Total hip arthroplasty
Total hip arthroplasty (THA) or total hip replacement (THR) is an orthopedic procedure that involves the surgical excision of the femoral head and cartilage of the acetabulum and replacement of the joint with articulating femoral and acetabular components. It is a commonly performed procedure usually with very good results and return to function.
- osteoarthritis (most common indication)
- can be considered in a case by case basis in neck of femur fracture (displaced intracapsular) in active and healthy patients
- osteonecrosis/avascular necrosis
- idiopathic or trauma-related
- developmental dysplasia of the hip
Most common surgical approach for total hip arthroplasty is a posterior approach to the hip. Skin incision is made 10-15 cm centered on the posterior aspect of the greater trochanter. Dissection includes splitting fascia lata and gluteus maximus in line with its fibers. This will uncover the short external rotators, which are dissected off the femur and retracted back over the sciatic nerve to protect the nerve throughout the operation. A capsulotomy is then performed and the hip dislocated.
Bearing surfaces are the surfaces which articulate in the prosthetic joint. The femoral head and the acetabular liner can be used in different combinations. These will give different appearance on radiograph depending on the configuration. Options for bearing surfaces include:
- metal-on-metal (metal head and a metal acetabular component)
- metal-on-polyethylene (metal head with a polyethylene acetabular liner)
- ceramic-on-ceramic (ceramic head with a ceramic acetabular liner)
Femoral component or stem: this refers to the prosthesis which is implanted into the femur. They can be described by length, taper, and presence of a collar. Attached to the femoral component is the neck and head which in most prostheses can be altered in size to create a stable joint.
Acetabular component: main variations in acetabular component on radiograph are related to fixation method and position.
Stability of the prosthetic joint is the primary goal in choosing the position of implanted prosthesis. Multiple patient and surgical factors can alter the ideal position of the components.
The overall position of the prosthetic hip replacement should ideally be in a mild degree of valgus, but should not exceed 140°. Excessive valgus creates strain on the knee, can lengthen the leg, and can lead to superior dislocation with adduction of the leg. A hip positioned in varus can cause increased loosening and stem failure and can also lead to dislocation.
The acetabular component should be positioned at 20° of anteversion with an inclination of 45°. Femoral component position should be in 10-15° of anteversion. Excessive anteversion or retroversion can lead to dislocation.
Femoral stem fixation can be either cemented or non-cemented (biological) fixation. There is a tendency to use non-cemented femoral stems in younger patients, due to higher reported rates of loosening of cemented stems in long term followup. Most common fixation for the acetabular component is non-cemented.
Biologic fixation uses either porous coated metallic surface to stimulate bone in growth or grit-blasted surface to allow bone on growth. The prosthesis can also be coated in hydroxyapatite, which is an osteoconductive agent.
Grading of cement technique on radiograph for femoral stem
- A: medullary canal completely filled with cement
- B: a slight radiolucency exists at the bone cement interface
- C: radiolucency >50% of bone cement interface
- D: radiolucency involving 100% of the interface between bone and cement in any projection, including absence of cement distal to the stem tip
Gruen zones are around the femoral stem from zone 1-7 from the proximal lateral aspect counterclockwise with zone 4 distal to the tip.
Charnley zones are around the acetabular component. There are 3 zones indicating the superior, middle, and inferior third of the acetabulum.
Primary vs revision
When the prosthesis fails, a revision THA is performed which involves removing the primary components. This may involve osteotomy of the femur, which requires internal fixation. There are many options for revision prosthesis including longer femoral stem.
- See separate article: complications of total hip arthroplasty