Mean Contact Pressure Research Articles

Influence of First Ray Positioning on Ankle Contact Stresses in the Setting of a Subtalar Arthrodesis

Category: Ankle; Hindfoot Introduction/Purpose: Subtalar arthrodesis is a commonly performed procedure for primary and post-traumatic subtalar arthritis and in the setting of deformity correction. Following arthrodesis, loss of motion at the subtalar joint alters loads experienced at the ankle joint. The influence of the first ray, an essential aspect of the foot tripod, in protecting or overloading the ankle joint in the setting of a fused subtalar joint is unknown. The purpose of this study was to assess the influence of first ray positioning on cartilage contact mechanics in the ankle joint in a native subtalar joint and following subtalar arthrodesis. Methods: Twelve below-knee cadaveric specimens (6 L/6 R) were mechanically loaded in neutral ankle flexion with an MTS-applied axial load of 600N and a pneumatically applied 45N of Achilles tendon tension to simulate 2-legged standing. A calibrated piezoresistive pressure sensor placed within the ankle joint measured cartilage contact pressure on the talar dome under load for 10 frames at 2Hz. In each specimen, contact pressure was measured pre- and post-subtalar fusion and after the application of two progressively larger dorsal opening wedges (4mm and 8mm Cotton osteotomy) in both the fused/unfused conditions. Computed tomography scans of specimens were segmented to generate 3D talus surfaces on which to overlay measured contact pressures. Peak and mean contact pressure, contact area, and center of pressure were compared between fused and unfused conditions with each first-ray correction. Results: Fusion of the subtalar joint increased contact pressures and shifted the contact patch laterally on the talar dome. First ray plantarflexion increased contact pressure and medialized the contact patch in both the fused and unfused conditions. (Figure 1). Joint contact area significantly decreased (by an average 18±9.3%) in all fused conditions compared to the unfused baseline (p < 0.001). Peak pressure in the unfused conditions increased over baseline by an average of 4±11.5% with the 4mm wedge and by 11±17.9% with the 8mm wedge, however neither increase was significant. Compared to the unfused baseline, subtalar fusion significantly increased peak contact pressures (32±21.7%, 38±23.4% and 49±30.5%) and mean contact pressures (23±22.2%, 23±19.8%, 21±19.1%) for the fused baseline, fused 4mm, and fused 8mm, respectively. Conclusion: This cadaveric biomechanical study found that subtalar fusion has a much stronger effect on ankle cartilage contact stress than changes in first-ray position. Alterations of the first ray position in specimens with subtalar fusion resulted in an increase in peak and mean contact pressure relative to the native ankle joint. Plantarflexion of the first ray modestly shifted the location of peak cartilage pressure in both the subtalar-fused and unfused foot but did not significantly increase contact pressure over the non-dorsiflexed condition.

Lateral Column Pressures Following Combined Subtalar/Talonavicular Fusion with and without Calcaneocuboid Fusion

Category: Hindfoot; Other Introduction/Purpose: There are few comparative studies between the traditional triple arthrodesis and the commonly performed modified procedure in which the calcaneocuboid joint is spared. Earlier investigations proposed that stiffening the lateral column of the hindfoot can increase lateral plantar pressures, potentially precipitating lateral column pain. We used a cadaver model to quantify the potential for reduced lateral column overload achieved by sparing the calcaneocuboid joint in a cadaveric model of triple arthrodesis simulating standing on both a neutral and sloping surface. Methods: Nine fresh-frozen, thawed cadaver legs were tested in 2 positions of the loaded surface: neutral and 10 degrees of eversion. Tendon loading and axial pressure was applied according to standard cadaveric models. Medial and lateral forefoot pressures were recorded using a pressure-sensitive plate (F-scan). The data were normalized as a percentage of the native state. Results: In neutral loading, the mean contact pressures under the fifth metatarsal head increased dramatically compared to the native state in both the standard (76.3%, p<.005) and calcaneocuboid-sparing (103.5%, p<.007) procedures. Pressure was reduced underneath the first metatarsal head medially in both standard (-52.8%, p< 0.017) and calcaneocuboid-sparing (-57.2%, p< 0.008). Loading on an everted surface yielded larger overall pressures laterally, but the further increases compared to the native state following both standard (39.0%, p < 0.04) and calcaneocuboid-sparing (57.9%, p< 0.03) procedures were smaller than in the neutrally loaded foot. No statistically significant difference was observed between the arthrodesis groups when loaded either on a neutral or sloped surface. Conclusion: In our cadaveric model, both traditional triple arthrodesis and the calcaneocuboid-sparing procedure resulted in significant elevation of lateral forefoot plantar pressures compared to the native state, but there was no identifiable protective effect from sparing the calcaneocuboid joint even in the provocative condition of loading on a sloping surface. Clinical Relevance: While other factors may guide the choice to spare the calcaneocuboid joint when performing hindfoot fusion, there is no evidence that preserving its motion significantly reduces the risk of lateral column overload.

Partial weight-bearing and range of motion limitation significantly reduce the loads at medial meniscus posterior root repair sutures in a cadaveric biomechanical model.

