Skeletal Radiol DOI 10.1007/s00256-016-2527-z

SCIENTIFIC ARTICLE

A cadaver study validating CT assessment of acetabular component orientation: the Perth CT hip protocol Varun Arora 1 & Richard Hannan 1 & Richard Beaver 2 & Timothy Fletcher 2 & Paul Harvie 1

Received: 19 October 2016 / Accepted: 27 October 2016 # ISS 2016

Abstract Objective The aim was to develop a CT-based protocol to accurately measure post-operative acetabular cup inclination and anteversion establishing which bony reference points facilitate the most accurate estimation of these variables. Materials and methods An all-polyethylene acetabular liner was implanted into a cadaveric acetabulum. A conventional pelvic CT scan was performed and reformatted images created in both functional and anterior pelvic planes. CT images were transferred to a Freedom-Plus Graphics software package enabling an identical, virtual 3D model of the cadaveric pelvis to be created and definitive acetabular cup orientation established. Using coronal and axial slices of the CT scans, acetabular cup inclination and anteversion were measured on five occasions by ten radiographers using differing predetermined bony landmarks as reference points. The intra- and inter-observer variation in measurement of acetabular cup orientation using varying bony reference points was assessed in comparison to the elucidated definitive cup position. Results and conclusion Virtually derived definitive acetabular cup orientation was measured showing cup inclination and anteversion as 41.0 and 22.5° respectively. Mean CT-based measurement of cup inclination and anteversion by ten

* Varun Arora [email protected]

1

Department of Orthopaedics, Royal Hobart Hospital, Tasmania 7000, Australia

2

Department of Orthopaedics, Royal Perth Hospital, Perth, WA, Australia

radiographers were 43.1 and 20.8° respectively. No statistically significant difference was found in intra- and inter-observer recorded results. No statistically significant differences were found when using different bony landmarks. CT assessment of acetabular component inclination and anteversion is accurate, reliable and reproducible when measured using differing bony landmarks as reference points. We recommend measuring acetabular inclination and anteversion from the inferior acetabular wall/teardrop and posterior ischium respectively. Keywords Anteversion . Inclination . Cup position . Acetabular component . Total hip replacement

Introduction Correct placement of acetabular components in total hip replacement arthroplasty is important to prevent complications such as dislocation, impingement and component loosening. Several studies have established guidelines for safe zones of cup anteversion and inclination [1, 2]. Accurate measurement of the position of the acetabular component is essential to correlate component positioning with outcome and to assess potentially improved methods of cup placement [3]. Plain radiographs are inexpensive and easily obtained post-operatively; however multiple studies have demonstrated that they are unreliable for accurate determination of cup position, especially anteversion [4–7]. Measurement using CT imaging is not well described, nor has it been validated for accuracy and reproducibility. Most studies evaluating CT-based assessment of cup position utilise 3D reconstructions and complex image processing software, which limits their routine use in the clinical setting. Measurements on coronal and axial CT slices are more

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amenable in the routine clinical setting; however the planes and landmarks to use are not well studied. Furthermore, exact definitions of the inclination and anteversion of cup position used in studies is variable, limiting their comparison across different studies. Murray [8] defined multiple methods of describing cup anteversion and inclination. Anatomic inclination is the angle between the acetabular axis and the longitudinal axis of the body; operative inclination is the angle between the acetabular axis and the sagittal plane; radiographic inclination is the angle between the longitudinal axis of the body and the acetabular axis when projected onto the coronal plane. Anatomic anteversion is the angle between the acetabular axis and the transverse axis of the body when the acetabular axis is projected onto the transverse plane; operative anteversion is the angle between the longitudinal axis of the patient and the acetabular axis when projected onto the sagittal plane; radiographic anteversion is the angle between the acetabular axis and the coronal plane [9]. The most frequently used method of referencing the acetabulum in CT measurements is the anterior pelvic plane (APP), defined by the two anterior superior iliac spines (ASIS) and the pubic tubercles, with the APP and the coronal plane being parallel when there is zero pelvic tilt [6, 10–12]. The aims of this cadaver-based study were to (1) Develop a CT-based protocol that is easily reproducible in the clinical setting to accurately measure cup position; (2) Establish which bony reference points facilitate the most accurate estimation of these variables; (3) The inter- and intra-observer reliability of these measurements.

