Limb salvage surgery is major surgical technique for a malignant bone tumor in the extremities [39, 40]. Currently, prosthetic reconstruction is widely used in clinical practice after tumor resection. Despite continuous improvement in the material and design of the prosthesis, the implantation failure rate of tumor prostheses remains higher than that of nontumor prostheses. Previous literature reviews showed that the 5-year survival rate of patients with knee tumor prostheses ranged from 57–93%, and the 10-year survival rate of the same was 50–88%. [1, 4, 8, 41] Aseptic loosening is a common prosthesis failure mode , and its occurrence may be related to age, tumor resection length, prosthetic stem size, and the biomechanical instability [9, 22–26].
Distal femoral replacement (DFR) comprises a large proportion of prosthetic replacement and is prone to aseptic loosening and revision surgery. [9, 42, 43] Aseptic loosening of DFR prostheses often results in bone absorption to varying degrees and even destruction. Consequently, residual bone mass is lost, and the proximal femur is shortened, inevitably causing challenges to revision surgery. [32, 44] Therefore, correct understanding and evaluation of bone defects after aseptic loosening are important prerequisites for revision surgery.
We reviewed the data of 29 patients with aseptic loosening and revision after DFR from 3 bone tumor diagnostic and treatment centers in China. The aseptic loosening of the distal femoral tumor prosthesis was classified by 1 expert in bone and soft tissue tumor imaging and 2 bone tumor surgeons. On the basis of the aforementioned classification, the differences in the performance of prosthetic displacement, periprosthetic bone, femoral residual bone mass, and length on X-ray, a new classification of bone defects was proposed. Under the new classification system, the defects were divided into 4 types and 8 subtypes. We also summarized the revision techniques performed on 29 patients by different types and suggested specific revisions. We hope that the proposed classification of aseptic loosening is expected to elucidate the aseptic loosening of distal femoral tumor prostheses, as well as provide a new revision strategy for DFR.
Aseptic loosening under Type Ia usually occurs shortly after surgery. The literature shows that tumor prosthesis failure has the highest incidence rate in the early stages, comprising 69% of the total within 5 y . Five patients with Type Ia loosening underwent revision surgery, with a median of 3 y after surgery; of this number, 3 patients had osteosarcoma. Early loosening may be attributed to perioperative chemotherapy and extensive resection boundaries. Aseptic loosening under Type Ia has no displacement, and the bone structure around the prosthesis is normal; the revision is relatively simple and similar to the initial prosthetic replacement. Similarly, aseptic loosening under Type Ib has no displacement, but the bone around the prosthesis is absorbed and thinned. Seven patients with Type IB loosening underwent prosthesis revision, with a median of 11 y postoperatively. Although the bone around the prosthesis becomes thinner, the bone inclusivity is still complete, providing a good bone implantation environment for the prosthesis. Moreover, the bone structure in the proximal femur has a certain normal length, which can be revised by longer-stem tumor prosthesis. In Type 1 aseptic loosening, the prosthesis has no displacement, the bone defect caused by loosening is less, and the amount of bone available for fixation in the proximal femur is sufficient. Therefore, the use of original or extended stems prostheses for reconstruction should be considered in revision surgery. In addition, the use of extended stem prosthesis, combined with an autologous bone or allogeneic bone graft, may also be considered by a small number of patients with severe bone loss but no displacement of the prosthesis.
The prosthesis with Type IIa loosening showed displacement and absorption of bone absorption, but its inclusion still existed. Under this type, 9 patients underwent revision surgery, with a median of 8 y postoperatively, and the affected limbs had a certain degree of shortening. The limb shortening that can be corrected by revision surgery is limited, which should be communicated to the patient before the operation. Generally, Type Ⅱ loosening can be repaired with longer-stem prostheses. The prosthesis with Type IIb loosening showed a large displacement, and the prosthesis stem pierced the broken bone cortex. Seven patients were reported to show this type of bone defect in 11 y postoperatively. This type is usually accompanied by a bad force line and limb shortening. In this case, an allogeneic bone graft, combined with a lengthened stem prosthesis, is a more reliable approach to revision. The allogeneic bone can effectively help to fill the bone defect. Lengthening the stem can avoid the stress concentration point of the original prosthesis, as well as reduce the possibility of stem of prosthesis penetrating the cortex of bone once again. In revision and reconstruction, the longer and larger stem, combined with autogenous bone or allogeneic bone plate fixation, should be considered first for type Ⅱ aseptic loosening.
In Type Ⅲ aseptic loosening, the residual normal bone length of the proximal femur is less than 5 cm, the isthmus bone cortex is absorbed by more than 50%, and the bone cortex is fragile. This type of loosening often occurs when long segmental bone defects are reconstructed with prostheses after large segmental resection of distal femoral tumors. The reason is that the proximal end of the femur is short, the contact surface between the prosthesis stem and bone is reduced, and the holding force of the stem is inadequate. Only 1 patient was classified under Type III loosening in our study; the normal bone length of the proximal femur was less than 5 cm, and the bone defect of the isthmus was serious; thus, the defect was classified as Type Ⅲb. After the original prosthesis was removed, the prosthesis with a longer and thicker stem was implanted. Postoperative X-ray examination showed that the stem of prosthesis pierced out of the proximal femur, the limb shortening was noticeable, and the MSTS function score was 20.
The revision of type Ⅲ loosening presents a challenge; thus, a good implantation environment is necessary for the prosthesis. Currently, the commonly used methods reported in the literature are the telescope tube-like allograft prosthesis [45–47] and the Compress® Compliant Pre-stress (CPS) prosthesis [50–52]. The telescope technology was first proposed by Healey et al.  and succeeded allogeneic bone grafts used in reconstruction and revision. [48, 49] With telescope technology, the grafted bone and the host bone are overlapped to maximize the surface contact between the host bone and the allogeneic bone to realize a stable fixation of the distal end of the prosthesis. Hindiskere et al  confirmed the effectiveness of telescope technology. In 14 patients with endoscopic bone allograft, the bone healing rate was 100%, and the MSTS score was 27 at the final follow-up. Another commonly used technique reported in the literature is compressive osseointegration, which uses the axial pressure between the implant and the bone surface for initial implant fixation . With this approach, the large prosthesis can be fixed to a considerably short backbone segment. In addition, compressive osseointegration can effectively avoid stress shielding, which can induce bone hypertrophy and inward growth at the interface between bone and prosthesis. [52–54] CPS prosthesis can be used to repair distal femoral prosthesis with a short femoral stump and numerous bone defects. [50, 51]
In Type Ⅳ loosening, the bone defects have involved the lesser trochanter of the proximal femur, and the loosening of this type is rarely reported in the clinical study. Its occurrence is usually observed in the middle and late stages of prosthesis survival. This failure belongs to large-segmental or ultralong bone defects. Revision of this type of defect is challenged by three issues: (1) The bone defect is serious, and the residual bone is too short to support and fix the traditional lengthened stem prosthesis; (2) The defect area is too long, and conventional bone transplantation fails to meet reconstruction needs; (3) Severe limb shortening and unequal length of both lower limbs were observed in the late stages of unreserved epiphysis. Therefore, this revision is faced with considerable challenges, and whether to retain the hip joint determines the choice of revision. The main methods applied for the reconstruction of this kind of femoral defect include customized lateral plate locking femur prosthesis , 3D-printed short femoral stem prosthesis , and total femoral replacement .