The aim of this study was to investigate the influence of medial meniscus posterior root avulsion (MMPRA) before and after surgical treatment on the biomechanics of the knee joint, including suture repair forces during daily and crutch-assisted gait movements. MMPRA were investigated in eight human cadaver knee joint specimens by a dynamic knee joint simulator with daily (normal gait, gait with additional rotational movement, standing up, sitting down) and rehabilitation-associated movements (crutch-assisted gait with limited flexion range of motion [30°] and 30% [toe-touch weight-bearing, TTWB] and 50% of body weight [partial weight-bearing, PWB]) with simulated physiologic muscle forces. Each specimen was tested in intact, torn and repaired (transtibial suture) state. The biomechanical parameters were: medial mean contact pressure and area, knee joint kinematics, medial displacement of the posterior meniscus horn and loading on the anchoring suture. Significant reduction of the contact area due to the avulsion was observed in all movements except for PWB and sitting down. MMPRA repair significantly increased the contact areas during all movements, bringing them to levels statistically indistinguishable from the initial state. MMPRA resulted in a medial displacement up to 12.8 mm (sitting down) and could be reattached with a residual displacement ranging from 0.7 mm (PWB) to 5.7 mm (standing up), all significantly (p < 0.001) reduced compared to the torn state. The mean peak anchoring suture load increased from TTWB (77 N), PWB (91 N) to normal gait (194 N), gait rotation (207 N), sitting (201 N; p < 0.01) and to standing up (232 N; p = 0.03). Surgical treatment of MMPRA allows restoration of physiological knee joint biomechanics. Crutch-assisted movements reduce the loading of the repair suture, thus likewise the risk for failure. From a biomechanical point of view, crutch-assisted movements are recommended for the early rehabilitation phase after MMPRA repair. Level V.

Evaluation of Tibiofemoral Contact Mechanics After a Novel Hybrid Procedure for Femoral Osteochondral Defect Repairs With a Subchondral Implant and Dermal Matrix.

There is a lack of procedures that adequately address the subchondral bone structure and function for reconstructing osteochondral defects in the femoral condyles. To biomechanically evaluate the tibiofemoral joint contact characteristics before and after reconstruction of femoral condylar osteochondral defects using a novel hybrid reconstructive procedure, which was hypothesized to restore the contact characteristics to the intact condition. Controlled laboratory study. Tibiofemoral contact areas, contact forces, and mean contact pressures were measured in 8 cadaveric knees (mean age 52 ± 11 years; 6 women, 2 men) using a custom testing system and pressure mapping sensors. Five conditions were tested for each condyle: intact, 8-mm defect, 8-mm repair, 10-mm defect, and 10-mm repair. Medial femoral condylar defects were evaluated at 30° of knee flexion and lateral condylar defects were evaluated at 60° of knee flexion, with compressive loads of 50, 100, and 150 N. The defects were reconstructed with a titanium fenestrated threaded implant countersunk in the subchondral bone and an acellular dermal matrix allograft. Repeated-measures analysis of variance with Bonferroni correction for multiple comparisons was used to compare the results between the 5 testing conditions at each load. Medial condylar defects significantly increased mean contact pressure on the lateral side (P < .042), which was restored to the intact levels with repair. The lateral condylar defect decreased the mean contact pressure laterally while increasing the mean pressure medially. The lateral and medial mean contact pressures were restored to intact levels with the 8-mm lateral condylar defect repair. The medial mean contact pressure was restored to intact levels with the 10-mm lateral condylar defect repair. The lateral mean contact pressure decreased compared with the intact state with the lateral condylar 10-mm defect repair. Tibiofemoral joint contact pressure was restored to the intact condition after reconstruction of osteochondral defects with dermal allograft matrix and subchondral implants for the repair of both 8- and 10-mm lateral condylar defects as well as 8-mm medial condylar defects but not completely for 10-mm medial condylar defects. The novel hybrid procedure for osteochondral defect repair restored tibiofemoral joint contact characteristics to normal in a cadaveric model.

Load Distribution After Serial Resection of the Posterior Horn of the Lateral Meniscus and Subsequent Meniscal Allograft Transplant: A Biomechanical Study

