A Possible Case of Selection Bias? Sergiy Voznesenskyy, Hermano Miguel Foundation, Quito, Ecuador 2 October 2006 The systematic review of McCarthy, Callaghan, and Oldham  is very timely, as the last substantive amendment to the Cochrane systematic review of Hulme et al. on electromagnetic fields for the treatment of osteoarthritis  was made in 2001, while pulsed electromagnetic fields (PEMF) appear to be disease-modifying in cellular and animal models of osteoarthritis [3,4].The review  has included 5 randomised controlled clinical trials (RCTs) focusing on pain (as measured by the visual analogue scale (VAS) or the WOMAC pain scale) and disability (as mesured by WOMAC, EuroQol, AIMS or SF-36). Several studies have been excluded from the review as not meeting the inclusion criteria, including three studies reported in previous systematic reviews, which have been excluded, according to the reviewers, due to their use of non-validated outcome measures. Among those three studies, two RCTs of Trock et al. [5,6] have, in fact, used a standard VAS pain scale as one of their outcome measures, as also did two studies included in the review [7,8]. Although it is true that the studies of Trock et al. [5,6] did not utilise any of the validated outcome measures of function mentioned in the review inclusion criteria, neither did one of the studies  included in the review, in which, like in the studies of Trock et al. [5,6], the VAS was used to assess pain outcome. By a coincidence, in contrast to the overall conclusion of the review, in the studies of Trock et al. [5,6] PEMF therapy has been found highly effective in patients with clinically and radiologically confirmed knee osteoarthritis. In addition, at least two other RCTs apparently meeting the inclusion criteria have not been included in the review of McCarthy, Callaghan, and Oldham , the study of Nicolakis et al.  and the study of Tejero Sánchez et al. , both studies including patients with symptomatic radiologically confirmed knee osteoarthritis. In the study of Nicolakis et al. , WOMAC has been used to assess main outcomes for pain and function, while in the study of Tejero Sánchez et al.  the VAS pain scale and SF-36 have been used. If these two studies had been actually retrieved and subsequently excluded from the review as not meeting the inclusion criteria, at the least, the reason for exclusion deserves to be mentioned. In conclusion, the review of McCarthy, Callaghan, and Oldham  appears to have failed to include at least some RCTs meeting the inclusion criteria. It is likely that inclusion of these studies in the review would have changed the results obtained therein, as well as the review’s conclusions and practical implications. References1. McCarthy CJ, Callaghan MJ, Oldham JA. Pulsed electromagnetic energy treatment offers no clinical benefit in reducing the pain of knee osteoarthritis: a systematic review. BMC Musculoskeletal Disorders 2006, 7:51. DOI: 10.1186/1471-2474-7-51.2. Hulme JM, Judd MG, Robinson VA, Tugwell P, Wells G, de Bie RA. Electromagnetic fields for the treatment of osteoarthritis. The Cochrane Database of Systematic Reviews 2002, Issue 1. Art. No.: CD003523. DOI: 10.1002/14651858.CD003523.3. Fioravanti A, Nerucci F, Collodel G, Markoll R, Marcolongo R. Biochemical and morphological study of human articular chondrocytes cultivated in the presence of pulsed signal therapy. Ann Rheum Dis 2002; 61:1032-1033.4. Ciombor DM, Aaron RK, Wang S, Simon B. Modification of osteoarthritis by pulsed electromagnetic field: a morphological study. Osteoarthritis Cartilage. 2003; 11:455-462.5. Trock DH, Bollet AJ, Dyer RH, Fielding P, Miner WK, Markoll R. A double blind trial of the clinical effects of pulsed electromagnetic fields in osteoarthritis. J Rheumatology 1993; 20:456-460.6. Trock DH, Bollet AJ, Markoll R. The effect of pulsed electromagnetic fields in the treatment of osteoarthritis of the knee and cervical spine. Report of randomized, double blind, placebo controlled trials. J Rheumatol 1994; 21:1903-1911.7. Klaber Moffett JA, Richardson PH, Frost H, Osborn A. A placebo controlled double blind trial to evaluate the effectiveness of pulsed short wave therapy for osteoarthritic hip and knee pain. Pain 1996; 67:127.8. Callaghan MJ, Whittaker PE, Grimes S, Smith L. An evaluation of pulsed shortwave on knee osteoarthritis using radioleucoscintigraphy: a randomised, double blind, controlled trial. Joint Bone Spine 2005; 72:150-155.9. Nicolakis P, Kollmitzer J, Crevenna R, Bittner C, Erdogmus CB, Nicolakis J. Pulsed magnetic field therapy for osteoarthritis of the knee: a double-blind sham-controlled trial. Wien Klin Wochenschr. 2002; 114:678-684.10. Tejero Sánchez M, Muniesa Portolés M, Díaz Santos P, Belmonte Martínez R, Mateu Sans S, Bonafont i Vidal M, Duarte Oller E, Ester Marco Navarro E, Escalada Rector F. Efectos de la magnetoterapia en gonalgia secundaria a gonartrosis. Estudio prospectivo a doble ciego [Effects of magnetotherapy in knee pain secondary to knee osteoarthritis. A prospective double-blind study]. Patología del Aparato Locomotor, 2003; 1:190-195. Competing interests The author declares that he has no competing interests.