1 Introduction

The importance of proper acetabular cup positioning in total hip arthroplasty is well recognized. Positioning the acetabular cup within Lewinnek's “safe zone” (40° ± 10° abduction and 15° ± 10° anteversion) has been shown to decrease rates of hip dislocations and lower biomechanical stresses.^1–5 Although this “safe zone” has been challenged,^6,7 it remains a recognized surgical target regarding acetabular positioning.

The use of intraoperative fluoroscopy (IF) is suggested to increase the accuracy and precision of cup positioning during AA total hip arthroplasty (THA).^8–12 It is important to understand, however, the limitations of fluoroscopy and how the projected image of the acetabular cup behaves as the position of the fluoroscopic beam changes in relation to the hip or pelvis in order to use fluoroscopy accurately. Changes in fluoroscopy beam position and direction can alter the projected image of the acetabular component and while studies indicate that increased surgical experience with IF increases the accuracy of cup positioning for surgeons performing AA THA with IF,^9,10,12 this understanding takes time to develop and general guidelines regarding the relationship between beam positioning and projected cup position are limited. While technologies that supplement IF guidance regarding cup targeting, leg length, and hip offset assessment are currently available and have demonstrated excellent results achieving targeted cup position,^13,14 such resources represent additional costs and may not be available to most surgeons. Furthermore, such additional technologies typically base measurement on the projected images obtained.

Understanding the relationship between the fluoroscopic beam position and the projected acetabular cup image during AA THA is not intuitive. Furthermore, there are no guidelines to appreciate the magnitude that intraoperative IF beam changes have on the projected image. Therefore, the purpose of this study was to examine how the projected acetabular cup position changes with varying positions of the fluoroscopic beam in relation to the hip and pelvis using a static pelvic model implanted with an acetabular cup in a predefined anatomic position. A secondary goal is to provide an estimate of the magnitude and direction of change regarding cup abduction and anteversion angles with differing beam positions and grossly quantify the difference in abduction and anteversion angles measured when the fluoroscopy beam is centered over the hip or pelvis.

2 Materials and methods

Key radiographic landmarks on an anatomic model of a female pelvis (Sawbones®, A Pacific Research Company, Vashon Island, WA, USA) were labeled with radiopaque markers (Fig. 1). These landmarks included posterior superior iliac spines, anterior superior iliac spines, anterior inferior iliac spines, pubic tubercles, superior and inferior aspects of the obturator foramen, ischial tuberosities, ischial spines, coccyx, and superior sacrum. Using fluoroscopy, a true anteroposterior (AP) view of the pelvis as described by Welton et al. was obtained.¹⁵ The fluoroscopy beam was then moved to obtain a perfect AP of the right hip. A 54 mm acetabular shell (Legend® Acetabular Shell, Ortho Development Corporation, Draper, UT, USA) was then optimally positioned under fluoroscopy using a digital radiographic IF supplementation system (Radlink Inc., Los Angeles, CA, USA) targeted to position the cup with an abduction angle of 45° and anteversion of 20°. Spot images were taken with the fluoroscopy beam positioned at 0° centered over the hip for the AP hip view and with the beam repositioned to obtain images at 10° and 20° in the caudal direction (outlet view) then 10° and 20° in the cranial direction (inlet view) (Fig. 2). The beam was then centered over the pubic symphysis to obtain the AP pelvis view and similarly repositioned at 10° and 20° in the caudal and cranial directions. Four independent observers used the digital radiographic IF supplementation system to measure the abduction and anteversion of the projected acetabular cup position at all of the positions described above. Measurements by all observers were recorded and the mean value of the abduction and anteversion angles measured was calculated for each of the fluoroscopy beam positions described above. Mean and standard deviation (SD) were used for all measurements of abduction and anteversion for both the AP hip and AP pelvis views. The differences in the measured cup abduction and anteversion angles from AP hip to AP pelvis views were calculated.

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Fig. 1 Anatomic female pelvis model labeled with radiopaque markers on anatomical landmarks. Sawbones model of female pelvis with lead markers at key anatomic locations and acetabular cup positioned at 45° of abduction and 20° of anteversion using computer aided imaging guidance software (Radlink Inc., Los Angeles, CA, USA).

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Fig. 2 Schematic of fluoroscopic images obtained at different inlet and outlet angles.

