Abstract
Purpose
Muscle enhancement and support therapy (MEST) using bidirectional barbed polydioxanone filaments has been introduced for knee osteoarthritis, but the original technique relies on surface anatomical landmarks without imaging guidance. This study aimed to establish a standardized ultrasound-guided protocol for MEST targeting the vastus medialis obliquus (VMO) and report preliminary clinical outcomes in patients with refractory chronic knee pain following total knee arthroplasty (TKA).
Patients and Methods
This single-center retrospective pilot study included three patients (five treated knees) with chronic knee pain after TKA who underwent ultrasound-guided MEST between August 2025 and January 2026. The standardized protocol involved identifying the VMO midpoint in the short-axis view, rotating the probe to visualize muscle fibers to guide needle insertion, and using color Doppler to identify major vessels along the planned needle path. A needle was advanced in-plane along the VMO long axis to traverse the muscle belly while avoiding vascular structures for filament placement. Outcomes were assessed at baseline and at 4, 8, and 12 weeks using the Numeric Rating Scale (NRS), functional scores (walking and stair-climbing), ultrasonographic effusion grading, and patient-reported global effectiveness.
Results
The protocol was successfully executed in all five cases, allowing clear visualization of the VMO and filament placement spanning its full thickness. Four cases achieved a ≥50% reduction in NRS at 8 weeks. Walking and stair-climbing scores improved in most cases at 8 weeks and in all cases at 12 weeks. Complications were minimal, including mild ecchymosis and filament protrusion. No serious adverse events occurred.
Conclusion
The standardized ultrasound-guided MEST protocol targeting the VMO appears technically feasible and safe. These preliminary findings suggest that MEST may be a potential treatment option for refractory chronic pain following TKA and warrant further prospective investigation.
Keywords: total knee arthroplasty, chronic pain, ultrasound-guided intervention, vastus medialis obliquus, quadriceps muscle
Introduction
Knee osteoarthritis (OA) is a prevalent degenerative joint disease characterized by progressive deterioration of articular cartilage and subchondral bone, leading to pain, functional limitation, and reduced quality of life.1 When the disease reaches an end stage and no longer responds to conservative management, total knee arthroplasty (TKA) is widely accepted as the gold standard treatment, offering substantial pain relief and functional restoration for most patients.2–4 Nevertheless, a considerable proportion of patients continue to experience persistent knee pain following surgery.5 Previous studies have reported that approximately 20% or more of patients experience chronic knee pain within 3–24 months after TKA despite the absence of infection, implant loosening, or other mechanical complications.6–10 Persistent pain after TKA remains a clinically significant problem, and effective interventional treatment options for these patients are limited.
The etiology of persistent pain after TKA is often difficult to identify in individual patients and is generally considered multifactorial, involving peripheral and central sensitization, neuropathic mechanisms, periarticular soft tissue dysfunction, and biomechanical factors such as altered joint loading and patellar maltracking.11–13 Among these, quadriceps muscle weakness may represent one possible contributor. The vastus medialis obliquus (VMO), the oblique distal fibers of the vastus medialis muscle within the quadriceps femoris, plays an important role in dynamic patellar stabilization, proper patellar tracking, and load distribution across the knee joint.14 Weakness or atrophy of the VMO may therefore disrupt patellofemoral mechanics and contribute to uneven joint loading, both of which may be associated with sustained knee pain. VMO weakness is commonly observed in knee OA and may be further exacerbated after TKA by surgical and postoperative factors.11,13,15,16 Persistent VMO dysfunction may therefore represent a plausible mechanism contributing to the refractory chronic pain after TKA despite an otherwise uncomplicated surgical outcome.
Recently, muscle enhancement and support therapy (MEST) has been introduced as a novel treatment modality for patients with knee OA. MEST involves the insertion of bidirectional barbed polydioxanone (PDO) filaments into the VMO, providing mechanical reinforcement of the muscle in a manner that may complement conventional rehabilitation.17 Although current evidence remains limited, preclinical and clinical studies have suggested potential benefits of MEST, including promotion of muscle growth, improvement of muscle strength, and reduction of pain in patients with mild-to-moderate knee OA.17–19 Given the potential role of VMO dysfunction in persistent pain after TKA, it is reasonable to hypothesize that MEST targeting the VMO could have therapeutic potential in selected patients with refractory postoperative pain. However, no prior study has explored the feasibility or clinical application of MEST in this population.
