Ultracongruent inserts avoid some of the drawbacks of central spine postero-stabilized inserts. However, early wear has been reported, and may be due to increased sagittal laxity. The principal objective of the present study was to compare sagittal laxity in rotating platform total knee replacements (TKR) according to insert design: ultracongruent versus central spine. The principal hypothesis was that insert design influences global sagittal laxity.

A retrospective comparative study recruited 3 consecutive series of patients treated for primary osteoarthritis of the knee, with a minimum 1 year's follow-up. The UC series comprised 35 knees in 34 patients, receiving a Total Knee Triathlon™ (Stryker Orthopaedics, Mahwah, NJ) TKR with ultracongruent insert, at a mean 2.0 years’ follow-up. The UC+ series comprised 36 knees in 34 patients, receiving the BalanSys™ (Mathys Ltd, Bettlach, Switzerland) TKR with ultracongruent insert, at a mean 2.5 years’ follow-up; in this model, the anterior edge of the insert is higher than in the UC series (“deep-dish” design). The PS series comprised 43 knees in 40 patients, receiving a Total Knee Triathlon™ (Stryker Orthopaedics, Mahwah, NJ) TKR with central spine posterior stabilization, at a mean 1.5 years’ follow-up. The principal assessment criterion was sagittal laxity at 90° flexion as measured by the Telos Stress Device^® (Metax GmbH, Hungen, Germany).

Sagittal laxity did not significantly differ between the UC and UC+ series: mean 8.2mm (range: 0–19.5mm) and 8.4mm (4.5–15.8mm), respectively. Sagittal laxity in the PS series was significantly less: 1.4mm (0.2–3.9) (P<0.0001).

Sagittal laxity was greater in ultracongruent than central spine posterior stabilized TKR. This anteroposterior movement may induce polyethylene wear. The ideal degree of sagittal laxity for ultracongruent inserts remains to be determined.

IV – retrospective study.