Patient enrollment
This study is a prospective series. The medical and radiographic records of patients with advanced OA knee who received TKA in our hospital between August 1st, 2015 to October 31th, 2016 were collected prospectively.
The inclusion criteria were patients with mental health that could signed the informed consent form and completed all the functional status questionnaires and measurements of this study. The exclusion criteria were patients undergone simultaneous bilateral TKA and revised TKA.
Data collection
All angles of lower extremity alignment were measured on the long leg radiograph. The hip-knee-ankle angle (HKAA), knee alignment angle (KAA), tibial joint line obliquity angle (TJLA), lateral distal femur angle (LDFA), medial proximal tibial angle (MPTA), angle between the mechanical axis of femur and anatomical axis (AA) were measured preoperatively [9] (Fig. 1). All measurements were performed independently by two blinded observers using the GeoGebra 5.0 software (International GeoGebra Institute, Austria, 2016). When the measurement value was different, the revised one was determined after re-measurement and discussion by the two observers.
The phenotypes of the knee were categorized mainly according to the difference of mechanical alignment of the femur and the tibia (LDFA and MPTA), as described below:
-
1.
The mechanical alignment of the femur was divided into varus, neutral, and valgus alignment. Varus was defined as LDFA≧90°, neutral as 87° ≤ LDFA < 90°and valgus as LDFA< 87° [9].
-
2.
The mechanical alignment of the tibia was defined as varus, neutral and valgus alignment of the tibia. Varus was defined as MPTA < 87° and neutral as 90° > MPTA≧87° and valgus as MPTA≧90°.
Based on the above different alignments of tibia and femur, five most common knee phenotypes could be categorized (Fig. 2), as described by our previous study [9]: Type 1 knee had neutral alignments in lower limb, the femur and the tibia (Mean HKAA: 0.6°, LDFA 88.0°, MPTA 87.0°). Type 2 knee also had a neutral alignment of the lower limb, but a high degree of joint obliquity of the knee (Mean HKAA: -0.4°, LDFA 85.0°, MPTA 85.1°). Type 3 knee had a varus alignment of the lower limb, but the distal femur remained valgus or neutral (Mean HKAA 4.2°, LDFA 88.0°, MPTA 83.5 °). The main cause of varus alignment of the lower limb was due to proximal tibia varus alignment. Type 4 knee had concomitant varus alignment of the tibia and femur (Mean HKAA 5.6°, LDFA 91.4°, MPTA 85.2°). A high degree of varus lower limb alignment and lateral bowing of the femur are usually observed in this type of knee. Type 5 knee had a valgus alignment of the lower limb with femur exhibiting a valgus alignment, but the tibia had a neutral or valgus alignment (Mean HKAA: -4.2°, LDFA 84.6°, MPTA 88.8°). Patients with valgus tibia alignment (MPTA≧90°) were included in type 5 knee for their number was very small and the operation method was similar to type 5 knee [9]. The lower limb alignment was valgus, which was mainly contributed by the femur.
We set different alignment targets for LDFA and MPTA according to each original phenotype of the knee. The preoperative planning, surgical technique, pain management, and rehabilitation program were as follows.
Preoperative planning
We first determined the original phenotype of the knee according to the alignment of the tibia and femur as described above. For type 1 knee, which has a neutral alignment and transverse joint line, we cut the distal femur and proximal tibia parallel to the original joint line. No adjustment of alignment was made. For type 2 knee, which is characterized by a high oblique joint line, we adjusted the LDFA and MPTA 2–3° to decrease the joint line obliquity. The target of LDFA and MPTA was set at 87°. For type 3 knee, which has a high degree tibia varus, we cut the distal femur according to the original joint line, and the target of tibia alignment adjustment for MPTA was 85–87°. For type 4 knee, which is characterized by a concomitant varus tibia and femur, the femur usually has lateral bowing with LDFA> 95°. We adjusted the LDFA to 90–93° and the MPTA to 85–87° to correct the varus alignment of the lower limb. For type 5 knee, which has a valgus femur, the target of LDFA was set at 87°, and the target of MPTA was set at 90°. The targets of alignment adjustment for each phenotype are described in Fig. 3.
Intraoperative surgical technique
All surgeries were performed by a team lead by single experienced surgeon utilizing a posterior stabilized knee (Zimmer Biomet, LPS flex, Warsaw Indiana, USA) using midline skin incision with a medial parapatellar arthrotomy. After removal of all the osteophytes, the superficial and deep medial collateral ligaments were released for better exposure. The distal femur was first resected to achieve the target LDFA. A designed cutting guide was used to evaluate the thickness of the distal femur cut (Fig. 4). In a varus knee, the ideal distal femur cut should be 7 mm and 9 mm for the medial and lateral sides, respectively. If adjustment of the distal femur cut is made, each 1° change requires a cut of 0.65–0.7 mm [10]. The thickness of bone cut should be measured again until the target range is achieved. Then, the knee was flexed to 90°. The femoral axial rotation was set parallel to the preoperative-planned MPTA target. A designed cutting guide was used to evaluate the thickness of posterior femur cut. In a varus knee, the ideal posterior femur cut should be 11 mm and 10 mm for the medial and lateral sides, respectively. Each 1 millimeter adjustment can be estimated to correspond to 1.2°-1.5° rotational change of the femoral axis [10]. The posterior femur cut should be 2 mm less than the thickness of the prosthesis to reduce the flexion instability (the posterior thickness of a Zimmer LPS flex is 12 mm), as the release of the posterior cruciate ligament increases the flexion gap by 2-3 mm [11]. Femoral component sizing was measured using a posterior condyle referencing device. After cutting the femur bone, the proximal tibia was cut according to the target MPTA with 5° of posterior slope. Trial implants were placed, and soft tissue releases were performed to create balanced extension and flexion gaps. A balanced gap is usually achieved with subperiosteal release of superficial and deep medial collateral ligament (MCL), transverse, and longitudinal incision of the posteriomedial capsule without release of pes anserinus and pie crust of the deep MCL.
In a valgus knee, the distal femur bone was cut to form an LDFA of 87°. The proximal tibia was cut with a jig perpendicular to the mechanical axis of the tibia, posterior slope 5°, such that the MPTA was 90°. Then the knee was placed in the extended position, and a trial insert was placed. Release of the iliotibial band, arcuate ligament, and lateral posterior capsule were performed to balance the extension gap. Then the knee was flexed to 90°, the femoral axial rotation was set parallel to the tibial plateau, which was cut perpendicular to the tibial mechanical axis. For a valgus knee, the ideal posterior femur cut should be 11 mm and 8 mm for the medial and lateral sides, respectively.
Post-operative evaluation
Patients were followed-up at 2 weeks, 1 month, 3 months, 6 months, 1 year, and 2 year after operation. Postoperative HKAA, LDFA, MPTA, KAA, and TJLA were measured in a long leg radiograph at 6 months follow-up. ROM, OKS and CKSS were recorded at 12 months after the operation.
Statistical analysis
Data plotting and statistics were processed using the GraphPad Prism software for Mac OS X. Values represent the mean ± SD. All data were processed for Gaussian distribution with D’Agostino & Pearson normality test initially. The patients’ demographics, knee alignment angles (HKAA, KAA, TLJA, MPTA, LDFA), and functional knee scores (OKS, CKSS, ROM) were assessed with the Wilcoxon matched-pair signed rank test and one-way ANOVA. The level of significance was set at p < 0.05.