Global research trends and hotspots in patellofemoral pain syndrome from 2000 to 2023: a bibliometric and visualization study

Background

Patellofemoral pain syndrome (PFPS) is a prevalent condition in sports medicine, and as sports competitions become more popular, the incidence of sports injuries is on the rise. Despite the increasing research on PFPS, there remains a lack of bibliometric analyses on this topic. The aim of this study was to identify the research hotspots and trends in the field of PFPS by reviewing 23 years of literature in this field.

Methods

By analyzing the literature on PFPS research from 2000 to 2023 in the core dataset of the Web of Science database and utilizing bibliometric tools like CiteSpace 6.1, VOSviewer 1.6.18, R-bibliometrix 4.6.1, Pajek 5.16, and Scimago Graphica 1.0.26, our aim was to gain insights into the current status and key areas of PFPS research. The study examined various aspects including the number of publications, countries, institutions, journals, authors, collaborative networks, keywords, and more. Through the visualization of relevant data, we also attempted to forecast future trends in the field.

Results

There were 2,444 publications were included in this visualization study, published in 322 journals by 1,247 authors from 818 institutions in 67 countries. The Journal of Orthopaedic and Sports Physical Therapy had the highest number of publications, with the USA leading in article count. La Trobe University contributed the most articles, while Rathleff MS and Barton CJ emerged as the most prolific authors. Hip and knee strength and core strength, lower extremity kinematics and biomechanics, females (runners), muscle activation, risk factors, gait retraining, clinical practice guidelines, and rehabilitation were research hotspot keywords.

Conclusion

Current research suggests that there is still significant potential for the development of PFPS research. Key areas of focus include the clinical effectiveness of combined hip and knee strengthening to address PFPS, characterization of lower limb kinematics and biomechanics, gait retraining, risk factors, and clinical practice guidelines. Future research could explore the effectiveness of innovative exercise therapies such as blood flow restricting training, gait retraining, and neuromuscular control training for PFPS improvement. Further investigation into gait retraining for runners, particularly females, and clinical efficacy study of a novel PRP formulation for the treatment of PFPS.

Signs of Nervous System Sensitization in Female Runners with Chronic Patellofemoral Pain

Background

Patellofemoral pain (PFP) is a common overuse injury among runners, affecting females at a higher rate than males. PFP can often become chronic, with evidence suggesting it may be linked to both peripheral and central sensitization of the nervous system. Sensitization of the nervous system can be identified through quantitative sensory testing (QST).

Hypothesis/Purpose

The primary objective of this pilot study was to quantify and compare pain sensitivity as identified through QST measures, in active female runners with and without PFP.

Study Design

Cohort Study.

Methods

Twenty healthy female runners and 17 female runners with chronic PFP symptoms were enrolled. Subjects completed the Knee injury and Osteoarthritis Outcome Score for Patellofemoral Pain (KOOS-PF), University of Wisconsin Running Injury and Recovery Index (UWRI), and the Brief Pain Inventory (BPI). QST consisted of pressure pain threshold testing to three local and three distant sites to the knee, heat temporal summation, heat pain threshold, and conditioned pain modulation. Data was analyzed utilizing independent t-tests for comparison of between-group data, effect sizes for QST measures (Pearson's r), and Pearson's correlation coefficient between pressure pain threshold values at the knee and functional testing.

Results

The PFP group exhibited significantly lower scores on the KOOS-PF (p<0.001), BPI Pain Severity and Interference Scores (p<0.001), and UWRI (p<0.001). Primary hyperalgesia, identified through decreased pressure pain threshold at the knee, was detected in the PFP group at the central patella (p<0.001), lateral patellar retinaculum (p=0.003), and patellar tendon (p=0.006). Secondary hyperalgesia, a sign of central sensitization, was observed via differences in pressure pain threshold testing for the PFP group at the uninvolved knee (p=0.012 to p=0.042), involved extremity remote sites (p=0.001 to p=0.006), and uninvolved extremity remote sites (p=0.013 to p=0.021).

Conclusion

Compared to healthy controls, female runners with chronic PFP symptoms exhibit signs of both peripheral sensitization. Despite actively participating in running, nervous system sensitization may contribute to continued pain in these individuals. For female runners with chronic PFP, physical therapy management may need to include interventions which address signs of central and peripheral sensitization.