Materials and methods Ethics approval from the HREC of the Royal Perth Hospital was obtained. Cup placement An all-polyethylene acetabular liner was implanted into a cadaveric acetabulum (Fig. 1). The fresh frozen cadaver was without pelvic anomalies or previous surgery to create a more realistic surgical simulation for necessary soft tissue releases. A direct lateral approach to the hip was performed with the cadaver in the lateral decubitus position. The acetabulum was reamed concentrically to 52 mm using the transverse acetabular ligament (TAL) and labrum as anatomical guides for the

Fig. 1 All polyethelene cemented cup placement in the cadaver

positioning of the cup. A 48-mm all-polyethene liner was cemented into the acetabulum to prevent any metal artefact. CT protocol and determination of the reference cup position A conventional pelvic CT scan was performed on the cadaver after cup placement, taking care to ensure the pelvis was square with the table and 1.25-mm slices at 0.6-mm intervals (Kv120) created. The CT slices were then reformatted to produce coronal and axial slices in the anterior pelvic plane (APP). The CT images were transferred to a Freeform-Plus modeling graphics software package (Version 10, 32 bit edition) enabling an identical, virtual, three-dimensional model of the cadaveric pelvis to be created. Using a computer interface, this model could be ‘palpated’ with the aid of a joystick, which allows tactile feedback for accurate mapping in any three-dimensional orientation, and bony landmarks accurately identified within a 0.25-mm error range. Using this software two planes were created. The APP was mapped by palpating the anterior superior iliac spine (ASIS) and pubic tubercles bilaterally and the plane of the liner by palpating multiple points on the surface of the liner (Figs. 2 and 3). The use of these 3D-CT models for cup position has been validated [11, 13–16]. Four operators acquired these data, and each operator performed the analysis five times. The planes were then identified using a simple average of these multiple points and used as a ‘gold-standard’ reference method of definitive cup orientation, against which the radiographic analyses of the coronal and axial CT slices were compared. Radiographic analysis Cup inclination (Fig. 4) was measured from coronal slices in the APP (radiographic inclination) and was measured in

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Fig. 2 Inclination assessment on Freeform-Plus 3D modelling software Fig. 4 Radiographic inclination tangent used on coronal slices

respect to four different horizontal landmark planes: the line between the (1) (2) (3) (4)

ASIS (Fig. 5); Inferior aspect of the sacroiliac joints (SIJs) (Fig. 6); Inferior acetabular wall (teardrop) (Fig. 7); Ischial tuberosities (Fig. 8).

Cup anteversion (Fig. 9) was measured from axial slices perpendicular to the APP (anatomic anteversion) and in respect to three different landmark planes: a line between the

occasions over 5 separate days using the varying bony reference points. Angle measurements were conducted on the AGFA IMPAX WEB1000 digital image viewing platform.

Statistical analysis

Ten experienced radiographers performed repeated measurements of the acetabular inclination and version on five

Two one-way analysis of variance (ANOVA) tests were used to determine differences among the five trials (one-way repeated measures ANOVA), among the ten radiographers and among the different landmark planes. An estimate of intraand inter-observer error was calculated using one-way random effects ANOVA by separating the variance components contained in the model. The errors were expressed as ±2 SD, which is equivalent to 95 % confidence intervals.

Fig. 3 Anteversion assessment on Freeform-Plus 3D modelling software

Fig. 5 Horizontal line through both ASISs for inclination assessment

(1) ASIS (Fig. 10); (2) Anterior SIJ (Fig. 11); (3) Posterior ischium (Fig. 12).

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Fig. 6 Horizontal line through the inferior SIJ for inclination assessment

Fig. 8 Horizontal line through both ischial tuberosities for inclination assessment

Results

observers there is only a small increase in the degree of error for both abduction and anteversion.

