To the Editor:

In a recent article published in Arthritis Care & Research(Purser JL, Golightly YM, Feng Q, Helmick CG, Renner JB, Jordan JM. Association of slower walking speed with incident knee osteoarthritis–related outcomes. Arthritis Care Res [Hoboken] 2012;64:1028–35), Purser et al report the effect of baseline walking speed on osteoarthritis (OA)–related outcomes. The effect measure was the adjusted odds ratio (OR) for the incidence of knee and/or hip OA per each clinically meaningful increment of walking speed. Radiographic OA was defined as a Kellgren/Lawrence (K/L) score ≥2.

The authors' principal result was that a slower walking speed was associated with a higher incidence of radiographic and symptomatic knee OA, while a faster speed appeared to be associated with a lower incidence. The adjusted ORs are shown in Table 2 of the original article; however, the authors have not adjusted the ORs for the baseline severity of OA, i.e., the baseline K/L score (K/L score 0 or 1). Previous studies have reported that subjects with different baseline radiographic severity of OA have different risks regarding progression of disease (Bruyere O, Honore A, Ethgen O, Rovati LC, Giacovelli G, Henrotin YE, et al. Correlation between radiographic severity of knee osteoarthritis and future disease progression: results from a 3-year prospective, placebo-controlled study evaluating the effect of glucosamine sulfate. Osteoarthritis Cartilage 2003;11:1–5). If the baseline severity of disease was to be included in the analysis, it could lead to very different results.

Alex N. Bastick MD*, * Erasmus University Medical Center, Rotterdam, The Netherlands.