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Patellofemoral Pain Syndrome

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Other Names

  • Patellofemoral Pain Syndrome (PFPS)
  • Patellofemoral Pain (PFP)
  • Anterior knee pain
  • Idiopathic knee pain

Background

  • This page refers to patellofemoral pain, a spectrum of anterior knee pain originating at the Patellofemoral Joint
  • The term 'patellofemoral pain' is not well defined
    • It generally refers to anterior knee pain
    • It covers all conditions related to anterior knee

History

Epidemiology

  • PFPS is the most common cause of knee pain in individuals under the age of 50[1]
    • 2 to 10 times more common in women than men[2]
    • Most common cause of anterior knee pain in pediatric, adolescent patients[3]
  • Prevalence
    • Reported as high as 25% in sports[4]
    • Up to 43% in military trainees (need citation)
    • 11-17% of patients who present to general practitioners[5]
    • 30% of adolescents will develop PFPS[6]
  • Incidence
    • 22 per 1000 person years among naval academy recruits[7]
  • Sports
    • Among adolescent basketball players: overall prevalence is 25%, with ~26% of female and 18% of male players affected[8]

Pathophysiology

  • General
    • PFPS is a common cause of anterior knee pain
    • It is most commonly seen in young women without any structural or pathological changes to the articular cartilage
    • The underlying etiology is often considered to be multifactorial
  • Patellar Tracking/ Malalignment
    • Patella maltracking has long been implicated as a caause
    • Witvrou et al: hypermobile patella had a significant correlation with the incidence of PFPS[9]
    • Draper et al: patients with PFPS squat with increased lateralization, increased lateral tilt of patella[10]
  • Vastus Lateralis and Vastus Medialis (VMO)
    • Cowan et al demonstrated by EMG delayed onset of VMO activation relative to vastus lateralis[11]
    • Patients with PFP tend to exhibit atrophy of the VMO as well[12]
  • Quadriceps Dysfunction
    • Several studies have shown that quadriceps muscle size, strength, and force are impaired in patients with patellofemoral OA[13]
  • Dynamic Valgus/ Q angle
    • The role of the Q-angle as a cause or predictor of PFPS is controversial
    • Some authors have demonstrated a relationship between the Q-angle and development of PFPS[14]
    • Other authors have not recreated that relationship[15]
    • Likely related to dynamic or functional malalignment than a structural problem
    • Multiple studies have demonstrated increased knee abduction, dynamic valgus stressors on the knee joint in PFPS among athletes[16]
  • Hip Stability, abductor strength
    • Multiple studies have shown weakness of hip external rotators, abductors lead to functional malalignment at the patellofemoral joint
    • Prins et al systematic review: strong evidence that females with PFPS have a decreased hip abduction, external rotation and extension strength[17]
    • Note that this is in female athletes, the evidence for male athletes is not strong
  • Disorders of the foot
    • Barton et al: PFPS associated with delayed timing of peak rear-foot eversion, increased rear-foot eversion at heel strike and reduced rear-foot eversion range[18]
    • Early rear-foot eversion appears to increase risk of developing PFPS[19]
    • Abnormalties with navicular bone, such as increased navicular drop, navicular drift and dorsiflexion also seem to contribute[20]
  • Hamstring imbalance, tightness
    • Several studies have identified a significant association between PFPS, hamstring tightness[21]
    • Besier et al: patients with PFPS have greater co-contraction of the quadriceps and hamstrings compared to controls[22]
  • Iliotibial Band
    • The IT band may have an influence on patellar tracking[23]
    • Kaplans fibers connect the IT band to the patella
  • "Knee-Spine Syndrome"
    • Significant difference in sacral inclination between subjects with and without anterior knee pain[24]
    • Otherwise not well described or understood
  • Psychological factors
    • Jensen et al: patients with PFPS have higher level of mental distress compared to healthy controls[25]
    • Coping mechanism of patients with PFPS similar to other groups of patients with chronic pain, PFPS more likely to catastrophize their pain[26]
    • Fear avoidance belief about physical activity associated with pain, function in PFPS patients[27]
    • Domenech et al: high incidence of psychological distress such as anxiety and depression[28]
  • Triggers for PFPS
    • Overload of the patellofemoral joint (e.g. highintensity training)
    • Dynamic valgus and functional lateralization of the patella may lead to overuse
  • Neurological etiology of pain in patients with PFPS
    • Most pain probably develops in the insertions of the extensor mechanism or within the subchondral bone
    • Increased expression of neurofilament protein, S-100 protein, neural growth factor and substance P in the lateral retinacula of PFPS[29]
    • Draper demonstrated increased metabolic bone activity in patients with PFPS using PET/CT[10]
  • Implicated pain sources
    • The medial retinaculum and MPFL[30]
    • Patellar compression of subchondral bone[31]
    • Fat pad

