Introduction / Definition

Patellofemoral Pain Syndrome (PFPS), frequently designated 'runner's knee' or anterior knee pain (AKP), is a clinical entity defined by insidious, diffuse retro- or peripatellar pain provoked by loading the patellofemoral joint (PFJ) in flexion. Standard provoking tasks include running, squatting, ascending or descending stairs, and prolonged seated knee flexion ('movie-goer's' or 'theatre sign'). PFPS is a diagnosis of exclusion: it describes retropatellar symptoms in the absence of distinct intra-articular pathology (such as high-grade chondral loss, meniscal tears, or ligamentous instability), frank patellofemoral instability, or patellar tendinopathy.

Epidemiology

• Incidence: PFPS accounts for 25% to 40% of all knee presentations in sports medicine and outpatient orthopaedic clinics.
• Demographics: Demonstrates a pronounced female predilection (approximately 2:1 female-to-male ratio), peaking in adolescents and young active adults aged 12–35 years.
• Natural History & Prognosis: Once historically considered self-limiting, prospective studies reveal that over 50% of individuals demonstrate recalcitrant or recurrent symptoms beyond 5 to 8 years post-diagnosis, predisposing to chronic activity limitation and potential secondary patellofemoral osteoarthritis.

Relevant Anatomy & Biomechanics

Articular & Osseous Congruence

• The patella is the largest sesamoid bone in the human body, acting as a fulcrum to enhance the mechanical advantage of the quadriceps lever arm by up to 30%.
• It features the thickest articular cartilage in the musculoskeletal system (up to 5–6 mm).
• Engagement: At 0° extension, the patella sits superior to the trochlear groove; initial engagement into the trochlea occurs between 10° and 20° of knee flexion. As flexion progresses beyond 90°, the contact zone shifts from inferior to superior on the patellar facets and bridges across the odd facet and medial femoral condyle.

Joint Reaction Forces

Patellofemoral joint reaction force (PFJRF) is a vector sum of quadriceps pull and patellar tendon tension:

• Level gait: 0.5 × body weight (BW).
• Stair ascent/descent: 3.0 to 4.0 × BW.
• Deep squatting / jumping: 7.0 to 8.0 × BW.
• Kinetic chains: During closed-chain exercises (e.g. leg press, squatting), PFJ stress increases with flexion (peak stress 60°–90°). In open-chain exercises (e.g. seated knee extension), PFJ stress peaks near terminal extension (0°–30°).

Restraints & Stabilisers

• Static Restraints: Osseous architecture (sulcus angle, lateral trochlear inclination), joint capsule, medial patellofemoral ligament (MPFL; primary restraint to lateral translation from 0°–30°), and lateral patellar retinaculum.
• Dynamic Restraints: Quadriceps musculature, particularly the vastus medialis obliquus (VMO; primary active medial dynamic stabiliser), vastus lateralis (VL), and the proximal kinetic chain (gluteus medius and minimus, hip external rotators).

Biomechanical Malalignment & The 'Miserable Malalignment' Syndrome

Lateral vector translation of the patella increases contact pressures over the lateral patellar facet and lateral trochlea. Contributors include:

• Increased Q-Angle: Normal values are 10°–14° in males and 15°–18° in females. Measured from the anterior superior iliac spine (ASIS) to the patellar centre, and patellar centre to the tibial tubercle.
• Kinetic Torsional Deformities: 'Miserable Malalignment' describes excessive femoral anteversion, increased external tibial torsion, compensatory subtalar hyperpronation, and genu valgum. Proximal hip abductor and external rotator weakness leads to dynamic femoral internal rotation and adduction during weight-bearing, functionally driving the trochlea laterally beneath the patella.

Classification Systems

While PFPS lacks a single universally accepted staging system, multiple frameworks delineate underlying drivers:

1. Holmes and Clancy Classification

Categorises anterior knee pain into:

• Patellofemoral instability (subluxation/dislocation).
• Patellofemoral pain with malalignment (abnormal tilt or subluxation without gross dislocation).
• Patellofemoral pain without malalignment (pure biomechanical overload/dysfunction).

2. Merchant Classification (Radiographic)

Measures congruency on axial views:

• Congruence Angle: Normal is -6° (medial); angles > +16° represent pathological lateral subluxation.

