft weave was stiffer than the spiral linking technique: mean stiffness of 11.1 and 6.7 N/mm, respectively. The spiral linking technique also absorbed considerably more energy: energy absorbed prior to failure to 90% of peak load, 1.75 and 1.13 kN mm, respectively. In conclusion, the spiral linking technique appears as strong as the Pulvertaft weave and we believe it is easier to perform.