Presented By:
M. Zare Mehrjardi, M.D. 1, 2 DOI: 10.13140/RG.2.2.23231.10400/1 1 Department 2 Section
(April, 2014)
of Radiology, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
of Musculoskeletal Imaging, Division of Clinical Research, Climax Radiology Education Foundation, Tehran, Iran
PRESENTS…
© 2013 Climax RC All Rights Reserved
• Modern Total Hip Arthroplasty (THA) systems are modular. This means that the femoral stem, head, acetabular shell and liner are separate pieces. • This modularity allows for greater flexibility in customizing prosthesis sizing and fit. • The acetabular part is usually a polyethylene liner with or without metal backing. Fixation is with cement, spikes, screws or cementless with porous coating for bone ingrowth.
• The femoral part is composed of a metal stem (chromium cobalt or titanium) and a femoral head of metal or ceramic. Stem-fixation is also either with cement or cementless with porous coating for bone ingrowth.
• Hybrid total hip replacements are a combination of cement and cementless fixation. • As cemented acetabular components have a tendency to loosen over time, the combination of a cementless acetabular component with a cemented femoral component is sometimes used.
Overall there is a tendency to use preferably non-cemented THA, which have better long-term results. • Most modern non-cemented THA have a femoral stem with only proximal coating, as this results in a better long-term outcome than fully coated (less loosening). • Some of the non-cemented THA have femoral stems with additional hydroxyapatite coating which further improve bone ingrowth.
• This coatings are not visible on radiographs.
LEFT: Assembled cementless Mallory Head prosthesis. RIGHT: Femoral stem with proximal porous coating for bone ingrowth, separate metal femoral head, polyethylene acetabular liner with a porous coated metal backing.
LEFT: Hybrid THA with cemented femoral stem and noncemented acetabular cup. RIGHT: Bone ingrowth arthroplasty. Density lateral to femoral stem in Gruens zone-I is a bone graft.
• The initial films serve as a baseline study and are used as reference films for comparison with all future studies, since sequential radiography is the most valuable method for detecting complications.
• The initial postoperative films are obtained to look for possible dislocation or fracture and to see if the prosthesis is good positioned.
• Dislocation can occur as a late complication in prostheses that are not well positioned, but it is most common in the immediate postoperative period (incidence: 3%).
• Fractures may be seen postoperatively in patients with poor bone stock and long stem revision prostheses or when the anatomy is abnormal as in hip dysplasia or prior surgery.
• They are also more common in non-cemented femoral stems, as these have to fit exactly and can cause a fracture during insertion.
• The incidence of fractures ranges from 0.1 to 1.0 percent for cemented components and 3 to 18 percent for uncemented components.
• Most intraoperative fractures occur on the femoral side.
• When the acetabulum is prepared for placement of the cup a perforation may occur. This defect is filled with bone chips, cement or bone transplant.
• Cement extrusion is usually asymptomatic.
• Rare complications include bowel fistulas, encasement of neurovascular structures and bladder wall burn.
LEFT: Revision THA with a large femoral stem with periprosthetic fracture.
RIGHT: Cement extrusion intrapelvic through acetabular defect.
• Acetabular and femoral component positioning should mimic normal anatomy. • The distance from center of the femoral head to teardrop (or other identifiable landmark) should be equal bilaterally. • This is called the horizontal center of rotation.
• Excessive lateral positioning of the acetabular component increases the risk for dislocation and may cause limping. • The transischial line is used as a reference to measure the lateral inclination of the acetabular cup (30-50º). • This line is also used to measure any leg length discrepancy. Leg length discrepancy up to 1cm is well tolerated. • A high positioned cup is better tolerated than a lateral positioned cup.
LEFT: Femoral head with large collar. Dislocation due to increased lateral inclination of acetabular cup.
RIGHT: Different patient at risk for dislocation. High and lateral position of a steep acetabular cup. Notice polyethylene wear due to increased forces on the superolateral side of the cup.
