Impact of Gluteus Medius Strengthening Based Knee Rehabilitation on Radiographic Knee Alignment and Muscle Function in Patients Following Total Knee Arthroplasty

1. Introduction

Knee osteoarthritis (OA) is the most common joint disease worldwide [1] and is a widespread musculoskeletal disorder that affects articular cartilage and synovial joints, leading to a decline in physical function [2,3]. In Asian countries, the prevalence of knee osteoarthritis has been reported as 46.3% in Thailand, 39.6% in Japan, 36.1% in the Republic of Korea, and 21% in India. The relatively high prevalence of knee osteoarthritis in Asia is thought to be associated with frequent knee-bending and squatting postures, as well as genetic and environmental factors [4]. According to the 2026 National Health Insurance Statistical Yearbook of the Republic of Korea, the number of total knee Arthroplasty (TKA) procedures increased steadily from 782,904 cases in 2014 to 882,824 cases in 2024, representing an increase of approximately 100,000 cases over the past decade [5]. Previous studies have also reported that knee osteoarthritis is more prevalent in women, and that women have a lower quality of life compared with men of the same age [6].

It has been estimated that approximately 40–80% of individuals with radiographic evidence of knee osteoarthritis develop clinical symptoms [7]. The Kellgren–Lawrence classification system categorizes knee radiographs into four grades: Grade 1 is characterized by doubtful joint space narrowing with possible osteophyte formation; Grade 2 indicates possible joint space narrowing with definite osteophyte formation; Grade 3 demonstrates definite joint space narrowing, moderate osteophyte formation, some sclerosis, and possible bony end deformity; and Grade 4 is defined by large osteophyte formation, severe joint space narrowing, marked sclerosis, and definite bony end deformity [8,9]. This condition represents a major public health concern, as it causes chronic pain and knee joint instability, ultimately progressing to end-stage osteoarthritis that markedly reduces range of motion (ROM), impairs physical function, and diminishes quality of life [10].

For the management of degenerative knee osteoarthritis, conservative treatments such as pharmacological therapy, physical therapy, and exercise are primarily recommended. However, in cases of severe symptoms accompanied by structural joint deformities, including extensive articular surface damage and varus or valgus malalignment of the knee, surgical intervention is considered as a final treatment option. In such advanced stages, total knee arthroplasty (TKA) is regarded as an effective therapeutic approach [11,12].

Surgery is generally performed by making an anterior incision in the knee to expose the joint, removing damaged tissues and bone, and creating space for insertion of the prosthetic components. During this process, the bone surfaces are resected to adjust the joint gap and ensure balanced ligament tension during knee flexion and extension [13]. Metal prosthetic components are then fixed to the femur and tibia, and a plastic insert is placed between the metal components to allow smooth joint movement [13].

Prosthetic components are fixed using either a cemented or cementless technique. Cemented fixation provides immediate stability using bone cement, whereas cementless fixation relies on bone ingrowth into a porous metal surface to achieve long-term biological fixation [14]. The femoral component is typically made of titanium, which enhances bone–implant integration and effectively distributes mechanical load [15].

Most patients who undergo TKA experience postoperative relief from knee pain, improvements in knee function, and enhanced quality of life [16]. However, despite these benefits, rapid declines in muscle strength due to postoperative muscle atrophy are commonly observed, along with increased joint stiffness and instability, which may limit activities of daily living and increase the risk of injury following TKA [17]. Therefore, effective postoperative rehabilitation programs incorporating progressive muscle strengthening exercises are essential to promote recovery and facilitate a return to daily activities. Early postoperative exercise is particularly important for reducing knee pain, improving muscle strength, decreasing swelling, preventing joint contracture, and restoring knee ROM [18].

The gluteus medius plays a critical role in hip abduction and contributes to the stabilization of the pelvis and trunk during functional movements such as walking and balance tasks [19]. However, patients with end-stage knee osteoarthritis (OA) have been reported to exhibit reduced gluteus medius strength due to decreased physical activity [20]. This reduction in muscle strength adversely affects gait function, and patients with knee OA often demonstrate altered gait patterns characterized by reduced walking speed, shorter stride length, and decreased cadence. These gait adaptations are considered compensatory strategies aimed at minimizing joint pain and mechanical loading during ambulation [21,22,23].

