AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

INTRODUCTION

Section 2 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Background

Arthroplasty of the hip may be categorized as a total hip arthroplasty or a hemiarthroplasty. In a total hip arthroplasty, the articular surfaces of both the acetabulum and femur are replaced. This involves either replacement of the femoral head and neck (conventional total hip arthroplasty) (see Images 1-7) or replacement of the surface of the femoral head (resurfacing total hip arthroplasty) (see Image 8); both procedures also replace the acetabulum. 

In contrast to a total hip arthroplasty, a hemiarthroplasty involves replacement of the articular surface of the femoral head without surgical alteration to the acetabular articular surface. This may involve replacement of the femoral head and neck (unipolar hemiarthroplasty) (see Images 9-10), replacement of the femoral head and neck with an additional acetabular cup that is not attached to the pelvis (bipolar hemiarthroplasty) (see Images 11-12), or replacement of the surface of the femoral head (resurfacing hemiarthroplasty) (see Image 35). With a bipolar hemiarthroplasty, there is normal motion between the femoral head and acetabular cup, and between the acetabular cup and native acetabulum (see Image 13). 

The hip joint may be replaced with a variety of materials, including metal, polyethylene, and ceramic. There are also various methods of arthroplasty fixation, such as polymethylmethacrylate (PMMA) cement and screw fixation, although cementless press fit and porous ingrowth arthroplasties may also be used. Arthroplasty materials and fixation are discussed in the Anatomy section, below. 

Imaging of a hip arthroplasty and its complications primarily relies on the information that is obtained from routine radiography, although there are specific roles for other imaging techniques, such as arthrography, computed tomography (CT) scanning, magnetic resonance imaging (MRI), ultrasonography, and nuclear medicine. 

For excellent patient education resources, visit eMedicine's Arthritis Center and Foot, Ankle, Knee, and Hip Center. Also, see eMedicine's patient education article Total Hip Replacement.

Related eMedicine topics:
Acetabulum Fractures
Fractures, Hip
Osteonecrosis, Hip 

Related Medscape topics:
Resource Center Fracture
Resource Center Joint Disorders
Resource Center Rheumatoid Arthritis
CME Glucosamine May Be No Better Than Placebo for Hip Osteoarthritis
CME/CE New Guidelines Issued for Management of Hip and Knee Osteoarthritis

Pathophysiology

Common indications for a total hip arthroplasty include conditions that affect both the articular surfaces of the acetabulum and femur, such as osteoarthritis. A resurfacing total hip arthroplasty is considered in younger patients with osteoarthritis and good bone stock (ie, no osteopenia or excessive bone loss). A hemiarthroplasty is commonly used when avascular necrosis of the femoral head is present, or if there is a displaced femoral neck fracture with significant risk of developing avascular necrosis, without hip joint osteoarthritis. 

Complications of hip arthroplasty include implant fracture, dislocation, mechanical loosening, infection, heterotopic bone formation, and particle disease (also termed aggressive granulomatosis, which represents a foreign-body reaction to implant debris that causes focal osteolysis).¹

Related eMedicine topics:
Acetabulum Fractures 
Avascular Necrosis, Femoral Head
Heterotopic Ossification 
Osteoarthritis
Osteonecrosis, Hip

Related Medscape topic:
Specialty Site Orthopaedics

Frequency

United States

More than 120,000 total hip arthroplasties are performed annually.²

International

Worldwide, approximately 800,000 total hip arthroplasties are performed annually.²

Mortality/Morbidity

A 3% prevalence of prosthetic loosening is observed at 11 years after hip replacement, and there is a 1% prevalence of prosthetic infection.

Anatomy

Arthroplasty Components

Acetabulum components include the following:

• Polyethylene (with or without metal backing) (see Images 1-4 and 11-13)
• Cobalt-chromium metal (see Image 5)
• Ceramic (see Images 6-7).

• Metal femoral stem (cobalt-chromium or titanium alloy) (see Images 1-13)
• Metal (see Image 5) or ceramic (see Images 6-7) femoral head. 

Preferred Examination

Radiography is the primary imaging method for the evaluation of hip arthroplasties (See Radiograph, below).

Limitations of Techniques

Specific issues exist that are related to the choice of the hip arthroplasty material that is used and to the type of the arthroplasty itself.

The most common scenario for a replaced femoral head and acetabular articulation in a total hip arthroplasty is to use a cobalt-chromium alloy metal femoral head and a polyethylene cup (with metal backing) articulation. A complication related to this combination is polyethylene wear and subsequent inflammation and osteolysis from particle disease. In order to reduce component wear, ceramic femoral heads have also been used with a polyethylene acetabular cup, as well as a combination of a ceramic head and a ceramic cup. Although not exclusive to ceramic-on-ceramic articulation, patients have complained of "squeaking" during ambulation.³ Ceramic head components may also fail or fragment. 

