• Hip and knee surgery

Total knee arthroplasty (TKA) is the standard treatment of the terminal stadium of osteoarthritis in the knee joint. The good results achieved with TKA are based on lasting pain relief, misalignment correction and improved function. For more than 30 years, orthopaedic surgeons have had the choice between a traditional fixed bearing tibial polyethylene (PE) insert design and various designs with mobile bearing tibial PE. Several gait analysis studies have shown the good functional results of TKA with improvements in temporospatial, kinematic, and kinetic measurements. In spite of these improvements, TKA patients still show gait abnormalities. The majority of clinical studies in this field showed no major differences between fixed bearing and mobile bearing articulation designs. The longevity of fixed and mobile bearing articulation designs has been found to be comparable.
The central study aim was to compare a fixed bearing (FB) versus a mobile bearing (MB) articulation (P.F.C. Sigma, Depuy Int., Leeds, UK) to determine which showed the better performance. This central question was attacked from different angles in the 3 studies included in this thesis with the following focus points and results in summary.

Study I validated a new knee-specific software for Dual-energy X-ray Absorptiometry (DXA) scans prior to use in the randomized, clinical trial (Study III). We found cementation to increase the measured bone mineral density (BMD) without negative influence on the reproducibility. Leg rotation around a vertical axis changed the measured BMD; hence careful placement of the leg at follow-up examination was shown to be crucial for good reproducibility. We also compared the knee-specific software to traditional DXA software and found similar performances of the 2 softwares with regard to point typing the implant and the bone edges correctly on DXA scans.
Study II was based on a randomized clinical trial which focused on a comparison of the patients’ gait function, surface electromyography, and knee scores between the 2 articulations groups before and after 6 and 12 months’ follow-up. Further, we compared the patients’ gait function to that of a height/weight, gender and age matched control group. Fifty-one patients underwent gait analysis 12-months after surgery (26 FB/25 MB). The gait analysis showed normalized cadence for the MB group only. Both the FB and the MB groups improved their gait towards a more asymptomatic gait pattern, but neither the FB nor the MB group achieved a totally asymptomatic gait pattern. Results pointing towards a normalized gait pattern in both groups were the kinematic values, the minimal valgus moment and the decreased co-contraction measured by EMG.
For the FB group, additional improvements were seen in knee extension and maximal extension moment in mid stance.

Study III was based on a randomized clinical trial in which the focus points were a comparison of 1) the migration pattern of the 2 tibial articulation types from baseline and up to 12 months’ follow-up assessed by model-based radiostereometric analysis and 2) the BMD in proximity of the tibial implants assessed by DXA scans using the knee-specific DXA software, which was validated in study I. In study III, 50 patients attended a 12 months follow-up (26 FB/24 MB). The FB implants migrated significantly more than the MB implants after 3, 6 and 12 months’ follow-up, but the expected decrease in periprosthetic BMD was similar for both implants.
In favor of the FB articulation count normalized kinematics and better kinetic results towards a more asymptomatic gait pattern than with the MB articulation. The normalized cadence and significantly lower migration at all follow-ups count in favor of the MB articulation. The choice between the two articulation designs is not made crystal clear from this work; both the FB and the MB articulation have their advantages.
Future research regarding migration and wear assessment could provide valuable information that will aid the surgeons and patients in making a decision.

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