For decades, Body Mass Index has been the go-to shorthand for obesity in surgical planning. But anyone who has reviewed a lumbar fusion file knows the frustration: BMI treats a 260-pound powerlifter and a 260-pound sedentary claimant as identical risks. As Orthopedics This Week put it, BMI is “about as precise as estimating blood loss by ‘eyeballing the suction canister.’” Now, a new tool is aiming to change that calculus.
The Lumbar Subcutaneous Adipose Classification (LSAC) is an MRI-based system that goes beyond simply weighing a patient or calculating a ratio. Rather than producing a single number, the LSAC maps the distribution of subcutaneous fat across the lumbar surgical corridor. According to the OTW report, the system doesn’t just measure fat—it classifies its pattern in a way that turns out to be a powerful predictor of post-operative infections and other complications following lumbar interbody fusion.
The LSAC is the latest evolution in a line of research that has steadily chipped away at BMI’s dominance. In 2018, Shaw and colleagues published their Subcutaneous Lumbar Spine (SLS) Index, which measured the ratio of subcutaneous adipose depth to spinous process height at the surgical site using preoperative MRI. Studying 285 patients who underwent laminectomy or lumbar fusion, the team found that the SLS Index was significantly associated with total complications, perioperative complications, and the need for revision surgery—outperforming both BMI and raw fat depth measurements alone.
Then came the Spine Adipose Index (SAI), published in The Spine Journal in 2021. A multicenter case-control study of posterior instrumented lumbar fusion patients found that the SAI was more sensitive than either BMI or subcutaneous fat thickness in predicting deep surgical site infections, and it demonstrated excellent inter-observer reliability—meaning different radiologists reading the same MRI would reach the same conclusion.
A 2024 study in Global Spine Journal further validated the approach, finding that the Subcutaneous Lumbar Spine Index (SLSI) was a superior predictor of early surgical site infection after transforaminal lumbar interbody fusion across a cohort of over 3,600 patients. The researchers noted that each millimeter increase in subcutaneous fat thickness corresponded to roughly a six percent increase in infection odds—but that accounting for the spinous process height relationship made the prediction even stronger.
Where the SLS Index and SAI each produced a single numerical ratio, the LSAC takes the concept further by generating a classification of adipose distribution pattern across the lumbar corridor. Think of the difference between taking a patient’s temperature (one data point) versus mapping the inflammation across an entire joint (a diagnostic picture). The LSAC leverages data already sitting in every preoperative lumbar MRI—no additional imaging, no extra cost—and converts it into an actionable risk category.
This matters for the workers’ compensation world because the research base is now clear: site-specific fat distribution is a far better predictor of lumbar surgical complications than BMI. A recent meta-analysis pooling data from seven studies confirmed that localized adiposity measures showed stronger associations with post-operative infection than BMI in spinal fusion procedures.
For the Workers’ Compensation Industry, this development touches several pressure points. First, preoperative risk stratification: carriers and utilization review teams may soon have a tool that more precisely identifies which claimants face elevated surgical risks—well before the scalpel touches skin. That creates opportunities for prehabilitation protocols, targeted weight-management programs, or frank conversations about risk-benefit tradeoffs that go beyond a generic BMI threshold.
Second, causation disputes. When a post-fusion infection develops, the question of whether the infection was a foreseeable surgical complication versus an independent intervening cause is already a common battleground. A preoperative LSAC classification showing high-risk adipose distribution could strengthen the argument that the complication was predictable—and perhaps even that authorization should have required additional safeguards.
Third, the “obesity defense” gets more nuanced. Defense counsel have long pointed to BMI as a comorbidity that contributed to poor outcomes. But the LSAC’s ability to differentiate between patients at the same BMI—one with favorable fat distribution, one without—could undercut blanket arguments and demand more granular expert testimony on both sides of the aisle.
The LSAC doesn’t replace clinical judgment. No classification system does. But what it does is extract meaningful, reproducible risk information from imaging that is already being ordered in virtually every lumbar fusion case. For an industry that spends billions annually on spinal surgery claims, a better way to identify which patients are heading toward complications—before they get there—is the kind of incremental advance that compounds over thousands of files.