There has been a renewed interested in metal-on-metal articulations with the use of a larger femoral head component, which has the advantage of a lower frequency of hip dislocations. The metal-on-metal articulation also has less wear compared with that of polyethylene components, and the particles are smaller and associated with less inflammation. Of concern, however, are reports that chromium and cobalt metal ions have been found in the blood and urine after metal-on-metal arthroplasties. Although a study by Miller et al quoted an increased incidence of leukemia from such procedures, there were no statistical differences between the metal-on-metal and the metal-on-polyethylene articulations.⁴

With regard to the type of arthroplasty, resurfacing metal-on-metal arthroplasties have been considered for younger patients who have osteoarthritis and normal proximal femur bone quality.⁵ The advantage of this surgery is that the femoral neck is preserved, which may be advantageous to the patient, who may later need a conventional arthroplasty. Patient selection is key to the success of resurfacing arthroplasties. Thinning of the femoral neck has been described after this type of surgery (possibly related to stress shielding), although this finding is not associated with failure.⁵ The incidence of femoral neck fracture after hip resurfacing is approximately 1.25%.⁵

DIFFERENTIALS

Section 3 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Other Problems to Be Considered

Acetabulum Fractures
Avascular Necrosis, Femoral Head
Femoral Neck, Fractures
Fractures, Hip
Immune Response to Implants
Infection
Intertrochanteric Hip Fractures
Osteoarthritis
Osteonecrosis, Hip
Total Joint Replacement Rehabilitation

RADIOGRAPH

Section 4 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Postoperative Assessment

Radiographs are essential for the evaluation of hip arthroplasties. It is important that the entire prosthesis is included on 2 orthogonal radiographs of acceptable technique. When evaluating the acetabular component on the frontal view, there is normally 30-50° of lateral inclination (see Image 17).⁶ On a cross-table (Manfredi) or true lateral radiograph, there is normally 5-25° of anteversion (see Images 18-19).⁶ The femoral component should be assessed for symmetry relative to the contralateral normal hip, when present. In the vertical direction, the center of the femoral head is assessed relative to the ischial tuberosities and the greater trochanter (see Image 20).⁶ In the horizontal direction, the center of the femoral head is assessed relative to the lateral margin of the acetabular tear drop (see Image 21).⁶

Normal Radiographic Findings

With a cemented prosthesis, normal radiographic findings include a lucency that is less than 2 mm thick at the bone-cement interface; the lucency represents fibrous tissue and is outlined by a thin, sclerotic demarcation line (see Image 22).⁷ A lucency at the metal-cement interface of a cemented arthroplasty and one at the metal-bone interface of a cementless arthroplasty are typically related to surgical technique. These lucencies are normal if they are stable over time, but they generally should be less than 2 mm thick (see Image 23). Lucencies should be followed up on radiographs because progression can indicate loosening. 

Another normal finding at porous ingrowth surfaces is the presence of focal sclerosis or spot welds (see Image 15).⁷ Bone resorption may also be seen beneath the femoral flange with a cementless femoral component and with a cemented femoral component, in which a lucency may be up to 4 mm thick (see Image 24). Focal osteopenia of the trochanteric regions due to stress shielding (diverted stress causes bone resorption) is considered a normal finding when the femoral component is secure (see Images 25-26). 

As an isolated finding, sclerosis at the tip of a cementless femoral component, or pedestal formation, is of unclear significance (see Image 27). A radiopaque cement restrictor or centralizer may be used with cemented femoral components (see Image 14), and cables or wires may be used after a trochanteric osteotomy or after a total hip arthroplasty revision (see Image 16). 

Wire fractures occur in up to 33% of hips and are usually insignificant without greater  trochanteric displacement.⁸ However, fractured wires may cause an adjacent soft-tissue abnormality such as bursitis. A patient with more than 2 cm of trochanteric displacement may need a repeat operation. The normal radiographic findings described above can also be applied to other imaging modalities, such as CT scanning and MRI.  

Early Complications

Early complications include improper component placement, dislocation, and cement extrusion. Increased acetabular cup inclination and abnormal version—as well as a femoral component that is too long (causing muscle spasm)—may predispose the hip to a dislocation between the acetabulum and the femoral head (see Image 28). Most such dislocations occur in the immediate postoperative period. Varus angulation between the femoral stem and femoral diaphysis predisposes to femoral fracture at the femoral stem tip (see Image 29). Cement extrusion is typically asymptomatic (see Image 30).

