A system of patellar prostheses (20,44), comprising: a first patellar prosthesis comprising: a first patellar prosthesis perimeter defining a first patellar prosthesis perimeter shape; and a first patellar prosthesis securement feature (28a',b',c') having a size and a shape; a second patellar prosthesis comprising: a second patellar prosthesis perimeter defining a second patellar prosthesis perimeter shape, said second patellar prosthesis perimeter shape dissimilar from said first patellar prosthesis perimeter shape; and a second patellar prosthesis securement feature (28a,b,c) having said size and said shape, whereby said first patellar prosthesis and said second patellar prosthesis are securable to a natural patella having a patella securement feature compatible with said size and said shape shared by said first patellar prosthesis securement feature (28a',b',c') and said second patellar prosthesis securement feature (28a,b,c) to allow interchangeable securement of said first patellar prosthesis and said second patellar prosthesis to a natural patella, wherein said first patellar prosthesis securement feature consists of three patellar prosthesis fixation posts (28a',b',c') arranged in a pattern and wherein said second patellar prosthesis securement feature consists of three second prosthesis patellar fixation posts (28a,b,c) arranged in said pattern, characterised in that the second patellar prosthesis is elongate and that said pattern of the three second prosthesis patellar fixation posts is offset from a centroid of the elongate second patellar prosthesis. a first patellar prosthesis comprising: a first patellar prosthesis perimeter defining a first patellar prosthesis perimeter shape; and a first patellar prosthesis securement feature (28a',b',c') having a size and a shape; a first patellar prosthesis perimeter defining a first patellar prosthesis perimeter shape; and a first patellar prosthesis securement feature (28a',b',c') having a size and a shape; a second patellar prosthesis comprising: a second patellar prosthesis perimeter defining a second patellar prosthesis perimeter shape, said second patellar prosthesis perimeter shape dissimilar from said first patellar prosthesis perimeter shape; and a second patellar prosthesis securement feature (28a,b,c) having said size and said shape, whereby said first patellar prosthesis and said second patellar prosthesis are securable to a natural patella having a patella securement feature compatible with said size and said shape shared by said first patellar prosthesis securement feature (28a',b',c') and said second patellar prosthesis securement feature (28a,b,c) to allow interchangeable securement of said first patellar prosthesis and said second patellar prosthesis to a natural patella, wherein said first patellar prosthesis securement feature consists of three patellar prosthesis fixation posts (28a',b',c') arranged in a pattern and wherein said second patellar prosthesis securement feature consists of three second prosthesis patellar fixation posts (28a,b,c) arranged in said pattern, characterised in that the second patellar prosthesis is elongate and that said pattern of the three second prosthesis patellar fixation posts is offset from a centroid of the elongate second patellar prosthesis. a second patellar prosthesis perimeter defining a second patellar prosthesis perimeter shape, said second patellar prosthesis perimeter shape dissimilar from said first patellar prosthesis perimeter shape; and a second patellar prosthesis securement feature (28a,b,c) having said size and said shape, whereby said first patellar prosthesis and said second patellar prosthesis are securable to a natural patella having a patella securement feature compatible with said size and said shape shared by said first patellar prosthesis securement feature (28a',b',c') and said second patellar prosthesis securement feature (28a,b,c) to allow interchangeable securement of said first patellar prosthesis and said second patellar prosthesis to a natural patella, wherein said first patellar prosthesis securement feature consists of three patellar prosthesis fixation posts (28a',b',c') arranged in a pattern and wherein said second patellar prosthesis securement feature consists of three second prosthesis patellar fixation posts (28a,b,c) arranged in said pattern, characterised in that the second patellar prosthesis is elongate and that said pattern of the three second prosthesis patellar fixation posts is offset from a centroid of the elongate second patellar prosthesis.