3. Outerbridge Classification (Arthroscopic/MRI Chondral Grading)

• Grade 0: Normal cartilage.
• Grade I: Cartilage softening and swelling.
• Grade II: Fissuring and fragmentation <1.25 cm diameter.
• Grade III: Fissuring and fragmentation >1.25 cm diameter.
• Grade IV: Full-thickness articular cartilage loss exposing subchondral bone.

Clinical Presentation & Examination

History

• Insidious onset, ache-like anterior/retropatellar pain.
• Exacerbated by ascending/descending stairs, deep squats, downhill running, and prolonged knee flexion with feet trapped (theatre sign).
• Transient sensations of 'giving way' (typically pseudogiving-way from quadriceps reflex inhibition due to pain, distinct from mechanical ligamentous laxity or true patellar dislocation).
• Mild intermittent puffiness, but frank effusion should alert the clinician to intra-articular derangement.

Physical Examination

• Alignment & Gait: Assess standing coronal alignment (genu valgum), pelvic tilt, foot arch morphology (pes planus with hindfoot valgus), and single-leg squat dynamic mechanics (evaluating dynamic valgus and contralateral pelvic drop [Trendelenburg]).
• Patellar Tracking (J-Sign): Observe the patella moving from 90° flexion to full terminal extension. Abrupt lateral deviation near terminal extension denotes an inverted 'J-sign', reflecting lateral retinacular tightness, lateralised vector pull, or trochlear dysplasia.
• Palpation & Retinacular Mobility: Palpate medial and lateral patellar facets, retro-trochlear region, patellar tendon origin/insertion, and pes anserinus. Retinacular quadrants of translation are assessed at 20°–30° flexion (normal patellar excursion = 2 quadrants medial/lateral; <1 quadrant medially indicates a tight lateral retinaculum).
• Provocative Tests:
  • Clarke's Test (Patellar Grind): Directing posterior-inferior pressure on the superior pole while the patient contracts the quadriceps reproduces anterior pain (high sensitivity, notoriously low specificity).
  • Patellar Tilt Test: Inability to elevate the lateral patellar border to horizontal or beyond confirms a contracted lateral retinaculum.
  • Eccentric Step-down Test: Stepping down from a 20 cm block yields high clinical diagnostic utility.

Investigations

Plain Radiography

Standard first-line workup requires weight-bearing radiographs:

• AP & Lateral (at 30° flexion): Assess joint spaces, patellar height via the Insall-Salvati ratio (patellar tendon length to patellar diagonal length; normal 0.8–1.2; >1.2 = patella alta; <0.8 = patella baja) or Caton-Deschamps index.
• Axial / Skylines (Merchant view at 45°, or Sunrise/Laurin at 20°–30°):
  • Trochlear Sulcus Angle: Normal <145° (>145° indicates flat trochlear morphology / dysplasia).
  • Congruence Angle of Merchant: Bisecting the sulcus angle creates a zero reference; normal is medial (-6°); values >+16° denote lateral subluxation.
  • Lateral Patellofemoral Angle of Laurin: Formed between the intercondylar line and the lateral patellar facet line; should diverge laterally. Parallelism or medial opening denotes abnormal lateral tilt.

Computed Tomography (CT)

• Gold standard for rotational profiling and measuring the Tibial Tubercle–Trochlear Groove (TT-TG) distance.
• Assessed on axial CT/MRI slices by superimposing the deepest point of the trochlear groove and the centre of the patellar tendon insertion at the tibial tubercle.
• Normal <15 mm; 15–20 mm is borderline; >20 mm is unequivocally abnormal and reflects lateralised vector alignment.

Magnetic Resonance Imaging (MRI)

• Indicated when symptoms resist conservative treatment or when ruling out chondral shear defects, osteochondritis dissecans (OCD), MPFL injury, pigmented villonodular synovitis (PVNS), or meniscal pathology.
• Depicts retropatellar cartilage signal change, subchondral bone marrow oedema (focal overload), and Hoffa's fat pad impingement.