Etiology

  • Overuse
    • Including tendinitis, insertional tendonosis
  • Patellar instability
  • Osteochondral damage
  • Trauma

Associated Conditions

Anatomy of the patellofemoral mechanism[32]

Pathoanatomy

  • Knee Extensor Mechanism
  • Patellofemoral Joint
    • Characterized by the articulation of the patella within the condylar groove of the femur
    • Stabilized by the medial and lateral retinaculae
  • Patella
    • Functions as a lever, increasing the moment arm of the extensor mechanism[33]
    • Acts as a pulley, controlling the direction of the quadriceps
    • Articular cartilage, which is the thickest in the body, provides a smooth sliding surface with the femoral trochlea[34]
    • Acts as a shock absorber

Risk Factors

  • Non-modifiable
    • Female gender
    • Poor shock absorption (i.e. footwear, surface, muscles)
  • Sports
    • Running, Jumping sports
    • Endurance athletes
    • Basketball
    • Soccer
    • Lacrosse
  • Training
    • Training errors or overuse
    • Increased running mileage
    • Increased jumping
  • Muscular
    • Weak knee extension strength, especially VMO[35]
    • Weak hip abduction strength[36]
    • Weak hip external rotation strength
    • Poor flexibility of quadriceps, hamstring, iliotibial band
    • Poor core muscle endurance[37]
    • Foot pronation
  • Biomechanical/ Anatomic
  • Other
    • Trauma
  • Not associated
    • Foot arch height index[38]

Differential Diagnosis

Clinical Features

Clinical demonstration of the patellar grind test
  • History
    • Pain is universally anterior
    • They may describe it as “behind,” “underneath,” or “around” the patella
    • Usually insidious, but can be acute in nature
    • Pain is typically worse after prolonged sitting, squatting, kneeling, and stair climbing[43]
    • Patients often report clicking, popping, snapping and cracking
    • May report buckling, which is typically transient inhibition of quadriceps due to pain or deconditioning[44]
    • Theatre sign: Pain in the patellofemoral joint after prolonged period of sitting with knee flexed
    • Pain is often bilateral
  • Physical Exam: Physical Exam Knee
    • Absence of joint effusion; if present consider other etiology
    • Patellar J Sign: lateral tracking of patella shifts medially as knee brought into flexion
    • Compare quadriceps muscle tone, bulk of VMO to contralateral limb
    • On palpation, crepitus may be present
    • Palpate medial/lateral and superior/inferior patellar poles
    • May be ttp to medial or lateral retinaculum
    • May be ttp medial or lateral facets
  • Special Tests

Evaluation

  • Note: Diagnosis is primarily clinical and imaging is not generally required to make the diagnosis

Radiographs

Ultrasound

  • Can be used to evaluate extensor mechanism

MRI

  • Not routinely indicated in the diagnosis of PFPS
  • May be helpful to evaluate for:
    • Malalignment
    • Trochlear dysplasia
    • Patella tilt
    • Articular chondral injuries
  • Potential findings
    • Enlarged fat pad
    • Subchondral bone edema

