The purpose of this study was to investigate the energy cost of walking after total ankle arthroplasty (TAA), and to investigate if possible differences could be attributed to changes in external mechanical work required for the step-to-step transition.

Eleven patients, 6 months to 4 years after successful unilateral TAA, and 11 healthy controls walked on a treadmill at a self-selected speed (SWS) and a fixed walking speed (FWS, 1.25 m/s). Ground reaction forces and oxygen uptake were measured. External mechanical work was analyzed using the double inverted pendulum model.

At SWS, velocity in the TAA group was reduced (v=1.29 vs 1.42 m/s, p=0.05) but metabolic energy cost was not different (E=2.50 vs 2.24 J/kg/m, p=0.32). At FWS, metabolic energy cost in the TAA patients was significantly higher (E=2.58 vs 1.96 J/kg/m, p=0.003). The difference in metabolic energy cost at FWS coincided with an increased negative work in the leading leg and reduced positive work in the trailing leg with TAA during double support. Although this indicates that the mechanical work for the step-to-step transition increases, the total external mechanical work over a complete stride was not different between the TAA and the control group.

TAA patients walk at a higher metabolic energy cost. This cannot be explained by differences in external mechanical work. Other factors, such as changes in muscle function, should be taken into account.

Introduction Mobile-bearing total ankle arthroplasty has gained more interest in recent years. Clinical results show favourable but varying results, with survival rates between 70% and 90% at 10-year follow-up. Design-specific differences in early migration patterns might explain differences in result and possible modes of failure.

Methods Prospective study of a cementless mobile-bearing total ankle arthroplasty by radiostereometric analysis (RSA). Fifteen total ankle arthroplasties were performed in patients with rheumatoid arthritis. The American Orthopaedic Foot and Ankle Society ankle score and radiostereometric radiographs were evaluated at regular intervals throughout the follow-up period: immediately postoperatively, 6 weeks postoperatively, 3 months, 6 months, and 12 months postoperatively and yearly thereafter.

Results The postoperative clinical results improved. We observed increased migration of the tibial component during the first 3 months, but this stabilized by the 6-months followup. The mean lateral-medial migration was 0.8 mm, distal-proximal migration was 0.9 mm, and posteroanterior migration was −0.5 mm. The latter implicated that the total resultant migration was in anterior and valgus tilting of this tibial component. This resulted in a main mode of migration proximal, anterior and valgus tilting of the tibial component.

Discussion This pilot study showed initial migration of this mobile-bearing ankle prosthesis into upward anterior and valgus tilting. However, migration stabilized at 6 months postoperatively. We think the surgical technique (anterior cortical window for placement) and the method of tibial fixation likely explain this migration.

Aims: To evaluate the results of a novel modular press-þt acetabular cup in primary total hip arthroplasty (THA) for osteoarthrosis (OA) and inßammatory arthritis (IA). Methods: From February 1996 to June 1999 in 324 patients (249 women, 75 men) 355 THA using a novel cup has been carried out. The shell is non-hemispherical on cross-section and has a hydroxyapatite coating on porous titanium for osseointegration. Diagnosis was: osteoarthrosis (OA) 236, developmental dysplasia (DDH) 21, post-traumatic arthrosis 27, avascular necrosis 6, IA 65 Ð mainly rheumatoid arthritis (RA). Average age at operation was 65.8 years. The patients were studied prospectively using Harris Hip Score (HHS), by measuring any radiolucency around the cup and by looking for signs of migration. Results: Median follow-up was 4.5 years. At follow-up, 21 patients had deceased. Revision for deep infection was carried out in 5 hips (3 low-grade infections). No septic loosening occurred with low-grade infection. Recurrent dislocations required revision of 1 cup and exchange of 2 inserts. Only 1 cup in a RA patient with severe superior bone loss became unstable after a fall 4 months postoperatively. Survival with aseptic loosening of the cup as endpoint was 1 in OA and 0.98 in IA. In all 290 patients with 318 THA in follow-up the cup was functioning well, both clinically and radiographically. HHS increased from 44.5 to 90.3. Conclusion: Press-þt þxation using a modern acetabular component is an excellent treatment option in primary total hip arthroplasty for all diagnostic groups.

Aims: Patient-related risk factors of dislocation after total hip arthroplasty (THA) that have been identified are previous hip surgery, old age and female gender. However, there have been no prospective reports whether inflammatory arthritis (IA) is an independent risk factor. Methods: Prospective evaluation of the incidence of early (< 2 year post-surgery) dislocation in a consecutive series of primary THA. From January 1996 to December 1999 341 THAs in 311 patients with osteoarthrosis (OA) and 69 THAs in 59 patients with IA (mainly rheumatoid arthritis) were included in this study. One type of prosthesis having a 28 mm. ball head was implanted in every hip through an anterior appoach. Results: Both groups were comparable with respect to the following risk factors: gender, position of the acetabular component and experience of the surgeon. Average age was lower in the IA group than in the OA group: 61.0 vs 68.1 years. Furthermore, the incidence of previous hip surgery was higher in the OA group. Despite the presence of these risk factors in the OA group, the incidence of dislocation was higher in IA than in OA: 10.1% vs. 2.9% (p=0.006). All dislocations in IA where posterior, in OA 5 were posterior and 4 were anterior (1 unknown). No other mechanical factors leading to an increased instability of the hip in IA, such as trochanteric fractures, could be identified. Conclusions: Inflammatory arthritis is an independent risk factor of dislocation after THA. Both the polyarticular impairments and the lower quality of the soft tissues in IA could explain this increased risk.