The morphology of the proximal femur differs according to age, race, sex, and lifestyle [2, 3, 29]. Some pathologic factors may also affect the geometry of the femur, such as rheumatoid arthritis, atrophic osteoarthrosis, osteoporosis, and some metabolic bone diseases [7, 30, 31]. Type C bone is found predominantly in women of older age and with lower body weight  and it has both structural and cellular compromise. The cortices are thin with correlated loss of the medial and posterior cortices resulting in a “stovepipe” shape of the intramedullary canal . The significant decrease in cortical indices and the increase in CC ratios reflects these structural changes. These structural changes, in combination with cellular abnormalities, create a less favorable environment for implant fixation. These changes also complicate joint replacement and may negatively affect the immediate fixation and long-term survival of the prosthetic implant [1, 3–6, 28]. Dorr C bone also poses challenges for the insertion of cementless stems because type C bones often presents a straightened femoral canal with a wide isthmus and thin cortex [3, 31]. Most primary conventional cementless stem implants cannot provide a close geometric matching to the stovepipe canals with a wide isthmus [2–6, 13, 32], and the enlarged isthmus can lose its metaphyseal hoop stress for the most primary conventional cementless taper stems . A sufficiently tight fill and fit cannot be achieved using even the largest femoral prosthesis, which can lead to stem slippage or excessive motion that prevents stem ingrowth. Cementless revision femoral prosthesis has been applied to achieve endosteal stem fit and fill the primary THA in Type C femoral bones . However, there are some concerns that more stress shielding may occur  because of the large stem placed in the proximal femur. In addition, more erosion of the femoral inner canal can occur by placing a large length and dimeter stem in an overall weakened bone, which could decrease the reserve of the bone stock.
The Dorr C stovepipe femur can be seen with severe osteoporosis because some conditions can create osteoporotic bones. In practice, patients requiring long-term steroid use from organ transplantation or rheumatological conditions may simultaneously develop osteonecrosis and osteoporosis . Type C femoral bone combined with severe osteoporosis is becoming a common challenge faced by surgeons performing primary hip arthroplasty. Besides the poor bone quality with a more enlarged femoral canal complicating stem matching and fixation, surgery is also associated with a range of adverse outcomes, such as intraoperative fracture, periprosthetic osteolysis with implant migration, and postoperative periprosthetic fracture [33, 34]. Thus, these shape and structural changes result in the preferred use of cemented implants in Dorr C osteoporotic bone [4, 6, 35] because some mismatches can be accommodated using the avoid-filling capacity of the cement layer in a cemented hip system . The only limitation is when the femoral canal becomes so wide that even the largest cemented stems begin to have a very thick cement mantle . Recommended mantles are 1–2 mm, and severely undersized femoral components may show early loosening [36, 37]. Even cementation becomes difficult in these severe Dorr C osteoporotic bones with profoundly wide and thin cortices of femur . Furthermore, fixation loosening of the cemented femoral component has remained the leading revision problem, particularly in young patients . Meanwhile, fat embolism, pulmonary microemboli, and cardiac arrest associated with cementing are still potential risks [40, 41] because a large amount of cement is required to fill the widened canal in the enlarged femoral canal.
To address this difficult issue, a case series was performed with many other choices of hip arthroplasty or other types of femur stems to address Dorr C osteoporotic bone in young adults [33, 34]. Total hip resurfacing arthroplasty is a good alternative to conventional total hip arthroplasty and can provide excellent results in terms of implant survivorship after careful patient selection [42, 43]. However, the poor bone mineral density may contribute to increased fractures of the femoral neck and head or pathological fractures in patients with severe osteoporosis, particularly in patients comorbid with rheumatologic disorders [44, 45]. Osteopenia or weakness of the femoral head may lead to femoral component loosening because it is mechanically unfavorable to have resistance on the femoral head and neck by stresses transmitted through the implant. Meanwhile, hip resurfacing arthroplasty is not indicated in young women who wish to become pregnant in the future as pregnancy-related complications and adverse effects of metal on metal debris on the fetus are unknown .
With current advances in prosthetic designs, there is growing interest in bone-conserving short stems to preserve proximal bone stock and provide physiologic loading to the proximal part of the femur [46, 47]. The short, metaphyseal fitting cementless femoral stem was designed to require less resection of the upper femur and/or less reaming of the femoral shaft . This serves a dual purpose of facilitating future revision while providing a postoperative state closely mimicking the original functioning hip. Preservation of the femoral neck provides greater torsional stability and reduces distal migration of the femoral stem . The absence of any diaphyseal fixation can be used to achieve proximal load transfer to reduce stress shielding and ignore the type of femoral canal. It also preserves the femoral canal and femoral elasticity, allowing for easy revision.
The short bone-conserving cementless stems have been introduced in young patients with type C bone . The short stem is suitable for most femur types because the initial stability of the stem can be achieved by neck filling and metaphyseal fixation, which does not depend on isthmus hoop stress compared to conventional primary femoral prostheses. In fact, most primary conventional implants cannot provide a close geometric matching to the extremely stovepipe canals [2–6, 13, 32]. We used the Type-4 stem  in primary total hip arthroplasty in young adult osteoporotic patients with Type C bones. The Type-4 stem is a shortened conventional design with primary fixation in the proximal femoral metaphysis. As opposed to other types of short bone-conserving stems , Type-4 stems are rarely neck-preserving and often extend to the upper diaphysis. With their tapered-wedge design, they achieve fixation in the proximal femur. These are similar to conventional, proximally porous-coated tapered designs with a shorter length or reduced distal end of the stem [16–19]. The “fit and fill” of the stem can be achieved by impaction of cancellous bone in the metaphyseal region with a larger proximal geometry. In our practice, Type-4 stems are suitable for severe osteoporotic young patients with Type C bones who present with a correspondingly straightened femoral canal profiled with a wide isthmus and thin cortex. These changes increase the difficultly of performing replacement using other types of hip arthroplasty or other conventional femoral stems with a stovepipe canal combined with severe osteoporosis . There are several proposed advantages of this type of short stem, including easier insertion through smaller incisions and less invasive techniques , simpler femoral preparation with a ‘‘broach-only’’ system, and the basic inherent bone conserving nature allowing for more favorable conditions in the potential revision setting.
This study had certain limitations. First, it was retrospective in nature and included a relatively small series of patients. Second, the duration of follow-up was short and insufficient to allow conclusions to be drawn because our prostheses only showed good results at 3 to 8 years (mean, 5.5 ± 1.1 years) after the operation. Third, our migration analyses of the stem did not use more precise methods such as roentgen stereophotogrammetry (rSA) or Ein-Bild-roentgen-femoral component analyses . Meanwhile, we did not use dual-energy X-ray absorptiometry to study bone mineral density changes around the femoral stem, which is considered the most reliable tool for evaluating bone remodeling after THA using different stem designs and is more sensitive and precise than conventional X-ray. Fourth, bone remodeling around the short stem was not observed comprehensively in long-term follow-up. Finally, we did not perform interobserver variability studies of the radiographic results to confirm the measurements by the single observer, which may lead to bias in interpreting the radiographs, leading to underestimation or overestimation.