CORR Insights®: Is Quantitative Radiographic Measurement of Acetabular Version Reliable in Anteverted and Retroverted Hips?

Where Are We Now? Abnormalities or extremes of acetabular version can lead to altered hip biomechanics, predisposing patients to joint instability and impingement. These in turn may lead to early arthritic changes, which may negatively impact quality of life [11]. Even though history and physical exam can direct the orthopaedic surgeon toward the hip as the source of pathology, the clinical assessment, although highly sensitive, may not be specific enough to detect the exact cause of the symptoms and dysfunction [5]. Thus, radiologic imaging of hip anatomy is an essential tool for orthopaedic surgeons and allied musculoskeletal specialists to help us identify both the primary dysmorphisms and the secondary problems that may ensue [27]. Radiographs are the first line of imaging for investigating the hip because they are easy to perform, they are relatively inexpensive, and they result in less radiation exposure than CT. Although acetabular morphology, specifically version, can impact hip function and can be evaluated on radiographs, both qualitatively and quantitatively [18, 19], it is affected by technical factors such as x-ray beam projection angle, spinopelvic relationship and alignment, supine versus standing positions, and variable pelvic orientation, whether patient inherent or positional. These factors may lead to over- or underestimation of acetabular version [6, 26]. In addition, radiographs may over- or underestimate assessment of acetabular coverage of the femoral head [8]. Finally, the version of acetabulum is not fixed, but rather variable, increasing from superior to inferior, and thus, deducing the net or average value of version on two-dimensional radiographs is challenging [7]. All these issues can make surgical decision-making more difficult and less precise, and having accurate and reliable measures to differentiate normal from abnormal is essential [19]. For these reasons, CT is generally considered a more accurate and reliable imaging technique to detect and measure version [29]. The performance of imaging tests often focuses on the diagnostic accuracy. However, the reliability of the evaluation technique in question is also important for determining its value and whether it can be broadly extended into practice beyond the project’s host investigators, patient population, and sites. In this month’s Clinical Orthopaedics and Related Research®, Yonga et al. [28] addressed the known challenges in radiographic version assessment and compared the technique with CT. More specifically, they carried out a subanalysis of anteverted and retroverted hips, which has not been previously performed. Based on their results, CT is a more reliable study, and radiographs alone should not be used to diagnose or measure version, or perhaps should only be used as a very preliminary assessment tool. Surgeons should consider CT when radiographic findings are questionable and where the diagnosis and/or management could be substantially affected. Where Do We Need to Go? There are several other questions to resolve before we assume that we have a validated approach to acetabular version assessment. First, Yonga et al. [28] employed supine radiographs, although standing projections have also been proposed, and some have suggested that loading the hip for these images is important [12]. However, supine and standing AP radiographs are not equal in terms of the morphologic appearance of the hip. The standing position results in greater posterior pelvic tilt, which reduces anterior coverage and masks the true degree of retroversion and/or overestimating anteversion. Standing radiographs have been found to have a lower incidence of ischial spine and crossover signs and crossover percentage [12, 25]. Furthermore, upright standing radiographs have been found to be of relatively lower image quality and with an associated 47% higher radiation dose [10]. Although most centers image hips supine, and even though some of late advocate for and have switched to a standing position, it has also been proposed that imaging be performed in both supine and upright positions to have two imaging points [13]. Given that no consensus imaging protocol exists at present, it would be interesting if future studies examined which of the two positions has the strongest correlation with joint function, physical exam, the incidence of hip disease, and eventual orthopaedic intervention. Second, while considering the effectiveness of imaging methods such as radiography and CT for detection and characterization of acetabular morphology, it is important to consider the radiation dose to which the patient is exposed. When employing CT, low-dose protocols suffice to determine gross joint morphology and should become the standard of care. When strategically constructed, preoperative hip preservation CT protocols have been shown to reduce the radiation exposure by 90% and with an effective radiation dose to the patient of less than an AP and lateral radiograph [24]. Many centers employ standard-dose CT protocols but have not yet implemented newer, low-dose versions, which can generate diagnostic-quality images, multiplanar reformations, and three-dimensional (3D) models. Given that many conditions such as hip dysplasia and femoroacetabular impingement are diagnosed and treated in young adults during their reproductive years of life, low-dose CT must become the norm and not the exception. Although radiography and CT are longstanding accepted imaging modalities, we should continue to explore more novel imaging techniques in evolution. Third, there is a need to define and determine