In addition to the uncertainty regarding its clinical application after TKA, the procedural technique for MEST has not yet been fully optimized or standardized. MEST has primarily been performed using a blind technique based on surface anatomical landmarks.17 Although major adverse events have not been reported with this approach,17,20 reliance on surface anatomical landmarks alone could compromise procedural accuracy because of the considerable anatomical variability of the VMO and the proximity of neurovascular structures.21,22 In contrast, ultrasound guidance has been shown to improve the accuracy of musculoskeletal interventions compared with landmark-based techniques.23 An ultrasound-guided approach may therefore be advantageous for MEST by enabling real-time visualization of the target muscle, needle trajectory, and surrounding structures. However, a standardized ultrasound-guided protocol for MEST targeting the VMO has not yet been described.
Accordingly, the primary aim of this study was to establish a standardized ultrasound-guided protocol for MEST targeting the VMO. As a secondary objective, we evaluated the technical feasibility and preliminary clinical outcomes of this protocol in a pilot application involving patients with refractory chronic knee pain following TKA. Given the exploratory and retrospective nature of this single-center investigation, the findings are intended to be hypothesis-generating and to inform the design of future prospective studies.
Materials and Methods
Study Design and Ethics Statement
This was a single center retrospective observational pilot study of patients with refractory chronic knee pain following TKA who underwent ultrasound guided MEST. The study protocol was reviewed and approved by the Institutional Review Board of Seoul National University Hospital (IRB No. 2602–039-1716) and conducted in accordance with the Declaration of Helsinki. Given the retrospective nature of the study, the requirement for written informed consent for study participation was waived. All patients had previously provided written informed consent for the interventional procedure and for the use of their anonymized clinical data for research and publication purposes.
Ultrasound-Guided MEST Procedure
Anatomy of the Vastus Medialis Obliquus (VMO)
The VMO is the distalmost portion of the vastus medialis muscle and forms a prominent muscular bulk on the medial aspect of the distal thigh, immediately superomedial to the patella (Figure 1). It arises from the medial lip of the linea aspera, the medial supracondylar line of the femur, and the tendon of the adductor magnus, and inserts onto the medial border of the patella and the medial patellar retinaculum.24 The muscle fibers of the VMO are oriented at approximately 50–55° relative to the longitudinal axis of the femur, which is considerably more oblique than the proximal fibers of the vastus medialis longus, typically oriented at approximately 15–18°.25–27 This unique fiber orientation enables the VMO to exert a medially directed force on the patella and contributes to dynamic stabilization against lateral patellar displacement during knee extension.
Figure 1.
Anatomy of the VMO and PDO filament insertion site. (A) Anatomical illustration of the distal medial thigh showing the VMO and patella. (B) Schematic illustration of PDO filament insertion within the VMO. Green crosses indicate insertion points, and yellow arrows indicate the direction of the PDO filament placement parallel to the VMO muscle fibers.
Abbreviations: PDO, polydioxanone; VMO, vastus medialis obliquus.
Anatomically, the VMO lies medial to the rectus femoris and vastus intermedius and superficial to the medial intermuscular septum, which separates the quadriceps compartment from the adductor compartment (Figure 2). Several vascular structures course in close proximity to the VMO. The muscle receives arterial supply primarily from muscular branches of the femoral artery and the profunda femoris (deep femoral) artery, as well as branches from the descending genicular artery. The descending genicular artery arises from the distal femoral artery just proximal to the adductor hiatus and descends between the vastus medialis and the adductor magnus, giving off articular and muscular branches that supply the medial knee region.28 The femoral artery passes through the adductor canal and becomes the popliteal artery after traversing the adductor hiatus; therefore, the femoral artery within the adductor canal lies in close proximity to the medial aspect of the VMO.29,30 Small intramuscular vessels and perforating arterial branches may also be present within the VMO muscle belly.
Figure 2.
Cross-sectional anatomy of the distal thigh.
Procedure Preparation and Materials
The patient was positioned supine on the procedure table with the ipsilateral knee flexed to approximately 20–30°, supported by a small pad placed beneath the popliteal fossa. The ultrasound machine was positioned on the ipsilateral and cranial side of the patient so that the operator could comfortably view the screen while inserting the needle in a caudal-to-cephalad direction. A high-frequency linear array transducer (LA2-14A, 2–14 MHz; Samsung RS80 EVO, Samsung Medison, Seoul, Republic of Korea) was used for ultrasound guidance. The probe was covered with a sterile sheath, and an assistant was available to assist with probe manipulation during the procedure.
