The radiology assistant hip arthroplasty

Publicationdate February 1, 2006
Radiography is the primary imaging method for the evalua-
tion of Total Hip Arthroplasty.
This overview focusses on the normal findings and compli-
cations of cemented versus non-cemented hip arthroplas-
ties.
Interactive cases are presented in the menubar to test
your knowledge on hipprostheses.
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.
Hip - Arthroplasty
Normal and abnormal imaging findings
Iain Watt, Susanne Boldrik, Evert van Langelaan and Robin Smithuis
from the Radiology Departments of the Leids University Hospital, Leiden; the Medical Centre Alkmaar, Alkmaar and the
Orthopedic and Radiology Department of the Rijnland Hospital, Leiderdorp, the Netherlands
Total Hip Arthroplasty systems
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.
The Radiology Assistant : Hip - Arthroplasty http://www.radiologyassistant.nl/en/p431c8258e7ac3/hip-arthroplasty.html
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Hybrid total hip replacements are a combination of ce-
ment- and cementless fixation.
As cemented acetabular components have a tendency to
loosen over time, the combination of a cementless acetab-
ular component with a cemented femoral component is
sometimes used.
Overall there is a tendency to use preferably non-cemented
THA, which have better logterm results.
On the left we see a hybrid THA with bone-ingrowth ac-
etabular 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 de-
tecting complications.
The initial postoperative films are obtained to look for pos-
sible 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 unce-
mented 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 usu-
Initial Evaluation
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.
LEFT: Revision THA with a large femoral stem with periprosthetic
fracture.RIGHT: Cement extrusion intrapelvic through acetabular
defect.
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ally 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 discrep-
ancy.
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 de-
gree 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 compo-
nent also may increase the risk of polyethylene wear of the
acetabular liner (see figure).
Measurement of lateral acetabular inclination. Right trochanter
minor is lower in position than the left indicating leg length
dicrepancy.Normal horizontal center of rotation (red line).
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).
LEFT: Femoral head with large collar. Dislocation due to increased
lateral inclination of acetabular cupRIGHT: 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.
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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 lucen-
cies 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 prox-
imal lateral aspect of the femoral stem may be seen on the
initial postoperative radiograph as a reflection of subopti-
mal 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.
Normal Findings at Follow up
Varus position of femoral stem leading to loosening and fracture.
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.
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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 that
should be 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 com-
mon 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 osteo-
porosis 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 cement-
less 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 combi-
nation of bone ingrowth and fibrous tissue ingrowth, that
provides the fixation in most cases.
This fibrous tissue presents as a lucent zone at the inter-
face.
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.
Acetabular zones according to De Lee and CharnleyFemoral zones
according to Gruen
Manipulated image showing normal reactions to the some of the
uncemented hip prostheses.
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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 scle-
rotic line parallel to it.
If they stay stable for 2 years than fixation by a strong fi-
brous 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 loosen-
ing, particle disease and infection.
These complications however may have similar imaging
findings and overlap exists.
Complications at Follow Up
Normal lucent zone:
Progressive calcar resorption during folllow up.
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Mechanical loosening presents as diffuse lucency.
Particle disease presents as focal lucency.
Evidence of polyethylene wear, which appears as asymmet-
ric positioning of the femoral head within the acetabular
cup, often coexists with particle disease. .
Infection presents as irregular lucency with periosteal reac-
tion, 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 for-
ward (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.
Illustration of the typical radiographic changes in Loosening (left) -
Particle disease (middle) - Infection (right)
Progressive lucent zone around acetabular component in zone I and
II. Steeper position of the cup indicates migration. Subtle excentric
positioning of the femoral head is indicative of polyethylene wear.
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Loosening (2)
As migration can be very subtle, it requires carefull com-
parison 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 (fig-
ure)
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 loos-
ening.
During follow up upward migration with increased tilting is
seen causing the fixation screw to break.
Same case as above with white marks on the tear drop figure.