• The anteversion of the acetabular cup should be 5-25º. • Exact measurement of this angle on a cross-table or true lateral radiograph is not possible , since the apparent degree of angulation on a radiograph is affected by pelvic or thigh rotation. • Measurement with CT is more accurate, but you still have to compensate for pelvic angulation.
Different anteversion of the acetabular cup in the same patient due to different rotation on a cross table view (left) compared to a lateral view (right).
• The following conditions predispose to dislocation: - Increased lateral inclination of the acetabular cup. - Decreased or increased anteversion of the cup. - Excessive lateral positioning of the acetabular cup.
- Increased or decreased anteversion of the femoral stem.
• The preferred position of the femoral component is with the stem centered in the femoral canal. • The center of rotation of the femoral head should be at the level of the tip of the greater trochanter. • Varus position of the femoral stem predisposes to loosening and fracture.
Cemented THA: •
Normal findings in cemented-THA are different from non-cemented prostheses, as the native bone shows more reactive changes to non-cemented prostheses.
•
In cemented THA ideally you would not expect any lucencies at the bone-cement or cement-prosthesis interface, but even in stable cemented prostheses they do occur.
•
A lucency at the metal-cement interface along the proximal lateral aspect of the femoral stem may be seen on the initial postoperative radiograph as a reflection of suboptimal metal-cement contact at the time of surgery.
•
A stable lucent zone is acceptable, but if the lucency enlarges or develops at the metal-cement interface during follow up, then it is a sign of loosening.
•
Ideally there is only a 3-4mm layer of cement around the prosthesis. Abundant cement packing leads to loosening.
Cemented THA: •
At the bone-cement interface a thin fibrous layer may form as response to local necrosis of osseous tissue due to the heat of the cement-polymerization.
•
It becomes stable by 2 years.
•
On radiographs this layer is seen as a lucent zone that should be especially in acetabular zone-I a 1-2mm lucency. This is a normal finding provided it is stable.
•
If also other zones are involved and the lucency widens, it is however a sign of loosening.
• In your report always indicate which zones are involved. • In the acetabulum you have three zones marked I - III. It is quite common to see a radiolucent line in zone I, but you shouldn't see it in zone II and III. • Similarly in the femur there are zones 1 - 7. It is very common to see radiolucency in zone 1, occasionally in zone 7, but it should not occur in the subtrochanteric region zones 2-6.
Acetabular zones according to: DeLee & Charnley.
Femoral zones according to: Gruen.
Cemented THA:
LEFT: Normal cement-metal interface (yellow arrow). However, loosening at cement-bone interface (orange curved arrow).
RIGHT: At follow-up also loosening at cement-metal interface.
Non-Cemented THA: •
The implantation of a bone ingrowth prosthesis results in altered stress distribution to the native bone, especially in the older models with non tapered and fully coated femoral stems.
•
Stress shielding proximally may result in proximal osteoporosis and calcar resorption.
•
Stress loading distally may result in cortical thickening and bridging sclerosis at the tip of the prosthesis ( called pedestal).
•
In an effort to avoid these changes, most modern cementless prosthesis only have fixation proximally, so you usually will not find proximal stress shielding. The distal part of the femoral prosthesis is not 'loaded', so there will be no distal stress loading.
Non-Cemented THA:
Normal reactions to the some of the uncemented hip prostheses.
Non-Cemented THA: •
In stable non-cemented hip arthroplasties lucent zones at the metal-bone interface do occur, as it usually is a combination of bone ingrowth and fibrous tissue ingrowth, that provides the fixation in most cases.
•
This fibrous tissue presents as a lucent zone at the interface and are therefore common (80%).
•
Again it should be stable and well within a range of 1-2mm and accompanied by a sclerotic line parallel to it.
•
If they stay stable for 2 years than fixation by a strong fibrous tissue has taken place.
Progressive calcar resorption during follow-up: •
Stress shielding or bone resorption is seen in areas that are relatively unstressed.
•
The forces are transmitted through the relative stiff femoral stem and is seen as osteoporosis in the proximal femur with thinning of the cortex and bone resorption of the femoral neck.
•
This is seen medially as calcar resorption, as the calcar has lost it's function.
