Post-total hip arthroplasty imaging optimization through clinical scenarios in practice

Scenario 1: Asymptomatic patients on routine follow-up after THA

A pelvic radiograph is often the primary imaging modality used to evaluate implant positioning following THA. It provides essential information regarding acetabular component orientation, the center of rotation, leg length discrepancies, and femoral stem alignment. These radiographic parameters are critical for identifying patients at risk of implant failure. We routinely obtain baseline radiographs immediately after surgery, ensuring at least two projections for accurate future comparisons. Serial radiographs in asymptomatic patients are not routinely advisable. Hacking et al. also found that none of the 110 asymptomatic patients who underwent revision surgery benefited from serial radiographic follow-up imaging. They also underlined how standard serial radiography follow-up imaging for asymptomatic individuals has significant financial and resource consequences.^[16] Other modalities are usually not essential in asymptomatic patients. Bone scans can show periprosthetic normal nonspecific tracer activity even up to 1 year after arthroplasty; hence, it is not useful in the initial follow-ups.^[17] In addition to immediate postoperative check X-ray for implant positioning, we routinely perform pelvic radiographs at 2 time points with 6 months interval in asymptomatic patients after arthroplasty. The serial radiographic follow-up in asymptomatic patients is not routinely advisable after a year or sufficient implant integration with native bone.

Scenario 2: Symptomatic patients with hip pain and negative infective profile

Periprosthetic aseptic loosening remains the most common reason for revision arthroplasty, accounting for 66.6% of cases.^[18] Stable periprosthetic radiolucency <2 mm often represents fibrous fixation in THA which is usually outlined by a thin sclerotic line, but close monitoring should be done to preclude loosening.^[19] Serial radiographs play a crucial role in assessing the early subtle changes of loosening. To localize the periprosthetic radiolucency, the acetabulum is divided into three zones (I–III) by DeLee and Charnley.^[20] Similarly, Gruen et al.^[21] divided the femur into 14 zones, with each seven zones represented in the AP and lateral radiographs, as shown in Figure 6. Periprosthetic osteolysis and bone resorption are documented using the DeLee and Gruen zones. Diagnosing aseptic loosening can be particularly challenging, as many patients remain asymptomatic despite significant osteolysis.^[22] Compared to conventional radiographs, computed tomography (CT) offers greater sensitivity for detecting osteolytic changes.^[23] Therefore, advanced imaging modalities such as CT, magnetic resonance imaging (MRI), or single-photon emission CT (SPECT)/CT are often required. Among these, metal artifact reduction sequence (MARS) MRI is the most sensitive technique for identifying periprosthetic osteolysis, demonstrating a sensitivity and specificity of 83% and 98% for acetabular component loosening, and 75% and 100% for femoral component loosening, respectively.^[24,25] Circumferential increased T2 short-tau inversion recovery (STIR) signal intensity at the metal-bone and cement-bone interfaces is the most typical manifestation of component loosening on MRI.^[26] A hyperintense margin indicating osteolysis, which corresponds with findings of loosening as seen on radiography and CT.^[27] MRI features demonstrated in aseptic loosening and PJI includes STIR hyperintense signal margin (optimal MRI cutoff values of 1.5 mm at the acetabulum and 3.5 mm at the femur) at the metal-bone interface, bone marrow, periosteum, and contrast enhancement at the metal-bone interface and adjacent periosteum, which occurs at both acetabulum and femoral components. To differentiate between these two conditions, MRI findings in the acetabulum region are more specific for PJI, particularly regarding contrast enhancement at the metal-bone interface and adjacent periosteum. In contrast, the imaging findings in the femoral region are highly sensitive and can be observed in both aseptic loosening and PJI. In addition to these changes, PJI also demonstrates STIR signal alteration or contrast enhancement in surrounding soft tissues.^[28] Findings indicating definite loosening include tilted or migrated acetabular cups, rotated or migrated femoral stems; and excessively subsided stems. Subsidence up to 10 mm in the 1^st year after uncemented femoral stem arthroplasty is acceptable. However, more than 15 mm and progressive subsidence are all signs of implant loosening.^[29] SPECT-CT showed a promising role in improving the diagnostic sensitivity and accuracy of aseptic loosening by providing both an anatomical map and functional activity. On SPECT-CT, loosening shows increased periprosthetic tracer activity with corresponding periprosthetic lucency or osteolysis on CT especially in delayed phase.^[30,31] A simple radiograph with MRI ± CT hip is usually sufficient enough to diagnose periprosthetic aseptic mechanical loosening in a patient with hip pain and a negative infective profile.