Late Complications

Late complications include dislocation, hardware failure, fracture, heterotopic ossification, prosthetic loosening, infection, and particle disease. Causes of dislocation have been discussed above (Early Complications).

Hardware failure may consist of metal (see Image 31), ceramic (see Image 32), or polyethylene (see Image 33) component fracture and displacement. Osseous fractures may involve the greater trochanter (see Image 34), femoral neck (see Image 35), acetabulum (see Images 36-37), and femoral diaphysis; these fractures may be related to trauma, stress shielding (see Images 25-26), or component loosening.²⁰ 

Heterotopic ossification is usually asymptomatic, but it is seen in up to 39% of total hip arthroplasties⁸ and may begin by 2-3 weeks after surgery, with possible ankylosis by 12 weeks (see Images 38-39). 

Brooker and Bowerman classified heterotopic ossification as the following⁸:

Class 1 – Islands of bone in soft tissues
Class 2 – >1 cm gap in heterotopic ossification between the femur and pelvis
Class 3 – <1 cm gap
Class 4 – Bony ankylosis

Loosening

With a cemented component, loosening is suggested by the presence of component migration (see Images 40-41) or tilt (see Images 42-44) or a new cement fracture (see Image 45). With a cementless component, a femoral component subsidence that is >10 mm or an increased number of metal beads that are displaced from the surface of a bone ingrowth prosthesis over time (bead shedding) also indicates abnormal component motion and loosening (see Image 46).

A finding that is common to all types of component fixation is a >2 mm or progressive periprosthetic lucency (see Images 47-49), although this may be due to coexisting infection or particle disease (see discussion below). The location of periprosthetic lucencies can be described by their zones, which are based on anteroposterior and lateral hip radiographs. On an anteroposterior radiograph, the femoral zones are numbered 1-7, and the acetabular zones are referred to as I, II, and III (see Image 50). On a lateral hip radiograph, additional femoral zones are numbered from 8-14 (see Image 51).

Infection and particle disease

The identification of a lucency around a prosthesis (>2 mm or increasing lucencies) raises the clinical concern for an infection (see Image 52) or particle disease (also called aggressive granulomatosis) (see Images 53-54), with possible coexisting component loosening.^{6, 8} Infection, particle disease, and isolated mechanical loosening may appear similarly on radiographs. However, a diffuse lucency suggests mechanical loosening or infection; multifocal lucencies can suggest particle disease or infection. With mechanical loosening, a diffuse lucency around the femoral component can be seen with a pistoning effect (see Image 41), or focal lucencies may be seen at the proximal and/or distal aspects from a toggling effect (see Image 48). 

Evidence for polyethylene wear, which appears as an asymmetric position of the femoral head within the acetabular cup, is an important finding that also suggests particle disease (see Image 53). None of the above radiographic findings are specific for infection, and a normal-appearing radiograph does not exclude infection⁹; therefore, hip aspiration is indicated when excluding infection from the differential diagnosis. 

Arthrography

Arthrography is primarily used to document intra-articular needle placement during fluoroscopic arthrocentesis to exclude infection. Dedicated arthrography can also be performed to evaluate prosthetic loosening. Normally, intra-articular contrast medium extends from the rim of the acetabular cup to the intertrochanteric line (see Image 55); thus, intra-articular contrast extension at the bone-cement interface can indicate component loosening (see Image 56).⁷ However, lack of abnormal contrast extension does not exclude component loosening. Arthrography has also been shown to be unreliable in the evaluation of a noncemented hip arthroplasty. 

When filling of the bursae or cavities around the hip occurs during arthrography, irregularity of the margins may indicate infection.¹⁰ Although the iliopsoas bursa may communicate with the hip joint in approximately 10% of patients (see Images 57-58), in a series of painful hip arthroplasties, Berquist et al reported that contrast filling of the trochanteric bursa was the most common finding (see Image 59).¹⁰ 

Ultrasonography should be considered before the performance of a fluoroscopic aspiration of a hip joint in order to exclude infection and to screen for any adjacent and overlying soft-tissue abscesses; needle placement at fluoroscopy could theoretically be passed through an unsuspected soft-tissue abscess, thus contaminating a sterile joint.

Degree of Confidence

Radiography is reliable in the diagnosis of a dislocation, an osseous fracture, and hardware failure. Because abnormal lucencies around a prosthesis that are caused by an infection may appear similar to that which is seen with prosthetic loosening or particle disease, arthrocentesis is typically used to exclude a diagnosis of infection.

