Abstract
Aim of the study: To prove that tapered pegs are effective in reducing tibial tray subsidence in vitro and that this effect is related to the dimensions of these pegs.
Methods: The peg designs were drawn up mathematically to allow for a unified surface area – Three different designs were used. The pegs were made from cobalt chrome, were conical in shape with a cut off tip and had a variable base and height and an equal surface area. These pegs were fixed with screws to an IB 11 HA coated tibial tray.
Wet foam was used as bone substitute, this is an open cell foam that is fairly fragile but has the benefit of being constant and is cheap and readily available. This foam is not desired to have cancellous bone characteristics but is useful in observing the relative effect of adding these pegs. Two different settings in vivo were mimicked: that of a tibial tray and pegs resting fully on cancellous bone, in which case a central vertical force was applied, and that of the tray resting on the cortex on one side with a lateral vertical force applied over the other side in both the proud and flush setting (2& 4 pegs respectively). The investigation was undertaken using a home made system allowing a crude estimate of the forces producing initial subsidence, which was identified by initial fracture of the foam, and total subsidence which was identified as total failure of the foam. Each test was carried out three times. Controls were carried out on the tray with no pegs and on the pegs individually before attaching these to the tray and repeating the tests for each design.
Results: Using this crude approach, the mean control force for total subsidence of the pegs was as follows: Short with wide base 550.3 g (± 45.3 g), medium length and base 475.6 g (± 24.25 g), long with narrow base 364.5 g (± 24.25 g). The mean control force for initial subsidence of the tray without pegs when subjected to a vertical central force was 4.3 kg (4–4.5 ± 0.27 kg) and the total subsidence force for the tray was 7.32 Kg (5.5–8, ± 0.84 kg). The mean central vertical force for initial subsidence of the tray with the tapers mounted was 7.16 kg (7–7.5 ± 0.28), for the short wide pegs, 5.33 kg (5–5.5 ± 0.28) for the medium pegs and 5.33 kg (5–6 ± 0.57) for the long pegs. The mean central vertical force for total subsidence of the tray with the tapers mounted was 9 kg (8.5–9.5 ± 0.5) for the short wide pegs, 9.8 kg (8–11 ± 1.6) for the medium pegs and 9.6 kg (8.5–11.5 ± 1.6) for the long pegs. The mean lateral control force for total subsidence of the proud tray with pegs resting on the wooden ledge was 5 kg (4–6 ± 0.75). The mean lateral vertical force for total subsidence with all pegs mounted was 7.16 kg (7–7.5 ± 0.28) for the short pegs, 5.8 kg (5.5–6 ± 0.28) for the medium pegs and 5.5 (5.5–5.6 ± 0.05) for the long pegs. No definite initial subsidence force could be identified. The mean lateral control force for total subsidence of the flush tray resting on the wooden ledge was 13.16 kg (12.5–14 ± 0.76).
The mean lateral vertical force for total subsidence with pegs mounted on the foam side was 12.3 kg (11.5–13 ± 0–76) for the short pegs, 13.5 kg (12–15.5 ± 1.8) for the medium pegs and 13.83 kg (12–15.5 ± 1.7) for the long pegs. Again no definite initial subsidence force could be identified.
Conclusion: The addition of tapered conical pegs to the tibial tray increases the resistance to subsidence when subjected to a central vertical force with the tray sitting fully on foam. The initial subsidence resistance was more marked in the case of the short wide variety. In the case of the tray resting on the hard edge and a lateral force applied, the proud tray showed improved resistance to total subsidence with the short pegs while the flush tray did not show improvement with pegs and was marginally worse with the short pegs. This is probably due to a higher margin of observer error.
The abstracts were prepared by Mr R. B. Smith. Correspondence should be addressed to him at the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