Background: Data are lacking as to when a meniscal allograft transplant (MAT) may be biomechanically superior to a partially resected lateral meniscus. Hypothesis: Lateral MAT using a bone bridge technique would restore load distribution and contact pressures in the tibiofemoral joint to levels superior to those of a partial lateral meniscectomy. Study Design: Controlled laboratory study. Methods: Eleven fresh-frozen human cadaveric knees were evaluated in 5 lateral meniscal testing conditions (native, one-third posterior horn meniscectomy, two-thirds posterior horn meniscectomy, total meniscectomy, MAT) at 3 flexion angles (0°, 30°, and 60°) under a 1600-N axial load. Pressure sensors were used to acquire contact pressure, contact area, and peak contact pressure within the tibiofemoral joint. Results: Limited (one-third and two-thirds) partial lateral posterior horn meniscectomy showed no significant increase in mean and peak contact pressures as well as no significant decrease in contact area compared with the intact state. Total meniscectomy significantly increased mean contact pressure at 0° and 30° (P = .008 and P < .001, respectively), increased peak contact pressure at 30° (P = .04), and decreased mean contact area in all flexion angles compared with the native condition (P < .01). Lateral MAT significantly improved mean contact pressure compared with total meniscectomy at 0° and 30° (P = .002 and P = .003, respectively) and increased contact area at 30° and 60° (P = .003 and P = .009, respectively), although contact area was still significantly smaller (24.1%) after MAT relative to the native meniscus (P = 0.015). However, allograft transplant did not result in better tibiofemoral contact biomechanics compared with limited partial meniscectomy (P > .05). Conclusion: The peripheral portion of the lateral meniscus provided the most important contribution to the distribution of contact pressure across the tibiofemoral joint in the cadaveric model. Total meniscectomy significantly increased mean and peak contact pressure in the cadaveric model and decreased contact area. Lateral MAT restored contact biomechanics close to normal but was not superior to the partially meniscectomized status. Clinical Relevance: Surgeons should attempt to preserve a peripheral rim of the posterior lateral meniscus. Meniscal allograft transplant appears to improve but not normalize mean contact pressure and contact area relative to total lateral meniscectomy.

The Effect of First Metatarsal Shortening and Sagittal Displacement on Forefoot Pressure in Minimally Invasive Hallux Valgus Correction.

Minimally invasive (MIS) treatment of hallux valgus (HV) deformity is increasing in popularity. A 2-mm diameter burr is used to create a distal first metatarsal osteotomy prior to capital fragment translation and fixation. The metatarsal will shorten by the burr's diameter (2 mm). Plantar or dorsal capital fragment displacement may also cause load transference and possibly transfer metatarsalgia. The purpose of this study is to examine the effect of MIS HV on forefoot loading mechanics with respect to metatarsal shortening and sagittal plane displacement. Four lower-limb cadaveric specimens were studied. A pedobarography pressure-sensing mat was used to record forefoot plantar pressure in a controlled weight-bearing stance position. Control and postosteotomy measurements were obtained with the capital fragment fixated in 3 possible positions: 0 mm, 5 mm dorsal, and 5 mm plantar displacement. Pedobarography data yielded pressure data within measurable graphical depictions. Raw mean contact pressure measurements were taken under the first and fourth metatarsal heads to establish medial and lateral forefoot loading pressure ratios. An a priori power analysis was performed based on previous peer-reviewed pedobarographic data, and our study was adequately powered. Around 40 measurements were recorded, and ratios of medial-to-lateral forefoot loading were constructed. Medial forefoot pressure control versus 0 mm displacement, and control versus dorsal displacement were not found to be statistically significant (p = 0.525, p = 0.55, respectively). Medial pressure significantly increased when comparing control versus plantar displacement (P = .006). Lateral pressure significantly increased with dorsal displacement of the osteotomy (P = .013). Our study found that MIS HV correction did not cause an increase in lateral forefoot pressure loading when sagittal plane displacements were controlled. Plantar displacement increased medial loading, and dorsal displacement increased lateral loading. It may be valuable for surgeons to consider metatarsal head position postosteotomy, as a decrease in medial loading and subsequent increase in lateral loading may lead to lateral forefoot pain and transfer metatarsalgia. IV.

Wedge indentation of elastoplastic solids — from single indentation to interaction between indenters

Wedge indentation of elastoplastic solids — from single indentation to interaction between indenters

Tribological behavior of alumina (Al2O3) and zirconia (ZrO2) plungers used in high pressure pumps

Ceramic materials are commonly used in plungers due to high resistance to wear, abrasion, and low coefficient of thermal expansion. Also, they are commonly used in dry conditions without permanent damage, ceasing of pump head and better corrosion resistance than metals due to their inert characteristics. Zirconia (ZrO2) ceramic is used due to its high strength whereas, alumina (Al2O3) is commonly used in industries for high pressure pumps. The toughness of zirconia ceramics is higher than alumina ceramics as it overcomes the inherent brittleness of ceramic materials. It also has higher wear resistance and extends the life of the product. Whereas alumina has better mechanical characteristics such as hardness compared to Zirconia. In this research work tribological behavior of Alumina and Zirconia ceramics used in high pressure pumps have been studied. The wear test using end face wear testing apparatus has been conducted under flat contact for both alumina and ceramic material considering a mean contact pressure of 10 N, 20 N, 30 N and 40 N and sliding velocity of 40 mm/s. The wear test was conducted for 30 min considering a total sliding distance of 1500 m and 3000 m. The wear test results indicate that both alumina and ceramic exhibit lower wear factors and superior mechanical properties. The findings also reveal that the wear rates of Al2O3 and ZrO2 are influenced by friction forces, subsequently impacting the overall wear rate. Also, as the load increases the surface contact area also increases which in turn increases the wear rate. However, zirconia could be a potential substitute for alumina due to its high strength and fracture toughness.

