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Use patient-specific acetabular component safe zones for hip replacement
Surgeons should assess a patient’s sagittal-pelvic mobility preoperatively to optimize component position at the time of THR.
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Sagittal-pelvic mobility is highly variable in populations of patients with osteoarthritis who are scheduled to undergo total hip replacement. For that reason, universal acetabular component position safe zones do not exist, and orthopedic surgeons need to be as familiar a possible with the relationship between the spine, pelvis and hip, according to a presenter at Orthopedics Today Hawaii.
“[Sagittal-pelvic] mobility varies from patient to patient,” Andrew M. Shimmin, MBBS, FRACS, said. “This means that a single acetabular safe zone does not exist. It also sheds light on why dislocations frequently [happen] when components are oriented within that historical [Lewinnek] safe zone.”
Therefore, the concept of the Lewinnek safe zone is dead and that surgeons need to focus on the relationship between the spine, pelvis and hip when they determine the position of the acetabular component during THR, he said.
“The sagittal motion of the pelvis affects the functional orientation of the acetabular component in total hip replacements,” Shimmin said. “This can affect wear and stability, which are the common causes of failure of total hip replacements.”
Patients who undergo THR should have their pelvic-sagittal mobility assessed preoperatively because it will aid optimization of the component position during THR and subsequently reduce failure due to wear and instability, according to Shimmin. It is especially the case in patients with extreme sagittal mobility, he said.
“It is important to identify patients whose sagittal-pelvic mobility changes by 13° or more (extreme) when changing from the supine to flexed-seated or supine to standing positions,” Shimmin told Orthopedics Today. “In these situations, the patient’s functional cup position is changing 10°.
Patients who anteriorly rotate when sitting are at-risk in flexion posture and, in those cases, increased anteversion should be planned. Similarly, patients who posteriorly rotate when standing are at risk in extension postures and should have a reduction in anteversion considered intra-operatively, Shimmin said.
“Therefore, based on the individual variability in this sagittal-pelvic mobility, we need to understand the need for individual, patient-specific safe zones,” he said.
Shimmin said current research into this topic indicates preoperative mobility is the best predictor of postoperative sagittal mobility, but it is not 100% predictable. Therefore, Shimmin and colleagues are collecting postoperative data to help increase the accuracy of predicting the effect on sagittal mobility as a result of performing THR. – by Monica Jaramillo
Reference:
Shimmin AM. Relevance of sagittal pelvic mobility in THR. Presented at: Orthopedics Today Hawaii; Jan. 13-17, 2019; Waikoloa, Hawaii.
For more information:
Andrew M. Shimmin, MBBS, FRACS, can be reached at Melbourne Orthopaedic Group, 33 The Avenue, Windsor VIC 3181, Australia; email: ashimmin@mog.com.au.
Disclosure: Shimmin reports he is a consultant for and receives royalties from Corin and MatOrtho and is a consultant for Smith & Nephew.
Perspective
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Aaron J. Buckland, MD
Dr. Shimmin’s research highlights the complex issue of instability after THR. Although Lewinnek’s “safe zone” was the first accepted target for reducing dislocation risk, it has become evident each patient has a different functional relationship between the spine, pelvis and hip, which depends on their native anatomy, as well as their hip and spinal pathologies. Spinal pathologies, including lumbar flatback and lumbar fusion, have been demonstrated to increase THR dislocation risk due to changes in acetabular orientation and altered postural lumbopelvic mobility.
Traditionally the ‘safe zone’ was assessed on supine radiographs, however this is not a functional posture in which dislocation occurs. The hip-spine relationship is a dynamic relationship which changes with posture and pathology and each patient functions differently. Assessment of flexed-seated pelvic tilt preoperatively helps identify those patients who are at higher risk of posterior dislocation. Increased anterior pelvic tilt (>13°) compared to supine pelvic tilt increases the risk of anterior femoroacetabular impingement and posterior dislocation. By determining the functional lumbopelvic mobility of a patient, implant orientation can be customized to provide the individual patient’s “safe zone,” rather than a “one-size-fits-all approach.” Those patients whose implant “safe zone” is particularly narrow may require increased implant constraint. Continued collaboration between hip and spine surgeons is required to delineate this complex relationship.
Director of Spine Research
NYU Hospital for Joint Diseases
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