“Patient specific instrumentation” means instruments such as alignment jigs particularly for knee replacements made specifically for a patient.
CT scans or MRI scans are performed & the implant companies use these scans to make instruments specifically for a particular patient.
This will be the way forward for all knee replacements in future but at present the costs of the scans, delays & costs of making the instruments mean that we use it only when absolutely indicated.
Some indications for using PSI are old fractures around the knee, overweight patients etc where obtaining good alignment with conventional Jigs becomes difficult.
The CT scans done before the operation allows planning for accurate size & alignment . Images below are examples of scans for PSI