The Effect of Lower Limb Alignment on Tibiofemoral Joint Contact Biomechanics after Medial Meniscus Posterior Root Repair: A Finite-Element Analysis.

The purpose of this study was to determine how variations in lower limb alignment affect tibiofemoral joint contact biomechanics in the setting of medial meniscus posterior root tear (MMPRT) and associated root repair. A finite-element model of an intact knee joint was developed. Limb alignments ranging from 4° valgus to 8° varus were simulated under a 1,000 N compression load applied to the femoral head. For the intact, MMPRT, and root repair conditions, the peak contact pressure (PCP), total contact area, mean and maximum local contact pressure (LCP) elevation, and total area of LCP elevation of the medial tibiofemoral compartment were quantified. The PCP and total contact area of the medial compartment in the intact knee increased from 2.43 MPa and 361 mm 2 at 4° valgus to 9.09 MPa and 508 mm 2 at 8° of varus. Compared with the intact state, in the MMPRT condition, medial compartment PCP was greater and the total contact area smaller for all alignment conditions. Root repair roughly restored PCPs in the medial compartment; however, this ability was compromised in knees with increasing varus alignment. Specifically, elevations in PCP relative to the intact state increased with increasing varus, as did the total contact area with LCP elevation. After root repair, medial compartment PCP remained elevated above the intact state at all degrees tested, ranging from 0.05 MPa at 4° valgus to 0.27 MPa at 8° of varus, with overall PCP values increasing from 2.48 to 9.09 MPa. For varus alignment greater than 4°, root repair failed to reduce the total contact area with LCP elevation relative to the MMPRT state. Greater PCPs and areas of LCP elevation in varus knees may reduce the clinical effectiveness of root repair in delaying or preventing the development of tibiofemoral osteoarthritis.

Higher contact pressure of the lateral tibiofemoral joint in lateral extra-articular tenodesis with tensioned graft in external rotation than in neutral rotation: A biomechanical study

ObjectiveTo determine the mean contact pressure, peak contact pressure, and mean contact area of the lateral tibiofemoral joint in lateral extra-articular tenodesis (LET) with tension on the graft in tibial neutral and external rotation. MethodsA total of eight Thiel-embalmed cadaveric knees were prepared and divided into two groups (4 knees in each group): the LET-NR group (lateral extra-articular tenodesis tension in neutral rotation) and (2) the LET-ER group (lateral extra-articular tenodesis tension in external rotation). Each knee was prepared according to the corresponding technique. A hydraulic testing system (E10000, Instron) simulates an axial load of 735 ​N for 10 ​s in each group. ResultsThe LET-ER group exhibited a statistically significant higher peak contact pressure compared to the LET-NR group. The peak contact pressure values in the LET-NR and LET-ER groups were 702.3 ​± ​233.9 ​kPa and 1235.5 ​± ​171.4 ​kPa, respectively (p ​= ​0.010, 95% CI, −888.0 to −178.5). The mean contact pressure values in the LET-NR and LET-ER groups were 344.9 ​± ​69.0 ​kPa and 355.3 ​± ​34.9 ​kPa, respectively (p ​= ​0.796, 95% CI, −105.1–84.2). The mean contact area values in the LET-NR and LET-ER groups were 36.8 ​± ​3.1 mm2 and 33.3 ​± ​6.4 mm2, respectively (p ​= ​0.360, 95% CI, −5.2–12.2). ConclusionsThe peak contact pressure of the lateral tibiofemoral joint is greater in LET when the graft is tensioned in external rotation than in neutral rotation. However, no statistically significant difference in the mean contact pressure or the mean contact area was observed between the two groups. Level of evidenceIII.

The Effect of First Metatarsal Shortening and Sagittal Displacement on Forefoot Pressure in MIS Hallux Valgus Correction

Introduction/Purpose: Minimally invasive surgical (MIS) treatment of hallux valgus (HV) deformity is increasing in popularity. A 2mm-diameter burr is used to create a distal first metatarsal osteotomy prior to capital fragment translation and fixation. The metatarsal will shorten by the burr’s diameter (2mm). Plantar or dorsal capital fragment displacement may also cause load transference and possibly transfer metatarsalgia. In this study, we examine the effect of MIS HV on forefoot loading mechanics with respect to metatarsal shortening and sagittal plane displacement. Methods: Four lower-limb cadaveric specimens were studied. A pedobarography pressure-sensing mat was used to record forefoot plantar pressure in a controlled weightbearing stance position.10 Control and post-osteotomy measurements were obtained with the capital fragment fixated in 3 possible positions: 0mm, 5mm dorsal, and 5mm plantar displacement. Pedobarography data yielded pressure data within measurable graphical depictions. Raw mean contact pressure measurements were taken under the first and fifth metatarsal heads to establish medial and lateral forefoot loading pressure ratios. A priori power analysis was performed based on previous peer-reviewed pedobarographic data and our study was adequately powered. Results: 40 measurements were recorded and ratios of medial-to-lateral forefoot loading were constructed. Medial forefoot pressure change control versus 0mm displacement, and control versus dorsal displacement was not found to be statistically significant (p=0.525, p=0.55 respectively). Significant medial pressure increase was identified comparing control versus plantar displacement (p=0.006). Lateral pressure increased significantly with dorsal displacement of the osteotomy (p=0.013). Conclusion: MIS hallux valgus correction does not appear to cause increase in lateral forefoot pressure loading when sagittal plane displacements are controlled. Plantar displacement increases medial loading, and dorsal displacement increases lateral loading. The clinician must consider metatarsal head position post-osteotomy, as decrease in medial loading and subsequent increase in lateral loading may lead to lateral forefoot pain and transfer metatarsalgia.