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3 Results

3.1

3.1 AP hip

As the fluoroscopy beam was moved from 0° in the caudal direction (outlet view) to 10° and 20°, the mean abduction angle of the acetabular cup increased from 43.5 ± 1.0° to 46.5 ± 1.3° and 51.0 ± 0°, respectively (Table 1, Fig. 3). The mean anteversion angle of the acetabular cup increased from 19.5 ± 1.7° to 26.8 ± 0.5° and 32.3 ± 1.9°, respectively. At the 20° caudal (outlet) view there was an average perceived gain of 7.5° in abduction angle and a gain of 12.8° in perceived anteversion when compared to the neutral AP hip view. As the fluoroscopy beam was moved in the cranial direction (inlet view) from 0° to 10° and 20° the mean abduction angle of the acetabular cup decreased from 43.5 ± 1.0° to 40.3 ± 0.5° and 39.3 ± 0.5° and the mean anteversion angle of the acetabular cup decreased from 19.5 ± 1.7° to 10.8 ± 1.5° and 0°, respectively. At the 20° cranial (inlet) view, there was an average loss of perceived abduction angle of 4.2° and an average loss of 19.5° of anteversion when compared to the AP hip view at 0°.

Table 1 Measured cup abduction and anteversion angles with fluoroscopy beam centered over the hip (AP hip view) with differing outlet and inlet beam angulations - mean (SD).

Angle of IF Beam	Abduction (°)	Anteversion (°)
0°	43.5 (1.0)	19.5 (1.7)
10° outlet	46.5 (1.3)	26.8 (0.5)
20° outlet	51.0 (0.0)	32.3 (1.9)
10° inlet	40.3 (0.5)	10.8 (1.5)
20° inlet	39.3 (0.5)	0°

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Fig. 3 AP Hip with adjusted inlet views. Anteroposterior (AP) intraoperative fluoroscopic images of a left hip following cup insertion targeted for 40° abduction and 20° degrees anteversion. (A) Image of cup with 20° cranial tilt (Inlet view) showing dramatic decrease in perceived anteversion. (B) AP image of hip at 0°, and (C) image taken with 20° caudal tilt (Outlet view) showing dramatic increase of perceived cup anteversion.

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3.2

3.2 AP pelvis

As the fluoroscopy beam was moved from 0° in the caudal direction (outlet view) to 10° and 20°, the mean abduction angle of the acetabular cup increased from 40.5 ± 1.0° to 43.3 ± 1.0° and 49.0 ± 2.0° respectively (Table 2, Fig. 4). The mean anteversion angle of the acetabular cup increased from 27.5 ± 1.7° to 34.0 ± 1.6° and 44.3 ± 0.5°respectively. At the 20° caudal (outlet) view there was an average perceived gain of 8.5° in abduction angle and a gain of 16.8° in perceived anteversion when compared to the neutral AP pelvis view. As the fluoroscopy beam was moved in the cranial direction (inlet view) from 0° to 10° and 20°, the mean abduction angle of the acetabular cup decreased from 40.5 ± 1.0° to 38.3 ± 0.5° to 37.0 ± 0.8° and the mean anteversion angle of the acetabular cup decreased from 27.5 ± 1.7° to 18.3 ± 1.0° and 4.0 ± 1.8°, respectively. At the 20° cranial (inlet) view, there was an average loss of perceived abduction angle of 3.5° and an average loss of 23.5° of anteversion when compared to the AP pelvis view at 0°.

Table 2 Measured cup abduction and anteversion angles with fluoroscopy beam centered over the pelvis (AP pelvis view) with differing outlet and inlet beam angulations - mean (SD).

Angle of IF Beam	Abduction (°)	Anteversion (°)
0°	40.5 (1.0)	27.5 (1.7)
10° outlet	43.3 (1.0)	34.0 (1.6)
20° outlet	49.0 (2.0)	44.3 (0.5)
10° inlet	38.3 (0.5)	18.3 (1.0)
20° inlet	37.0 (0.8)	4.0 (1.8)

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Fig. 4 AP Pelvis with adjusted outlet views Anteroposterior (AP) intraoperative pelvic fluoroscopic images cup insertion targeted for approximately 40° abduction and 20° degrees of anteversion. (A) Image of cup with 20° cranial tilt (Inlet view) showing dramatic decrease in perceived anteversion. (B) AP image of pelvis at 0°, and (C) image taken with 20° caudal tilt (Outlet view) showing dramatic increase of perceived cup anteversion.

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4 Discussion

The importance of proper positioning of the acetabular component in THA in preventing complications is well described.^1,4,16 Use of IF during AA THA was reported to result in improved implant positioning, however, there is conflicting data to support this claim.^16–21 Previously published reports have indicated that the use of IF does not necessarily result in improved acetabular positioning^17,18,20,21 and more recent studies examining this topic have indicated that surgeons unaware of these relationships may mistakenly accept cup positioning outside of their intended target zone.^2,3,22–24 The purpose of this study was to provide a basic appreciation of how the perceived cup position (abduction and anteversion) will change depending on the position of the fluoroscopic beam in relation to the surgical hip.

