How unattended lateral pelvic tilt during total hip arthroplasty will affect acetabular cup inclination angle, a simplified demonstration on a sawbone pelvis model

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BACKGROUND

It is undeniable that proper acetabular cup placement (including inclination and anteversion) during total hip arthroplasty (THA) is an essential factor for obtaining proper long-term functional outcomes, decreasing instability risk, and improving implant survival [1–6].

Regardless of the surgical approach, most surgeons operate while patients are in a lateral decubitus position; furthermore, surgeons using freehand technique or mechanical alignment guides (MAG) for acetabular cup placement rely on external landmarks such as the floor level or local anatomical landmarks for proper cup positioning [5, 7–9].

Acetabular cup positioning and the safe zone for placement have evolved since considering the spine-pelvis-hip interrelationship and the effect of spine mobility on acetabular cup orientation in different positions [10–12]. It is now recommended that spinopelvic mobility is considered during preoperative planning to obtain a more personalized cup position to guard against impingement and early postoperative instability [9, 13, 14].

For adjusting the cup inclination angle while utilizing freehand or MAG-assisted techniques, the surgeon assumes that the pelvis is in an optimum position where the pelvis transverse axis of the pelvis (trans-ischial or trans-tear drop lines) is perpendicular to the operative table of the floor level [1, 5]. However, this is not true for all patients, as various factors could affect the pelvis position in the coronal plane after patient positioning, leading to unde lateral pelvic tilt affecting the cup inclination angle [1, 10, 15, 16].

AIM

The current experiment aimed to demonstrate how the unattended lateral pelvic tilt affects the acetabular cup inclination angle during THA on a pelvis sawbone model.

METHODS

A sawbone model of a normal pelvis (no deformities) was utilized for this demonstration. After ensuring that the operative table was parallel to the floor level, the pelvis model was fixed to the table. In a lateral decubitus position situation, the pelvis could assume one of three positions (Fig. 1): first is the neutral position, where the line connecting the anterior superior spine (ASIS) bilaterally is perpendicular to the floor level (90°). Second, the adduction position, where the upper ASIS (the side to be operated upon) is tilted toward the patient's feet (caudad), has an angle of less than 90° (minus value). Last is the abduction position, where the upper ASIS is tilted toward the patient's head (cephalad), and the angle is more than 90° (positive value).

Fig. 1. Possible pelvis orientation in the coronal plane while a patient in a lateral decubitus position. a, neutral position where the angle between the pelvis transverse axis and the table level is 90°. b, pelvis adduction (the upper hip is rotated caudad) where the angle is < 90°. c, pelvis abduction (the upper hip is rotated cephalad) where the angle is > 90°.

Рис. 1. Возможные варианты ориентации таза во фронтальной плоскости при положении пациента на боку. a — нейтральное положение, при котором угол между поперечной осью таза и уровнем операционного стола составляет 90°; b — приведение таза (верхний тазобедренный сустав ротирован каудально), при котором угол < 90°; c — отведение таза (верхний тазобедренный сустав ротирован краниально), при котором угол > 90°.

The sawbone pelvis model was fixed to the operative table, mimicking the previously mentioned positions [neutral = 0° (90°), adduction of -10° (80°), and abduction of +10°(100°)], and the angles were confirmed using a smartphone-free protractor application (Fig. 2, a). Then, an acetabular cup attached to its inserter was used to mimic cup application inside the sawbone model acetabulum; the insertion angle was kept at 40° (within the safe zone for acetabular cup inclination [17]) in relation to the floor level, confirmed by a smartphone-free spirit level application. The sawbone pelvis model was blurred to resemble an intraoperative situation where a surgeon using a manual instrument and freehand technique relies on the visual assessment of the angle in relation to the floor level (Fig. 2, b).

Fig. 2. Experiment performed on a normal pelvis sawbone model fixed to an operative table parallel to the floor. a, the model is attached to the operative table in three different positions (1: neutral, 2: adduction 10°, and 3: abduction 10°). b, an acetabular cup is applied to the sawbone model acetabulum in the three specimens at the same intended inclination angle (40°), confirmed using a smartphone spirit-level application. c, the final acetabular cup inclination angle measured for the three specimens as the angle between the acetabular cup and the trans-ischial line.

