This randomized controlled trial (RCT) was approved by our Institutional Review Board and based on a parallel trial design with a 1:1 allocation ratio. Two treatment methods where used and executed by the senior surgeon in all cases enrolled; of these, treatment A was primary TKA performed with conventional instrumentation with IM guides for the femur and an extramedullary guide for the tibia using a posterior-stabilized (PS) knee system (Vanguard®; Zimmer Biomet, Warsaw, IN, USA) (conventional group), while treatment B was the PSI (Signature™; Zimmer Biomet, Warsaw, IN, USA) group treated using the same Vanguard PS knee system. In each patient, one knee received treatment A and the other received treatment B by simple randomization of treatment six weeks prior to surgery. The surgeon and evaluator were blinded until the date of surgery while the patient was blinded until after the surgery of both knees. No changes throughout the trial were made. All patients enrolled agreed to join the study and signed an informed consent form. Statistical analysis was performed using Microsoft® Excel® for Microsoft 365 MSO (Microsoft Corporation, Redmond, Washington, USA) (16.0) with the analysis toolPack. All entries and statistical analysis was done by one person to preserve integrity of data.
The femoral bowing angle on the coronal plane was measured using the acute angle between the mid-endosteal canal axes drawn at the proximal and distal femoral shaft quarters. Here, the proximal femoral shaft quarter extended from 0 to 5 cm below the lesser trochanter and the distal quarter extended from 5 to 10 cm above the lateral femoral condyle’s lowest part. Femoral bowing was confirmed when the bowing was at least 5° [2, 9, 10, 12, 13].
This study was conducted from February 2017 to August 2020 in our hospital, including a total of 336 patients eligible for simultaneous or staged bilateral TKAs. All patients going for bilateral primary TKAs—whether simultaneous or staged—were screened for lateral femoral bowing using the method described by Xiaojun et al., who defined lateral femoral bowing as the crossing of the medial cortex of the femur by a line extending between the apex of the intercondylar notch and the greater trochanter of the femur [14]. All cases of lateral femoral bowing identified by screening were then measured to include bowing of at least 5° bilaterally and to exclude those who did not meet this criterion using the method described by Lasam et al. [2, 9, 15]. Lasam et al. defined femoral bowing as the angle between two lines passing through the mid-endosteal canal of the femur, where the first line is at the level of 0 and 5 cm distal to the lesser trochanter and the other is at 5 and 10 cm proximal to the lowest point of the lateral femoral condyle [2, 9, 15].
All patients booked for bilateral staged or simultaneous TKA with Kellgren–Lawrence grade 3 or 4 osteoarthritis of the knee and bilateral lateral femoral bowing of at least 5° were eligible for inclusion. Meanwhile, all patients with deformities due to trauma or tumors, previous lower-limb surgery with implants, or femoral bowing not amenable to intra-articular correction were excluded. The feasibility of intra-articular correction was judged by drawing a line perpendicular to the mechanical axes of the femur representing the distal femoral resection on long film radiographs preoperatively. If the line passed through the insertion of the collateral ligament on the distal femoral condyles, then intra-articular resection alone could not be applied to correct the pathology and, hence, the case was not a candidate for this study [7, 16, 17].
Among 336 patients planed for bilateral TKA assessed for eligibility, 31 patients were selected (Fig. 1). Two of these patients were subsequently excluded for severe ankylosis warranting a tibial tubercle osteotomy and PSI converted to conventional guides due to a manufacturing mistake, respectively. We obtained postoperative computed tomography (CT) scans for 27 patients and postoperative long film radiographs for 28 patients. This RCT was ended when we exceed the number of participants required as suggested by a priori power analysis.
The surgical technique was standardized by the senior surgeon, who performed all operations. A neutral mechanical alignment on the coronal plane was aimed for both surgical techniques. The subvastus approach was applied in all cases. Preoperative planning using long film anteroposterior (AP) standing radiographs of the lower limb on the side receiving treatment A incorporated the following steps. First, on an AP-view long film radiograph, three lines were drawn, which were the mechanical axis of the femur, the IM guide pathway from the apex of the intercondylar notch, and the isthmus of the femur, while another line was drawn along the lateral cortex representing the most capable path passing through the IM canal. Second, the lateral entry point was marked at a distance equal to half the diameter of the IM guide medial to the lateral cortex line, doted in yellow (Fig. 2a). Third, the distance from the entry point to the apex of the intercondylar notch was noted. Finally, the cutting angle was calculated by subtracting 90 from the angle formed between the perpendicular line to the mechanical axis and the lateral cortex line (Fig. 2b).