The MEST device consisted of a biodegradable PDO filament with a bidirectional barbed design (OV MEDI, Seongnam, Republic of Korea). The filament was preloaded over the insertion needle in a folded configuration, with the filament draped over the needle and threaded through an outer sheath. When the needle was advanced to the desired depth within the target muscle and subsequently withdrawn, the bidirectional barbs engaged the surrounding muscle tissue, allowing the PDO filament to remain anchored within the muscle. The approximate length of filament deposited within the muscle was 6.5 cm, measured from the tip of the needle to the hub of the outer sheath. The PDO filaments are bioabsorbable and undergo complete hydrolytic degradation within 4 to 6 months.17
MEST Procedure
The skin was prepared with antiseptic solution using standard aseptic technique, and sterile draping was applied. First, the superior border of the patella was palpated, and a horizontal reference line was drawn along its superior contour. Two reference points were then established: (1) a point located 5 cm directly proximal to the midline of the superior patellar border along the longitudinal axis of the thigh; and (2) a second point located 5 cm proximal to the superomedial corner of the patella, directed approximately 40–50° from the midline toward the medial side. Dots were then marked at 0.5 cm intervals between these two reference points (Figure 3A).
Figure 3.
Determination of entry points for ultrasound-guided MEST. (A) Surface landmark identification. Reference points are marked at 0.5 cm intervals along a straight line connecting two points located 5 cm proximal to the superior border of the patella. (B) Short-axis ultrasound view of the VMO, with the midpoint indicated by a yellow asterisk(*). (C) Six insertion points, indicated by white arrows, are selected on the short-axis view at 1 cm intervals extending bilaterally from the VMO midpoint, spanning approximately 6 cm.
Abbreviations: MEST, muscle enhancement and support therapy; SFA, superficial femoral artery; SFV, superficial femoral vein; VMO, vastus medialis obliquus.
The ultrasound probe was placed along this marked line in the short-axis orientation to visualize the VMO and surrounding structures, including adjacent vessels, and to assess the size of the muscle. The VMO appeared as a well-defined hypoechoic muscular structure located superficially on the medial side of the distal thigh (Figure 3B). Its lateral border was delineated by the rectus femoris, and its deep boundary was defined by the medial intermuscular septum, which appeared as a hyperechoic linear structure separating the VMO from the adductor compartment. Deep to the intermuscular septum, the femoral artery and vein were identified. The midpoint of the VMO was determined on this short-axis view. Based on this midpoint, six insertion points were selected along the marked line at 0.5–1 cm intervals, extending medially and laterally depending on the width of the VMO in each patient (Figure 3C).
Prior to needle insertion, color Doppler ultrasonography was performed to identify intramuscular vascular structures along the planned needle trajectory (Figure 4). These included small arterial branches such as branches of the descending genicular artery, accompanying intramuscular veins, and other vessels coursing within or adjacent to the VMO. Identification of these vessels allowed adjustment of the planned needle trajectory to avoid vascular injury. Local anesthesia was then administered using 0.5% lidocaine with a total volume of 20 mL through a 25-gauge spinal needle under ultrasound guidance (Figure 5A). The local anesthetic was injected subcutaneously and into the VMO at each planned insertion point. An interval of 3–5 minutes was allowed for the anesthetic to take effect before commencing the MEST procedure. In addition to providing procedural analgesia, this step allowed the operator to assess the optimal needle trajectory and insertion depth under ultrasound guidance.
Figure 4.
Pre-procedural assessment of intramuscular vascular structures within the VMO. (A) Short-axis color Doppler image demonstrating small arterial branches, including branches of the descending genicular artery. (B) Long-axis view showing an intramuscular vein coursing within the VMO. (C) Long-axis color Doppler image confirming vascular flow within intramuscular vessels adjacent to the planned needle trajectory.
Abbreviation: VMO, vastus medialis obliquus.
Figure 5.
Ultrasound-guided MEST needle insertion. (A) Local anesthesia under ultrasound guidance. A 25-gauge spinal needle is advanced to infiltrate 0.5% lidocaine into the subcutaneous and perimuscular tissues at each planned insertion point. (B) MEST needle insertion using an in-plane technique along the orientation of the VMO muscle fibers. (C) Long-axis ultrasonographic view during MEST needle insertion. In-plane visualization of the MEST needle (indicated by green arrows) traversing the full thickness of the VMO.
Abbreviations: MEST, muscle enhancement and support therapy; SFA, superficial femoral artery; VMO, vastus medialis obliquus.
