Tantalum Components in Difficult Acetabular Revisions Have Good Survival at 5 to 10 Years: Longer Term Followup of a Previous Report.

BACKGROUND: The best method for acetabular revisions in patients with severe bone loss remains controversial; typical approaches include jumbo cups, cages, cup-cages, and custom components. Tantalum (TM) components have good results at midterm followup, but the longer term survival and complications are not available. We previously reported on a series of such reconstructions; here we provide additional followup on that group along with a larger study cohort. QUESTIONS/PURPOSES: (1) What is the survival at a minimum followup of 5 years of these components implanted in difficult revisions and what is the mechanism of failure? (2) What is the most common complication of these reconstructions, and what hip scores did these patients achieve? METHODS: Between 2001 and 2010, one surgeon performed 150 acetabular revisions, of which 74 (49%) were done using TM components. The general indication for use of these devices was the surgeon's perception that there was sufficient bone loss to place the reconstruction at a high risk of mechanical failure (generally Paprosky Type 3 or 4 or lower Paprosky type with < 50% host bone coverage or morbid obesity). No cages were used during this time. Fifteen patients died and 11 were lost or did not return, leaving 48 hips (46 patients) with a mean followup of 8 years (range, 5-14 years). Of these, 39 were reported on in our previous series, and nine are new in the present series; the overall group here has an additional median of 5 years followup (range, 3-7 years). Five hips had six augments placed to obtain stability. Patients were evaluated by the Harris hip score and standard radiographs; survivorship was estimated using the cumulative incidence competing risks survival analysis. The primary outcome was fixation and survival of the TM component and the secondary outcome was complications. RESULTS: Cumulative incidence competing risks estimate survival free from aseptic loosening was 92% at 10 years (95% confidence interval [CI], 0.81-0.98). Dislocation, the most common complication, occurred in seven of 48 (15%) patients, and five of 48 (10%) had a reoperation for it. Survival free from any reoperation was 84% at 10 years (95% CI, 0.72-0.92). The Harris hip score improved from a mean of 50 points (SD 17) before surgery to 85 points (SD 10; p < 0.001) at latest followup. CONCLUSIONS: Given the findings of this study, TM components appear to provide durable fixation at midterm followup in complex acetabular revisions. Steps to minimize dislocation, the most frequent complication of these revisions, may include the routine use of larger femoral heads. Future studies likely will need to be multisurgeon or multicenter and should evaluate different techniques and components for long-term fixation and the prevention of dislocation. LEVEL OF EVIDENCE: Level IV, therapeutic study.

Large Metal Heads and Highly Cross-Linked Polyethylene Provide Low Wear and Complications at 5-13 Years.

BACKGROUND: Highly cross-linked polyethylene (XLPE) is reported to have low rates of linear and volumetric wear at 10-14 years. In a prior study, larger (36 and 40 mm) femoral heads were associated with more volumetric wear, but there were only 12 hips with these heads. METHODS: We evaluated 107 hips (93 patients, with a mean age of 76 years) with one design of uncemented acetabular component, a 36 (90 hips) or 40 mm (17 hips) metal femoral head, and one electron beam 100 kGy irradiated and remelted XLPE at a mean follow-up of 8 years (range 5-13 years). Selection of these femoral heads was based on several factors, including the perceived risk of dislocation, the outer diameter size of the acetabular component, and liner availability. Measurements of linear and volumetric wear were performed in one experienced laboratory by the Martell method and analyzed using the first-to-last method. Standard radiographs, with additional Judet views, were used to detect periprosthetic osteolysis. Clinical records were used to determine the complications of dislocation, liner fracture, and painful trunnion corrosion. RESULTS: For the entire cohort, the median linear wear rate was 0.041 mm/y (95% confidence interval, 0.031-0.057) and the median volumetric wear rate was 34.6 mm3/y (95% confidence interval, 31.4-53.5). With the numbers available, there was no difference in linear or volumetric wear between the 36 and 40 mm head sizes. Small, asymptomatic osteolytic lesions were noted in 3 hips (2%). There were 3 patients (3%) with dislocation (2 early and 1 late), but these have not had a revision. There were no revisions for loosening, no liner fracture, and no patient with symptomatic trunnion corrosion. CONCLUSION: This acetabular component and XLPE with large metal heads had low rates of linear and volumetric wear. Large metal femoral heads did not lead to liner fracture, loosening, or symptomatic trunnion corrosion in this patient population. However, we recommend longer clinical follow-up studies and caution in the routine use of larger metal femoral heads in other, younger patient populations.

