Purpose
The objective of this work was to characterise the dimensions of knee focal cartilage defects, generate a heat map of cartilage defect locations, and characterise the native articular surface morphology at the most common defect sites to aid in the design of generic focal knee resurfacing implants (FKRIs).

Methods
Healthy femoral cartilage was segmented from knee magnetic resonance imaging scans of 70 patients eligible for FKRI surgery. Cartilage defects were manually reconstructed to create three-dimensional (3D) native cartilage and cartilage defect models. A 3D statistical shape model of the native cartilage was created and 200 artificial models with varying shapes were generated. A heat map showing the frequency of defect occurrence location was created, and the size and aspect ratio of the defects were determined. Local radii of curvature were calculated at five locations that exhibit high defect occurrence for the medial and lateral condyles and in the trochlear groove.

Results
The median defect size and aspect ratio were 2.7 and 1.6, respectively. Cartilage defect location frequency heat maps were successfully generated for the medial and lateral condyles. The median mediolateral and anteroposterior radius of curvature at the medial condyle hotspot were 17.5 mm and 40.8 mm respectively, and 22.6 mm and 33.8 mm at the lateral condyle hotspot.

Conclusions
Characteristics of knee focal cartilage defect properties such as the size, their spatial distribution and local articular cartilage surface morphology offer valuable insights for the design of generic FKRIs.