Influence of a Modified Procedure of Joining Ceramic Head and Adapter Sleeve on the Stem Taper in Revision: An Experimental Study.

In revision operations, ceramic heads of modular hip implants can be replaced. As the surface of the stem taper can be damaged, additional adapter sleeves are applied. The components are usually connected manually by the surgeon in a one-step procedure by hammer impacts. In this study, we investigated a two-step joining procedure with reproducible impaction force. First, the adapter sleeve and head were joined quasi-statically with a force of 2 kN using an assembly device. In the second step, these components were applied to the stem taper using a pulse-controlled instrument. For reference, the joints were assembled according to standard conditions using a tensile testing machine. An average pull-off force of 1309 ± 201 N was achieved for the components joined by the instrument, and the average measured values for the components joined by the testing machine were 1290 ± 140 N. All specimens achieved a force >350 N when released and therefore met the acceptance criterion defined for this study. This study showed that a modified procedure in two steps with a defined force has a positive effect on the reproducibility of the measured joining forces compared to previous studies.

Evaluating the Feasibility and Reproducibility of a Novel Insertion Method for Modular Acetabular Ceramic Liners.

Modern hip implants have a modular design. In case of wear or other damage it allows surgeons to change the tribological partners, i.e., the acetabular liner and femoral ball. In both revision and primary surgery, the secure joining of the implant components is important for the success of the operation. The two components, namely the ceramic liner and hip cup, are connected via a conical press connection and should be concentrically aligned to avoid chipping. Malseated liners can reduce the life span in situ. The amount of the joining force, which is usually applied via a hammer, depends on the surgeon. In this study, an alternative joining method for acetabular ceramic liners in hip cups was investigated, which intends to make the process more reproducible and thus safer. For this purpose, a handpiece was used to apply a defined force impulse of 4 kN. For the concentric alignment of a ceramic liner in the hip cup, an adapter was developed based on findings via a qualitative finite element (FE) analysis. Insertion and pushout tests of the acetabular cup-liner connection were performed in the laboratory with the new instrument (handpiece with the connected adapter) to evaluate the functionality of the instrument and the reproducibility of the new insertion method. For comparison, liners and acetabular cups were joined using a testing machine according to the standard. The presented results demonstrate the technical proof-of-concept of the new joining method under laboratory conditions. They meet the acceptance criteria of established manufacturers, which proves the equivalency to proven methods for joining modular implant components. To verify the improvement of the new joining method compared to the conventionally used joining method, an application-oriented study with different surgeons and the new joining instrument under clinical conditions is necessary.

Experimental validation of adaptive pedicle screws-a novel implant concept using shape memory alloys.

Pedicle screw fixation is a standard procedure in spine surgery. A secure anchoring in deficient (e.g., osteoporotic) bone or in a secondary supply after a revision is a major challenge. This study aimed to test the primary stability of a new self-expanding shape memory actuator screw concept with the traditional screw design. The actuator part braces itself against the osseous environment after implantation by heating to body temperature. Thirty screws and twenty-four actuator sheets were manufactured and tested in vertebrae from seven human cadavers in vitro. The pullout force and mechanical work for the pullout were evaluated. Bone quality was determined from µCT. The mean actuator sheet compression force of the used shape memory alloy (SMA) sheets averaged - 8.2 ± 0.6 N. Seven of the SMA screws activated correctly after the implantation, four activated unilaterally only. The pullout force averaged 868 ± 392 N for the standard screws and 828 ± 353 N for the SMA screws. The mechanical work was conducted after the first 100 N, and the loss of strain resistance of 40% of the pullout force averaged 2.2 ± 1.6 Nm for the standard screws, and 1.7 ± 0.9 Nm for the SMA screws. Consequently, the novel concept showed non-inferiority compared with the traditional screw designs. Graphical abstract.

Novel concept of a modular hip implant could contribute to less implant failure in THA: a hypothesis.