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Figure 6:

DeLee and Gruen classification systems for radiographic assessment of prosthetic loosening. The acetabulum is divided into three zones (I–III) according to DeLee and Charnley. The femoral component is assessed using the Gruen classification, which divides the femur into 14 zones – seven zones on the anteroposterior view and seven zones on the lateral view of pelvic radiographs.

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Scenario 3: Suspected cases of infected hip prosthesis

PJI is the most devastating complication of THA, with a short-term risk of 0.2% and an occurrence rate of 1–5%.^[32] In addition, in comparison with primary THA, the incidence is higher after revision arthroplasty.^[33] X-rays are the first-line modality used to detect periprosthetic loosening. In cemented THA, loosening is defined as periprosthetic lucency >2 mm at the cement-bone interface. Similarly, in uncemented THA, the periprosthetic lucency >2 mm at the implant-bone interface is considered as periprosthetic loosening. As aseptic mechanical loosening can closely mimic septic loosening, serial radiographs are necessary to detect rapid progression of loosening with irregular bone margins and extensive bone destruction which usually favors PJI^[34] [Figure 7]. Radiographic features of loosening may include non-focal lucency, focal lucency, and periostitis [Figure 8a and b]. However, not all cases of septic loosening are radiographically apparent. In such instances, CT imaging plays a crucial role by providing detailed evaluation of subtle bone changes, periosteal reactions, and component interface abnormalities, aiding in the differentiation between septic and aseptic loosening.^[35] Among these changes, the periosteal reaction exhibits 100% specificity for periprosthetic infection, despite having a low sensitivity of 16%. Apart from periostitis, fluid collection in the muscle and perimuscular regions shows 100% specificity and positive predictive value (PPV). Other soft-tissue markers are joint distension [Figure 8c and d] and fluid-filled bursae. The absence of joint distension on CT showed 96% negative predictive value.^[36] Since MRI has superior soft-tissue resolution compared to CT, it is the second-line investigation of choice for symptomatic patients who have a strong suspicion of periprosthetic infection after radiographs. Modern, MARS-based MRI techniques eliminate post-THA imaging artifacts.^[37] MRI is the most sensitive modality to pick up numerous PJI imaging features, including pericapsular or soft-tissue edema, lamellated T2 hyperintense synovial thickening, fistula or sinus tracts, soft-tissue abscess, and regional lymphadenopathy^[38] [Figure 9]. In their study, Plodkowski et al. demonstrated that lamellated synovitis, when present in isolation, exhibited high specificity, sensitivity, and a high PPV for diagnosing PJI. Moreover, the finding showed strong inter- and intra-observer reliability, reinforcing its diagnostic utility. On MRI, lamellated synovitis in PJI is characterized by hyperintense, layered synovial thickening relative to skeletal muscle, distinguishing it from the hypointense, non-lamellated synovitis observed in cases of wear-induced synovitis or adverse local tissue reactions (ALTRs) to metal debris. Furthermore, the specificity of lamellated synovial thickening for PJI increased when secondary findings, such as extracapsular edema or fluid collections and regional lymphadenopathy, were also taken into account^[39] Size asymmetry of iliac nodes compared to the unaffected side has high sensitivity and specificity for infection. Particularly, the difference of node number and the ratio of node number showed accuracy rates of more than 90%.^[40] The standard procedure for identifying the causative organisms from a soft-tissue abscess or joint fluid is ultrasound (USG)-guided aspiration, which influences the use of specific antimicrobials. USG-guided synovial biopsy has low sensitivity but high specificity for PJI compared to fluid aspiration.^[41] Radionuclide scans often serve as the final diagnostic imaging to confirm the presence of septic loosening. The major drawbacks of radionuclide scans include their high cost, time consumption, the unavailability of in vitro labeling in most institutions, and their limited usefulness during the early post-operative period. Bony remodeling can cause physiological uptake up to 36 months after arthroplasty.^[42] Techniques such as triple-phase Methylene Diphosphonate bone scintigraphy, fluorodeoxyglucose positron emission tomography, and gallium citrate have high sensitivity and low specificity for PJI. Differentiating between septic and aseptic mechanical loosening based on standard uptake value is also difficult, as these tracers accumulate in both infective and inflammatory conditions. Therefore, leukocyte-labeled and bone marrow (sulfur or nanocolloid) scintigraphy are the gold-standard diagnostic tools for PJI due to their high specificity and sensitivity. One of the major limitations of a leukocyte-labeled scan is its inability to effectively detect infections that do not predominantly involve neutrophils, such as tuberculosis. In addition, radiolabeled leukocytes may accumulate in normal bone marrow, potentially complicating image interpretation and leading to false-positive results. This limitation can be addressed through the use of Technetium 99 metastable sulfur colloid or nanocolloid imaging, which demonstrates preferential uptake in infective foci due to specific avidity but not in normal bone marrow. When used in conjunction with leukocyte scans, this approach improves differentiation between infective and non-infective causes of prosthetic loosening, enhancing diagnostic accuracy.^[43] However, a negative nuclear medicine scan completely rules out periprosthetic infection.^[42,43] If there is strong clinical suspicion for PJI, MRI is the most sensitive modality to confirm diagnosis in most of the cases. If diagnosis is inconclusive even after equivocal results of image-guided aspiration, radionuclide scans are often required to confirm or exclude the diagnosis of PJI.

