2.1 Participants and surgical procedure

This study was designed as a prospective, comparative study. Patients who met the inclusion criteria were assigned alternately in a predetermined sequence to receive either a SR or a MR femoral component in a manner intended to minimize selection bias. The allocation process was determined before the start of the study, and the implant design was not disclosed to the evaluators of postoperative outcomes. This study included 11 patients (15 knees) undergoing SR-TKA (GetAroundKnee™, Stryker Orthopedics, Mahwah, NJ) and 13 patients (16 knees) undergoing MR-TKA (Balanced Knee® System, Ortho Development Corporation, Draper, UT). Both implants used were of the posterior-stabilized design, and all patients underwent patellar resurfacing. Table 1 presents patient demographics, including sex, side, age, height, body weight, and body mass index. A single board-certified surgeon performed all surgeries under general anesthesia using a medial parapatellar approach. The surgical technique aimed for mechanical alignment in all cases. Both groups received the same postoperative rehabilitation protocol. The preoperative diagnosis of all patients were OA. In cases where OA was present in the contralateral limb, it had not yet progressed to cause pain or functional impairment. The inclusion criteria were: (i) age <75 years at the time of surgery; (ii) no history of lower extremity fracture, osteotomy, or joint replacement; (iii) unilateral or simultaneous bilateral TKA for OA; and (iv) ability to walk without assistance. Patients with valgus deformities, post-traumatic or post-surgical knees, previous osteotomies, or rheumatoid arthritis were excluded from the study.

This study was approved by the Western Institutional Review Board® (WIRB), United States (Approval Date: July 15, 2016; Protocol No. 2014-018; Study No. 1147713). The research was conducted in accordance with the ethical standards of the Declaration of Helsinki and the principles outlined in the Belmont Report. Written informed consent was obtained from all participants prior to enrollment.

2.2 Clinical scores

Clinical and functional outcomes were evaluated using The University of California Los Angeles (UCLA) Activity Questionnaire, a validated predictor of routine function in patients with TKA.¹⁹ Data were analyzed preoperatively within 2 weeks preoperatively and at 6 months and 1 year postoperatively.

2.3 Strength and biomechanical assessment

All strength and biomechanical analyses were conducted at the university gait laboratory. These assessments, including the collection of clinical scores, were performed by three experienced laboratory staff members. The evaluators were blinded to the specific implant design (SR or MR) used for each patient. Gait biomechanics were assessed during self-paced walking using a 29-marker retro-reflective array. Markers were placed on the thorax (xiphoid process, jugular notch, C7 and T10 spinous processes, inferior angle of the scapula, and bilateral acromioclavicular joints), pelvis (bilateral anterior superior and posterior superior iliac spines), and lower extremities (bilateral first, second, and fifth metatarsal heads, base of the fifth metatarsal, posterior calcaneus, medial and lateral malleoli, and medial and lateral femoral epicondyles). Four marker arrays were attached bilaterally to the midthigh and midshank. The medial femoral epicondyle, medial malleolus, and first metatarsal head were used for static calibration only and were removed during dynamic trials.

Kinematic data were collected at 240 Hz using a Vicon motion capture system (Nexus 2.5, Vicon, Inc., Centennial, CO, USA) and synchronized with kinetic data. Kinetic data from the operated limb were collected at 960 Hz using a floor-embedded force plate (Advanced Mechanical Technology Inc., Boston, MA, USA). Kinematic data were smoothed using a low-pass Butterworth filter with a 10 Hz cutoff frequency, whereas ground reaction forces were filtered at 50 Hz. Joint moments were calculated using inverse dynamics based on marker trajectories and filtered at 10 Hz cutoff frequency.²⁰ The first peak knee flexion moment (KFM) was defined as the initial local maximum occurring during the stance phase. Three successful walking trials were conducted to ensure that the participants made full-foot contact with the force plate without altering their gait patterns.

Following the movement trials, knee extensor strength was assessed using a MicroFET2 handheld dynamometer (Hoggan Health Industries, West Jordan, UT, USA). Participants were seated with their knees flexed at 60° and trunks reclined. The dynamometer was placed on the anterior shank at 80 % of the distance from the lateral knee joint line to the lateral malleolus and secured with a strap to improve measurement reliability.²¹ This method is recommended as an alternative to isokinetic dynamometry for assessing knee strength.²² Participants were instructed to gradually build force over 3 s and maintain maximum contraction for 2 s. Two maximal effort trials were performed, with a third trial added if the second trial deviated by more than 10 % from the first. The average of the valid trials was calculated as Newton-meters per kilogram (N·m/kg). Verbal encouragement was provided to ensure maximal effort during strength testing.

