The design of PSI was based on full-length computed tomography (CT) of the lower extremities, whereas the surgical technique of calipered KA followed the technique recommended by Howell et al. [22]. Regardless of which assistant alignment instrument was used, an articular surface-based bone cut approach was adopted [9]. A Vernier caliper was used to measure the thickness of the resected bone pieces of the distal femoral condyles, posterior condyles, and tibial plateau. The general principle is that the sum of the thickness of the resected bone piece, the compensated thickness of the worn cartilage, and the width of the saw kerf is equal to the thickness of the component.

To manufacture PSI, CT data (slice thickness, 0.625 mm) were collected and imported into Mimics (version 17, Materialise NV, Belgium) for 3D reconstruction. Then, the solid models were imported into NX 9.0 (Siemens PLM Software, TX, US) for the design of PSI (Fig. 1). Rapid prototyping technology (Formiga P 110, EOS, Krailling, Germany) was used for 3D printing of the PSI. The printing material was medical nylon (PA2200 Polymer powder, EOS, Krailling, Germany), which can be sterilized using autoclaving.

Computer-aided design (a) and intraoperative application (b) of PSI

For the intraoperative application of PSI, all the residual articular cartilage should be removed using a curette before the PSI is secured to its unique position. In contrast to the management of articular cartilage in PSI-KA, calipered KA only removed the residual cartilage on the severely worn side. If the contralateral articular cartilage was intact, then it was kept in place. Stacked neodymium magnets (1 mm of thickness each) were used to compensate for the cartilage thickness on the severely worn side (Fig. 2). An intramedullary rod was introduced to ensure medial-lateral and flexion-extension orientation of the distal femoral cutting jig. An extramedullary referencing jig was applied on the tibia side.

Stacked neodymium magnets (white arrows) are used to compensate for the thickness of worn articular cartilage (2 mm) in calipered KA

The distal femoral resection was parallel to the joint line of the distal femur, and the posterior condylar resection was parallel to the posterior condylar axis (PCA). The ‘posterior referencing’ technique was used in femoral bone preparation. The tibial plateau resection was based on the original inclination of the proximal tibial joint line in the coronal plane. The tibial rotation alignment was consistent with the anteroposterior axis of the lateral plateau, and the posterior tilt of resection was consistent with the posterior slope of the medial plateau. A single posterior cruciate retained (CR)-designed prosthesis (Gemini MK II, Link, Hamburger, Germany) was used in the current study. When the undercoverage of the lateral trochlear resection is too obvious, depending on the coverage of distal femoral resection, the femoral component is appropriately lateralized to alleviate the undercoverage of the lateral trochlea. The patella was not resurfaced in any case.

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