Abdominal wall tension after spinal deformity correction compromises postoperative biomechanics and may contribute to proximal junctional kyphosis.

Purpose: Proximal junctional kyphosis (PJK) is a common complication after fusion surgery for adult spinal deformity (ASD). Tissue adaptation to deformity may cause abdominal wall shortening/stiffening. This study evaluated, using musculoskeletal modeling, the effect of these adaptations and sarcopenia on trunk muscle effort required to maintain postoperative alignment in PJK patients versus controls.

Methods: ASD patient data was grouped by mechanical complication status: PJK (N=44), other (N=56), none (N=260). Spinopelvic landmarks were annotated in pre-op, post-op, and follow-up X-ray images. Patient-specific musculoskeletal models of corresponding alignments were built. Forces due to stretching of the abdominal wall beyond pre-op length (assumed slack) were applied, representing abdominal wall stiffness. Sarcopenia was implemented by reducing paraspinal muscle strength based on patient age and gender. Inverse-static simulations predicted overall muscle effort by summing muscle activities.

Results: Postoperatively, the abdominal wall was more elongated in the PJK group (+8.4%[-0.3;20.0]) versus the no-complication group (+2.4%[-4.9;10.2], p<0.01) due to larger preoperative deformities and greater surgical correction. This elongation correlated more with pelvic tilt change (r=-0.53) than lumbar lordosis correction (r=0.16). Greater muscle effort was estimated for post-op alignment in the PJK group (12.40[6.42;28.6]) versus the no-complication group (8.42[4.34;13.3], p<0.05). Muscle effort was reduced at follow-up in groups with mechanical complications.

Conclusion: Alignment restoration tensions abdominal structures, requiring increased extensor muscle forces to maintain postoperative alignment. Patients might develop PJK to reduce unsustainable muscle effort or due to spinal structure failure. More attention should be given to pelvic reciprocal changes to improve surgical planning and perioperative rehabilitation.