Classification

  • Proposed clinical classification[45]
    • Patellofemoral instability, ie., subluxation or dislocation
    • Patellofemoral pain with malalignment but no episodes of instability
    • Patellofemoral pain without malalignment

Management

Nonoperative

  • Indications
    • Vast majority of cases
  • General goals
    • Reduce total patellofemoral compressive forces
    • Alter the distribution of stress forces on the patella
  • Relative rest and activity modification
    • Patients may need to temporarily discontinue offending activity
    • They may need to alter their sport or training habits
    • This could include avoiding/ limiting stairs, running, jumping, squats
  • Physical Therapy
    • Quadriceps strengthening is the gold standard treatment[46]
    • Increased quadricep strength has been show to reduce PFPS pain[47]
    • Bolga et al systematic review: Targeting hip abductors, external rotators generated a modest reduction in pain[48]
    • Harvie et al: 2008 meta-analysis showed positive effects on pain reduction[49]
    • Exercises should address hip muscles, trunk stability, quadriceps, hamstrings and the iliotibial tract

Pharmacotherapy

  • NSAIDS
    • A Cochrane review found limited evidence for the effectiveness of short term pain reduction in PFPS[50]
  • Topical NSAIDS
    • Cochrane review found topical NSAIDS are as effective as oral NSAIDS for chronic musculoskeletal pain[51]
  • Vitamin D
    • Anecdotal evidence that patients with PFPS have low vitamin D and osteopenia by DEXA[52]
    • It is unknown whether supplementation with vitamin D is beneficial

Bracing and Taping

  • Patellar Taping or McConnel Taping
    • Goal: modify patella tracking with adhesive tape, typically a medially directed force
    • When combined with physical therapy and daily home exercises, patellar taping was superior to the control group[53]
    • Warden et al: 2008 meta-analysis showed functional improvement, decreaesed pain when combined with exercise[54]
  • Patellar Brace
    • Goal: apply an external, medially directed force to counteract patella maltracking
    • Lun et al found patellar bracing non-superior to home exercise program with or without a patellar brace or knee sleeve[55]
    • D’hondt et al: 2002 meta-analysis showed patellar brace had positive effects on pain, function, patellofemoral congruence angle[56]
    • Overall, literature is weak and better designed studies are needed.
  • Knee Brace
    • Standard hinged knee brace
  • Foot Orthosis
    • Goal: insoles could improve rear-foot eversion or pes pronatus
    • Collins et al reported moderate improvement in pain in patients who used a corrective orthosis and participated in physical therapy[57]
    • Other studies have shown mixed results
    • Overall, literature is weak for PFPS and better designed studies are needed

Other Modalities

Operative

  • Indications
    • Unknown
    • Likely reserved for refractory cases
  • Technique
    • Arthroscopy
    • Percutaneous
    • Lateral reticular release
    • MPFL repair or reconstruction
    • Anteromedialization of the tibial tubercle
  • Research
    • Kettunen compared arthroscopy plus exercise to exercise alone in patients with chronic PFPS and found no difference[61]

Rehab and Return to Play

Rehabilitation

  • Needs to be updated

Return to Play

  • The athlete can return to sport when the following criteria are met[62]
    • No swelling
    • No pain in squatting and in ascending or descending stairs • Good quadriceps strength (especially vastus medialis obliques)
    • Proper hamstring flexibility
    • Normal gait biomechanics
    • Proper core stability strength
    • Good performance in challenging functional tests (vertical jumping, anteromedial lunge, step-down, single-leg press, and balance and reach tests)
    • The patient feeling that he/she is ready and has confidence in the injured knee

Complications and Prognosis

Prognosis

  • Kannus et al studied chronic PFPS over 7 years[63]
    • 2/3 of patients had complete recovery at 7 years
    • Approximately 30% of the non-recovery PFPS patients had persistent complaints
  • Predictor of poor long term prognosis[64]
    • PFP > 2 months duration
    • Anterior knee pain score < 70
    • Higher levels of usual/resting
    • High worst/activity-related pain