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Figure 7:

(a) Extensive periprosthetic osteolysis with irregular margins involving all seven Gruen zones of the femoral component, suggestive of an infective etiology. Following diagnosis, Stage I (b) management involved removal of the prosthesis and placement of a cement spacer. (c) The outcome of Stage II revision total hip arthroplasty performed after adequate infection control.

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Figure 8:

(a) Focal periprosthetic lucency in Gruen zones 5 and 6 (blue thin arrow), along with periostitis along the lateral femoral cortex (blue thick arrow). (b) Periosteal thickening over the acetabular cortex (blue thick arrow) with evidence of peri-implant osteolysis (blue thin arrow). (c and d) Joint distension (blue thin arrow) and muscular and perimuscular fluid collections (blue thick arrows), suggestive of a possible infectious process.

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Figure 9:

Axial T2 short-tau inversion recovery sequences demonstrate various imaging findings of periprosthetic joint infection. (a) Joint effusion causing capsular distension (blue thin arrow) involving left hip with prosthesis in situ, in comparison with the contralateral normal hip (blue thick arrow). (b) Marrow edema in the acetabulum (blue thin arrow), along with periosteal edema (blue thick arrow) and a complex soft-tissue collection with synovial thickening (red notched arrow), suggestive of an infectious process.

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Scenario 4: Post-THA cases with history of acute trauma

The primary purpose of imaging in symptomatic arthroplasty patients after acute injury is to rule out periprosthetic fracture. Periprosthetic fracture is the third most common cause for revision arthroplasty.^[44] The accumulated incidence of periprosthetic fracture is 0.4% of primary THA and 2.1% after revision arthroplasty.^[45] The Vancouver classification by Duncan and Masri^[46] is the most widely used and reliable classification system for periprosthetic fracture [Figure 10], which includes implant stability and bone stock loss other than fracture location [Table 2]. CT has higher sensitivity for these periprosthetic fracture assessments compared to plain radiographs. Metal artifacts limit CT’s ability to accurately characterize periprosthetic fractures and evaluate their extent, resulting in diagnosis uncertainty. However, modern techniques, such as iterative reconstruction and CT with metal artifact reduction, can be used to improve image quality by suppressing metal artifacts.^[47] Other modalities usually have a limited role in fracture assessment. As an adjunct, USG can reveal surrounding soft-tissue contusion/hematoma in displaced fractures. The role of MRI and nuclear medicine scans in the evaluation of periprosthetic fractures is generally limited to the identification of occult fractures. MRI typically demonstrates bone marrow edema, while nuclear scans reveal corresponding tracer uptake in the affected area.