2.4 Data analyses

All statistical analyses were performed using the EZR software.²³ Data are presented as mean and standard deviation. The UCLA activity scores, knee extension strength, and sagittal biomechanical variables were compared between the groups using the Mann–Whitney U test. A chi-square test was used to compare sex and side in patient demographics. Spearman's rank correlation coefficients were calculated to assess the relationship between knee extension strength and sagittal plane biomechanical variables. The strength of the correlation (r) was interpreted using the following thresholds: 0.1 ≤ |r| < 0.3 as weak, 0.3 ≤ |r| < 0.5 as moderate, and |r| ≥ 0.5 as strong.²⁴ Statistical significance was set at P < 0.05.

A sensitivity power analysis for the correlation analysis was performed using EZR software. Given our sample sizes (n = 15 to 16) and an alpha level of 0.05 (two-tailed), the statistical power to detect a conventional medium effect size (r = 0.30) was 26–29 %, and the power to detect a large effect size (r = 0.50) was 64–70 %.

This study was designed as a prospective exploratory cohort study conducted in a specialized gait laboratory. Comprehensive three-dimensional motion capture analysis after TKA requires substantial technical resources, trained personnel, and participant burden, which inherently limits the feasibility of large-scale enrollment. Accordingly, the present sample size reflects the practical constraints of conducting high-resolution biomechanical assessments in a clinical population.

Rather than aiming to detect small between-group differences with high statistical power, the primary objective of this study was to explore associations between quadriceps strength and sagittal-plane knee biomechanics, particularly external KFM, within each implant design. This exploratory approach was considered appropriate for generating clinically relevant hypotheses regarding implant-dependent biomechanical behavior following TKA.

Accordingly, the absence of statistically significant between-group differences should not be interpreted as evidence of equivalence between implant designs.

3.1 Baseline demographics and clinical scores

Preoperative demographic data, including age (MR: 65.8 ± 6.8 years; SR: 66.3 ± 3.8 years) and body mass index (MR: 29.1 ± 6.4 kg/m²; SR: 31.2 ± 4.3 kg/m²), were comparable between the groups (Table 1). Clinical outcomes, as measured by the UCLA activity score, improved in both groups over one year but were not significantly different between the MR and SR groups at any time point (e.g., 1 year post-TKA: 6.6 ± 1.5 [95 % CI: 6.0–7.2] vs. 6.1 ± 1.5 [95 % CI: 5.1–7.1], p = 0.417) (Table 2).

3.2 Knee extensor strength and sagittal biomechanical variables

Knee extensor strength improved from preoperative levels in both groups by 6 months. However, no significant differences were observed between the groups at any evaluation (1 year post-TKA: 0.88 ± 0.33 N m/kg [95 % CI: 0.70–1.06] vs. 0.80 ± 0.27 N m/kg [95 % CI: 0.61–0.99], p = 0.800) (Table 3). Similarly, all sagittal biomechanical variables during level walking and stair climbing, including PKFM and peak knee flexion angle, showed no significant intergroup differences at any time point (Table 4).

3.3 Associations between knee extension strength and sagittal-plane biomechanics

The primary findings of this study were revealed in the correlation analysis (Table 5). No significant correlations were observed in the SR group at any time point during any task. In contrast, the MR group showed specific, task-dependent correlations. At 6 months postoperatively, a moderate, significant positive correlation was found between knee extension strength and KFM impulse during stair descent (r = 0.498, p < 0.05). This association was not observed at 1 year postoperatively (p = 0.226). Conversely, during stair ascent, a correlation was not present at 6 months (p = 0.080) but emerged as a strong, significant positive correlation at 1 year postoperatively. This was observed for both 1st PKFM (r = 0.606, p < 0.05) and PKFM (r = 0.606, p < 0.05).