Migration is shown more easily. Blue arrow indicates acetabular
fracture.
Migration of acetabular cup cranially with tilting and subsequent
acetabular fracture
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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 re-
action 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 re-
sponse.
These characteristics help to distinguish small particle dis-
ease from infection, which often has more aggressive fea-
tures, 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 ec-
centric 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 pros-
thesis 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.
Eccentric position of femoral head within cup consistent with
polyethylene wear.Focal osteolysis with endosteal scalloping in
proximal femur due to particle disease.
Subtle eccentric position of femoral head. Even more subtle focal
osteolysis around screw in acetabulum.
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Another case on the left.
Again there is focal osteolysis around screws after migra-
tion 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 peripros-
thetic tissue.
- Presence of a sinus tract communicating with the pros-
thesis.
Creep is normal remoulding and is superomedial. Wear is
superolateral and pathologic
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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 im-
munoglobulin 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 prosthe-
sis are acquired during implantation. The risk of intraoper-
ative infection is less than 1% due to the use of antimicro-
bial 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 cer-
clage 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.
Irregular periprosthetic bone resorption with periosteal reaction
typical for infection.
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Dislocation
As discussed above, dislocation or subluxation of the com-
ponents 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 se-
vere degenerative changes or dysplasia.
Dislocation can be in posterior, anterior or lateral direction.
On the left another case with dislocation as a result of tilt-
ing of the cup due to loosening.
Component fracture
Component fracture is uncommon.
The case on the left is probably secondary to severe poly-
ethylene wear resulting in cup and cement fracture.
Component dissociation, as opposed to component frac-
ture, most commonly develops when the plastic liner of the
acetabulum slips from its backing.
Lateral dislocation of THA
Tilting of loose cup resulting in dislocation
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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 Grade IV = radiographic
ankylosis of the hip.
Heterotopic Ossification occurs when primitive mesenchy-
mal 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 com-
mon complaint and pain is rarely a problem.
Destruction of polyethylene liner
Classification of heterotopic ossification according to Brooker
Various degrees of heterotopic ossification
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Arthrography and infection
Arthrography plays a role in the evaluation of possible in-
fection.
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 cul-
ture.
The sensitivity for infection is 66-90%.
Arthrography and loosening
Lack of abnormal contrast extension does not exclude loos-
ening 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 in-
growth may persist allowing contrast-passage in stable
hips.
In cemented THA contrast extension at the bone-cement
interface can indicate prosthesis loosening.
Arthrography
Arthrogram used to confirm intra-articular position of needle in
possible infected prosthesis.
Sutraction arthrography reveals contrast leakage in Gruen zone 1
(yellow arrow).Movement of the patient simulates leakage in zone 2
and 3 (red arrow indicates white stripe on medial side which is as
broad as black stripe on lateral side ( yellow arrow).
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Communication with the trochanteric bursa, which is com-
mon, 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 anaes-
thetic.
This is to see if the painful hip is due to the prosthesis and
not something else.
Imaging of total hip replacement ,BN Weissman, Radiology 1997; 202: 611.1.
From the RSNA refresher courses. Total hip arthroplasty: radiographic evaluation, BJ Manaster, RadioGraphics 1996; 16: 645.2.
Prosthetic-Joint Infections: current concepts
Glatt, A. E., Melamed, E., Cohen, I., Robinson, D., Zimmerli, W., Trampuz, A. (2005).. N Engl J Med 352: 95-97
3.
Imaging of prosthetic joints
S Ostlere, FRCR and S Soin, MB BChir. Nuffield Orthopaedic Centre and Oxford Radcliffe Hospital, Oxford, UK
4.
Complications of total hip arthroplasty. Saleh, KJ, Kassim, R, Yoon, P, Vorlicky, LN. Am J Orthop 2002; 31:4855.
Communication between intra-articular space and the trochanteric
bursa
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The radiology assistant hip arthroplasty

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