False Positives/Negatives

Both radiography and arthrography would not detect a soft-tissue infection; thus, a normal-appearing radiograph does not exclude the presence of an infection. Arthrography is not reliable in the evaluation of a noncemented hip arthroplasty. In addition, a normal arthrogram does not exclude the possibility of a loosening prosthesis.

CT SCAN

Section 5 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Although the initial evaluation of a hip arthroplasty should begin with radiography, there is a definite role for CT evaluation in several situations. When there is concern about infection, CT scanning is complementary to radiography in that CT scans can show soft-tissue abscesses (see Image 60). A significant role for CT scanning is in the evaluation of osteolysis that is related to particle disease (see Image 54). Although radiography is effective in identifying large, abnormal periprosthetic lucencies, CT scans better characterize and show the extent of the osteolysis.¹¹ With multiplanar reformation in multiple imaging planes, CT scans also display the location of the osteolysis and assess the status of the adjacent normal bone before surgery. Lastly, CT scans can show the location of fragmented or failed arthroplasty components and periprosthetic fractures (see Image 37), as well as assess the acetabular component version.

To reduce metal artifacts when imaging a hip prosthesis with CT scanning, it is important to optimize various technical parameters (see Image 7). The milliamperes (mAs) are increased (350-450 mAs in adults; up to 600 mAs if there are bilateral hip arthroplasties), but one must also take the radiation dose into consideration, especially if one is imaging children. Additional methods to reduce artifacts include the use of lower pitch settings (to reduce cone beam artifacts with multichannel scanners), narrow detector element collimation, increased peak killivoltage (kVp) (140 kVp), and a smoother image reconstruction algorithm (eg, use of a standard soft-tissue filter vs a bone filter).¹² The original data are reconstructed using 1.0-1.5–mm thick slices with a 50% overlap, and then multiplanar reformations are created using 1.5-2–mm thick slices in the coronal and sagittal planes. 

Degree of Confidence

As a complementary imaging study to radiography, CT scanning has a role in the evaluation of soft-tissue abnormalities and osteolysis. Artifacts can be reduced with optimized technical parameters.

False Positives/Negatives

Soft-tissue abnormalities immediately adjacent to a metal prosthesis may not be seen on CT scans due to the presence of artifacts, which can potentially cause a false-negative examination result; however, this depends on the quality of the image and the technician's success in reducing such artifacts.

MRI

Section 6 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

MRI has some limitations due to the artifacts produced by the prosthesis that may obscure the adjacent soft tissue and any bone abnormalities. However, the role of MRI in the evaluation of hip arthroplasty has been investigated.² Modifications to standard MRI sequences can improve imaging of a hip arthroplasty, including using thin sections or reducing the voxel size, increasing the frequency-encoding gradient strength, using spin-echo or fast spin-echo (FSE) sequences rather than gradient echo, using short TI inversion recovery (STIR) rather than spectral fat saturation, using broader band width, and using lower magnetic field strength, as well as imaging of the prosthesis with its long axis longitudinal to the static magnetic field.^{2, 13} 

In addition, because artifacts are most pronounced in the frequency-encoding direction, it is important to place the frequency-encoding gradient in a direction that is away from suspected pathology. Similar to CT scanning, MRI may show soft-tissue abnormalities, such as abscesses and bursae (see Images 61-62), as well as osseous abnormalities, such as osteolysis from particle disease. Component loosening appears as periprosthetic low T1 and high T2 signals, whereas particle disease will show low T1 and low to intermediate T2 signals.¹³

Degree of Confidence

Similar to CT scanning, artifacts that are produced by a prosthesis on MRI may obscure any adjacent soft-tissue and bone abnormalities. However, these artifacts can be reduced by optimizing the MRI's technical parameters. Radiography remains an important imaging method to evaluate hip arthroplasty.

False Positives/Negatives

Artifacts that are produced by a prosthesis may obscure the immediate adjacent soft tissue and any bone abnormalities, resulting in a false-negative MRI examination.

ULTRASOUND

Section 7 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Unlike CT scanning or MRI, with ultrasonography, the artifacts produced by metal occur deep to the prosthesis; therefore, periprosthetic fluid collections can be visualized with this modality. The plane of the ultrasound beam or the long axis of the transducer is positioned along the long axis of the femoral neck of the prosthesis. Often, a lower frequency transducer is needed to optimize the image resolution (<10 MHz); a curvilinear transducer or a linear transducer with a trapezoidal function is helpful to increase the field of view. The superficial contours of the arthroplasty and the adjacent acetabulum and femur allow identification, and the metal components will appear hyperechoic with posterior reverberation artifacts (see Image 63). The native bone of the acetabulum and femur will also appear hyperechoic but with posterior acoustic shadowing (see Image 63). 

