Unworn cartilage in total knee arthroplasty strongly deviates from 2 mm: MRI, robotic-assisted and ruler assessment

Kinematic alignment (KA) in total knee arthroplasty (TKA) aims to resurface the articular surface of the pre-arthritic knee. One cornerstone of KA TKA and the basis for conventional instrumentation (CI) is a 2 mm cartilage thickness of the healthy distal femur. This study of image-based (CT) robotic-assisted surgery (RAS) TKA determined (1) the difference in measuring cartilage thickness using preoperative MRI, the RAS registration probe and MRI and (2) the percentage of knees with a <1.5 or >2.5 mm thickness of the unworn cartilage.

Forty-two patients undergoing a TKA using an image-based RAS were included. Preoperative long-leg radiographs provided the limb phenotype. The two zones of interest for cartilage thickness were the distal unworn (DU) and posterior unworn (PU) condyle. The preoperative MRI provided cartilage thickness values for both zones. Intraoperatively, the surgeon identified the zones without cartilage wear (healthy cartilage) and measured the distance between the cartilage and bone with the RAS registration probe. After resecting the condyles, a ruler was provided to measure the cartilage thickness reference.

Mean patient age was 68.2 ± 4.1 years, and gender was evenly distributed. According to the phenotype classification, there were 8 NEU_HKA0°, 13 VAR_HKA3°, 9 VAR_HKA6°, 5 VAR_HKA9°, 4 VAL_HKA3°, 2 VAL_HKA6° and 1 VAL_HKA9° patient. The difference between measurement techniques was not statistically significant for both zones (DU p = 0.960, PU p = 0.267). In seven patients (16.7%), the DU cartilage thickness was >2.5 mm measured with any of the three techniques, and up to 50% of patients had unworn cartilage thickness <1.5 mm, depending on the zone and measurement technique.

There is a significant variation of unworn cartilage thickness from the assumed 2 mm, with a significant proportion (up to 67.7%) having >2.5 or <1.5 mm. If KA is applied, additional preoperative or intraoperative measurements might provide valuable information to avoid the risk of femoral component malposition.

Level II, diagnostic study.