Artificial hip arthroplasty has developed rapidly in recent years, with the primary goal of improving function and mobility and reducing pain. Its most common surgical indications include ischemic necrosis of the femoral head, end-stage hip disease, and developmental or post-traumatic joint deformity.
It generally includes total hip arthroplasty and hemiarthroplasty.
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From left to right are hemi hip replacement, total hip replacement and hip surface replacement
There are two main types of prosthesis fixation: cemented and uncemented.
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From left to right, the acetabular cup is cemented, the femoral stem is cemented, and the neck collar is uncemented.
X-rays of artificial hip arthroplasty
X-rays have good spatial resolution and can show the alignment of the prosthesis, which is important for assessing complications such as prosthesis loosening, displacement, periprosthetic fracture, and heterotopic ossification.
Radiographic positions.
(1) Anterior-posterior pelvic radiograph (including both hips and the upper 1/2 of the femur): the X-ray bulb is aligned with the pubic symphysis, with bilateral symmetry.
(2) Anterior-posterior view of the affected hip: the X-ray tube is aligned with the hip joint and used to analyze the anterior/posterior tilt of the acetabular prosthesis.
(iii) Lateral view of the affected hip: X-ray projected through the femoral head.
Normal femoral prosthesis.
The femoral prosthesis should be neutral or mildly valgus (tip of the prosthesis pointing medially) relative to the femoral stem. The femoral prosthesis can be partitioned on anteroposterior and lateral X-rays (used to characterize prosthetic loosening and other complications).
Normal acetabular prosthesis.
The acetabular prosthesis should be positioned at an angle of 45° (30° to 50°) to the iliac seating line in the orthopantomograph and should have an anterior inclination of 5° to 25° in the lateral view. The acetabulum is divided into three zones on the orthopantomograph.
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Initial assessment.
① Vertical and horizontal position of the acetabular prosthesis
A. Vertical position: the distance from the line of the lower edge of the acetabular teardrop (or the center of the femoral head) to the line of the sciatic tuberosity. It is normally less than 1 cm and is related to the length of the lower extremity; a higher one may lead to shortening of the limb and low utilization of the muscles around the hip joint, which may easily lead to dislocation.
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B. Horizontal position: the distance from the center of the femoral head to the edge of the tear drop shadow. Failure to maintain the horizontal position will cause abnormal position of the iliopsoas tendon, which will easily lead to dislocation.
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②Acetabular abduction angle.
The angle between the surface of the cup and the horizontal pelvic orthostatic measurement: the angle between the line connecting the edge of the cup and the line connecting the sciatic tuberosity. The normal angle is 30°~50°. When the angle is small, the hip joint is stable but the abduction is limited; when the angle is large, the risk of hip dislocation is high.
(iii) Acetabular anteversion angle.
The angle between the cup axis of the socket and the coronal plane. Lateral view of the hip joint: the angle between the line connecting the edge of the cup and the vertical line of the horizontal plane. The normal angle is 5°~20°. Posterior tilt of the acetabulum is prone to hip dislocation.
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④ Position of the femoral prosthesis.
The angle between the long axis of the stem of the prosthesis and the long axis of the femur is determined by measuring the angle between the long axis of the prosthesis and the long axis of the femur on an orthogonal x-ray, with the angle ≤3° being central fixation and beyond being inversion or valgus fixation. Femoral prosthesis internal rotation is easy to cause prosthesis loosening.
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Figure A: Femoral prosthesis body line (green line) is consistent with the femoral stem axis. Figure B: Femoral prosthesis (green line) points outward with respect to the femoral stem axis (red line) and is mildly inwardly rotated.
⑤ Femoral stalk positioning.
Position of the femoral stalk (red line) compared to the longitudinal axis of the femoral neck (green line). Normal 5°~10°.
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(vi) Femoral prosthesis (stem)/cemented medullary cavity match.
Prosthesis-bone interface (non-cemented), bone-cemented and prosthesis-cemented interfaces (cemented) need to be evaluated for each zone.
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Stalk-medullary cavity ratio: the ratio of the width of the prosthesis to the width of the medullary cavity is examined. Orthopantomogram: ratio > 80%, lateral: ratio > 70%, then the match is satisfactory.
Bone cement: acetabular side bone cement is generally 3-4mm thick; femoral side bone cement is generally 4-7mm proximal and 1-3mm distal.
