The main causes of prosthesis loosening are osteolytic prosthesis loosening caused by wear particles, infection, prosthesis placement error, traumatic mechanical loosening, bone structure decay of patients, etc. Aseptic loosening caused by biological debris reaction is the most common complication with long-term, slow and slight inflammation, while PJI is the most serious with severe inflammatory reactionsgenerally, which include inflammatory exudation, synovial reaction, pus formation and even joint capsule rupture. The inflammatory reaction of PJI is generally more obvious. In this study, ultrasonography could reveal all of PJI with extracapsular effusion or sinus, simultaneously PJI group had large amount of effusion, poor acoustic window, and hypertrophic synovium with rich blood flow signal. The area under ROC curve of effusion depth in the two groups was 0.82 (0.7–0.9), which could explain the reference value of effusion depth in the judgment of disease better. There were all PJI when the patients with effusion volume ≥ 25.5mm (sensitivity 53.5%, specificity 100%) or with extracapsular effusion or sinus. The positive predictive value of ultrasound in the diagnosis of PJI could be as high as 95% when 17.0 mm was carried as the cut-off value of fluid volume (sensitivity 71.1%, specificity 83.3%), combined with hypertrophic synovium with flow signal. The results of this study indicated that ultrasound played an important role in the differential diagnosis of PJI and aseptic loosening.
Guidelines recommend that white blood cell (WBC) count, C-reactive protein (CRP) level, and erythrocyte sedimentation rate (ESR) be determined as non-invasive methods, however, no laboratory test is sensitive or specific enough to confirm or rule out a periprosthetic joint infection definitively[15, 16]. Current studies [17, 18] have found that MRI is associated with artefacts caused by metal implants,although ultrasound and MRI both have enough high sensitivity and specificity for the soft tissue.Takashi Nishii MD and his team had showed that there were 9 cases of MRI found abnormity but US missed, 8 of 9 cases were located in the lateral area, while 11 cases of US picked up abnormal echography but MRI could not detect. Ultrasound and MRI had certain limitations for the specific area in metal-on-metal THA, however,MRI could not be used as a routine examination tool ,since MRI was more expensive, time-consuming, and there were many contraindications. The artifacts of prosthesis severely degrade the image quality and decrease the diagnostic value of CT, and dual-energy CT technique alone does not sufficiently remove the artifacts either[19, 20].Bone scintigraphy and FDG-PET are less specific, leukocyte scintigraphy has only moderate specificity༌and the existing research data related to radiological diagnostic procedures such as PET and 99mTc bone scintigraphy are inconsistent, so further validation of the value of nuclear medicine examination is needed . As a new diagnostic mode, SPECT/CT combines the sensitivity of bone imaging with the high specificity of CT [22, 23], but there is still limited literature on its efficacyand clinical validity. Ultrasound has the characteristics such as total reflection and attenuation, so the lesions with deep location and covered by bone or metal prosthesis are easy to be missed for the loss of sound beam. On the other hand, ultrasound can clearly show the situation of effusion, synovium and other soft tissues ,since it has enough high resolution and can be scanned dynamically from multiple angles. When necessary, different auxiliary skills or changing body position in real time can be used,and adjusting the energy output, frequency and other conditions timely according to the situation of different patient can improve the probability of finding abnormal signs. In the past, it was thought that infection and aseptic loosening were difficult to distinguish. Our research found that the accuracy of ultrasound was 100% for extra-articular capsule infection. Meanwhile, in the part of the situation that PJI containing large effusion (greater than 17mm) and obvious synovial hyperplasia, the detection rate of ultrasound was also high. Therefore, it could be cautiously optimistic that ultrasound could better distinguish the two.
In clinical work, ultrasound is not only used to routinely observe joint and prosthetic peripheral lesions, but also to guide joint aspiration. Many researchers such as Randelli F had found that joint aspiration guided by ultrasound could provide direct vision when the articular fluid volume was relatively less in the early stage of inflammation. Sonicated fluid could significantly improve the positive rate of joint fluid culture where required. Some studies [25, 26] compared the capabilities of culture and broad-range polymerase chain reaction (PCR)using joint fluid (JF), periprosthetic tissue (PT), and sonicated fluid(SF) for the diagnosis of periprosthetic joint infection(PJI),the results showed that the specificity and sensitivity of sonicated fluid culture were not lower than other methods, and SF culture had theadvantage of detecting polymicrobial or fungal infections.
Our study had some limitations: In some cases the blood flow signals of the synoviam of obese patients were not easy to display, and it was difficult to distinguish synoviam from effusion because of the deep location of hip joint, which might affect the results; it was not suitable to classify hyperplastic synovium according to Szkudlarek semi standard for evaluation for the joint structure changing after operation; In addition, some studies had shown that aseptic loosening usually occured long after operation, but we were unable to further group on this aspect due to the limited number of cases in this study, which did not affect the utility of ultrasonography. We need to strengthen the cooperation with clinical in the future and make further efforts to accumulate experience.