•
It is also called calcar round off.
• There are many complications in THA. • Radiographic follow-up and comparison with the oldest films available is the most valuable method of detecting these complications.
• The most important complications are mechanical loosening, particle disease and infection. • These complications however may have similar imaging findings and overlap exists.
• Mechanical loosening presents as diffuse lucency. • Particle disease presents as focal lucency. • Evidence of polyethylene wear, which appears as asymmetric positioning of the femoral head within the acetabular cup, often coexists with particle disease. • Infection presents as irregular lucency with periosteal reaction, but may be difficult to differentiate from loosening and particle disease. Infection is often low grade and is difficult to detect with any imaging method. In more aggressive cases there will be irregular osteolysis, no sclerotic border, cortical bone resorption and a periosteal reaction. • In typical cases the imaging findings of loosening, particle disease and infection are straight forward.
Illustration of the typical radiographic changes in Loosening (left) - Particle disease (middle) - Infection (right)
• Mechanical loosening remains the most common indication for revision. Patients are usually symptomatic, although asymptomatic radiographic changes may be seen.
• The most common radiographic manifestation of loosening are: - Lucent zone > 2 mm at interface (indicative). - Component migration (diagnostic).
• A lucent zone of more than 2 mm at the bone-prosthesis interface or at the bone-cement interface is very indicative of loosening. Especially if more zones are involved and if there is progression.
Progressive lucent zone around acetabular component in zone I and II. Steeper position of the cup indicates migration. Subtle eccentric positioning of the femoral head is indicative of polyethylene wear.
• Component migration is diagnostic for loosening. • It is seen as tilting or cranial migration of the acetabular cup or as subsidence (>10mm) and varus tilting of the femoral stem.
The case on the left shows progressive subsidence, which is diagnostic for loosening, with subsequent break of the screws.
• As migration can be very subtle, it requires careful comparison with the initial postoperative films. • Do not just compare to the prior examination.
The case on the left shows migration of the acetabular cup, which is better appreciated if a reference point is used (next page).
• If we look at the same radiographs and we use the tear drop figure as a landmark, the migration becomes more evident. • Migration of the cup in cranial direction has resulted in a fracture in the acetabular wall (blue arrow).
Same case as last page with white marks on the tear drop figure. Migration is shown more easily. Blue arrow indicates acetabular fracture.
• Migration of acetabular components is never acceptable. • It is seen as upward movement or tilting of the cup.
Migration of acetabular cup cranially with tilting and subsequent acetabular fracture.
• The case on the left is for several reasons not ideal : - High and very lateral positioning of the cup.
- Too much lateral inclination. - Abundant cement packing.
- Screws are positioned too horizontally (too much stress). - Lucency in zone II and III > 2 mm (very indicative of loosening).
During follow up upward migration with increased tilting is seen causing the fixation screw to break.
• Originally this was called cement disease or aggressive granulomatosus. • It is a histiocytic response that occurs as a result of macrophage reaction to any of the components, that are shed of the surface of the components.
• Nowadays, it is mostly seen in non-cemented hips as a reaction to small polyethylene wear particles. • Radiographically these aggressive granulomatous lesions present as focal radiolucencies around the prosthesis. • The condition tends to occur between 1 and 5 years after surgery and is associated with smooth endosteal scalloping. • The key feature is that it produces no secondary bone response (despite infection).
• Although particle disease is a result of polyethylene wear, you will not always see evident findings of polyethylene wear in the acetabular cup, but whenever you see an eccentric position of the femoral head within the cup, look for focal lucencies.
• Large focal defects may be seen while the prosthesis is still stable. • Particle Disease is relentlessly progressive with loosening, fracture and destruction of bone.
• Sometimes revision of a stable THA is needed because more bone loss would make revision surgery impossible.
Eccentric position of femoral head within cup consistent with polyethylene wear.
Focal osteolysis with endosteal scalloping in proximal femur due to particle disease.
• The small wear-particles of the polyethylene liner are shed into the joint fluid and can be transported around the prosthesis through small channels even in stable hips. • They have a tendency to be transported through screw holes. This is why surgeons are more and more reluctant to use screws for the fixation of acetabular cups.