Traditional rehabilitation programs following TKA have primarily focused on improving knee range of motion and strengthening the quadriceps muscle [24]. However, previous studies have reported that functional limitations may persist after TKA even when conventional rehabilitation programs are applied [17]. In particular, preoperative weakness of the gluteus medius has been shown to persist after surgery, and this deficit is not sufficiently improved by rehabilitation programs that emphasize quadriceps strengthening alone [25]. Furthermore, the contribution of gluteus medius strength to functional limitations in patients following TKA has been reported to be more strongly associated with functional outcomes than quadriceps strength [26].

Despite this evidence, rehabilitation practice continues to focus predominantly on the knee joint, and studies examining the relationship between gluteus medius strengthening and improvements in knee joint function after TKA remain limited. Although recent studies have applied rehabilitation programs incorporating hip abductor strengthening exercises in patients following TKA, these interventions were initiated at least four weeks postoperatively [27].

This study aimed to evaluate the impact of early postoperative gluteus medius strengthening–based knee rehabilitation, initiated 2 days after surgery and performed for 12 weeks, on radiographic knee alignment and muscle function following TKA.

2. Materials and Methods

2.1. Participants

A total of 100 patients who underwent total TKA due to OA at A Hospital were initially recruited for this study. The required sample size was calculated using the G-Power software (version 3.1.9.7) with a significance level (α) of 0.05, statistical power (1−β) of 0.80, and an effect size (f) of 0.34, resulting in a required sample size of 100 participants. During the course of the study, 12 participants withdrew due to personal reasons such as time constraints and non-compliance with the intervention. Consequently, a total of 88 participants completed the 12-week intervention program and post-intervention assessments. The attrition rate was approximately 12%, which is within the generally acceptable threshold of less than 20% for clinical and exercise-based intervention studies [28].

Participants were randomly assigned, regardless of the affected side, to either a Conventional Knee Rehabilitation (CKR, n = 45) or a Gluteus medius–Based Rehabilitation (GBR, n = 43). All participants received a full explanation of the study objectives and procedures and provided written informed consent prior to participation. Eligible participants were adults aged 50 years or older, of either sex, with no history of knee surgery within the previous six months. Exclusion criteria included the presence of severe postoperative complications, severe pain that limited participation, neurological impairment due to spinal disorders, or a diagnosis of rheumatoid arthritis. This study utilized data collected from participants under the approval of the Institutional Review Board of Sangmyung University (protocol code: SMUIRB Ex-2022-010). Unlike previous studies, however, this study independently analyzed lower-limb alignment variables measured via weight-bearing full-length lower extremity radiographs (X-rays), aiming to elucidate the effects of early postoperative gluteus medius–based rehabilitation on the restoration of lower-limb alignment and symmetry in patients. The physical characteristics of the participants are shown in Table 1.

2.2. Hip-Knee-Ankle (HKA) Angle Measurement

The hip–knee–ankle (HKA) angle was measured using weight-bearing full-length lower extremity radiographs. The HKA angle was defined as the angle formed between the mechanical axis of the femur, drawn from the center of the femoral head to the center of the knee joint, and the mechanical axis of the tibia, drawn from the center of the knee joint to the center of the ankle joint [29,30]. This measurement was used as a standard parameter to evaluate coronal plane lower limb alignment. Neutral alignment was defined as the condition in which the mechanical weight-bearing axis passed through the center of the femoral head, the center of the tibial plateau, and the center of the ankle joint [29]. All radiographic images were obtained under standardized conditions, and angle measurements were performed using digital image analysis software.

In healthy adults, the HKA angle—defined as the mechanical axis angle between the femur and tibia measured on weight-bearing full-length radiographs—is generally considered neutral when it falls within approximately 0° ± 3° of coronal alignment, with values beyond this range indicating varus or valgus deviation.

Figure 1. Preoperative and postoperative measurement of the hip–knee–ankle angle. The HKA angle was determined on weight-bearing full-length lower-extremity images as the angle formed between the mechanical axis of the femur, extending from the center of the femoral head to the center of the knee, and the mechanical axis of the tibia, extending from the center of the knee to the center of the ankle. (A) Preoperative varus alignment with medial-compartment knee osteoarthritis. (B) Postoperative alignment following total knee arthroplasty, demonstrating restoration toward neutral mechanical alignment. HKA, hip–knee–ankle; TKA, total knee arthroplasty.

Figure 1. Preoperative and postoperative measurement of the hip–knee–ankle angle. The HKA angle was determined on weight-bearing full-length lower-extremity images as the angle formed between the mechanical axis of the femur, extending from the center of the femoral head to the center of the knee, and the mechanical axis of the tibia, extending from the center of the knee to the center of the ankle. (A) Preoperative varus alignment with medial-compartment knee osteoarthritis. (B) Postoperative alignment following total knee arthroplasty, demonstrating restoration toward neutral mechanical alignment. HKA, hip–knee–ankle; TKA, total knee arthroplasty.