Complications

  • Knee Osteoarthritis
    • Proposed, although not well described, that PFPS increases likelihood of developing patellofemoral OA[65]
  • Patellofemoral Arthritis
  • Inability to return to sport
    • Up to 25% of recreational athletes diagnosed with PFP will stop participating in sports because of knee pain[66]

See Also

References

  1. ↑ Davis IS, Powers CM. Patellofemoral pain syndrome: proximal, distal, and local factors, an international retreat, April 30–May 2, 2009, Fells Point, Baltimore, MD. J Orthop Sports Phys Ther 2010;40:A1–16.
  2. ↑ Taunton JE, Ryan MB, Clement DB, McKenzie DC, Lloyd-Smith DR, Zumbo BD. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med. 2002;36:95-101.
  3. ↑ Kodali, Pradeep, Andrew Islam, and Jack Andrish. "Anterior knee pain in the young athlete: diagnosis and treatment." Sports medicine and arthroscopy review 19.1 (2011): 27-33.
  4. ↑ Devereaux MD, Lachmann SM. Patello-femoral arthralgia in athletes attending a Sports Injury Clinic. Br J Sports Med. 1984;18:18-21.
  5. ↑ Crossley KM, van Middelkoop M, Callaghan MJ, et al. Patellofemoral pain consensus statement from the 4th International Patellofemoral Pain Research Retreat, Manchester. Part 2: recommended physical interventions (exercise, taping, bracing, foot orthoses and combined interventions) Br J Sports Med. 2016;50(14):844–852
  6. ↑ McNerney, Michelle L. Gorman, and Elizabeth A. Arendt. "Anterior knee pain in the active and athletic adolescent." Current sports medicine reports 12.6 (2013): 404-410.
  7. ↑ Boling M, Padua D, Marshall S, Guskiewicz K, Pyne S, Beutler A (2010) Gender differences in the incidence and prevalence of patellofemoral pain syndrome. Scand J Med Sci Sports 20(5):725–730
  8. ↑ Foss KD, Myer GD, Magnussen RA, Hewett TE. Diagnostic differences for anterior knee pain between sexes in adolescent basketball players. J Athl Enhanc. 2014;3(1):1814–1820.
  9. ↑ Witvrouw E, Lysens R, Bellemans J, Cambier D, Vanderstraeten G (2000) Intrinsic risk factors for the development of anterior knee pain in an athletic population. A two-year prospective study. Am J Sports Med 28:480–489
  10. ↑ 10.0 10.1 Draper CE, Besier TF, Santos JM, Jennings F, Fredericson M, Gold GE, Beaupre GS, Delp SL (2009) Using real-time MRI to quantify altered joint kinematics in subjects with patellofemoral pain and to evaluate the effects of a patellar brace or sleeve on joint motion. J Orthop Res 27(5):571–577
  11. ↑ Cowan SM, Bennell KL, Hodges PW, Crossley KM, McConnell J (2001) Delayed onset of electromyographic activity of vastus medialis obliquus relative to vastus lateralis in subjects with patellofemoral pain syndrome. Arch Phys Med Rehabil 82(2):183–189
  12. ↑ Pattyn E, Verdonk P, Steyaert A, Vanden Bossche L, Van den Broecke W, Thijs Y, Witvrouw E (2011) Vastus medialis obliquus atrophy: does it exist in patellofemoral pain syndrome? Am J Sports Med 39(7):1450–1456
  13. ↑ Hart HF, Ackland DC, Pandy MG, Crossley KM. Quadriceps volumes are reduced in people with patellofemoral joint osteoarthritis. Osteoarthritis Cartilage. 2012;20(8):863–868.
  14. ↑ Kaya D, Doral MN (2012) Is there any relationship between Q-angle and lower extremity malalignment? Acta Orthop Traumatol Turc 46(6):416–419
