Multicycle mechanical performance of titanium and stainless steel transpedicular spine implants.

STUDY DESIGN: This was a prospective in vitro study comparing titanium alloy and stainless steel alloy in transpedicular spine implants from two different manufactures. OBJECTIVE: To compare the multicycle mechanical performance of these two alloys, used in each of two different implant designs. SUMMARY OF BACKGROUND DATA: Transpedicular spine implants primarily have been manufactured from stainless steel, but titanium alloy offers imaging advantages. However, the notch sensitivity of titanium alloy has caused concern regarding how implants made from this material will compare in stiffness and fatigue life with implants made from stainless steel. METHODS: Twenty-four implants (two alloys, two designs, six implants per group) were mounted in machined polyethylene wafers and repetitively loaded (up to 1 million cycles) from 80 N to 800 N using a 5-Hertz sinusoidal waveform. Load and displacement data were automatically and periodically sampled throughout the entire test. RESULTS: Implant stiffness increased with cycle load number, reached a steady state, then declined just before fatigue failure. Stiffness varied less in titanium transpedicular spine implants than in their stainless counterparts. All stainless steel implant types were stiffer (steady-state value, P < 0.0001) than their titanium alloy counterparts. One titanium implant design failed with fewer (P < 0.05) load cycles than its stainless steel counterpart, whereas a stainless steel implant of another design failed with fewer (P < 0.002) load cycles than its titanium counterpart. Overall, fatigue life, i.e., the total number of load cycles until failure, was related to implant type (P < 0.0001), but not to implant material. CONCLUSIONS: A transpedicular spine implant's fatigue lifetime depends on both the design and the material and cannot be judged on material alone. Stainless steel implants are stiffer than titanium alloy implants of equal design and size; however, for those designs in which the fatigue life of the titanium alloy version is superior, enlargement of the implant's components can compensate for titanium's lower modulus of elasticity and result in an implant equally stiff as its stainless steel counterpart. Such an implant made from titanium alloy would then be clinically preferable because of titanium's previously reported imaging advantages.

Comparison of lumbar sagittal alignment produced by different operative positions.

STUDY DESIGN: This study is a prospective evaluation of the effects of commonly used spinal tables on lumbar sagittal alignment. OBJECTIVES: The objective was to determine the differences, if any, in lumbar sagittal alignment produced by different positions on routinely used spinal operating tables. SUMMARY OF BACKGROUND DATA: Earlier studies have documented the advantages of the knee-chest position in lumbar decompressive procedures. When simultaneous fusion is performed and augmented with internal fixation, intraoperative position is the critical determinant of sagittal plane balance. Other investigators have documented an association between the knee-chest position and decreased lumbar lordosis. METHODS: Ten asymptomatic volunteers underwent a series of four lateral lumbar radiographs, as follows: standing, prone on the Jackson (Orthopaedic Systems, Inc., Hayword, CA) spinal table, and prone on the Andrews (Orthopaedic Systems, Inc.) table with the hips flexed 60 degrees and 90 degrees, respectively. Intervertebral body angle measurements were obtained from L1 to S1. Lordosis values were compared and analyzed for each of the positions. Standing lordosis was assumed to be physiologic. RESULTS: Physiologic lordosis values were produced only on the Jackson operative table. Both positions on the Andrews table resulted in a statistically significant decrease in lumbar lordosis. Decreasing hip flexion on the Andrews table from 90 degrees to 60 degrees produced a statistically significant increase in lumbar lordosis. However, this change did not reproduce physiologic values. CONCLUSION: When instrumentation is used to augment lumbar fusions, positions incorporating hip flexion should be avoided to ensure maintenance of sagittal plane balance.

Primary lymphoma and Paget's disease of the femur.

