The most significant findings of the present study were that the CGs using PS PCT, both in extension and flexion, were not different from the final CGs set with trial femoral components. The final CGs were smaller than the estimated CGs obtained from the BGs without femoral components during knee extension. Moreover, we showed that the extension gaps increased by releasing the soft tissue around the intercondylar notch. Thus, the CGs obtained by PS PCT were the same as the final CGs in both knee extension and flexion. Moreover, the final CGs by the CG control using PS PCT were predictable before final bone resection.
The release of the femoral intercondylar notch capsule significantly increased the CG in only knee extension, compared with that before release. The difference between the CR- and PS-type femoral components is the presence of a cam structure in the PS femoral component. The cam occupies a large portion of the intercondylar notch, and the soft tissue around the intercondylar notch is subjected to tension. Okamoto et al. showed that 30 out of 54 knees in PS TKA required capsular release around the intercondylar notch to prevent flexion contracture, and noted that the extension gaps with the femoral component were dependent on the capsular tension around the cam [19]. In our study, release was not necessary in only one case, which met the exclusion criteria. The posterior protrusion of the cam in the femoral component may be the reason why most of our cases needed release; this is dependent on the shape of the femoral component, as determined by each manufacturer. The extension CGs made by PS PCT after release of the femoral intercondylar notch capsule were the same and were predictable as the final extension CGs with the trial femoral component. In contrast, the flexion CGs were unchanged.
Several studies have reported that the flexion gaps in TKA increased after resection of the PCL [6, 8, 17, 22]. In a pre-cut system, a 4-mm pre-cut surface on the lateral posterior femoral condyle was placed in the posterior part, (thickness: 4 mm) of the CR PCT [7]. However, in the pre-cut system of PS PCT, a 4-mm pre-cut surface on the femoral condyle was placed in the posterior part (thickness: 5 mm). The PS PCT has a 1-mm-thicker posterior part than the CR PCT; thus, the PS PCT fills an additional 1 mm of the gap in flexion compared with CR PCT (when not posterior of the cutting level). Our cases showed that an average of 1.1 mm posteriorly of cutting level was needed as flexion gap control; thus, a total of 2.1 mm was required (1.1 plus 1.0 mm) after pre-cutting to fill the flexion gap looseness compared with that of CR PCT (when not posterior of the cutting level). Therefore, gap control at 90° knee flexion is required during the PS TKA. The estimated final CGs obtained by PS PCT were not different from the final CGs with trial components after flexion gap control. These results suggest that release of the posterior capsule did not affect the CGs in flexion. Several studies demonstrated that the CGs were reduced for setting with trial femoral components only in extension, and were not affected in flexion [2, 16]. Our results support the notion that the final CGs with trial femoral components predicted the CGs, in both knee extension and flexion, using PS PCT.
We measured the BGs during extension and flexion using the OFR tensor. The CGs with PS PCT and final CGs were smaller than those estimated from BGs at knee extension only. Muratsu et al. showed that the CGs after placement trial femoral components of PS TKA (NexGen LPS Flex®, Zimmer Biomet Holdings) were significantly decreased by as much as 5.3 mm at knee extension only [16]. Minoda et al. showed that BGs in flexion tightened by over 1 mm compared with BGs in extension, and the BG differences between knee extension and flexion decreased; this suggested that the BGs in flexion should be made smaller than the BGs in extension before implantation, in order to minimise the mid-flexion laxity after implantation [15]. Our results suggest that final CGs with the trial femoral component can be estimated from BGs in knee flexion, but not in extension. In other words, when PS TKA using PS PCT is performed, surgeons are not required to estimate the final CGs from the BGs in both knee extension and flexion.
Further, we showed the relationship between the gap control amount and final CGs in knee extension. Although the final CGs at knee extension were not statistically different from the CGs with PS PCT, the final CGs with trial femoral components ≥ 2 mm posterior to the cutting level for flexion gap control were significantly smaller than those with PS PCT in the sub-analysis of extension CGs. Onodera et al. demonstrated that the posterior offset ratio depends on the shape of the femoral component, determined by each manufacturer, and the protrusion of the posterior condyle may cause knee flexion contracture due to the relative shortening of the posterior capsule [21]. Tsubosaka et al. confirmed that a larger posterior condylar offset reduced the CGs during knee extension, but not always in flexion [24]. Here, we quantitatively demonstrated that gap control of over 2 mm posterior to the cutting level (total under 7-mm cut) compared with the amount of femoral posterior condyle (total 9-mm cut) by the measured resection technique reduced the final CGs in knee extension. It is beneficial for surgeons to quantitatively estimate the final CGs before bone resection in knee extension and flexion.
The improvement of pre- to postoperative JOA scores was not related to the CG differences between the final extension and flexion gaps. The changes in ROM from pre- to post-surgery were not related to CG differences (data not shown). Although the influence of the difference between the flexion and extension gaps on the ROM and clinical outcome remains controversial [5, 13, 25], our results suggest that the average 1.4-mm gap difference between the extension and flexion gaps was not related to the clinical result.
This study has several limitations. First, the design of PS PCT is implant-specific and the results cannot be extended to all prosthesis designs. In detail, the posterior protrusion size of the cam in the femoral component depends on the component design and concept of each manufacturer. Although, the release of intercondylar notch of the posterior capsule was not needed in only one knee in the exclusion criteria, a previous report contrarily showed that 44% of knees required release of intercondylar condylar notch of the posterior capsule [19]. The influence of the cam on the extension gap should be considered when using PS TKA; however, it is not necessary in all cases. Analysis of the cam design for each femoral component is required in future studies. Second, the posterior reference guide for femoral resection is classified into three types according to the rotation centre for femoral component rotation, medial rotation, centre rotation and lateral rotation [14]. The pre-cut guide for initial femoral condylar resection is a posterior reference guide for the lateral rotation type for determining the femoral rotation [7]. The posterior reference guide for Persona® PS TKA using the measured resection technique is a centre rotation type. Thus, the resection was performed 9 mm from the femoral posterior centre, and the femoral component was set. Thus, the position of the PCT after pre-cutting of the femoral condyle is slightly different from the position of the femoral component after using the measured resection technique; however, we believe that a slight difference in the amount of femoral resection did not affect the final component position because the position of the femoral component was decided after the gap control in flexion. Third, the JOA score used as a clinical score in this study is commonly used in Japanese clinical practice [3, 18, 20]. The scoring system is an observer-based scoring system, and has already proven a significant correlation with other validated patient-reported outcomes [20]. However, we did not directly measure patient-reported outcomes, such as the Knee Injury and Osteoarthritis Outcome Score (KOOS) and Knee Society Score (KSS 2011, modified version), which are used internationally [9, 10]. The patient satisfaction scores in the KOOS and KSS 2011 may correlate with the final CG differences between extension and flexion.