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adiographic Imaging of Hip Replacement Hardwareatherine C. Roberts, MD,* and Felix S. Chew, MD, MBA†

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ip joint replacement surgery has been the most com-mon type of elective and semi-elective orthopedic op-

ration for nearly 25 years. As the science and engineering ofmplants have evolved, their appearances on imaging and therequency of common complications have also evolved. Ourbjectives in writing this article are to explain the postoper-tive appearance of hip joint replacement hardware in termsf materials, design, and function, and to review the imagingppearance of common complications. Our emphasis will ben radiographic imaging because that modality remains theainstay of follow-up and evaluation.

ndications for Hip Replacementor most joints, the indication for replacement is articularisease involving that joint, most commonly osteoarthritisnd rheumatoid arthritis. In these cases, the goal of jointeplacement is twofold: first, the restoration of joint function,nd second, the elimination of arthritic pain. Joint replace-ent has been proven to be highly efficacious in meeting

hese goals for most patients with arthritis.1-3 At the hip,dditional common indications include femoral head dis-ase, such as osteonecrosis, and femoral neck fractures, suchs displaced subcapital fractures, where the risk of posttrau-atic osteonecrosis is high.2-4 In the latter circumstance, hip

eplacement may not really be elective, as the morbidity andortality from hip fractures in elderly patients who are not

reated with hip replacement are both high. Patients withongenital hip disease can benefit from hip replacement.3 Aip replacement may also be one component of a larger re-onstruction following tumor treatment.4

aterials Usedn Hip Replacementhe principal material used in joint replacements is metal.he types of metal that are commonly used for implant com-

Assistant Professor of Radiology, Radiology Department Education Coor-dinator, Mayo Clinic College of Medicine, Scottsdale, AZ.

Professor of Radiology, University of Washington, Seattle, WA.ddress reprint requests to Catherine C. Roberts, MD, Radiology Department

Education Coordinator, Mayo Clinic College of Medicine, 13400 E. Shea
mBoulevard, Scottsdale, AZ 85259. E-mail: [email protected]
20 0037-198X/05/$-see front matter © 2005 Elsevier Inc. All rights reserved.doi:10.1053/j.ro.2005.01.019

onents are pure titanium and titanium alloys (titanium–luminum–vanadium), and cobalt–chromium alloys. Obvi-usly, all metal components are radiopaque on radiographs,ut titanium is less dense than cobalt–chromium, and bothre less dense than lead. (The atomic numbers are 22 foritanium, 24 for chromium, 27 for cobalt, and 82 for lead.)he exact appearance on radiographs will depend on theadiographic technique. Titanium alloys are more commonlysed in implants of recent vintage, but since the average life ofjoint implant easily exceeds 10 years, there are many pa-

ients being followed who have cobalt–chromium implants.tainless steel or titanium may be used for screws and wash-rs.

Bone cement is commonly used to fix components to boner to fill voids in bone to provide a closer fit with compo-ents. Bone cement is an acrylic plastic (polymethylmethac-ylate) that is polymerized in situ at the time of implant.ecause plastic is a hydrocarbon with the same primarytomic constituents as fat (carbon and hydrogen), it is radi-lucent in radiographs. However, bone cement is typicallyixed or premixed with barium powder to render it ra-iopaque, and thus it is radiopaque on radiographs, but lessense than metal. Bone cement can be used as both an adhe-ive as well as a space-filler.

Polyethylene is commonly used as a bearing surface, sohat most total joint replacements have a convex metal com-onent articulating with a concave polyethylene liner. Theype of polyethylene used in joint replacements is not theommon, pliable material used in kitchenware or other lightonsumer products, but it is an ultrahigh molecular weightaterial (molecular weight of 3 to 6 million Daltons) that is

lso used for bullet-proof vests and as the lining for skatinginks and ship docks. A high degree of crosslinking providesncreased strength and resistance to surface abrasion.

