This study aims to examine whether individuals with unilateral patellofemoral pain (PFP) have increased subchondral bone remodelling and thus increased 18F Sodium Fluoride (18F-NaF) uptake in the painful knee compared to the opposite knee measured by simultaneous positron emission tomography and computed tomography (PET/CT) and to examine whether the increased uptake is associated with pain intensity, with patient-reported function and with pain localization. Furthermore we aim to investigate the effect of an acute patellofemoral loading bout on the bone remodelling.

Patellofemoral pain (PFP) is a highly prevalent musculoskeletal condition mainly affecting younger people, causing pain, physical disability, and reduced quality of life. The cause of PFP is still unknown. It is, i.e., unclear whether there is a link between PFP and changes in the cartilaginous (subchondral) bone in the joint between the patella and the femur.

Our goals are to use 18F NaF PET/CT to evaluate whether subjects with unilateral patellofemoral pain exhibit elevated bone metabolic activity in the painful knee compared to the pain free knee and to determine whether bone metabolic activity correlates with pain intensity and location of pain. Furthermore, we aim to investigate the effects of acute loading on the bone metabolic activity.

The primary objective of this cross-sectional study is to assess whether patients with unilateral PFP have increased bone metabolic activity in the painful knee compared to the pain free knee. We will assess quantitative parameters indicative of bone metabolic activity (Standardized Uptake Value (SUV) and kinetic data) and compare values between the painful knee and the pain free knee. The secondary objectives are to 1) determine whether a bout of single leg weight bearing knee bends standing on a wedge result in increased bone metabolic activity acutely in the painful knee compared to the pain free knee, and 2) identify "hot" regions of abnormal tracer uptake and compare area and SUV values between the painful knee and the pain free knee.

An explorative objective is to determine the correlation between bone metabolic activity and pain intensity and location of pain.

Inclusion Criteria:

• A clinical diagnosis of PFP in at least one knee confirmed by an experienced sports medicine doctor.

• Visual analogue score rating of pain during activities of daily living during the previous week at a minimum of 3 on a 10 cm scale.

• Insidious onset of symptoms unrelated to trauma and persistent for at least 4 weeks.

• Pain in the anterior knee associated with at least 3 of the following:

  • During or after activity
  • Prolonged sitting
  • Stair ascent or descent
  • Squatting

Exclusion Criteria:

• Meniscal or other intra-articular injury
• Cruciate or collateral ligament laxity or tenderness
• Patellar tendon, iliotibial band, or pes anserine tenderness
• Osgood-Schlatter or Sinding-Larsen-Johansson syndrome
• History of recurrent patellar subluxation or dislocation
• History of surgery to the knee joint
• History of head injury or vestibular disorder within the last 6 months
• Pregnancy
• Breastfeeding

Adults (n = 30) with unilateral anterior knee pain / PFP (3> 10 NRS) will be included from the main study.