Concerns with modularity in total hip arthroplasty.

Modularity is being diversified in total hip prostheses to increase surgical latitude in optimizing implant fixation and adjusting hip biomechanics. However, several problems have been clearly identified with implant modularity. First generation metal-backed acetabular components have shown deficiencies in the locking mechanism, the congruency and extent of polyethylene liner support, and polyethylene thickness, all of which have been implicated in accelerated polyethylene wear and failure. Evidence of screw motion against the metal backing, release of particulate material, and focal osteolysis have also been observed. At the head/neck junction evidence of corrosion and fretting has been documented with both similar-metal and mixed-metal taper combinations. Femoral prostheses with other sites of modularity present additional concerns with regard to mechanical integrity and generation of particulate debris by fretting. The modular junctions of three hip prostheses, the S-ROM, Infinity, and RMHS, were subjected to wet environment high cycle mechanical testing in a worst-case loading scenario. Preliminary results at relatively low loads up to three times body weight indicated gross stability of the modular junctions with evidence of minor fretting damage. Analysis of water solutions surrounding the modular junctions after ten to 20 million loading cycles yielded counts of one to three micron sized particles totalling several hundred thousand to several million. It is unknown what quantity of particulate material is sufficient to cause macrophage-mediated osteolysis or whether the debris from modular junctions can cause third-body wear of the articulating surfaces. Modular hip prostheses should be examined under stringent test conditions in order to characterize their fretting behavior and establish their mechanical limitations.

Fatigue behavior of titanium femoral hip prosthesis with proximal sleeve-stem modularity.

Modular hip prostheses are increasing in variety and utilization. Component stability, high endurance limit, and minimal particulate debris generation are critical for long-term clinical success. The purpose of this study was to characterize the fatigue response and evaluate the in vitro potential for component motion and wear of the S-ROM¿, a Ti-6Al-4V hip prothesis with a modular design based on a Morse taper connection. A fatigue jib was designed to simulate fixation of the device at the sleeve-bone interface only with distal support mainly against the lateral endosteal cortex. Two series of tests were performed in air at room temperature: one with direct vertical loading (to produce high bending moments in the coronal plane) and one with a compounding loading angle directed at 15 degrees out-of-plane (to include torsional physiological loads). Applied loads using a servohydraulic test machine ranged from 5 x BW (body weight) to 9 x BW (1 x BW = 73 kg, approximately 160 lb) at 10 Hz on an Instron apparatus. No mechanical failures were observed on the 11-mm size stems below 6 x BW for in-plane vertical loading, and at or below 7 x BW for out-of-plane loading. Using displacement monitoring with a sensitivity of 35 mum, no measurable slippage or relative motion was detected between the stem and sleeve when they were properly assembled. Examination of the contact areas with scanning electron microscopy releaved random surface modification (an indication of fretting or burnishing) with occasional evidence of transfer of material between stem and sleeve.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical compatibility of noncemented hip prostheses with the human femur.

It is generally accepted that more flexible implants are needed to reduce stress shielding and postoperative thigh pain. However, there is no detailed information on the stiffness of currently used implants relative to the human femur. The purpose of this study was to determine the stiffness characteristics (bending, torsional, and axial) of human femora relative to commercially available prostheses as a first step in assessing the mechanical compatibility of the implants. This was achieved by computerized tomography scanning of a collection of human femora from proximal to distal at 10 mm intervals, digitizing the cross-sectional contours, and calculating the stiffness characteristics of each section using standard beam theory. The results show that significant stiffness mismatches exist, especially for larger stem sizes and for stems fabricated from cobalt-chrome alloy. Interestingly, certain implant stiffness values are lower than those of the femur for stems up to 15 mm in diameter, substantially so if the implant is made from titanium alloy and incorporates design features that reduce area and moments of inertia. The data suggest that only larger implant sizes need to be adjusted for increased flexibility compared with current stands.

The physiological basis for a flexible condylar tibial plateau design.

Knee resurfacing is a successful treatment for osteo- and rheumatoid arthritis in elderly patients. The application of this treatment to younger more active and obese persons has the potential to produce premature wear, loosening, and undesirable bone remodelling. A new generation of more physiologically compatible components is required for these situations. This paper discusses the design and analysis of a prototype tibial base plate aimed at physiological load transfer. Incorporated in the design are mechanisms to alleviate lift-off phenomena, bone stress concentrations, stress shielding, and micromotion at the bone-implant interface. The design requires viable cancellous bone stock, so that the bone may respond by remodelling to the dynamic loading during normal ambulatory activities.

Correlation between leuteinizing hormone responses to luteinizing hormone-releasing hormone (LH-RH) and to D-leu-6-LH-RH-ethylamide in patients with hypothalamo-pituitary disease.

Seventeen patients (eleven females and six males) with organic hypothalamo-pituitary disease were subjected to a test consisting of a rapid intravenous injection of 50 mug of luteinizing hormone-releasing hormone (LH-RH) at 8:00 A.M., with blood sampling before and 30 and 60 minutes afterward. Two to four weeks later, a rapid intravenous injection of 50 mug of D-Leu-6-des-Gly-10-LH-RH-ethylamide (D-Leu-6-LH-RH-EA) was given under similar conditions, with blood sampling before and 30, 60, and 90 minutes afterward. Serum LH levels were determined by radioimmunoassay. D-Leu-6-LH-RH-EA caused a greater and more sustained rise in serum LH levels than did an equal dose of LH-RH. However, functional classifications of patients were similar with either preparation. This finding suggests that acute administration of D-Leu-6-LH-RH-EA does not cause a higher number of relevant responses as compared with LH-RH, but only a greater stimulation of LH releases in responsive patients.

Correlation between catecholamine content of the human Fallopian tube and the uterus and plasma levels of estradiol and progesterone.

The content of catecholamines in the Fallopian tube and the uterus and the plasma levels of estradiol and progesterone were studied in cycling women. During the follicular phase norepinephrine levels were 33.4+/-7.1, 50.5+/-8.0, and 145.7+/-43.6 ng. per gram of wet tissue in the external, middle, and internal segments of the Fallopian tube, respectively. During the luteal phase norepinephrine content increased significantly in the external and middle portions (219.3+/-57.0 and 206.2+/-34.3 ng. per gram) whereas it remained unchanged in the internal one (185.2+/-53.6 ng. per gram). The NE content of the external and middle segments correlated significantly with plasma progesterone levels (r = 0.76 and 0.82, respectively, whereas oviductal epincphrine levels did not show significant changes as a function of the stage of the menstrual cycle. Uterine epinephrine content decreased by 67 per cent during the luteal phase whereas norepinephrine remained unchanged.