Subtle eccentric position of femoral head. Even more subtle focal osteolysis around screw in acetabulum.
Focal osteolysis around screws after migration of wear particles through the screw holes.
Eccentric position of femoral head within acetabular cup as a result of polyethylene wear.
• Normal loading of the polyethylene cup comes up the femoral shaft, along the femoral neck towards the lumbar spine. • So it is normal to see slight thinning in the area of the weight bearing as the plastic molds itself. This remolding of the cup is called creep. • Abnormal loading leads to pressure more lateral, resulting in polyethylene wear on the supero-lateral side.
Creep is normal remoulding and is superomedial. Wear is superolateral and pathologic.
Destruction of polyethylene liner
• Radiologic findings in patients with low grade infection may be unremarkable or may mimic loosening or small particle disease. • With more aggressive organisms, progression can be rapid, with bone destruction and sinus tract formation. • Uniform criteria for the diagnosis of infection associated with prostheses have not been established. • In several studies infection was diagnosed if at least one of the following criteria was present: - Same microorganism in two cultures of synovial fluid. - Purulence of synovial fluid at the implant site. - Inflammation on pathological examination of periprosthetic tissue.
- Presence of a sinus tract communicating with the prosthesis.
• Radionuclide bone scans are very sensitive for infection, but not specific as they may show findings similar to those occurring in loosening. • Negative findings on a bone scan suggest that no infection exists.
• The role of dedicated radionuclide techniques for infection such as gallium scanning or indium-labeled WBC or immunoglobulin G is not clear, but they tend to be a bit more specific compared to normal Technetium bone scan.
• Most researchers advocate fluoroscopic or sonographic guided joint aspiration to assess infection. • Several samples should be taken to minimize confusion caused by skin contaminants.
• Infections up to one year after the insertion of the prosthesis are acquired during implantation. The risk of intraoperative infection is less than 1% due to the use of antimicrobial prophylaxis and laminar airflow surgical environment. • Late infections are acquired by hematogenous seeding from respiratory tract, dental and urinary tract infections.
Irregular periprosthetic bone resorption with periosteal reaction typical for infection.
• Incidence post-operative: - cemented THA: 0.4% - press fit prosthesis: 2.5%
- revision hip arthroplasty: 7.2% • Usually it does not affect outcome, but may require cerclage cables. • Sometimes a control perforation is placed by the surgeon during revision to aid in removal of the previously placed femoral component. • Fractures during follow-up are a result of loosening, particle disease, infection or severe cases of stress shielding.
• Dislocation or subluxation of the components may occur because of patient factors including poor muscle tone or trauma or because of surgical factors such as a posterior (rather than lateral) surgical approach. • Another factor is difficulty in achieving ideal angulation of the acetabular component. This is usually the result of severe degenerative changes or dysplasia. • Dislocation can be in posterior, anterior or lateral direction.
Lateral dislocation of THA
Tilting of loose cup resulting in dislocation
• Component fracture is uncommon. • Component dissociation, as opposed to component fracture, most commonly develops when the plastic liner of the acetabulum slips from its backing.
The case is probably secondary to severe polyethylene wear resulting in cup and cement fracture.
Fracture of the metal head of the femoral component.
• Heterotopic Ossification occurs when primitive mesenchymal cells in the surrounding soft tissues are transformed into osteoblastic cells, that form mature lamellar bone.
• It typically occurs around the femoral neck and adjacent to the greater trochanter and occurs in 15-50% of patients.
• Many patients with radiographically low-grade heterotopic ossification are asymptomatic.
• If it becomes symptomatic, hip stiffness is the most common complaint and pain is rarely a problem.
• The classification of heterotopic ossification includes four grades based on an AP radiograph of the pelvis and hip: 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
Classification of heterotopic ossification according to Brooker.
Classification of heterotopic ossification according to Brooker.
• Zare Mehrjardi M. Total hip arthroplasty: diagnostic imaging. 2014. doi: 10.13140/RG.2.2.23231.10400/1.
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