Virtually derived definitive acetabular cup orientation was measured showing cup inclination and anteversion as 41.0 (±0.53) and 22.5 (±2.4) degrees respectively. Mean CTbased measurements of cup inclination and anteversion by ten radiographers were 43.1 (±0.15) and 20.8 (±0.16) degrees respectively. Hence, compared to the previous softwaregenerated angles the degree of error was 2.1° for inclination and 1.7° for anteversion (Table 1). No statistically significant difference was found between the different measurement techniques using the different bony landmarks to calculate inclination (p = 0.255) (Table 2) or anteversion (p = 0.324) (Table 3). No statistically significant difference was found in intra- and inter-observer recorded results. Furthermore, as can be seen from Tables 2 and 3, with the presence of additional

Fig. 7 Horizontal line through the inferior acetabular walls for inclination assessment

Discussion Post-operative radiographs are the mainstay of follow-up imaging post THA. While assessment of acetabular cup inclination on plain radiographs has shown good inter- and intraobserver reliability together with good accuracy [9, 17, 18], assessment of anteversion still presents problems. These inaccuracies are secondary to multiple factors while taking the radiograph, such as uncontrollable rotation of the pelvis around the longitudinal craniocaudal axis, the uncontrollable inclination around the mediolateral transverse axis, varying pelvic tilt between patients and variations between the supine/sitting/standing positions [5, 17, 19]. Furthermore, errors of analysis of anteversion angles of the cup measured on cross-table lateral radiographs are larger in patients having a stiffer contralateral hip joint than those having a contralateral hip with good flexion [20]. Even though Widmer [19] has proposed a method for assessment of anteversion on plain radiographs that has shown reproducible results and good correlation with more elaborate trigonometric calculations of anteversion [17, 21, 22], multiple studies have still shown error ranges of more than 10° when anteversion in assessed using these methods on conventional, two-dimensional plain radiographs [23–27]. CT-based methods have shown more accuracy and reproducibility compared to plain radiographs [5, 6, 10, 11, 28] and are considered an acceptable standard when precise assessment of cup position is needed for therapeutic/planning purposes, owing to control over pelvic rotation and/or tilt. Cadaver studies evaluating CT-based 3D models against

Skeletal Radiol Fig. 9 Anatomic anteversion assessment on axial slices

post-operative direct digitisation of bony landmarks using both imageless or CT-based navigation systems have shown 3D-CT to be valid for accurately assessing cup position [16]. Hence, it was used as a gold standard reference method in our study. These methods utilised in research studies to assess cup position on CT involve creating time-consuming 3D models by incorporating CT data into costly and complex image processing or CT-based navigation system software, limiting their practicality in the clinical setting. Furthermore, comparison of interobserver reliability using these 3D-CT models demonstrated that training, experience and practice were necessary to use the method reproducibly and achieve reliable determination of angles of cup position [16]. Hence, validated CT protocols for measuring cup position using landmarks on coronal and axial slices formatted to the APP are needed to apply them simply, reproducibly and inexpensively in a practical clinical setting. In our study, the degree of error comparing the mean measurements across all landmarks on coronal/axial CT slices against the virtually derived cup position on the 3D

software was small, being only 2.1° for inclination and 1.7° for anteversion. This highlights the accuracy of these measurement methods, which are quick and easily reproducible and can be used as a surrogate measure of accurate cup position, rather than the complexity or cost of the reference method. The reference landmarks used in other studies have not been well stated and comparison between different landmarks is also limited in the literature. Wines et al. defined acetabular component version as the angle between a line connecting the lateral anterior and posterior margins of the acetabular component and the sagittal plane, the sagittal plane being defined as the plane perpendicular to a line connecting two identical points on either side of the pelvis [12]. Ybinger et al. [29] measured cup inclination relative to a tangent applied to the ischial tuberosities (ischial tuberosity line) as well as a line through the anterior superior iliac spines to detect differences between the two landmarks. The mean difference in inclination measured with a tangent to the ischial tuberosities as reference line vs. a line connecting the iliac spines was 1.58°

