Acetabular Orientation With Different Pelvic Registration Landmarks

Abstract

This study determined the relationship of various pelvic landmarks to the pelvic plane in the lateral position. Five whole-body cadavers were used in this study. All navigation data were collected using the OrthoPilot navigation system (B. Braun Aesculap, Tuttlingen, Germany) to register all landmarks. The pelvic plane was registered with the body in the supine position for comparison. The ipsilateral posterosuperior iliac spine (PSIS) and contralateral PSIS were registered for comparison of the line made by the 2 anterosuperior iliac spines (ASIS) in the supine position. Registration points along the acetabular rim at the 12-, 3-, and 9-o’clock positions were recorded, and the transverse acetabular ligament (TAL) was registered using the ends and middle of the ligament for the 3 registration points. Inclination as determined by the TAL was 49.7°±25.8°, whereas the acetabular rim resulted in an inclination of 36.3°±7.2°. When the interspinal posterior line was calculated, the difference in inclination compared with the pelvic plane was 1.8°±1.7°. Anteversion using the acetabular rim resulted in a difference of 8.1°±4.9° and using the TAL resulted in a difference of 13.4°±7.9°. The difference in anteversion using the pelvic plane and the posterior interspinal line was 1.2°±1.3°. This study determined that the alternative landmarks of the acetabular rim and the TAL were not as accurate as using the posterior interspinal line as determined by registering the PSIS.

The number of total hip arthroplasty (THA) procedures performed every year is significantly increasing.¹ By the year 2018, the number of primary THA cases performed is expected to have increased exponentially. As health care costs also increase, surgeons should concentrate on how to best perform these procedures to provide the greatest longevity of the THA.

Malpositioning of an acetabular component increases wear, increases pressure on the bearing surface, and may be responsible for runaway wear on hard-on-hard bearing surfaces in THA.² Because these are known effects of malpositioning, it is even more important to ensure that the acetabular implant is well aligned to maximize the longevity of the implant. Currently, the pelvic plane is the most used anatomic point of registration upon which the position of the acetabular component is based.^3,4 There are drawbacks to using the pelvic plane, however, such as hyperlordosis or hypolordosis of the lumbosacral spine,^5,6 aberrant registration in patients with large body habitus, and problems registering the pelvis in the lateral position for a posterior approach after the patient is prepped and draped (Figure 1).⁷

Figure 1: The lateral position after draping for a THA, even in a nonobese patient, can limit the access to the contralateral anterosuperior iliac spine and pubic symphysis for registration of the pelvic plane.

Because the posterior approach is used by most surgeons in the United States and is being used abroad, especially in the United Kingdom, for hip resurfacing procedures, it would be helpful to have more accessible and reliable landmarks for registration of the pelvis in the lateral position.

This study investigates the use of the posterior interspinal line (ie, the line drawn between the posterosuperior iliac spines [PSIS]) as a reference rather than registering the contralateral anteroposterior iliac spines (ASIS), which may be difficult to access due to body habitus or draping. The hypothesis was that, in the lateral position, the interspinal line would give results comparable to using the ASIS in the supine position.

Materials and Methods

Five whole-body cadavers were used in this study. The specimens were fresh-frozen and allowed to defrost overnight before being used. Navigation data were collected using the OrthoPilot navigation system (B. Braun Aesculap, Tuttlingen, Germany) to register all landmarks. Data were then manipulated and analyzed using CATIA software (Dassault Systemes, Suresnes Cedex, France) to compare the difference in the resulting acetabular orientation for each set of registration landmarks.

Normal pelvic plane landmarks (the ASIS and the pubic symphysis) were measured first with the body in the supine position (Figure 2) and were used as control data. To obtain these measurements, the tip of the registration probe was buried under the skin and placed on the bone to represent a true pelvic plane. The same landmarks measured from the skin surface but with the body in the lateral position were obtained to quantify the effects of less accessibility of the contralateral ASIS.

The acetabular rims were recorded at the 12-, 3-, and 9-o’clock positions to represent quarters of the rim. The plane of the face of the acetabulum was then calculated, and its resulting orientation was compared with values obtained using other methods as well.

Figure 2: The pelvic plane in this study is defined by the ipsilateral and contralateral anterosuperior iliac spines and the pubic symphysis.

The transverse acetabular ligament was registered at 3 points (at the attachments and at midsubstance) and the plane made by these registered points was also used for comparison.