Ceramics have become more common as prosthetic femo-al heads and prosthetic acetabular bearing surfaces. The twoaterials in use are Zirconia,5 a strong material with an elasticodulus similar to metals that is more widely known for faux

ewelry, and alumina, a hard and rigid material that is moreidely known as an ingredient for antacids.

ixation of Hip Replacementsixation of components to bone may occur through direct
echanical fixation, passive interference fit, bone cement,

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Radiographic imaging of hip replacement hardware 321

nd porous ingrowth.6 Methods of direct mechanical fixationuch as screws are generally obvious on radiographs. Passiventerference fit or press fit components are held in position byhe shape of the components and the space into which theyre tightly fitted. Bone cement may be used as an adhesive,iterally gluing the component to bone, or it may be used toll spaces and contribute to a closer interference fit. Porous

ngrowth fixation is based on the principle that remodelingone can attach itself directly to the component, holding it inlace. The key to porous ingrowth is providing a surface thatone can grow into. Special surface treatments that have beensed include layers of small metallic spheres, fine wire mesh,nd hydroxyapatite crystals. On radiographs, it is difficult ifot impossible in most cases to identify the specific type ofurface treatment.

ypes of Hip Replacementshere are two general types of hip replacement: hemiarthro-lasty and total hip arthroplasty. The clinical indications forach type are different, and these general types can be iden-ified from radiographs.

Hemiarthroplasty is an operation that replaces one surfacef a joint. In the case of the hip, it is always the femoral sidehat is replaced, so that the resulting articulation is between a

igure 1 Austin–Moore unipolar hemiarthroplasty. Femoral headnd neck are replaced with single-piece prosthesis that articulatesith native acetabulum. This prosthesis is loose, as seen by lateral

ilt of prosthesis tip (black arrow) and focal lucency at prosthesis–one interface along proximal, lateral aspect of remaining nativeemur (white arrowheads).

rosthetic femoral head and the native acetabular cartilage. a

igure 2 Bipolar hemiarthroplasty. Two-piece prosthesis replacingemoral head and neck articulates with native acetabulum. Unlikenipolar arthroplasty, bipolar prosthesis has an additional region ofotion between prosthetic neck and prosthetic head. Similar to therst case, this prosthesis is loose, with lateral tilt of the prosthesis tipblack arrow) and lucency at cement–bone interface along the prox-mal, lateral aspect of the remaining native femur (white arrow-

igure 3 Femoral head resurfacing arthroplasty. Damaged portionsf femoral head have been removed and femoral head remodeled to
ccept a thin metal cup as a new articulating surface.

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322 C.C. Roberts and F.S. Chew

emiarthroplasty at the hip is generally performed for fem-ral head disease, not hip joint disease. Common clinicalndications for hip hemiarthroplasty include osteonecrosis ofhe femoral head (before secondary osteoarthritis has devel-ped), displaced subcapital fracture of the femoral neck, andesection for tumor. Hemiarthroplasty can be recognizedrom radiographs because the native acetabular cartilage wille present. One surgical strategy for hemiarthroplasty is re-lacement of the femoral head and neck by a one-pieceunipolar) prosthesis that is inserted into the medullary canalf the proximal femur (Fig. 1). In this operation, the femoralead and neck are resected and the prosthesis is secured tohe proximal femur by cement or press fit. A more advancedype of hemiarthroplasty is called the bipolar hip replace-ent, in which a two-piece prosthesis is inserted into theedullary canal of the proximal femur (Fig. 2) following

emoval of the femoral head and neck. The bipolar prosthesis

igure 4 Total hip arthroplasty with cemented femoral and acetab-lar components.

as a femoral stem and a separate head, so that two articula- t

ions result: an articulation between the prosthetic head andhe native acetabulum, and an articulation between the twoomponents of the prosthesis. Both the unipolar and theipolar arthroplasty are commonly used today. Recent stud-

es recommend the use of unipolar prostheses instead of bi-olar prostheses in elderly patients with femoral neck frac-ures.7,8 A third type of hemiarthroplasty is one in which therticular surface of the femoral head is replaced by a metalup (Fig. 3). This is referred to as a cup arthroplasty or aemoral head resurfacing arthroplasty. Arthroplasties such ashese have been in use since the 1920s.

A total hip arthroplasty (THA) is an operation that replacesoth surfaces of the hip joint. The clinical indication for totalip arthroplasty is typically hip joint disease such as osteoar-hritis or rheumatoid arthritis. Patients with osteonecrosis areore commonly treated with hemiarthroplasty but total hip

rthroplasty is also an effective treatment.9 Specifically, pa-ients with Ficat grade III osteonecrosis have a better clinicalutcome when treated with a total hip arthroplasty rather

igure 5 Total hip arthroplasty with noncemented femoral and ace-
abular components.

