Initial stability comparison of modular hip implants in synthetic femurs.

Synthetic femurs were used to assess the initial bone-implant interface stability of three total hip systems: Wright Medical Technology's Infinity smooth trochanteric module (S-TM), Infinity porous-coated trochanteric module (PC-TM), and Johnson and Johnson S-ROM with a porous surface. The hips were implanted into synthetic femurs, rigidly fixed, and subjected to internal rotation and cyclic, axial compressive loads. The results showed that all three implants achieved good initial implant stability and would be expected to permit bone ingrowth. The porous-coated implants showed greater initial implant stability with less axial micromotion compared with the smooth implants. This finding suggests that surface texture plays a role in initial stability of uncemented prostheses if the bone behaves similar to the material used in this study.

The mechanical properties of skeletally mature rabbit anterior cruciate ligament and patellar tendon over a range of strain rates.

The effect of strain rate on the mechanical properties of the rabbit anterior cruciate ligament (ACL) and patellar tendon (PT) was evaluated. The medial portion of the ACL was loaded to tensile failure at rates of 0.003, 0.3, and 113 mm/s, and the middle third of the PT was loaded at rates of 0.008, 0.8, and 113 mm/s. The load was recorded with a high-speed measurement plotting system, and each test was videotaped for strain analysis. The nonlinear portion of the stress-strain curve was curve-fit to an exponential function having two nonlinear constants, representing the initial modulus and rate of change of the modulus. The modulus of the rabbit PT was found to be 89% higher than that of the ACL. The initial modulus and rate of change of the modulus also were greater for the PT than for the ACL. The modulus of the PT was shown to be more sensitive to strain rate than that of the ACL; a 94% increase was observed for the PT, and a 31% increase was observed for the ACL. There was no effect of strain rate on the mode of failure of either the ACL or the PT; all but three of the specimens failed at the insertion site.

The use of a laser micrometer system to determine the cross-sectional shape and area of ligaments: a comparative study with two existing methods.

Determination of the tensile stresses in ligaments and tendons during uniaxial loading depends on accurate measurement of the cross-sectional area. In this study, a laser micrometer system was employed to evaluate the cross-sectional shape and area of the medial collateral ligament (MCL) at three locations and anterior cruciate ligament (ACL). In a New Zealand White (NZW) rabbit, morphologic sections of the ligaments were made to verify the cross-sectional shape reconstructed by the laser micrometer system. The areas obtained by the laser micrometer system from ten additional NZW rabbits were compared with those obtained by two other methods commonly used to measure the cross-sectional area of ligaments and tendons: one method uses digital calipers and the other a constant pressure (0.12 MPa) area micrometer. For the MCL, the digital calipers yielded results very similar to those of the laser micrometer, but the constant pressure area micrometer yielded values 20 percent lower. The area measured at the proximal site of the MCL was 13 percent greater than the area measured at the joint line and distal line. For the ACL, the values obtained by the digital calipers and constant pressure area micrometer were 16 and 20 percent lower, respectively. Because of the irregular shape exhibited by the rabbit ACL, the digital calipers could not accurately measure the cross-sectional area. The constant pressure area micrometer yielded lower values for the cross-sectional area of both the MCL and ACL, presumably due to the applied pressure which caused changes in both the cross-sectional shape and area.

The effects of strain rate on the properties of the medial collateral ligament in skeletally immature and mature rabbits: a biomechanical and histological study.

The effects of strain rate on the structural properties of the femur-medial collateral ligament-tibia complex (FMTC) and on the mechanical (material) properties of the medial collateral ligament (MCL) of skeletally immature and skeletally mature rabbits were studied. The FMTCs were tested in tension to failure, at five extension rates (0.008 mm/s-113 mm/s). For the FMTCs from the skeletally immature animals, values of load, elongation, and energy absorbed at failure increased substantially with extension rates. Such increases also existed for skeletally mature animals, but they were much less in magnitude. All samples from the skeletally immature animals failed by tibial avulsion, whereas samples from the skeletally mature animals failed within the ligament substance. The mechanical properties of the ligament substance were minimally strain-rate sensitive for both groups. Histological sections of the ligament substance and insertion sites from the failed samples were examined, and these observations were correlated with the biomechanical findings. For the rabbit model used in this study, we conclude that skeletal maturity has more influence on the biomechanical properties of the MCL than does strain rate.