Depletion of the upper mantle by convergent tectonics in the Early Earth.

Partial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2-3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300-1550 °C, ∆T = T - Tmodern = 0-250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite-Trondhjemite-Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200-250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100-200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels.

Building cratonic keels in Precambrian plate tectonics.

The ancient cores of continents (cratons) are underlain by mantle keels-volumes of melt-depleted, mechanically resistant, buoyant and diamondiferous mantle up to 350 kilometres thick, which have remained isolated from the hotter and denser convecting mantle for more than two billion years. Mantle keels formed only in the Early Earth (approximately 1.5 to 3.5 billion years ago in the Precambrian eon); they have no modern analogues1-4. Many keels show layering in terms of degree of melt depletion5-7. The origin of such layered lithosphere remains unknown and may be indicative of a global tectonics mode (plate rather than plume tectonics) operating in the Early Earth. Here we investigate the possible origin of mantle keels using models of oceanic subduction followed by arc-continent collision at increased mantle temperatures (150-250 degrees Celsius higher than the present-day values). We demonstrate that after Archaean plate tectonics began, the hot, ductile, positively buoyant, melt-depleted sublithospheric mantle layer located under subducting oceanic plates was unable to subduct together with the slab. The moving slab left behind craton-scale emplacements of viscous protokeel beneath adjacent continental domains. Estimates of the thickness of this sublithospheric depleted mantle show that this mechanism was efficient at the time of the major statistical maxima of cratonic lithosphere ages. Subsequent conductive cooling of these protokeels would produce mantle keels with their low modern temperatures, which are suitable for diamond formation. Precambrian subduction of oceanic plates with highly depleted mantle is thus a prerequisite for the formation of thick layered lithosphere under the continents, which permitted their longevity and survival in subsequent plate tectonic processes.

Early failures in total knee arthroplasty.

Total knee arthroplasty is a predictable operation. Unfortunately, there is a subset of patients who do not do well and require revision surgery within the first 5 years. The purpose of the current study was to analyze the mechanisms of failure in patients who had revision surgery within 5 years of their index arthroplasty. Between 1986 and 1999, 440 patients with total knee arthroplasties were referred for revision surgery. An analysis of patients in whom the arthroplasties failed within 5 years of the index arthroplasty and the reasons for early failure were documented. Of the 440 patients who had revision surgery, 279 (63%) had revision surgery within 5 years of their index arthroplasty: 105 of the 279 patients with early failures (38%) had revision surgery because of infection; 74 (27%) had revision surgery because of instability; 37 (13%) had revision surgery because of failure of ingrowth of a porous-coated implant; 22 (8%) had revision surgery because of patellofemoral problems; and 21 (7%) had revision surgery because of wear or osteolysis. Only eight of the 279 patients with early failures (3%) had revision surgery because of aseptic loosening of a cemented implant. The remaining 12 patients had revision surgery because of miscellaneous problems. Host factors may prevent infection from ever being eradicated totally. The two other major patterns of failure in this series were failure of cementless fixation and instability. If all of the arthroplasties in the patients in this early failure group would have been cemented routinely and balanced carefully, the total number of early revisions would have decreased by approximately 40%, and the overall failures would have been reduced by 25%.

Non-chondritic distribution of the highly siderophile elements in mantle sulphides.

The abundances of highly siderophile (iron-loving) elements (HSEs) in the Earth's mantle provide important constraints on models of the Earth's early evolution. It has long been assumed that the relative abundances of HSEs should reflect the composition of chondritic meteorites--which are thought to represent the primordial material from which the Earth was formed. But the non-chondritic abundance ratios recently found in several types of rock derived from the Earth's mantle have been difficult to reconcile with standard models of the Earth's accretion, and have been interpreted as having arisen from the addition to the primitive mantle of either non-chondritic extraterrestrial material or differentiated material from the Earth's core. Here we report in situ laser-ablation analyses of sulphides in mantle-derived rocks which show that these sulphides do not have chondritic HSE patterns, but that different generations of sulphide within single samples show extreme variability in the relative abundances of HSEs. Sulphides enclosed in silicate phases have high osmium and iridium abundances but low Pd/Ir ratios, whereas pentlandite-dominated interstitial sulphides show low osmium and iridium abundances and high Pd/Ir ratios. We interpret the silicate-enclosed sulphides as the residues of melting processes and interstitial sulphides as the crystallization products of sulphide-bearing (metasomatic) fluids. We suggest that non-chondritic HSE patterns directly reflect processes occurring in the upper mantle--that is, melting and sulphide addition via metasomatism--and are not evidence for the addition of core material or of 'exotic' meteoritic components.