the true clinical impact of excessive anteversion or retroversion. A plethora of evidence exists on detecting abnormalities of acetabular version, although there is less on the clinical significance of the measurements and the label of “retroversion.” Anatomic retroversion or overcoverage of the femoral head does not necessarily equate to functional abnormality. Morphologies that are at the extremes of the spectrum may be asymptomatic and inconsequential, whereas those within normal ranges may actually predispose to joint dysfunction [14]. This is because there are many established and newly recognized factors that can determine whether a given acetabular version leads to joint dysfunction such as femoral version, head-neck shape and morphology, spinopelvic relationships, and neuromuscular factors. There may be critical combinations and weightings of these parameters that lead to clinically significant version measure. Thus, version should perhaps not be looked at in isolation but rather in combination with other hip and nonhip factors. How Do We Get There? We need to establish and disseminate standardized, effective imaging protocols through multidisciplinary collaboration and education among orthopaedic surgeons, radiologists, and all allied musculoskeletal specialists. Consensus is required on several parameters of technique, position limitations of radiographs and CT, and a validated method for qualitatively and quantitatively evaluating acetabular version while also factoring in image quality, radiation dose, diagnostic accuracy, reliability, as well as the correlation with physiology and biomechanical function. The process of generating consensus will need to be led by orthopaedic societies specializing in the young adult hip and joint-preserving surgical management, along with academic groups such as ANCHOR (Academic Network of Conservational Hip Outcomes Research). If the standing radiograph is to be adopted more widely, then studies will need to be repeated to establish the new thresholds for the qualitative and quantitative parameters traditionally used to establish retroversion on supine films. Reliability studies, as in Yonga et al. [28], should also be repeated in standing radiographs as the reduction in pelvic tilt may reduce the degree of visual retroversion, rendering the qualitative findings more subtle and challenging to detect, and the quantitative measures smaller and more challenging to perform. Comparative studies looking at how supine and standing images predict and/or correlate with CT, clinical assessment and physical exam, patient-reported outcome scores, and surgical outcomes could help determine which of the two positions should be favored. The field needs to continue to explore newer, advanced imaging techniques. The emerging EOS imaging system generates radiographic and 3D CT model-like images using novel technology that allows for a radiation dose that is lower than both radiography and CT. Furthermore, the EOS scan is acquired in the standing position, which some consider advantageous over supine imaging [2]. A recent systematic review and meta-analysis comparing EOS with CT found high correlation between the two techniques for measuring femoral version, tibial torsion, acetabular coverage, as well as THA cup version and inclination. However, studies are lacking on the efficacy of EOS specifically for measuring acetabular version [15]. Further technological evolution will also require collaboration with physicists and imaging specialists to make advances in image acquisition, processing, and reconstruction to generate high-fidelity representations of the hip. Larger population studies are required to define with greater precision the normal range of acetabular version and learn under what circumstances will those at the extremes of the version spectrum have clinically important alterations in hip biomechanics. This will allow for adequately powered studies and regression analyses to control for, and evaluate the impact of, each of the factors contributing to hip dysfunction. High-quality, large datasets would also allow for employment of artificial intelligence (AI) processes to analyze the “big data” that could be generated through international multicenter collaborations [9]. This may facilitate the determination of which features determine whether a given version is clinically important. AI also has the potential to automate measurements of version in both the clinical and research settings. To date, AI has been applied to radiographs in the classification of hip fractures, hip arthroplasty component identification and alignment, as well as to identify morphologies predisposing to femoroacetabular impingement and AVN of the femoral head [1, 3, 4, 16, 20, 22, 23]. Recently, AI-powered software was shown to have moderate-to-strong agreement with manual measurements of several hip parameters on radiographs; although retroversion was not specifically evaluated, it could be in future studies. AI-driven automation may allow for rapid and accurate measurement of key hip parameters on imaging [21], advancing clinical and research endeavors related to acetabular version. Read This Next Read this article to understand basic spinopelvic biomechanics and appreciate why supine and upright radiographs can provide different assessments of hip version [13]. To learn more about low-dose CT, read this article on how it can be implemented in imaging the young adult hip, thereby reducing the radiation dose to equal to or less than a standard radiographic series [24]. This article provides an introduction to artificial intelligence, including potential orthopaedic applications [17].

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