2.1 Study design and participants

This is a pre-post study design conducted at the Department of Physiotherapy and Department of Orthopedics, Kasturba Hospital, Manipal, Karnataka, India. Ethical approval was sought by the Institutional Ethics Committee (IEC) of Kasturba Hospital, Manipal (IEC1 - 20–2022). The study protocol was registered in the Clinical Trial Registry – India (CTRI/2022/07/043642). The study was done as per the World Medical Association (Declaration of Helsinki) - Code of Ethics. Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines were followed.²⁵

Thirty-four participants with severe knee OA who underwent TKA were recruited. The sample size was determined (G∗Power, version 3.1.9.7) considering an effect size of 0.4, an alpha of 0.05, and a beta of 0.80. The inclusion criteria were patients aged 50–80 years of either sex with primary severe (Kellgren-Lawrence grade 4) knee OA who were scheduled for TKA using the medial parapatellar approach. Individuals who had inflammatory arthritis with multiple joint involvement, neurological deficits, or posttraumatic arthritis and those who declined to participate were excluded from the study.

2.2 Instrumentation and procedure

The screening of the participants was done as per the eligibility criteria. The demographic information of the participants, including age (in years), sex, height (in cm), weight (in kg), and body mass index (BMI) (in kg/m2), was collected. The participants were explained about the procedure of the study, and their consent was acquired. The outcomes were assessed by a musculoskeletal physical therapist with clinical experience in knee assessment and rehabilitation. The Q-angles were measured using standardized procedures,¹⁴ and the values were noted. Pain, quadriceps strength, and knee flexion ROM were measured, and the values were noted. The measurements were taken preoperatively (one day before surgery), and postoperatively at six weeks and three months following TKA.

2.3 Intervention program

All the participants underwent a standard postoperative TKA rehabilitation program post-TKA.^26,27 The stage 1 exercises were administered from day 1–2 weeks in the early function phase following TKA. Along with the standard rehabilitation program, specific exercises targeting VMO muscle activation, such as isometrics of the quadriceps, terminal knee extension short arc, straight leg raise, knee extension in sitting (90°-0), and the McConnell method of VMO contraction in high sitting, were included and were performed thrice a day. The stage 2 exercises were administered in the progressive function phase from 3 to 6 weeks following TKA. Strengthening exercises with cuff weights ranging from 0.5 to 1 kg, which included terminal knee extension short-arc, straight leg raising, knee extension in sitting (90°-0), and VMO contraction in high sitting using the McConnell method, were done twice a day, with 3 sets of 20 repetitions each. The closed-chain activity of VMO contraction in standing was done twice a day. The stage 3 exercises were administered in the advanced function phase from 7 to 12 weeks following TKA. The strengthening exercises progressed with cuffed weights of 2 kg or 60 % of 1 RM were done twice a day, with 3 sets of 30 repetitions. The progressive stages included other general exercises, mobility-based and stretching exercises, functional training, balance training, and aerobic conditioning.²⁶

2.4 Clinical measurement of the Q-angle before and after TKA

The participants laid supine on a couch with a straight spine and legs parallel to each other with the quadriceps relaxed and the knees fully extended. The hip joints and feet were in a neutral position. The pelvis was squared, both the anterior superior iliac spines (ASISs) were at the same level, and the lower limbs were in a plane at 90° to the line joining the two ASISs. The ASIS, tibial tubercle, and center of the patella bony landmarks were adequately exposed while the measurements were taken and marked. Palpation and visual estimation were used to locate the anatomical landmarks for measurement. Two lines were drawn, one from the ASIS to the center of the patella and the other from the center of the patella to the center of the tibial tubercle. The acute angle formed between the two lines was measured using a goniometer (Fig. 1).¹⁴

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Fig. 1 Clinical measurement of Q-angle using goniometric method. One line has been drawn from ASIS to the center of patella (line a) and another line has been drawn from the center of patella to the tibial tubercle (line b). The angle between the two lines was measured as the Q-angle of the knee joint.

