Total artificial hip arthroplasty (THA) is the most effective surgical treatment for hip diseases such as hip pain and dysfunction caused by advanced femoral head necrosis, advanced hip osteoarthritis, acetabular dysplasia and osteoarthritis, and hip ankylosis caused by ankylosing spondylitis. With the development of material science, changes in the design of artificial prosthesis and improvement of surgical techniques, the wear resistance of artificial hip joint, the firmness of prosthesis fixation and the speed of postoperative recovery have been greatly improved. The diversification of artificial hip prosthesis has created conditions for individualization of surgery, but also brought the problem of how to better select the prosthesis. Yan Xinfeng, Joint Specialized Center, Shandong Qianfo Mountain Hospital
The artificial total hip joint is generally composed of four parts: acetabular cup, liner, femoral head and femoral stem. The choice of the prosthesis fixation method is a matter of how the acetabular cup and the femoral stem are firmly fixed to the bone. The liner is fixed in the acetabular cup and the femoral head is fixed in the femoral stem, and the joint is formed by the movement between the two, so there is wear and tear.
1. Choice of prosthetic fixation method
1.1 Bone cement fixation vs. non-cemented fixation (biologic fixation)
Bone cement fixation is the filling of bone cement between the prosthesis and the bone bed at the time of installation, so that the bone cement is combined with the bone surface and forms a cement shell, and the prosthesis is fixed in the cement shell. Non-cemented biological fixation is a direct contact between the prosthesis and the bone bed, and fixation is achieved through a different way of bonding.
The cemented versus uncemented fixation debate has continued throughout the development of the artificial joint, but there is no denying that the use of bone cement and the use of metal-to-polyethylene interfaces were two of Dr. Charnley’s landmark contributions to artificial hip replacement. However, in the early years of cement fixation, loosening and osteolysis of the artificial joint often occurred, and the cement was considered the culprit and was called “cementosis”. Later studies proved that polyethylene wear particles played a greater role in osteolysis, while the cement technique has evolved from the first generation to the current third generation, i.e., placement of a medullary plug in the distal femoral cavity, adequate flushing of the bone bed, vacuum mixing of the cement, pressurized irrigation with a cement gun, and intermediate placement of the prosthesis, etc. Bone cement fixation provides immediate prosthetic stability.
Distal prosthetic loosening and osteolysis are also present with biologic fixation, again primarily caused by wear particles. The initial stability of a biologically fixed prosthesis is provided by the close press fit of the prosthesis to the bone bed, and after 1 to 3 months the prosthesis and bone surfaces are bonded together in different ways to provide long-term stability.
It is now generally accepted that young people choose biofixed prostheses, middle-aged and elderly people can use biofixed or cemented prostheses, and patients with severe osteoporosis choose cemented prostheses. Because of the greater amount of osteolysis during loosening of the cemented prosthesis, the biologically fixed prosthesis is more advantageous when considering patients who are likely to require revision.
When choosing a cemented fixed prosthesis use third generation bone cement technology to ensure that the cement is firmly bonded to the bone surface and forms an intact cement shell 2 to 3 mm thick; too thin and too thick bone cement can cause fracture of the bone cement, which can lead to loosening.
When choosing a biologically fixed prosthesis, try to choose a prosthesis with good bone growth, and do a tight press fit during installation to ensure firm initial stability and create conditions for the union of bone and prosthesis.
1.2 Surface microporosity vs. surface coating
Biofixed prostheses undergo two phases after implantation: the initial fixation phase and the secondary fixation phase. The initial fixation is mechanical and relies on the close press fit of the prosthesis to the bone bed; the secondary fixation phase is based on the close fit of the prosthesis to the medullary cavity, and the bone is tightly bonded to the prosthesis by the biologically inherent properties of its trabecular growth and ossification, and the secondary fixation provides long-term stability. The surface treatment of the prosthesis determines how the prosthesis is bonded to the bone and thus the degree of secondary fixation.
The surface treatment of the prosthesis can be roughly divided into two types of surface microporous and surface coating, pearl surface treatment of the macroscopic interlocking prosthesis has been largely eliminated.
