The Cup Conundrum in Total Hip Arthroplasty: Is There a Safe Zone?

Educational Objectives

The goal of this program is to improve the performance of total hip arthroplasties. After hearing and assimilating this program, the clinician will be better able to:

1. Elaborate on the mechanics of the relationship between the hip and spine.
2. Obtain appropriate imaging studies for diagnosis of hip pathology.
3. Implement appropriate surgical approaches to avoid dislocations after total hip arthroplasty.

Summary

Historical perspectives: a key article (Lewinnek; 1978) described a “safe zone” of 40 degree cup inclination and 15 degree anteversion with a 10 degree tolerance in either direction; drawbacks of the paper include methodology, with inclusion of only 9 dislocations, and failure to provide patient-specific targets; Abdel reported a series of consecutive dislocations, most of which fell within the safe zone; it is important to understand the limitations of Lewinnek’s paper and how current research is diverging from a homogenous target

The hip-spine relationship: when a person moves from a standing to a sitting posture, the hip flexes by ≈55 to 70 degrees, not 90 degrees as assumed previously; only ≈66% to 75% of the motion required to sit comes from the hip joint; the remainder comes from the lumbosacral junction; loss of lumbar lordosis with sitting and movement of the pelvis posteriorly lead to ≈10 to 35 degrees of motion from the lumbosacral junction; when arthritis develops, the kinematics of these synchronized joints are affected; degeneration of the lumbar spine leads to loss of motion at the lumbopelvic junction, which is then compensated by the hip joint, leading to hip hypermobility; patients with spinal pelvic disease who lose lordosis have a standing kyphotic deformity and a posterior pelvic tilt are more prone to anterior dislocations in terminal stance phase

Abnormal biomechanics: “stuck standing” refers to a pelvis with neutral anterior tilt and normal standing orientation; the pelvis does not roll posteriorly when a patient sits; this prevents the acetabulum from tilting posteriorly to accommodate flexion of the femur; “stuck sitting” relates to the natural history of lumbar disease; lumbar lordosis is lost as a patient develops degenerative disease in the spine; the lumbar spine becomes relatively kyphotic, and the pelvis tilts posteriorly when standing; such patients have a pseudoseated position when standing and lose motion at the lumbosacral junction; there is no motion between the standing and sitting position; patients with spinopelvic disease can have 2 very different, divergent deformities which can drive cup position in 2 different directions; treatment algorithm differs according to the deformity

Terminology: sacral slope is the angle between the sacral end plate of S1 and a horizontal line; it is reproducible and easy to identify; used to quantify the amount of pelvic motion between different postural positions; anterior pelvic plane (APP) describes standing pelvic tilt; APP should assume a vertical position in most people when standing; formed by the anterior superior iliac spine and symphysis pubis; measurements of tilt describe deviations from vertical in the standing position; spinal deformity surgeons define pelvic tilt as the angle formed by a vertical line and a second line from the femoral head to the center of S1; functional anteversion is a measure of anteversion while accounting for the standing sagittal position of the pelvis; if a cup is placed at 20 degrees anteversion relative to patient anatomy with a neutral standing position, the patient's functional anteversion is 20; patients have various sagittal deformities; altered orientation of the pelvis in the sagittal plane affects the amount of axial plane anteversion of the cup; loss of lordosis with a relatively kyphotic lumbar spine and a posteriorly tilting pelvis functionally increase the anteversion of the cup; sagittal alterations of cup position by virtue of the patient's pelvic tilt affect axial anteversion

Imaging: standing anteroposterior (AP) pelvic x-ray can be used as a screening tool; speaker orders lateral, standing, and sitting spine x-rays and spine, pelvis, and hip radiographs, which may be too much for the average surgeon; clinicians should develop parameters and guidelines in their practice to identify high-risk patients that warrant additional imaging; a minimum of 2 lateral x-rays are required; important to quantify pelvic motion laterally in the sagittal plane between 2 relevant physiologic states; speaker prefers standing and relaxed seated positions; speaker recommends lateral x-rays for patients with indications of clinically relevant lumbopelvic deformity on a standing AP pelvic x-ray, history of lumbar surgery, marked hip flexion deformity, ankylosing spondylitis, radicular pain, longstanding debilitating back pain, or hunched-forward gait; cross-sectional imaging adds very little

Surgical targets: with stuck standing deformity, the patient has a neutral sagittal position and loss of lumbopelvic motion with compensatory femoral hyperflexion in the seated position, placing the patient at increased risk for anterior impingement and posterior instability; speaker aims for an inclination of 40 to 44 degrees and anteversion of 25 to 29 degrees instead of a 40/20 target; to compensate for increased femur excursion, speaker uses a head size of 36 or 40 mm when possible; (Lieberman; 2018) optimization of femoral offset is crucial; over-restoration of offset is preferable to under-restoration; patients with a stuck sitting deformity are at risk for anterior and posterior instability; speaker increases anteversion by 5 degrees but restricts combined anteversion to <45 degrees; maximum cup anteversion 30 degrees

Intraoperative stability testing: for stuck sitting deformities, the femur is in relative hyperextension when standing; trial the patient in extension and external rotation; for all patients, test maximal hip flexion, maximal internal rotation, adduction, flexion, and internal rotation

Technological tools: navigation is useful to execute acetabular component positions within a few degrees of target; important to consider the patient’s anatomy when using navigation or robotic systems; technology facilitates precise anteversion within a few degrees; speaker finds technological tools particularly beneficial for stuck sitting deformities

Readings

Abdel MP, von Roth P, Jennings MT, Hanssen AD, Pagnano, MW. What safe zone? the vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position. Clin Orthop Relat Res. 2016 Feb;474(2):386–391; Lee SH, Lim CW, Choi KY, Jo S. Effect of spine-pelvis relationship in total hip arthroplasty. Hip Pelvis. 2019 Mar;31(1):4-10; Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978;60(2):217-220; Lieberman EG, Jansen K, Mast L, Brady JM, Yoo BJ. Comparison of fluoroscopic techniques for assessment of femoral rotational alignment. OTA Int. 2018 May 16;1(1):e004; Stefl M, Lundergan W, Heckmann N, et al. Spinopelvic mobility and acetabular component position for total hip arthroplasty. Bone Jt J. 2017 Jan;99-B(1 Suppl A):37–45.

Disclosures

For this program, the following relevant financial relationships were disclosed and mitigated to ensure that no commercial bias has been inserted into this content: Dr. Heckmann is a member of the scientific advisory board for Intellijoint Surgical and owns stock in Intellijoint Surgical. Dr. Aggarwal reported nothing relevant to disclose. Members of the planning committee reported nothing relevant to disclose.

Acknowledgements

Dr. Aggarwal and Dr. Heckmann were recorded for Audio Digest on November 28, 2021. Audio Digest thanks the speakers for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0 CE contact hours.

Lecture ID:

OR450501

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.