For MEST needle insertion, the probe was rotated 90° to align parallel to the VMO muscle fibers, providing a long-axis (longitudinal) view. In this orientation, the oblique fiber pattern of the VMO was clearly visualized, and the superficial and deep fascial boundaries were identified. Using an in-plane technique, the MEST needle was inserted along the orientation of the VMO muscle fibers (Figure 5B). The needle was introduced at each planned entry point and advanced proximally along the longitudinal axis of the VMO to engage the maximal extent of the muscle belly. Although the original blind technique described a fixed insertion angle of 30°,17 the insertion angle in this study was adjusted individually under ultrasound guidance to maintain a trajectory parallel to the muscle fibers while maximizing the length of the VMO engaged by the needle. The needle was advanced until it reached the deep fascial boundary of the VMO, aiming to span the full thickness of the muscle from the superficial fascia to the deep fascia overlying the medial intermuscular septum (Figure 5C). Care was taken not to advance the needle beyond the medial intermuscular septum to avoid entering the adductor compartment and injuring underlying vascular structures, particularly the superficial femoral artery. The depth of insertion was typically 3–5 cm but varied depending on the individual VMO thickness assessed by ultrasound.
Once the needle reached the appropriate depth, the inner needle was withdrawn while the outer sheath was held in place, allowing the bidirectional barbs of the PDO filament to engage the surrounding muscle tissue. The outer sheath was then withdrawn, leaving the filament anchored within the VMO (Figure 6A), and any protruding portion was trimmed flush at the skin entry site. This process was repeated at each of the six insertion points across the VMO. After completion of the procedure, the anchored PDO filaments were visualized within the VMO as echogenic linear structures on long-axis ultrasonography (Figure 6B), confirming successful intramuscular placement.
Figure 6.
Completion of the MEST procedure. (A) PDO filaments protruding from the insertion sites prior to trimming. (B) Long-axis ultrasonographic view demonstrating echogenic PDO filaments (indicated by green arrows) embedded within the VMO after completion of the procedure.
Abbreviations: MEST, muscle enhancement and support therapy; PDO, polydioxanone; VMO, vastus medialis obliquus.
Post-Procedure Management and Assessment
For post-procedural pain control, patients received Maxigesic (acetaminophen 500 mg and ibuprofen 150 mg; Kyongbo Pharmaceutical Co., Ltd., Asan, Republic of Korea) as a preemptive analgesic prior to the procedure. Immediately after the procedure, ultrasound examination was performed to confirm appropriate filament placement within the VMO, assess the integrity of adjacent vascular structures, and exclude the presence of hematoma at the insertion sites or within the muscle.
Antibiotic ointment was applied to the insertion sites, followed by sterile dressing and placement of an elastic compression bandage to minimize bleeding or oozing. Patients were observed for approximately 20 minutes, during which the insertion sites were monitored for bleeding, oozing, or swelling, and the lower extremity was assessed for motor deficits prior to discharge.
Before discharge, patients were informed that the PDO filaments undergo hydrolytic degradation and are gradually absorbed over approximately 4–6 months. They were also advised to avoid excessive or strenuous activity during the early post-procedural period, as excessive movement could potentially lead to filament protrusion at the insertion sites.
MEST for TKA Patients
Participants
Clinical data were retrospectively reviewed for patients with chronic knee pain after TKA who underwent ultrasound-guided MEST at the Pain Clinic of Seoul National University Hospital between August 2025 and January 2026. A total of three patients were included in the study. Two patients underwent MEST procedures on both knees; therefore, each treated knee was considered an independent procedural case, resulting in a total of five cases included in the analysis. Each knee was analyzed separately because the intervention and clinical outcomes were evaluated independently for each treated joint.
Eligible patients met the following inclusion criteria: age ≥18 years; history of primary TKA on the ipsilateral knee; persistent postoperative knee pain lasting at least 3 months with a numeric rating scale (NRS) score ≥4; and insufficient response to conventional treatments, including oral analgesics, physical therapy, or prior injection therapies. Patients with identifiable structural causes of postoperative pain requiring surgical intervention, such as prosthetic loosening, periprosthetic joint infection, fracture, or prosthetic malalignment, were excluded. Patients with systemic infection, coagulopathy, or other contraindications to interventional procedures were also excluded.
Outcomes
Clinical outcomes were evaluated using the NRS, patient-reported functional outcomes during walking and stair climbing, knee joint effusion assessed by ultrasonography, and patient-reported global treatment effectiveness.