One of the primary findings from this paper was the demonstration that an AP pelvic fluoroscopic image will make the acetabular component appear about 3° less abducted and 8° more anteverted compared to an AP hip view. Our findings suggest that for surgeons who prefer an AP pelvic view and are targeting Lewinnek's safe zone (40° abduction and 15° anteversion),¹ the measured position of the cup should have an abduction and anteversion that measures approximately 37° and 23°, respectively. Surgeons who use primarily an AP hip view should expect the projected abduction and anteversion angles to measure nearly identical to Lewinnek's defined targets as demonstrated in Table 1.

Our findings also demonstrate that when comparing AP hip to AP pelvic views, the magnitude of variation regarding cup abduction angle is negligible when comparing the spectrum of caudal and cranial beam angulations investigated and at maximum was less than approximately 3° (Table 3). Therefore, it is unlikely small variations in IF positioning will adversely affect cup abduction targeting. However, projected anteversion angles can differ by as much as 32°–40° depending on the degree of cranial or caudal angulation when utilizing the AP hip or pelvic view, respectively. Therefore, with both AP pelvic and AP hip techniques, excessive inlet (cranial) or outlet (caudal) beam angulation should be recognized and corrected to assure proper cup positioning. Recognition and awareness of this phenomena can help surgeons avoid devastating complications such as hip dislocation due to improper cup positioning as a result of underappreciation of this relationship between beam angulation and projected cup image (Fig. 5). A recent retrospective study compared intraoperative AP hip and AP pelvis abduction and anteversion measurements to a postoperative AP pelvis view.²³ Delagrammaticas et al. found that both AP hip and AP pelvis measurements of anteversion were overestimated compared to postoperative PA, 19.89°, 24.38°, and 13.36°, respectively. When examining abduction, they found that AP hip and AP pelvis measurements underestimated abduction, 40.95°, 38.87°, 41.73°, respectively. IF beam positioning and image distortion were used to explain the differences in measurements and their findings follow the same trends seen in the current study. Overestimation of anteversion was seen when moving the IF beam to an outlet view, and underestimation of abduction was seen if the IF beam was moved to an inlet view. Furthermore, Delagrammaticas et al. had nearly a 10° difference in anteversion measurements, highlighting the importance of IF beam positioning in anteversion measurement.

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Fig. 5 AP Pelvis with adjusted inlet views. Intraoperative fluoroscopy image during cup insertion (A) Note excessive outlet view with cup appearing to be in proper abduction and having approximately 15° of anteversion. (B) Immediate post-operative radiograph again with excessive outlet view due to lumbar arthrosis. Cup appears to be in good position but is relatively retroverted as the cup should appear to have greater anteversion with outlet views of the pelvis. Early post-operative dislocation (C). Post cup revision showing greater anteversion as should be expected with outlet views of the pelvis when the acetabular cup is properly positioned.

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A major limitation of this study is that only one model and a single fluoroscopic unit was used. Future studies should investigate if significantly different results can occur with various fluoroscopic systems or designs. Secondly, while cup position and model stability could be well controlled, a single digital radiographic supplementation system was used to target and define the cup reference position. While the model was secured to the operating table, exact replication of the position in which the reference measurement were recorded may have differed by a few degrees. Therefore, measurement corrections for targeting of cup positions are estimates. However, the magnitude of discrepancy is likely small and clinically insignificant. A strength of this study was the use of an anatomic model. A fixed anatomic model was felt to be ideal for this type of initial study as it allowed controlled cup positioning and served as a fixed known reference from which variations in cup measurements at differing fluoroscopic beam locations and angulations could be demonstrated. Furthermore, pelvic tilt and pelvic rotation could be well controlled and are known to be important clinical factors.²⁵

5 Conclusion

In conclusion, the current study demonstrates that for surgeons using AP pelvic views to guide cup positioning, the measured abduction and anteversion angles of the projected cup image should be approximately 3° less abducted and 8° more anteverted than the chosen target. With either AP pelvic or AP hip imaging, care should be taken to avoid excessive cranial (inlet) or caudal (outlet) angulation as this may significantly affect the assessment of true cup anteversion. Excessive cranial (inlet) or caudal (outlet) angulation may lead to excessive cup anteversion or retroversion, respectively.

Institutional ethical committee approval

This study was carried out by the protocol approved by an IRB board and in accordance with The Code of Ethics of the World Medical Association.

Author contributions

Tyler Thorne: Methodology, Validation, Formal Analysis, Data Curation, Writing-Original Draft, Writing-Revision and Editing, visualization Anne Wright: Conceptualization, Methodology, Writing-Original Draft, Writing-Review and Editing, Mariya Opanova: Methodology, Investigation Lee Mitsumori: Writing-Review and Editing Dylan Lawton: Investigation, Data Curation Emily Unebasami: Investigation, Data Curation Cass Nakasone: Conceptualization, Methodology, Writing-Review and Editing, Supervision.

Disclosures and declarations

Each author certifies that he or she has no commercial associations (e.g., consultancies, stock ownership, equity interest, and patent/licensing arrangements) that might pose a conflict of interest in connection with the submitted article.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.