Is There a Difference between Cemented and Uncemented Femoral Stem Extensions in Revision Knee Arthroplasty?

A stem is usually recommended for the femoral component in revision total knee arthroplasty (TKA) for reasons of alignment, fixation, and bone loss. However, the optimal method of fixation for the femoral component stem remains controversial. We queried the prospective revision TKA database of one surgeon and performed a clinical and radiographic evaluation of 84 knee revisions in which a femoral component stem extension was implanted. There was no established protocol for fixation of the femoral stem during this time. There were 34 knees with cemented (C) fixation and 50 with uncemented (UC) fixation. There were no significant differences in age, gender, body mass index (BMI), or Anderson's Orthopaedic Research Institute (AORI) defect between the two groups. Patients were evaluated using the classic Knee Society clinical and radiographic scores and followed for a mean of 6 years (range: 2-17 years). There was no statistically significant difference in prevalence of reoperation for loosening between cemented and uncemented stems (cemented 3.3% vs. uncemented 10%; p = 0.4). Post hoc power analysis showed that 203 knees in each group would be needed for statistical significance. With the numbers available, there was no difference in aseptic component loosening and radiographic loosening combined (one revision and two radiographic loosening, 9%, in the C group vs. five revisions and three radiographic loosening, 16%, in the UC group; p = 0.51). There was no difference between the groups in the overall rate of any reoperation. There were no differences in postoperative Knee Society pain score, change in pain score, Knee Society function score, or change in function score. Due to the numbers required, a large multicenter study will be needed to determine the optimal method of fixation of the femoral stem in revision TKA.

Simulating Groundcover Community Assembly in a Frequently Burned Ecosystem Using a Simple Neutral Model.

Fire is a keystone process that drives patterns of biodiversity globally. In frequently burned fire-dependent ecosystems, surface fire regimes allow for the coexistence of high plant diversity at fine scales even where soils are uniform. The mechanisms on how fire impacts groundcover community dynamics are, however, poorly understood. Because fire can act as a stochastic agent of mortality, we hypothesized that a neutral mechanism might be responsible for maintaining plant diversity. We used the demographic parameters of the unified neutral theory of biodiversity (UNTB) as a foundation to model groundcover species richness, using a southeastern US pine woodland as an example. We followed the fate of over 7,000 individuals of 123 plant species for 4 years and two prescribed burns in frequently burned Pinus palustris sites in northwest FL, USA. Using these empirical data and UNTB-based assumptions, we developed two parsimonious autonomous agent models, which were distinct by spatially explicit and implicit local recruitment processes. Using a parameter sensitivity test, we examined how empirical estimates, input species frequency distributions, and community size affected output species richness. We found that dispersal limitation was the most influential parameter, followed by mortality and birth, and that these parameters varied based on scale of the frequency distributions. Overall, these nominal parameters were useful for simulating fine-scale groundcover communities, although further empirical analysis of richness patterns, particularly related to fine-scale burn severity, is needed. This modeling framework can be utilized to examine our premise that localized groundcover assemblages are neutral communities at high fire frequencies, as well as to examine the extent to which niche-based dynamics determine community dynamics when fire frequency is altered.

Global change and the importance of fire for the ecology and evolution of insects.

Climate change is drastically altering global fire regimes, which may affect the structure and function of insect communities. Insect responses to fire are strongly tied to fire history, plant responses, and changes in species interactions. Many insects already possess adaptive traits to survive fire or benefit from post-fire resources, which may result in community composition shifting toward habitat and dietary generalists as well as species with high dispersal abilities. However, predicting community-level resilience of insects is inherently challenging due to the high degree of spatiotemporal and historical heterogeneity of fires, diversity of insect life histories, and potential interactions with other global change drivers. Future work should incorporate experimental approaches that specifically consider spatiotemporal variability and regional fire history in order to integrate eco-evolutionary processes in understanding insect responses to fire.

Emergence of ideal membrane cascades for downstream processing.

An algorithm is developed for describing ideal membrane cascades for fractionation of binary and pseudo-binary mixtures. It is shown that solvent management plays a key role in determining both purification and yield. Development of efficient diafilters is needed if membrane cascades are to achieve their full potential in competing with both chromatography and simulated moving bed operations in downstream processing of proteins. Such a replacement will also be important for fractionation of higher titers and larger substrates, such as plasmids, viruses, and even whole cells.