Arthroplasty of the Hip

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.

Most modern non-cemented THA have a femoral stem with only proximal coating, as this results in a better longterm 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 coating is not visible on radiographs.

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 longterm results.

On the left we see a hybrid THA with bone-ingrowth acetabular cup and cemented femoral component and next to it a non-cemented bone ingrowth THA.

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
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%).

Periprosthetic fractures
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.

Cement extrusion
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.

Alignment and Positioning

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 (30-50?).
This line is also used to measure any leg length discrepancy.
Leg length discrepancy up to 1 cm is well tolerated.
A high positionened cup is better tolerated than a lateral positioned cup.

The anteversion of the acetabular cup should be 5-25?.
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 (figure).
Measurement with CT is more accurate, but you still have to compensate for pelvic angulation.

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.

Due to increased forces on the superolateral margin of the cup, increased lateral inclination of the acetabular component also may increase the risk of polyethylene wear of the acetabular liner (see figure).

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 good, but if the lucency enlarges or develops at the metal-cement interface during follow up, then it is a sign of loosening (figure).
Ideally there is only a 3-4mm layer of cement around the prosthesis. Abundant cement packing leads to loosening.

At the bone-cement interface a thin fibrous layer may form as responce 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.
Especially in acetabular zone I a 1-2 mm lucency is frequently seen at the bone-cement interface, 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 (figure).
In the acetabulum you have three zones marked I-III.
It is quite common to see a radio lucent 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.

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.

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.
Again it should be stable and well within a range of 1 -2 mm.

The figure on the left sums all the findings in some of the non-cemented prostheses, that can be normal.
You have to be familiar with the normal and abnormal changes in the types of prostheses, that are used by your orthopaedic surgeons.

Thin lucent zones along the bone-metal interface due to fibrous tissue are therefore common (80%).
They should be less than 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.

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 (figure).
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.In typical cases the imaging findings of loosening, particle disease and infection are straight forward (figure).
Infection is often low grade and is difficult to detect with any imaging method.
In more agressive cases there will be irregular osteolysis, no sclerotic border, cortical bone resorption and a periosteal reaction.

Loosening

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.
A lucent zone

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.

Loosening (2)
As migration can be very subtle, it requires carefull 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 (see next figure)

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).

Migration of acetabular components is never acceptable.
It is seen as upward movement or tilting of the cup (figure)

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.

Especially lucency in these zones is very indicative of loosening.
During follow up upward migration with increased tilting is seen causing the fixation screw to break.

Particle Disease

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 of the arthroplasty.
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.
These characteristics help to distinguish small particle disease from infection, which often has more aggressive features, although the distinction is not always possible.

Although particle disease is a result of polyethylene wear, you will not always see evident findings of polyethyleen 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.

Particle Disease (2)
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 (figure).
This is why surgeons are more and more reluctant to use screws for the fixation of acetabular cups.

Another case on the left.
Again there is focal osteolysis around screws after migration of wear particles through the screwholes.
Eccentric position of femoral head within acetabular cup as a result of polyethylene wear.

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 moulds itself. This remoulding of the cup is called creep.
Abnormal loading leads to pressure more lateral, resulting in polyethylene wear on the supero-lateral side.

Infection

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, resulting in radiological findings as listed in the table on the left.

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.

On the left the typical radiographic findings of infection with irregular bone destruction and periosteal reaction.
In many cases however the infection is really low grade and difficult to establish.

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.

Fractures

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

As discussed above, 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.

On the left another case with dislocation as a result of tilting of the cup due to loosening.

Component fracture

Component fracture is uncommon.
The case on the left is probably secondary to severe polyethylene wear resulting in cup and cement fracture.
Component dissociation, as opposed to component fracture, most commonly develops when the plastic liner of the acetabulum slips from its backing.

The case on the left shows severe wear and fracture of the polyethylene liner.
The metal backing is still intact.

The case on the left shows a fracture of the metal head of the femoral component.

Heterotopic Ossification

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.

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.

Arthrography and infection
Arthrography plays a role in the evaluation of possible infection.
Its value in the evaluation of possible loosening and painful hips is limited.
The arthrogram is used to confirm intra-articular position of needle and fluid is aspirated for aerobic and anaerobic culture.
The sensitivity for infection is 66-90%.

Arthrography and loosening
Lack of abnormal contrast extension does not exclude loosening as fibrosis and cells may fill the interfaces preventing contrast passage.
In non-cemented THA arthrography is not accurate for the detection of loosening, as small channels between bone ingrowth may persist allowing contrast-passage in stable hips.
In cemented THA contrast extension at the bone-cement interface can indicate prosthesis loosening.

Communication with the trochanteric bursa, which is common, further reduces sensitivity as a good intra-articular pressure cannot be established (figure).

Arthrography and painfull hips
Sometimes arthrography is used to find out, if the patients symptoms subside by putting in a long lasting local anaesthetic.
This is to see if the painful hip is due to the prosthesis and not something else.

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