Biodegradable Composite Implants

Biocompatibility Assessment of Zinc Alloys as a New Potential Material for Bioabsorbable Implants for Osteosynthesis

In the last several years, zinc and its alloys have come into focus as bioabsorbable materials by qualifying themselves with an excellent corrosion rate, mechanical properties, anti-bacterial effects. and considerable biocompatibility. In this study, the biocompatibility of zinc-silver alloys containing 3.3 wt% silver (ZnAg3) was assessed by evaluating their cell viability, the proliferation rate, and the cell toxicity. Two alloys were investigated in which one was phosphated and the other was non-phosphated. The alloys were tested on human osteoblasts (hOb), which are, to a large extent, responsible for bone formation and healing processes. The performance of the phosphated alloy did not differ significantly from the non-phosphated alloy. The results showed a promising biocompatibility with hOb for both alloys equally in all conducted assays, qualifying ZnAg3 for further investigations such as in vivo studies.

Biomechanical evaluation of an allograft fixation system for ACL reconstruction

The purpose of this study was to compare the biomechanical stability, especially graft slippage of an allograft screw and a conventional interference screw for tibial implant fixation in ACL reconstruction. Twenty-four paired human proximal tibia specimens underwent ACL reconstruction, with the graft in one specimen of each pair fixed using the allograft screw and the other using the conventional interference screw. Specimens were subjected to cyclic tensile loading until failure. The two fixation methods did not show any statistical difference in load at graft slippage (p = 0.241) or estimated mean survival until slippage onset (p = 0.061). The ultimate load and the estimated mean survival until failure were higher for the interference screw (p = 0.04, and p = 0.018, respectively). Graft displacement at ultimate load reached values of up to 7.2 (interference screw) and 11.3 mm (allograft screw). The allograft screw for implant fixation in ACL reconstruction demonstrated comparable behavior in terms of graft slippage to the interference screw but underperformed in terms of ultimate load. However, the ultimate load, occurring at progressive graft slippage, may not be considered a direct indicator of clinical failure.

Evaluation of the degradation of two bioabsorbable interference screws: an in-vivo study in sheep

PURPOSE

To evaluate in-vivo degradation of two bioabsorbable interference screws.

METHODS

Twenty-two crossbred Santa Inês ewes were used. A poly-DL-lactide (PDLLA) screw (70%/30%) was inserted in the right pelvic limb, and a PDLLA screw (70%) + β-tri-calcium phosphate (β-TCP) (30%) in the left pelvic limb. Animals were euthanized at one, four, seven and a half and 18 months after surgery. Plain radiography, computed tomography (CT), microCT, and histological analysis were accomplished.

RESULTS

PDLLA screw was hypodense at all evaluation moments, but with progressive density increase along the central axis, whereas PDLLA/β-TCP was initially hyperdense and progressively lost this characteristic. No adverse reactions were observed on histological evaluation.

CONCLUSIONS

The inclusion of β-TCP favors screw degradation since the PDLLA/β-TCP screws evidenced a more intense degradation process than the PDLLA screws at the last evaluation. PDLLA screws showed higher bone production, evident around the screw thread, inside the lateral perforations, and in the central canal, whereas the PDLLA/β-TCP screws presented less bone tissue at the implantation site.

Degradation of poly-D-L-lactide (PDLLA) interference screws (Megafix ®)

INTRODUCTION

Interference screw fixation is a standard procedure in anterior cruciate ligament (ACL) replacement. Aim of this study was to evaluate the degradation process of Poly-D-L-lactide (PDLLA) interference screws used for tibial ACL graft fixation.

MATERIALS AND METHODS

We evaluated magnetic resonance imaging (MRI) scans of 18 patients who underwent ACL revision surgery at different time points after anatomic ACL reconstruction. At primary surgery, a tibial hybrid fixation was performed with a degradable interference (IF) screw made of PDLLA (Megafix(®)) and a button.

RESULTS

MRI revealed three different phases of degradation of the PDLLA screw. 6-8 months after surgery the IF screw was clearly visible as a well-defined structure on MRI and CT scan. After 12-16 months, the screws appeared less defined with central ingrowths' of connective tissue. In some cases only fragmented screw material was visible. At these time points, there was a slight edema surrounding the tunnel visible on MRI. After 22 months and later, the mean screw site densities were comparable with the surrounding bone density. There was no edema or signs of inflammation around the bone tunnels visible. Presence of cystic or osteolytic changes was not detected.