Intraoperative marking of the femoral guide entry point is shown in Fig. 3. First, the traditional entry point was marked; then, the distance calculated previously was used to mark the lateralized entry point as shown in Fig. 3a. Finally, the lateralized entry point was drilled in an enlarged fashion by rotating the drill bit in a conical fashion to provide wiggle space for the guide to enter (Fig. 3b). With lateralization of the femoral entry, we aimed to find the most open path for the IM guide to reach the femur’s isthmus, which, in turn, extends the utility of our instruments [12]. For the group receiving treatment B, PSI was ordered six weeks prior to surgery with a goal of neutral mechanical alignment. The process of PSI acquisition involved ordering a CT scan of the lower limb in accordance with the manufacturer’s protocol, then uploading said scan to their site and waiting for an implant proposal from their engineers, which was then approved by our senior surgeon with parameters similar to the conventional side except for the sagittal plane alignment of the femoral component, which was planned with an extra 3° flexion to avoid femoral notching. After approval by the senior surgeon, manufacturing of the PSI was completed by the company in their facilities and sent to our hospital.
Our primary outcome measurements were defined as follows. On the long AP radiograph of the lower limbs with weight-bearing, the following points were marked to draw the mechanical axes of the lower limb: the center of the femoral head, the mid-condylar point of the distal femur, the tibial plateau center marked by the interspinous midpoint, and the tibial plafond center (Fig. 4). Each axis was defined as follows: femoral mechanical axis (femoral head-distal femur), tibial mechanical axis (tibial plateau center-tibial plafond center), and overall mechanical axis (femoral head-tibial plafond center). The hip–knee–ankle (HKA) angle was formed by the acute angle between the femoral and tibial mechanical axes, with a negative sign for varus and a positive sign for valgus [9, 18].
Meanwhile, the mechanical lateral distal femoral angle (mLDFA) was formed between the mechanical axis of the femur and the distal femoral joint line [19], the anatomic lateral distal femoral angle (aLDFA) was formed between the distal femoral mid-IM axis and the distal femoral joint line [19], and the medial proximal tibial angle (MPTA) was formed between the tibial mechanical axis and the proximal tibial joint line [19]. The joint lines for the distal femur were drawn passing through the subchondral aspect of the distal femoral condyles, while, for the proximal tibia, a line was established passing the tibial plateau through the subchondral aspect of its concavities. Additionally, the joint line convergence angle (JLCA) was formed between both knee joint lines mentioned [19] and the distal femur valgus correction angle was formed between the mechanical axis and an axis extending from the intercondylar notch until the isthmus of the femur [3]. The component alignment was measured in accordance with the Knee Society Radiographic Evaluation System and Methodology for TKA [20].
The component alignment goals for the tibial component were 90° valgus on the coronal plane and 87° on the sagittal plane, while, for the femoral component, the sagittal angle for PSI was flexed by 3°, that for the conventional side was 0°, and the coronal angle was 90° [6]. On axial CT cuts of the distal femur, the posterior condylar angle was formed between the transepicondylar axis (TEA) and the posterior condylar line. The TEA was drawn between the medial and lateral epicondylar prominences of the distal femur [21, 22]. The posterior condylar line (PCL) was defined as a line passing through the subchondral border of the most posterior aspect of the femoral condyles [21, 22].
Statistics
A priori power analysis was performed using an online calculator provided freely by sealed envelope®. Twenty-eight patients (n = 56 knees) were estimated to detect for a 5% significance level with 80% confidence interval to exclude the HKA angle difference of means by 2.2 based on a SD assumption of 2.8 [23]. All data analyses involved continuous variables and regular descriptive tests were applied. A t-test assuming unequal variance was used to compare the means of the conventional to PSI side for every variable. These statistical modalities were easily available in a downloadable analytical pack by Microsoft for Excel (Microsoft Corporation, Redmond, Washington, USA).
