Quantitative histologic analysis of tissue growth into porous total knee components

Histologic and radiographic analysis was performed on 85 porous-coated, uncemented knee components (26 femoral, 34 tibial, and 25 patellar) retrieved from 45 patients for malposition or instability, unexplained pain, late infection, postmortem or postamputation, and posttrauma. No component was clinically or radiographically loose at the time of removal. The average patient age at removal was 62.1 years (range, 39–85 years), and the average time in situ was 12.4 months (range, 2 weeks to 53 months). Radiographs from 31 of the 45 patients (69%) were examined for implant alignment, fit, subsidence, loose beads, and the presence of sclerotic and lucent lines. Histologic and microradiographic sections of the implant and adherent tissue were examined for type, amount, and anatomic location of tissue ingrowth. Overall, 52% of the components showed no evidence of bone ingrowth, 29% showed minimal (<2%), 12% showed moderate (2–5%), and 7% showed extensive (5–10%) bone ingrowth. In no case was bone present in more than 10% of the available pore volume of any component. There were no significant differences in the incidence or extent of bone ingrowth among femoral, tibial, and patellar components, nor in the extent of bone ingrowth observed in comparing different implant designs, materials, or porous coating types. When bone ingrowth was observed, the anatomic location was consistent in each type of component. The presence of a lucent line radiographically was indicative of fibrous encapsulation, while a sclerotic line or absence of any radiographic abnormality was seen with both fibrous tissue and bone ingrowth.