Mechanical Analysis and Corrosion Analysis of Zinc Alloys for Bioabsorbable Implants for Osteosynthesis

Zinc alloys have recently been researched intensely for their great properties as bioabsorbable implants for osteosynthesis. Pure zinc (Zn) itself has relatively poor strength, which makes it insufficient for most clinical use. Research has already proven that the mechanical strength of zinc can be enhanced significantly by alloying it with silver. This study evaluated zinc silver alloys (ZnAg) as well as novel zinc silver titanium alloys (ZnAgTi) regarding their mechanical properties for the use as bioabsorbable implants. Compared to pure zinc the mechanical strength was enhanced significantly for all tested zinc alloys. The elastic properties were only enhanced significantly for the zinc silver alloys ZnAg6 and ZnAg9. Regarding target values for orthopedic implants proposed in literature, the best mechanical properties were measured for the ZnAg3Ti1 alloy with an ultimate tensile strength of 262 MPa and an elongation at fracture of 16%. Besides the mechanical properties, the corrosion rates are important for bioabsorbable implants. This study tested the corrosion rates of zinc alloys in PBS solution (phosphate buffered solution) with electrochemical corrosion measurement. Zinc and its alloys showed favorable corrosion rates, especially in comparison to magnesium, which has a much lower degradation rate and no buildup of hydrogen gas pockets during the process. Altogether, this makes zinc alloys highly favorable for use as material for bioabsorbable implants for osteosynthesis.

OUTCOMES OF BIOABSORBABLE FIXATION OF OSTEOCHONDRAL INJURIES IN THE PEDIATRIC AND ADOLESCENT KNEE

Background: Osteochondral lesions (OCL), including osteochondritis dissecans (OCD) and traumatic osteochondral fractures (OCF), are often encountered when treating injuries affecting the pediatric and adolescent knee. There are an array of treatment options depending upon lesion size, location, chronicity, and equally important, surgeon experience. There is a potential for healing in children and adolescents that may not be present in adults and the intent of internal fixation is to reduce and restore the injured hyaline cartilage and underlying bone. Stable fixation utilizing bioabsorbable implants offers a possibility of OCL healing without the need for implant removal or a major restorative procedure such as an osteochondral autograft transfer, osteochondral allograft, or autologous chondrocyte implantation, therefore avoiding the associated costs and potential morbidity. The purpose of our study was to review the mid-term outcomes of internal fixation with bioabsorbable implants of osteochondral injuries of the knee in children and adolescents. Methods: An IRB approved retrospective review of patients younger than 18 years old who had internal fixation of knee OCLs with bioabsorbable implants was performed. All those included had a minimum 2-year follow-up and completed validated outcome scores (Pedi-IKDC, Tegner, Lysholm). Medical records were reviewed from 2009 to 2015 for patient demographics, intraoperative details, and postoperative outcomes. Pedi-IKDC and Tegner-Lysholm functional outcome scores were used to assess overall outcomes at final follow-up. Return to the operating room was recorded. All cases were performed at a single tertiary pediatric institution by a single pediatric fellowship trained orthopedic surgeon. Results: There were a total of 35 children and adolescents that had a knee OCL that underwent internal fixation with bioabsorbable implants. 7 patients were lost to follow-up with a final cohort of 28 of the 35 patients (80%). Average follow-up was 44 months (range 24-97 months). Average age was 14.7 years. Out of the 28 children, 14 had OCD and 14 OCFs. The average number of bioabsorbable darts and screws used per intervention was 2 and 1, respectively. The average Pedi-IKDC and Lysholm scoring at final follow-up was 89.5 and 91, respectively. Five patients returned to the operating room following the primary procedure: 2 for manipulation under anesthesia related to OCL fixation, 1 for a planned staged ACL reconstruction, 1 for MPFL reconstruction for recurrent patella instability following the index procedure, and 1 patient returned secondary to a proud implant that was in-turn debrided. None of the 28 patients underwent a revision cartilage procedure. Conclusion: The use of bioabsorbable implants (screws and darts) in children and adolescents appears to be a safe and efficacious treatment with good functional outcomes at mid-term follow up and in this cohort there was no need for a revision cartilage procedure. Larger multi-center longer-term follow studies are needed.

