Decrease In Contact Pressure Research Articles

Tension, abduction, and surgical technique affect footprint compression after rotator cuff repair in an ovine model

Shoulder motion after rotator cuff repair may result in changes in tension and contact pressure at the repair site. Our goal was to determine how tension and motion affect a repair and what type of repair best tolerates these variables. Rotator cuff tears were created ex vivo in 30 ovine shoulders divided into 5 groups: single-row repair, double-row repair, tension-band repair, suture bridge repair, and double-row tension-band repair. Apressure probe was passed through a hole created in the footprint to dynamically measure footprint pressure. The rotator cuffs were repaired, and contact pressure was measured with variable tension placed on the repaired tendon from 10 to 30 N and variable shoulder abduction from -10° to +10°. Repair strength was determined by use of a pull-to-failure test. Increasing tension on the repaired tendon resulted in an increase in contact pressure whereas increasing the abduction angle resulted in a decrease in contact pressure in all 5 groups. For all abduction and tension combinations, the suture-bridge and double-row tension band groups recorded the highest contact pressures (P < .05), followed by the tension-band, single-row, and double-row repairs. Load to failure was greatest for the 2 double-row techniques, followed by the tension-band, suture-bridge, and single-row repairs. Contact pressure increases as tension increases across the repair and decreases as the shoulder is abducted. The double-row tension-band rotator cuff repair showed the best combination of contact pressure and repair strength.

Effects of Hub Contact Shape on Contact Pressure and Fatigue Life in a Press-Fitted Shaft

The objective of this study is to clarify the effect of hub contact shape on contact pressure and fatigue life with regard to the selection of a suitable taper design near the end of the fit. A numerical asymmetric-axisymmetric finite element model was developed in order to determine the contact stress state of press-fitted shaft by using four types of tapered contact surfaces on the hub. The variations of fatigue crack initiation life according to the change of tapered contact surfaces on the hub were evaluated by using the Smith-Watson-Topper (SWT) multiaxial fatigue criterion. As the result, comparing with the contact pressure and the fatigue crack initiation life, maximum decrease of contact pressure and maximum increase of fatigue crack initiation life were obtained for the 1/400 m/m tapered hub subjected to a bending load near the fretting fatigue limit. Furthermore, as the change of bending load, the optimal amout of taper in hub which fatigue life gets into maximum is varied. Therefore, we suggest that the best performance, in terms of pressure distribution and fatigue life of press fit, can be obtained by using a proper taper values for the hub element.

Biomechanical Evaluation of Arthroscopic Rotator Cuff Repairs Over Time

The aim of this study was to assess the contact pressure, force, and area over time for 4 common arthroscopic rotator cuff repair techniques. The transosseous-equivalent, single-row, triangle double-row, and suture-chain transosseous repair techniques were used to repair a full-thickness tear of the supraspinatus in 16 cadaveric shoulders. Continuous data points were collected immediately after repair and for 160 minutes at set time intervals by use of a custom thin film pressure sensor. Each of the 4 rotator cuff repair techniques showed decreased contact force, pressure, and area 160 minutes after the repair was performed. The transosseous-equivalent construct had the highest contact pressure and force initially and at all time points up to 160 minutes. Although the 3 double-row constructs had greater pressure and force at all time points compared with the single-row repair, only the transosseous-equivalent group showed a statistically greater pressure and force when compared with single-row repair (P < .05). Contact pressure, force, and pressurized footprint area decrease 160 minutes after repair regardless of repair technique. The transosseous-equivalent group had the highest contact pressure and force at all time points. The decrease in contact pressure and force after rotator cuff repair may have important implications in evaluating tendon-to-bone healing and determining the optimal rehabilitation protocol.