Management

Non-Operative Management (The Definitive Standard of Care)

Over 90% of patients with PFPS without frank recurrent instability are successfully treated non-operatively. Current consensus statements (e.g. International Patellofemoral Pain Research Retreat) outline core pillars:

• Exercise Therapy (Grade A Recommendation):
  • Combined quadriceps strengthening (especially closed-chain functional exercises in pain-free ranges, 0°–45°) with posterolateral hip muscle strengthening (gluteus medius, minimus, hip external rotators).
  • Hip strengthening reduces dynamic knee valgus and adduction moments.
• Patellar Taping (McConnell Taping): Provides short-term pain relief (2–4 weeks) to facilitate exercise compliance by realigning the patella and unweighting soft tissue structures.
• Foot Orthoses: Prefabricated or customised medial arch supports yield significant short-term pain relief, particularly in patients exhibiting excessive dynamic foot pronation or increased midfoot mobility.
• Activity Modification & Load Management: Temporary cessation of repetitive jumping, hill running, and deep lunges. High-cadence running retraining reduces ground reaction forces.
• Pharmacotherapy: Short courses of NSAIDs provide acute symptom control; joint injections (corticosteroid, hyaluronic acid) demonstrate poor sustained evidence and are generally discouraged in uncomplicated PFPS.

Operative Management

Surgery is contraindicated in primary, uncomplicated PFPS. It is entertained only after a minimum of 6–12 months of structured, compliant rehabilitation has failed, and when an identifiable, mechanically correctable anatomical deformity exists.

• Isolated Lateral Retinacular Release (LRR): Historically overutilised. Current consensus restricts LRR solely to patients with objective isolated lateral patellar tilt secondary to a clinically tight lateral retinaculum (negative tilt test) without patellar hypermobility or subluxation. Performing LRR in generalised hypermobility or normal tracking risks catastrophic medial subluxation and instability.
• Tibial Tubercle Osteotomy (TTO):
  • Elmslie-Trillat Procedure: Medial transposition of the tibial tubercle for patellofemoral maltracking with elevated TT-TG (15–20 mm).
  • Fulkerson Osteotomy: Anteromedialisation (AMZ) combining medial shift with anterior translation. Indicated for distal or lateral patellar facet chondrosis associated with an elevated TT-TG, thereby reducing PFJRFs and realigning tracking vectors.
  • Maquet Osteotomy: Extreme anteriorisation (up to 2 cm) using an iliac crest bone graft. Rarely utilised currently due to unacceptably high rates of skin necrosis, nonunion, and prominence.
• Cartilage Restoration Procedures: For focal Outerbridge Grade III/IV defects in refractory cases (e.g. autologous chondrocyte implantation [ACI], matrix-induced autologous chondrocyte implantation [MACI], or osteochondral autograft/allograft transfer [OATS]), often accompanied by unloading osteotomy.
• Patellofemoral Arthroplasty (PFA) / Total Knee Arthroplasty (TKA): Reserved for end-stage, isolated bicompartmental or tricompartmental patellofemoral osteoarthritis in older cohorts refractory to conservative care.

Complications

• Complications of Non-Operative Treatment: Chronic pain persistence, secondary patellofemoral osteoarthritis, and quadriceps atrophy.
• Complications of Lateral Release: Medial patellar subluxation/iatrogenic medial instability (often crippling and difficult to salvage), haemarthrosis (lateral superior genicular artery injury), reflex sympathetic dystrophy / CRPS.
• Complications of Tibial Tubercle Osteotomy: Tibial shaft fracture, tubercle nonunion or displacement, deep vein thrombosis (DVT), hardware prominence requiring metalwork removal (up to 40% of cases), anterior compartment syndrome.

Exam Pearls (Viva & Written Preparation)

• Mechanism of McConnell Taping: Works primarily via neurosensory modulation and mechanical medial glide, offloading irritable soft tissues rather than permanently modifying static osseous patellar tracking.
• Safe Exercise Zones: In PFPS, program closed-chain kinetic exercises between 0° and 45° of flexion (where PFJ stress is lowest) and open-chain kinetic exercises between 45° and 90° of flexion (avoiding the high-stress terminal 0°–30° arc).
• The Caton-Deschamps Advantage: Unlike the Insall-Salvati ratio, the Caton-Deschamps index is independent of patellar morphology and can be reliably used in skeletal immaturity and post-patellectomy reconstructions.
• Red Flag on Lateral Release: If an examiner presents an unstable patella with generalised ligamentous laxity (Beighton score ≥5/9), an isolated lateral retinacular release is contraindicated because it accelerates medial patellofemoral incompetence.