Effect of Varus-Producing Distal Femoral Osteotomy and High Tibial Osteotomy on Compartment Pressures and Contact Area at Varying Degrees of Knee Flexion.

In patients with valgus alignment and degenerative changes in the lateral compartment, both distal femoral osteotomy (DFO) and high tibial osteotomy (HTO) can be used to unload the lateral compartment. Prior studies have shown that in valgus knees, the tibial wear is posterior and DFO exerts the greatest effect in extension; however, its effect is decreased as flexion angle rises. Medial closing-wedge (MCW) HTO would significantly decrease contact area, mean contact pressure (MCP), and peak contact pressure (PCP) in the lateral knee compartment through knee flexion to a greater extent compared with lateral opening-wedge (LOW) DFO. Controlled laboratory study. MCWHTO and LOWDFO were performed, correcting a mean of 8° of valgus alignment, in 10 cadaveric knees using plate fixation. Tibiofemoral contact pressure of the medial and lateral compartments was measured in 0°, 30°, 60°, and 90° of knee flexion before and after osteotomy using thin electronic sensors and load applied through an Instron device. PCP, MCP, and contact area were measured for each condition. The lateral MCP was significantly decreased in the HTO state compared with the native state in 30° (P = .015), 60° (P = .0199), and 90° (P < .0001) of flexion. The lateral MCP was also significantly decreased in the HTO state when compared with the DFO state in 60° (P = .0093) and 90° of flexion (P < .0001). After DFO, the lateral MCP returned to that of the native state in 60° (P > .999) and 90° (P > .999) of flexion. The lateral PCP decreased for all test states in all degrees of flexion; the HTO state was significantly decreased when compared with the native state in 60° (P < .0001) and 90° (P < .0001). With varus corrections of 8°, MCWHTO was more effective at unloading the lateral compartment than LOWDFO. This effect was significant as the knee flexion angle increased. This study should be considered as one aspect of the surgical decision-making process. In patients with mild to moderate valgus deformity without hypoplastic lateral femoral condyle and without significant joint line obliquity, MCWHTO may improve offloading of the lateral compartment in flexion.

Middle trapezius tendon transfer using Achilles allograft for irreparable isolated supraspinatus tendon tears effectively restores the superior stability of the humeral head without restricting range of motion: a biomechanical study

BackgroundMiddle trapezius tendon (MTT) transfer has been suggested for promising treatment of irreparable isolated supraspinatus tendon tears (IISTTs). However, there have been no attempts to assess the biomechanical efficacy of MTT transfer. This study aims to evaluate the biomechanical efficacy of MTT transfer in the setting of IISTTs. MethodsEight fresh-frozen cadaveric shoulders were tested in three conditions (1) intact rotator cuff, (2) IISTTs, and (3) Middle trapezius tendon transfer (MTT) using Achilles allograft for IISTTs. Total humeral rotational range of motion, superior translation of the humeral head, and subacromial contact characteristics were measured at 0°, 20°, and 40° glenohumeral abduction (representing 0°, 30°, and 60° shoulder abduction). Superior translation and subacromial contact pressures were measured at 0°, 30°, 60° and 90° external rotation (ER). Two different MTT muscle loading conditions were investigated. A linear mixed-effects model and Tukey post hoc test were used for statistical analysis. ResultsTotal range of motion was significantly increased after IISTT at 20° abduction (P =.037). There were no changes in total ROM following MTT transfer compared to the IISTT condition (P>.625 for all comparisons). The IISTT condition significantly increased superior translation compared to the intact rotator cuff condition in 0° and 20° abduction with all ER angles (P<.001), 40° abduction/30° ER (P=.016), and 40° abduction/60° ER (P=.002). MTT transfer significantly decreased superior translation of the humeral head at all abduction angles compared to the IISTTs condition (P<.026). MTT transfer significantly decreased peak contact pressure by 638.7 kPa (normal loading) and 726.8 kPa (double loading) at 0° abduction/30° ER compared to the IISTTs condition (P<.001). Mean contact pressure was decreased by 102.8 kPa (normal loading) and 118.0 kPa (double loading) at 0° abduction/30° ER (P<.001) and 101.0 kPa (normal loading) and 99.2 kPa (double loading) at 0° abduction/60° ER (P<.001). MTT transfer at 20° abduction/30° ER with 24N loading significantly decreased contact pressure by 91.2 kPa (P=.035). ConclusionsThe MTT transfer biomechanically restored the superior humeral head translation and reduced the subacromial contact pressure in a cadaveric model of IISTT, while not restricting total ROM. These findings suggest that MTT transfer may have potential as a surgical treatment for IISTTs.