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2.5 Pain, quadriceps strength, and knee flexion ROM measurements before and after TKA

Pain severity was measured using a 0–10 scale, the Numerical Pain Rating Scale (NPRS).^28,29 Quadriceps muscle strength was measured with a Chatillon DMG series handheld dynamometer following standard procedures.³¹ Handheld dynamometry has excellent reliability and moderate-to-good validity for most groups of muscles,^32,33 with ICCs ranging from 0.932 to 0.984.³¹ A universal goniometer was used to measure knee flexion ROM using standardized procedures.³⁰

2.6 Statistical analysis

SPSS software (version 29.0) was used for statistical analysis. Demographic characteristics were summarized using descriptive statistics. The values of the Q-angle obtained are shown as the mean ± standard deviation (SD). The data normality was checked using Kolmogorov–Smirnov test, and the data were normally distributed. Hence, repeated-measures ANOVA was performed to compare the Q-angle before TKA, and at the sixth week and third month after TKA. The correlations of the Q-angle with pain, quadriceps strength, and knee flexion ROM were determined using Pearson's correlation coefficient. The level of statistical significance was set as p ≤ 0.05.

3.1 Demographics

Thirty-four individuals with severe knee OA with a mean age of 65.05 years (SD = 8.03) were included in the study. Among the thirty-four participants, 17 (50 %) were females, and 17 (50 %) were males. The mean BMI was 28.26 kg/m2 (SD = 7.31). The mean duration of knee pain was 15.9 (SD = 4.3) months. The demographic details are shown in Table 1.

3.2 Comparison of the Q-angle measurements before and after TKA

The mean Q-angle was 19.17° (SD = 1.92°) before TKA, 16.06° (SD = 1.87°) at the sixth week and 13.43° (SD = 1.77°) at the third month after TKA (Fig. 2). A significant difference in Q-angle was noted between the preoperative and postoperative values, at the sixth week and at the third month following TKA (p < 0.05) (Table 2).

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Fig. 2 Q-angle measured before (preoperatively one day before surgery) and after TKA (postoperatively sixth week and third month after surgery) in individuals with severe knee OA.

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3.3 Correlation of the Q-angle with pain, quadriceps strength, and knee flexion ROM following TKA

Post-TKA, pain significantly decreased (6.8 ± 2.9 to 2.6 ± 1.7, p < 0.05, t = 7.29, standard error of difference = 0.58). The quadriceps strength (31.87 ± 6.9 to 39.5 ± 7.2 pounds, p < 0.05, t = 4.46, standard error of difference = 1.71) and knee flexion ROM (110 ± 9.2° to 121.5 ± 6.8°, p < 0.05, t = 5.86, standard error of difference = 1.96) significantly improved post-TKA. Linear correlations were noted between the Q-angle and pain (r = 0.2; p < 0.05), quadriceps strength (r = −0.2; p < 0.05) and knee flexion ROM (r = −0.1; p < 0.05) following TKA.

4.1 Clinical measurement of the Q-angle before and after TKA

Akinbo et al. reported an increased Q-angle in individuals with knee OA.²¹ Our study showed a significant alteration in the Q-angle before and sixth week after TKA. The Q-angle determines the quadriceps muscles lateral pull on the patella. The vastus intermedius and rectus femoris which are located centrally, pull the patella through the femoral axis. The patella is pulled slightly laterally by the vastus lateralis. However, the patella is pulled in the distal direction by the patellar ligament. Due to the cumulative action of the forces acting on the patella, the resulting force is laterally directed to the patella. The obliquity of the VMO fiber provides a stabilizing force that opposes the lateral pull of the vastus lateralis.^34,35 VMO muscle weakness could cause increased lateral pull of the patella, thus increasing the Q-angle in individuals with knee OA. During TKA following the medial parapatellar approach, the VMO is cut, which tends to cause an immediate disruption of the orientation and mechanism of action of the muscle.^9,36

The Q-angle is of utmost importance to surgeons during TKA and to rehabilitation professionals after TKA. The Q-angle is altered by the tibial tray position and femoral component rotation on it. There is an acceptable rotational alignment to position the tibial tray. If the tibial tray is rotated more medially or laterally than the acceptable limits or if the femoral component is rotated more internally or externally than the acceptable limit, patellar tracking and the Q-angle will be affected, and the functioning of the knee will lead to early wear, implant loosening and pain. Hence, surgeons should consider maintaining the tibial tray within the acceptable rotation so that the Q-angle is within the normal limit, as most surgeons never measure the Q-angle on a table while performing TKA. Therefore, surgeons should maintain the Q-angle within the acceptable limits so that better outcomes can be achieved.^37,38