Рис. 2. Эксперимент, выполненный на стандартной модели таза Sawbone, зафиксированной на операционном столе параллельно полу. a — модель таза закреплена на операционном столе в трёх различных положениях (1 — нейтральное, 2 — приведение 10°, 3 — отведение 10°); b — вертлужный компонент установлен в вертлужную впадину модели таза во всех трёх положениях с одинаковым заданным углом инклинации (40°), что подтверждено с помощью мобильного приложения-уровня; c — итоговый угол инклинации вертлужного компонента, измеренный для трёх вариантов как угол между плоскостью вертлужного компонента и трансишиальной линией.

Then, the cup inclination angle within the pelvis sawbone model was assessed after rotating all the images, as the angle between the inter-ischial line and long axis of the eclipse formed by the cup (connecting the inferomedial and superolateral edges) [18]. All measurements were performed using the Surgimap program (Fig. 2, c).

Study Design

An experimental (proof-of-concept) simulation study was carried out on a pelvis sawbone model.

Research Question

How does lateral pelvic tilt (adduction, neutral, abduction) during THA in the lateral decubitus position affect the final radiographic acetabular cup inclination angle when it is inserted at a fixed angle relative to the floor level?

Hypotheses

• Null Hypothesis (H₀): unattended lateral pelvic tilt (adduction or abduction) during acetabular cup insertion does not affect the final acetabular cup inclination angle relative to the anatomical pelvic plane.
• Alternative Hypothesis (H₁): unattended lateral pelvic tilt during cup insertion significantly alters the final measured acetabular cup inclination angle relative to the anatomical pelvic plane.

Enrollment, Sampling and Randomization

• Enrollment / Sample: A single, normal (non-deformed) sawbone hemipelvis model was used. No human or animal subjects were involved.
• Sampling: This was a convenience sample of a standard anatomical model.
• Randomization: The conditions were tested in a fixed, non-randomized sequence, as this was a proof-of-concept demonstration.

Experiment and Data Collection Methods

A sawbone model of a normal pelvis (no deformities) was utilized for this demonstration. After ensuring that the operative table was parallel to the floor level, the pelvis model was fixed to the table. In a lateral decubitus position situation, the pelvis could assume one of three positions (Fig. 1): first is the neutral position, where the line connecting the anterior superior spine (ASIS) bilaterally is perpendicular to the floor level (90°). Second, the adduction position, where the upper ASIS (the side to be operated upon) is tilted toward the patient's feet (caudad), has an angle of less than 90° (minus value). Last is the abduction position, where the upper ASIS is tilted toward the patient's head (cephalad), and the angle is more than 90° (positive value).

The sawbone pelvis model was fixed to the operative table, mimicking the previously mentioned positions [neutral = 0° (90°), adduction of -10° (80°), and abduction of +10°(100°)], and the angles were confirmed using a smartphone-free protractor application (Fig. 2, a). Then, an acetabular cup attached to its inserter was used to mimic cup application inside the sawbone model acetabulum; the insertion angle was kept at 40° (within the safe zone for acetabular cup inclination [17]) in relation to the floor level, confirmed by a smartphone-free spirit level application. The sawbone pelvis model was blurred to resemble an intraoperative situation where a surgeon using a manual instrument and freehand technique relies on the visual assessment of the angle in relation to the floor level (Fig. 2, b).

Then, the cup inclination angle within the pelvis sawbone model was assessed after rotating all the images, as the angle between the inter-ischial line and long axis of the eclipse formed by the cup (connecting the inferomedial and superolateral edges) [18]. All measurements were performed using the Surgimap program (Fig. 2, c).

Ethical review

The ethical committee of our institution waived the approval for this study, as no human participants were involved.

Statistical analysis

Was not required, as a simple description of the results was sufficient.

RESULTS

When the pelvis was in the neutral position (0°), the final cup inclination angle was the same as the intended intraoperative angle (40°); however, when the pelvis was tilted, the resulting inclination angle was changed according to the direction of the lateral pelvic tilt. In the case of pelvic abduction, the inclination angle was reduced from 40° to 30° by the same amount of the pelvic tilt (10°). Moreover, the opposite was true for pelvic adduction, where the inclination angle was increased than the intended inclination angle by an amount equal to the lateral pelvic tilt (50° instead of 40°).

