The Hip Joint and Total Hip Replacement
This chapter presents a brief on hip joint and total hip replacement (THR). The hip joint is composed of soft and hard tissues. A joint comprises the femoral head, acetabulum, cartilage, and ligaments (Figure 1.1). The hip joint is classified as a ball- and-socket joint (Polkowski & Clohisy, 2010). The ball-and-socket joint provides three rotational movements, namely, flexion-extension, abduction-adduction, and internal-external rotation. The femoral head is connected to the femur via the femoral neck. The cartilage supplies a frictionless joint. The stability of the hip joint is supplied by the ligaments and muscles. This structure provides optimal stability for the stance and bipedal locomotion, but the hip joint endures complex dynamic and static loads (Bowman Jr et al., 2010).
Mechanical injury, chemical process, and/or their combination can cause degeneration and dysfunction in the articular hip joint (Bougherara et al., 2011). The most common causes of hip joint degeneration are osteoarthritis, fracture of the hip, inflammatory arthritis, femoral head necrosis, and rheumatoid arthritis (Figure 1.2).
The final recourse but the most successful procedure to remedy a severely degenerated hip joint is THR (Caeiro et al., 2011). This procedure alleviates the pain and
FIGURE 1.1 The hip joint (Stops et al.. 2011).
FIGURE 1.2 Three typical hip joint diseases: (a) osteoarthritis, (b) necrosis, and (c) neck fracture. (Reproduced with permission from Dunne and Ormsby (2011) and Ilesanmi (2010). Creative Commons Attribution 3.0 License 2012. IntechOpen.)
restores hip joint function. In THR, the natural hip joint is replaced with an artificial hip joint, which consists of the femoral head, acetabular cup (acetabular shell and liner), and femoral prosthesis (stem) (Figure 1.3). The artificial hip joint components are formed in a modular or monoblock structure. A femoral head may also be included in a femoral prosthesis in a monoblock structure.
Implant Fixation Methods
The implants are fixed inside the bone with or without cement (Figure 1.4). Cemented prosthesis fixation secures an orthopedic cement prosthesis within the bone. An orthopedic cement is made of polymethylmethacrylate, which is a self-curing and nonadhesive polymeric material (Pal et al., 2013). Therefore,
FIGURE 1.3 A typical artificial hip prosthesis. (Reproduced with permission from Li et al. (2014), Creative Commons Attribution 3.0 License 2012. IntechOpen.)
FIGURE 1.4 Typical cemented and uncemented fixation. (Reproduced with permission from Izzo (2012), Creative Commons Attribution 4.0 License 2012. IntechOpen.) interlocking the spongy bone-cement and cement-implant features provides fixation (Pal et al., 2013). However, in a cementless prosthesis, fixation is performed by press fitting or screwing the components in the bone. This procedure guarantees primary stability for the in-growth and on-growth of the bone to the implant surfaces, thus providing secondary fixation and long-term durability. Porous and hydroxyapatite (HA) coatings are applied on the surface of a cementless prosthesis to strengthen primary and secondary fixation.
Moreover, a hybrid THR is a process in which cementless and cemented methods are used to fix the artificial hip joint components in THR. Bone quality is the most influential criterion in selecting a fixation procedure. Young and more active patients have better bone quality than old and less active patients. Accordingly, a cementless prosthesis is more appropriate for young patients, whereas a cemented prosthesis is more suitable for older patients. Each implant fixation method has advantages and disadvantages. For example, cement provides instant fixation, but a cementless prosthesis bone must grow to secure the prosthesis in the bone. In addition, a cemented prosthesis requires a bigger hole or more reaming inside the bone than a cementless prosthesis. The revision rate of patients who underwent THR with cemented prosthesis is lower than that of patients with cementless prosthesis.
Total Hip Replacement Failure
Developments in the design, technology, and technical operation increased the success rate of THR. However, THR failure remains a problem, so revision surgery is essential and unpreventable. For example, 10% of all THR surgeries in the USA per year undergo THR revision surgery (Brown & Huo, 2002). Accordingly, the components of the old artificial joint are partially or totally replaced with new components. Mechanical factors are more common causes of THR failure than infection. Aseptic loosening is the most important cause of THR failure (Gross & Abel, 2001). The mechanisms leading to aseptic loosening remain ambiguous. Osteolysis, lack of sufficient primary stability, stress shielding, cement failure, and debonding are some of the main factors that contribute to the development of aseptic loosening and ultimately destruction of THR (Boyle & Kim, 2011; Sivarasu et al., 2011).