A Novel Method to Measure Equi-Biaxial Residual Stress by Nanoindentation

BackgroundThe accurate measurement of residual stresses (RS) is crucial for predicting the performance of mechanical components, as RS can significantly impact fatigue life, fracture, corrosion, and wear resistance. Different experimental methods were developed to measure RS, including non-destructive techniques. Among these methods, instrumented nanoindentation has emerged as a promising approach to assess equi- or non-equi-biaxial RS states. This technique analyzes variations in the mechanical response of indentation on a stressed or stress-free component to estimate residual stresses. Previous studies proposed different approaches to establish a relationship between RS and indentation parameters, such as contact area, peak load, mean contact pressure, indentation work, etc. However, the correlation between RS and peak load variation, commonly assumed to be linear, showed limitations, particularly when dealing with compressive RS.ObjectiveThe aim of this work is to develop a hybrid procedure, based on finite element (FEM) simulations and experimental analyses, to measure the equi-biaxial residual stresses. In particular, it is based on the analysis of the nanoindentation peak load variation generated by the presence of residual stresses on a component.MethodsTo overcome the limitations of the linear assumption, nanoindentation experiments were combined with finite element analyses (FEA). FEA simulations were used to estimate the correlation between RS and peak load variation, providing a better understanding of the non-linear relationship. A proper experimental setup, consisting in a stress generating jig, was designed and manufactured to perform nanoindentations on a sample, made by aluminium alloy AA 7050 T451, subjected to external mechanical stress with the aim to validate the FEA model. FEA and the digital image correlation (DIC) technique were also used to verify that the induced stress field was the expected one.ResultsObtained results revealed that the proposed method is a valid way to measure residual stresses. In fact, it offers an improved correlation between RS and peak load variation. In addition, by integrating nanoindentation experiments and FEA, a more accurate assessment of RS can be also achieved.ConclusionsThis research contributes to the development of a consistent methodology for RS measurement using instrumented nanoindentation.

A statistics view of contact pressure distribution for normal contact of fractal surfaces

Due to inherent nonlinear stiffness and damping characteristics, dry friction interfaces have a significant impact on the dynamics of jointed structures. When two surfaces are brought into purely normal contact, contact pressure distribution is of high concern. This work focused on the statistics of the contact pressure distribution between a rough surface and a smooth rigid plane, which provides new insight into the interface contact behaviour. First, the fractal rough surface is generated using the Weierstrass–Mandelbrot function with measured roughness parameters. With meshed rough surfaces, an elastic–plastic finite element contact analysis is performed to determine the contact pressure distribution. Then, the results of contact pressure are statistically analysed. The effects of roughness and contact load on contact area, contact stiffness and mean contact pressure are thoroughly investigated. The probability distribution of contact pressure is determined by fitting a continuous function using a twofold Weibull mixture model. The proposed probability distribution function is found to be capable of describing contact pressure. The contact pressure distribution is affected by the surface fractal characteristics and evolves with the contact load.

Biomechanical Analysis of Meniscotibial Ligament Tenodesis to Treat Meniscal Extrusion in the Setting of Posterior Medial Meniscus Root Repair

Background: Meniscal extrusion often persists after a medial meniscus root repair. If the meniscus is extruded, the function of the meniscus as a load-sharing device and secondary knee stabilizer is compromised. Hypothesis: It was hypothesized that repairing the meniscotibial ligament (MTL) would decrease meniscal extrusion in the settings of both an isolated MTL tear and a repaired medial meniscus root while also improving medial compartment contact mechanics. Study Design: Controlled laboratory study. Methods: Ten fresh-frozen cadaveric knees (mean age, 50.5 years) were tested in 5 conditions: intact, MTL deficiency, MTL deficiency + posterior medial meniscus root deficiency, MTL deficiency + posterior medial meniscus root repair, and MTL tenodesis + posterior medial meniscus root repair. Specimens were mounted to a load frame that applied a 1000-N axial load. Joint contact pressures were measured using thin pressure sensors, and the peak and mean pressures were analyzed. Ultrasound was used to measure meniscal extrusion. Results: The MTL tear in isolation resulted in significant meniscal extrusion compared with the intact state (P = 0.035) without a detectable difference in medial compartment pressures. The addition of a root tear to the MTL tear state resulted in significantly more extrusion (P = 0.001) and significant increases in medial compartment pressure (P = .030) compared to the MTL tear state. Root repair alone restored extrusion, mean contact pressure, and peak contact pressure back to the intact state (P > .05). Conclusion: This study showed that MTL disruption led to increased meniscal extrusion in a cadaveric model. Unlike the root tear state, MTL disruption did not change contact mechanics. Furthermore, root repair alone was sufficient in restoring intact biomechanics and extrusion. Clinical Relevance: This study may help clinicians understand the origin of medial meniscus root tears and aid in the decision-making process for whether to add an MTL tenodesis in the setting of root repair.