DISCUSSION

Proper acetabular cup placement remains crucial for obtaining optimum outcomes and longer survival of THA [5]. Various factors could affect the cup position, including surgeon factors such as experience [19, 20], patient factors such as body mass index, distorted local anatomical landmarks, severe deformities [21], and surgical factors including approaches, utilizing advanced technologies (computer-navigation and robotic-assisted surgery), and spinopelvic relationship [3, 22–24]. However, in most cases, it combines more than one of the previously mentioned factors [4, 5, 17, 19–22, 25].

Obtaining near anatomical acetabular cup position entails adjusting cup inclination, anteversion, and restoring the hip center of rotation (COR) [5, 9]. In the current sawbone pelvis model demonstration, the inclination position was only tested, which showed significant affection by lateral pelvic tilt, which is usually passed unnoticed by surgeons, especially after patient draping.

Various studies (clinical and cadaveric) evaluated the possible factors leading to lateral pelvic tilt [16, 26, 27]. In a clinical setting, Okutani et al. studied 363 unilateral THA patients (341 primary and 22 Revision), and an anteroposterior (AP) radiograph of the pelvis in the lateral decubitus position was performed on all patients. The authors found a lateral pelvic tilt of about 4° (ranging from -15.8 to 10) with the operated hip being located caudally (pelvis adduction), which they attributed to a high body mass index (BMI) and limited motion of the operated hip [16]. Furthermore, the authors reported that after considering the amount of lateral pelvic tilt, the percentage of their cups placed within the safe zone for inclination improved from 61.4% to 93.9% [16].

Hill et al. [28] reported a 13-degree difference between the cup inclination angle measured in the postoperative radiographs and the intended intraoperative angles; they attributed this difference to an unnoticed lateral pelvic tilt that occurred after patient positioning.

In a comparative study by Khalifa et al., where the authors compared the resultant cup inclination angle after using smartphone-assisted cup insertion to the freehand technique, they reported that the percentage of cups placed within the safe zone for inclination improved from 63% to 93% after utilization of smartphone assisted technique, however, one crucial step they reported preoperatively, which was the clinical assessment of the possible lateral pelvic tilt as an angle between line connecting marks placed on the ASIS bilaterally and the table level, the mean lateral pelvic tilt in this study was average pelvic tilt measured in the study group was +0.9° ± 4.52 (ranging from +7° to -5°) [26, 29].

To anticipate this possible lateral pelvic tilt and incorporate the value while positioning the cup, surgeons could measure it clinically, as reported by Khalifa et al. [26, 29], but this method could be less accurate. Another more precise method is to measure this possible tilt in a specific radiographic view, an AP pelvis view obtained while the patient is in a lateral decubitus position (mimicking the intraoperative position) (Fig. 3), which was proposed by some authors [1, 9, 16].

Fig. 3. A real case demonstration of how an anteroposterior (AP) view in a lateral decubitus position was obtained. a, an AP pelvis view (in supine position) of a female patient 41 years old showing bilateral hip osteoarthritis. b, and c, Schematic diagram and real-life image showing patient position on the radiology table for obtaining the AP view in lateral decubitus position. d, the resultant radiograph showing pelvis adduction of about 2°. e, the postoperative radiograph showing a cup inclination angle of 36° (within the safe zone for inclination).

Рис. 3. Клинический пример, демонстрирующий получение передне-задней рентгенограммы таза в положении пациента на боку. a — передне-задняя рентгенограмма таза (в положении лёжа на спине) у пациентки 41 года с двусторонним коксартрозом; b, c — схематическое изображение и реальное фото, демонстрирующие положение пациентки на рентгенологическом столе для получения передне-задней проекции таза в положении на боку; d — полученная рентгенограмма, демонстрирующая приведение таза приблизительно на 2°; e — послеоперационная рентгенограмма, на которой угол инклинации вертлужного компонента составляет 36° (в пределах «безопасной зоны» инклинации).

One major limitation of the current demonstration is the lack of considering the effect of anteversion on the resultant change in inclination angle, as it is postulated that the radiographic cup inclination is greater than the operative intended cup inclination, reaching up to 13 degrees more owing to the effect of anteversion on radiographic projection [28, 30]. Furthermore, in a real-surgery scenario, other factors leading to a discrepancy between the intended operative and the postoperative radiographic inclination were not evaluated; as the patient is placed in a lateral decubitus position, the pelvis tends to tilt posteriorly, which could affect the anteversion and subsequently the inclination [31]. The possible unnoticed motion of the pelvis during surgery leads to errors in obtaining the targeted angle [30, 32]. Moreover, the pelvis could move while reaming the acetabulum or during the first impaction [32]. Last, the current demonstration considered pelvic motion in only the coronal plane. At the same time, the acetabular cup position could be affected by pelvic motion in all three planes (sagittal, coronal, and axial) [1].

