A kit for use in knee replacement surgery, which comprises: a. a trial patella component (2, 50, 80) comprising: i. a backing plate (4, 56, 86, 122) having a backing surface for positioning against a patient's patella, and at least one sensor (130, 132, 134, 136) which can generate a signal corresponding to the compressive load applied to the trial patella component, and ii. a first bearing plate (6, 6a-d, 58, 68, 70, 88, 92, 94, 150, 152, 154, 170, 172, 174, 182, 184, 186) which can be fitted to the backing plate and which, when fitted to the backing plate, provides a bearing surface (14, 60, 90) for articulation against a bearing surface on the patient's femur, and b. a patella implant component having a backing surface for fixation to a patient's patella instead of the backing surface of the trial patella component, and a bearing surface for articulation against a femoral bearing surface, in which the thickness of the patella implant component is greater than the thickness of the trial patella component when the first bearing plate is fitted to the backing plate, the difference between the said thicknesses being at least about 1 mm, characterised in that the kit further comprises a first shim (72, 100, 158, 200), which can be fitted between the backing plate and the first bearing plate, and a second shim (72, 102, 160, 202), which can be fitted between the backing plate and the first bearing plate instead of or in addition to the first shim. a. a trial patella component (2, 50, 80) comprising: i. a backing plate (4, 56, 86, 122) having a backing surface for positioning against a patient's patella, and at least one sensor (130, 132, 134, 136) which can generate a signal corresponding to the compressive load applied to the trial patella component, and ii. a first bearing plate (6, 6a-d, 58, 68, 70, 88, 92, 94, 150, 152, 154, 170, 172, 174, 182, 184, 186) which can be fitted to the backing plate and which, when fitted to the backing plate, provides a bearing surface (14, 60, 90) for articulation against a bearing surface on the patient's femur, and i. a backing plate (4, 56, 86, 122) having a backing surface for positioning against a patient's patella, and at least one sensor (130, 132, 134, 136) which can generate a signal corresponding to the compressive load applied to the trial patella component, and ii. a first bearing plate (6, 6a-d, 58, 68, 70, 88, 92, 94, 150, 152, 154, 170, 172, 174, 182, 184, 186) which can be fitted to the backing plate and which, when fitted to the backing plate, provides a bearing surface (14, 60, 90) for articulation against a bearing surface on the patient's femur, and b. a patella implant component having a backing surface for fixation to a patient's patella instead of the backing surface of the trial patella component, and a bearing surface for articulation against a femoral bearing surface, in which the thickness of the patella implant component is greater than the thickness of the trial patella component when the first bearing plate is fitted to the backing plate, the difference between the said thicknesses being at least about 1 mm, characterised in that the kit further comprises a first shim (72, 100, 158, 200), which can be fitted between the backing plate and the first bearing plate, and a second shim (72, 102, 160, 202), which can be fitted between the backing plate and the first bearing plate instead of or in addition to the first shim.