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igure 6 Hybrid total hip arthroplasty. Femoral component is ce-
ented, while acetabular component is cementless.
Fylene liner to separate femoral and acetabular components.

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igure 7 Ceramic femoral head portion of THA. Ceramic head isore radiolucent than remainder of metallic femoral and acetabular

omponents. m

igure 8 Metal-on-metal total hip arthroplasty. There is no polyeth-

igure 9 Surface replacement arthroplasty. Damaged portions of ac-tabulum and femoral head have been removed and lined with
etal cups.

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han a bipolar arthroplasty.10 Because both surfaces of theoint are replaced, the resulting articulation is always be-ween the femoral and acetabular components of the pros-hesis. Total hip replacements can be recognized from radio-raphs by the presence of both femoral and acetabularomponents. The most common type of total hip replace-ent is one in which the femoral head and neck are replaced

igure 10 Acetabular reconstruction cage. Acetabulum has been re-ised with cage device to correct protrusio deformity, thus restoringnatomic positioning for femoral component of hip replacement.

igure 11 Modular hip prosthesis. Custom length prosthesis re-
laces proximal femur, which was resected for osteosarcoma. w
igure 12 Periprosthetic fracture. Subtle longitudinal fracture line iseen extending distally beyond tip of femoral component (arrow-eads). This fracture was noted at time of surgery and was stabilized

able 1 Potential Complications of Total Hip Arthroplasty

EarlyNerve palsy: sciatic, femoral, peronealHemarthrosisThromboembolism: deep venous thrombosis, pulmonary

embolismVascular injuriesHemorrhage and hematoma formationBladder infectionLimb length discrepancyFracture

LateLooseningStem failureOsteolysis (small particle disease)Heterotopic ossificationStress fractures: femur, pubisFracture of hardwareImplant sarcoma

EitherInfection of jointDislocation

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y a prosthesis that fits inside the medullary canal of theroximal femur, and the acetabulum is replaced by a metalup with a polyethylene liner. The articulation is thus metaln polyethylene. The components may be fixed to bone usingemented or cementless methods (Figs. 4 and 5). The com-ination of a cemented femoral component with a nonce-ented acetabular component is called a hybrid THA (Fig.

). Some total hip replacements use ceramic-bearing surfacesFig. 7), and recently, the use of metal-on-metal hip prosthe-is has become more widespread in the United States (Fig. 8).otal hip replacement by resurfacing both the acetabular and

he femoral sides of the joint may also be done. When bothhe femoral and the acetabular articular surfaces are reshapednd lined by thin metal cups, the procedure is referred to assurface replacement arthroplasty11 or a “double cup” ar-

hroplasty (Fig. 9).Specialized techniques and hardware may be used in cir-

umstances where the native bone stock is insufficient fortandard techniques. These circumstances include revisionsf failed arthroplasty, developmental hip dysplasia, tumoresection, and severe protrusio acetabuli in rheumatoid ar-hritis. Autograft or allograft bone may be used to buttress thecetabulum or the proximal femur.12 Autograft bone is com-only derived from the femoral head and neck that have

een resected during the course of hip arthroplasty. Allograftone is typically obtained from a bone bank and is usuallybtained from cadavers. Acetabular reconstruction cages areomponents that are used to reconstruct acetabular beds that

Figure 13 A and B. Dislocated THA. Femoral compo

re deficient in bone stock (Fig. 10). Patients with severe d

rotrusio acetabuli13 from conditions such as rheumatoidrthritis or patients with marked bone loss from a failed ac-tabular component are typical candidates for acetabular re-onstruction. Following the acetabular reconstruction, atandard total hip prosthesis may be implanted. When theroximal femoral shaft is resected along with the femoralead and neck, a modular prosthesis can be used for recon-truction. The proximal end of this prosthesis may be anipolar, bipolar, or total hip arthroplasty, while the distalortion that replaces the missing segment of femoral shaftonsists of modules customized for the individual patient.he distal portion has an intramedullary stem that fits into

he medullary canal and is cemented into place (Fig. 11).