  15. ↑ Park SK, Stefanyshyn DJ (2011) Greater Q angle may not be a risk factor of patellofemoral pain syndrome. Clin Biomech (Bristol, Avon) 26(4):392–396
  16. ↑ Myer GD, Ford KR, Barber Foss KD, Goodman A, Ceasar A, Rauh MJ, Divine JG, Hewett TE (2010) The incidence and potential pathomechanics of patellofemoral pain in female athletes. Clin Biomech (Bristol, Avon) 25(7):700–707
  17. ↑ Prins MR, van der Wurff P (2009) Females with patellofemoral pain syndrome have weak hip muscles: a systematic review. Aust J Physiother 55(1):9–15
  18. ↑ Barton CJ, Levinger P, Menz HB, Webster KE (2009) Kinematic gait characteristics associated with patellofemoral pain syndrome: a systematic review. Gait Posture 30(4):405–416
  19. ↑ Barton CJ, Levinger P, Webster KE, Menz HB (2011) Walking kinematics in individuals with patellofemoral pain syndrome: a case-control study. Gait Posture 33(2):286–291
  20. ↑ Mølgaard M (2011) Patellofemoral pain syndrome and its association with hip, ankle, and foot function in 16- to 18-year-old high school students: a single-blind case-control study. J Am Podiatr Med Assoc 101(3):215–222
  21. ↑ White LC, Dolphin P, Dixon J (2009) Hamstring length in patellofemoral pain syndrome. Physiotherapy 95(1):24–28
  22. ↑ Besier TF, Fredericson M, Gold GE, Beaupre´ GS, Delp SL (2009) Knee muscle forces during walking and running in patellofemoral pain patients and pain-free controls. J Biomech 42(7):898–905
  23. ↑ Wu CC, Shih CH (2004) The influence of iliotibial tract on patellar tracking. Orthopedics 27(2):199–203
  24. ↑ Tsuji T, Matsuyama Y, Goto M, Yimin Y, Sato K, Hasegawa Y, Ishiguro N (2002) Knee-spine syndrome: correlation between sacral inclination and patellofemoral joint pain. J Orthop Sci 7(5):519–523
  25. ↑ Jensen R, Hystad T, Kvale A, Baerheim A (2007) Quantitative sensory testing of patients with long lasting patellofemoral pain syndrome. Eur J Pain 11(6):665–676
  26. ↑ Thomee´ P, Thomee´ R, Karlsson J (2002) Patellofemoral pain syndrome: pain, coping strategies and degree of well-being. Scand J Med Sci Sports 12(5):276–281
  27. ↑ Piva SR, Fitzgerald GK, Irrgang JJ, Fritz JM, Wisniewski S, McGinty GT, Childs JD, Domenech MA, Jones S, Delitto A (2009) Associates of physical function and pain in patients with patellofemoral pain syndrome. Arch Phys Med Rehabil 90(2):285–295
  28. ↑ Domenech J, Sanchis-Alfonso V, Lo´pez L, Espejo B (2013) Influence of kinesiophobia and catastrophizing on pain and disability in anterior knee pain patients. Knee Surg Sports Traumatol Arthrosc 21(7):1562–1568
  29. ↑ Sanchis-Alfonso V, Rosello´-Sastre E (2000) Immunohistochemical analysis for neural markers of the lateral retinaculum in patients with isolated symptomatic patellofemoral malalignment. A neuroanatomic basis for anterior knee pain in the active young patient. Am J Sports Med 28(5):725–731
  30. ↑ Luhmann, Scott J., et al. "Adolescent patellofemoral pain: implicating the medial patellofemoral ligament as the main pain generator." Journal of children's orthopaedics 2.4 (2008): 269-277.
  31. ↑ Graf, J., et al. "Chondromalacia patellae und intraossärer Druck." Zeitschrift für Orthopädie und ihre Grenzgebiete 130.06 (1992): 495-500.