The NRS was scored on a scale of 0 (no pain) to 10 (worst imaginable pain). A treatment responder was defined as a case achieving a ≥50% reduction in NRS score from baseline at 8 weeks, in accordance with the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) recommendation for substantial clinical improvement.31 Knee joint effusion was evaluated by qualitative ultrasonographic assessment of the suprapatellar recess using a 4-point ordinal scale (grade 0 = none, grade 1 = mild, grade 2 = moderate, and grade 3 = severe).32
Functional outcomes during walking and stair climbing were each evaluated using a 5-point Likert scale reflecting subjective change from baseline (1 = much worse, 2 = slightly worse, 3 = no change, 4 = slightly improved, and 5 = much improved). Overall treatment effectiveness was evaluated using a 4-point patient-reported global assessment scale (1 = not effective, 2 = slightly effective, 3 = effective, and 4 = very effective). Clinical outcomes were recorded at baseline and during follow-up visits at 4, 8, and 12 weeks after the procedure.
Statistical Analysis
Given the exploratory and descriptive nature of this study and the very small sample size of three patients with five treated knees, no formal inferential statistical analysis was performed. Clinical and safety outcomes were summarized descriptively at each follow up time point. Individual case-level data are presented alongside summary measures. Continuous variables are reported as median (range) and categorical variables as number (%).
Results
A total of three patients (five cases) who underwent ultrasound-guided MEST were included in this study. All patients were female with a mean age of 72.6 years (range 66–79 years). Body mass index ranged from 26.4 to 31.6 kg/m2 (mean 28.9 kg/m2). Two patients underwent bilateral MEST procedures; therefore, five knees were treated in total. Baseline patient characteristics are summarized in Table 1.
Table 1.
Patient Characteristics
| Patient 1 | Patient 2 | Patient 3 | |
|---|---|---|---|
| Age, yr | 79 | 66 | 78 |
| Sex | Female | Female | Female |
| BMI, kg/m2 | 31.6 | 26.4 | 27.0 |
| Smoking | – | – | – |
| Alcohol use | – | – | – |
| Surgical history | |||
| TKA side | Bilateral | Bilateral | Bilateral |
| Time since TKA, Rt, mo | 108 | 14 | 72 |
| Time since TKA, Lt, mo | 109 | 50 | 62 |
| Pain duration, Rt, mo | 108 | 14 | 72 |
| Pain duration, Lt, mo | 109 | 50 | 62 |
| MEST side | Bilateral | Rt | Bilateral |
| Comorbidities | |||
| Hypertension | + | – | + |
| Diabetes mellitus | + | – | – |
| Current Medications | |||
| Strong opioid | – | – | – |
| Weak opioid | – | + | + |
| NSAIDs | + | + | + |
| Antidepressant | – | – | – |
| Anticonvulsant | + | + | – |
| SYSADOA | + | – | + |
Note: Values are presented as individual patient data. +, present; −, absent.
Abbreviations: BMI, body mass index; yr, years; TKA, total knee arthroplasty; Rt, right; Lt, left; mo, months; MEST, muscle enhancement and support therapy; NSAIDs, nonsteroidal anti-inflammatory drugs; SYSADOA, symptomatic slow-acting drugs for osteoarthritis.
Procedural Feasibility and Technical Success
The standardized ultrasound-guided protocol was successfully implemented in all five cases, yielding a technical success rate of 100%. In all cases, the VMO muscle belly was clearly visualized on ultrasound, allowing accurate identification of the muscle center despite interindividual anatomical variation. During needle insertion, color Doppler imaging was used to identify adjacent vascular structures, enabling safe advancement of the needle while avoiding these vessels. The needle trajectory was maintained within the VMO without crossing the medial intermuscular septum, and the PDO filaments were successfully deployed within the muscle belly without technical difficulty. Post-procedural ultrasound evaluation demonstrated no evidence of major hematoma, and the integrity of adjacent vascular structures remained intact.
Clinical Outcomes
Clinical outcomes for all five cases are summarized in Table 2 and Figure 7. The median baseline NRS pain score was 7, with a range of 5 to 10. By 4 weeks, NRS pain scores had decreased in all cases, with a median reduction of 4 points from baseline, with a range of 3 to 7. At 8 weeks, the median NRS pain score was 3, with a range of 1 to 3, and four of the five cases achieved a 50% or greater reduction from baseline. Pain reduction was generally maintained at the 12 week follow up.
Table 2.