Level of Evidence

Level 3.

Running gait modifications can lead to immediate reductions in patellofemoral pain

Gait modifications are commonly advocated to decrease knee forces and pain in runners with patellofemoral pain (PFP). However, it remains unknown if clinicians can expect immediate effects on symptoms. Our objectives were (1) to compare the immediate effects of gait modifications on pain and kinetics of runners with PFP; (2) to compare kinetic changes in responders and non-responders; and (3) to compare the effects between rearfoot strikers (RFS) and non-RFS. Sixty-eight runners with PFP (42 women, 26 men) ran normally on a treadmill before testing six modifications: 1- increase step rate by 10%; 2- 180 steps per minute; 3- decrease step rate by 10%; 4- forefoot striking; 5- heel striking; 6- running softer. Overall, there were more responders (pain decreased ≥1/10 compared with normal gait) during forefoot striking and increasing step rate by 10% (both 35%). Responders showed greater reductions in peak patellofemoral joint force than non-responders during all conditions except heel striking. When compared with non-RFS, RFS reduced peak patellofemoral joint force in a significant manner (P < 0.001) during forefoot striking (partial η ² = 0.452) and running softer (partial η ² = 0.302). Increasing step rate by 10% reduced peak patellofemoral joint force in both RFS and non-RFS. Forty-two percent of symptomatic runners reported immediate reductions in pain during ≥1 modification, and 28% had reduced pain during ≥3 modifications. Gait modifications leading to decreased patellofemoral joint forces may be associated with immediate pain reductions in runners with PFP. Other mechanisms may be involved, given that some runners reported decreased symptoms regardless of kinetic changes.

A simple, bed-side tool to assess evoked pressure pain intensity

OBJECTIVES

Existing equipment for quantitative sensory testing is generally expensive and not easily applicable in a clinical setting thus simple bed-side devices are warranted. Pressure hyperalgesia is a common finding in patients with musculoskeletal pain and an experimental model is delayed-onset muscle soreness (DOMS). DOMS is characterised by muscle hyperalgesia and some studies report facilitation of temporal summation of pain. This study aimed to detect DOMS induced muscle hyperalgesia and temporal summation of pain using a newly developed bed-side quantitative sensory testing device to deliver standardised pressure.

METHODS

Twenty-two healthy participants participated in two sessions with the second session approximately 48 h after baseline. Pressure pain intensities were assessed from the gastrocnemius muscle with four probes calibrated to apply 2, 4, 6 and 8 kg, respectively. Temporal summation of pain (10 stimuli delivered at 0.5 Hz using the 6 kg probe) intensities were assessed from the same location. DOMS was evoked in the gastrocnemius muscle by an eccentric exercise. Sleepiness and physical activity were measured with the Epworth Sleepiness Scale and the Global Physical Activity Questionnaire to investigate if they were associated with the quantitative sensory testing measures.

RESULTS

Pressure pain intensity was significantly increased 48 h after induction of DOMS when compared to baseline for all four probes (p<0.05). Temporal summation of pain was not statistically significant affected by DOMS and sleep quality and physical activity did not associate with any of the measures.

CONCLUSIONS

This study introduces a simple, bed-side assessment tool for the assessment of pressure pain intensity and hence hyperalgesia and temporal summation of pain.

Chronic Postoperative Pain after Total Knee Arthroplasty: The Potential Contributions of Synovitis, Pain Sensitization, and Pain Catastrophizing - An Explorative Study

Background

A subset of osteoarthritis patients will experience chronic postoperative pain after total knee arthroplasty (TKA), but the source of pain is unclear. The aim of this exploratory study was to assess patients with and without postoperative pain after TKA using magnetic resonance imaging (MRI), quantitative sensory testing (QST), clinical assessment of pain and assessments of catastrophizing thoughts.

Methods

Forty‐six patients completed the 6‐month postoperative assessment. MRI findings were scored according to the MRI Osteoarthritis Knee Score recommendation for Hoffa synovitis, effusion size and bone marrow lesions. QST included assessment of pressure pain thresholds (PPTs), temporal summation of pain (TSP) and conditioned pain modulation (CPM). Pain catastrophizing was assessed using the Pain Catastrophizing Scale (PCS). Clinical pain assessment was conducted using a visual analogue scale (VAS, 0–10 cm), and groups of moderate‐to‐severe (VAS > 3) and none‐to‐mild postoperative pain (VAS ≤ 3) were identified.