Long-term contact-coupled wear prediction for metal-on-metal total hip joint replacement

The major long-term failure identified for metal-on-UHMWPE is aseptic loosening due to wear generation. Metal-on-metal hip joint replacement has been found to have minimal wear compared to metal-on-UHMWPE. The objective of this study was to apply the computational simulation in predicting wear of metal-on-metal total hip joint replacement. Finite-element model of half ball and the 45° inclined cup was developed to represent the femoral head and acetabular cup, respectively. A simple Archard's equation was used to simulate the wear process combining with the finite-element model. The wear simulation of 28 mm femoral head with 60 μm diametral clearance was carried out for up to 50 million cycles, and subsequently analysed. The contact pressure decreased dramatically from the initial cycle to the first million cycles accompanied by an increase of contact area; however, the decrease of contact pressure was very small between 30 and 50 million cycles. There was relatively good agreement in volumetric wear between the present and hip simulator study. The total predicted volumetric wear for 50 million cycles was 4.2 mm3. It was found that the post-wear bearing surface would be advantageous to the hip implant by increasing the lubricating film and thus decreasing wear.

Loss in mechanical contact of cementless acetabular prostheses due to post-operative weight bearing: A biomechanical model

The primary stability of cementless acetabular components is a prerequisite for their clinical success. The target of the present study was to analyse possible effects of post-operative joint loading on the initial mechanical stability of a press-fitted acetabular prosthesis. For this purpose, a three-dimensional finite element model of the pelvic bone with acetabular reconstruction was set-up. The analysis included two steps: (1) simulation of the prosthesis press-fit implantation and (2) simulation of the instant of peak resultant hip loading during the one-legged stance. The difference between the contact pressures at the bone/implant interface, at the end of the second step and those at the end of the first step was calculated and assumed as an index of variation in mechanical contact due to post-operative weight bearing. The results show that, due to hip loading, contact pressures given by press-fit increase in the postero-superior acetabular region but decrease in the antero-inferior acetabular region. The calculated area in which the contact pressures decrease extend to about 30% of the total contact surface. These results imply that post-operative joint loading significantly reduces the mechanical stability given by press-fit. The decrease in contact pressures at the bone/implant interface may result in a lack of osteointegration, possibly hindering the implant secondary stability. It may also create a route for wear debris, possibly favouring periprosthetic osteolysis, which may lead to further loss in contact and clinical failure of the implant due to loosening.

Influence of polyethylene creep behavior on wear in total hip arthroplasty

After total hip arthroplasty (THA), polyethylene acetabular liner creep occurs quickly and serves to increases head-liner contact area and decrease contact pressures. What effect these early changes in contact mechanics will have on the wear behavior of the articulation remains unclear, and hence, selection or modification of polyethylene materials for optimal creep and wear performance is impossible. The objective of this study was to determine the influence of polyethylene creep behavior on volumetric wear and linear creep and wear penetration during simulated gait loading conditions. A finite element model of THA articulation was developed, and simultaneous numerical creep and wear simulation was performed to 10 million gait cycles with three levels of polyethylene creep behavior. Long-term volumetric wear and penetration were surprisingly unaffected by the polyethylene creep behavior due to the competing decrease in contact pressures coupled with increased contact area. In addition, variation in contact mechanics with the creep levels studied was only noteworthy in the initial postoperative period; after 1 million gait cycles, peak contact pressures and areas were within 13% regardless of the creep material behavior selected. Femoral head size had considerable impact on wear and penetration, while liner thickness primarily affected only early penetration. These results suggest that polyethylene creep behavior plays a major role in early penetration, but has little influence on the more important long-term volumetric wear.

THE RESEARCH INTO THE DISAXIAL CAM MECHANISM FOR DIESEL FUEL ‐ INJECTION PUMP

A cam mechanism is named disaxial due to the offset of a plunger axis concerning the axis of a camshaft. The named offset allows to reduce the maximal angle of pressure. There is decrease of contact pressure in a pair of a cam and a roller and the pressure upon the side face of a guide sleeve. Theoretical researches into the influence of the offset and other elements of a cam mechanism on the specified pressure and kinematics of a plunger are resulted. Examples of practical realization of disaxial designs are shown. The results of comparative tests of fuel‐injection pumps with disaxial and without disaxial are shown.

Thermal contact conductance for cylindrical and spherical contacts

A prediction methodology based on Monte-Carlo simulation model, developed for flat conforming surfaces in contact, is modified and extended to predict contact conductance between curvilinear surfaces like cylinders and spheres. Experiments are also conducted in vacuum for the measurement of contact conductance between stainless steel and aluminium cylindrical contacts and stainless steel spherical contacts over a range of contact pressures. The contact conductance between cylindrical and spherical bodies is, in general, about an order of magnitude lower than for flat surfaces in contact. Increase of surface roughness and decrease in contact pressure lowers the contact conductance. However, the influence of these parameters is larger than those obtained for flat surfaces. The prediction for different parametric conditions agree closely with those measured in the experiments.