omplicationsomplications related to hip replacements are mostly related

o fractures, dislocations, component loosening, componentailure, and infection. They may occur early during the hos-ital admission or years after the surgery (Table 1). Fracturesf the proximal femoral shaft may occur as a complication ofnsertion of the stem of the femoral component during sur-ery, particularly if it is a press fit component.14 Most of theseractures are non-displaced and are often recognized at theime of surgery. Cerclage cables can be used to secure theracture fragments (Fig. 12). In some cases a nutrient arteryanal near the tip of a femoral stem can be confused withracture.15 Comparison with preoperative imaging can help

es anterior and superior to acetabular component.

ifferentiate between an acute fracture and a vascular groove.

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eriprosthetic fracture after surgery is most often secondaryo prosthesis loosening, occurring at the tip of the femoralndoprosthesis. Fractures occurring after sports-related inju-ies are rare,16 likely due to the more sedentary nature of theopulation receiving the implants. Periprosthetic fracturesften require revision of the hip arthroplasty.17

Dislocations are more common in total hip replacementhan in hemiarthroplasty and occur typically between theemoral component and the acetabular component. Theroximal femur tends to rotate externally, and the head of therosthesis dislocates anteriorly (Fig. 13). Recurrent disloca-ions often indicate malposition of components that requireevision.18 Dislocation of the hip during transfer from theperating table to the gurney for transport to the recoveroom may occur as an early complication. Postprocedureadiographs that are typically performed in the recoveryoom can exclude this possibility and document the imme-iate postsurgical appearance of the hip.Loosening of components may occur from the cyclic me-

hanical loading that occurs during weight bearing or may

igure 14 Osteolysis and polyethylene wear. Femoral component isccentrically located in acetabular cup, indicating wear of polyeth-lene liner. Wide regions of lucency surround acetabular compo-ent and proximal femoral component likely secondary to a granu-

omatous response to polyethylene particles.

esult from biologic action. As implanted, joint replacement o

aterials are biologically inert. However, repetitive mechan-cal stress on the surface of bearing surfaces causes the abra-ion of small particles. These particles are released from thertificial joint surfaces into the joint capsule, where they mayncite a foreign body granuloma response. The particles thatave been particularly implicated in this granulomatous re-ponse are particles of polyethylene. This granulomatous re-ponse results in the resorption of bone at its interface, andhis resorption may be visible on radiographs as osteolysis ift is extensive enough (Fig. 14). Osteolysis may be evident asregion of lucency around a component at its interface withone; this may be seen at the cement–bone interface in thease of a cemented prosthesis or at the metal–bone interfacen the case of a noncemented prosthesis. Locations of osteol-sis surrounding arthroplasty components can be describedsing the seven femoral zones described by Gruen and threecetabular zones (Fig. 15).19 As the zone of osteolysis extendslong the interface between bone and component, the com-onent loses its mechanical attachment. Once gross motion isossible, the zone of osteolysis may become progressively

arger, and often a fine zone of sclerosis may form at theargins of the cavity. This process may occur slowly over

everal years. In general, a zone of osteolysis greater than 2m around a component is abnormal20 and suggests loosen-

ng from foreign body granulomatous reaction. Any gap be-

igure 15 Periprosthetic zones used to describe location of osteolysis
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ween metal and cement is abnormal. The presence of oste-lysis does not necessarily correlate with clinical symptoms,nd clinical symptoms are the usual indication for revision,ot radiographic appearance. Because polyethylene particlesave been implicated in osteolysis, research has focused ex-ensively on how to reduce the generation of these particles.21

he results of such research have included improved forms ofolyethylene with greater resistance to abrasive wear as wells implants that do not use polyethylene.22,23 These includemplants with ceramic-on-ceramic bearing surfaces as well as

etal-on-metal bearing surfaces (Fig. 8).Infections may occur in the immediate postoperative pe-

iod or at a subsequent time. There are no specific radio-raphic features that indicate early infection, but dislocationay be associated with large effusions. In some cases, intra-

rticular foci of air, beyond the immediate postoperativetate, suggest the presence of infection (Fig. 16). Periostealeaction can also suggest infection.23 Imaging-guided arthro-entesis is the best radiological method for identifying infec-ion. In most cases, this may be performed under fluoros-opy. Following informed consent, the patient is placed onhe fluoroscopy table in the supine position. The hip capsuleill extend from the native bone of the acetabulum to the

igure 16 Infected THA. Small foci of intraarticular gas (arrowheads)re only radiographic indication of Streptococcus milleri infection inhis patient.

