Evaluation of Aerosol Electrospray Analysis of Metal-on-Metal Wear Particles from Simulated Total Joint Replacement.

Wear is a common cause for aseptic loosening in artificial joints. The purpose of this study was to develop an automated diagnostical method for identification of the number and size distribution of wear debris. For this purpose, metal debris samples were extracted from a hip simulator and then analyzed by the electrospray method combined with a differential mobility analyzer, allowing particle detection ranging from several nanometers up to 1 µm. Wear particles were identified with a characteristic peak at 15 nm. The electrospray setup was successfully used and validated for the first time to characterize wear debris from simulated total joint replacement. The advantages of this diagnostic method are its time- and financial efficiency and its suitability for testing of different materials.

Biomechanical analysis of two fixation methods for proximal chevron osteotomy of the first metatarsal.

PURPOSE: The proximal chevron osteotomy provides high correctional power. However, relatively high rates of dorsiflexion malunion of up to 17 % are reported for this procedure. This leads to insufficient weight bearing of the first ray and therefore to metatarsalgia. Recent biomechanical and clinical studies pointed out the importance of rigid fixation of proximal metatarsal osteotomies. Therefore, the aim of the present study was to compare biomechanical properties of fixation of proximal chevron osteotomies with variable locking plate and cancellous screw respectively. METHODS: Ten matched pairs of human fresh frozen cadaveric first metatarsals underwent proximal chevron osteotomy with either variable locking plate or cancellous screw fixation after obtaining bone mineral density. Biomechanical testing included repetitive plantar to dorsal loading from 0 to 31 N with the 858 Mini Bionix(®) (MTS(®) Systems Corporation, Eden Prairie, MN, USA). Dorsal angulation of the distal fragment was recorded. RESULTS: The variable locking plate construct reveals statistically superior results in terms of bending stiffness and dorsal angulation compared to the cancellous screw construct. There was a statistically significant correlation between bone mineral density and maximum tolerated load until construct failure occurred for the screw construct (r = 0.640, p = 0.406). CONCLUSION: The results of the present study indicate that variable locking plate fixation shows superior biomechanical results to cancellous screw fixation for proximal chevron osteotomy. Additionally, screw construct failure was related to levels of low bone mineral density. Based on the results of the present study we recommend variable locking plate fixation for proximal chevron osteotomy, especially in osteoporotic bone.

The role of the peroneal tendons in passive stabilisation of the ankle joint: an in vitro study.

PURPOSE: Peroneal tendons are known as active stabilizer in acute ankle sprain while an intact ankle mortise and intact lateral ligaments are required for passive stability of the ankle joint. The goal of this study is to determine the peroneal tendons as passive stabilizer in case of lateral ligament instability. METHODS: Twelve (12) human lower leg cadaver specimens underwent a torsion simulation in the testing system, 858 Mini Bionix(®) (MTS(®) Systems Corporation, Eden Prairie, MN, USA) and a specially designed mounting platform for the specimens. The preset torsion between tibia and calcaneus was primarily set at 30° of internal rotation during plantar flexion and hindfoot inversion. The resisting torque around mechanical tibial axis was recorded which ensures stability in ankle sprain trauma. The first series of measurements were performed on healthy specimens and the following after transecting structures in following order: ATFL (anterior talofibular ligament) in combination with CFL (calcaneofibular ligament), followed by peroneus longus tendon and finally peroneus brevis tendon. RESULTS: The combined lateral ATFL and CFL instability shows a decrease of the resisting torque which ensures stability in ankle sprain trauma. Only a transection of PLT with existing lateral dual-ligament instability results in a significant decrease in torque (p < 0.0001). CONCLUSION: The PLT has a substantial effect on passive stability at a present lateral ligament lesion in ankle sprain trauma. A deficiency in viscoelastic properties of the peroneus longus tendon must be considered in diagnostic and treatment for ankle instability.

Effects of a surface matching articular resurfacing device on tibiofemoral contact pressure: results from continuous dynamic flexion-extension cycles.

INTRODUCTION: The application of a defect-size metal implant for the treatment of focal articular cartilage lesions of the femoral condyle is of potential concern resulting in cartilage damage to opposing biological structures. This in vitro study aims to determine the tibiofemoral contact pressure with a contoured articular partial femoral resurfacing device under continuous dynamic pressure loads. METHODS: Peak and area contact pressures were determined in eight fresh-frozen cadaveric specimens using a pressure-sensitive sensor placed in the medial compartment above the menisci. All knees were tested in the untreated condition and after implantation of the prosthetic device in the weight-bearing area of the medial femoral condyle. A robotic knee simulator was used to test each knee under continuous pressure load for 400 s during 40 dynamic knee bending cycles (5°-45° flexion) with body weight ground reaction force (GRF). The GRF was adjusted to the living body weight of the cadaver donor and maintained throughout all cycles. RESULTS: Comparison of the untreated condition to focal inlay resurfacing showed no statistically significant differences (P ≤ 0.05) between all testing conditions. The average maximum peak contact pressure across all 40 flexion cycles increased by 5.1% after resurfacing compared to the untreated knees. The average area contact pressure essentially stayed the same (+0.9%). CONCLUSION: The data suggest that resurfacing with the contoured articular prosthetic device does not pose any immediate deleterious effects to the opposing surfaces based on peak and area contact pressure in a continuous dynamic in vitro application. However, long-term in vivo effects remain to be evaluated.

Biomechanical comparison of knotless vs. knotted suture anchors in the acetabular rim with respect to bone density.

