1 Introduction

Acetabular component malpositioning during total hip arthroplasty (THA) can lead to complications such as wear, osteolysis, impingement, and instability.^1–3 To minimize component malpositioning, anatomic safe zones for acetabular inclination and anteversion have been described.^4,5 Intra-operative assessment of these parameters is determined by a number of factors and can be influenced by the position of the pelvis at the time of implantation.

For optimal lateral decubitus positioning, the pelvis should be positioned ‘neutral’ for the duration of the procedure with the axes of the pelvis parallel to the operating room table.^{6 and 7} Previous studies have shown significant intra-operative motion during total hip arthroplasty, with rotation across all three axes.^7–10 Rotation across each of these planes has been previously shown to influence acetabular inclination and version, potentially leading to cup malposition.¹¹ A previous survey among the British Hip Society noted up to a 35.7% of orthopedic surgeons had issues with their support in the lateral decubitus position.¹¹ While the effect of pelvic rotation on acetabular orientation has been well-studied, comparative data on the effects of specific hip positioners on pelvic rotation and translation is limited.

The purpose of this study is to quantify and compare the stability of four commercially available hip positioners (Beanbag, Stulberg, Pegboard and ExactFit) by tracking the translation and rotation of the pelvis using an infrared marker and camera system. We hypothesize that different positioners will have significant differences in motion with various movements of the hip.

2 Methods

2.2

2.2 Hip positioners

Four commercially available hip positioners were examined in this study: Beanbag, Pegboard, Stulberg, and ExactFit (Fig. 1). The Beanbag (Allen® Hug-u-Vac® Lateral Positioner, Hill-Rom Allen Medical, Acton, MA) consists of small beads within a bag which conforms to the sides of the torso when air is vacuumed out. The Pegboard (MorphBoard®, IMP, Plainville, CT) consists of a baseboard into which pegs are inserted; pegs are placed anterior to the pubis and pectoral muscle and posterior to the sacral body and thoracic spine. The Stulberg (Stulberg Hip Positioner, Innomed, Savannah, GA) provides stability by compressing both anterior superior iliac spines (ASIS) and the sacrum with small foam pads. The ExactFit (De Mayo Hip Positioner®, IMP, Plainville, CT) provides stability similarly to the Stulberg but allows for more refined motion of the ASIS pads and has a crossbar which connects both sides of the positioner, providing added stability.

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Fig. 1 Four Commercially Available Hip Positioners Utilized. (A) Beanbag positioner (Allen® Hug-u-Vac® Lateral Positioner, Hill-Rom Allen Medical, Acton, MA) (B) Pegboard positioner (MorphBoard®, IMP, Plainville, CT) (C) Stulberg positioner (Stulberg Hip Positioner, Innomed, Savannah, GA) (D) ExactFit positioner (De Mayo Hip Positioner®, IMP, Plainville, CT).

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2.3

2.3 Coordinate system setup

An infrared marker and camera system (Optotrak 3D Investigator, NDI, Waterloo, Canada) was used to track the motion of the pelvis compared to the surgical table. Two markers were used: one marker was mounted on a rod inserted into the iliac crest and the second marker on the side rail of the operating table. The camera was positioned superior to the head of the patient and the markers were pointed in the direction of the camera to avoid the surgeon or patient blocking the line of sight of the system (Fig. 2).

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Fig. 2 Coordinate system utilized for the pelvis and optical tracking system setup.

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The hip coordinate system was defined using an infrared probe via fluoroscopic guidance according to ISB standards.20 The origin was set at the center of the femoral head by taking the centroid of two equidistant points on a line passing through the center of the femoral head. The z-axis was defined as the line parallel to the line connecting the right and left ASIS. The x-axis was orthogonal to z, lying in the plane connecting the right and left ASISs and the midpoint of the posterior superior iliac spines, pointing anteriorly. Finally, the y-axis was perpendicular to both z and x, pointing crainally.

3 Results

All hip positioners resulted in some degree of motion of the pelvis during the various movements. Internal rotation and flexion created the greatest total rotation of the pelvis (Fig. 3). The ExactFit hip positioner resulted in the least amount of overall rotation of the pelvis, with a maximum rotation of the pelvis of up to 3.2°. The Stulberg and Pegboard positioners showed intermediate stability, with rotations up to 7.8° and 17.3°, respectively. The Beanbag showed poor stability, allowing rotation up to 41.5° (Fig. 3).

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Fig. 3 Maximum rotation (in degrees) for each hip positioner that occurred during each movement.

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The greatest amount of rotation was present in the axial plane, followed by the sagittal and coronal planes, respectively (Fig. 4). Axial rotation was greatest with the Beanbag, followed by the Pegboard, Stulberg, and Exact Fit positioners. The largest amount of motion in this plane resulted from internal rotation of the hip. Sagittal and coronal rotation were similarly greatest with Beanbag, followed by the Pegboard, Stulberg, and Exact Fit positioners. Specific measurements of total rotation and rotation in the sagittal, coronal, and axial planes with different motions of the hip are included in supplemental Table 1.

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Fig. 4 Maximum rotation (in degrees) of the hip that occurred around each axis (Rz: Anteversion, Rx: Lateral Bending, Ry: Axial Rotation) for each hip positioner that occurred during each movement.