A survey and biomechanical analysis of the feasibility of the thumb test for determining the cancellous bone quality for stemless shoulder prosthesis.

The effect of the thumb test for assessing the cancellous bone quality at the resection plane of the proximal humerus on determining the application of a stemless shoulder prosthesis remains unclear. This study was conducted to survey the current utilization of the thumb test among surgeons and to investigate biomechanical features of the thumb test. A survey among shoulder surgeons who had experience with stemless prostheses was conducted to investigate the current utilization of preoperative assessments and intraoperative thumb test when applying stemless prosthesis. Biomechanical experiments for the thumb test using artificial bone models were performed to assess the compression force, contact pressure and area. According to the preliminary survey, three compression techniques were assessed: compression perpendicular to the surface with thumb pad (P-pad technique) or tip of the thumb (P-tip technique), or compression in the vertical direction simulating compression along the longitudinal axis of the humeral shaft with tip-pad of the thumb (H-axis technique). The contact area was separated into three subregions (proximal, middle and distal) to assess the distribution of contact pressure. Among 38 surgeons, 66% utilized the thumb test intraoperatively. The P-pad technique was more frequently applied than the P-tip or H-axis techniques (80%, 4% and 16%, respectively). Although with wide variation among the examiners, biomechanical assessments revealed the P-pad technique showed larger contact area and less compression force than the P-tip technique. The P-pad technique provided no significant localized differences in the mean contact pressure on the compressed plane, whereas the P-tip and H-axis techniques showed significant differences among subregions. This survey demonstrated relatively frequent application of the thumb test on applying the stemless shoulder prosthesis. Biomechanical assessment revealed the thumb test can hinder objective reproducibility among examiners; therefore, further investigations to identify feasible assessments of the bone quality is required.

Biomechanical Analysis Evaluating Meniscal Extrusion After Knotless Suture Anchor Fixation for Segmental Medial Meniscal Allograft Transplantation.

Segmental medial meniscal allograft transplantation (MAT) has been shown to restore knee biomechanics; however, stable fixation of the transplantation is critical to avoid extrusion and maximize healing. To evaluate the degree of meniscal extrusion and biomechanical function of segmental medial MAT performed with meniscocapsular sutures versus repair augmentation with knotless suture anchors. Controlled laboratory study. Segmental midbody medial meniscectomy and subsequent segmental medial MAT were performed on 10 fresh-frozen cadaveric knees. The knees were then loaded in a dynamic tensile testing machine to 1000 N for 60 seconds at 0°, 30°, 60°, and 90° of flexion, and 4 conditions were tested: (1) intact, (2) segmental defect, (3) inside-out segmental repair, and (4) anchor plus inside-out segmental repair of the medial MAT. Meniscal extrusion was measured using high-fidelity ultrasound imaging. The mean contact area and the mean and peak contact pressures were assessed with submeniscal pressure-mapping sensors. Data from testing conditions were compared with 2-way repeated-measures analysis of variance, with pairwise comparison using the Bonferroni method. At 90° of flexion, the segmental defect state showed a higher degree of meniscal extrusion compared with all other states (P ≤ .012). There was no difference in the degree of meniscal extrusion between the intact state and the inside-out repair or anchor plus inside-out segmental repair states at all knee flexion angles (P > .05). There was no significant difference in the mean and peak contact pressures among the 4 states at all flexion angles except that at 0° of knee flexion there was significantly lower peak contact pressure at the medial compartment after anchor plus inside-out segmental repair compared with the segmental defect state (P = .048). Meniscal extrusion was not significantly increased at any flexion angle after segmental resection. The addition of knotless anchors did not improve meniscal extrusion or contact pressures/area compared with capsular repair alone. The addition of knotless anchors did improve contact mechanics from the segmental defect state, but only at 0° of flexion. The addition of knotless suture anchors to segmental meniscal transplantation increased stabilization of the meniscus at full extension compared with repair with sutures alone. This increased stabilization may lead to better long-term outcomes.

Paper 23: Taking the Load Off: Effect of Varus Producing Distal Femoral Osteotomy and High Tibial Osteotomy on Compartment Pressures and Contact Area in Varying Stages of Knee Flexion