Motion at the modular acetabular shell and liner interface. A comparative study.

Excessive polyethylene debris generated from a total hip arthroplasty can lead to osteolysis and premature revision. Most of this polyethylene debris comes from the concave articulation. However, abrasive wear on the convex side of a modular polyethylene component also may play a role in this problem. Motion of a modular polyethylene liner with respect to its shell can lead to such abrasive wear. Six samples of modular acetabular components from eight manufacturers were tested for motion between the shell and the liner. Motion at the shell liner interface was detected by five fiberoptic sensors in the x, y, and z planes. Micromotion occurred at the shell liner interface in all designs tested. The magnitude of motion varied between manufacturers, ranging from 5 to 311 microns. Design features that limit motion may be advantageous in limiting the amount of polyethylene debris.

Cementless fixation in 2-stage reimplantation for periprosthetic sepsis.

Twenty-five patients with documented infection of the hip were reviewed. All patients underwent reconstruction in a 2-stage fashion with cementless implants. The average follow-up in this group was 41 months. The average time to reimplantation was 4.8 months. Of the 25 living patients, 22 retained their implants. There were 2 recurrences of infection, for an infection recurrence rate of 8% (2 of 25). The average postoperative Harris Hip Score was 81. Bone ingrowth was confirmed radiographically via the Engh fixation score in all but 1 of the surviving implants. Cementless fixation in 2-stage reimplantation can result in acceptable eradication rates, while supplying predictable fixation, provided that appropriate cementless revision implants are used.

Revision of failed cementless total knee implants with cement.

Between 1986 and 1996, 268 revision total knee replacements were performed. Revisions for aseptic loosening were reviewed prospectively. Sixty-three patients had implants revised for aseptic loosening. Thirty-six of the patients had revisions of cemented constructs, whereas 27 of the patients had revisions of cementless constructs. The average time before revision for the cemented group was 86 months. The average time to revision for the cementless group was only 43 months. Fifty-two percent of the patients in the cementless group had revision of their implants within 2 years of their index arthroplasty. The average pain free interval for the cementless group was only 11 months with 63% of the patients having no pain relief after their index arthroplasty. The average postoperative score for the cementless group was 88, whereas the average postoperative score for the cemented group was 82. Revision of failed cementless total knee implants with cement is a reliable procedure. Results are similar to those of cemented knees revised with cement.

Bone loss associated with the use of spacer blocks in infected total knee arthroplasty.

Twenty-five knees in 24 patients with infected total knee replacements were treated with debridement, component removal, and insertion of an antibiotic impregnated cement spacer block. Intravenous antibiotics were administered for 6 weeks during which time the patients' knees were kept in a knee immobilizer nonweightbearing. These cases were reviewed retrospectively to determine radiographically the amount of bone loss that occurred during the period before reimplantation. Tibial and femoral bone loss occurred frequently from invagination of the cement spacer block into the femoral and tibial cancellous bone. Tibial bone loss was present in 10 (40%) cases and averaged 6.2 mm. Femoral bone loss was present in 11 (44%) cases and averaged 12.8 mm. Bone loss was more common when spacer blocks were undersized. None of the 15 spacer blocks that were made with a small intramedullary stem displaced. Three of the remaining 10 spacer blocks made without an intramedullary stem did displace with associated bone loss. Antibiotic spacer blocks used in the two-stage treatment of infected total knee replacements can be associated with subsequent tibial and femoral bone loss.

