Strengthening of the hamstrings is often recommended following injury and reconstruction of the anterior cruciate ligament. It has been suggested that hamstrings activity stabilizes the knee and reduces anterior cruciate ligament load during weight-bearing flexion; however, the effects of hamstrings cocontraction on the kinematics and mechanics of the normal knee have not been assessed at physiological load levels. The aim of this study was to determine whether the addition of hamstrings force affects knee rotations, translations, and joint and quadriceps force during flexion with loads at physiological levels applied to the muscles and joints. Eight cadaveric knee specimens were tested with a servohydraulic mechanism capable of applying controlled dynamic loads to simulate quadriceps and hamstrings muscle forces throughout a physiological range of motion. A constant vertical load of physiologic magnitude was applied to the hip, and quadriceps force was varied to maintain equilibrium throughout flexion. Two conditions were tested: no hamstrings force and a constant hamstrings force equivalent to the vertical load. Hamstrings force significantly reduced internal rotation (p<0.0001) and anterior translation (p<0.0001), increased quadriceps force (p<0.0001) and normal resultant force on the tibia (p<0.0001), and reversed the direction of the shear force on the tibia (p<0.0001). These results suggest that hamstrings strengthening following anterior cruciate ligament injury may benefit anterior cruciate ligament-deficient and reconstructed knees by reducing the load in the ligament; however, they also imply that this comes at the expense of efficiency and higher patellofemoral and joint forces.

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Function and wear of total knee arthroplasties were compared by analysis of damage patterns on polyethylene tibial inserts retrieved from patients (Group R) with inserts obtained after in vitro force-controlled knee joint wear simulation. Two simulator input profiles were evaluated, including standard walking (Group W), and combined walking and stair descent (Group W þ S), simulating varied activities and a more severe physiological environment. Damage regions on all inserts were quantitatively assessed. On average, inserts in all groups had internally rotated damage patterns and the greatest articular deformation in the lateral compartment. These patterns were more pronounced in Group W þ S compared to Group W. Deformation rates of simulated inserts were analogous to about six years of physiologic function. However, both groups of simulated inserts generally underestimated the magnitude of damage area and extent observed on retrieved inserts, consistent with differences in the simulator's tibiofemoral contact mechanics and those known to occur in patients during functional activities. Modification of simulator inputs, such as the increased anteroposterior excursion and more severe loading conditions in Group W þ S, can generate greater wear volume, larger damage areas, and increased surface deformation rates compared to standard inputs. ß

Although implanted medical devices (IMDs) offer many benefits, they are susceptible to bacterial colonization and infections. Such infections are difficult to treat because bacteria could form biofilms on the implant surface, which reduce antibiotics penetration and generate local dormant regions with low pH and low oxygen. In addition, these infections are hard to detect early because biofilms are often localized on the surface. Herein, an optical sensor film is developed to detect local acidosis on an implanted surface. The film contains both upconverting particles (UCPs) that serve as a light source and a pH indicator that alters the luminescence spectrum. When irradiated with 980 nm light, the UCPs produce deeply penetrating red light emission, while generating negligible autofluorescence in the tissue. The basic form of the pH indicator absorbs more of upconversion luminescence at 661 nm than at 671 nm and consequently the spectral ratio indicates pH. Implanting this pH sensor film beneath 6-7 mm of porcine tissue does not substantially affect the calibration curve because the peaks are closely spaced. Furthermore, growth of Staphylococcus epidermidis on the sensor surface causes a local pH decrease that can be detected non-invasively through the tissue.

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