Letter in response to Shaw KE, Charlton JM, Perry CKL, et al. The effects of shoe-worn insoles on gait biomechanics in people with knee osteoarthritis: a systematic review and meta-analysis. Br J Sports Med 2018;52:238-253.

Footwear has long been considered a conservative management strategy for a variety of medical conditions. From tendon dysfunction to diabetic ulcers to degenerative joint disease, the goal has always been the same: modify the patient’s biomechanics to mitigate potentially injurious loading. In particular, footwear has become a very hot topic when it comes to conservative management of knee osteoarthritis, with several randomized controlled trials and meta-analyses on the subject. Specifically, the biomechanical goal for knee osteoarthritis footwear has been to reduce the knee adduction moment (KAM), as this variable is thought to serve as a proxy for medial compartment loading.

Recently, Shaw et al.¹ provided a summary of the mean biomechanical effects of footwear insoles in patients with knee osteoarthritis; however, the extent to which there is variability in some of these biomechanical measures between patients was not discussed.  In footwear interventions, many patients experience changes opposite to the group mean change.² This may account for the generally low-medium effect sizes seen in their meta-analysis,¹ and this highlights the importance of also considering individual patient results when it comes to footwear for knee osteoarthritis.

In the case of the knee adduction moment (KAM) – the variable most consistently found to be affected by insoles in the study by Shaw et al.¹ – up to 25% of patients may actually experience an increased KAM with wedged insoles.² The mechanism by which a patient experiences increased or decreased KAMs may differ across patients as well. For instance, relationships between KAM change and frontal plane centre-of-pressure shifts or ankle kinematics have not been strong.^3,4

From inverse dynamics equations for KAMs,⁵ there are many ways in which altered KAMs can be achieved including isolated changes in ground reaction force magnitudes, centre-of-pressure locations, ankle joint loads, foot/ankle kinematics,  leg/knee kinematics or any combination of these. These possible mechanisms cannot all be captured by analyzing mean data alone. Likely, the “bio” in biomechanics is critical: anatomy, neuromuscular physiology, skin/foot sensation, comfort and other biological factors could have an impact on mechanical variables in different ways for different patients. That is, there is probably no one unifying mechanism in terms of response to footwear.

This is unlike some other areas of medicine, where there is little variability in the mechanism by which an intervention takes effect. For instance, beta-blockers (a type of antihypertensive) reduce blood pressure for all patients by reducing heart rate, but the mechanism by which wedged insoles (a type of insole) affect KAMs differs across patients.  For hypertension, control can be achieved by a variety of different medications, but the medication chosen for a specific patient often depends on individual patient factors,⁶ and so too should the selection of footwear interventions.

I therefore propose that while mean data can describe important general trends, it is imperative that researchers and clinicians consider individual patient data as well. This may allow for precision-medicine approaches whereby we match the right footwear to the right patient, and ensure the desired biomechanical and clinical response is achieved on an individual patient basis.

Conflicts of Interest: RTL has filed a US Patent (15/189,830) for a method of predicting biomechanical responses to wedged insoles.

Funding: RTL has received funding from Alberta Innovates.

Ryan Lewinson completed his PhD in biomedical engineering at the University of Calgary’s Human Performance Laboratory where he conducted research on the biomechanics of footwear orthotics and knee osteoarthritis. He continues to do musculoskeletal research and is completing his MD at the University of Calgary in 2018.

References

1. Shaw KE, Charlton JM, Perry CKL, et al. The effects of shoe-worn insoles on gait biomechanics in people with knee osteoarthritis: a systematic review and meta-analysis. Br J Sports Med 2018;52:238-253.
2. Lewinson RT, Stefanyshyn DJ. Wedged insoles and gait in patients with knee osteoarthritis: a biomechanical review. Ann Biomed Eng 2016;44(11):3173-3185.
3. Hinman RS, Bowles KA, Metcalf BB, et al. Lateral wedge insoles for medial knee osteoarthritis: effects on lower limb frontal plane biomechanics. Clin Biomech 2012;27(1):27–33.
4. Chapman GJ, Parkes MJ, Forsythe L, et al. Ankle motion influences the external knee adduction moment and may predict who will respond to lateral wedge insoles?: an ancillary analysis from the SILK trial. Osteoarthritis Cartilage 2015;23(8):1316-1322.
5. Nigg BM, Herzog W. Biomechanics of the Musculo-Skeletal System (3rd ed.). Hoboken, NJ: Wiley, 2007.
6. National Institute for Health and Clinical Excellence. Hypertension in adults: diagnosis and management. NICE guideline (CG127) 2011, updated 2016.