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    The Hip-Spine Relationship: What Hip Surgeons Need to Know

    Spinal deformities such as excessive kyphosis and degenerative lumbar spine disease and even prior lumbar fusion can have an impact on the success of a total hip arthroplasty. Here’s what surgeons need to keep in mind when planning and performing hip replacement in patients with spinal pathology.

    Authors

    Jonathan Vigdorchik, MD; Edward DelSole, MD; Aaron Buckland, MD; and Ran Schwarzkopf, MD

    Disclosures

    The authors have no disclosures relevant to this article.

    Introduction

    The spine’s mobility is intimately associated with the alignment and motion of the hip after total hip arthroplasty (THA). It is also sometimes associated with complications following the procedure.

    Therefore, hip surgeons must understand and incorporate spinal pathology into the surgical plan, keeping in mind the following points:

    • Acetabular anteversion and inclination change during human motion (sitting and standing) as the pelvis flexes and extends in the sagittal plane.
    • Sagittal spinal deformity and lumbar spondylosis can affect pelvic mobility and, therefore, can affect the functional acetabular position.
    • Abnormal pelvic position and mobility can predispose patients to edge-loading, impingement, and instability/dislocation.
    • Performing lumbar spinal fusion in patients who have undergone THA can alter the functional position of the acetabular cup and reduce the normal protective change in posterior pelvic tilt in sitting.
    • Pelvic position and stiffness needs to be taken into account when performing THA on a patient with prior lumbar fusion surgery.

    Impact of Postural Changes on the Hip

    In normal patients, the lumbopelvic junction is flexible in the sagittal and coronal planes. As a patient moves from standing to sitting, the pelvis tilts posteriorly to accommodate hip flexion. Similarly, as the patient moves from sitting to supine, the pelvis tilts anteriorly.

    As the pelvis moves within the sagittal plane, the orientation of the acetabulum is altered in a predictable, linear way. It has been demonstrated that for 1° of increasing pelvic tilt, acetabular anteversion will increase 0.7 or 0.8°. [1,2] In normal patients, this translates into an average anteversion decrease of 15.6° when moving from sitting to standing.

    Acetabular inclination also increases with pelvic tilt, and it does so in a non-linear manner that depends on the overall cup version and inclination at the time of implantation. [3] An increase in posterior tilt of the pelvis causes an increase in cup inclination, which is protective of anterior impingement as the hip flexes.

    Similarly, acetabular cup inclination has been found to increase a mean of 25° when patients move from standing to sitting as the pelvis naturally extends. [4] Postural changes in anteversion and inclination can be protective of anterior impingement as the native hip is flexed.

    Pathologic changes in the mobility of the pelvis or spinal balance, therefore, can predispose patients to mechanical abnormalities with their THA.

    Impact of Spinal Deformity on THA

    In the patient with a degenerative lumbar spine or sagittal spinal deformity, the spinopelvic junction may become more rigid from arthritis, or assume abnormal compensatory tilt as the patient attempts to maintain an upright posture. The spatial orientation of the pelvis is a function of the overall sagittal plane deformity of the patient.

    Patients with excessive kyphosis, for example, will increase their posterior pelvic tilt by extending the hips in order to maintain an upright posture. This will increase acetabular anteversion in the standing position and may cause hyperextension of the hip, resulting in posterior impingement, edge loading, asymmetric polyethylene wear, and anterior instability.

    In the patient with degenerative lumbar spine disease or with a lumbar fusion, the pelvis becomes less mobile. This decreases the ability of the pelvis to accommodate the extremes of hip motion and can increase the risk of impingement events during high flexion activity (Figure 1).

    Figure 1. Lateral stereoradiographic (EOS) imaging of a patient with recurrent dislocation of a total hip arthroplasty. The patient had undergone an L4-S1 instrumented fusion, with negligible change in pelvic tilt when transitioning from the standing (A) to the seated (B) position.

    Spinal surgery for deformity correction has a direct impact on acetabular cup position. These corrections tend to decrease pelvic tilt, reducing anteversion accordingly. [5] In other words, for every degree of pelvic tilt lost during deformity correction, there is a concomitant loss of 0.9° of anteversion.