Malignant neoplasms associated with Paget's disease of the bone are most commonly metastatic and rarely primary malignant transformations. Sarcomatous transformations, such as osteogenic sarcoma, predominant in Paget's disease. The authors discuss the synchronous and synostotic occurrence of a primary lymphoma of bone in association with Paget's disease. The simultaneous occurrence of isolated primary lymphoma of bone and its phenotype in association with Paget's disease in the femur, which had not been rigorously documented in the previous literature, is reported.

Lumbosacral chordoma resection: image integration and surgical planning.

Clinical, radiographic, and functional outcome parameters were analyzed before and after treatment of eight individuals with lumbosacral chordomas studied over a 3-year period. Emphasis was placed upon correlation of multiplane images and the pathoanatomy of the gross specimen. Surgical resections achieving wide margins can be curative. Magnetic resonance imaging of lumbosacral chordomas has revolutionized the preoperative anatomic localization of these tumors. Therefore, it is critical that the operating surgeon scrutinize the three-dimensional location of the tumor and its spatial relationship to surrounding structures prior to surgical eradication. A systematic approach for the radiographic analysis of tumor extent in lumbosacral chordomas is presented. Four strategic areas of preoperative analysis are identified for sacral chordomas and two for lumbar neoplasms. Functional outcome after resection is predicted by Stener's work. Contamination-free surgery is facilitated by accurate preoperative radiographic interpretation of the pathoanatomy by the surgeon.

Short segment transpedicular Cotrel-Dubousset instrumentation: a porcine corpectomy model.

This study examines changes in stiffness in a three-level spinal construct utilizing transpedicular Cotrel-Dubousset instrumentation. In addition, we evaluated the effect of adding offset laminar hooks at the same level as the superior transpedicular fixation. Porcine lumbar spines (L-1 to L-5) were tested as: (a) intact spine, (b) intact spine with instrumentation, and (c) instrumented spine after L-3 corpectomy. Instrumented constructs were tested with and without laminar hooks placed at the level of superior transpedicular fixation. Constructs were tested in modes of axial compression and torsion in a MTS materials testing machine. The fully instrumented intact spines demonstrated a 28.0% stiffness increase over the intact spines in torsion (p < 0.005) and a 23.1% increase in the mode of axial compression (p < 0.0002). There was no statistically significant difference in axial or torsional stiffness in the instrumented intact spine constructs when hooks were added. In the corpectomy model, addition of the laminar hooks increased stiffness in axial compression an average of 26.9% (p < 0.002) and in torsion an average of 28.1% (p < 0.0005). This increase in stiffness may aid in preventing the postoperative progression of kyphosis noted in studies utilizing similar constructs in clinical application for the treatment of lumbar burst fractures.

Segmental fixation of lumbar burst fractures with Cotrel-Dubousset instrumentation.

A retrospective review of 17 patients who underwent bilateral transpedicular decompression, instrumentation with a Cotrel-Dubousset construct, and posterolateral fusion with iliac crest bone graft for treatment of lumbar burst fracture is presented. All patients were followed to fusion with an average follow-up of 18.9 months. Fifteen of sixteen patients returned to preinjury occupation and/or activity. All patients reported good to excellent clinical results. The average postoperative progression of kyphosis was 11.9 degrees. There was no significant change in anterior vertebral height between the preoperative and postoperative periods. We conclude that although excellent early clinical results can be obtained using this operative strategy, the long-term effect of residual kyphosis at the fracture site is unknown.

Transpedicular short-segment fusions for treatment of lumbar burst fractures.

A retrospective review of 13 patients who underwent decompression and transpedicular instrumentation for lumbar burst fractures is presented. Average follow-up was 22.5 months. Eighty-four percent of patients reported little or no pain at follow-up. Sixty-nine percent of patients returned to full preinjury activity. Radiographic review demonstrated an average postoperative progression of kyphosis of 8.7%. Anterior vertebral body height was unchanged between preoperative evaluation and follow-up. Although short-segment posterior transpedicular instrumentation with VSP plates did not reestablish or maintain anatomic alignment of the lumbar spine after burst fractures, the clinical outcome was excellent.