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Figure 10:

Coronal computed tomography sections show prosthetic and periprosthetic fractures. (a) Shows a linear prosthetic fracture (blue thin arrow) at the mid-portion of the femoral stem component. (b) An oblique periprosthetic femoral fracture around the stem (blue thin arrow), with a stable stem and adequate bone stock, consistent with a Vancouver B1-type fracture. (c) An extensively comminuted periprosthetic fracture with an unstable stem and inadequate bone stock, suggestive of a Vancouver B3-type fracture.

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Although these imaging findings can confirm the presence of a fracture not visible on conventional radiographs, they rarely alter the course of conservative management, which remains the standard approach for occult periprosthetic fractures. Implant stability not only depends on the location of the fracture but also depends on the time of injury post-THA. Stability varies for the same type of fracture occurring at different points. For example, in uncemented THA, a B1 type of Vancouver fracture during intraoperative or within the first few weeks would be considered unstable since the stem usually would not have integrated with native bone.^[44] X-ray hip ± CT hip are the appropriate investigation of choice in those patients with history of acute trauma after arthroplasty at any given point of time. Other modalities are not essential in this clinical setting after arthroplasty.

Scenario 5: Symptomatic patients with suspected ALTRs

The literature collectively refers to conditions such as metallosis, ALVAL, pseudotumor formation, and trunnionosis under the umbrella term ALTRs. Among the various types of surface arthroplasty, metal-on-metal (MoM) implants were initially favored due to their low wear rates, increased joint stability, and absence of polyethylene debris. However, MoM arthroplasty is associated with a high incidence of ALTRs, largely due to the release of cytotoxic metal ions and particles, edge loading, and increased volumetric wear.^[48] ALTRs primarily occur as a type IV delayed hypersensitivity response to metal ions released locally, resulting in perivascular lymphoid infiltrate, macrophage activation, and local tissue necrosis.^[49] Several studies have explored the role of metal ion levels, particularly cobalt and chromium, in evaluating the risk of pseudotumor formation following THA. Proposed risk factors for elevated serum metal ion concentrations include steep acetabular inclination angles (>50°), excessive combined anteversion (>40°), large femoral heads, and small component sizes. These factors are associated with an increased risk of ALTRs.^[50,51]

However, despite these associations, high serum metal ion concentrations have not demonstrated sufficient sensitivity or specificity for reliably predicting pseudotumor development.^[52,53] Importantly, ALTRs can closely mimic PJI in both clinical presentation and serological findings. As such, imaging plays a pivotal role in differentiating ALTRs from other post-operative complications and is essential for accurate diagnosis and management. Radiographs are often normal or inconclusive, particularly in early or subtle presentations. CT is valuable for detecting focal or extensive osteolysis [Figure 11a], especially around the prosthesis. However, MRI remains the most sensitive modality for characterizing soft tissue and bone involvement in ALTR. After MoM THA, MRI frequently identifies pseudotumor-related conditions in asymptomatic patients.^[54]

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Figure 11:

The imaging characteristic appearance of a pseudotumor. (a) Axial computed tomography section shows extensive focal osteolysis involving the acetabulum (blue thin arrow) and a round circumscribed soft tissue lesion (blue thick arrow) just lateral to the femoral vessels consistent with pseudotumor formation. (b) coronal T2 WI shows T2 hyperintense periprosthetic collection (blue thick arrow) with hypointense peripheral lining noted around left hip prosthesis. (c) Axial T2 WI and (d) coronal T2 WI are showing a T2 heterogeneously hyperintense collection with T2 dark synovial thickening (blue thick arrows in c and d) around the right hip in the same patient.

