Height or Weight, Which Is a Bigger PJK Risk Factor? (Not a Trick Question)

Key Takeaways

• Height predicts PJK risk more than weight
• Risk peaks around 179 cm height
• Taller patients may need enhanced junctional protection
• BMI alone insufficient for PJK risk stratification
• Consider UIV selection based on patient height

Summary

A multicenter retrospective review of 904 adult spinal deformity patients found that height, not weight, independently predicts proximal junctional kyphosis (PJK) after surgery. The risk rises with stature, peaking near 179 cm, and then plateaus. Weight and the height‑weight interaction showed no significant association. The study used a generalized additive model and adjusted for age, osteoporosis, UIV level, fused levels, and postoperative alignment parameters.

Pulse Analysis

Proximal junctional kyphosis remains one of the most dreaded complications after adult spinal deformity (ASD) surgery, traditionally linked to factors such as age, bone quality, and body mass index. While BMI has dominated pre‑operative risk models, the International Spine Study Group’s recent analysis shifts the focus to an often‑overlooked variable—patient height. By examining a diverse cohort of 904 ASD cases collected between 2008 and 2020, the researchers uncovered a clear, independent relationship between stature and PJK, suggesting that taller individuals face a biomechanical disadvantage that BMI metrics fail to capture.

The study employed a generalized additive model, allowing it to detect nonlinear trends that standard linear regressions might miss. After controlling for age, osteoporosis, upper instrumented vertebra (UIV) level, number of fused segments, and postoperative alignment measures like PI‑LL mismatch, height emerged as a statistically significant predictor (p = .03). Notably, the risk curve rose sharply up to approximately 179 cm (about 5 ft 10 in) before leveling off, indicating a threshold effect rather than a simple “taller‑equals‑higher‑risk” rule. Biomechanically, longer spinal columns generate greater moment forces at the proximal junction, increasing stress on the transition between rigid fixation and mobile segments.

For spine surgeons, these findings prompt a reassessment of pre‑operative planning protocols. Height should be incorporated into risk calculators, influencing decisions such as UIV selection, the use of prophylactic hooks or tethers, and postoperative monitoring intensity. Tailoring junctional protection strategies to taller patients could reduce revision rates and improve long‑term functional outcomes. Future research may explore whether customized instrumentation or novel alignment techniques can mitigate the height‑related risk, potentially redefining standards of care across the ASD community.