This study shows large improvements in self-rated pain, function and quality of life at 2 years for patients operated on for knee OA by tibial osteotomy using the hemicallotasis technique. Surprisingly, substantial improvements were seen already during the immediate postoperative period when the external fixation was still used.
To our knowledge, this follow-up study is the first evaluating the patients' perspective of the HCO including the treatment period. Clinical scores have been used by Magyar et al in a randomized study comparing close wedge osteotomy and HCO, and by Gerdhem et al when evaluating the HCO [2, 6]. Both studies showed significant improvements and good to excellent results as evaluated by the Hospital for Special Surgery Score (HSS), at 2 years and at 12 to 28 months respectively. Our results are in line with these previous reports.
Magyar et al  found no differences in clinical scores between two methods of high tibial osteotomy and remarked that the clinical scores used, seemed to be too blunt to detect the differences in younger active patients. For this reason we choose an outcome measure validated for younger or physically active subjects with knee OA. The Knee injury and Osteoarthritis Outcome Score (KOOS) evaluates pain, other symptoms and activities of daily living but also includes sport and recreational activities and quality of life, dimensions that have been shown to be more sensitive in younger and/or physically active with knee OA than the more commonly used Western Ontario and MacMaster Universities Osteoarthritis Index (WOMAC) . The patients filled in the questionnaires themselves in their homes. In studies using this administration mode and frequent follow-ups a high number of dropouts could be expected. In the present study the 2-year follow-up rate was 90%, which must be considered high.
Most of the improvements in all subscales of the KOOS, except for sport/recreational function, were obtained during the treatment period, when using the external fixation. The KOOS questionnaire was sensitive enough to detect significant changes over just a few weeks. Four weeks postoperatively, pain related to the correction was common, but the pain gradually diminished. Seven weeks postoperatively, the correction was completed and the external fixator was locked. When visiting the outpatient clinic, the patients told that they felt improvements almost day by day, as confirmed by the improvements in all KOOS subscales except for sports/recreation function. During the later part of the treatment, patients gained more knee stability and gradually decreased the use of crutches, but were still prevented from certain activities due to the external fixator. This clinical improvement was detected by the KOOS, especially the subscale ADL. One week after finishing the treatment with external fixator knee-related QOL was further improved, probably due to the extraction of the fixator and the pins. The similar decrease of number of patients with moderate to extreme pain during walking and at rest reported during the treatment in external fixation, indicate that the treatment by the HCO effected activity-induced pain as well as pain at rest.
Most probably the early pain reduction seen is due to the gradually corrected alignment of the leg. Alternative explanations to the early improvements seen include decrease of intraosseous pressure [15, 16].
The gender distribution in our study was almost even reflecting that knee injuries, and thus post-traumatic OA due to knee injuries, are more common in men , whereas elderly women more commonly have OA. The patients with known knee injury in this study were mostly men (28%). They were, on average, 8 years younger than men without known knee injury, and had performed more high/medium joint-loading sport activities. This reflects that patients with joint injury have an increased risk of developing knee OA requiring surgery [17–19].
Patients having tibial osteotomy in the current study and patients having knee arthroplasty in a previous study report similar preoperative pain and function . This is remarkable, taking into account the tibial osteotomy patient being on average 17 years younger. It should also be noted that patients developing OA at a younger age often have high demands of their knee function in both working life and leisure time.
Long-term results have been shown to depend on the achieved correction of the healed osteotomy [21–24]. 57/58 patients in our study achieved the intended correction. The mean HKA-angle at the 2-year follow-up was acceptable in 38/52 patients. These results are comparable or even better than studies with similar evaluation period [2, 5, 6].
As a predictor for poor improvements in pain over time, worse preoperative pain accounted for 21% of the variance. This is a factor that should be taken into consideration when selecting patients for high tibial osteotomy. This may indicate that operating earlier would give a better result as also discussed for total joint replacement .
Ten patients had complications and not unexpectedly, these complications were predictors of poor improvement in pain over time.