4.1 Clinical outcomes and knee extension strength

Consistent with previous studies and meta-analyses comparing SR and MR femoral designs, no significant differences were observed in clinical outcomes or quadriceps strength between the two groups.²⁵ UCLA Activity Scores were comparable at all time points, suggesting similar functional recovery regardless of femoral component geometry. However, given the relatively small sample size, the absence of statistically significant differences should be interpreted cautiously and should not be considered evidence of equivalence between implant designs. Previous investigations of quadriceps recovery after TKA have reported conflicting results, with some studies suggesting more efficient quadriceps activation or short-term strength advantages in SR designs,^26–28 whereas others found no meaningful differences between SR and MR implants.^12,29 A strength of the present study is the inclusion of both preoperative and postoperative strength assessments, demonstrating recovery to preoperative levels by six months in both groups, supporting the concept that postoperative rehabilitation and neuromuscular recovery play major roles in strength restoration after TKA.³⁰

4.2 Sagittal-plane biomechanics

No significant between-group differences were detected in key sagittal-plane biomechanical variables during walking or stair activities, including KFM, peak KFM, KFM impulse, trunk flexion angle, or knee flexion angle. These findings suggest that theoretical biomechanical differences between SR and MR designs were not clearly reflected in absolute joint mechanics during functional tasks within the statistical power of this study. Previous gait analyses have reported that SR implants may demonstrate kinematic or kinetic patterns more closely resembling those of healthy knees, particularly during weight acceptance or stair negotiation.^31–33 The discrepancy between those findings and the present results may reflect differences in patient populations, implant systems, alignment strategies, or analytical methodologies. Importantly, sagittal-plane knee biomechanics after TKA are influenced by multiple interacting factors, including muscle strength, neuromuscular control, and compensatory movement strategies, which may attenuate implant-related effects in clinical settings.

4.3 Association between quadriceps strength and sagittal-plane biomechanics

The most clinically relevant finding of this study was the implant design–dependent difference in the relationship between quadriceps strength and sagittal-plane knee biomechanics. In the MR group, significant correlations between quadriceps strength and KFM-related parameters were observed in a task- and time-specific manner, whereas no such associations were identified in the SR group. These findings suggest that sagittal-plane knee mechanics may be more strength-dependent in MR designs, while SR designs may be associated with relatively strength-independent biomechanics during functional activities.

The task-specific nature of these associations in the MR group may reflect differing neuromuscular demands between stair descent and ascent. Stair descent predominantly requires eccentric quadriceps control to regulate knee flexion and body weight lowering, whereas stair ascent relies more heavily on concentric quadriceps force generation to produce knee extension moments.³⁴ The early postoperative association between quadriceps strength and KFM impulse during stair descent and the later association observed during stair ascent may therefore reflect sequential recovery patterns of eccentric and concentric quadriceps function following TKA. These interpretations remain speculative and should be regarded as hypothesis-generating rather than explanatory.

In contrast, the absence of significant strength–biomechanics associations in the SR group suggests that sagittal-plane knee mechanics may be less sensitive to variations in quadriceps strength in this implant design. This characteristic may be clinically relevant for patient populations prone to persistent postoperative muscle weakness, such as older or less active individuals, in whom age-related declines in quadriceps strength are well documented.^17,34,35 From a clinical perspective, selection of an implant design associated with more strength-independent biomechanics may contribute to more consistent functional performance during daily activities in such patients.

Given the exploratory nature of this study, no correction for multiple comparisons was applied. Accordingly, all observed correlations should be interpreted cautiously as hypothesis-generating rather than confirmatory. To our knowledge, this is the first study to examine implant design–specific associations between quadriceps strength and external KFM after PS-TKA. Rather than demonstrating superiority of one femoral design over another, the present findings highlight a novel distinction between strength-dependent and strength-independent sagittal-plane knee biomechanics, which may inform future studies on patient selection, implant choice, and postoperative rehabilitation strategies.

4.4 Limitations

The primary limitation of this study is the relatively small sample size, which limits the statistical power to detect subtle between-group differences and increases the risk of Type II error. However, this limitation should be interpreted within the context of the study design. The present investigation was conducted as a prospective exploratory cohort study using three-dimensional motion capture analysis, a methodology that is inherently resource-intensive and difficult to scale to large patient populations. Therefore, the sample size was considered acceptable for the primary aim of evaluating associations between quadriceps strength and sagittal-plane knee biomechanics rather than establishing definitive causal or comparative conclusions. In addition, this study was performed at a single specialized gait laboratory and included only patients undergoing posterior-stabilized total knee arthroplasty, which may limit the generalizability of the findings to other implant designs or clinical settings. Although a post hoc sensitivity power analysis indicated sufficient power to detect large correlations, the study was underpowered to identify small to moderate effects. Consequently, non-significant findings—particularly for between-group comparisons—should not be interpreted as evidence of equivalence between implant designs. Finally, given the exploratory nature of this study, no adjustment for multiple comparisons was performed. Accordingly, the observed associations should be interpreted cautiously as hypothesis-generating rather than confirmatory.