Surface microporosity is obtained by titanium bead sintering, titanium wire sintering, tantalum metal bone trabeculae, etc. They form 0.2 ~ 0.5 mm mesh-like voids on the metal surface, which can make the bone trabeculae grow into them so that the prosthesis and bone firmly combined, called “bone long into”.
Surface coating mainly includes titanium spray rough surface, hydroxyapatite (HA) coating, titanium spray + HA double coating, etc.. The HA-coated prosthesis can bond with bone through both chemical bonding and biological bonding, and can also provide good secondary stability; the rough surface prosthesis cannot provide bone ingrowth, but only the “bone ingrowth” phenomenon, and the bonding area is significantly less, so the secondary fixation effect is poor.
In patients with acetabular dysplasia, it is important to choose a prosthesis with good bone integration for the acetabular cup.
1.3 Proximal fixation vs. distal fixation
Biologically fixed femoral stem prostheses differ between proximal fixation and distal fixation. Proximally fixed prostheses only have a microporous surface for approximately 1/3 of the proximal length, whereas prostheses with microporous surfaces for the entire length are distally fixed prostheses. In general, postoperative thigh pain is less common with proximal fixation prostheses because they avoid stress masking, so if the proximal femur does not have a bone defect and can provide good initial fixation of the prosthesis, choose a proximal fixation prosthesis if possible.
When the proximal femur cannot provide good initial fixation of the prosthesis, e.g., femoral head necrosis or rheumatoid arthritis combined with intertrochanteric fracture, then a distal fixation prosthesis should be selected. High hip dislocations requiring femoral osteotomy shortening also require distal fixation of the femoral stalk. In the case of severe osteoporotic intertrochanteric fractures, femoral head replacement is gradually accepted in order to resume activities as soon as possible, and distal fixation of the femoral stem can also be obtained with the use of an extended cement-based prosthesis.
2. Selection of friction interface
2.1 Wear and tear of different femoral heads and liners
The femoral head of the total hip joint is mainly made of metal (M) and ceramic (C), and the lining material is mainly made of polymer polyethylene (PE), high cross-linked polyethylene (XPE), ceramic (C), and metal (M), except for the metal femoral head which cannot be matched with ceramic lining, there are 7 types of matching.
If the wear of CoC (i.e. ceramic femoral head to ceramic liner) is 1, the wear of CoXPE and MoM is 10 times, MoXPE is 20 times, and MoPE is 200 times. Moreover, the wear particles of PE and XPE cause the strongest reaction to the surrounding tissues and tend to cause osteolysis, while the wear particles of M are the second most reactive and the tissue reaction of C wear particles is the weakest. Therefore, the CoC interface should be the best choice, but there is a fragmentation rate of 1 to 4/10,000, MoM does not fragment, and the same diameter cup can be used with a larger femoral head than other combinations, so it was once considered the first choice for young patients, but in recent years, clinical applications have found that MoM combinations have a metal particle allergic reaction rate of about 9/1000, and patients with renal insufficiency, pregnant women or women with fertility requirements should be avoided.
2.2 Large diameter femoral head vs. small diameter femoral head
The larger the diameter of the femoral head, the lower the dislocation rate and the greater the range of motion of the joint, which can better meet the requirements of young people. In terms of joint wear, as the diameter of the femoral head increases, the linear wear rate of the joint decreases slightly due to the decrease in pressure, while the volumetric wear rate gradually increases. The two offset, the large-diameter femoral head still has a clear advantage over the more active young people, especially the large-diameter ceramic-to-ceramic prosthesis.
3. Special hip prostheses
The mating femoral stem prosthesis is an excellent tool for hip replacement in acetabular dysplasia. The grouped femoral prosthesis breaks down the original femoral stem into two or three components, allowing adjustment of the anterior femoral tilt angle during application and, if a short femoral osteotomy is required, the distal end of the grouped prosthesis has a raised ridge that effectively prevents rotation of the osteotomy. Although it is possible to correct the anteversion angle by femoral osteotomy with a distal fixed prosthesis, the group-mounted prosthesis not only corrects the anteversion angle without osteotomy but is a proximal fixed prosthesis and theoretically has a lower incidence of postoperative thigh pain.