2.3. Isokinetic Assessment of Knee Muscle Function

Knee muscle function was assessed using an isokinetic dynamometer (Biodex System 4, Inc., Shirley, NY, USA) in accordance with the manufacturer’s user manual. Isokinetic strength of the knee extensors and flexors was measured through the maximal available range of motion. Testing was performed at two angular velocities: 60°/s to assess muscle strength and 180°/s to assess muscular endurance. A 30-second rest period was provided between tests at different angular velocities. At an angular velocity of 60°/s, five repetitions were performed, whereas at 180°/s, ten repetitions were completed. Peak torque values obtained during knee extension and flexion were used as the primary outcome measures to evaluate muscle strength and muscular endurance [31].

2.4. Rehabilitation Exercise Program

In this study, both the CKR and the GBR participated in a 12-week rehabilitation program conducted twice per week. Each session consisted of a 5-minute warm-up, a 30-minute main exercise session, and a 5-minute cool-down. During the first postoperative week, knee joint ROM was maintained at a minimum of 90°, after which ROM was progressively increased according to each participant’s recovery status. Exercise intensity was individually adjusted and progressively increased based on a rating of perceived exertion (RPE) of 11–13.

2.4.1. Conventional Knee Rehabilitation (CKR)

The CKR program was modified from a previously described rehabilitation protocol and included exercises designed to strengthen the quadriceps and improve knee joint strength, flexibility, and balance [32] (Table 2).

2.4.2. Gluteus Medius–Based Rehabilitation (GBR)

The GBR program was developed by modifying previously validated gluteus medius strengthening protocols [33,34]. In addition to the CKR program, participants in the GBR group performed an additional 15 minutes of gluteus medius strengthening exercises. These exercises were designed to enhance hip abductor strength and improve lower extremity stability in a progressive manner (Table 2).

2.5. Statistical Analysis

Data was analyzed using IBM SPSS Statistics version 22.0 (SPSS Corp., Chicago, CA, USA). Means and standard deviations were calculated for all measurement variables in both groups. A two-way mixed analysis of variance (ANOVA) was performed to analyze the two independent variables, time (pre-intervention and post-intervention) and group (CKR and GBR), during the 12-week training period and to analyze the interaction effect between the two independent variables. Post hoc tests for the interaction effect included paired-sample t-tests for pre- and post-differences within groups and independent-sample t-tests for differences between groups. The statistical significance level for all analyses was set at p < 0.05.

3. Results

3.1. Effects of Rehabilitation on HKA Alignment

Both groups demonstrated significant changes in the HKA angle following the 12-week intervention. In the CKR, the HKA angle increased significantly from pre-intervention (171.03 ± 6.06°) to post-intervention (177.28 ± 2.41°), with a significant paired t-test result (t = −7.411, p = 0.001). Similarly, the GBR showed a significant increase in HKA angle from 170.86 ± 5.89° at baseline to 178.24 ± 1.38° after the intervention (t = −8.009, p = 0.001).

Two-way repeated measures ANOVA revealed a significant main effect of time (F = 119.408, p = 0.001), indicating overall improvement in HKA angle across both groups. However, no significant main effect of group was observed (F = 0.308, p = 0.58), and the group × time interaction was not significant (F = 0.814, p = 0.369), suggesting that the magnitude of change over time did not differ significantly between the CKR and GBR groups. Changes in HKA angle are presented as shown in Table 3.

3.2. Changes in Isokinetic Knee Muscle Strength

Changes in isokinetic knee muscle strength at an angular velocity of 60°/s are presented in Table 4. For knee extension strength, the CKR showed a non-significant decrease from pre-intervention (41.81 ± 15.37 N·m) to post-intervention (37.34 ± 15.80 N·m) (t = 1.960, p = 0.056). In contrast, the GBR demonstrated a significant increase in knee extension strength from 36.73 ± 15.25 N·m to 51.15 ± 23.60 N·m (t = −6.128, p = 0.001).

Two-way repeated measures ANOVA revealed a significant main effect of time for knee extension strength (F = 9.222, p = 0.003), whereas no significant main effect of group was observed (F = 1.624, p = 0.206). A significant group × time interaction was identified (F = 33.248, p = 0.001), indicating that changes in knee extension strength over time differed significantly between the two groups.