CONCLUSION

After 22 months, a PDLLA screw may not interfere with ACL revision surgery. Regarding the degradation process of PDLLA screws, we noted three different phases. Furthermore, the degradation process observed by MRI resembles to that described by animal studies. The PDLLA screws fully absorb and are partially replaced by bone. The degradation process in humans seems to be longer than that described in animals.

Strengthening of Dense Bioceramic Samples Using Bioresorbable Polymers – A Statistical Approach

Mechanical properties of bioceramics are poor and need to be improved for biomedical applications. In order to do this, bioceramics may be strengthened by bioresorbable polymers. In this study, the mechanical properties of poly(ε-caprolactone), PCL, coated dense bioceramic pellets made of silica-contained calcium phosphates were studied and analyzed using a statistical experimental design in conjunction with Taguchi methods for optimization. The aim of this experimental work was to maximize the pellet flexural strength and minimize the amount of deposited PCL. The most important factors affecting the strengthening of the ceramic pellets were evaluated. Four independent processing variables (a removal technique of an excess polymer solution, concentration of PCL in the solution, a heat treatment temperature and the number of dipping) with three levels of variability were tested using an L9 (34) orthogonal array. A statistical experimental design using the analysis of means and orthogonal array was applied to optimize the responses of these variables. The optimal conditions for achieving the maximal flexural strength of the coated pellets at the minimal amount of the deposited PCL were determined. A high quality dense bioceramic pellets with ~ 10.5 MPa flexural strength and ~ 80 μm thickness (~ 21 mg weight) of the deposited PCL coating were manufactured as a result.

Toughening of porous bioceramic scaffolds by bioresorbable polymeric coatings

The mechanical properties of poly(c-caprolactone) (PCL)-coated porous bioceramic scaffolds made of calcium phosphates were studied and analysed using a statistical experimental design and Taguchi methods. In this study, both the flexural strength of the coated scaffolds and the amount of deposited PCL were considered as the measured responses. A statistical experimental design using the analysis of means and orthogonal array was applied to optimize these responses. The removal technique of excess polymer solution, the concentration of PCL in the solution, a heat treatment temperature, and the number of times that the scaffolds were dipped in the solution were chosen as the significant processing variables. The removal technique of excess polymer solution and the number of times that the scaffolds were dipped in the solution showed the major effects on the flexural strength, while the technique for removal of excess polymer solution was found to have the major effect on the mass of the deposited PCL. The optimal conditions for achieving the maximal flexural strength of the coated scaffolds at the minimal amount of the deposited PCL were determined and tested. High-quality porous bioresorbable scaffolds with approximately 19 MPa flexural strength and approximately 0.4g of the total (PCL coating+calcium phosphates) mass with dimensions of 8.5 mm diameter and 13 mm width were manufactured as a result.

Influence of scaffold stiffness on subchondral bone and subsequent cartilage regeneration in an ovine model of osteochondral defect healing

BACKGROUND

In osteochondral defects, subchondral bone, as a load-bearing structure, is believed to be important for bone and cartilage regeneration.

HYPOTHESIS

A stiff scaffold creates better conditions for bone formation and cartilage regeneration than does a softer one.

STUDY DESIGN

Controlled laboratory study.

METHODS

Critical osteochondral defects were created in the femoral condyles of 24 sheep. Subchondral bone was reconstructed with a stiff scaffold or a modified softer one, with untreated defects serving as controls. The repair response was evaluated with mechanical, histological, and histomorphometrical techniques at 3 and 6 months postoperatively.

RESULTS

The elastic modulus of regenerated fibrocartilage over the stiff scaffold tended to be higher than in the soft scaffold group (61% vs 46% of healthy cartilage) at 3 months. No difference was determined at 6 months; all were well below healthy cartilage. Treated defects showed substantial degradation of the soft scaffold with surrounding sclerotic bone at 3 and 6 months. In contrast, degradation of the stiff scaffold was slower and occurred together with continuous osseous replacement.

CONCLUSION

Stiff scaffolds were found to improve bone regeneration. In contrast, soft scaffolds provided less support, and consequently subchondral bone became sclerotic. Although regenerated cartilage formed over the stiff scaffolds at 3 months, and these exhibited better mechanical properties than did the soft scaffold group, the mechanical properties in both treated groups were the same at 6 months, not dissimilar to that of tissue formed in the untreated specimens and inferior to native articular cartilage.

CLINICAL RELEVANCE

The results imply that subchondral defect filling in clinical settings advances bone regeneration and should have a comparable stiffness to that of healthy subchondral bone rather than being too flexible. Degradation of resorbable materials and consequently the loss of stiffness may compromise the healing of critical defects.