Reconstruction of the distal femoral hemi-articular defect presents huge challenges with regards to the further elongation of the affected limb, especially for skeletally immature patients. Numerous reconstruction approaches have been reported in the past; however, the optimal choice remains controversial. Expandable prosthesis is a reasonable choice to equalize limb length for growing children [1, 4]. However, the problem involved this method is the relatively high rate of complications due to frequent lengthening procedures and overstretching of the soft tissue. Zou et al. reported complications in 36% of patients who underwent expandable prosthetic replacement in their study [5]. Besides aseptic loosening and deep infection, certain specific complications were reported on using expandable prosthesis, including the patellar tendon tear, and injury of nervous peroneus communis [5]. Furthermore, a high revision rate with expandable prosthesis has been reported in previous studies; up to 48% of cases required the revision surgery [14, 15]. Hence, hemiarthroplasty with the advantage of tibial physis preservation is a reasonable option for assuring further growth of the affected limb in children. Allograft transplantation for hemiarthroplasty after tumor resection could maximize the preservation of limb growth. However, inherent limits in allograft, such as limited donor sources, potential infection transmission, and immunological rejection, are hard to solve [16]. Moreover, the relatively low survival rate of osteoarticular allograft transplantation is another concern [3]. Recently, prosthetic hemiarthroplasty, which can not only prevent allogeneic complications, but also preserve potential limb growth, has been rapidly developed for the reconstruction of the distal femur defect. Modern technology, such as computer-aided design programs and additive manufacturing, offers the possibility of designing and fabricating prostheses with complex shapes, especially for preventing a mismatch with the proximal tibia. To improve the stability of the knee joint after hemiarthroplasty, our 3D-printed uncemented unipolar prosthesis was designed with JSRS for ligament reconstruction.
In our study, satisfactory result with the mean MSTS score of 25.8 was achieved. In detail, our patients’ postoperative limb function met the requirement for the activities of daily living, such that walking and even running was possible for the patients. Chung et al. also reported the use of hemiarthroplasty after the removal of the distal femoral tumor in 12 children (aged 8–12 y) [4]. The MSTS scores of these children are comparable to those of children in our study; however, our cases involved younger age with greater levels of difficulty in the surgical procedure and postoperative management. With regard to the complications, they clarified that the instability of the knee joint after hemiarthroplasty was a serious problem, and the risk of anterior subluxation was high (up to 17%) [4]. Additionally, Yao et al. highlighted the potential risk of hemiarthroplasty impairing the joint stability in their study, with the movement and limb function limited by the multi-directional instability of the knee joint [17]. To the best of our knowledge, in previous studies on prosthetic hemiarthroplasty, the sacrifice of the ligaments around the knee was inevitable due to the loss of suture position. Consequently, the stability of the knee was impaired, and patients had to suffer from an unstable knee. Contrary to previous studies, no stability-related complications were detected in our patients after ligament reconstruction using JSRS. Furthermore, the results of the drawer, Lachman, and pivot shift tests were negative, indicating that this technique helped achieve adequate strength to maintain joint stability. In 2019, Ji et al. adopted a thin stem passing through the tibial physis to constrain the prothesis, thereby improving knee stability after hemiarthroplasty [2]. However, the growth capacity of the tibial physis was weakened in most of the patients, with only a quarter of patients having equal tibial length. In the present study, JSRS did not involve the adjacent tibia, with the LARS ligaments anchored in the shallow grooves on the surface of the prosthesis through steel plates and screws. Nevertheless, we observed painless joint space narrowing in two cases (2/7, 28.6%), and the screw for LARS ligaments fixation loosened in one case (1/7, 14.3%). Hence, joint space narrowing was the most common complication in our study, which was believed to be related to the mild wear of the tibia plateau cartilage caused by the prosthesis [18]. Regardless, the patients in this study achieved satisfactory limb function without pain, indicating that this method is worth considering for the reconstruction of distal femur defect in children.
In addition, the 3D-printed customized prosthesis helped achieve a perfect fit with the proximal tibia and was of appropriate size to cater to the small joints of children. According to previous studies, the consistency between the reconstruction implant and the joint is essential for postoperative limb function [9, 11]. However, the complex shape of the defect and the small joints of children pose challenges in achieving acceptable consistency in children using traditional prosthesis manufacturing technology. Therefore, some studies have attempted to prevent the mismatching of the implant to the knee joint by compositing the prosthesis with resurfaced allograft, in which femoral condyles were modified according to the size and shape of the defect [9, 11]; however, the composite could not satisfactorily fit the proximal tibia. Additionally, Errani et al. reported a relatively low success rate with this method, with 50% of transplants removed due to infection and allogeneic fracture [9]. In the present study, using 3D printing technology, the prosthesis was custom-designed for each patient according to the mirror model of the contralateral healthy femur. It was characterized by prosthetic femoral condyles, which helped prevent the mismatching of the prothesis to the proximal tibia. The appropriate size of prothesis for the small joints of children was also achieved with this technique.
Another reason for the satisfactory outcomes in our cases was the minimal LLD. The prosthetic hemiarthroplasty following tumor resection in the distal femur performed in this study prevented the loss of the proximal tibial physis, minimizing the potential LLD at the end of growth. In actual, the observed LLD at the last follow-up was 1 cm lower than the expected LLD, because the prosthesis was manufactured 1 cm longer than the length of the resection to compensate for potential growth of contralateral distal femoral physis. Although the prosthesis was longer than the bone defect and resulted in LLD after surgery, patients would be able to adapt to this small degree of LLD [19]. Moreover, the expected LLD and consequent damage to the postoperative limb function would consequently decrease. In the present study, the prosthesis for the distal femoral hemiarthroplasty was assembled, providing an opportunity to compensate for the LLD by replacing or adding the prosthetic component.
Our study has several limitations. First, this study was retrospective and had a small sample size. Second, our mean follow-up period was short, and the outcome of the study could possibly change with a longer follow-up duration. Finally, the present study only focused on the osteosarcoma in the distal femur, future research about different types of bone tumors with randomized controlled study need to be conducted.