Background
Fractures involving the ankle are increasing and are one of the most common fractures in the lower extremity with an incidence rate of 101 fractures per 10 person-years. Ankle fractures occur in all ages and during different types of daily activities. The age-adjusted incidence rate for the two genders has been reported as equal, but in younger ages the incidence rate is higher among men and at the age of 50 the gender ratio reverses. The number of ankle fractures in the elderly is increasing and in women over 65 years of age it has been found to be 300 per 10 person-years. Most fractures are surgically treated with open reduction and internal fixation due to dislocation. After surgery the ankle is normally immobilized in a below knee plaster cast or in a brace for six to eight weeks.
Many investigators have evaluated both short- and long-term results after surgery. Radiographic assessments, ankle mobility and muscle strength have been studied. It has however become more common to use patient-reported scores to evaluate functional results, as has been done by many authors. As suggested by the International Classification of Functioning, Disability and Health (ICF), the degree of impairments, disabilities, participation problems and health related quality of life should be described from the patient's perspective. Patient-reported instruments such as questionnaires are appropriate instruments for this purpose.
Several scores have been developed in order to evaluate function after ankle injuries. The Karlsson score is a patient self-reported questionnaire while the Kaikkonnen score has to be completed by both patient and clinician as six of the nine items consist of clinical tests. Both are mainly intended to evaluate function after ankle ligament injuries and both have been found to be valid. The Foot and Ankle Outcome Score (FAOS) is a self-reported questionnaire and was developed to assess function in a variety of foot and ankle-related problems (http://www.koos.nu). FAOS has been found valid and reliable.
The Olerud-Molander Ankle Score (OMAS) is a disease-specific questionnaire devised for patients with ankle fractures and has been frequently used to evaluate subjectively scored function in this patient group. OMAS has been validated against: Linear Analogue Scale (LAS) assessing subjective evaluation of ankle function on a 15 cm long linear analogue scale with the ends marked "perfectly normal ankle" and "totally disabling ankle" (p < 0.01), range of motion in dorsal extension (p < 0.05), presence of osteoarthritis grade II-IV (p < 0.001) and presence of dislocations on radiographs (p < 0.05) after an ankle fracture. It has also been found to discriminate for subjectively experienced ankle instability (p < 0.02) and muscle strength in the ankle dorsi- and plantar flexors (p < 0.02). Wees et al. examined concurrent validity in patients with acute ankle ligament injuries and compared the Ankle Function Score (AFS) and the OMAS. The concurrent validity between the two scores at baseline and at follow-up was found to be good (rp = 0.82 and 0.70). Furthermore OMAS has been reported capable of recording change over time in the short-term after an acute ankle ligament injury. The day seven effect size was reported to 1.3 and day 14 to 2.3. No floor or ceiling effects have been found when evaluating patients with acute ankle ligament injuries. Although frequently used, few methodological studies regarding OMAS have been performed and to the best of our knowledge no studies have been published assessing the reliability of the OMAS instrument, effect size in the long-term or validity using another disease-specific questionnaire or self-rated function with a graded rating scale in patients with surgically treated ankle fracture. The aim of this study was therefore to evaluate the test-retest reliability and the validity of the self-reported questionnaire OMAS in subjects after an ankle fracture.