ESSR 2017 / P-0219
Normal radiologic findings and detection of complications in Total Hip Arthroplasty
Congress: ESSR 2017
Poster No.: P-0219
Type: Educational Poster
Keywords: Prostheses, Surgery, Conventional radiography, Musculoskeletal joint, Musculoskeletal bone
Authors: A. L. Proenca, A. P. Caetano, L. Bogalho; Lisbon/PT
DOI:10.1594/essr2017/P-0219

Imaging findings OR Procedure Details

       Acetabular and femoral prosthetic components should mimic normal radiographic anatomy.
        It is important to perform an initial evaluation to use as reference for comparison in serial studies.

        The anteroposterior pelvic radiograph is the best imaging modality to assess de hip arthroplasty.

    Alignment and positioning, periprosthetic radiolucencies, bone remodeling and component migration should always be assessed. Charnley and Gruens zones present an anatomic division to report the location of lucent areas.

 

 

Normal postoperative findings in THA

 

1. Alignment and Positioning

   A - Vertical center of rotation of the acetabular head:  Vertical distance between the center of the femoral head and the transischial tuberosity line should be equal bilaterally. Fig. 4

   B - Horizontal center of rotation: Center of the femoral head to teardrop distance should be equal bilaterally. The center of the femoral head should be at the level of the tip of the greater trochanter. Fig. 4

   C - Lateral acetabular inclination: Angle between the transischial line and a line drawn across the superior and inferior acetabular rims (Normal values between 30-50º). Lesser angulations result in limited abduction and greater predispose to dislocation. Fig. 4

   D - Acetabular anteversion is more accurately measured with CT. However, sharp edges of the cup suggest no version, rounded edges suggest version. Fig. 5

   E - Leg length discrepancy: Distance of the top of each lesser trochanter to the transischial line is used as reference. Well tolerated up to 1cm of discrepancy. Fig. 4

   F - Position of the femoral stem - The femoral stem positioning should be aligned with the longitudinal axis of the shaft. Varus malpositioning (distal tip against the lateral cortex) increases the risk of failure. Fig. 4

 

 

Fig. 4: Alignment and positioning: A –Vertical center of rotation B –Horizontal center of rotation C –Lateral acetabular inclination E –Leg length F –Position of femoral stem
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

Fig. 5: Acetabular anteversion - sharp edges of the cup suggest no version (A),rounded edges suggest version (B).
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

 

2. Periprosthetic radiolucencies

    Periprosthetic radiolucencies may be normal or a sign of loosening.  These findings differ between cemented and non-cemented prosthesis.

 

Cemented

Non-cemented

Ideally no lucencies at the bone-cement or cement-prosthesis interface.

Metal-cement interface - lucency along the proximal lateral aspect of the femoral stem may be seen in early postoperative radiographs.

Lucency should be stable, enlargement or development at the metal-cement interface is a sign of loosening.

The cement layer around the prosthesis should be about 3-4mm thick. Larger layers predispose to loosening.

Bone-cement interface – a fibrous radiolucent layer may develop due to local necrosis of osseous tissue. This is a normal finding if stable for 2 years, in acetabular zone I and with 1-2mm. In other zones or enlargement, indicate loosening.

Native bone shows more reactive changes to non-cemented prostheses.

Metal-bone interface commonly shows lucent zones that should be stable for 2 years and within 1-2mm. It should be demarcated by a sclerotic line, parallel to it. Fig. 6

Proximal stress shielding or bone resorption is seen in areas without stress. Osteoporosis develops in the proximal femur cortex and femoral neck. Medially there is calcar resorption. Fig. 7

Distal stress loading may promote sclerosis at the tip of the prosthesis creating a pedestral.

Modern prosthesis with only proximal fixation can avoid proximal stress shielding and distal stress loading changes.

 

 

 

Fig. 8: Non-cemented THA. Distal stress loading (pedestral (green arrow) and cortical thickening (blue arrow)). Proximal stress shielding – bone resorption at greater trochanter (yellow arrow).
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

 

3. Cemented THA

    Charnley and Gruens zones present an anatomic division to report the location of lucent areas.

     Acetabular lucency in zone I is common, but not it shouldn´t be seen in zone II or III.

     Femoral lucency in zone 1 is common, and also occasionally in zone 7, but it shouldn´t be seen in zones 2-6. Fig. 10

 

 

Fig. 9: Charnley zones - around the Acetabulum– 3 zones (I-superior, II - middle and III -inferior thirds of the acetabulum). Gruen zones - Around the femoral stem – 1-7 zones, starting from the lateral aspect and progressing anticlockwise. Zone 1 and zone 7 are around the greater and lesser trochanters, respectively and zone 4 is around the distal prosthesic tip.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

4. Bone Remodeling

    The mechanical loading changes after THA, promoting:

  •        Loss of bone mineralization  - in areas with reduced loading (stress shielding)
  •         Bone sclerosis, cortical thickening and periosteal reaction - in areas with bone ingrowth or with more stress (below the tip of the stem in zone 4 -bone pedestral).

    These changes imply stability of the prosthesis and occur within the first 2 years after surgery.

 

 

5. Component Migration

    Within the first 2 years, it may be normal that some types of prostheses subside around 1-2mm. Progressive movement after 2 years or more that 10mm is not normal.

 

 

Complications in THA

 

 

Early Complication

Delayed complications

Periprosthetic fracture

Dislocation

Infection

Cement extrusion

Leg length discrepancy

 

Loosening

Polyethylene wear

Particle disease

Heterotopic ossification

Others: Haematoma, thromboembolism, nerve injuries, vascular injuries, trochanteric bursitis, metal-on-metal pseudotumour, component fracture

 

    The most common complications are loosening, periprosthetic fractures and infection.