A normal hip arthroplasty may show minimal hypoechoic tissue along the femoral neck component or no tissue at all.¹⁴ Abnormal fluid will appear anechoic or hypoechoic over the femoral neck component (see Images 64-65). Synovitis may also appear hypoechoic (see Image 66), but this condition is more variable in echotexture, with possible flow on color or power Doppler imaging.¹⁴ It is important to scan around the entire hip region to adequately assess for the possible presence of a soft-tissue fluid collection or bursae (see Images 67-68); in addition, imaging deep to the skin incision is very important as fluid collections often occur here. 

The patient may indicate a focal area of symptoms to guide scanning. Although the findings of infection are often nonspecific, extensive fluid collection with a pseudocapsule-to-bone distance of >3.2 mm, especially if it extends beyond the femoral neck area and there is hyperemia, is often a clue that infection may exist (see Image 69).¹⁵ In a patient who has a large body habitus, the ultrasonographic resolution decreases, and anechoic fluid may appear hypoechoic.¹⁴ 

Ultrasonography may be used in conjunction with fluoroscopy in the setting of infection to evaluate for soft-tissue abscesses and other extra-articular fluid collections.¹⁵ It is important to exclude an extra-articular fluid collection before fluoroscopic arthrocentesis because there is a theoretical risk of seeding a sterile joint by passing a needle through an overlying soft-tissue abscess. Ultrasonography can also be used to guide percutaneous needle aspiration of a joint or soft-tissue abscess, as well as guide injection or aspiration of a bursa. Ultrasound-guided injection of anesthetic agents and steroids deep to the iliopsoas tendon can be completed in the case of an iliopsoas tendon impingement from an acetabular cup (see Image 70).¹⁶

Another advantage of ultrasonography is the ability to dynamically assess the hip joint and the adjacent structures; this modality can evaluate any snapping or symptomatic condition that requires joint movement or unusual positioning.

Degree of Confidence

Significant joint effusions or extra-articular fluid collections can be identified easily with ultrasonography. In a patient with a large body habitus, it may be difficult to visualize or exclude a small joint effusion. In this setting, percutaneous joint aspiration is needed if the clinical suspicion for infection is high, preferably with the use of fluoroscopic guidance so that intra-articular needle placement can be confirmed with an iodinated contrast medium.

False Positives/Negatives

Infected and sterile joint effusions or soft-tissue fluid collections may appear similar on ultrasonograms. Aspiration is required in this setting. In a patient with a large body habitus, it may be difficult to visualize a small joint effusion and differentiate it from normal postoperative changes. Consider performing an arthrocentesis if the index of suspicion for infection is high.

False-negative and false-positive arthrocentesis results for infection have been described. One study found false-negative aspirations in 11 of 19 infected arthroplasties.⁷ Similarly, false-positive aspirations have been described in up to 21% of arthroplasties. Both clinical and radiographic evidence for infection can help to differentiate true-positive from false-positive cases; the type and quantity of an organism found in aspirates has not been determined to be helpful.⁷

NUCLEAR MEDICINE

Section 8 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Findings

Nuclear medicine studies have also been used to diagnose prosthetic loosening or infection of hip arthroplasties. A cemented component may normally demonstrate radionuclide uptake on a bone scan within 1-2 years (see Images 71-72). Increased tracer uptake after this period can indicate infection, prosthetic loosening (see Image 73), or fracture (see Image 74); the sensitivities range from 50-100%.⁷ With a cementless component, increased radionuclide uptake on a bone scan may persist secondary to bone ingrowth.⁴  In general, a negative bone scan suggests infection or component loosening is unlikely.   

Tracer uptake on a gallium scan that correlates with bone scan findings indicates infection with few false-positive results. Labeled white blood cell scans are more specific than gallium scans, but false-negative results are possible in these cases in the presence of chronic infection. To exclude cellulitis, the radionuclide uptake on labeled white blood cell scans should correspond to the bone scan findings; to exclude normal bone marrow, white blood cell scan uptake should not correspond to sulfur colloid uptake.

Fluorodeoxyglucose positron emission tomography (FDG-PET) scanning has a variable accuracy in the diagnosis of infection, ranging from 43-78%.⁴

Degree of Confidence

Tracer uptake on a bone scan indicates loosening of a prosthesis or infection with a 50-100% sensitivity. Concordant uptake on a bone scan and gallium or labeled white blood cell scan localizes the abnormality to the bone. Correlation with radiographs is important. Percutaneous aspiration is often required to confirm the presence of an infection.

False Positives/Negatives

The combination of gallium with a bone scan reduces false-positive results. Chronic infection, however, may produce false-negative results with labeled white blood cells.

