Widely used “locking” bone fracture plates include standard monoaxial locking plates (MLPs) that allow a single screw insertion angle relative to the plate, as well as newer polyaxial locking plates (PLPs) that permit various screw insertion angles relative to the plate. This is the first review of all studies that compared the mechanical performance of MLPs vs. PLPs. PubMed and Web of Science were searched using the phrases “locked plate polyaxial” and “locked plate variable.” Eligibility criteria were applied:mechanical studies, rather than clinical studies, that directly compared MLPs vs. PLPs; bone fracture studies, rather than spine fusion or joint arthroplasty studies; studies published in English; and studies from any date. The 32 studies used experimental testing or computational modeling to compare MLPs vs. different types of PLPs either as implant-bone constructs or isolated implants. The studies employed different bone sites (humerus, radius, femur, tibia, calcaneus, “generic” long bone, implant only), bone qualities (normal, osteoporotic, unknown), fracture sites (proximal, midshaft, distal), and loading modes (axial, bending, torsional). The studies reported mechanical outcome metrics like stiffness, strength, fragment or fracture motion, bone or plate peak stress, and cyclic fatigue life; but no studies reported screw peak stress or bone “stress shielding” risk. The highest mechanical stability was obtained by different implants for the humerus (PLP with “cap” locking), radius (MLP or PLP with “bushing” or “flange” locking), femur (PLP with “cap” locking), tibia (PLP with “bushing” locking), calcaneus (PLP with “flange” or “score” locking), “generic” long bone (PLP with “cap” locking), and isolated implant without bone (PLP with “cap” locking). General trends in results, tabulated numerical data, practical “key concepts,” clinical aspects, and recommendations for future work were also given in this review. PLPs almost always provided greater mechanical stability than MLPs for various bone fracture applications, but the specific type of PLP depended on the bone site.