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 testyour knowledge on hipprostheses.

Modern Total Hip Arthroplasty (THA) systems are modular.This means that the femoral stem, head, acetabular shelland liner are separate pieces.This modularity allows for greater flexibility in customizingprosthesis sizing and fit.The acetabular part is usually a polyethylene liner with orwithout metal backing.Fixation is with cement, spikes, screws or cementless withporous coating for bone ingrowth.The femoral part is composed of a metal stem (chromiumcobalt or titanium) and a femoral head of metal or ceramic.Stem-fixation is also either with cement or cementless withporous coating for bone ingrowth.

Most modern non-cemented THA have a femoral stem withonly proximal coating, as this results in a better longtermoutcome than fully coated (less loosening).Some of the non-cemented THA have femoral stems withadditional hydroxyapatite coating which further improvebone ingrowth. This coating is not visible on radiographs.

Hip - ArthroplastyNormal and abnormal imaging findings

Iain Watt, Susanne Boldrik, Evert van Langelaan and Robin Smithuisfrom 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 toloosen over time, the combination of a cementless acetab-ular component with a cemented femoral component issometimes used.Overall there is a tendency to use preferably non-cementedTHA, 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 toit a non-cemented bone ingrowth THA.

The initial films serve as a baseline study and are used asreference films for comparison with all future studies, sincesequential 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 isgood positioned.

DislocationDislocation can occur as a late complication in prosthesesthat are not well positioned, but it is most common in theimmediate postoperative period (incidence 3%).

Periprosthetic fracturesFractures may be seen postoperatively in patients withpoor bone stock and long stem revision prostheses or whenthe anatomy is abnormal as in hip dysplasia. or priorsurgery.They are also more common in non-cemented femoralstems, as these have to fit exactly and can cause a fractureduring insertion.The incidence of fractures ranges from 0.1 to 1.0 percentfor cemented components and 3 to 18 percent for unce-mented components. Most intraoperative fractures occur onthe femoral side.

Cement extrusionWhen the acetabulum is prepared for placement of the cupa perforation may occur. This defect is filled with bonechips, 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 ofneurovascular structures and bladder wall burn.

Alignment and Positioning

Acetabular and femoral component positioning shouldmimic 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 componentincreases the risk for dislocation and may cause limping.

The transischial line is used as a reference to measure thelateral 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 lateralpositioned cup.

The anteversion of the acetabular cup should be 5-25?.Exact measurement of this angle on a cross-table or truelateral radiograph is not possible , since the apparent de-gree of angulation on a radiograph is affected by pelvic orthigh rotation (figure).Measurement with CT is more accurate, but you still haveto 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 thecup, increased lateral inclination of the acetabular compo-nent also may increase the risk of polyethylene wear of theacetabular 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 withthe stem centered in the femoral canal.The center of rotation of the femoral head should be at thelevel of the tip of the greater trochanter.Varus position of the femoral stem predisposes to looseningand fracture.

Cemented THA

Normal findings in cemented-THA are different fromnon-cemented prostheses as the native bone shows morereactive 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 theinitial 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 ordevelops 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 theprosthesis. 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 mayform as responce to local necrosis of osseous tissue due tothe heat of the cement-polymerization.It becomes stable by 2 years.On radiographs this layer is seen as a lucent zone thatshould be Especially in acetabular zone I a 1-2 mm lucencyis frequently seen at the bone-cement interface, this is anormal finding provided it is stable.If also other zones are involved and the lucency widens, itis 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, butyou 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 zones2-6.

Non Cemented THA

The implantation of a bone ingrowth prosthesis results inaltered stress distribution to the native bone, especially inthe older models with non tapered and fully coated femoralstems.Stress shielding proximally may result in proximal osteo-porosis and calcar resorption.Stress loading distally may result in cortical thickening andbridging sclerosis at the tip of the prosthesis ( calledpedestal).In an effort to avoid these changes, most modern cement-less prosthesis only have fixation proximally, so you usuallywill not find proximal stress shielding.The distal part of the femoral prosthesis is not 'loaded', sothere will be no distal stress loading.

In stable non-cemented hip arthroplasties lucent zones atthe metal-bone interface do occur, as it usually is a combi-nation of bone ingrowth and fibrous tissue ingrowth, thatprovides 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 -2mm.

The figure on the left sums all the findings in some of thenon-cemented prostheses, that can be normal.You have to be familiar with the normal and abnormalchanges in the types of prostheses, that are used by yourorthopaedic 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 tofibrous 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 thatare relatively unstressed.The forces are transmitted through the relative stifffemoral stem and is seen as osteoporosis in the proximalfemur with thinning of the cortex and bone resorption ofthe femoral neck.This is seen medially as calcar resorption, as the calcarhas 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 oldestfilms available is the most valuable method of detectingthese complications.The most important complications are mechanical loosen-ing, particle disease and infection.These complications however may have similar imagingfindings 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 acetabularcup, often coexists with particle disease. .Infection presents as irregular lucency with periosteal reac-tion, but may be difficult to differentiate from looseningand particle disease.In typical cases the imaging findingsof loosening, particle disease and infection are straight for-ward (figure).Infection is often low grade and is difficult to detect withany imaging method.In more agressive cases there will be irregular osteolysis,no sclerotic border, cortical bone resorption and a periostealreaction.

