Analysis of the resected proximal tibia reveals that the cortical bone surface area is an average of 6% of the total tibial surface, and that cancellous bone accounts for 18% of the total area, with bone marrow space comprising 76% of the remaining surface area.21 The implication is that some form of "cement" is required to increase the surface attachment between the tibial component and the resected cancellous bone, and thus eliminate loosening and subsidence and provide durable fixation. The authors advocate the routine use of autograft cancellous bone chips22-25 as a biologic "cement" to improve bone ingrowth by reconstructing the subchondral bone region creating a dense neocortex at the implant interface, and to increase the cancellous bone surface attachment of porous-coated tibial components to host bone.24,25 The autolo-gous bone chips are prepared using the patellar reaming instruments on the cut surface of the tibial wafer.
The use of morselized autogenous bone chips appears to enhance the fixation of cementless components. An experimental study was performed in which paired porous-coated devices were implanted with and without the addition of morselized autogenous bone chips into the contralateral medial femoral condyle of patients undergoing the first stage of bilateral total knee arthroplasty.25 The devices were removed en bloc at the second total knee arthroplasty. Backscattered electron imaging revealed significantly more bone in the implant with autogenous bone chips. Tetracycline labeling demonstrated that this was living bone. In a postmortem retrieval study23 of tibial components implanted with and without supplementary morselized autograft bone chips, the tibial components implanted with bone chips had a clear advantage in bone ingrowth and bone apposition to the porous-coated surface. Postmortem retrieval analysis of 10 porous-coated tibial components implanted with autograft cancellous bone chips revealed bone in contact with 64% of the porous-coated interface, and backscattered electron imaging revealed bone ingrowth within 8 to 22% of the porous coating by volume24 (Fig. 7.1).
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