BACKGROUND: The modularity in total hip arthroplasty (THA) allows orthopaedic surgeons for an exact reconstruction of hip biomechanical parameters especially in revision and tumor arthroplasty. Modular structured femoral stems using taper junctions showed increased implant breakage in the recent past. PRESENTATION OF THE HYPOTHESIS: We hypothesize that a novel modular stem-neck-interface leads to less implant breakage compared to conventional femoral stems. TESTING OF THE HYPOTHESIS: For this purpose, a novel modular femoral stem for THA was to design and manufacture. Therefore, three different variants of interface mechanisms were developed that enable a simple connection between the stem and the neck modules and allow for intra-operatively adjustment. Three prototypes A, B and C were manufactured and subsequently dynamic fatigue (ISO 7206-6) and body donor tested. IMPLICATION OF THE HYPOTHESIS: Modularity in THA is mainly applied in THA as well as in revision and tumor arthroplasty. Modular implants are barely used because of the high risk of breakage. Another risks in this context are taper fretting, corrosion and disconnection. With the novel design, it should be possible to detach the stem and neck module intra-operatively to adapt the anatomical situation. The novel coupling mechanism of the rotating interface seems to be the most suitable for a secure stem-neck connection and is characterized by good intraoperative handling.

Biocompatibility and Inflammatory Potential of Titanium Alloys Cultivated with Human Osteoblasts, Fibroblasts and Macrophages.

The biomaterials used to maintain or replace functions in the human body consist mainly of metals, ceramics or polymers. In orthopedic surgery, metallic materials, especially titanium and its alloys, are the most common, due to their excellent mechanical properties, corrosion resistance, and biocompatibility. Aside from the established Ti6Al4V alloy, shape memory materials such as nickel-titanium (NiTi) have risen in importance, but are also discussed because of the adverse effects of nickel ions. These might be reduced by specific surface modifications. In the present in vitro study, the osteoblastic cell line MG-63 as well as primary human osteoblasts, fibroblasts, and macrophages were cultured on titanium alloys (forged Ti6Al4V, additive manufactured Ti6Al4V, NiTi, and Diamond-Like-Carbon (DLC)-coated NiTi) to verify their specific biocompatibility and inflammatory potential. Additive manufactured Ti6Al4V and NiTi revealed the highest levels of metabolic cell activity. DLC-coated NiTi appeared as a suitable surface for cell growth, showing the highest collagen production. None of the implant materials caused a strong inflammatory response. In general, no distinct cell-specific response could be observed for the materials and surface coating used. In summary, all tested titanium alloys seem to be biologically appropriate for application in orthopedic surgery.

Concept of patient-specific shape memory implants for the treatment of orbital floor fractures.

PURPOSE: We will aim to develop implants made of a Ni-Ti shape memory alloy which can be applied for the treatment of midface fractures, such as isolated orbital floor fractures. These can then be implanted in a compressed form and unfold automatically in the body. With the help of newly developed application instruments, the implants can be applied along transnasal and transantral approaches into the maxillary sinus. Our objective is to evaluate the operation process and the functionality of these implants, already in a pre-investigation by an experienced surgeon on a phantom. METHODS: The functionality of the surgical procedure and an implant prototype were both evaluated with the help of a realistic phantom. The minimally invasive application was carried out using the transnasal and transantral approach. Instruments and implant were rated individually on a scale, from -2 (not at all) to +2 (very good) for vaious criteria, such as the implants functionality or the ergonomics of the entire procedure. For a geometric comparison between the manufactured implant and the planned target geometry, the implants were scanned by micro-computed tomography. CAD models were derived from the scans by using reverse engineering. RESULTS: Both the implants and the application procedure were assessed as good; thus, the implant concept is suitable for further development. CONCLUSIONS: Implants made of shape memory alloys could allow in the future and allow less invasive access to treat orbital floor fractures. The implant design has to be modified that the implant can be stabilized and fixed with screws or a suture to avoid dislocation or implant loosening. The complication rates and risks of conventional orbital reconstructions should be lowered by this new method.