INTERVENTION

Section 9 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Aspiration

Aspiration of the hip joint is often required to exclude the presence of a joint infection. This can be accomplished with either fluoroscopic or ultrasonographic guidance. The benefit of fluoroscopic guidance is the ability to confirm intra-articular needle placement with the use of an iodinated contrast medium. The benefit of ultrasonographic guidance is the portability and accessibility of this modality. However, ultrasonography may be technically difficult to perform in patients with a large body habitus because both the fluid and needle may be difficult to identify. The accuracy of a preoperative hip joint aspiration after arthroplasty in the diagnosis of infection has been reported to be as high as 96%.¹⁷

A complete assessment of a joint infection optimally utilizes both ultrasonography and fluoroscopy. Ultrasonography may be performed initially to screen for the presence of soft-tissue abscesses, bursae, and joint fluid. If a large joint effusion is identified, immediate ultrasonographic guidance can be used for percutaneous aspiration. If no joint effusion is present and there is significant concern about an infection, joint aspiration may proceed with either ultrasonography or fluoroscopy. However, heterotopic ossification around a hip arthroplasty may make ultrasound-guided aspiration difficult. 

Regardless of the imaging method that is used to guide percutaneous aspiration, it is important to consider using ultrasonography to evaluate the surrounding soft tissues. Extra-articular fluid collections would not be identified with the sole use of fluoroscopy. Additionally, there is the theoretical risk of passing a needle through a soft-tissue abscess, thereby seeding a sterile joint during percutaneous aspiration. Ultrasonography thus has a role in the evaluation for a soft-tissue abscess or fluid collection.

Ultrasound-Guided Joint Aspiration

With ultrasound-guided aspiration, the long axis of the transducer and needle are in the plane of the femoral neck or the oblique sagittal plane. Once the fluid is identified, the skin is marked at the inferior end of the transducer with an "X" (to indicate the puncture site), and a line is placed at the superior aspect (to indicate the plane of the transducer). The transducer is then removed and the skin is cleansed. The use of a sterile probe cover is optimal to avoid infection. The transducer is returned and an 18- or 20-gauge spinal needle with a stylet is placed approximately 1-2 cm in the soft tissues, directed from inferior to superior. 

It is important to identify the needle before advancing; the needle should only be advanced if the entire needle, including the tip, is visualized on the ultrasound image. Once this occurs, the needle angle can be adjusted as the needle is advanced superiorly and posteriorly to the femoral neck. As the needle makes contact with the prosthesis, the metal will feel like an extremely hard and smooth object. If no fluid can be aspirated, the needle can be repositioned along other areas of the femoral neck. Be aware that a hypoechoic synovitis may appear similar to a complex joint fluid on ultrasonograms.

Fluoroscopy-Guided Joint Aspiration

After the femoral artery is palpated and its course marked on the skin surface, the skin directly over the femoral neck is marked. An 18- or 20-gauge spinal needle with a stylet is then advanced perpendicular to the skin from anterior to posterior and makes contact with the femoral neck. This is an advantage over the oblique approach in which the needle is advanced superiorly and medially from the greater trochanteric region, where the needle may course anterior or posterior to the femoral neck. 

If the femoral artery is in close proximity to the needle puncture site, the patient may be rolled away from the ipsilateral hip. With this maneuver, the femoral artery, which is located anteriorly, moves medially away from the hip; then, without the need for an oblique needle course, the needle can still be directly advanced down to the femoral neck perpendicular to the skin surface. Once the needle makes contact with the femoral neck of the prosthesis, aspiration is attempted. If no fluid is aspirated, repositioning of the needle tip along the femoral neck may be successful. At the completion of the aspiration, an iodinated contrast medium is injected to confirm the needle location. It is important to visualize contrast medium along the femoral neck to indicate the needle's intra-articular placement.

Other Interventional Studies

In addition to hip joint aspiration, any soft-tissue fluid collection may be aspirated or injected. Ultrasound-guided injection around the iliopsoas tendon and acetabular cup has been used to treat tendon impingement.¹⁸

Medical/Legal Pitfalls

• It is important to note that no single imaging method can accurately diagnose or exclude infection in all cases. This is why percutaneous aspiration of a hip joint, regardless of imaging guidance, is critical. Also, consider using ultrasonography to evaluate the surrounding soft tissues for the presence of an abscess.