Characterization of a broad-scope amino acid transport system in sand dollars.

Both echinoderm embryos and adults take up alpha-amino acids by an apparent broad-scope transport system. This transporter can be characterized as follows: alanine transport is not blocked by alpha-(methylamino)isobutyric acid. Leucine and other lipophilic neutral amino acids are preferentially transported. Transport is sodium dependent and blocked by 2-aminobicyclo-[2,2,1]heptane-2-carboxycylic acid. Lysine and aspartate transport is inhibited by lipophilic neutral amino acids. Taurine, a beta-neutral amino acid, is translocated via a second and independent carrier.

Active Uptake of Amino Acids by Leaves of an Epiphytic Vascular Plant, Tillandsia paucifolia (Bromeliaceae).

Specialized epidermal trichomes on the leaves of the epiphyte, Tillandsia paucifolia (Bromeliaceae) accumulate amino acids from solution. Simultaneous net uptake of 17 amino acids was determined using high performance liquid chromatography. Uptake occurs against concentration gradients at least as high as 10(4).

Regulation of net amino acid exchange in sea urchin larvae.

Bacteria-free larvae of Strongylocentrotus purpuratus remove each of 14 free amino acids (FAA) from very dilute solution in seawater. When larvae are maintained in seawater, influx of radiolabeled FAA and net removal of substrate as determined by high-performance liquid chromatography (HPLC) occur at the same rate in those substrates examined. Exposure of larvae to high concentrations of glutamate, glycine, or serine (greater than 50 microM) produces large increases in these substrates in the internal pool of FAA. After incubation of larvae in high concentrations of serine, the rate of influx of 14C-labeled serine is decreased compared with unincubated controls. Comparison of influx of radiolabeled serine with net change in ambient serine as determined by HPLC indicates that the apparent change in rate of influx is a consequence of an increase in ambient serine with time and the resulting decrease in the specific activity of labeled serine. Analysis of data indicates that abnormally high internal serine leads to carrier-mediated efflux of serine as well as other neutral amino acids. The rate of such efflux is increased in the presence of increased levels of ambient serine.

Uptake of free amino acids by bacteria-free larvae of the sand dollar Dendraster excentricus.

Larvae of Dendraster excentricus were produced by collecting gametes and carrying out fertilization under aseptic conditions. Since gametes are free of bacteria in the gonad, bacteria-free (axenic) suspensions of larvae result. Net rates of entry of 14 amino acids and the rate of production of ammonia were simultaneously determined by high-performance liquid chromatography. The net rates of uptake of neutral amino acids were an order of magnitude greater than rates for basic and acidic amino acids. Influx of 14C-labeled leucine, arginine, and glutamate accurately reflects the net entry rate of these substrates. Uptake of amino acids by axenic suspensions of larvae was compared with uptake by suspensions prepared without aseptic precautions. There was no significant difference in net uptake of the 14 amino acids or in the pattern of oxidation and assimilation of [14C]leucine during short-term experiments of 4-h duration or less.

Simultaneous determination of net uptake of 16 amino acids by a marine bivalve.

High-performance liquid chromatography was used to measure the simultaneous transport of 16 amino acids from seawater into the mussel Mytilus edulis. All the substrates, including representative neutral, basic, and acidic amino acids, were accumulated from ambient concentrations as low as 5 nM. Influx of four radioactively labeled amino acids was compared with their net flux. In each case the influx of the 14C-labeled substrate, determined from the removal of isotope from test solutions, accurately represented the net flux of that substrate as measured by direct chemical analysis. Ammonia was the only compound observed to be routinely lost from these animals. Rates of uptake of individual amino acids, each at concentrations of 10 nM, ranged from 2.4 to 7.0 nmol . g dry flesh wt-1 . min-1. The studies show that M. edulis is capable of a net uptake of dissolved amino acid from the very low concentrations characteristic of seawater. Moreover, net accumulation can occur against chemical gradients calculated to be in excess of one million to one. The rates of amino acid uptake may contribute significantly to the animals' energy requirements.

