Internal-locking intramedullary nails (talons, claws, wings, wedge-locks, endopins): a mechanics-guided scoping review of designs, failure signatures, and technique imperatives.

background"Internal-locking" concepts-deployable talons/claws and tanged anchors, winged/wedge-wing head elements, blade-expandable and wedge-locked distal modules, inside-to-out internal distal locking pins (endopins), and shape-memory (nitinol) wings-aim to streamline intramedullary (IM) fixation by reducing reliance on freehand distal interlocking screws while maintaining stability.

methodsA scoping, mechanics-informed review was conducted using PubMed/MEDLINE, Scopus, Web of Science, and Google Scholar (January 1980-October 2025; last search October 25, 2025). Studies evaluating intramedullary systems with internal-locking mechanisms for the humeral shaft/diaphysis, hip (intertrochanteric, basicervical, or subtrochanteric), femoral shaft/distal femur, tibia, fibula, clavicle, and selected pelvic/foot indications were eligible. Bench/FEA, cadaveric, clinical (comparative and single-arm), technique, and health-economic reports were included. For human clinical studies, English-language full texts were eligible; during revision, non-English full texts with an English abstract and extractable outcome data were re-evaluated. Screening was performed by a single reviewer, and data were charted by anatomic pattern, key technique covariates (reduction quality, central-inferior head position, tip-apex distance [TAD]), outcomes, and complications. Sixty-three sources of evidence met eligibility criteria and were charted; details are compiled in Supplementary Table S1 (Master Evidence Table). The remaining 10 references are background/methodological or definitional citations-for example, foundational work informing technique covariates such as tip-apex distance (TAD) and calcar-referenced metrics (e.g., Calcar-TAD)-and therefore are not listed or appraised in the supplementary evidence tables.

resultsFor the humeral shaft, internal distal-locking strategies reduce the distal-targeting burden of antegrade nailing and may shorten surgery while avoiding an additional distal incision, but outcomes remain strongly technique-dependent. Historical Seidel data and more recent humeral internal-locking series suggest that the main risks are rotational instability, technical locking failure, proximal hardware-related symptoms, and shoulder morbidity rather than the distal mechanism alone. Bench and cadaveric studies indicate that inferior subchondral deployment of talons increases torsional resistance (~ 2-3×) and interfragmentary compression (~ 2×) versus retracted configurations, providing a plausible basis for reduced lag-element sliding when TAD is 15-25 mm and the head element is central-inferior. Finite-element models suggest that wedge-locked constructs can approximate screw-locked axial/torsional stiffness when deployment torque and friction are adequate, albeit with stress concentrations near the locking windows. Clinically, in geriatric intertrochanteric fractures, union and functional outcomes are generally similar among talon-equipped, helical-blade, and dual-screw cephalomedullary nails when reduction/TAD are optimized; internal-locking designs are often associated with shorter operative and fluoroscopy times, and distal talon locking has been reported to be associated with lower cutout in unstable patterns. In femoral/tibial shafts, internal distal locking shortens procedures and radiation without consistent union penalties, although simple diaphyseal patterns distal to the isthmus show a greater risk of axial shortening/malrotation without adjuncts. In the fibula and clavicle, modern intramedullary systems achieve high union rates (~ 98-100%) with low wound morbidity; in benchtop/synthetic models, talon screws have reproduced plating-like axial stiffness while remaining torsionally inferior. Early health-economic signals suggest the potential cost-effectiveness of talon-locking constructs.

conclusionsAcross indications, operative technique remains the dominant determinant of outcome; thoughtfully selected internal locking mechanisms can deliver efficiency and soft tissue advantages without compromising stability when fundamentals (reduction quality, central-inferior head position, TAD 15-25 mm) are respected. A practical taxonomy and decision framework are proposed, along with priorities for pattern-stratified comparative trials and engineering refinements.