Clinical Outcomes Following Ultrasound-Guided MEST
| Outcome | Case 1 (Patient 1, Lt) |
Case 2 (Patient 1, Rt) |
Case 3 (Patient 2, Rt) |
Case 4 (Patient 3, Lt) |
Case 5 (Patient 3, Rt) |
All Cases (n=5)* |
|---|---|---|---|---|---|---|
| Baseline | ||||||
| NRS | 7 | 7 | 10 | 5 | 5 | 7 (5–10) |
| Effusion grade† | 1 | 1 | 2 | 2 | 0 | 1 (0–2) |
| 4 weeks | ||||||
| NRS | 3 | 3 | 3 | 2 | 2 | 3 (2–3) |
| Effusion grade† | 1 | 1 | 2 | 0 | 1 | 1 (0–2) |
| Walking score‡ | 4 | 5 | 5 | 4 | 3 | 4 (3–5) |
| Stair score‡ | 4 | 5 | 4 | 3 | 3 | 4 (3–5) |
| Global Effectiveness§ | 4 | 4 | 4 | 4 | 4 | 4 (4–4) |
| 8 weeks | ||||||
| NRS | 3 | 1 | 3 | 2 | 3 | 3 (1–3) |
| Effusion grade† | 1 | 1 | 2 | 0 | 1 | 1 (0–2) |
| Walking score‡ | 4 | 5 | 5 | 3 | 5 | 5 (3–5) |
| Stair score‡ | 4 | 5 | 4 | 3 | 4 | 4 (3–5) |
| Global Effectiveness§ | 4 | 4 | 4 | 4 | 4 | 4 (4–4) |
| 12 weeks | ||||||
| NRS | 1 | 2 | 3 | 5 | 3 | 3 (1–5) |
| Effusion grade† | 1 | 1 | 2 | 3 | 1 | 1 (1–3) |
| Walking score‡ | 4 | 4 | 5 | 5 | 5 | 5 (4–5) |
| Stair score‡ | 4 | 4 | 4 | 4 | 4 | 4 (4–4) |
| Global Effectiveness§ | 4 | 4 | 4 | 4 | 4 | 4 (4–4) |
| Positive responder at 8 weeks¶ | ||||||
| NRS ≥50% reduction | + | + | + | + | – | 4 (80) |
| Complications | ||||||
| Ecchymosis | – | – | + | – | – | 1 (20) |
| Hematoma | – | – | – | – | – | 0 (0) |
| Filament protrusion | – | – | + | + | – | 2 (40) |
Notes: +, present; −, absent. *The “All cases (n = 5)” column presents continuous outcomes as the median (minimum–maximum) and categorical outcomes as n (%). †Effusion grade: grade 0 = none, grade 1 = mild, grade 2 = moderate, grade 3 = severe, assessed by ultrasonographic evaluation of the suprapatellar recess. ‡Walking and stair scores were assessed using a 5-point Likert scale reflecting change from baseline (1 = much worse, 2 = slightly worse, 3 = no change, 4 = slightly improved, 5 = much improved). These scores were not assessed at baseline as they represent subjective change from baseline. §Global effectiveness was assessed using a 4-point patient-reported scale (1 = not effective, 2 = slightly effective, 3 = effective, 4 = very effective). Positive responder was defined as ≥50% reduction in NRS from baseline at 8 weeks post-MEST, in accordance with IMMPACT recommendations. Case 1 and Case 2 correspond to the left and right knees of Patient 1, respectively. Case 3 corresponds to the right knee of Patient 2. Cases 4 and 5 correspond to the left and right knees of Patient 3, respectively.
Abbreviations: NRS, Numeric Rating Scale (0–10); MEST, muscle enhancement and support therapy.
Figure 7.
Individual changes in NRS pain scores over time following ultrasound-guided MEST. NRS pain scores for five cases were assessed at baseline and at 4, 8, and 12 weeks after the procedure.
Abbreviations: MEST, muscle enhancement and support therapy; NRS, numeric rating scale.
Functional outcomes also improved after the procedure. Walking and stair climbing scores improved in most cases by 4 weeks and were maintained or further improved at subsequent follow-up visits. Patient reported global effectiveness was rated as “very effective” in all cases at each follow-up assessment. Ultrasonographic knee effusion grades remained stable in most cases; one case demonstrated a reduction by 8 weeks, whereas another showed a mild increase at later follow-up.
Procedural-Related Complications
No serious adverse events occurred in any of the five treated cases. No infection, motor dysfunction, neurovascular injury, vascular puncture, or hematoma formation was observed, as confirmed by post-procedure ultrasound and follow-up evaluations. Ecchymosis occurred in three cases but was mild and resolved spontaneously within 2 weeks to 1 month without intervention. Filament protrusion beyond the skin entry point occurred in two cases (Cases 3 and 4). The protruding filaments were trimmed flush with the skin surface at the bedside without difficulty, and no further protrusion or wound-related complications were observed.