CONCLUSION

The lateral pelvic tilt is one of many factors affecting cup placement during THA, especially on cup inclination. It should be evaluated preoperatively, and if present, its value should be considered during acetabular cup placement, especially for young, less experienced surgeons and surgeons using manual instruments and relying on external or local anatomical landmarks for cup positioning.

ADDITIONAL INFORMATION

Author contribution: Ahmed A. Khalifa. carried out the conception, performed the experiment, carried out a literature search, drafted the manuscript, and prepared the images. Author have approved the final version before publication and have also agreed to be responsible for all aspects of the work, ensuring that issues relating to the accuracy and integrity of any part of it are properly addressed and resolved.

Acknowledgments: The author would like to thank Dr. Mohamed Khaled and Mrs. Aziza Hassan for their help and assistance.

Ethics approval: Not applicable

Funding sources. No funding.

Disclosure of interests: The author declares the absence of relationships, activities and interests (personal, professional or financial) related to third parties (commercial, non-profit, private), whose interests may be affected by the content of the article, as well as other relationships, activities and interests over the past three years, which must be reported.

Provenance and peer-review: This paper was submitted to the journal on an initiative basis and reviewed according to the usual procedure. Two external reviewers, a member of the editorial board and the scientific editor of the publication participated in the review.

ДОПОЛНИТЕЛЬНАЯ ИНФОРМАЦИЯ

Вклад авторов: Ахмед А. Халифа — разработка концепции исследования, проведение эксперимента, поиск и анализ литературы, подготовка рукописи и иллюстраций. Автор одобрил рукопись (версию для публикации), а также согласился нести ответственность за все аспекты настоящей работы, гарантируя надлежащее рассмотрение и решение вопросов, связанных с точностью и добросовестностью любой её части.

Благодарности. Автор выражает благодарность д-ру Мохамеду Халеду и г-же Азизе Хассан за оказанную помощь и содействие.

Этическая экспертиза. Не применимо.

Источники финансирования. Отсутствуют.

Раскрытие интересов: Автор заявляет об отсутствии отношений, деятельности и интересов за последние три года, связанных с третьими лицами (коммерческими и некоммерческими организациями), интересы которых могут быть затронуты содержанием статьи.

Рассмотрение и рецензирование. Настоящая работа подана в журнал в инициативном порядке и рассмотрена по обычной процедуре. В рецензировании участвовали два внешних рецензента, член редакционной коллегии и научный редактор издания.

About the authors

Ahmed A. Khalifa

Author for correspondence.
Email: ahmed_adel0391@med.svu.edu.eg
ORCID iD: 0000-0002-0710-6487

MD, FRCS, MSc., Assistant Professor, South Valley University Hospital

References

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2. Fig. 1. Possible pelvis orientation in the coronal plane while a patient in a lateral decubitus position. a, neutral position where the angle between the pelvis transverse axis and the table level is 90°. b, pelvis adduction (the upper hip is rotated caudad) where the angle is < 90°. c, pelvis abduction (the upper hip is rotated cephalad) where the angle is > 90°.

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3. Fig. 2. Experiment performed on a normal pelvis sawbone model fixed to an operative table parallel to the floor. a, the model is attached to the operative table in three different positions (1: neutral, 2: adduction 10°, and 3: abduction 10°). b, an acetabular cup is applied to the sawbone model acetabulum in the three specimens at the same intended inclination angle (40°), confirmed using a smartphone spirit-level application. c, the final acetabular cup inclination angle measured for the three specimens as the angle between the acetabular cup and the trans-ischial line.

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4. Fig. 3. A real case demonstration of how an anteroposterior (AP) view in a lateral decubitus position was obtained. a, an AP pelvis view (in supine position) of a female patient 41 years old showing bilateral hip osteoarthritis. b, and c, Schematic diagram and real-life image showing patient position on the radiology table for obtaining the AP view in lateral decubitus position. d, the resultant radiograph showing pelvis adduction of about 2°. e, the postoperative radiograph showing a cup inclination angle of 36° (within the safe zone for inclination).

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