Related Medscape topic:
Resource Center Medical Malpractice and Legal Issues

MULTIMEDIA

Section 10 of 11  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT Scan
• MRI
• Ultrasound
• Nuclear Medicine
• Intervention
• Multimedia
• References

Media file 1:  Image from a patient who had a normal, hybrid-type total hip arthroplasty. This anteroposterior radiograph shows the femoral (metal, cemented) and acetabular (polyethylene cup with metal backing, cementless) components. (See Images 1-7.)
	View Full Size Image
Media type:  Radiograph

Media file 2:  Image from a patient who had a normal total hip arthroplasty. This frog-leg radiograph shows the femoral (metal, press fit, cementless) and acetabular (polyethylene cup with metal backing, screw fixation) components. (See Images 1-7.)
	View Full Size Image
Media type:  Radiograph

Media file 3:  Image from a patient who had a normal total hip arthroplasty. This frog-leg radiograph shows the femoral (metal, cemented) and the acetabular (polyethylene, cemented [yellow arrow]) components. (See Images 1-7.)
	View Full Size Image
Media type:  Radiograph

Media file 4:  Image from a patient who had a normal total hip arthroplasty. This anteroposterior radiograph shows the femoral (ceramic head [yellow arrow], metal stem, cementless, porous) and acetabular (polyethylene cup, metal backing, cementless, porous) components. Note the extensive reaming that is deep to the acetabular cup, creating a lucency of the rim fit of the acetabular component. (See Images 1-7.)
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Media type:  Radiograph

Media file 5:  Image from a patient who had a normal total hip arthroplasty. This frog-leg radiograph shows the femoral (metal head and stem, cemented) and acetabular (metal, cementless) components. Note the large metal femoral head articulating with the metal acetabular cup (arrows). (See Images 1-7.)
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Media type:  Radiograph

Media file 6:  Image from a patient who had a normal total hip arthroplasty. This frog-leg radiograph shows the femoral (ceramic head [arrow], metal stem, cementless, cable) and acetabular (ceramic cup, metal backing, cementless) components. Note the density of the ceramic head, which is less dense than metal. (See Images 1-7.)
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Media type:  Radiograph

Media file 7:  Image from a patient who had a normal total hip arthroplasty. This coronal computed tomography reformatted image shows the femoral (ceramic head [right yellow arrow], metal stem, cementless, cable) and acetabular (ceramic cup [top yellow arrow], metal backing, cementless) components. (See Images 1-7.)
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Media type:  CT

Media file 8:  Image from a patient who had a normal resurfacing total hip arthroplasty. This anteroposterior radiograph shows the femoral (metal head and stem) and acetabulum components (metal, cementless). Note the large femoral head and the preserved femoral neck.
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Media type:  Radiograph

Media file 9:  Image from a patient who had a normal unipolar hemiarthroplasty (same patient in Images 9 and 10). This anteroposterior radiograph shows the femoral component (metal, cementless) and the normal, native acetabulum.
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Media type:  Radiograph

Media file 10:  Image from a patient who had a normal unipolar hemiarthroplasty (same patient in Images 9 and 10). This frog-leg radiograph shows the femoral component of the unipolar hemiarthroplasty (metal, cementless). Note the normal sclerotic bone of the acetabulum (which indicates no surgical reaming) and the normal lucent hyaline cartilage between the acetabulum and the femoral head (arrow).
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Media type:  Radiograph

Media file 11:  Image from a patient who had a normal bipolar hemiarthroplasty. This anteroposterior radiograph shows the femoral (metal head and stem, cemented) and acetabular components (polyethylene cup, metal backing). Note the normal appearance of the acetabulum without reaming (arrow) and the lucency between the acetabulum and cup, which represents the normal acetabular articular cartilage.
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Media type:  Radiograph

Media file 12:  Image from a patient who had a normal bipolar hemiarthroplasty. This anteroposterior radiograph shows the femoral (metal head and stem, cemented) and acetabular components (polyethylene cup, metal backing). Note the angle between the edge of the acetabular cup and the horizontal in comparison to Image 13.
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Media type:  Radiograph

Media file 13:  Image from a patient who had a normal bipolar hemiarthroplasty. This frog-leg radiograph shows the femoral (metal head and stem, cemented) and acetabular components (polyethylene cup, metal backing). Note the angle between the edge of the acetabular cup and the horizontal in comparison to Figure 12. The motion between the acetabular cup and the native acetabulum is normal.
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Media type:  Radiograph

Media file 14:  Image from a patient who had a normal total hip arthroplasty. This anteroposterior radiograph shows the cement restrictor or centralizer (arrow) distal to the cemented femoral stem.
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Media type:  Radiograph

Media file 15:  Image from a patient who had a normal total hip arthroplasty. This anteroposterior radiograph shows "spot welds" (arrows) that indicate bone ingrowth at the cementless femoral stem.
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Media type:  Radiograph