Objectives: Study Objective: Evaluate mean contact area, mean contact pressure, and peak contact pressure in the medial and lateral compartment of the knee following varus producing high tibial osteotomy and distal femoral osteotomy in varying states of knee flexion (0*, 30*, 60*, 90*). Background: Genu valgum causes mechanical pressure to be transferred to the lateral compartment. In patients with valgus alignment and a diseased lateral compartment, both lateral opening wedge distal femoral osteotomy (DFO) and high tibial closing wedge osteotomy (HTO) can be used to unload a diseased lateral compartment of the knee. To the best of our knowledge, there are no biomechanical studies investigating how a DFO and HTO affect lateral compartment contact pressure and area in varying states of flexion. Prior anatomic studies have shown that, in valgus knees, the tibial wear is posterior. Additional, biomechanical studies have indicated that DFO exerts the greatest effect in full extension but it’s effect is decreased as flexion angle rises. Hypothesis: Medial closing wedge high tibial osteotomy will significantly decrease contact area, mean contact pressure and peak contact pressure in the lateral knee compartment through knee flexion to a greater extent compared to lateral opening wedge distal femoral osteotomy. Methods: Lateral opening wedge DFO and medial closing wedge HTO was performed, correcting a mean of 8°, in 10 cadaveric knees using plate fixation. Tibio- femoral contact pressure of the medial and lateral compartment was measured in 0°, 30°, 60°, and 90° of knee flexion before and after osteotomy using thin electronic sensors (K-scan Model 4000, 1500 psi; Tekscan Inc, South Boston, MA) and load applied through an Instron device (Instron, Norwood, MA). Peak contact pressure (PCP), average contact pressure (ACP), and contact area (CA) were measured for each condition. Mixed-effects regression analyses were used to compare the change in variables before and after osteotomy and between flexion angles. Results: The lateral ACP was significantly decreased in the HTO state when compared to native in 30° (p=0.015), 60° (p=0.0199), and 90° (p&lt;0.0001) of flexion. The lateral ACP was also significantly decreased in the HTO state when compared to the DFO state in 60° (p=0.0093) and 90° of flexion (p&lt;0.0001). Following DFO, the lateral ACP returned to that of the native state in 60° (p=1.000) and 90° (p=1.000) of flexion. The medial ACP was significantly increased when compared to both the native and HTO state in 60° (p&lt;0.001; p=0.015) and 90° (p&lt;0.0001; p=0.0435) of flexion. Conclusions: With varus corrections of &lt;10°, medial closing HTO is more effective at unloading the lateral compartment than lateral opening DFO. This effect is amplified as the knee flexion angle increases.

Poster 286: Biomechanical Analysis Evaluating Meniscal Extrusion Following Knotless Suture Anchor Fixation for Segmental Medial Meniscus Allograft Transplantation

Objectives: Segmental medial meniscus allograft transplantation (MAT) has been shown to restore knee biomechanics, however, stable fixation of the transplant is critical to avoid extrusion and maximize healing. The purpose of this study was to evaluate the degree of meniscal extrusion and biomechanical function of segmental MAT performed with meniscocapsular sutures versus repair augmentation with knotless suture anchors. Methods: Ten fresh frozen cadaveric knees underwent central midbody medial meniscectomy (15-mm defect) and subsequently segmental medial MAT. Knees were loaded in a dynamic tensile testing machine to 1000 N for 20 seconds at 0°, 30°, 60°, and 90° of flexion. Four conditions were tested: (1) intact, (2) segmental defect, (3) inside-out segmental repair, and (4) anchors plus inside-out segmental repair of the MAT. Meniscal extrusion was measured for each test state using high-fidelity ultrasound imaging by three independent observers. Submeniscal medial and lateral pressure-mapping sensors assessed the mean contact pressure, peak contact pressure, mean contact area, and pressure mapping. Assessment of inter- and intraobserver reliability of measurements were done using intraclass correlation coefficients. A two-way repeated measures ANOVA was computed in order to evaluate whether there is a significant interaction between meniscal states and knee flexion angles on the outcome variables. If the P value was ≤ 0.05, a pairwise comparison of the means was determined using the Bonferroni method. Results: At 0° of knee flexion, the segmental defect demonstrated the statistically significant higher degree of meniscal extrusion (2.08 mm ± 0.78 mm) compared with intact (1.02 mm ± 0.86 mm, P = 0.039) and anchor plus inside-out segmental repair (1.44 mm ± 0.76 mm, P = 0.044), but not for an inside-out segmental repair (1.55 mm ± 0.49 mm, P = 0.08). The segmental defect state also showed a higher degree of meniscal extrusion compared with all other states at 90° of knee flexion (P &lt; 0.01). There was no difference in the degree of meniscal extrusion between the intact state and the inside-out repair or anchor plus inside-out segmental repair in all knee flexion angles (P &gt; 0.05). Three observers showed good-to-strong intra-rater and moderate-to-strong inter-rater reliabilities for extrusion measurements. There was no significant difference in the mean contact pressure and peak contact pressure among four states in all knee flexion angles for both medial and lateral compartments except for that at 0° of knee flexion, there was a significantly lower peak pressure at the medial compartment after anchor plus inside-out segmental repair compared with the segmental defect state (P = 0.048). The medial contact area of the intact state was significantly higher than the other states at 0° of knee flexion (P &lt; 0.05). However, the lateral contact area after anchor plus inside-out repair showed no significant difference compared with the intact state (P = 0.13). Conclusions: Segmental meniscus transplantation mechanics may be improved at full extension (0° of knee flexion) with the addition of knotless anchors. However, in terms of meniscus extrusion, the knotless anchors provided limited benefit over inside-out repair. Overall, a robust repair is achieved of a segmental meniscus with sutures and knotless anchors. Although, additional work is needed to assess long-term in vivo results.