Fixation of the cemented tibial component : a radiostereometric analysis

Abstract

Aseptic loosening of the tibial component was the major mode for failure in early uni-(UKA) and total condylar knee arthroplasty (TKA). Metal-backing (MB) was introduced to solve the fixation problem of the tibial component, based on biomechanical experiments. However MB added new sources for potential failures such as polyethylene insert locking mechanism failure, backside wear and osteolysis. Metal backing decreases the polyethylene thickness at a given tibial component height and since a polyethylene thickness of less than eight millimetres has inferior resistance to wear, a thin component might be associated with an increased revision risk due to wear. If metal backing does not improve tibial component fixation compared to all polyethylene only, the risks associated with a thin tibial metal backed component are difficult to justify. A stem is generally accepted to enhance fixation in tibial components. Extra cement in tibia methaphyse negatively influence viable bone-stock and make revision more complicated. No clinical studies have assessed the effect on component fixation when the stem is included in the cementmantle, either in all polyethylene (AP), or in MB "cemented" tibial component.

The aim of this study was to evaluate if MB improves fixation in primary cemented UKA/TKA and if cement around the stem enhance tibial component fixation in TKA Fixation was evaluated with radiostereometry (RSA) up to 2 years after surgery. RSA allows, by accuracy about 10 times that of conventional radiography, for conclusive evaluation of migration/fixation with relatively few patients and within a relatively short time period. Clinical scores (HSS) preoperatively and after 2 years and conventional post-operative radiographs including measurement of the Hip-Knee-Ankle (HKA) angle were analysed.

45 knees (42 patients) were randomly allocated to either MB or AP tibial component in cemented UKA (Miller-Callante®). We could not detect any differences in migration with RSA. There was no difference in clinical score or radiographic evaluation between the two groups. In conclusion MB did not improve fixation and consequently AP can be recommended in cemented UKA.

In the TKA-series 80 knees (77 patients), all osteoarthrosis, were operated using the AGC-prosthesisÆ with an 8 mm thick tibial component. The knees were randomly allocated to MB or AP with or without cement around the. All components were cemented beneath the tibial tray. Accordingly, 20 knees in each group were followed with HSS-score and RSA for two years. No major complications occurred; one revision was performed due to patellar pain. Clinical score did not show any differences between the four groups. We detected the following differences in migration with RSA: - A cemented stem improved fixation compared to an uncemented stem in the metal backed tibial component group; - There was no difference in fixation in the all-polyethylene group when comparing cemented to uncemented stem; -There was no difference in fixation between the all-polyethylene and the metal-backed-group when the stem was cemented; - With uncemented stem the all-polyethylene had better fixation than the metal-backed group.

In summary MB did not enhance fixation compared to AP regardless mode of fixation. Only when the MB component was cemented around the stem, fixation on par with AP tibial component was achieved. AP component with uncemented stem seem to obtain the same early fixation as when the stem was cemented. Our results could be design and material specific; however the AGC® represents AP design "at risk" and favours MB since monobloc excludes all potential insert-associated failures.