A dilator retractor and the dilators that are used for minimally invasive spinal surgery or other surgery are configured to accommodate the anatomical structure of the patient as by configuring the cross sectional area in an elliptical shape, or by forming a funnel configuration with the wider end at...http://www.google.com.au/patents/US7008431?utm_source=gb-gplus-sharePatent US7008431 - Configured and sized cannula