Results

Patients with moderate‐to‐severe postoperative pain (N = 15) demonstrated higher grades of Hoffa synovitis (p < 0.001) and effusion size (p < 0.001), lower PPTs (p = 0.039), higher TSP (p = 0.001) and lower CPM (p = 0.014) when compared with patients with none‐to‐mild postoperative pain (N = 31). No significant difference was found in PCS scores between the two groups. Multiple linear regression models found synovitis (p = 0.036), effusion size (p = 0.003), TSP (p = 0.013) and PCS (p < 0.001) as independent parameters contributing to the postoperative pain intensity.

Conclusion

These exploratory findings could indicate that chronic postoperative pain after TKA is a combination of joint‐related synovitis and effusion, sensitization of central pain mechanisms and potentially pain catastrophizing thoughts, but larger studies are needed to confirm this.

Significance

The end‐stage treatment of knee osteoarthritis is total knee arthroplasty. Some patients experience chronic postoperative pain after total knee arthroplasty, but the mechanism for chronic postoperative pain is widely unknown. The current study indicates that higher levels postoperative of synovitis and effusion, higher temporal summation of pain and higher pain catastrophizing scores could be associated with higher chronic postoperative pain.

A mechanism-based proof of concept study on the effects of duloxetine in patients with painful knee osteoarthritis

BACKGROUND

The global burden of osteoarthritis (OA) is steadily increasing due to demographic and lifestyle changes. The nervous system can undergo peripheral and central neuroplastic changes (sensitization) in patients with OA impacting the options to manage the pain adequately. As a result of sensitization, patients with OA show lower pressure pain thresholds (PPTs), facilitated temporal summation of pain (TSP), and impaired conditioned pain modulation (CPM). As traditional analgesics (acetaminophen and non-steroidal anti-inflammatory drugs) are not recommended for long-term use in OA, more fundamental knowledge related to other possible management regimes are needed. Duloxetine is a serotonin-noradrenalin reuptake inhibitor, and analgesic effects are documented in patients with OA although the underlying fundamental mechanisms remain unclear. The descending pain inhibitory control system is believed to be dependent on serotonin and noradrenalin. We hypothesized that the analgesic effect of duloxetine could act through these pathways and consequently indirectly reduce pain and sensitization. The aim of this mechanistic study is to investigate if PPTs, TSP, CPM, and clinical pain parameters are modulated by duloxetine.

METHODS

This proof of concept study is a randomized, placebo-controlled, double-blinded, crossover trial, which compares PPTs, TSP, and CPM before and after 18 weeks of duloxetine and placebo in forty patients with knee OA. The intervention periods include a titration period (2 weeks), treatment period (60 mg daily for 14 weeks), and a discontinuation period (2 weeks). Intervention periods are separated by 2 weeks.

DISCUSSION

Duloxetine is recommended for the treatment of chronic pain, but the underlying mechanisms of the analgesic effects are currently unknown. This study will investigate if duloxetine can modify central pain mechanisms and thereby provide insights into the underlying mechanisms of the analgesic effect.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04224584 . Registered on January 6, 2020. EudraCT 2019-003437-42 . Registered on October 22, 2019. The North Denmark Region Committee on Health Research Ethics N-20190055. Registered on October 31, 2019.

Exploring the Pain in Patellofemoral Pain: A Systematic Review and Meta-Analysis Examining Signs of Central Sensitization

BACKGROUND

Patellofemoral pain (PFP) has high recurrence rates and minimal long-term treatment success. Central sensitization refers to dysfunctional pain modulation that occurs when nociceptive neurons become hyperresponsive. Researchers in this area of PFP have been increasingly productive in the past decade.

OBJECTIVE

To determine whether evidence supports manifestations of central sensitization in individuals with PFP.

DATA SOURCES

We searched MeSH terms for quantitative sensory testing (QST) pressure pain thresholds (PPTs), conditioned pain modulation (CPM), temporal summation, sensitization, hyperalgesia, and anterior knee pain or PFP in PubMed, SPORTDiscus, CINAHL, Academic Search Complete, and EBSCOhost.