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The largest translation observed was with the beanbag positioner, followed by the Pegboard, Stulberg, and ExactFit positioners (108.1 mm vs. 52.2 mm vs. 17.0 mm vs. 13.8 mm) (Fig. 5). Overall translation was greatest medial to lateral, followed by anterior to posterior translation and superior to inferior translation (Fig. 6).

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Fig. 5 Maximum resultant translation (in mm) for each hip positioner that occurred during each movement.

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Fig. 6 Maximum translation (in mm) of the hip that occurred along each axis (Z: Medial-Lateral, X:Anterior-Posterior, Y:Superior-Inferior) for each hip positioner that occurred during each movement.

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4 Discussion

In this comparative laboratory study, we found that the ExactFit hip positioner was associated with the least pelvic rotation and translation during simulated movements of total hip arthroplasty, followed by the Stulberg, Pegboard, and Beanbag positioners.

Variation in multiplanar tilt of the pelvis in the lateral decubitus position is a known cause for component malposition. Sagittal tilt of the pelvis has been found to most significantly affect anteversion of the acetabulum, with a reported increase between 0.70 and 0.80° for every degree of posterior pelvic tilt.^12–18 The effect of sagittal tilt on cup inclination is less pronounced, with a smaller, non-linear increase (0.20–0.50°) with each degree of posterior tilt.^12,13,16 Coronal tilt of the pelvis, or lateral bending can also increase abduction and anteversion of the acetabulum.^14,19 Axial rotation or “roll” of the pelvis is also believed to alter both acetabular version and inclination, with a reported changes of 0.5–0.6° of version with each degree of rotation.^12,20 Rotation along the three axes during total hip arthroplasty in the lateral decubitus has been reported to be up to 32° of axial rotation or roll, 25° of sagittal tilt, and 10° of coronal tilt, or pitch. A recent study found each degree of pelvic roll, tilt, and pitch to influence anteversion by −0.69°, +1.00°, and +0.26° and inclination by 0.29°, 0°, and 0.95° respectively.¹² Inadvertent alteration of these parameters intra-operatively occurs frequently and can significantly increase the risk for cup malpositioning.^{9,13,16,17,21,22}

The effects of intra-operative pelvic translation in the lateral decubitus position during THA has been less studied. We saw the greatest amount of translation in the coronal plane, followed by translation in the sagittal plane. While it is unlikely that pelvic translation in isolation would cause significant implant malpositioning, greater translation likely increases the potential for pelvic rotation and may lead to greater difficulties with THA.

The bean bag was associated with the highest amount of pelvic rotation and translation overall. Unlike the other devices used, the bean bag lacks any form of rigid fixation to the operating room table, predisposing the pelvis to greater motion. The pegboard positioner had the second largest amount of pelvic rotation and translation present. The most rotation was seen in the axial plane, followed by motion in the coronal and sagittal planes, respectively. The higher amount of rotation and translation may be due to a smaller amount of compression provided on the pelvis with the pegboard when compared to the Stulberg and ExactFit positioners. The Stulberg positioner has two points of fixation anteriorly, using the anterior superior iliac spine (ASIS) and pubis. This type of positioner with three-point stabilization has been shown to have greater compression across the pelvis compared to conventional positioners with lesser intra-operative motion, particularly sagittal tilt.^7,23 In our study, we similarly found less rotation and translation in all planes using the Stulburg positioner compared to the bean bag and pegboard positioners. The ExactFit hip positioner consists of two anterior pads with a posterior pad as well as a cross arm to add additional stability. We found that the ExactFit was associated with the least overall rotation and translation among the different positioners. The addition of the cross arm with this positioner likely confers additional stability to the pelvis with movement.

Of the simulated motions performed, internal rotation and flexion generally resulted in the greatest rotation of the pelvis. These are two of the most frequently performed motions during THA.⁸ Awareness of the potential for pelvic motion during these motions may help better anticipate pelvic rotation intra-operatively and facilitate corrections in positioning. It is also important to note that optimal patient positioning is only one of multiple strategies to ensure accurate implant placement in THA. Intra-operative landmarks, inclinometers, intra-operative fluoroscopy, and computer assisted navigation are important tools that can further aid with optimal implant placement.^6,24–26 These should be used when available and needed to further ensure desirable implant placement.

5 Conclusion

The four hip positioners tested allowed for varying degrees of rotation of the pelvis over the course of the study. The ExactFit positioner had the least rotation and translation, followed by the Pegboard, Stulberg and Beanbag positioners. Further studies are required to better correlate these findings with clinical outcomes.

Funding/sponsorship

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

Informed consent

There were no patients included in this study, thus informed consent was not sought.

Institutional ethical committee approval

No institutional ethical committee approval was sought as this study did not involve inclusion of any patients or patient data.

Author Contribution

Ashish Mittal: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing, Visualization, Project administration Nikole Chetty: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft Tuan Pham: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft Richard Raji: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft Ishan Shah: Conceptualization, Writing – original draft, Writing – review & editing Jeremi Leasure: Conceptualization, Methodology, Software, Validation, Resources, Writing – review & editing, Supervision William McGann: Conceptualization, Writing – review & editing, Visualization, Supervision Edward DeMayo: Conceptualization, Methodology, Investigation, Resources, Writing – review & editing, Visualization, Supervision, Project administration