Media file 16:  Image from a patient who had a normal total hip arthroplasty. This frog-leg radiograph shows cables (arrow) at the site of a trochanteric osteotomy.
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Media type:  Radiograph

Media file 17:  Illustration of the lateral inclination of an acetabular component (adapted from Manaster BJ. Radiographics. 1996;16(3):648.).6 The angle between the rim of the acetabular cup and the ischial tuberosities should normally be from 30-50º.
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Media type:  Illustration

Media file 18:  Illustration of an anteversion of an acetabular component (adapted from Manaster BJ. Radiographics. 1996;16(3):649.).6 Normal anteversion of the acetabular cup is 5-25º on a lateral hip radiograph.
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Media type:  Illustration

Media file 19:  Image from a patient who had a normal total hip arthroplasty. This true lateral radiograph shows the normal degree of acetabular component anteversion (dashed line). Courtesy of Dr JC Hodge, New York.
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Media type:  Radiograph

Media file 20:  Illustration of the vertical assessment of a femoral component (adapted from Manaster BJ. Radiographics. 1996;16(3):648.).6 The distance from the center of each femoral head to the lines that intersect the ischial tuberosities (between lines A and C) and to the greater trochanters (between lines B and C) should be symmetric.
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Media type:  Illustration

Media file 21:  Illustration of the horizontal assessment of a femoral component (adapted from Manaster BJ. Radiographics. 1996;16(3):648.).6 A line drawn from the center of each femoral head to the adjacent acetabular tear drop (which represents the acetabular margin of the medial hip joint space) should be symmetric.
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Media type:  Illustration

Media file 22:  Image from a patient who had a cemented total hip arthroplasty. This anteroposterior radiograph shows a <2 mm-thick normal periprosthetic lucency (arrow) at the bone-cement interface, which is demarcated by a thin, sclerotic line (ie, demarcation line).
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Media type:  Radiograph

Media file 23:  Image from a patient who had a cemented total hip arthroplasty. The lucency about the femoral stem is demarcated by a sclerotic line (arrows) that measures <2 mm thick. The lucency was stable over time.
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Media type:  Radiograph

Media file 24:  Image from an asymptomatic patient who had bone resorption under the femoral flange (arrow) that measured 4 mm thick.
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Media type:  Radiograph

Media file 25:  Image from a patient who had a cemented total hip arthroplasty and stress shielding. This anteroposterior radiograph shows localized osteopenia (arrow) of the greater trochanter as a result of diverted mechanical stress from the secure arthroplasty.
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Media type:  Radiograph

Media file 26:  Image of a patient who had a cemented total hip arthroplasty and stress shielding. This frog-leg radiograph shows focal osteopenia involving both the lesser and greater trochanteric regions (arrows) as a result of diverted stress.
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Media type:  Radiograph

Media file 27:  Image of pedestal formation from a patient who had a cementless total hip arthroplasty. This radiograph of the distal femoral stem shows an adjacent sclerosis.
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Media type:  Radiograph

Media file 28:  Image from a patient who had a cementless total hip arthroplasty. This anteroposterior radiograph shows an acute posterosuperior dislocation of the femoral head from the acetabulum.
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Media type:  Radiograph

Media file 29:  Image from a patient who had a cementless bipolar hemiarthroplasty. This anteroposterior radiograph shows a varus angulation between the femoral stem and the femoral shaft (black line) that predisposed the bone to stress and fracture at the lateral femoral cortex (arrow).
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Media type:  Radiograph

Media file 30:  Image from a patient who had a cemented total hip arthroplasty. This frog-leg radiograph shows medial extrusion (arrow) of polymethylmethacrylate (PMMA) cement.
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Media type:  Radiograph

Media file 31:  Image from a patient who had a prosthesis failure from a cemented unipolar hemiarthroplasty. This anteroposterior radiograph shows a femoral stem fracture (arrow).
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Media type:  Radiograph

Media file 32:  Image from a patient who had a prosthesis failure from a total hip arthroplasty. This anteroposterior radiograph shows a fractured and displaced ceramic femoral head (arrow). The femoral neck is in direct contact with the superolateral aspect of the polyethylene acetabular liner. Note the fracture of an acetabular screw and the ceramic fragments within the joint.
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Media type:  Radiograph

Media file 33:  Image from a patient who had a prosthesis failure from a total hip arthroplasty. This frog-leg radiograph shows the fragmented and displaced acetabular component. The lucent polyethylene liner has rotated and appears as an oval lucency that projects over the femoral neck (open arrow), whereas the associated metal ring (arrow) is disrupted and displaced. The femoral head is protruding into the native acetabulum.
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Media type:  Radiograph