|ESSR 2019 / P-0166|
|Radiographic evaluation of hip arthroplasty|
Imaging findings OR Procedure Details
Despite the widespread use of MRI, CT and sonography in joint imaging, the radiography remains the primary imaging method for the evaluation of hip arthroplasty and its complications.
Radiographic view (Fig. 4)
The initial postoperative protocol is an antero-posterior (AP) pelvic radiograph. The AP view of the pelvis is taken with the patient supine, hips in extension and 15° internal rotation. The centre of the x-ray beam should be focused on the pubic symphysis to ensure the inclusion of the entire hip prothesis.
Assessment of the initial postoperative radiograph is an important part of hip replacement surgery. The initial radiograph provides information on the type of prosthesis, the component positioning and fixation and on early complications.
Specific anatomical landmarks and measurements are used to verify correct placement.
The evaluation of a hip arthroplasty should include the following parameters:
1. Leg length (Fig. 5)
Leg length inequality is very common after hip arthroplasty. A discrepancy of up to 1 cm is well tolerated. Moderate inequalities are usually corrected with a shoe orthosis.
The leg length is measured by drawing a line transversely connecting the inferior borders of the acetabular tear drops - the pelvic reference line. The lesser trochanters are used as the femoral reference line. Perpendicular lines are drawn from the pelvic reference line to the femoral reference line, the difference between the distances being the leg length discrepancy.
2. Horizontal centre of rotation (Fig. 6)
The horizontal centre of rotation is defined by the distance between the centre of the femoral head and the teardrop shadow. The distance should be equal to that of the contralateral hip.
Excessive lateral positioning of the acetabular component increases the risk for dislocation and may cause limping.
3. Vertical centre of rotation (Fig. 7)
The vertical center of rotation of the acetabular component is evaluated by measuring the vertical distance between the center of the femoral head and the transischial tuberosity line. This distance should be similar to that of the contralateral hip.
4. Acetabular inclination (Fig. 8)
The acetabular inclination is the angle between the articular side of the acetabular cup and the transverse axis. The measure of this angle is done by drawing a line through the medial and lateral margins of the cup and measuring the angle with the transischial tuberosity line.
The normal range of acetabular inclination is between 30 and 50°. Smaller angles are associated with reduced abduction and greater angles with greater risk of hip dislocation.
5. Femoral stem positioning (Fig. 9)
The position of the femoral stem should be in neutral alignment with the longitudinal axis of the femoral shaft, and the tip situated in the centre of the shaft. Failure of the femoral stem is associated with varus malpositioning.
6. Cement mantle (Fig. 10)
The cement-bone interface, the cement-prosthesis interface and the cement thickness should be analyzed for the presence of lucencies.
Femoral cement mantles should ideally be 2–3 mm thick as this thickness
The most common system for assessing radiolucencies within the acetabular and femoral components are the Charnley-Delee and Gruen systems, respectively, which are depicted in Figure 10.
Follow up radiographs are a major part of the ongoing assessment of a prosthetic joint and are of significant diagnostic value in detecting changes in the appearance of the prosthetic components and bone which may indicate impending failure.
Radiological evaluation at 6 weeks and 12 months after surgery should be assessed, unless pain or clinical symptoms warrants more early investigation. After this period, radiographic evaluation should be performed if the patient is symptomatic
The radiographic complications features can be classified into three categories:
1. Periprosthetic lucencies
a. Aseptic loosening or osteolysis (Fig. 11)
Osteolysis may be observed radiographically as a thin zone of radiolucency that may slowly extend around the bone-cement or bone-prosthesis interface. Osteolysis leads to aseptic loosening and periprosthetic fractures.
Periprosthetic lucencies wider than 2 mm or progressive lucencies are signs of abnormality.
Radiographic findings are diverse, can vary from completely normal to periprosthetic zones of lucency or to frank bone destruction. A distinction between septic and aseptic loosening often cannot be made on a single radiograph. Usually, previous radiographs are necessary for comparison. Aseptic loosening usually takes a slowly progressive course, whereas infection usually occurs with a rapid time course and an aggressive appearance,
2. Sclerosis and bone proliferation
a. Heterotopic ossification
Abnormal formation of true bone within extraskeletal soft tissues. Despite being a frequent complication, clinically significant limitation of motion is rare.
b. Spot welding
Formation of new bone originating from the endosteal surface and reaching the prosthesis. This is mostly seen in cementless femoral stems and is a strong indicator of stability.
c. Stress shielding (Fig. 13)
The transference of the normal load from the femoral neck and intertrochanteric region to the proximal femoral diaphysis causes bone resorption on the lateral side of the proximal femur, most commonly seen in Gruen zone 1, as well as bone hypertrophy at the medial side of the proximal femur. This process implies stability.
3. Component failure/ fracture
Most dislocations occur in the early postoperative period, during the initial weight bearing. Abnormal acetabular inclination, acetabular retroversion or an incorrect center of rotation, among others, increase the probability of dislocation.
b. Periprosthetic fractures (Fig. 16)
Occur more often around the femoral than the acetabular component. The Vancouver classification divides the periprosthetic postoperative fractures of the femur into three types:
- Type A: peritrochanteric fractures (subtypes: AL = lesser trochanter and AG = greater trochanter).
- Type B: around or just below the tip of the stem (subtypes: B1 = well-fixed stem, B2 = not-wellfixed stem, B3 = poor bone stock in the proximal femur and not-well-fixed stem)
- Type C: below the femoral stem
c. Prosthetic fractures
Occur mostly in the femoral stem of the implant, representing a metal-fatigue stress fracture. The main risk factors are increased body mass index and varus malpositioning.