Core decompression of the osteonecrotic femoral head.

The results in 114 hips of ninety-two patients who had osteonecrosis of the femoral head were assessed after treatment with core decompression. The average duration of follow-up was three years and four months (range, two years to six years and six months). The average age of the patients was forty-one years (range, fifteen to sixty-seven years). The presumed risk factors were the use of corticosteroids (thirty-seven hips), excessive use of alcohol (thirty-two hips), trauma (seven hips), and various other factors (seven hips). No specific risk factor was identified for thirty-one hips, and the osteonecrosis was considered to be idiopathic. The preoperative evaluation consisted of clinical assessment, magnetic resonance imaging, and radiographic staging according to a modification of the system of Ficat. Thirty-two hips were in stage I; thirty-eight, in stage IIA; twenty-five, in stage IIB (transition stage, with a crescent sign); and nineteen, in stage III. Clinical failure was defined as the performance of a subsequent operation. Over-all, sixty-four hips (56 percent) failed clinically. Fifty-seven were treated with a hip replacement; four, with a femoral osteotomy; and three, with a vascularized fibular graft. Clinical failure was seen in five (16 percent) of the thirty-two hips in stage I, twenty (53 percent) of the thirty-eight hips in stage IIA, twenty (80 percent) of the twenty-five hips in stage IIB, and in all nineteen of the hips in stage III.(ABSTRACT TRUNCATED AT 250 WORDS)

Cementless acetabular components.

Four hundred and fifteen patients with cementless acetabular components of either a smooth threaded (130) or porous surfaced (285) variety were compared for clinical symptoms and radiographic signs of component loosening. At a mean 4.8 year follow-up none of the patients with porous acetabular components had signs of component instability. At a mean 3.9 year follow-up 27 (21%) of the patients with a smooth threaded acetabular component showed radiographic signs of instability and 33 (25%) had clinical symptoms. The disappointing short-term results with these threaded cups in our hands have prompted us to abandon their use in favour of the porous surfaced hemispherical cups.

Results of cementless revision for failed cemented total hip arthroplasty.

The goals of revision total hip arthroplasty (THA) are to reestablish and maintain stable implant fixation. Based upon promising early results in primary THA, porous-surfaced implants designed for bone ingrowth fixation are being increasingly used in hopes of more successfully achieving these goals than has been the case using cement. One hundred and sixty such revisions were followed for a mean of 4.4 years, with specific reference to implant fixation. Roentgenographic evaluation of implant fixation suggested four categories of femoral and acetabular components: (1) bone ingrown, (2) stable fibrous encapsulation, (3) questionable, with signs of impending instability, or (4) definitely unstable implant migration, indicative of the need for rerevision. Not surprisingly, success in achieving and maintaining stable implant fixation following revision THA is dependent upon component design, surgical technique, and preexistent bone stock damage. This classification according to bone stock damage should be borne in mind when critically evaluating the results from various revision series.

A retrospective study of traumatic tympanic membrane perforations in a clinical practice.

A retrospective study of 227 traumatic tympanic membrane perforations was made. These perforations were treated in a clinical practice of otolaryngology between January 1969 and July 1977. The following conclusions were supported by the data obtained: 1. Larger perforations are less likely to heal spontaneously. 2. Lightning and molten metal injuries are less likely to heal spontaneously. 3. Final healing and hearing results are good regardless of the mode of therapy employed. 4. Spontaneous healing gives better results than early surgical intervention. 5. Infection adversely affects healing in traumatic perforations. Based on information delineated in the paper, a logical plan of treatment for traumatic perforations is presented.

Symptomatic ossification of the pterygomandibular raphe.

A case of symptomatic ossification of the pterygomandibular raphe, believed to be the first reported case of such an entity, was relieved by surgical removal of the ossified tissue. Ossification of the pterygomandibular raphe should, in the future, be considered in the differential diagnosis in cases of obscure lateral pharyngeal pain.