Aim

Non-steroidal anti-inflammatory drugs (NSAIDs) are a cornerstone of perioperative pain management in orthopedic trauma surgery, although concerns persist regarding the potential impact of these drugs on fracture healing. Furthermore, NSAIDs may also exert an influence on host immune defenses, which may also be important in the context of infection treatment. However, this has been very much under-investigated in the clinical and scientific literature. The aim of this study was to determine the impact of NSAIDs on the course of an orthopedic device-related infection (ODRI) and its response to antibiotic therapy in a rat model.

Three Cannulated Screws (3CS), Dynamic Hip Screw (DHS) with antirotation screw (DHS–Screw) or with a Blade (DHS–Blade) are the gold standards for fixation of unstable femoral neck fractures. Compared to 3CS, both DHS systems require larger skin incision with more extensive soft tissue dissection while providing the benefit of superior stability. The newly designed Femoral Neck System (FNS) for dynamic fixation combines the advantages of angular stability with a less invasive surgical technique. The aim of this study is to evaluate the biomechanical performance of FNS in comparison to established methods for fixation of the femoral neck in a human cadaveric model.

Twenty pairs of fresh–frozen human cadaveric femora were instrumented with either DHS–Screw, DHS–Blade, 3CS or FNS. A reduced unstable femoral neck fracture 70° Pauwels III, AO/OTA31–B2.3 was simulated with 30° distal and 15° posterior wedges. Cyclic axial loading was applied in 16° adduction, starting at 500N and with progressive peak force increase of 0.1N/cycle until construct failure. Relative interfragmentary movements were evaluated with motion tracking.

Highest axial stiffness was observed for FNS (748.9 ± 66.8 N/mm), followed by DHS–Screw (688.8 ± 44.2 N/mm), DHS–Blade (629.1 ± 31.4 N/mm) and 3CS (584.1 ± 47.2 N/mm) with no statistical significances between the implant constructs. Cycles until 15 mm leg shortening were comparable for DHS–Screw (20542 ± 2488), DHS–Blade (19161 ± 1264) and FNS (17372 ± 947), and significantly higher than 3CS (7293 ± 850), p<0.001. Similarly, cycles until 15 mm femoral neck shortening were comparable between DHS–Screw (20846 ± 2446), DHS–Blade (18974 ± 1344) and FNS (18171 ± 818), and significantly higher than 3CS (8039 ± 838), p<0.001.

From a biomechanical point of view, the Femoral Neck System is a valid alternative to treat unstable femoral neck fractures, representing the advantages of a minimal invasive angle–stable implant for dynamic fixation with comparable stability to the two DHS systems with blade or screw, and superior to Three Cannulated Screws.

This longitudinal microCT study revealed the osteolytic response to a Staphylococcus epidermidis-infected implant in vivoand also demonstrates how antibiotics and/or a low bone mass state influence the morphological changes in bone and the course of the infection.

Colonisation of orthopaedic implants with Staphylococcus aureusor S. epidermidisis a major clinical concern, since infection-induced osteolysis can drastically impair implant fixation or integration within bone. High fracture incidence in post-menopausal osteoporosis patients means that this patient group are at risk of implant infection. The low bone mass in these patients may exacerbate infection-induced osteolysis, or alter antibiotic efficacy. Therefore, the aims of this study were to examine the bone changes resulting from a S. epidermidisimplant infection in vivousing microCT imaging, and to determine if a low bone mass stateinfluences the course of the infection and the efficacy of antibiotic therapy. An in vivomodel system using microCT scanning [1], involving the implantation of either a sterile or a S. epidermidis-colonised PEEK screw into the proximal tibia of 24 week-old female Wistar rats, was used to investigate the morphological changes in bone following infection over a 28 day period. In addition, the efficacy of a combination antibiotic therapy (rifampin and cefazolin: administered twice daily from days 7–21 post-screw implantation) for affecting osteolysis was also assessed. A subgroup of animals was subjected to ovariectomy (OVX) at 12 weeks of age, allowing for a 12 week period for bone loss prior to screw implantation at 24 weeks. Bone resorption and formation rates, bone-implant contact and peri-implant bone volume in the proximity of the screw were assessed by microCT scanning at days 0, 3, 6, 9, 14, 20 and 28 days post-surgery. Following euthanasia at day 28, the implanted screw, bone and soft tissues were subjected to quantitative bacteriology as a measure of the efficacy of the antibiotic regimen. In non-OVX animals S. epidermidisinfection induced marked osteolysis, which peaked between 9 and 14 days post-screw implantation. Peak bone resorption was detected at day 6, before recovering to baseline levels at day 14. Infection also resulted in extensive deposition of mineralised tissue, initially within the periosteal region (day 9–14), then subsequently in the osteolytic region at day 20–28. Quantitative bacteriology indicated all non-OVX animals remained infected. Rifampin and cefazolin successfully cleared the infection in 5/6 non-OVX animals group although there was no difference observed in CT-derived bone parameters. OVX resulted in extensive loss of trabecular bone but this did not alter the temporal pattern of infection-induced osteolysis, or mineralised tissue deposition, which was similar to that observed in the non-OVX animals. Similarly, there was no difference in bacterial counts between non-OVX and OVX animals (39,005 colony-forming units (CFU) [range: 3,675–156,800] vs 37,665 CFU [range 3,250–84,000], respectively). Interestingly, antibiotic treatment was less effective in the OVX animals (3/5 remained infected), suggesting that antibiotics have reduced efficacy in OVX animals. This study demonstrates S. epidermidis-induced osteolysis displays a similar temporal pattern in both normal and low bone mass states, with comparable bacterial loads present within the localised infection site.

Summary Statement

In vivo microCT allows monitoring of subtle bone structure changes around infected implants in a rat model.