  32. ↑ musculoskeletalkey.com
  33. ↑ Grelsamer, Ronald P., and John R. Klein. "The biomechanics of the patellofemoral joint." Journal of Orthopaedic & Sports Physical Therapy 28.5 (1998): 286-298.
  34. ↑ Eckstein, Felix, Magdalena Müller-Gerbl, and Reinhard Putz. "Distribution of subchondral bone density and cartilage thickness in the human patella." Journal of anatomy 180.Pt 3 (1992): 425.
  35. ↑ Lankhorst, Nienke E., Sita MA Bierma-Zeinstra, and Marienke van Middelkoop. "Risk factors for patellofemoral pain syndrome: a systematic review." journal of orthopaedic & sports physical therapy 42.2 (2012): 81-94.
  36. ↑ Robinson RL, Nee RJ. Analysis of hip strength in females seeking physical therapy treatment for unilateral patellofemoral pain syndrome. J Orthop Sports Phys Ther. 2007;37(5):232–238
  37. ↑ Ferber R, Bolgla L, Earl-Boehm JE, Emery C, Hamstra-Wright K. Strengthening of the hip and core versus knee muscles for the treatment of patellofemoral pain: a multicenter randomized controlled trial. J Athl Train. 2015;50(4):366–377.
  38. ↑ 38.0 38.1 Lankhorst, Nienke E., Sita MA Bierma-Zeinstra, and Marienke van Middelkoop. "Factors associated with patellofemoral pain syndrome: a systematic review." British journal of sports medicine 47.4 (2013): 193-206.
  39. ↑ Powers CM, Maffucci R, Hampton S. Rearfoot posture in subjects with patellofemoral pain. J Orthop Sports Phys Ther. 1995;22(4):155–160.
  40. ↑ Kannus P, Niittymaki S. Which factors predict outcome in the nonoperative treatment of patellofemoral pain syndrome? A prospective follow-up study. Med Sci Sports Exerc. 1994;26(3):289–296
  41. ↑ al-Rawi Z, Nessan AH. Joint hypermobility in patients with chondromalacia patellae. Br J Rheumatol. 1997;36(12):1324–1327.
  42. ↑ Thijs Y, Van Tiggelen D, Roosen P, De Clercq D, Witvrouw E. A prospective study on gait-related intrinsic risk factors for patellofemoral pain. Clin J Sport Med. 2007;17(6):437–445.
  43. ↑ Cirkel JW, Klaassen WRC, Kunst JA, et al. NHGStandaard Niet-traumatische knieproblemen bij kinderen en adolescenten. Huisarts Wet. 1998;41:246-251.
  44. ↑ Post WR. Clinical evaluation of patients with patellofemoral disorders. Arthroscopy. 1999;15:841–51.
  45. ↑ Holmes Jr, S. Wendell, and William G. Clancy Jr. "Clinical classification of patellofemoral pain and dysfunction." Journal of Orthopaedic & Sports Physical Therapy 28.5 (1998): 299-306.
  46. ↑ Bolgla LAMalone TR. Exercise prescription and patellofemoral pain: evidence for rehabilitation. J Sport Rehabil. 2005;14(1):72–88
  47. ↑ Crossley, Kay, et al. "A systematic review of physical interventions for patellofemoral pain syndrome." Clinical Journal of Sport Medicine 11.2 (2001): 103-110.
  48. ↑ Bolgla, Lori A., and Michelle C. Boling. "An update for the conservative management of patellofemoral pain syndrome: a systematic review of the literature from 2000 to 2010." International journal of sports physical therapy 6.2 (2011): 112.
  49. ↑ Harvie D, O’Leary T, Kumar S (2011) A systematic review of randomized controlled trials on exercise parameters in the treatment of patellofemoral pain: what works? J Multidiscip Healthc 4:383–392
