In modern cementless total hip arthroplasty (THA), securing reliable, rigid mechanical stability for the acetabular cup demands a precise interference fit (or press-fit) technique against host bone. To enhance long-term biologic fixation and expand implant longevity, contemporary hardware designs have rapidly transitioned toward ultraporous metal surfaces manufactured from three-dimensional structural titanium or tantalum. Biomechanically, these highly porous interfaces boast an exceptionally high coefficient of friction against cancellous structures.
However, this enhanced structural friction introduces an unintended surgical paradox: the high resistance encountered during cup impaction can severely impede smooth seating into the hemispherical socket, depending on the manufacturer’s reamer tolerances. This incomplete seating routinely leaves a focal air space or polar gap at the absolute apex of the cup, designated as zone 2 under the classic DeLee and Charnley radiographic mapping matrix.