Pathology of Bioabsorbable Implants in Preclinical Studies

Bioabsorbable implants can be advantageous for certain surgical tissue bioengineering applications and implant-assisted tissue repair. They offer the obvious benefits of nonpermanence and eventual restoration of the native tissue’s biomechanical and immunological properties, while providing a structural scaffold for healing and a route for additional therapies (i.e., drug elution). They present unique developmental, imaging, and histopathological challenges in the conduct of preclinical animal studies and in interpretation of pathology data. The bioabsorption process is typically associated with a gradual decline (over months to years) in structural strength and integrity and may also be associated with cellular responses such as phagocytosis that may confound interpretation of efficacy and safety end points. Additionally, as these implants bioabsorb, they become increasingly difficult to isolate histologically and thus imaging modalities such as microCT become very valuable to determine the original location of the implants and to assess the remodeling response in tandem with histopathology. In this article, we will review different types of bioabsorbable implants and commonly used bioabsorbable materials; additionally, we will address some of the most common challenges and pitfalls confronting histologists and pathologists in collecting, handling, imaging, preparing tissues through histology, evaluating, and interpreting study data associated with bioabsorbable implants.

Fixation of Traumatic Chondral-Only Fragments of the Knee in Pediatric and Adolescent Athletes: A Retrospective Multicenter Report

Background: While traditional biological principles have suggested that fragments consisting of cartilage alone cannot be reaffixed to bone with expectable long-term healing, case reports of successful healing after fixation in younger patients indicate that this concept remains incompletely explored. Purpose: To evaluate the presenting features, techniques, healing rates, and clinical and radiological results in a cohort of pediatric and adolescent athletes who underwent fixation of traumatic chondral-only fragments in the knee. Study Design: Case series; Level of evidence, 4. Methods: Patient registries at 2 tertiary care children’s hospitals were reviewed to identify patients ≤18 years old who underwent fixation of a “chondral-only” fragment in the knee, defined as the inability to visualize the fragment on injury radiographs or discern bone on the articular portion of a fragment intraoperatively. The mechanism of injury, fragment features, fixation technique, and postoperative clinical course, including timing of sports clearance, healing on postoperative magnetic resonance imaging (MRI), and any complications or reoperations, were assessed. Results: Fifteen patients with a median age at surgery of 12.7 years (interquartile range [IQR], 11.7-14.2 years) and median follow-up of 12.0 months (IQR, 6.0-19.2 months) were analyzed. All patients sustained an acute knee injury before surgery. The injured sites, as assessed on MRI, were the patella (n = 6), trochlea (n = 5), and lateral femoral condyle (n = 4). The median fragment surface area was 492.0 mm2 (IQR, 400.0-787.5 mm2). Fixation with bioabsorbable implants was performed in all patients at a median of 1.6 weeks (IQR, 1.0-2.6 weeks) after the injury. One patient (7%) sustained a fall 8 weeks postoperatively, requiring secondary surgery for excision of a dislodged fragment, and 1 patient (7%) underwent unrelated patellar stabilization surgery 3.4 years postoperatively, at which time the fragment was found to be stable. MRI was performed in 9 of 14 patients with retained fragments (median, 12.0 months postoperatively), with 5 patients (56%) showing restoration of the cartilage contour and the resolution of subchondral edema; 2 patients showed thinning but intact cartilage, 1 had cartilage thickening, and 1 had subchondral edema, fissuring, and cystic changes. The median time to return to sports for all 15 patients was 26.0 weeks (IQR, 22.8-40.9 weeks), including 2 patients who required second surgery and returned to sports at 26.1 and 191.1 weeks. Conclusion: Fixation of traumatic chondral-only fragments using bioabsorbable implants may result in successful short-term healing in the majority of pediatric and adolescent athletes.