  50. ↑ Heintjes E, Berger MY, Bierma-Zeinstra SM, Bernsen RM, Verhaar JA, Koes BW (2004) Pharmacotherapy for patellofemoral pain syndrome. Cochrane Database Syst Rev 3:CD003470
  51. ↑ Derry S, Moore RA, Rabbie R. Topical NSAIDs for chronic musculoskeletal pain in adults. Cochrane Database Syst Rev. 2012;(9):CD007400.
  52. ↑ Björkström, Sven, and Ian F. Goldie. "Hardness of the subchondral bone of the patella in the normal state, in chondromalacia, and in osteoarthrosis." Acta orthopaedica Scandinavica 53.3 (1982): 451-462.
  53. ↑ Crossley, Kay, et al. "Physical therapy for patellofemoral pain: a randomized, double-blinded, placebo-controlled trial." The American journal of sports medicine 30.6 (2002): 857-865.
  54. ↑ Warden SJ, Hinman RS, Watson MA Jr, Avin KG, Bialocerkowski AE, Crossley KM (2008) Patellar taping and bracing for the treatment of chronic knee pain: a systematic review and metaanalysis. Arthr Rheum 59(1):73–83
  55. ↑ Lun, Victor MY, et al. "Effectiveness of patellar bracing for treatment of patellofemoral pain syndrome." Clinical Journal of Sport Medicine 15.4 (2005): 235-240.
  56. ↑ D’hondt NE, Struijs PA, Kerkhoffs GM, Verheul C, Lysens R, Aufdemkampe G, Van Dijk CN (2002) Orthotic devices for treating patellofemoral pain syndrome. Cochrane Database Syst Rev (2):CD002267
  57. ↑ Collins NCrossley KMBeller EDarnell RMcPoil TVicenzino B. Foot orthoses and physiotherapy in the treatment of patellofemoral pain syndrome: randomised clinical trial. Br J Sports Med. 2009;43(3):169–171
  58. ↑ Jensen, Roar, et al. "Acupuncture treatment of patellofemoral pain syndrome." The Journal of Alternative and Complementary Medicine 5.6 (1999): 521-527.
  59. ↑ Lake DA, Wofford NH. Effect of therapeutic modalities on patients with patellofemoral pain syndrome: a systematic review. Sports Health. 2011;3(2):182–189
  60. ↑ Dos Santos RL, Souza ML, Dos Santos FA. Neuromuscular electric stimulation in patellofemoral dysfunction: literature review. Acta Ortop Bras. 2013;21(1):52–58.
  61. ↑ Kettunen JA, Harilainen A, Sandelin J, Schlenzka D, Hietaniemi K, Seitsalo S, Malmivaara A, Kujala UM (2007) Knee arthroscopy and exercise versus exercise only for chronic patellofemoral pain syndrome: a randomized controlled trial. BMC Med 13(5):38–45
  62. ↑ Halabchi, Farzin, et al. "Patellofemoral pain in athletes: clinical perspectives." Open access journal of sports medicine 8 (2017): 189.
  63. ↑ Kannus P, Natri A, Paakkala T, et al. An outcome study of chronic patellofemoral pain syndrome. Seven-year follow-up of patients in a randomized, controlled trial. J Bone Joint Surg Am 1999;81:355–63
  64. ↑ ollins NJ, Bierma-Zeinstra SM, Crossley KM, van Linschoten RL, Vicenzino B, van Middelkoop M. Prognostic factors for patellofemoral pain: a multicentre observational analysis. Br J Sports Med. 2013;47(4):227–233.
  65. ↑ Thomas MJ, Wood L, Selfe J, Peat G (2010) Anterior knee pain in younger adults as a precursor to subsequent patellofemoral osteoarthritis: a systematic review. BMC Musculoskelet Disord 11:201–208
  66. ↑ Rathleff MS, Rasmussen S, Olesen JL. Unsatisfactory long-term prognosis of conservative treatment of patellofemoral pain syndrome. Ugeskr Laeger. 2012;174(15):1008–1013
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