Discussion
This study describes a standardized ultrasound-guided protocol for MEST targeting the VMO and reports preliminary clinical outcomes in patients with refractory chronic knee pain following TKA. The protocol was developed to address limitations of the originally described blind technique by enabling precise identification of the VMO,17 optimization of the needle trajectory along the muscle fiber orientation, and real-time avoidance of adjacent vascular structures. In addition to establishing the technical protocol, this pilot study showed that MEST was associated with pain reduction in most treated knees at 8 weeks, suggesting potential clinical utility in this population.
Although previous clinical studies have reported promising outcomes with MEST,17,20 the technical aspects of the procedure warrant careful consideration. The original technique described by Kwon et al relied on surface anatomical landmarks without imaging guidance, with six entry points marked along the VMO at 10–15 cm proximal to the superior patellar border and the needle inserted at a 30° angle to a depth of 3–5 cm.17 Such an approach has inherent limitations, as surface landmarks alone cannot account for individual variability in VMO morphology, muscle thickness, or the proximity of critical neurovascular structures. In contemporary pain medicine practice, ultrasound guidance has become widely adopted for virtually all percutaneous interventional procedures because it allows real-time visualization of target structures, identification and avoidance of adjacent vessels and nerves, and confirmation of accurate needle placement.33 Furthermore, a recent umbrella review comparing ultrasound-guided and landmark-guided musculoskeletal interventions reported generally favorable findings for ultrasound guidance.23 Although direct comparative evidence in MEST is currently lacking, these findings support the rationale that MEST may benefit from ultrasound guidance, particularly with respect to procedural accuracy and avoidance of adjacent neurovascular structures.
Our ultrasound-guided protocol was designed to modify several key aspects of the previously described blind technique. First, the entry points are positioned more distally, approximately 5 cm proximal to the superior patellar border, rather than 10–15 cm proximal as described in the original technique.17 This region corresponds to the area of greatest muscle bulk and the most pronounced oblique fiber orientation, which may enhance the biomechanical effect of filament reinforcement on patellar stabilization. Second, short-axis ultrasonographic scanning allows precise identification of the VMO center, enabling symmetrical distribution of insertion points adjusted to individual muscle size. Third, the in-plane long-axis needle approach provides continuous visualization, facilitating confirmation of full-thickness muscle engagement while avoiding penetration of the medial intermuscular septum. Finally, color Doppler evaluation before each insertion helps identify intramuscular and adjacent vascular structures along the planned needle path. As demonstrated in our ultrasound assessment, numerous vascular structures exist both within and adjacent to the VMO, including the descending genicular artery, intramuscular arterial branches, and accompanying veins.28 The superficial femoral artery within the adductor canal also lies in close proximity to the medial aspect of the VMO.14,29,30 Therefore, the protocol described in this study provides a structured and reproducible approach that may improve procedural accuracy while reducing the risk of vascular injury.
The precise mechanism by which MEST exerts its clinical effects has not been fully elucidated. Nevertheless, available preclinical and clinical evidence suggests that MEST may have potential as a non-surgical approach for symptom relief in knee OA.17,20,34 A recent multicenter, randomized, double-blind, sham-controlled trial by Kwon et al involving 131 patients with mild-to-moderate knee OA demonstrated that MEST using PDO filament insertion into the VMO significantly improved weight-bearing pain, walking pain, isometric maximal contractile strength of the quadriceps compared with the control group.17 Similarly, a Korean clinical study by Kim et al, using PDO filament inserted into the vastus medialis, reported significant improvement in isometric maximal contractile strength and weight-bearing pain following the procedure, with significant improvements in the patient global impression of change at 20 and 30 weeks.20
The proposed mechanism underlying these effects may involve both biomechanical reinforcement and biological muscle recovery. The rationale for targeting the quadriceps is well supported by existing literature demonstrating that quadriceps weakness is an important contributor to the development, progression, and symptom burden of knee OA.18,35 Quadriceps strengthening exercises have consistently been shown to alleviate pain and improve functional outcomes in this population.36 The VMO, in particular, plays a critical role in patellar stabilization and load distribution across the knee joint, and its weakness has been associated with exacerbation of pain and mobility limitation in knee OA.14 When the VMO is weakened, impaired dynamic stabilization may contribute to altered load distribution and increased joint stress. MEST is hypothesized to address this mechanism by inserting biodegradable PDO filaments into the VMO, thereby providing direct mechanical support and inducing controlled micro-injury within the muscle tissue.18,19 In a rat model of monoiodoacetate-induced knee OA, this minor injury was shown to trigger a transient inflammatory response with activation of macrophages and satellite cells in the atrophied quadriceps, leading to subsequent recovery of muscle mass and contractile function that correlated with clear reduction in OA-related pain.18 A related study by the same group further demonstrated that MEST attenuated peripheral sensitization at the L3/L4 dorsal root ganglion, with reduced overexpression of TRPA1 and NaV1.7 receptors, which are key components in mechanical pain perception.19 Collectively, improved patellar stabilization, more even load distribution, and reduced joint stress, together with these biological mechanisms, may underlie the pain-relieving effects of MEST reported in the literature.