      Some complications may have similar imaging findings and overlap exists. Radiographic follow up and comparison with oldest films is the most important method for detecting complications. 

      Early complications occur during surgery, immediately after surgery or up to one year after surgery. Infection and aseptic loosening are the two main delayed complications.

 

 

1. Periprosthesic Fractures

      Fractures can occur during surgery or months or years later. They are more frequent with non-cemented prosthesis intraoperativelly and on the femoral side.

      Factors contributing to fractures include: trauma, female gender, old age, loosening, particle disease, infection, poor bone stock or severe cases of stress shielding.

      Cerclage cables may be needed to stabilize the fracture.

      Vancouver classification of periprosthetic femoral fractures after THA has been modified to include intraoperative fractures. Type A are located at the trochanteric region, Type B around or just distal to femoral stem and Type C well below the stem.

 

 

Fig. 11: Periprosthetic femoral fractures.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

 

2. Dislocation

     More common as an early postoperative complication but it can also occur later.

     Dislocation can occur either in posterior, anterior or lateral direction at any of the components (acetabular cup, femoral stem or femoral head out of the cup).

      Eccentric position of femoral head within the acetabular cup is a sign of polyethylene wear and predisposes to dislocation.

     It may be due to patient factors (poor muscle tone or trauma) or surgical factors (posterior surgical approach, inappropriate angulation of the acetabular component, small head size).

 

 

Fig. 12: Left - Acetabular dislocation in an early post-operative THA. Right - Femoral dislocation. Also, cerclage at the femoral shaft holding a periprosthetic fracture.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

 

3. Infection

      Early infections (up to one year after surgery) are usually related to the surgical procedure.

      Late infections may be due to haematological seeding from dental, respiratory or urinary infections.

     Infection can be difficult to detect with imaging and radiographic findings may include:

  • Normal radiographs;
  • Lucencies indistinguishable from loosening;
  • Progressive irregular periprosthetic radiolucencies, ill defined bone resorption or not concentric lucencies;
  • Periosteal reaction, irregular bone destruction with no sclerotic margins.

      Infection is more frequent in patients with diabetes and rheumatoid arthritis.

 

 

  4. Cement extrusion

    Intraoperative perforation of acetabulum may occur during preparation of acetabular cup placement and the defect may be filled with cement, bone transplant or bone fragments. This is usually asymptomatic but rare complications may develop.

 

Fig. 13: Cement extrusion.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

5. Leg length discrepancy

     It is well tolerated up to 1cm discrepancy but greater differences may cause limping and pain.

      There are several intraoperative limb length measurements, usually based on the distance between 2 reference points in the pelvis and femur (greater or lesser trochanter to the transischial line can be used as reference).

 

 

Fig. 14: Symptomatic leg length discrepancy.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

 

6. Loosening

     It is a macrophage-induced inflammatory response that results in peri-prosthetic bone loss.

     Criteria for diagnosing loosening of either femoral or acetabular components are not well defined, but several features suggest the diagnosis:

  •  radiolucent line greater than 2 mm in width (including focal osteolysis);
  • progression of lucencies;
  • appearance of new lucency after 2 years from  insertion;
  • subsidence of components seen months or years after surgery.

     Loosening predisposes to dislocation and fractures. Thus, to diagnose loosening is important to compare current radiographs with previous ones.

 

 

Fig. 10: Evidence of abnormal lucencies, more pronounced at zone 3 and 5 of Gruen and zone III of Charnley.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

7. Component wear /Polyethylene wear

     Results of local mechanical damage by friction of components, with loss of material and generation of wear particles. It often coexists with particle disease. 

     Abnormal loads on the lateral side result in polyethylene wear, which appears as asymmetric positioning of the femoral head within the acetabular cup.

     Superiorly located femoral head, even slightly, is not normal. Fig. 15

 

 

8. Particle disease

   Consists of a hystiocytic  response to anyof the prosthetis components, as a reaction to small plyethylene wear particles. It is more frequently seen in non-cemented hips.

   Radiographic evidence of wear may not be evident. Focal radiolucencies around the prosthesis are the main presentation. Other features include smooth endosteal scalloping and no sclerotic reaction. It may be difficult to distinguish particle disease form low grade infection, but the first produces no secondary bone response.

       It can appear between 1-5 years after surgery and slow but steady progression of the lytic lesions may lead to loosening, fracture and destruction of the bone.

      Screw holes can transport particles, therefore nowadays surgeons try to avoid screws for acetabular fixation.

 

 

Fig. 15: Particle disease. Exuberant acetabular osteolysis leading to acetabular protusion in a cemented THA. Notice the eccentric position of the femoral head at the acetabulum, a sign of polyethylene wear.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

Fig. 16: Particle disease. Acetabular osteolysis leading to screw dislocation in a non-cemented. Notice the eccentric position of the femoral head at the acetabulum, a sign of polyethylene wear.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

 

9. Heterotopic ossification

      It may occur when mesenchymal cells in the surrounding soft tissues are transformed into osteoblastic cells, producing lamellar bone.

        Most typical locations are around the femoral neck and adjacent to the greater trochanter.

      Classification of extent of bone formation  (AP radiograph of the pelvis):

 

  • Grade I = islands of bone within soft tissues.
  • Grade II = bone spurs leaving > 1 cm between opposing bone surfaces.
  • Grade III = bone spurs leaving < 1 cm between opposing bone surfaces.
  • Grade IV = radiographic ankylosis of the hip.

      The symptoms may range from asymptomatic to hip stiffness. There is a higher risk of heterotopic ossification in patiens with ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis (DISH) and hypertrophic osteoarthritis.

 

Fig. 17: Exemples of heterotopic ossification.
References: Radiology, Centro Hospitalar Lisboa Central, Hospital de Curry Cabral - Lisbon/PT

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