CT examination of artificial hip arthroplasty
CT has high density resolution and can show fine bone structures and early bone changes. It is more sensitive and specific in diagnosing bony complications and is better than X-ray in showing subtle fractures and osteolysis. It can accurately assess the extent of osteolysis, the presence of cystic or solid masses around the prosthesis, joint effusion and the presence of surrounding abscess formation.
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MRI of artificial hip arthroplasty
MRI has the advantages of multiple sequences, multiple orientations, high soft tissue resolution and no ionizing radiation, and can clearly display soft tissue, bone cortex and bone marrow tissue, etc. MRI is becoming the Gold standard for diagnosing soft tissue lesions around prosthesis.
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Nuclear bone imaging for artificial hip arthroplasty
Current research in nuclear Medicine is focused on identifying aseptic prosthesis loosening and infection after arthroplasty.
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Complications after artificial hip arthroplasty
As the number of artificial joint replacement cases increases, the incidence of postoperative complications such as aseptic loosening of the prosthesis, osteolysis, periprosthetic infection, heterotopic ossification, pseudotumor and fracture is gradually increasing.
- Dislocation/subluxation.
Incidence 3%~7%. Immediate postoperative dislocation is more common. It can be dislocated in posterior, anterior and lateral directions.
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- aseptic loosening of the prosthesis.
The incidence of loosening is 6% to 18% for femoral prostheses and 6% to 28% for acetabular prostheses. Loosening is the most common cause of revision.
Imaging features.
① radiolucent lines at the bone-cement interface/metal-cement interface greater than 2 mm; progressive widening or irregularity of the radiolucent lines.
(ii) sinking/displacement of the femoral prosthesis and displacement/inversion of the acetabular prosthesis.
(iii) fracture of the prosthetic stem; fracture of the bone cement.
④Shedding of the coating material on the surface of the prosthesis.
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- Infection.
Incidence 2% to 3%.
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One month after left hip arthroplasty, a cystic bag-like fluid-like density shadow was seen in the operative area, and the adjacent fatty space was blurred; the puncture was an abscess
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A case of infection after hip arthroplasty, PET scan was performed showing increased radial uptake in the left acetabulum and proximal femur.
- Osteolysis.
Incidence 5% to 9%. Unlike translucent lines, the map-like translucent areas are due to tissue inflammatory reactions induced by prosthesis-related bone cement, metal or polyethylene particles (granulomatosis).
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Osteolysis and resorption of bone in the right acetabulum and pubic ramus, with a periarticular mass of soft tissue surrounding the femoral head and femoral neck
- Metal-on-metal (MOM) disease.
Pseudobursa, a tissue reaction to metal-on-metal hip replacements; manifests as a bursa around the hip prosthesis. The most common sites are: the superior acetabular rim, the greater trochanter, and the lesser trochanter.
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Artificial hip arthroplasty. Cystic mass around the prosthesis (arrow)
- periprosthetic fracture.
Incidence 1% to 2%. Can occur in the book or postoperatively. Delayed fractures occur mostly at the tip of the femoral prosthesis. Intraoperative fractures are more likely to occur with non-cemented prosthetic implants and revisions.
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Ag type (involving the greater trochanter)
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Al type (involving small ridge)
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B1 type with firmly fixed stem
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B3 type with loose stem and osteoporosis
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Type C fracture is located distal to the tip of the femoral prosthesis and the prosthesis is stable.
- Prosthetic failure.
Can occur in the acetabulum or femoral prosthesis.
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After total hip ceramic prosthesis replacement: ceramic fracture in the acetabular internal fixation part and ceramic fragment in the femoral trochanter (arrow)
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After total hip replacement on the right side:polyethylene wear, asymmetric position of the femoral head in the acetabular cup (arrow)
- Heterotopic ossification.
Male, over 65 years of age, with a high incidence of heterotopic ossification in patients with spondylolisthesis or DISH.
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Grade I
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Grade IV
Summary
Imaging is important in the evaluation of hip arthroplasty; X-ray is still the preferred method, CT examination can detect the fine structure of the bone, MRI can detect the soft tissue around the joint at an early stage, and nuclear bone imaging can be used as a supplement to the above methods to better identify the infection and loosening around the joint prosthesis.
In conclusion, imaging can better evaluate the complications after prosthesis replacement and guide orthopedic surgeons to develop appropriate treatment to further improve the survival rate of the prosthesis and improve the quality of Life of patients.
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