Although these mechanisms have primarily been studied in knee OA, they provide a biologically plausible basis for exploring MEST in other clinical settings characterized by VMO dysfunction. Persistent pain after TKA may represent one such setting. Patients undergoing TKA frequently exhibit preoperative quadriceps weakness and VMO atrophy,37 and after surgery, quadriceps weakness is further aggravated by arthrogenic muscle inhibition, a neural inhibition of voluntary muscle activation, and by disuse atrophy related to postoperative pain and reduced activity.15,16 Quadriceps strength has been reported to decrease by approximately 60% in the early postoperative period.38 These mechanisms may impair recovery of VMO function and dynamic patellar stabilization, potentially contributing to refractory chronic pain in selected patients. It is therefore reasonable to hypothesize that MEST, by mechanically reinforcing the VMO and potentially promoting muscle recovery, may help address these deficits in patients with refractory post-TKA pain.
The preliminary observations from the present study are consistent with this hypothesis and broadly align with the clinical outcomes previously reported in knee OA. Although objective functional measures were not systematically recorded in our retrospective dataset, NRS pain score decreased in all treated knees, and patient-reported walking, stair ambulation, and global effectiveness improved after the procedure and were maintained at 12 weeks. These findings suggest a potential clinical effect of MEST in post-TKA patients. Given these preliminary outcomes and the limited treatment options for refractory post-TKA pain, MEST may warrant further investigation as a potentially useful adjunctive treatment option. Although intra-articular corticosteroid injections are commonly used for knee OA, their use in a prosthetic knee is generally discouraged because of the potential risk of periprosthetic joint infection.39 The efficacy of genicular nerve radiofrequency ablation in post-TKA pain also remains inconclusive.40 In contrast, MEST is an extra-articular and minimally invasive intervention that does not directly access the prosthetic joint space. This feature may offer a potential safety advantage and provides further rationale for evaluating MEST in selected patients with refractory post-TKA pain.
This study has several limitations. First, the retrospective design and very small sample size limit the ability to draw definitive conclusions regarding treatment efficacy. Second, the absence of a control group precludes determination of whether the observed improvements can be attributed solely to the MEST intervention. The natural course of post-TKA pain and placebo effects cannot be excluded as potential contributors to the observed clinical outcomes. Third, the follow-up period of 12 weeks may be insufficient to evaluate the long-term durability of treatment effects, particularly considering that the PDO filaments are expected to be absorbed within approximately 4–6 months. Fourth, as a retrospective pilot investigation, outcome variables were not pre-specified, and the analysis was limited to data routinely collected during clinical follow-up. Consequently, functional outcomes were assessed using a subjective Likert scale rather than validated outcome instruments such as the Western Ontario and McMaster Universities Osteoarthritis Index or the Knee Injury and Osteoarthritis Outcome Score, and quadriceps muscle strength was not objectively measured. Future studies should ideally incorporate objective and validated outcome measures such as handheld dynamometry, ultrasonographic assessment of muscle morphology, gait analysis, and validated functional instruments to more rigorously evaluate the efficacy of MEST in this population. Beyond outcome assessment, future studies should also investigate potential determinants of MEST response by integrating demographic characteristics, ultrasound-based muscle imaging, and neuroimmune-related pain phenotypes, as recently suggested in the context of personalized musculoskeletal interventions.41
Conclusion
This study describes a standardized technical approach for ultrasound guided MEST targeting the VMO and explored its preliminary clinical application in patients with persistent pain following TKA. In this pilot study, the procedure was associated with observed pain reduction in most treated knees. These preliminary observations support the technical feasibility of this approach and suggest that ultrasound-guided MEST may warrant further investigation as a potential approach for refractory post-TKA pain. Given the small sample size, retrospective design, and absence of a control group, these findings should be interpreted as hypothesis-generating and viewed with caution. Larger prospective studies with appropriate control groups and longer follow-up durations are needed to establish the efficacy and safety of this approach.
Abbreviations
MEST, muscle enhancement and support therapy; NRS, numeric rating scale; OA, osteoarthritis, PDO, polydioxanone; TKA, total knee arthroplasty; VMO, vastus medialis obliquus.
Disclosure
The authors report no conflicts of interest in this work.
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