Meniscal substitutes should ideally distribute loads across adjacent articular cartilage much in the way of the native tissue, thereby protecting the cartilage from degeneration. The objective of this study was to quantify the loads transferred to the tibial plateau after implantation of a porous polyurethane scaffold. Our hypothesis was that scaffold implantation would improve the contact mechanics of a partial meniscectomy knee. Six sheep cadaver knees were mounted in a load-controlled knee-joint simulator. A pressure-sensor sheet, secured to the tibial plateau, dynamically recorded pressure data over 10 gait cycles for the following conditions: (i) intact, (ii) partial meniscectomy, (iii) implanted scaffold, and (iv) total meniscectomy. Implantation of the scaffold increased the contact area and decreased the contact pressures compared to the meniscal defect knee. No significant difference was found between the average and peak contact pressures of the intact knees and scaffold-implanted knees; however, contact area was significantly lower for the scaffold-implanted knees compared to intact knees. Our hypothesis was accepted; replacement with a polyurethane scaffold improved knee-contact mechanics relative to a partially meniscectomized knee. On the basis of these findings, the scaffold will be evaluated for its chondroprotective capacity in an in vivo ovine model.
Rapid implant fixation could prove beneficial in a host of clinical applications from total joint arthroplasty to trauma. We hypothesized that a novel self-assembled monolayer of phosphonate molecules (SAMP) covalently bonded to the oxide surface of titanium alloy would enhance bony integration. Beaded metallic rods were treated with one of three coatings: SAMP, SAMP + RGD peptide, or hydroxyapatite. Rods were inserted retrogradely into both distal femurs of 60 rabbits. Fifteen rabbits were sacrificed at 2, 4, 8, and 16 weeks. At each time, seven specimens for mechanical pull-out testing and three for histomorphometric analysis were available for each coating. At four weeks, both SAMP groups had significantly higher failure loads when compared to hydroxyapatite (p < 0.01). No significant differences were found among groups at other times, though the SAMP-alone group remained stronger at 16 weeks. Histology showed abundant new bone formation around all the three groups, though more enhanced formation was apparent in the two SAMP groups. With this novel treatment, with or without RGD, the failure load of implants doubled in half the time as compared with hydroxyapatite. Where early implant fixation is important, the SAMP treatment provides a simple, cost-effective enhancement to bony integration of orthopaedic implants.
Although modularity affords various options to the orthopedic surgeon, these benefits come at a price. The unintended bearing surface between the back surface of the tibial insert and the metallic tray results in micromotion leading to polyethylene wear debris. The objective of this study was to examine the backside wear of tibial inserts from three modern total knee designs with very different locking mechanisms: and Advance\ . A random sample of 71 inserts were obtained from our institution_s retrieval collection and examined to assess the extent of wear, depth of wear, and wear damage modes. Patient records were also obtained to determine patient age, body mass index, length of implantation, and reason for revision. Modes of wear damage (abrasion, burnishing, scratching, delamination, third body debris, surface deformation, and pitting) were then scored in each zone from 0 to 3 (0 = 0%, 1 = 0-10%, 2 = 10-50%, and 3 = >50%). The depth of wear was subjectively identified as removal of manufacturing identification markings stamped onto the inferior surface of the polyethylene. Both Advance \ and IB II \ polyethylene inserts showed significantly higher scores for backside wear than the Optetrak \ inserts. All IB II \ and Advance \ implants showed evidence of backside wear, whereas 17% (5 out of 30) of the retrieved Optetrak \ implants had no observable wear. There were no significant differences when comparing the depth of wear score between designs. The locking mechanism greatly affects the propensity for wear and should be considered when choosing a knee implant system.
The clinical goal of spinal fusion is to reduce motion and the associated pain. Therefore, measuring motion under loading is critical. The purpose of this study was to validate four-point bending as a means to mechanically evaluate simulated fusions in dog and rabbit spines. We hypothesized that this method would be more sensitive than manual palpation and would be able to distinguish unilateral vs bilateral fusion. Spines from four mixed breed dogs and four New Zealand white rabbits were used to simulate posterolateral fusion with polymethyl methacrylate as the fusion mass. We performed manual palpation and nondestructive mechanical testing in four-point bending in four planes of motion: flexion, extension, and right and left bending. This testing protocol was used for each specimen in three fusion modes: intact, unilateral, and bilateral fusion. Under manual palpation, all intact spines were rated as not fused, and all unilateral and bilateral simulated fusions were rated as fused. In four-point bending, dog spines were significantly stiffer after unilateral fusion compared with intact in all directions. Additionally, rabbit spines were stiffer in flexion and left bending after unilateral fusion. All specimens exhibited significant differences between intact and bilateral fusion except the rabbit in extension. For unilateral vs bilateral fusion, significant differences were present for right bending in the dog model and for flexion in the rabbit. Unilateral fusion can provide enough stability to constitute a fused grade by manual palpation but may not provide structural stiffness comparable to bilateral fusion.
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