For knee flexion strength, both groups exhibited significant improvements following the intervention. The CKR group showed a significant increase from 19.52 ± 12.02 N·m to 25.64 ± 10.00 N·m (t = 2.467, p = 0.018), while the GBR group demonstrated a greater increase from 16.99 ± 10.55 N·m to 28.43 ± 16.25 N·m (t = −6.735, p = 0.001). Two-way repeated measures ANOVA indicated significant main effects of group (F = 4.683, p = 0.033) and time (F = 10.474, p = 0.002), as well as a significant group × time interaction (F = 43.699, p = 0.001), suggesting that the magnitude of improvement in knee flexion strength was significantly greater in the GBR compared with the CKR (Table 4).

3.3. Changes in Isokinetic Knee Muscle Endurance

Changes in isokinetic knee muscle endurance at an angular velocity of 180°/s are presented as shown in Table 5. For knee extension endurance, the CKR demonstrated a significant decrease from pre-intervention (29.50 ± 10.00 N·m) to post-intervention (23.67 ± 8.28 N·m) (t = 5.094, p = 0.001). In contrast, the gluteus medius–based rehabilitation (GBR) group showed a significant increase in knee extension endurance from 25.42 ± 10.66 N·m to 36.60 ± 19.37 N·m (t = −5.293, p = 0.001).

Two-way repeated measures ANOVA revealed a significant main effect of time for knee extension endurance (F = 5.107, p = 0.026), whereas no significant main effect of group was observed (F = 3.276, p = 0.074). A significant group × time interaction was identified (F = 51.354, p = 0.001), indicating that changes in knee extension endurance over time differed significantly between the two groups.

For knee flexion endurance, both groups exhibited significant changes following the intervention. The CKR group showed a significant decrease from 14.49 ± 9.85 N·m to 11.86 ± 8.77 N·m (t = 2.405, p = 0.020), whereas the GBR group demonstrated a significant increase from 11.15 ± 8.21 N·m to 22.39 ± 14.80 N·m (t = −6.833, p = 0.001). Two-way repeated measures ANOVA indicated a significant main effect of time (F = 19.299, p = 0.001), while the main effect of group was not significant (F = 3.054, p = 0.084). A significant group × time interaction was also observed (F = 50.126, p = 0.001), suggesting that improvements in knee flexion endurance were significantly greater in the GBR group compared with the CKR group (Table 5).

4. Discussion

This study investigated whether adding early gluteus medius strengthening to conventional knee rehabilitation would improve knee muscle function and coronal-plane lower-limb alignment after TKA. The principal finding was that the GBR group demonstrated significantly greater improvements in knee extensor and flexor strength and endurance than the CKR group, as indicated by significant group × time interactions (Table 4 and Table 5). Both groups also showed significant time-related changes in the HKA angle; however, neither the group effect nor the group × time interaction was significant (Table 3). Therefore, the present data support a specific benefit of GBR for knee muscle function, whereas superiority for HKA correction cannot be concluded.

4.1. Effects of Gluteus Medius–Based Rehabilitation on Knee Muscle Function

The GBR group showed significant gains in knee extension and flexion peak torque at 60°/s and in extension and flexion performance at 180°/s, whereas the CKR group showed no significant improvement in extension strength and reductions in both endurance outcomes (Table 4 and Table 5). Marked quadriceps weakness commonly develops after TKA because of postoperative atrophy and voluntary activation deficits, and this weakness can persist despite improvements in pain and joint structure [17,35,36]. Contemporary rehabilitation evidence therefore supports progressive strengthening rather than relying only on range-of-motion exercises or low-intensity knee-focused activity [18,24,37].

The greater improvement in the GBR group is clinically plausible because the gluteus medius provides hip abduction torque and frontal-plane pelvic control during single-limb support [19]. Hip abductor weakness is present in individuals with knee OA and may persist after TKA [20,25]. Hip abductor strength has also been independently associated with walking performance, stair negotiation, and other functional outcomes after TKA [25,26]. A randomized pilot study further reported that adding hip abductor strengthening after TKA improved hip strength and selected physical-function outcomes, supporting the inclusion of proximal strengthening in postoperative care [38].

From a kinetic-chain perspective, improved proximal control may allow patients to perform closed-chain rehabilitation tasks with less pelvic drop and better control of femoral motion [19,25,26]. This explanation is consistent with the direction of the present strength findings but remains inferential because hip strength, pelvic kinematics, electromyographic activity, and movement quality were not directly measured. Accordingly, the results should be interpreted as evidence that the multicomponent GBR program enhanced knee muscle performance, not as proof that isolated gluteus medius activation was the sole physiological mechanism.

