In this study, primary TKA using ASBG was performed on 17 knees with uncontained defects ≥ 10 mm in depth. Bone union was achieved in 94% of the knees at 96.4 months follow-up. In addition, knee score and HKA angle showed sustained improvement until the final follow-up when compared to the preoperative scores, and there were no major complications.
There are two types of bone defects in the proximal medial tibia: (i) contained defects showing that preservation of the cortical rim can support the tibial prosthesis, and (ii) uncontained defects without the cortical rim and cannot support the prosthesis. Uncontained defects are often observed in patients with severe varus deformity . Dorr et al noted that prosthesis loosening and postoperative knee deformities are likely to occur in uncontained defects; they found that repairing the defect with bone cement was not adequate and promoted the use of bone graft as a useful treatment method . Since then, although several authors reported satisfactory results following an autologous bone graft in primary TKA, most reports were of uncontained defects ≥ 5 mm in depth [6–8]. Generally, uncontained bone defects ≥ 10 mm in depth are treated using metal augmentation , and some reports [9, 10] recommend TKA with metal augmentation as a useful method. However, the use of additional metal augmentation requires further bone cutting, including the cortical rim and can make future revision TKA difficult. On the other hand, there are only a few studies on treatment with autologous bone grafts for uncontained tibial bone defects ≥ 10 mm in depth [2, 3], and no reports on primary TKA with allogenous bone graft.
The advantage of a bone graft is that once incorporated, it becomes a part of the proximal tibia and is, therefore, durable. The load transfer to the underlying tibia is more physiologic if the bone grafts are successful compared to using metal custom components without a bone cement composite. Moreover, structural bone grafts are superior to morselized bone grafts in the initial fixation and are reported to yield good results in primary TKA [5, 6, 11]. However, with the combined use of autologous bone graft from a resected femoral or tibial bone, it is difficult to obtain a bone block ≥ 10 mm thick and reconstruct the rim, including the cortex . Allogenous bone grafts can be freely sized and shaped, and it is possible to reconstruct a rim, including the cortex, with almost the same shape as bones used from the same site. In the present study, we were able to reconstruct the rim for any form of defect using ASBG.
Several authors [11–13] have associated allogenous bone graft with an increased risk of transmission of infection, nonunion, and resorption. However, almost all reports are on revision TKA with allogenous bone grafts. As primary TKA is less complicated with a shorter operation time than revision TKA, we considered that the risk of infection might be lower. In this study, we observed no infection and achieved bone union in 94% of the cases.
Generally, the grafted bone is fixed with screws [6, 8]. Although screws can achieve rigid initial fixation, insertion of multiple screws may lead to fragmentation of the grafted bone, resulting in early failure of knee replacement . As a device that does not use screw fixing, Chon et al  used the oblique structural peg bone, while Yoon et al  applied a cement to the medial surface of the tibia to fix the grafted bone. We temporarily fixed the allogenous grafted bone with Kirschner wires and withdrew them after fixation of the tibial component with extension stems. Extension stems may result in a future loss of bone stock when performing revision surgery. In addition, Scott et al  reported that extension stems might have disadvantages, including stress shielding along the length of the stem with associated reduction in bone density and a theoretical risk of subsidence and loosening, peri-prosthetic fracture, and end-of-stem pain. However, Rawlinson et al  used cadaver knees to biochemically confirm that using a tibial extension stem reduced bone stress and limited micromotion between the metal wedge and surrounding bone, and hence, recommended using a tibial extension stem. Baek et al  reported that bone grafting could not provide sufficient stability for severe bone defects ≥ 10 mm in depth in primary TKA; thus, requiring an extension stem, particularly for uncontained bone defects where fixation of the grafted bone was technically difficult. We did not fix the grafted bone because we considered that we could obtain the initial strength with a combination of the tibial extension stem and performing ASBG, including the rim; we did not observe dislocation of the ASBG and/or implant.
We acknowledge that the present study had certain limitations. First, this was a single-center study, and the study sample (17 cases) was small. Although Chon et al  and Sugita et al  reported 40 cases and 44 cases, respectively, in the literature using autologous bone graft for bone defects ≥ 10 mm, in this study, only 21 (4%) of 525 primary TKAs from January 2004 to December 2014 had a medial tibial defect ≥ 10 mm in depth; thus, such defects can be considered rare. Second, we have not been able to compare this with other methods, such as autologous bone graft or metal augmentation. Sugita et al  reported 14% non-progressive RL with autologous bone graft, Beak et al  and Tsukada et al  reported non-progressive RL in 10% and 33%, respectively, combined with metal augmentation. In this study, nonunion and presence of RL were 5.9% each, which did not differ from the other methods. Despite the absence of a control group, we believe that our findings are adequate; thus, important conclusions can be drawn from them.