Patellar tracking in total knee arthroplasty: inset versus onset design.

Patellar components come in onset and inset designs. Kinematic differences between these designs were studied in a cadaver model of closed kinetic chain knee extension. Seven frozen knees were implanted with a standard posterior cruciate-retaining design. Each knee first was tested with the inset design, followed by the onset design in the Oxford Knee Rig. Three-dimensional tracking of the femur, tibia, and patella was performed using an electromagnetic system during active knee extension under load. No statistically significant differences were seen in knee kinematics between the 2 designs. The inset patella tended to shift laterally and tilt laterally more than the onset patella. This difference may be clinically significant.

Automatic control design for a dynamic knee-brace system.

A self-contained electronically controlled dynamic knee-brace system (DKBS) has been designed and tested which allows knee flexion during swing phase, but restricts flexion during the stance phase of gait. Cardiovascular energy measurements indicate that DKBS use allowed a more energy efficient gait.

Optimization and application of a wrap-spring clutch to a dynamic knee-ankle-foot orthosis.

A dynamic knee-brace system (DKBS) has been designed which provides stance phase stability and swing phase freedom. A wrap-spring clutch controls knee flexion. Clutch optimization was performed minimizing clutch length. Kinematic tests on a normal subject using the DKBS document nearly normal dynamic knee flexion during swing (38 degrees versus 53 degrees for normal).

Effects of patellar thickness on compression and shear forces in total knee arthroplasty.

Ten unembalmed cadaveric knees were dissected to the capsule preserving the ligamentous and tendinous structures. A posterior cruciate ligament sparing total condylar knee arthroplasty was implanted routinely. A force transducer that measured compression force and shear was implanted into each patella. For each knee, 3 thicknesses of the patellar composite (osteotomized patella, transducer, polyethylene component) were evaluated: (1) precut patellar thickness, (2) precut plus 2 mm, and (3) precut plus 4 mm. The knees were tested in an Oxford Knee Testing Rig, which allowed dynamic testing with 6 degrees of freedom. Values of patellar forces were obtained throughout a range of motion of 0 degrees to 95 degrees flexion. At higher flexion angles (45 degrees and above), the total patellofemoral shear forces for the precut plus 2 mm and the precut plus 4 mm composites were altered significantly from the precut thickness. Increasing the patellar thickness results in a significant increase in shear forces, potentially leading to early loosening of the component or increased wear or both. Therefore, bone conserving resections that increase the patellar composite thickness above the precut thickness should be avoided.

Instrumented implant for measuring tibiofemoral forces.

Experimental measurement of loads occurring in the human knee joint will allow validation of analytical models and provide data for the design of total knee implants. A customized transducer was developed to measure the dynamic tibiofemoral force and center of pressure after total knee arthroplasty. The transducer consists of a standard tibial component to which four uniaxial load cells and an additional tibial tray have been added. The transducer was calibrated using a loading device traceable to the National Institute of Standards and Technology (NIST). The transducer was accurate to within 1% in magnitude, 0.07 mm in medial/lateral location and 0.24 m in anterior/posterior location.

The effects of patellar thickness on patellofemoral forces after resurfacing.

The purpose of this study was to determine the effect of patellar bone and patellar implant thickness on patellofemoral forces after resurfacing in total knee arthroplasty. Seven cadaver knees were tested using an Oxford Knee Testing Rig. This model gave the specimens 6 degrees freedom while dynamic data were collected. Each knee was tested from full extension to 95 degrees knee flexion. Knees were implanted with Press Fit Condylar femoral, tibial, and patellar implants. The effect of varying patellar thickness on patellofemoral forces was determined by using custom modular oval-domed polyethylene patellar components with progressive thickness increments of 2 mm. Patellofemoral forces were measured by a custom-designed uniaxial patellar load cell. Statistically significant increases in patellofemoral compression forces were found from 70 degrees to 95 degrees flexion with increased patellar bone and patellar implant thickness.

Arterial blood sampling devices influence ionized calcium measurements.

OBJECTIVE: To determine the effect of commercially available arterial blood sampling devices on ionized calcium measurements. DESIGN: Prospective study. SETTING: Neurosurgical and shock-trauma intensive care units (ICU) at a tertiary care teaching hospital. PATIENTS: Fourteen patients admitted to the ICU. Each patient had an indwelling arterial catheter. INTERVENTIONS: Arterial blood sampling. MEASUREMENTS AND MAIN RESULTS: In 14 ICU patients, measurements of arterial blood ionized calcium concentrations were performed, using 12 different commercially available arterial blood sampling devices. Significant underestimation of ionized calcium in blood samples compared with the reference test tube (Vacutainer 45) was seen in seven of the devices. Arterial blood ionized calcium concentrations measured, using one commercially available syringe, were significantly higher compared with the reference test tube. There was no correlation between either the amount or type of heparin in the arterial blood sampling devices and arterial blood ionized calcium measurement. CONCLUSIONS: This study demonstrates that various commercially available arterial blood sampling devices alter arterial blood ionized calcium measurements. These alterations are clinically important because ICU patients may be treated with inappropriate calcium supplementation.

The effect of trochlear design on patellofemoral shear and compressive forces in total knee arthroplasty.

Biomechanical testing was performed on 5 cadaveric knees to evaluate the effect of patellofemoral design on shear and compressive force at the patellar component-bone interface. Three patellofemoral designs with identical tibiofemoral articular surfaces were tested. The knees were tested under dynamic loading conditions from 0 degree to 100 degrees flexion. A transducer that measured 3 orthogonal force components was mounted between the patellar component and the patella. The combination of an oval dome patella with a 2-mm deeper trochlear groove and associated increased femoral resection was compared with a biconcave patella with standard trochlea and normal femoral resection. Alignment of the trochlear groove in 7 degrees valgus (anatomic) decreased medial-lateral shear by 10% to symmetric trochlear groove alignment.

The anterior cruciate ligament in controlling axial rotation. An evaluation of its effect.

Changes in axial tibial rotation after anterior cruciate ligament sectioning were evaluated in 14 fresh human knee joints. Simulation of vertical stance in a quadriceps-stabilized knee was performed. Internal and external rotational torques were applied before and after anterior cruciate ligament sectioning. Pivot shift tests were done in the intact and anterior cruciate ligament sectioned knee. Results of pivot shift tests were all negative before sectioning and positive after isolated sectioning. No significant change in axial rotation occurred between the intact and sectioned knee for external rotation (P = 0.24) or internal rotation (P = 0.12). Presence of a load at the femoral housing in both the intact and ligament-sectioned knees caused a significant change in external rotation (P < 0.0001). No significant change was noted in internal rotation between loaded and unloaded states (P = 0.70). Total tibial rotation in the intact knee was noted to vary between 31 degrees at 0 degree of flexion and 42 degrees at 60 degrees of flexion. These results suggest that the anterior cruciate ligament does not play a significant role in limiting axial rotation and that rotational instability is not a major factor after isolated anterior cruciate ligament rupture.

Hemolytic uremic syndrome and metastatic malignancy.

We report a case of hemolytic uremic syndrome (HUS) with advanced renal failure and metastatic adenocarcinoma of the prostate. The HUS rapidly improved after treatment with blood transfusions, Plasmanate infusions, aspirin and dipyridamole, and hemodialysis for uremia. No cancer chemotherapy was given. This case illustrates that, in adults, HUS may have an excellent prognosis even with advanced renal failure, and that the presence of metastatic cancer does not necessarily imply a relationship between the two diseases.