CUSTOM AND MODULAR COMPONENTS IN PRIMARY TOTAL HIP-REPLACEMENT

Cementless custom implants attempted to enhance fit and fill of variab le hip geometry. Fabrication of custom implants is referenced from a c omputed tomography scan, thus allowing three dimensional specification s of femoral anatomy. However, the aggregate charge of manufacturing t he implant and obtaining the computed tomography scan is prohibitive i n today's healthcare climate. Clinical studies have not shown that cus tomized implants incrementally improve clinical success or implant lon gevity. Modular prostheses allow the surgeon intraoperative versatilit y, allowing adjustment of leg length, offset, neck length, anteversion , and fixation. This is particularly helpful in developmental dysplasi a of the hip and posttraumatic arthritis. Other advantages of modulari ty include decreased implant inventory and the ability to remove the f emoral head at revision surgery to improve exposure or change head siz e without component removal. Subsequent clinical experience has witnes sed significant drawbacks associated with modularity. These include co rrosion, especially with mixed metals, fretting, dissociation, implant fracture below the head and neck taper joint, and reduced range of mo tion. In addition, thin acetabular polyethylene contributes to higher wear rates, earlier failure, local or distal debris particles, and ost eolysis. Finally, the cost of modular implants is generally higher tha n a comparable monolithic prosthesis. In primary hip arthroplasty, use of custom or modular implants should be judicious. Modularity beyond the head and neck junction should be reserved for those cases where a comparable monolithic implant would not suffice.