STUDY SELECTION

Peer-reviewed studies that were written in English and published between 2005 and 2020 and investigated QST or pain mapping in a sample with PFP were included in this review.

DATA EXTRACTION

The initial search yielded 140 articles. After duplicates were removed, 78 abstracts were reviewed. The full text of 21 studies was examined, and we included 15 studies in our evaluation: 6 in the meta-analysis, 4 in the systematic review, and 5 in both the meta-analysis and systematic review.

DATA SYNTHESIS

A random-effects meta-analysis was conducted for 4 QST variables (local PPTs, remote PPTs, CPM, temporal summation). Strong evidence supported lower local and remote PPTs, impaired CPM, and facilitated temporal summation in individuals with PFP compared with pain-free individuals. Evidence for heat and cold pain thresholds was conflicting. Pain mapping demonstrated expanding pain patterns associated with long duration of PFP symptoms.

CONCLUSIONS

Signs of central sensitization were present in individuals with PFP, indicating altered pain modulation. The etiologic and treatment models of PFP should reflect the current body of evidence regarding central sensitization. Signs of central sensitization should be monitored clinically, and treatments with central effects should be considered as part of a multimodal plan of care.

Fear of movement and (re)injury is associated with condition specific outcomes and health-related quality of life in women with patellofemoral pain

OBJECTIVES

Investigate the association of fear of movement and (re)injury with clinical outcomes in women with patellofemoral pain (PFP).

METHODS

This cross-sectional study included 92 women with PFP who completed the TAMPA scale for kinesiophobia. The TAMPA score and its two subscales - activity avoidance and somatic focus were correlated with BMI, physical activity level, pain catastrophizing scale, health-related quality of life, pain sensitivity via pressure pain threshold, self-reported disability, and worst knee pain in last month.

RESULTS

Greater fear of movement and (re)injury, activity avoidance, and somatic focus were correlated with lower local pain sensitivity (rho = -0.29 to -0.55), lower health-related quality of life (rho = -0.38 to -0.42), greater pain catastrophizing (rho = 0.41 to 0.47), and greater self-reported disability (rho = -0.31 to -0.52). Greater fear of movement and (re)injury and activity avoidance were correlated with adjacent and remote pain sensitivity (rho = -0.24 to -0.39). Greater fear of movement and (re)injury and somatic focus were correlated with greater worst knee pain in last month (rho = 0.21 to 0.32). Fear of movement and (re)injury predicted pain measures, disability, and health-related quality of life (p ≤ 0.010).

CONCLUSION

The relationship of greater fear of movement and (re)injury with greater disability, pain catastrophizing, pain sensitization, and poorer health-related quality of life highlights the potential importance of considering this psychological feature of PFP during assessment and management.

Update of Risk Factors, Diagnosis, and Management of Patellofemoral Pain

PURPOSE OF REVIEW

Patellofemoral pain is the most common cause of anterior knee pain. The purpose of this review is to examine the latest research on risk factors, physical examination, and treatment of patellofemoral pain to improve accuracy of diagnosis and increase use of efficacious treatment modalities.

RECENT FINDINGS

The latest research suggests patellofemoral pain pathophysiology is a combination of biomechanical, behavioral, and psychological factors. Research into targeted exercise therapy and other conservative therapy modalities have shown efficacy especially when used in combination. New techniques such as blood flow restriction therapy, gait retraining, and acupuncture show promise but require further well-designed studies. Patellofemoral pain is most commonly attributed to altered stress to the patellofemoral joint from intrinsic knee factors, alterations in the kinetic chain, or errors in training. Diagnosis can be made with a thorough assessment of clinical history and risk factors, and a comprehensive physical examination. The ideal treatment is a combination of conservative treatment modalities ideally individualized to the risk factors identified in each patient. Ongoing research should continue to identify biomechanical risk factors and new treatments as well as look for more efficient ways to identify patients who are amenable to treatments.