4.2. Interpretation of Improvements in Hamstring Muscle Function

A clinically important finding was the significant increase in knee flexor peak torque at both 60°/s and 180°/s in the GBR group, with significant group × time interactions for both outcomes (Table 4 and Table 5). Knee flexors contribute to dynamic tibiofemoral stability and help control the knee during weight acceptance and functional transitions [39]. Because deficits after TKA are not limited to the quadriceps, assessment and rehabilitation of both extensor and flexor muscle groups are important for restoring balanced knee function [31,35,39].

One possible explanation is that improved hip and pelvic control enabled more effective performance of weight-bearing exercises that simultaneously recruit the hip extensors, knee extensors, and knee flexors [19,25]. Another possibility is that the additional 15 minutes of exercise performed by the GBR group increased the overall training volume and exposure to functional loading. The present design cannot separate the specific effect of gluteus medius strengthening from the effect of additional exercise time; therefore, the mechanism should be tested in future trials using time-matched programs and direct measures of hip strength, muscle activation, and three-dimensional movement.

4.3. Clinical Interpretation of Changes in HKA Alignment

The HKA angle moved toward neutral in both groups over the 12-week period, with a significant main effect of time (Table 3). Restoration of coronal alignment is primarily achieved during TKA through bone resection, component positioning, and soft-tissue balancing [29,30,40]. Postoperative rehabilitation may improve the patient’s ability to stand with symmetrical loading and better neuromuscular control, but it is not expected to substantially alter the fixed position of the prosthetic components [40,41].

Although the post-intervention standard deviation was numerically smaller in the GBR group than in the CKR group (1.38° vs. 2.41°), the absence of a significant group × time interaction means that this descriptive difference should not be interpreted as evidence that GBR produced superior HKA correction. Moreover, a standing radiographic HKA measurement reflects both structural alignment and measurement conditions, including limb rotation, knee position, weight-bearing strategy, and image acquisition [29,30,41]. The smaller post-intervention dispersion in the GBR group may suggest more consistent standing alignment, but this interpretation is exploratory and requires confirmation with reliability testing and dynamic gait measures.

These findings also distinguish static radiographic alignment from dynamic frontal-plane control. Hip abductors can influence pelvic stability and lower-limb loading during gait [19,20,25], but a static HKA radiograph does not directly quantify hip abductor function, knee adduction moment, or dynamic mechanical-axis behavior. Future studies should therefore combine full-length weight-bearing radiographs with hip abductor dynamometry, three-dimensional gait analysis, and patient-reported and performance-based outcomes to determine whether improved proximal strength translates into more favorable dynamic loading.

4.4. Clinical Implications and Study Limitations

Clinically, the present results support including progressive hip abductor exercises alongside quadriceps, hamstring, range-of-motion, and balance training during early rehabilitation after TKA [18,24,38]. This approach treats the lower limb as an integrated kinetic chain and may be particularly useful for patients who demonstrate pelvic instability, hip abductor weakness, or difficulty with walking and stair tasks [25,26]. Nevertheless, exercise prescription should remain individualized according to pain, wound status, swelling, balance, and recovery of knee motion.

Several limitations should be considered. First, because the GBR group completed 15 additional minutes of exercise, the observed between-group differences may partly reflect unequal training volume rather than the isolated effect of gluteus medius strengthening. Second, hip abductor strength and neuromuscular activation were not measured, so the proposed proximal-stability mechanism could not be verified. Third, HKA was assessed only with static radiographs, and measurement reliability and dynamic coronal-plane loading were not evaluated. Fourth, the 12-week follow-up does not establish whether the gains in muscle performance or the descriptive difference in HKA variability were maintained over the longer term. Finally, the absence of a significant group × time interaction for HKA precludes a conclusion that GBR was superior to CKR for radiographic alignment.

5. Conclusions

This study demonstrated that a GBR program initiated early after TKA resulted in superior improvements in knee muscle function compared with conventional knee-focused rehabilitation. In particular, participants who performed gluteus medius strengthening exercises showed greater gains in knee flexor (hamstring) and extensor strength and endurance. Although both rehabilitation approaches led to significant improvements in HKA alignment toward neutral, the gluteus medius–based rehabilitation group exhibited greater post-intervention alignment stability.

These findings suggest that incorporating proximal hip strengthening into postoperative rehabilitation may enhance functional recovery of the knee and support more stable coronal plane lower-limb alignment following TKA. Rehabilitation strategies that extend beyond isolated knee exercises and emphasize coordinated recovery of the hip–knee kinetic chain may therefore provide meaningful clinical benefits for patients undergoing TKA.