Jack-knife Posture After Correction Surgery for Degenerative Sagittal Imbalance-Does Spinopelvic Parameter Always Matter in Preventing Stooping Posture?

STUDY DESIGN: A retrospective comparative cohort study.

OBJECTIVE: To investigate the causes of jack-knife posture (JKP) without proximal junctional kyphosis (PJK) and to compare the causes of PJK and JKP without PJK.

SUMMARY OF BACKGROUND DATA: PJK causing stooping posture could be inevitable after long instrumented lumbar fusion for degenerative sagittal imbalance. However, few studies have addressed JKP without PJK, which is a different type of sagittal malalignment.

METHODS: Ninety-four adult patients who underwent long instrumented lumbar fusion for degenerative sagittal imbalance were divided into three groups depending on the presence of a postoperative sagittal deformity: JKP, PJK, or none. JKP was defined as a sagittal vertical axis of at least 8 cm anterior to the posterosuperior corner of S1 upper endplate without PJK. PJK was defined as a proximal kyphotic angle ≥10° and at least 10° greater than the preoperative measurement.

RESULTS: Seventeen patients (18.1%) showed JKP, and 39 patients (41.5%) showed PJK. The mean age at the surgery of JKP and PJK patients was 70.0 and 65.5 years, respectively (p = .05). JKP developed more frequently if the preoperative pelvic tilt was <25° or the sagittal vertical axis was >8 cm (p = .048, 0.004, respectively). PJK developed more frequently if the preoperative pelvic tilt was >25°, surgically corrected lumbar lordosis was >30°, or the lumbar lordosis to thoracic kyphosis angle was <0° (p = .002, .010, .031, respectively). JKP was more likely to develop in patients without sacropelvic fixation (p = .042), although they had a more degenerative L5-S1 segment (p = .010).

CONCLUSIONS: Postoperative sagittal deformity can be classified into two types: JKP and PJK. JKP is caused mainly by hip and back extensor muscle weakness even though the cause of postoperative sagittal decompensation could be multifactorial. However, PJK developed mainly due to spinopelvic malalignment. Therefore, hip and back extensor weakness, which is age-dependent, may explain why older patients experienced more postoperative sagittal deformity.

LEVEL OF EVIDENCE: Level IV.