Thorn Implant: A Novel Mechanism for Interamedullary Stem Fixation

Existing non-cemented prostheses fixation mechanisms use press-fit techniques to obtain initial stability between bone and implant. For example, the primary fixation of non-cemented total hip stem prosthesis is achieved by carefully impacting the implant to the broached, contoured proximal femur. A successful primary fixation will be followed by the secondary fixation caused by bone ingrowth into the porous surface of implant. However, the primary postoperative fixation of the implants is usually far from ideal [1, 2]. Theoretically, for bone ingrowth to occur, a porous coated implant must be rigidly fixed with close apposition to the bone, without causing excessive stresses and stains at the bone-implant interface. However, this would require the medullar cavity to be prepared to very tight tolerances. In practice, this press fit technique requires interference fit between the prosthesis and the bone, thus creating large stresses and strains. In this case severe impacts for fitting the prosthesis may also fracture the bone. Therefore, it is important that the broaching and contouring of the femur be carried out carefully and exactly. This results in operations being lengthier and more complex. Furthermore, reaming and broaching damage bone vascular system delaying bone formation. The objective of this study was to introduce a novel fixation method, which can achieve immediate initial stability at the operation without having the mentioned problems of contemporary prosthetic designs. In this method, instead of waiting for the bone ingrowth to occur, pins from the inside of a hollow prosthesis stem are driven into the bone making a thorn implant. Therefore, fixation is achieved by so-called metal ingrowth into the bone instead of bone ingrowth into metal.