Altered pain processing and sensitisation is evident in adults with patellofemoral pain: a systematic review including meta-analysis and meta-regression

Background and aims

Previous systematic reviews have reported manifestations of pain sensitisation as a feature of painful knee disorders, in particular osteoarthritis, with moderate evidence for pain sensitisation in patellofemoral pain (PFP). However, despite past studies recruiting female mostly adolescent PFP patients, it is unclear if sex or age plays a role. Investigation is required to determine if altered pain processing is a key feature of PFP and if a subgroup of patients is at an increased risk to help provide targeted management. The primary aim of this systematic review was to examine evidence investigating pain processing in PFP. Secondary aims were to evaluate the relationship between pain processing and (1) sex, (2) age and (3) symptom duration.

Methods

The protocol was prospectively registered with PROSPERO (CRD42019129851). PubMed, CINAHL, Web of Science and EMBASE were systematically searched from inception to April 2019 for studies investigating pain processing in PFP patients compared to controls using quantitative sensory testing. Each included paper was assessed for methodological quality using a modified version of Downs and Black. Means and standard deviations were extracted to calculate standardised mean differences (SMD) and 95% confidence intervals (95% CI). Where possible meta-analysis and meta-regression were performed using a random effects model.

Results

Eleven studies were identified, two medium and nine high quality. Meta-analysis indicates moderate evidence for decreased pressure pain thresholds (SMD −0.68, 95% CI −0.93 to −0.43), increased tactile detection thresholds (SMD 1.35, 95% CI 0.49–2.22) and increased warmth detection thresholds (SMD 0.61, 95% CI 0.30–0.92) in PFP patients compared to controls. Secondary analysis indicates moderate evidence for decreased pressure pain thresholds in female compared to male patients (SMD −0.75, 95% CI −1.34 to −0.16). Meta-regression indicates a moderate correlation between decreasing local and distal pressure pain thresholds and decreasing patient age (local R2 = 0.556, p = 0.0211; distal R2 = 0.491, p = 0.0354) but no correlation with symptom duration (p > 0.05).

Conclusions

Evidence from this systematic review with meta-analysis and meta-regression appears to suggest the presence of altered pain processing and sensitisation in patients with PFP with increased sensitivity indicated in female patients and younger patients.

Implications

With evidence of altered pain processing and sensitisation in PFP, it may be beneficial for clinicians to consider management approaches that aim specifically at adressing neuropathic pain, for example neuroscience education, to improve patients outcomes. With female patients and younger patients indicated as experiencing greater degree of sensitivity, this may be a good demographic to start screening for sensitisation, in order to better identify and treat those most affected.

Pressure pain thresholds in adults with patellofemoral pain and patellofemoral joint osteoarthritis: a case-control study

Background and aims

Patellofemoral pain (PFP) and patellofemoral joint osteoarthritis (PFJOA) are common non-self-limiting conditions causing significant pain and disability. The underlying pain pathologies lack consensus with evidence suggesting reduced pressure pain thresholds (PPTs) in adolescent females with PFP and individuals with knee osteoarthritis. A paucity of evidence exists for mixed-sex adults with PFP and PFJOA in isolation. Exploring if pain sensitisation is a dominant feature of PFP and PFJOA may have important implications for the delivery of a patient centred management approach. The primary aim was to measure local and remote PPTs in PFP and PFJOA patients compared to matched controls. Secondary aims were to evaluate the relationship between PPTs and (1) condition severity and (2) knee function.

Methods

13 PFP patients plus 20 matched controls and 15 PFJOA patients plus 34 matched controls were recruited from a UK mixed-sex adult population. Controls were matched on age, sex and activity level. Demographic details, Tegner activity level score, symptom duration, condition severity (Kujala and KOOS-PF scores for PFP and PFJOA, respectively) and knee function (Modified Whatman score rating of five single leg squats) were recorded. PPTs were measured at six sites: five local around the knee, one remote on the contralateral leg. Between-group differences were tested using a two-way mixed model analysis of variance with repeated measures. Strength of association between PPTs and condition severity and knee function were tested using Spearman’s rank order correlation.

Results

No statistically significant difference in PPTs were observed between the PFP patients [F(1,31) = 0.687, p = 0.413, η2 = 0.022] or PFJOA patients [F(1,47) = 0.237, p = 0.629, η2 = 0.005] and controls. Furthermore, no correlation was found between PPTs and condition severity or knee function in PFP or PFJOA (p > 0.05).

Conclusions

Results suggest mechanical pain sensitisation is not a dominant feature of UK mixed-sex adults with PFP or PFJOA.

