Modular Determinism in Shoulder Hemi arthroplasty: BG-Guided Fixed-Version, Proximal Cement-Restricted Short-Stem Design for Complex Proximal Humerus

Background: In fracture hemiarthroplasty, functional failure is frequently driven by kinematic reconstruction errors—particularly malreconstruction of head height and version—which contribute to tuberosity malposition/migration and poor outcomes [2]. Contemporary humeral stems may inadvertently couple diaphyseal fixation with articular reconstruction, increasing intraoperative variability (“system noise”).Methods: We propose an anchor–interface framework that decouples these tasks. The anchor is a proximal cement‑restricted short stem designed to generate a circumferential 1.5–2.0 mm cement mantle and a low‑stress polished distal segment. Fixation is achieved through a controlled fluid–thermal cementation protocol that standardizes cement viscosity and pressurization, informed by evidence that pressurization influences cement penetration and fixation strength [8] and by micromechanical modeling demonstrating that cement–bone interdigitation/contact area governs interface strength [9]. Version control is encoded as fixed retroversion referenced to the bicipital groove (BG) [6,7], allowing the surgeon to set retroversion by an anatomic alignment step during cementation. The interface consists of modular heads that permit independent adjustment of head height and offset using the pectoralis major tendon reference [3], with explicit attention to preserving subacromial clearance (avoid overstuffing) [10]. Integrated suture windows and longitudinal slots enable bridging plus cerclage tuberosity fixation [4].Conclusions: By rendering diaphyseal fixation more deterministic through process control, the proposed system reduces the intraoperative “error budget” to its essential kinematic variables: version, height, and clearance. Preclinical testing and prospective clinical evaluation are required to determine whether reducing variability improves tuberosity healing and mitigates glenoid erosion.