Bacteria-free sea urchin larvae: selective uptake of neutral amino acids from seawater.

Bacteria-free suspensions of larvae of Strongylocentrotus purpuratus (Stimpson) were prepared without the use of antibiotics. Net rates of removal of 18 amino acids, each supplied at 125 nanomoles per liter, and the appearance of ammonia were measured by high-performance liquid chromatography. Taurine and acidic and basic amino acids were not taken up. Removal of neutral amino acids from the medium occurred at rates adequate to contribute to the carbon and nitrogen balance of the larvae at ecologically relevant substrate concentrations.

Transport of Dissolved Amino Acids by the Mussel, Mytilus edulis: Demonstration of Net Uptake from Natural Seawater.

High-performance liquid chromatography provides direct evidence for substantial removal of naturally occurring specific free amino acids during a single passage of water through the mantle cavity of mussels. This occurs during the few seconds required for passage of the water across the gill, and removal proceeds unabated at ambient substrate concentrations as low as 38 nanomoles per liter.

Influence of temperature and unstirred layers on the kinetics of glycine transport in isolated gills of Mytilus californianus.

The effect of temperature on the kinetics of glycine influx in isolated gills of Mytilus californianus was studied. Increases in temperature resulted in significant increases in the Jimax for glycine influx over the range 7--23 degrees C (Q10 = 5). Concomitant with this increase in Jimax was an increase in experimentally determined Kt, from 23 microM to 134 microM. The relationship between changes in Jimax and measured Kt was adequately described by an equation that takes into account the influence of unstirred layers on the kinetics of carrier-mediated transport. Use of this equation indicated that the changes in measured Kt could be explained by the presence of an unstirred layer approximately 400 micrometer thick over the transporting surfaces(s) of the isolated gill, and that the "true" Kt of the transport process is 1 microM. The very low values of Kt for amino acid uptake measured in intact muscles (1--3 microM) are revealed to be the product of transport processes of high affinity for substrate and a perfusion system which effectively minimizes the influence of unstirred layers, thereby permitting efficient utilization of substrate at the low concentrations characteristic of these animals' environment.

Characteristics of influex and net flux of amino acids in Mytilus californianus.

1. Intact mussels and an in vitro preparation of isolated gill tissue were employed to study characteristics of the influx and net flux of amino acids in Mytilus californianus. 2. The kinetics of influx of 14C-labelled amino acids were complicated by the presence of a rapidly labelled extracellular compartment. 3. Correction of influx data for the extracellular compartment revealed influx of 14C-cycloleucine and 14C-glycine to be mediated by a transport mechanism adequately described by Michaelis-Menten kinetics. Passive diffusion plays no significant role in influx. 4. Influx and net flux of glycine into intact Mytilus were examined. From high concentrations (greater than 20 micronM) the influx of 14C-glycine was equivalent to the net influx of primary amine as determined by fluorescamine. At low ambient concentrations (greater than 20 micronM), influx of 14C-glycine occurred more rapidly than net influx as determined by fluorescamine. The data suggest that influx of labelled substrate is accompanied by efflux of unknown primary amine(s). In the absence of labelled substrate, efflux continues until a steady-state concentration of 2-5 micronM in the medium is achieved.5. The rate of influx of 14C-cycloleucine into isolated gill tissue, and the concentration gradient which can be developed by gill tissue are reduced when sodium is replaced in the medium. 6. The efflux of primary amines from isolated gill tissue is stimulated by the replacement of sodium in the medium with choline. 7. The data are consistent with a sodium-coupled mechanism for the transport of amino acid into gill tissue. However, energetic considerations cast doubt on the adequacy of such a mechanism to account fully for the observed trans-epidermal transport.