BACKGROUND: Acetabular labral tears are managed with suture anchors providing good clinical outcomes. Knotless anchors are easier to use and have a quicker insertion time compared to knotted anchors. The purpose of this study was to compare the biomechanical behavior of two different anchor designs (knotted vs. knotless) in ultimate load testing in correlation with bone density in the acetabular rim. METHODS: Eighteen knotted Bio-FASTak and seventeen knotless PushLock anchors (both: Arthrex Inc., Naples, FL, USA) were inserted in the rims of two human acetabula, with known bone density distribution. The anchors were subjected to load-to-failure tests. Anchors were additionally tested in solid polyurethane foam with defined densities. FINDINGS: The Bio-FASTak group showed higher survival rates at 1, 2, and 3 mm displacement and was able to withstand significantly higher loads at 3 mm displacement (p = 0.031). There was no statistically significant difference in stiffness (p = 0.087), yield- (p = 0.190), and ultimate load (p = 0.222) between the two groups. Only the PushLock group showed correlation between bone volume over total volume (BV/TV) and stiffness (R = 0.750, p = 0.086) and between BV/TV and yield load (R = 0.838, p = 0.037). Experiments on solid polyurethane foam confirmed the correlation between the mechanical properties and tissue density for the same anchor. INTERPRETATION: PushLock shows similar biomechanical properties to the Bio-FASTak, but eliminates knot tying and potentially abrasive knots. In addition, biomechanical properties of the PushLock are governed by local bone density.

Mechanical testing of different hip protectors according to a European Standard.

UNLABELLED: The effect of hip protectors depends on two factors: (1) mechanical features and (2) wearing time, which depends on the user's compliance. Various biomechanical studies have been performed to test hip protectors' ability in reducing impacts. However, none of these has shown the ability to simulate a fall in a natural way. Therefore, we tested conventional and newly designed hip protectors that might raise user's compliance using a standardised mechanical test. MATERIALS AND METHODS: Mechanical testing was performed according to a European Standard testing method for protective impact clothing (EN 1621-1) where an impact is released vertically onto the hip protectors with an energy of 50J. Seven different hip protectors were tested three times each. RESULTS: Using this test, four of the hip protectors were able to reduce the impact below a suggested fracture threshold. The two newly designed hip protectors using special absorption materials were superior (9.10kN, 12.65kN) to the other commercially available hip protectors (21.97-50.62kN), which differ in their mechanical testing performance. DISCUSSION: This is the first study using a standardised mechanical test on hip protectors and allows an objective comparison because only mechanical properties are tested. New hip protectors with improved mechanical properties are superior to conventional hip protectors. Furthermore, they allow a more appealing design that increases the comfort of the wearer. Mechanical testing should be performed as a first step and has to be followed up by clinical trials to determine and clarify their overall effect.

Hip fractures and the contribution of cortical versus trabecular bone to femoral neck strength.

Osteoporotic fractures are caused by both cortical thinning and trabecular bone loss. Both are seen to be important for bone fragility. The relative contributions of cortical versus trabecular bone have not been established. The aim of this study was to test the contribution of cortical versus trabecular bone to femoral neck stability in bone strength. In one femur from each pair of 18 human cadaver femurs (5 female; 4 male), trabecular bone was completely removed from the femoral neck, providing one bone with intact and the other without any trabecular structure in the femoral neck. Geometrical, X-ray, and DXA measurements were carried out before biomechanical testing (forces to fracture). Femoral necks were osteotomized, slices were analyzed for cross-sectional area (CSA) and cross-sectional moment of inertia (CSMI), and results were compared with biomechanical testing data. Differences between forces needed to fracture excavated and intact femurs (DeltaF/F mean) was 7.0% on the average (range, 4.6-17.3%). CSA of removed spongiosa did not correlate with difference of fracture load (DeltaF/F mean), nor did BMD. The relative contribution of trabecular versus cortical bone in respect to bone strength in the femoral neck seems to be marginal and seems to explain the subordinate role of trabecular bone and its changes in fracture risk and the effects of treatment options in preventing fractures.

Tibiofemoral contact mechanics with a femoral resurfacing prosthesis and a non-functional meniscus.

BACKGROUND: Increased contact stress with a femoral resurfacing prosthesis implanted in the medial femoral condyle and a non-functional meniscus is of concern for potential deleterious effects on tibiofemoral contact mechanics. METHODS: Peak contact pressures were determined in seven fresh frozen human cadaveric specimens using a pressure sensitive sensor placed in the medial compartment above the menisci. A knee simulator was used to test each knee in static stance positions (5 degrees/15 degrees/30 degrees/45 degrees) and through 10 dynamic knee flexion cycles (5-45 degrees) with single body weight ground reaction force which was adjusted to the living body weight of the cadaver donor. All specimens were tested in three different conditions: untreated knee (A); flush implantation of a 20mm resurfacing prosthesis (HemiCAP) in the weight bearing area of the medial femoral condyle (B); complete radial tear at the posterior horn of the medial meniscus with the femoral resurfacing device in place (C). FINDINGS: On average, flush device implantation resulted in no statistically significant differences when compared to the untreated normal knee. The meniscal tear resulted in a significant increase of the mean maximum peak contact pressures by 63%, 57%, and 57% (all P< or =0.05) at 15 degrees , 30 degrees and 45 degrees static stance positions and 78% (P< or =0.05) through the dynamic knee flexion cycle. No significant different maximum peak contact pressures were observed at 5 degrees stance position. INTERPRETATION: Although the condition of a meniscal tear without the resurfacing device could not be compared, possible effects of reduced meniscal tissue and biomechanical integrity of the meniscus must be considered in an in vivo application.