My leg length difference is causing my back pain
He came in wearing a heel lift prescribed after a podiatrist found his left leg was 7mm shorter. He'd worn it for eight months. His back pain hadn't changed. 'The insole isn't working,' he said. 'Maybe I need a bigger one.'
What Patients SayA podiatrist measured my legs and one is apparently 8mm shorter. He said this is tilting my pelvis and causing all my back pain. I've been told I need special insoles.
Where Did This Come From?
Leg length inequality (LLI) is real — most people have some degree of difference between their two legs. The assumption that this difference causes pelvic tilt, which causes lumbar stress, which causes back pain is a logical chain that many practitioners follow. The trouble is that the chain breaks at almost every link when tested against actual evidence.
A leg length difference makes for a satisfying diagnosis. It's measurable, it sounds structural, it implies a clear solution (orthotics). Patients feel they've found the cause of their pain. Orthotics manufacturers have a product to sell. The appeal is understandable.
What the Science Actually Says
Studies in large populations consistently find that small LLI (under 10–15mm) does not significantly predict back pain. The spine has remarkable capacity to accommodate small asymmetries — it adapts, bends slightly, adjusts. This is normal and functional, not pathological.
Large LLI (above 20–30mm) is more clinically significant and can contribute to functional gait changes and, potentially, over very long timeframes, spinal issues. But this is a different magnitude from the subtle differences most people are told about after a podiatry assessment.
The reliability of clinical leg length measurement is also poor. Different measurement methods give different results. The same practitioner measuring the same patient twice often gets different results. This makes acting on small measured differences even more questionable.
Orthotics and heel lifts do have a role for significant, true anatomical LLI. For small, possibly-functional differences — the evidence for treating them is weak.
The Verdict
Significant leg length differences (20mm+) can be clinically relevant. Small differences (under 10–15mm) — which most people have — are unlikely to be the primary cause of back pain, and treating them with orthotics has weak evidence.
What To Do Instead
- Ask specifically: "How large does a leg length difference need to be to cause back problems?" — any clinician should be able to answer this
- For differences under 10mm, treat the symptoms rather than the measurement
- For significant differences (20mm+), orthotics do have evidence — this is a legitimate intervention at that scale
- Address overall fitness, muscle balance, and biomechanics — these contribute more to back pain than small LLI in most cases
Yellow Flags — Worth Monitoring
- LLI measured at 20mm or more — worth discussion with an orthopaedic specialist about whether treatment is warranted
- Back pain that changes significantly with and without a specific shoe or insert — practical evidence that the difference may be relevant for you
Red Flags — Get Checked Immediately
- Sudden or rapidly progressing LLI — this could indicate a hip or pelvic problem requiring investigation rather than just insoles
- LLI in a child that is progressively worsening — needs paediatric orthopaedic assessment
- Grundy PF & Roberts CJ, "Does unequal leg length cause back pain? A case-control study," Lancet, 1984. Brady RJ et al., "Reliability and validity of three methods of determining leg length," Journal of Orthopaedic & Sports Physical Therapy, 1994.