ative bone of the proximal femur, encompassing all of the n

xposed components in between (Fig. 17). The easiest targeto hit is the prosthetic femoral neck, right in the middle.ollowing localization of this target under fluoroscopy andreparation of the skin with cleansers and local anesthetic, an8- or 20-gauge spinal needle can be advanced through theoft tissues until it touches metal. At that point, fluid withinhe joint may be aspirated. If no fluid is aspirated, a smallmount of contrast may be instilled into the joint to confirmocation. Washing the joint with an injection of 10 mL ofreservative-free saline followed by aspiration will provideaterial for microbiological analysis, but cultures of fluid

rom saline wash when the initial aspiration is dry are rarelyositive. Because the majority of injected fluid will accumu-

ate posteriorly, it is generally possible to re-aspirate only 1 tomL. Flexing the hip while aspirating may improve the yield.he definitive method for identifying infection and the caus-tive agents is considered to be surgical biopsy of the syno-ium, a procedure typically beyond the scope of the radiol-gist. It is sometimes possible to obtain a small piece ofynovium using a 22-gauge Sure-Cut (Bauer Medical, Clear-ater, FL) biopsy needle placed through the spinal needle.his small piece of tissue can be sent for culture and sensi-

ivity.Confirmed infections require surgical debridement and

rolonged antibiotic therapy. Because it is difficult to achievehe requisite concentrations of antibiotics when they are

igure 17 Aspiration of THA. Conventional subtraction image withntraarticular contrast (black) shows joint space extending from
ative acetabular bone to native femoral bone.

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iven systemically, methylmethacrylate beads that have beenmpregnated with antibiotics can be placed directly into theurgical bed for better results (Fig. 18). In most cases, therosthesis itself must be removed (or retrieved24 as the sur-eons refer to it). With the prosthesis gone, patients are notble to bear weight on the affected extremity. To preventoft-tissue contractures and preserve the limb for possibleeimplantation, spacers fabricated from polymethylmethac-ylate may be placed into the joint bed. The PROSTALAC®Prosthesis of Antibiotic-Loaded Acrylic Cement) device is aemporary implant embedded with antibiotics tailored to thenfecting agent (Fig. 19). This implant acts to help treat thenfection and to maintain function of the hip between re-

oval of the infected hardware and revision of the hip whenhe infection has resolved. Infections may require treatmentor many months before definitive reconstruction can be per-ormed. The ultimate outcome of infected hip replacements

ay be reconstruction with another prosthesis, surgical ar-hrodesis of the joint, or permanent pseudoarthrosis of theoint (Fig. 20).

Heterotopic bone may form in the surgical bed follow-ng total hip replacement. Although small amounts of het-rotopic bone formation are unimportant, large amountsf heterotopic may interfere mechanically with motion athe hip and require resection (Fig. 21). Heterotopic bone

igure 18 Antibiotic beads. Polymethylmethacrylate beads are im-regnated with antibiotics and placed at site of infection, in this caselong side plate with dynamic compression screw fixating subtro-hanteric fracture.

ormation appears to be more common in patients who w

ave other evidence of excessive bone formation such asypertrophic osteoarthritis or diffuse idiopathic skeletalyperostosis.25,26 Therapeutic irradiation of the surgicaled following surgery may prevent recurrence of hetero-opic ossification.27

igure 19 PROSTALAC implant. This temporary antibiotic-impreg-ated implant is placed after removal of infected hardware.

igure 20 Permanent pseudoarthrosis (Girdlestone procedure) ofeft hip joint following infection. Femoral head has been removed,
ithout replacement.