Loosening

Mechanical loosening remains the most common indicationfor revision. Patients are usually symptomatic, althoughasymptomatic radiographic changes may be seen.The most common radiographic manifestation of looseningare:

- Lucent zone > 2 mm at interface (indicative)- Component migration (diagnostic).

A lucent zone of more than 2 mm at the bone-prosthesisinterface or at the bone-cement interface is very indicativeof loosening. Especially if more zones are involved and ifthere is progression.A lucent zone

Component migration is diagnostic for loosening.It is seen as tilting or cranial migration of the acetabularcup or as subsidence (>10mm) and varus tilting of thefemoral stem.The case on the left shows progressive subsidence, whichis diagnostic for loosening, with subsequent break of thescrews.

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 (seenext figure)

If we look at the same radiographs and we use the teardrop figure as a landmark, the migration becomes moreevident.Migration of the cup in cranial direction has resulted in afracture 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 isseen 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 aggressivegranulomatosus.It is a histiocytic response that occurs as a result ofmacrophage reaction to any of the components, that areshed 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 lesionspresent as focal radiolucencies around the prosthesis.The condition tends to occur between 1 and 5 years aftersurgery 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 polyethyleenwear in the acetabular cup, but whenever you see an ec-centric position of the femoral head within the cup, look forfocal lucencies.Large focal defects may be seen while the prosthesis is stillstable.Particle Disease is relentlessly progressive with loosening,fracture and destruction of bone.Sometimes revision of a stable THA is needed becausemore bone loss would make revision surgery impossible.

Particle Disease (2)The small wear-particles of the polyethylene liner are shedinto 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 screwholes (figure).This is why surgeons are more and more reluctant to usescrews 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 asa result of polyethylene wear.

Polyethylene wear

Normal loading of the polyethylene cup comes up thefemoral shaft, along the femoral neck towards the lumbarspine.So it is normal to see slight thinning in the area of theweight bearing as the plastic moulds itself. This remouldingof the cup is called creep.Abnormal loading leads to pressure more lateral, resultingin polyethylene wear on the supero-lateral side.

Infection

Radiologic findings in patients with low grade infection maybe unremarkable or may mimic loosening or small particledisease.With more aggressive organisms, progression can be rapid,with bone destruction and sinus tract formation, resultingin radiological findings as listed in the table on the left.

Uniform criteria for the diagnosis of infection associatedwith prostheses have not been established.In several studies infection was diagnosed if at least one ofthe 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 infectionwith irregular bone destruction and periosteal reaction.In many cases however the infection is really low gradeand difficult to establish.

Radionuclide bone scans are very sensitive for infection,but not specific as they may show findings similar to thoseoccurring in loosening.Negative findings on a bone scan suggest that no infectionexists.The role of dedicated radionuclide techniques for infectionsuch as gallium scanning or indium-labeled WBC or im-munoglobulin G is not clear, but they tend to be a bit morespecific compared to normal Technetium bone scan.Most researchers advocate fluoroscopic or sonographicguided joint aspiration to assess infection.Several samples should be taken to minimize confusioncaused 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 seedingfrom 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 surgeonduring revision to aid in removal of the previously placedfemoral component.Fractures during follow up are a result of loosening, particledisease, 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 includingpoor muscle tone or trauma or because of surgical factorssuch as a posterior (rather than lateral) surgical approach.Another factor is difficulty in achieving ideal angulation ofthe 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 theacetabulum 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 thepolyethylene liner.The metal backing is still intact.

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

Heterotopic Ossification

The classification of heterotopic ossification includes fourgrades 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 opposingbone surfaces.Grade III = bone spurs leaving Grade IV = radiographicankylosis of the hip.

Heterotopic Ossification occurs when primitive mesenchy-mal cells in the surrounding soft tissues are transformedinto osteoblastic cells, that form mature lamellar bone.It typically occurs around the femoral neck and adjacent tothe greater trochanter and occurs in 15-50% of patients.

Many patients with radiographically low-grade heterotopicossification 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 infectionArthrography plays a role in the evaluation of possible in-fection.Its value in the evaluation of possible loosening and painfulhips is limited.The arthrogram is used to confirm intra-articular position ofneedle and fluid is aspirated for aerobic and anaerobic cul-ture.The sensitivity for infection is 66-90%.

Arthrography and looseningLack of abnormal contrast extension does not exclude loos-ening as fibrosis and cells may fill the interfaces preventingcontrast passage.In non-cemented THA arthrography is not accurate for thedetection of loosening, as small channels between bone in-growth may persist allowing contrast-passage in stablehips.In cemented THA contrast extension at the bone-cementinterface 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-articularpressure cannot be established (figure).

Arthrography and painfull hipsSometimes arthrography is used to find out, if the patientssymptoms subside by putting in a long lasting local anaes-thetic.This is to see if the painful hip is due to the prosthesis andnot 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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Health & Medicine

The radiology assistant hip arthroplasty