    Case reports of hip dislocations in patients with THA who undergo subsequent spinal deformity corrections have lead some surgeons to suggest that sagittal spinal deformity should be corrected prior to performing THA. [6]

    Considering Balance and Flexibility

    When planning for THA in the patient with spinal disease, it is helpful to consider the overall balance of the spine and the flexibility of the lumbopelvic junction.

    Phan et. Al grouped patients into 1 of 4 categories with respect to their lumbopelvic motion and overall sagittal alignment: [7]

    • Flexible and balanced
    • Rigid and balanced
    • Flexible and unbalanced
    • Rigid and unbalanced

    Each group has unique challenges to address when planning THA:

    • For the flexible and balanced patient, THA proceeds as usual (Figure 2).
    • For the rigid and balanced patient, the surgeon must account for a pelvis that will not flex or extend during regular activity, which may force the coxofemoral joints into the positions of maximum flexion and extension in order to accomplish tasks that require bending (Figure 3). With an inflexible lumbar spine, this may cause anterior impingement and posterior dislocation events.
    • For the flexible and unbalanced spine, the surgeon must accept that the orientation of the acetabular cup determined at the time of surgery may not be the same when the patient returns to function (Figure 4). Acetabular cups apparently placed into Lewinnek’s “safe zone” during surgery may be excessively anteverted and abducted post-operatively when the patient is standing in the compensated (increased posterior pelvic tilt) position. [8] Unpublished data from our institution show wide variability in safe zone positioning, with only 42% of patients with spinal deformity having safely positioned cups in the standing position.
    • For the patient with the rigid and unbalanced spine, care must be taken to appropriately position cups – the pelvis will remain abnormally extended, with no accommodative tilt changes regardless of human motion (Figure 5).

    Figure 2. Graphical rendering of a flexible lumbopelvic junction without sagittal deformity. Movement from standing (A) to sitting (B) position results in an anticipated posterior pelvic tilt with a corresponding increase in acetabular anteversion. Anteversion is expected to increase 0.7-0.8° for each degree of posterior pelvic tilt. Reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Bone Joint J 2015;97-B:1017-1023].

    Figure 3. Graphical rendering of a patient without sagittal spinal deformity and an L3-S1 instrumented fusion demonstrating pelvic tilt (PT) and acetabular anteversion (AA). When moving from the standing (A) to seated (B) position, there is no accommodative posterior PT, and therefore no change in functional AA. This increases the risk of anterior impingement events. Reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Bone Joint J 2015;97-B:1017-1023].

    Figure 4. Graphical rendering of a patient with a flexible sagittal spinal deformity demonstrating pelvic tilt (PT) and acetabular anteversion (AA). The lumbar spine has hypokyphosis. When moving from the standing (A) to seated (B) position, there is an accommodative increase in posterior PT and AA. Reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Bone Joint J 2015;97-B:1017-1023].

    Figure 5. Graphical rendering of a patient with sagittal spinal deformity and an L3-S1 fusion, demonstrating pelvic tilt (PT) and acetabular anteversion (AA). The lumbar spine has a fused hypokyphosis. When moving from the standing (A) to seated (B) position, there is only a minimal accommodative increase in posterior PT and AA, thus creating a situation of possible anterior prosthetic impingement in flexion (seated). Reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Bone Joint J 2015;97-B:1017-1023].

    Conclusion

    Hip surgeons must continue to seek understanding of the hip-spine relationship to optimize THA component orientation in the patient with spinal pathology.

    Each patient will present with a unique spinopelvic junction, and thus each will present challenges for predicting post-arthroplasty hip motion, cup orientation, and risk factors for failure.

    Author Information

    Jonathan M. Vigdorchik, MD, is an Assistant Professor of Orthopaedic Surgery and Associate Fellowship Director in Adult Reconstruction at NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York. Aaron J. Buckland, MD is an Assistant Professor of Orthoapedic Surgery in the Spine and Scoliosis Division at NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York. Edward DelSole, MD is a resident in Orthopaedic Surgery at NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York. Ran Schwarzkopf, MD is an Assistant Professor of Orthoapedic Surgery in the Adult Reconstruction Division at NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York.

    References

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    3. Maratt JD, Esposito CI, McLawhorn AS, Jerabek S a., Padgett DE, Mayman DJ. Pelvic Tilt in Patients Undergoing Total Hip Arthroplasty: When Does it Matter? J Arthroplasty. 2015;30(3):387-391. doi:10.1016/j.arth.2014.10.014.

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