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Fracture of the greater trochanter may occur during sur-ery or as a delayed complication. Because these fracturesisplace widely, they are virtually impossible to reduce andx, so they are generally left ununited (Fig. 22). The gluteusedius, one of the strong abductors of the hip, inserts on the

reater trochanter. Its loss of action results in a permanentimp.

igure 21 Heterotopic ossification. Prominent heterotopic bone hasormed adjacent to each THA, which likely limits range of motion.

igure 22 Ununited greater trochanter fracture. Greater trochanter
arrow) has been left unattached to femur. s
pproach toadiographic Reporting

adiographs in follow-up of hip replacements have two gen-ral functions, documentation of the procedure and the con-ition of the joint, and surveillance for complications. Jointeconstruction surgeons typically follow their patients sev-ral times in the first year following an implant, and thennnually after that. At the time of the first perioperative set ofadiographs, it is appropriate to identify the type of prosthe-is and to search for immediate complications or problems.ecause these radiographs are typically obtained using por-able equipment in the recovery room, their quality may beubstandard. At our institutions, AP pelvis and true lateralip radiographs are obtained, allowing assessment of thelacement of the components. On the AP radiograph, onean assess the orientation of the acetabular cup in the coronallane of total hip replacements. The angle between a linerawn across the opening of the acetabular cup and a hori-ontal reference line drawn tangential to the ischial tuberos-ties is called the lateral opening angle and typically measuresetween 30 and 50° (Fig. 23). On the true lateral projection,he angle between a line drawn against the opening of thecetabular cup and a vertical reference line is called the de-ree of anteversion and typically measures between 10 and5° (Fig. 24). The position of the center of rotation of the

igure 23 Normal postoperative total hip replacement shows mea-urement of lateral opening angle on AP pelvis.

igure 24 Normal postoperative lateral view of total hip replacement
howing measurement of degree of acetabular anteversion.

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rosthesis should be anatomic, and limb length should beymmetric with the other side. In some patients who have aHA subsequently revised, an extended trochanteric osteot-my28,29 may be necessary to remove the original femoralomponent. It is important not to confuse the osteotomy withfracture (Fig. 25).At the time of subsequent follow-ups, one should note the

esults of a comparison with the initial postoperative exami-ation and search for evidence of complications. In the earlyollow-ups, one should look specifically for fractures; in sub-equent follow-ups, it is important to search for the compli-ations described above, including asymmetric polyethyleneear (Fig. 14), osteolysis, loosening, heterotopic ossification,

ubsidence (Fig. 26), textured coating disintegration (Fig.7), and stress-related changes (Fig. 28). If there has been nohange in appearance and there is no evidence of complica-ions, it is sufficient to simply state that there has been nohange in appearance and that there is no evidence of com-lications.Table 2 summarizes the radiographic analysis of every total

ip arthroplasty.

igure 25 Laterally located linear lucency of extended trochanteric
Ssteotomy (arrow) should not be confused with fracture.
onclusionip replacement surgery has been one of the most common

lective and semi-elective orthopedic operations for the past5 years. Radiography remains the mainstay of imaging eval-ation of hip replacements. The various types of hip replace-ents can be recognized on radiographs, and many compli-

ations have specific appearances that the radiologist shoulde familiar with.

cknowledgementse extend special thanks to William W. Daniel, MD, and F.

igure 26 Right THA with cemented femoral prosthesis and nonce-ented acetabular prosthesis. Femoral prosthesis has subsided into

emur approximately 1.5 cm (arrow). Regions of lucency at cement–one interface (arrowheads) are consistent with loosening fromranulomatous small particle disease. Acetabular prosthesis is supe-iorly located and also appears loose.

pencer Chivers, MD, for contributing cases.

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able 2 Radiographic Analysis of Total Hip Arthroplasty

Is the entire prosthesis imaged?Check for acetabular versionCheck for polyethelene wear. Is femoral head centrally

located within acetabular prosthesis?Check for loosening

Widening of the lucent zone at the cement-to-boneinterface >2 mm

Widening of the lucent zone at metal-to-bone interface>2 mm

Migration of components from their original positionsDevelopment of a lucent gap between metal and

cementCement fracturePeriosteal reactive boneOsteolysis

igure 27 Textured coating of implant is disintegrating and free par-

igure 28 Stress fracture. Laterally located periosteal reaction alongemoral shaft, between tip of THA and total knee arthroplasty, in-

revision. J Bone Joint Surg Am 85:2156-2162, 2003

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replacements: lessons learned from retrieval studies. Clin Orthop420:10-17, 2004

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radiographic imaging of hip replacement hardware · treated with hip replacement are both high....

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