Fig. 10 Anteversion assessment on the ASIS landmark plane

Fig. 11 Anteversion assessment on the anterior SIJ landmark plane

Skeletal Radiol Table 2

Fig. 12 Anteversion assessment on posterior ischium plane

(range, 0.8–5.48). They measured radiographic anteversion by using a perpendicular plane to the centre of the largest cup diameter identified in the coronal plane. They further validated the precision of their direct angle measurements for both inclination and anteversion by comparison with two additional independent measurements (surgical anteversion and anatomic anteversion) that have a known co-relation to the radiographic anteversion and inclination and found differences of only 0.98 (range, 0.8–3.68) for inclination and 0.68 (range, 0.8–2.6) for anteversion. We compared four different landmark planes for radiographic inclination and three different landmark planes for anatomic anteversion and found no differences across all landmarks, suggesting that whichever landmark is used is equally accurate and can be up to the discretion of the user. Our preferred landmarks were the inferior acetabular wall/teardrop for acetabular and the posterior ischium for anteversion and incorporated as part of the Perth CT Hip Protocol. Studies validating the reference 3D-CT processed models have shown no significant intra- or inter-observer errors [11], and this was similarly seen with our measurement methods as well, validating its reproducibility and accuracy compared to the reference method. Similarly, Hernandez et al. [30], Ybinger et al. [29] and Wines et al. [12] reported negligible levels of inter- and intra-observer error for their 2D-CT methods. Furthermore, we utilised ten different radiographers, each conducting an analysis five times for the CT measurements and four different operators conducting measurements five times for the reference method. This is a much larger observer pool than used in the studies mentioned above.

Inclination landmarks

Mean

Intra-observer error

Inter-observer error

ASIS

43.4

±1.59

±1.61

Inferior SIJ

43.1

±1.63

±2.01

Inferior acetabular walls Ischial tuberosities

43.0 43.0

±1.82 ±2.38

±1.89 ±2.58

Using CT scans for judging cup position provides several advantages. Measurements are more precise, are not dependent on patient position, provide greater comfort to the patient as the opposite extremity does not have to be elevated, allows comparison of the version of the cup with the version of the anatomic acetabular cavity in unilateral arthroplasty cases and can also provide information on the femoral component anteversion. There are some practical limitations with routine use of CT post-operatively including the costs of CT scans, the length of time needed for measurements on CT data and greater radiation exposure compared to plain radiographs. Some post-operative CT protocols utilised in studies evaluating cup position have a radiation dose of up to 10 mSv, which is 14 times that of prevalent pelvis radiographs (0.7 mSv) [24]. However, Aria et al. demonstrated that minimal radiation dose CT protocols of 1.1 mSv (which is less than two times that of a radiograph) maintained image quality for measurements of cup orientation without increasing error compared to higher dose protocols [31]. Our study does have some limitations. This study was an in vitro study of a single cadaver on an all poly-cemented cup on a single CT scanner, which clearly deviates from imaging obtained in clinical practice by not accounting for the anatomic variation seen in the general population or the possible variation of measurements with different cup types, bearing surfaces or presence of hooded/lateralised liners, which consequently may impact on the reproducibility of the accuracy of our measurements. The observers in our study were experienced radiographers and may not be reproduced by experienced clinicians. Furthermore, even though our observer pool was larger than in previous studies these results may not be replicated if an even larger observer pool with varying experience is utilised.

Table 3 Table 1

Error difference on CT measurements and reference method

Inclination Anteversion

Reference method

CT measurement

Error

41.0 22.5

43.1 20.8

2.1 1.7

Measurements of inclination with different landmarks

Measurements of anteversion with different landmarks

Anteversion landmarks

Mean

Intra-observer error

Inter-observer error

ASIS Anterior SIJ Posterior ischium

20.7 20.7 21.0

±1.46 ±1.52 ±1.64

±1.64 ±1.64 ±2.45

Skeletal Radiol

Conclusions Post-operative CT assessment of acetabular component position is accurate, reliable and reproducible when measured using different bony landmarks as reference points. We recommend measuring acetabular inclination and anteversion from the inferior acetabular wall/teardrop and posterior ischium respectively. The Perth CT Hip Protocol is simple and easily reproducible in the clinical setting both in the routine assessment of hip arthroplasty patients and as a research tool; hence it can be utilised in future research to help compare different navigation softwareregistration programmes and compare results of navigated joints with conventional arthroplasty. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.

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