The most prominent aspects of the ipsilateral PSIS and contralateral PSIS were then registered and compared with the line made by the ipsilateral ASIS and contralateral ASIS. The interspinal posterior line was measured because of its accessibility in the lateral position and because patients with increased body mass index (BMI) tend to have greater anterior accumulation of fat tissue in the pelvis than they do posteriorly (Figure 3). The premise was that the line could be translated to the ipsilateral ASIS in the lateral position for a more reliable option if the patient’s body habitus made it difficult to obtain a reliable pelvic plane.

Figure 3: The line between the posterior-superior iliac spines is more accessible in the lateral position.

The differences in inclination and anteversion for each set of points were then determined by postprocessing and compared with the measurements of the pelvic plane in the supine position described earlier. Analysis of variance was used for comparison. P<.05 was used as the level of significance to compare each of the techniques described with the values produced by the pelvic plane in the supine position.

Results

Inclination as determined by the transverse acetabular ligament was 49.7°±25.8°. The acetabular rim resulted in an inclination of 36.3°±7.2°. When the interspinal posterior line was calculated, the difference in inclination compared with the pelvic plane was 1.8°±1.7° (42.4°±4.4°). Anteversion using the acetabular rim resulted in a difference of 8.1°±4.9° and using the TAL resulted in a difference of 13.4°±7.9°. The difference in anteversion using the pelvic plane and the posterior interspinal line was 1.2°±1.3° (19.1±°4.7°) and was statistically significantly different from the acetabular rim but not the TAL. There were no statistically significant differences between the posterior interspinal line and the pelvic plane.

Discussion

This study proves that using more accessible landmarks in the lateral position can be as accurate as using the pelvic plane in the supine position, giving a more accessible and more accurate alternative to the ASIS portion of the pelvic plane, especially in patients with large body habitus. Using this technique would provide reliable registration for acetabular positioning without having to prep and drape a patient in supine position before positioning in the lateral decubitus position for a posterior or direct lateral approach.

This technique does not preclude other problems with accurately measuring the pelvic plane, such as spinal lordosis and pelvic tilt. Other outliers of acetabular anteversion and inclination may be due to pelvic tilt and to differences in sitting and standing positions.^6-8

A functional analysis may still be necessary to optimize acetabular orientation. Because pelvic tilt has been shown to change after THA, it is difficult to know how to assess functional placement. For now, the pelvic plane, acetabular index,⁹ and transverse acetabular ligament¹⁰ are the landmarks for registration and placement of the acetabular component. The TAL can be quite distorted due to overlying osteophytes and, in our experience, has not been a good predictor of inclination. The acetabular center index depends on the face of the acetabular rim, and in severe cases this is also distorted from osteophyte formation.

Knowing that computer navigation aids in the proper placement of components during THA^11,12 makes its use in everyday orthopedics inevitable in some aspects. The fact that so many surgeons in the United States perform THA on an infrequent basis also gives credence to the use of these systems on regular basis.¹³ Given the problems with measuring the anterior pelvic plane and changes in pelvic tilt on a functional level, however, it seems likely that a move will be made toward functional positioning of implants on a patient-by-patient basis.¹⁴

Conclusion

This study shows that the posterior spinal line can be used as an alternative to the ASIS registration points for the pelvic plane and can be used in the lateral position with easier access than the contralateral ASIS in the lateral position. With the current trend of patients with larger BMI needing a THA and presenting earlier in life, it is more important to obtain proper implant positioning to assure maximum longevity of the bearing surface. With the posterior approach in the United States being the most prevalent and with obesity increasing in the patient population for this surgery, the use of the PSIS for registration can be significantly easier and may convince more surgeons to start using computer imageless navigation in this patient population. It may also convince more surgeons using the posterior approach to use computer navigation because registration is easier with the patient in the lateral position.

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Authors

Dr Mihalko is from Campbell Clinic Orthopaedics, InMotion Orthopaedic Research Laboratory, University of Tennessee, Memphis, Tennessee. Dr Kammerzell is from Aesculap Incorporated, Bethlehem, Pennsylvania. Dr Saleh is from the Department of Surgery, Division of Orthopaedics, University of Southern Illinois, Springfield, Illinois.

Drs Mihalko and Saleh are consultants for Aesculap. Dr Saleh also received a research grant from Aesculap. Dr Kammerzell is an employee of Aesculap Implant Systems.

Correspondence should be addressed to: William M. Mihalko, MD, PhD, Campbell Clinic Orthopaedics, 1458 West Poplar Ave, Ste 100, Collierville, Tennessee 38017.

doi: 10.3928/01477447-200915-51