153- Position du rachis et orientation du cotyle : implication pour le réglage des arthroplasties de hanche et conséquence des ostéosynthèses et ostéotomies rachidiennes sur le bassin. - Position of the spine and orientation of the acetabululum : Implication for the adjustment of total hip replacement and

JY Lazennec (Paris)

Position of the spine and orientation of the acetabululum : Implication for the adjustment of total hip replacement and consequences on the pelvis of ostheosynthesis and spinal osteotomies.

In the normal subject, standing, sitting or lying positions are “instantaneous” situations in a more complex adjustment including spinal movements, sacral tilt and hips flexion.

The “pelvic vertebra” adapts the orientation of the each acetabulum , and then , the functional range of motion of the hip joints . Lumbo-sacral orientation in the sagittal plane is of upmost importance, as it plays a critical role in the function of the spine and the hip joints.

Sagittal sacral tilt ( ST ) also described as sacral slope is a functional parameter that illustrates the importance of the relationship between pelvis and spine : it is linked to global lumbo sacral posture.
In the standing position , forward rotation of the pelvis uncovers the femoral head posteriorly and improves anterior coverage (increased ST ). In the sitting position , the relatively vertical position of the sacrum with backward rotation of the pelvis uncovers the femoral head anteriorly (small ST).
Those modifications of sagittal orientation are associated with variations of acetabular anteversion in the normal hip or with T.H.P. (less functional anteversion in standing position and more functional anteversion in sitting position).

Permanent decrease of ST is frequently observed in degenerative lumbar spine lesions : this situation results in marked anterior uncovering and can cause posterior impingement in the standing position.
In patients with limited spinal motion and, in many cases, permanent backward rotation of the pelvis (closed and fixed ST angle), changes in anteversion between the sitting and standing position may be very small. Selecting an inappropriate anteversion angle during hip replacement may induce catastrophic consequences in case of stiff lumbosacral junctions.

Each patient is characterized by a “morphological” parameter named incidence angle; it’s sagittal balance is the consequence of a postural adaptation for other functional parameters (pelvic tilt, sacral slope, lumbar lordosis, acetabular sagittal tilt, functional anteversion). Understanding variations of the sacral slope on lateral pelvic Xrays is essential for planning total hip arthroplasty and identifying patients at risk for impingement, as lumbosacral posture influences functional anteversion of the acetabulum.

Actual T.H.P. cannot reproduce the real mechanical characteristics and tolerances of normal hip joint . The evaluation of lumbo-pelvic alignment is mandatory for optimization in T.H.P. implantation . But those adjustments can be questionable in the future due to progressive changes in postural adaptation of the patients.

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