Press-fit fixation is a technique which uses physical force on the substance to be inserted so that maximal press fit can be achieved by surrounding material. For this application, the size of press-fit fixation acetabular cups are generally 1–4 mm bigger than reaming diameter so that viscoelasticity of acetabulum maximizes the binding force of acetabular cups3).

Theoretically, the press-fit method does not require additional fixation such as the use of screws since sufficient primary fixation can be achieved. However, additional screw fixation might be used in the certain cases if press-fit fixation is not satisfactory. Further, inexperienced orthopedic surgeons may judge the gap between acetabular bone and cup through screw holes thus it is recommended to select a cup design with screw holes.

Cementless acetabular cups have evolved through three generations based on their development periods and design characteristics4).

Generally speaking, femoral stems are designed to place the center of rotation at the native hip center. In order for this, vertical height, medial offset, and version of femoral region should be appropriate. The vertical height is often expressed as the height from the proximal margin of lesser trochanter to the center of femoral head. Medial offset is expressed as the distance between a line passing through the vertical axis of femoral stem and the center of femoral head. Last, the version indicates the rotation angle between proximal and distal parts of femurs. Depending on the length of implant neck, the medial offset and vertical height can be adjusted; similarly, the vertical height can be adjusted according to depths of femoral stem into femurs.

In addition, femoral stems represent approximately 125° – 135° of neck-shaft angle which also causes a difference in vertical height and medial offset. Moreover, depending upon their design, femoral stems may have different femoral stem lengths, diameter and width of proximal and distal parts, and the angle narrowing down as approaching to the proximal part (Fig. 4). The measuring systsm of the factors could vary according to respective manufacturer, thus one should not make mistakes by comparing stems as solely relying on numerical values on product brochure.

Depending on the shape of the cementless stem, the contact site between the implant and femoral cortical bones might differ and can result in differences in primary stability and following biologic fixation. All cementless stem designs aim to achieve best fixation by maximizing primary fixation forces and secondary biological fixation.

When they initially came to market, femoral stems were classified only as either straight or curved; similarly they were divided as either fixing proximally on metaphysis or distally on diaphysis. As additional types of femoral stems were developed and introduced, they were classified and named according to different perspective and classification criteria. In the absence of unified classification systems, it was difficult to make direct comparison and scientific analyses among femoral stems. For instance, Callaghan et al.5) classified femoral stems into 7 different types; modular, extensively coated, hydroxy apatite (HA) coated, proximally ingrown, tapered, press-fit, and custom design. The modular was named per its modularity and extensively coated per the range of surface coating, the HA coating per the surface treatment method, proximally ingrown per surface treatment range and method, and tapered per stem shape, the press-fit per primary fixation method, custom design per manufacture type, respectively.

Similarly there are other classification methods in which femoral stems were divided into 5 categories; fit and fill, modular, distal fitting, proximal tapered wedge, and press-fit tapered wedge. In this classification, the fit and fill was defined by primary fixation method, the modular by modularity, the distal fitting by fixation site, the proximal tapered wedge by the range of surface treatment and shape, the press-fit tapered wedge by fixation method and shape, respectively. As demonstrated here, in order to minimize any potential confusion from classification and nomenclature by different perspective and criterion, further objective and homogenous criteria might be warranted.

In 2011, the Mont group suggested a classification system for femoral stems with a total of 6 types defined based on the bone contact area and subdivisions of fixation sites (proximal to distal)1). This classification may not be without its limitations but it does provide a general criteria for femoral stem classification thereby making a direct comparison easier.

As the design with metaphyseal fixation are recently preferred, a question regarding the need to maintain a long distal part of femoral stems has led to the development of short femoral stem designs. Expected theoretical merits of this short femoral stems are as follows; 1) they are less invasive due to less loss of femoral bone stock, 2) patients experience less pain without stem parts stimulating diaphysis, 3) there is less stress shielding on proximal femurs. Although it has not been fully established with regards to definition of short femoral stems, Feyen and Shimmin23) previously defined it as stems with "total length less than twice tip of greater trochanter to base of lesser trochanter vertical distance".

Short femoral stems are not included in aforementioned classification of the Mont group, and their classification is not completely established yet. In 2014, the Mont group also proposed classifications for short femoral stems based on the loading sites on proximal part of femurs and stem fixation principles23). In this system, the short femoral stems are classified into 4 types depending on the increasing area for loading on the stem. Type 1 are femoral neck only, type 2 are calcar loading, type 3 are calcar loading with lateral flare, and type 4 have shortened tapered conventional stems. Other than this classification, Falez et al.24), classified the short femoral stems into collum, partial collum, trochanter sparing, and trochanter harming based on primary fixation principles, osteotomy level, and bone loss of femoral neck and greater trochanter.

The femoral neck only or collum-type stems showed excellent clinical outcomes in several studies but they are not fully investigated242526) (Fig. 13). Relatively longer types such as shortened tapered conventional stems or trochanter-sparing, trochanter-harming types show approximately 98% survival rates over more than 10-year follow ups, making these stem types as the most widely utilized femoral stem designs recently242627) (Fig. 14). Nonetheless, further long-term follow ups are required since the most studies regarding short femoral stem types are relatively short.

Recent trends in femoral stem designs for primary hip arthroplasty in patients without serious anatomical deformities are described as follows.

Recently, the most preferred stem design is proximally coated single wedge stems with wide yet thin proximal portions with no collars. The distal part of the design was shortened by approximately 4–5 cm, compared to conventional stems. The shoulder of the lateral part of a proximal stem is inclined to make a slope to encourage bone preservation which lowers the risk of fracture while stems are inserted. The neck is designed to minimize collision between liners and acetabular cups by making it slightly thinner. The femoral head is comparable for both metal and ceramic materials. Previously, a few attempts were made to achieve better elasticity of stems; 1) distal parts of stems were grooved, 2) thin plates or small bumps were extruded to reduce diameter of stem body, and 3) vertical flutes were made. Recently, for similar reasons, it was attempted to reduce elasticity by making long grooves on both the anterior and posterior sides of distal parts of stems, parallel to the vertical axis. Furthermore, the lateral part of stem tips was partially removed to reduce contacts with lateral cortical bone (Fig. 14). These attempts are all aiming to minimize thigh pain but clear scientific evidence has not been provided fully yet (Fig. 15).