Surface constrained plasticity: Oxide rupture and the yield point process

Abstract Nanoindentation of single crystals has been a topic of several recent investigations. This is a result of their ability to withstand near theoretical stresses without showing signs of plastic deformation. When plasticity occurs, it produces a yield point, a sudden discontinuous increase in indenter displacement and decrease in contact pressure. This study combines atomic force microscopy (AFM) with nanoindentation to focus on the roles that oxide and asperities play in the yield point process. Time dependent and instantaneous yield point properties were investigated for single crystals of tungsten and Fe–3 wt% Si in variable temperature and variable humidity environments. AFM observations indicate that the presence of asperities has a dramatic effect on the time dependent yield point properties. AFM measurements also provide evidence for plasticity in the absence of a yield point, suggesting that dislocation nucleation can occur well before a yield point is observed. Measurements on the dependenc...

Fretting Fatigue Behavior of an Aluminum Alloy.

Fretting fatigue tests were carried out using a 2618 aluminum alloy to investigate the effect of contact pressure and relative slip range between the specimen and the contact pad on fretting fatigue strength. Irrespective of contact pressure and relative slip range, the fretting fatigue strength was reduced to one quarter of the plain fatigue strength under the stress ratio R=0.0. However, a slight reduction in fretting fatigue strength occured with a decrease in contact pressure and with an increase in relative slip range, respectively. Friction force came to a constant value beyond a certain relative slip range (9 μm) under a pad-span of 30 mm and a contact pressure of 20 MPa. MoS2+PTFE coating improved fretting fatigue strength. And using fracture mechanics models, a prediction method for fretting fatigue endurance was assessed. The predicted fretting fatigue endurances were in good agreement with the experimental results of fretting fatigue tests.

The roles of brittle microfracture and plastic flow in the wear of MgO single crystals

The roles of brittle microfracture and plastic flow in the wear of MgO single crystals were investigated using a repeated rubbing apparatus. Wear after a small number of rubbing cycles occurs by brittle microfracture under the severe contact stress associated with the small contact area of the slider, although a plastically flowed layer is formed locally in a small region of the wear surface. Plastic flow occupies a larger region of the wear surface as wear increases, owing to the decrease in contact pressure caused by an increase in the contact area of the slider after many repeated slidings. Wear during this period, in which there is a very low wear rate, is caused by removal of the plastically flowed layer from the surface and corresponds to wear under a steady state process. When the tip of the slider approaches a subsurface parallel crack, a rectangular particle is formed; this particle is surrounded by the subsurface parallel crack, by two normal cracks K 1, which coincide with chevron cracking, and by two normal cracks K 2, which reach the subsurface parallel crack by a fatigue process after many repeated rubbing cycles. The rectangular particle is removed from the wear track and the subsurface parallel crack plane is locally exposed. The plastically flowed layer is readily produced after the material freed from the matrix is removed by frictional action although a new subsurface parallel crack below the previous subsurface parallel crack can be formed by further rubbing.

Wear Behavior of MgO Single Crystals under Vibrational Sliding (1st. Report)

From some microscopic observations, the appearance of many cavities (slip-shaped wear scars) was found on the wear surface of MgO in the steady state after the vibrational sliding of 104 cycles. The less rate of wear in this state not only depends on the decrease of fatigue damage on the wear surface, which is due to the decrease of contact pressure at a real contact point, but may be explained by a phenomenon that many of the wear fragments were packed into the cavities without being accumulated in the wear interface. It is suggested that rolling contact of the wear fragments in the interface play a significant role in making the cavities. The cavities, however, were not produced at the condition below the amplitude of 60μm because the wear was too less.Internal creack which appears to be independent from various types of surface damages was found within the areas about 50μm under the wear surface of MgO and was parallel to the surface (cleavage plane). In this case, there is a possibility that the surface material would be locally removed if the internal creack was connected with another type of creack perpendicular to it.