Abstract
Introduction
Ideally, a patient receiving a unicondylar knee replacement will have fully functional anterior and posterior cruciate ligaments. When at least one of the cruciate ligaments is not fully functional, femoral and tibial implant contact position can potentially increase along the anterior-posterior (AP) axis. Where unicondylar implant wear testing typically uses AP resistance assuming fully functional cruciate ligaments, the authors used reduced AP resistance intended to simulate deficient cruciate ligaments.
Methods
Optetrak Logic® Uni (Exactech Inc, Gainesville, FL USA) unicondylar test specimens featuring an all-UHMWPE tibial component and a cobalt chromium femoral component were used in this study. The system has a semi-constrained articular geometry. Testing was conducted at an independent testing facility (EndoLab GMBH, Thansau, Rosenheim, Germany). A four-station knee simulator was used (EndoLab knee simulator) with two unicondylar knee implants per station, giving a total of eight test specimens. Two different tibial fixation designs (keeled and peg) with identical articulating surfaces were tested. Tibial test specimens were 6 mm in thickness. Unloaded soak controls were stored in distilled water at 37°C. The test was conducted according to ISO 14243–1: 2009 [1]. Test specimens were immersed in calf serum (PAA GmBH, Cölbe, LOT B00111-5126) with a protein content of 20 g/l. Custom polyurethane molds allowed for individual component measurement. Per the ISO 14243-1, a 7% medial offset was incorporated into the set-up. The unicondylar knee implants were set at neutral position in extension. Tibial rotational restraint was 0.36 Nm/° and zero when the test specimen was within ± 6° of the reference position. This test was conducted with an AP resistance of 9.3N/mm to maximize AP displacement and simulate deficient cruciate ligaments. Typical unicondylar knee wear testing is conducted with an AP resistance of 44N/mm, which assumes functional cruciate ligaments.
Results
Wear data was separated by component design (keeled and peg) as well as for medial and lateral placement [Table 1]. There was no significant difference between lateral components but there was for medial components. This difference could be due to the small sample size. Contact area of the UHMWPE tibial components was elliptical, with the longer portion along the AP axis. Mean wear rates were comparable to historical unicondylar knee systems tested at the same laboratory using the standard AP resistance (i.e., 44 N/mm).
Discussion/Conclusion
This study demonstrated using an AP resistance 9.3 N/mm to simulate the presence of deficient cruciate ligaments in a unicondylar knee wear test produced similar wear rates and greater AP displacement when compared to testing using an AP resistance of 44 N/mm, which assumes functioning ligaments. This being said, design and material information about historical unicondylar knee systems tested are not known, so a direct comparison cannot be made. Performing unicondylar knee wear tests with reduced AP resistance could provide realistic wear information for devices implanted in patients without fully functioning cruciate ligaments.