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One of the MRI-based classification systems for pseudotumors^[55] categorizes them based on morphology and signal characteristics, ranging from simple cystic to complex solid lesions:

• Type 1: Simple, thin-walled cysts with a flat morphology. These lesions are hypointense on T1-weighted and hyperintense on T2-weighted sequences

• Type 2a: Complex, thick-walled or irregular cystic lesions that retain fluid-signal characteristics on conventional MRI sequences. These lesions are non-flat in more than half of their volume

• Type 2b: Thick-walled or irregular cystic lesions with variable shape and show atypical fluid signals with hyperintense signal on T1 and variable signal intensity on T2-weighted images

• Type 3: Complex, homogeneously solid lesions with mixed signal intensities and variable morphology, often indicating more aggressive or chronic pathology.

A T2 hyperintense periprosthetic collection with hypointense synovial thickening, often lined with metal particles that produce corresponding areas of magnetic susceptibility artifacts, is the most characteristic MRI appearance of ALTRs [Figures 11b-d]. In addition, T2 hypointense nodal enlargement may be observed, resulting from the deposition of metal debris secondary to decompressed synovitis.^[54] USG is the initial screening modality used to rule out the presence of asymptomatic pseudotumors. It is a widely available, non-ionizing, and non-invasive modality, and it aids in differentiating solid from cystic lesions around prosthetic implants. In the same setting, USG -guided biopsy or aspiration can be performed.^[56] According to the literature, the reported sensitivity and specificity of USG for detecting ALTRs, using MRI as the reference standard in MoM THA, are 74% and 92%, respectively.^[57] However, USG is operator-dependent, and its effectiveness is limited when evaluating deeper anatomical structures. Among various imaging modalities, USG is the initial imaging method of choice in case of ALTRs to look for periprosthetic solid or cystic lesions. MRI hip ± CT is the next modality of choice to characterize the ALTRs imaging manifestation. Role of nuclear medicine is limited in these conditions.

Scenario 6: Post-THA cases presented with lateral hip pain

In patients with THA, the most common causes of trochanteric lateral hip pain are local extrinsic factors rather than intrinsic pathology of the hip joint. These extrinsic causes include greater trochanteric bursitis, abductor tendon tears or tendinosis, and herniation of the vastus lateralis muscle. USG is the initial modality in the suspected case of bursal or tendon pathology, permitting dynamic assessment and guiding for delivery of therapeutic agents.^[58] The reported incidence of iliopsoas impingement is 4%, with patients presenting with hip or groin pain during active hip flexion activities. Risk factors include protruding large acetabular cups (>12 mm), prominent screw tip positioning, and the presence of cement debris anterior to the cup. These factors help explain why the iliopsoas tendon is most commonly affected at the anterior rim of the acetabulum. Other conditions such as iliopsoas bursitis, tenosynovitis, and tendonitis were also reported. Apart from the initial assessment, USG also helps confirm the diagnosis by providing immediate pain relief after injecting a local anesthetic ± steroid.^[59,60] Ischiofemoral impingement occurs secondary to post-operative reduction in ischiofemoral space, often manifesting as edema and fatty atrophy of quadratus femoris.^[61] MRI is highly sensitive in evaluating the soft-tissue structures around the hip and has demonstrated a diagnostic accuracy of 91% for tendon tears, according to Cvitanic et al. Direct MRI signs of tendon abnormalities include peritendinitis, tendinopathy, and partial or complete tendon tears. Indirect signs may consist of bursal fluid accumulation, bony changes, and fatty atrophy of the associated muscles.^[62]

In a study by Blankenbaker et al., patients presenting with trochanteric pain syndrome consistently showed peritrochanteric T2 signal abnormalities and were more likely to exhibit signs of abductor tendinopathy [Figure 12].^[63] USG is the initial investigation of choice in patient with lateral hip pain permitting both dynamic assessment and anesthetic/steroid injection for confirmation of diagnosis with immediate relief of pain. In diagnostic uncertainty, MRI is the next modality of choice. Other modalities are usually not essential in assessment of lateral hip pain

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Figure 12:

(a) Coronal short-tau inversion recovery (STIR) and (b) axial STIR images show peritrochanteric T2 STIR hyperintense signal intensity (blue thin arrows in a and b) with thin rim of fluid at greater trochanteric abductor insertion site.

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