A patellar clamp and reamer is used during a total knee arthroplasty procedure to prepare the posterior surface of a patellar bone to accept a prosthetic implant. A description of such a patella reamer may be had by reference to U.S. patent application Ser. No. 464,028 filed Jan. 12, 1990, incorporated herein by reference. As described in the incorporated application, the patella is clamped between a fixed jaw and a shiftable guide. The guide provides alignment for a shaft and blade to help avoid malalignment due to tilting during resection on the posterior surface of the patella. The guide, shaft and blade come in a plurality of sizes to accommodate a variety of patella sizes.
Typically, a surgeon will select to install a patellar prosthesis either by utilizing a surfacing technique wherein the prosthetic patella will be positioned on the prepared surface of the patella or by an insetting technique wherein the prosthetic patella will be recessed or inset into the prepared surface of the patella. Accordingly, two varieties of guides could be available for connection to the patellar clamp and could be interchangeably connectable to the clamp. The interchangeable guides permit a surgeon to use the patellar clamp during either a total patellar bone surfacing procedure or a patella insetting procedure.
In either a total resurfacing procedure or a patella insetting procedure, it is important that a sufficient amount of bone stock remain after resection to accept the fixation pegs of the patellar prosthesis and maintain the integrity of the remaining patellar bone. There may be some debate on the exact amount of patellar bone material that should be left after resection. At least one teaching requires that at least 11 millimeters (mm) should remain of the patellar bone stock after resection. For the purpose of this discussion it is assumed that 11 mm of bone stock should remain after resection. If too much bone material is removed, the remaining patellar bone may be structurally weak.
In order to prevent an excessive amount of bone material from being resected a prior art clamp and reamer included a stop ring carried by the shaft above the reamer blades. This type of stop controls the amount of bone removed from the patella, but does not control the minimum thickness of patella bone remaining. The stop ring engages with the upper surface of the reamer guide to stop the shaft and reamer from being further shifted into the patellar bone. A plurality of stop rings having a plurality of effective thicknesses may be interchangeably connected to the shaft of the reamer to accommodate the varying amount of bone to be resected as the thickness of the patellar bone varies between patients. The stop rings may be calibrated and may include measurement indicia thereon, in terms of the amount of bone material to be removed during resection. For example, if a surgeon needs to resect 10 mm of bone material, a ringed stop having the indication 10 mm, and correspondingly an effective thickness of 10 mm, would be turned onto the shaft. Therefore, the shaft and reamer would only be able to resect 10 mm of bone material before the ring stop engages the guide.