Implications

PFP and PFJOA remain persistent pain complaints which may not be well explained by objective measures of sensitivity such as PPTs. The findings suggest that peripheral pain processing changes leading to pain sensitisation is not a key feature in PFP or PFJOA. Instead the underlying pain pathway is likely to remain primary nociceptive, possibly with a subgroup of patients who experience pain sensitisation and might benefit from a more targeted management approach.

The effect of an individualised functional retraining intervention on pain, function and biomechanics in participants with patellofemoral pain: a series of n of 1 trial

[Purpose] To determine the effect of an individualised functional retraining intervention on pain, function, kinematics and self-reported recovery in participants with PFP. [Participants and Methods] Thirty-one participants with unilateral PFP between the ages of 14–40 were included. Data collection and treatment sessions were conducted at the Tygerberg 3D Motion Analysis Laboratory and Physiotherapy Clinic at the University of Stellenbosch Medical School in Cape Town, South Africa. Participants underwent motion analysis testing pre- and post-intervention and attended physiotherapy weekly for a 6-week individualised intervention. [Results] Thirty of the thirty-one participants (96.8%) demonstrated improved pain levels (NPRS) post intervention. Participants demonstrated a statistically significant improvement in function (AKPS) immediately post intervention and continued to improve with greater functional scores at 6-month follow up. Fifteen participants (48.4%) rated themselves as fully recovered on a 7-point Likert scale at 6-month follow up. Nineteen of the 31 participants (61.3%) demonstrated a clinically significant improvement in their priority kinematic outcome post intervention. [Conclusion] Individualised functional retraining may improve pain, function, kinematics and long-term recovery in participants with PFP presenting with kinematic contributing factors. Clinicians need to be educated on common biomechanical contributing factors and how to tailor treatment accordingly.

Current methodological approaches in conditioned pain modulation assessment in pediatrics

Conditioned pain modulation (CPM) paradigms have been used in various studies with healthy and non-healthy adult populations in an attempt to elucidate the mechanisms of pain processing. However, only a few studies so far have applied CPM in pediatric populations. Studies finding associations with chronic pain conditions suggest that deficiencies in underlying descending pain pathways may play an important role in the development and persistence of pain early in life. Twelve studies were identified using a PubMed search which examine solely pediatric populations, and these are reviewed with regard to demographics studied, methodological approaches, and conclusions reached. This review aimed to provide both clinicians and researchers with a brief overview of the current state of research regarding the use of CPM in children and adolescents, both healthy and clinical patients. The implications of CPM in experimental and clinical settings and its potential to aid in refining considerations to individualize treatment of pediatric pain syndromes will be discussed.

Adults with patellofemoral pain do not exhibit manifestations of peripheral and central sensitization when compared to healthy pain-free age and sex matched controls - An assessor blinded cross-sectional study

Patellofemoral Pain (PFP) is highly prevalent among adults and adolescents. Localized mechanical hyperalgesia around the knee and tibialis anterior have been observed in people with PFP, but limited knowledge of potential manifestations of central sensitisation exists. The aims of this study were to study conditioned pain modulation (CPM) and wide-spread hyperalgesia in adults with PFP. This assessor-blinded cross-sectional study design compared CPM and mechanical pressure pain thresholds (PPT) between 33 adults (23 females) diagnosed with PFP and 32 age and sex matched pain-free controls. The investigator taking the PPT measurements was blinded to which participants had PFP. PPTs were reliably measured using a Somedic hand-held pressure algometer at three sites: 1) The centre of the patella, 2) the tibialis anterior muscle and 3) a remote site on the lateral epicondyle. For the assessment of CPM, experimental pain was induced to the contralateral hand by immersion into a cold water bath (conditioning stimulus), and assessment of PPTs (the test stimulus) was performed before and immediately after the conditioning stimulation. On average, the CPM paradigm induced a significant increase in PPTs across the three sites (6.3–13.5%, P<0.05), however there was no difference in CPM between young adults with PFP compared to the control group, (F(1,189) = 0.39, P = 0.89). There was no difference in mechanical PPTs between the two groups (F(1,189) = 0.03, P = 0.86). Contrary to our a-priori hypothesis, we found no difference in CPM or PPT between young adults with PFP and age and sex matched pain-free controls.