Targeted intervention for patellofemoral pain (TIPPS): identifying potential clinical subgroups

Background: Patellofemoral pain (PFP) is a specific musculoskeletal disorder that causes significant pain and dysfunction around the knee cap and knee joint, which can lead to long term limitation in societal participation and physical activity. There is a consensus that conservative intervention should be the first treatment option so PFP is a condition that is commonly referred for physiotherapy. Unfortunately, however, current multi modal approaches to intervention in PFP are failing in the long term. Purpose: Given the poor outcome of current treatment regimens, identification of patellofemoral subgroups is an international priority. Large individual variation in outcome results is consistently reported in response to conservative management, therefore understanding whether subgroups have different outcomes has become imperative. However it is unknown whether subgrouping is possible in PFP. The main purpose of this study was to describe the distribution of PFP patients into different subgroups using six routine clinical assessment test criteria. Methods: Design: Cross-sectional with single point assessment prior to commencement of physiotherapy. Participants with PFP underwent clinical assessment of muscle strength (quadriceps and hip abductor), muscle length (quadriceps, hamstrings, gastrocnemius), patellar mobility and foot posture. The presence of subgroups was explored using two classification techniques: Hierarchical Clustering and Latent Profile Analysis. Results: One hundred and twenty seven of 130 recruited participants had complete assessment data: 66% were female, mean (SD) age was 26 (5.7) years, BMI 25.5 (5.8), and Modified Functional Index Questionnaire (MFIQ) was 34 (17). A three subgroup solution appeared optimal from a modelling and clinical perspective. The three suggested clinical subgroups were characterised as: Strong subgroup (n = 44), weak and tight subgroup (n = 58), pronated feet subgroup (n = 25). An ANOVA and post hoc tests confirmed significant differences between subgroups. Strong subgroup: quadriceps strength 1.5 Nm/kg (0.51), hip abductor strength 1.5 Nm/kg (0.51), lowest patellar mobility of the three groups 9.6 mm (3.43). Weak and tight subgroup: hip abductor strength 0.7 Nm/kg (0.27), shortest quadriceps length 118° (17.9). Pronated feet subgroup: highest FPI 7 (3.0), highest patellar mobility 17.6 mm (4.75). Conclusion(s): This study is the largest conducted to date on subgrouping of PFP. The results lend support to the idea that clinical subgroups exist within the patellofemoral population, with three potential subgroups emerging. The key clinical tests for identifying the three subgroups are strength measurement of the quadriceps and hip abductors, and foot posture index. Patellar mobility and quadriceps flexibility contributed to subgroup profiles, so also appear important to consider when identifying PFP subgroups. Implications: These results fit well with other findings in musculoskeletal research where subgrouping has been successful in informing targeted intervention in low back pain patients and is being increasingly adopted as a strategy for managing patients with shoulder problems. Our next step is explore variation in outcome across the identified PFP subgroups and whether subgroup targeted intervention improves overall patient outcomes.