Section 1: Natural History of PFP and Local (Knee Region) Factors That Influence Pfp

What have we learned?

1. 
   PFP is common in young adolescents, with a high point prevalence of PFP in adolescents between 12 and 17 years of age. There are no data regarding the prevalence or incidence in other populations, except in military personnel where the annual incidence in men is 3.8% and in women is 6.5%, with a prevalence of 12% in men and 15% in women.
   
2. 
   The sporting and general populations' true incidence is unknown, and the much cited figure of 25% is based on figures from sports clinics which have ascertainment bias. Thus, there are insufficient data to confirm the incidence of PFP in these populations. Despite the common assumption that PFP is more common in women, there are a few studies comparing incidence and prevalence between men and women; however, in adolescents, the prevalence is higher in women. Understanding the different incidence and prevalence rates of PFP between men and women will improve the design of case-control studies.
   The PFP/PFOA continuum
   
3. 
   A subject of major interest is the possibility of a continuum from PFP to PFOA and if there are subgroups/phenotypes of the population who may develop radiographic PFOA or pain, or neither or both. Although it is possible that there is a phenotype of PFP that goes on to develop PFOA, there is no evidence to support this view. There have been no new studies on this subject since the review by Thomas et al, which was only able to include retrospective evidence in the review.
   Patellofemoral osteoarthritis
   
4. 
   PFOA is now regarded as a subgroup of knee OA. PFOA prevalence has been described using radiographic evidence in studies of people with knee pain or in community settings or MRI definitions in large epidemiology groups such as the Multicenter Osteoarthritis Study (MOST) and Framingham Osteoarthritis study (FOA) cohorts. Regardless of the methods used to define PFOA, the prevalence is high. Notably, there are new data to challenge the traditional views that PFOA predominantly affects the lateral patellar facet. These authors demonstrated that medial patellofemoral cartilage damage is highly prevalent, and possibly more prevalent than cartilage damage in the lateral patellofemoral compartment. This finding was consistent across three large epidemiological studies (MOST, FOA and the Boston Osteoarthritis of the Knee Study). Thus, there is a need for research into different mechanisms and risk factors for lateral and medial PFOA.
   
5. 
   There are new cohorts of people with PFOA on which randomised controlled trials have been performed.. The interventions used have been bracing and targeted multimodal physiotherapy programmes.
   Local (knee region) factors that influence PFP
   
6. 
   There is new evidence that abnormal structure or alignment of the PFJ may lead to cartilage damage and focal areas of loading and stress manifested as bone marrow lesions (BMLs). Stefanik et al reported that knees in the MOST cohort with patella alta and abnormal trochlea morphology were associated with cartilage damage and BML, with similar findings reported from the Osteoarthritis Initiative cohort.
   
7. 
   The relationship between structure and biomechanics is not known. It is possible that structural abnormalities coupled with poor biomechanics will increase the likelihood of PFP. On the other hand if there is normal structure then the biomechanics may not matter. As yet, no study has examined PFJ structure and mechanics in the same cohort.
   
8. 
   A systematic review of prospective studies indicates the Q angle is not a risk factor for PFP casting further doubt on the Q angle's relevance in PFP.
   
9. 
   There is limited evidence that a variety of local structures contribute to nociception (and potentially pain) in PFP. These include the infrapatellar fat pad in PFP, increased water content in subchondral patellar bone in athletes and BMLs in PFOA. There is no new evidence that the retinaculum, which was previously a commonly cited soft tissue problem, contributes to nociception.
   

Future Advances Required to Understand PFP and its Treatment

1. 
   Large, long-term prospective cohort studies are needed to identify whether there are phenotypes of PFP that goes on to develop PFOA.
   
2. 
   Future studies should also evaluate the importance of psycho-social factors and central sensitisation in PFP.
   
3. 
   Identification of different subgroups of people with PFP remains a goal. These subgroups could be based on pain types (eg, nociceptive pain or central sensitisation), or on different structural or biomechanical features. Investigations could then investigate whether different subgroups influence the large individual variation in outcome results after a physiotherapy programme.
   
4. 
   Inter-relationships between these different psychological, structural and neuromechanical features will impact patient's presentations and treatment responses