Furthermore, our study also noted significant alterations in the Q-angle at the third month after TKA. This could be due to the VMO activation and strengthening exercises administered during the phase-wise postoperative rehabilitation program. VMO activation and improved strength help stabilize the patellar lateral pull by the vastus lateralis, thus, normalizing the Q-angle. Postoperative rehabilitation exercises gradually improve the VMO architecture and action, leading to a steady improvement in quadriceps function.^9,39,40 Postoperatively, rehabilitation should focus not only on the general mobility of the patient but also on the structural mobility of specific muscle groups. Post-TKA, rehabilitation professionals can further work on individual muscles, specifically the quadriceps femoris, to achieve appropriate VMO contraction for better biomechanical functioning of the knee joint. The rehabilitation process should be continuous and progressive to achieve better functioning of the muscle.

4.2 Correlation of the Q-angle with pain, quadriceps strength, and knee flexion ROM following TKA

A positive linear correlation between the Q-angle and pain was noted, which aligns with the findings of the study by Akinbo et al., who noted a positive correlation of Q-angle with pain intensity in persons with knee OA.²¹ This might be due to the biomechanical load on the medial joint compartment due to knee joint malalignment.^41,42 After TKA, the pain subsequently decreased, which correlated with the change in the Q-angle. A negative linear correlation between the Q-angle and quadriceps strength was found. This may be due to quadriceps femoris muscle disuse atrophy, which is usually associated with knee OA, and is sufficient to alter (i.e. increase) the Q-angle.^21,43 During TKA, there is significant trauma to the knee joint extensor mechanism, thereby causing quadriceps strength deficits.^7–9 A negative linear correlation of Q-angle with knee flexion ROM was noted, which might be due to abnormal knee kinematics due to pain and structural constraints in individuals with knee OA.¹² However, after TKA, the knee flexion ROM subsequently improved, which correlated with the change in the Q-angle. After TKA, the severity of pain decreased, and the quadriceps strength and knee flexion ROM improved, thereby correlating with a decreased Q-angle. A reduction in knee pain would have improved knee joint stability and enhanced the normal range of painless motion, which may have prevented exaggeration of the Q-angle.

4.3 Clinical implications

This is a preliminary analysis of long-term research in which we aimed to look at the late changes in the Q-angle post-TKA. This study demonstrated the applicability of the use of clinical methods for measuring the Q-angle before and after TKA. The Q-angle is an important aspect that surgeons must consider for better physiological and functional knee joint stability and to increase the durability of the implant by facilitating appropriate biomechanical functioning. Along with structural correction, dynamic structures such as muscles need to be assessed and properly rehabilitated so that the overall outcome of the patient is better. An elaborate dynamic movement evaluation is required after TKA for better clinical decision making.

Before TKA, genu varum is the major challenge influencing pain, functioning of the knee joint and overall quality of life. Therefore, patients with severe knee OA undergo TKA. TKA focuses mainly on correcting the structural alignment of the limb. Functional alignment, such as muscle imbalance, needs to be corrected in the long run following TKA to reduce pain, improve function of the joint and enhance quality of life. However, if there is a muscle imbalance following TKA, it can further influence the gait and other parameters. Therefore, in this study, we focused on three months of intensive rehabilitation, focusing exclusively on VMO exercises and taking the Q-angle as an outcome measure. Along with structural correction by TKA, progressive or graded VMO exercise helps in further aligning the Q-angle. It is imperative to evaluate any change in the Q-angle post-TKA, as individually tailored rehabilitation programs can be designed by rehabilitation professionals to address the muscle imbalances causing changes in the Q-angle and thereby, causing a reduction in pain, and improvement in quadriceps strength and knee flexion ROM.

4.4 Limitations and future recommendations

As this is a preliminary analysis, a small sample was used. Hence, the sample size might have impeded the statistical analysis of the study. The Q-angle measured using goniometry could have accounted for the goniometric measurement error.^44,45 Future studies may also shed light on therelationship of Q-angle and the hip-knee-ankle angle before and after TKA to better understand the influence of knee varus severity on the Q-angle. Follow-up of long-term changes in the Q-angle following TKA can be considered. Furthermore, changes in the Q-angle can be compared with changes in biomechanical variables following TKA at regular follow-up timepoints to better understand postoperative recovery.