For years, body mass index (BMI) has been the go-to metric when thinking about surgical risk. But what about height?
A multicenter study from the International Spine Study Group delivers a surprising conclusion: it’s not weight — it’s height that may matter more when it comes to proximal junctional kyphosis (PJK) after adult spinal deformity (ASD) surgery.
The Study Design: Separating Height From Weight
This retrospective review of prospectively collected data analyzed 904 ASD patients treated between 2008 and 2020. The cohort looked familiar — median age 65, predominantly female (76%), and packed with the usual suspects: varying frailty levels, comorbidities, and complex deformities.
The investigators asked a simple, but elegant question: Do height and weight independently (or together) influence the risk of PJK?
To answer it, they went beyond standard linear models and used a generalized additive model — a flexible statistical approach that can detect nonlinear relationships (because let’s be honest, spines rarely behave linearly).
They adjusted for all the heavy hitters: age, osteoporosis, UIV (Upper Instrumented Vertebra), number of fused levels, and postoperative alignment parameters like PI–LL mismatch and T1 pelvic angle.
The Results: Tall Patients, Higher Stakes
Here’s the data: PJK occurred in 14% of patients (131/904) — right in line with what keeps surgeons up at night. Baseline characteristics? Surprisingly similar between PJK and non-PJK groups. Height, however, emerged as a significant independent predictor of PJK (P = .03). Weight? Not significant. Height–weight interaction? Also, not significant.
Even more intriguing, the relationship wasn’t linear forever. The risk of PJK increased with height and peaked around 179cm, after which it plateaued.
So, the takeaway is not “the taller, the worse” indefinitely — but rather that there’s a threshold where biomechanics may start working against you.
Why Might Height Matter?
Let’s put on our biomechanical hats for a moment. Taller patients inherently have longer arms, greater moment forces across junctional segments and, potentially, higher mechanical stress at the proximal construct.
In other words, extra vertical real estate may come at a cost — especially when transitioning from a rigid fusion to a mobile spine.
Meanwhile, weight didn’t independently move the needle here. That’s a bit of a mic drop for anyone still obsessing over BMI cutoffs alone.
What This Means in Practice
Should you start measuring every ASD patient with a stadiometer before booking surgery? Not quite — but this study does nudge surgeons toward a subtle shift in thinking: height deserves a seat at the preoperative planning table and taller patients may benefit from:
- More thoughtful UIV selection
- Enhanced junctional protection strategies (hooks, tethers, soft landing zones)
- Closer postoperative surveillance
And perhaps most importantly, it reminds us that risk stratification isn’t always about complexity —sometimes it’s about perspective.
The Bottom Line
In the ongoing quest to predict and prevent PJK, this study suggests you may need to look up — literally.
BMI might still have its place, but when it comes to junctional failure, height stands tall as a more meaningful predictor.
Origin Study Title: Height, Not Weight, is an Independent Predictor of Proximal Junctional Kyphosis After Adult Spinal Deformity Surgery
Authors: Sulieman, Ahmed M.D.; Sahhar, Maxwell B.S.; Beeram, Indeevar M.D.; Diebo, Bassel G. M.D.; Lafage, Virginie Ph.D.; Lafage, Renaud M.S.; Line, Breton G. B.S.; Hamilton, D. Kojo M.D.; Hostin, Richard M.D.; Passias, Peter G. M.D.; Klineberg, Eric O. M.D.; Smith, Justin S. M.D., Ph.D.; Gum, Jeffrey L. M.D.; Mullin, Jeffrey M.D.; Buell, Thomas J. M.D.; Soroceanu, Alex M.D., MPH; Kim, Han Jo M.D.; Eastlack, Robert K. M.D.; Daniels, Alan H. M.D.; Mundis, Gregory M. Jr M.D.; Protopsaltis, Themistocles S. M.D.; Gupta, Munish C. M.D.; Anand, Neel M.D.; Okonkwo, David O. M.D., Ph.D.; Turner, Jay D. M.D., Ph.D.; Schwab, Frank J. M.D.; Shaffrey, Christopher I. M.D.; Lewis, Stephen J. M.D.; Mummaneni, Praveen V. M.D.; Ames, Christopher P. M.D.; Lenke, Lawrence G. M.D.; Bess, Shay M.D.; Lee, Sang Hun M.D.; Kebaish, Khaled M. M.D.; on behalf of the International Spine Study Group