Radiographic features of bone ingrowth in highly porous cementless total knee arthroplasty with minimum 2-year follow-up.

backgroundThe definition of successful osteointegration after cementless total knee arthroplasty (TKA) remains uncertain. We hypothesize that trabecular line formation, bone resorption, and radiolucent line (RLL) disappearance follow a temporal pattern after cementless TKA.

methodsThis is a single-institution retrospective radiographic study involving 221 consecutive patients (301 knees) who had undergone robotic-assisted cementless TKA for primary knee osteoarthritis. All patients had a minimum of 2 years of follow-up (mean 46.5 ± 12.4 months). All TKAs consisted of a 3D-printed titanium-coated tibial tray and cobalt chromium-coated femoral component, which were highly porous, allowing for biologic fixation. Primary outcomes were incidences of trabecular line formation, early radiolucent lines (RLL), and bone resorption. Secondary outcomes were (1) the temporal relationship in RLL disappearance, trabecular line formation, and bone resorption, (2) the relationship between factors with osteointegration, and (3) mid-term clinical outcomes in terms of radiographic alignment, postoperative complications, and revision rates.

resultsTrabecular line formation was observed in 90.4% of knees and was most commonly seen medial (67.2%) and lateral (36.9%) to the tibial keel. RLLs were observed immediately following surgery in 33.6% of knees and were most common at the femoral anterior chamfer region, while bone resorption was most commonly found at the femoral anterior chamfer (41.2%) and the medial tibia (36.5%), indicating a stress shielding effect. Overall, new trabecular line formation occurred early in the postoperative course at an average of 4.5 ± 4.1 months, with bone resorption at a far later stage of 16.2 ± 11.4 months. In knees with early RLL (n = 101), disappearance of RLL occurred at a mean of 8.9 ± 8.6 months postop and was preceded by trabecular bridging. In knees without early RLLs (n = 200), trabecular line formation occurred at an average of 4.0 ± 3.3 months postop, which occurred at an earlier time point than knees with early RLLs (p = 0.009). Timing of bone resorption did not differ significantly between groups (p = 0.221). Radiographic alignment was maintained at post-op > 2 years with only two cases (0.7%) of revisions recorded.

conclusionsIn highly porous cementless TKAs with a 3D-printed titanium-coated implant system, bone ingrowth in the form of new trabeculae was observed in > 90% of knees, pointing towards satisfactory osteointegration. Osteointegration typically occurred between 4.5 and 8.9 months, with trabecular line formation and RLL disappearance. Bone remodeling from the stress shielding effect continued beyond 14 months, leading to zones of bone resorption. LEVEL OF EVIDENCE: Level III (retrospective study).