Patients with RA often have severe osteoporosis [5, 6], which may seriously affect knee functions . TKA might be required [8, 9], but studies have yet to report the effectiveness of TKA in patients with RA, severe osteoporosis, and genu valgum. Therefore, the present study aimed to assess the clinical benefits of TKA in patients with severe osteoporosis and moderate genu valgum at a single center. The results suggest that TKA by the medial parapatellar approach with proper osteotomy and soft tissue balancing could restore knee functions in patients with RA, severe osteoporosis, and genu valgum.
Various criteria are available for the classification of genu valgum, such as the Ellison criteria , Krackow criteria , Ranawat criteria , and SOO criteria . Previous studies have reported that patients suitable for TKA are generally Ellison type I genu valgum. In this study, the Ellison classification of genu valgum in all the patients was type II, and lateral soft tissue release, medial soft tissue tightening, and application of post-stable prosthesis were conducted. All the patients were with high knee joint stability after the operation.
The operations in all patients were performed through the medial parapatellar approach. The medial parapatellar articular capsule approach could facilitate the operation procedures and exposure of the operating field. The complications of operations through the medial parapatellar approach are postoperative joint instability and prosthesis loosening [18-20]. The operations through the lateral parapatellar approach are generally accompanied by incision complications [21-24]. All patients in this study underwent operations through the medial parapatellar articular capsule approach, and a stepwise release was conducted and achieved adequate soft-tissue balance.
The mean postoperative FTA was 171.2°±6.2°, and some patients were still with residual genu valgum. In a previous study about TKA outcomes for patients with genu valgum by Nakano et al. , the medial parapatellar approach was used to treat 24 knee joints, of which the preoperative and postoperative FTA was 172.4°±2.7°. Residual genu valgum was found in some patients, supporting the present study. Femoral valgus resection could be one of the factors leading to residual valgus alignment, and conducting a 3°-5° femoral valgus resection could be performed to prevent insufficient correction [26-28]. Therefore, it is applicable to adjust the femoral osteotomy angle to improve the positioning and alignment of the graft.
The highest point is generally used as the reference point for the measurement for the osteotomy. Bone defects are very common at the lateral tibial plateau in patients with severe genu valgum, and they can be repaired by autogenous bone grafting or bone cement filling . In the present study, one patient was with severe lateral tibial defects. We speculated that if the highest lateral point were used as the reference, the medial side's osteotomy amount would be too important. Therefore, the highest medial point was selected as the reference for osteotomy, and the severe lateral defects were filled with bone cement. After the osteotomy at the distal femur and tibial plateau was completed, the knee joint's flexion-extension gap was assessed. The lower extremity alignment was calculated to assess the correction of the genu valgum, based on which the thickness of the polyethylene liner was decided. A second osteotomy could be conducted if necessary. The patella was not replaced routinely, while the surrounding osteophyte was resected, the articular surface was reconditioned to be smooth, and the nerves surrounding the patella were blocked.
In cases of genu valgum deformity, the lateral tissues are generally with different degrees of tension. Thus soft tissue release is among the most critical procedures of TKA for genu valgum, leading to postoperative joint stability and the survival of the prosthesis. Nevertheless, soft tissue release has also been acknowledged as the most challenging part of this operation . In the present study, the Ranawat technique  was adopted to release the lateral contracture structures step by step. The flexion-extension gap balancing and varum-valgum stability were assessed after each step of release to prevent over-releasing. The popliteus tendon was preserved as possible, and the integrity of the lateral stable structures was maintained to prevent postoperative lateral instability. For the loosened medial structures, tight suturing was adopted to improve stability. By using these procedures, adequate soft tissue balance was obtained, and no joint instability was found after the operation. Bremer et al.  also adopted these techniques and avoided semi-restrictive and restrictive prosthesis. Mullaji et al.  adopted similar techniques for the release of posterior cruciate ligament and iliotibial band, followed by computer-navigated posterosuperior femoral osteotomy, which helped obtain more accurate positions.
For patients found with medial knee joint loosening, enhancing lateral releasing and thickness of padding appropriately and using restrictive padding and tightening the medial ligaments, could help achieve medial and lateral balance, which was followed by the application of post-stable prosthesis. The post-stable prosthesis could facilitate the intraoperative soft tissue release, provide higher internal stability to match the articular surface, and allow the femoral and tibial prostheses' maximum lateral shifting to improve the patellar tracking. Therefore, a post-stable prosthesis is recommended for genu valgum patients using a non-condyle restrictive prosthesis.
Severe osteoporosis is not an absolute contraindication of TKA, but perioperative treatment and preparation are essential. The frequency of osteoporosis is high in patients with RA [5, 6], and TKA in such patients can be highly difficult [8, 9]. First, improper position, inappropriate traction, and rough operating during TKA could induce bone fracture [8, 9, 33]. Therefore, all surgical procedures in osteoporotic patients must be meticulous and gentle. The tools, including power saw and drill, should be used instead of a bone knife and bone chisel. The procedures of planting prosthesis and flexion-extension of the knee joint, should also be careful to avoid intraoperative bone fracture . For patients with underlying bone defects, impaction bone grafting is generally required to improve the prosthesis's fixation and reduce the amount of bone cement used. Still, the bone mass of osteoporotic patients is generally poor, and a suitable bone graft is lacking. Therefore, allogeneic bone is frequently used in such patients.
An essential aspect of TKA is postoperative functional training. In addition to routine postoperative function training, there are still some specificities in RA patients. For instance, such patients are generally of advanced ages and long disease courses, and their physical condition is often poor. In addition, such patients have reduced activities. Thus, they are generally with different degrees of osteoporosis and muscle atrophy, leading to imbalanced strength of the muscles surrounding the knee joint [34, 35]. Therefore, training of the quadriceps femoris muscle should be stressed and the training of the flexion-extension function of the knee joint.
The present study has limitations. Only 32 patients (44 knees) were included in this study. In addition, this was a single-center study, and it was a retrospective study with a follow-up of 1 year.