header advert
The Bone & Joint Journal Logo

Receive monthly Table of Contents alerts from The Bone & Joint Journal

Comprehensive article alerts can be set up and managed through your account settings

View my account settings

Open Access


Towards a better understanding of patellofemoral instability

a Tower of Babel challenge

Download PDF

A great deal of recent research and debate has advanced our understanding of instability of the patellofemoral joint (PFJ),1-3 but a definitive language, understanding, and approach to management remains elusive. Attempts at a consensus and at flushing out the unknowns and key research questions are therefore timely. In this issue of The Bone & Joint Journal, the authors of a modified Delphi study have integrated the opinions of 60 surgeons from 11 countries.4,5 While emphasizing areas of agreement, this research highlights persistent areas of debate.

The stability of the PFJ depends on an intricate balance between local and distant factors, with both static and dynamic stabilizers.6 Locally, the stability is derived from the bony and cartilaginous morphology, and the ligaments which offer static support.7 Distant static factors, of secondary significance, include femoral anteversion, the rotation of the knee, and external tibial torsion.8 Locally, dynamic balance is mainly provided by the extensor muscles including the vastus medialis obliquus. Distant dynamic influences include the iliotibial band complex, the abductors and external rotators of the hip, and pronation of the subtalar joint, which may generate a dynamic valgus force moving the patella laterally.

It is widely accepted that the medial patellofemoral ligament (MPFL) acts as the main passive check rein of the patella during the initial stages of knee flexion, from 20° to 30°.9 The stability of the PFJ in deeper flexion relies on the bony geometry and cartilaginous cover of the patella and trochlea.

The diagnostic criteria for patellar instability are opaque. Efforts to quantify instability and adopt an à la carte approach have been riddled with difficulty and are prone to misinterpretation.10 While MRI and CT can identify static abnormalities, they often fail to capture the complex dynamic interactions. Moreover, these anatomical variations might lead to PFJ instability in some patients and remain inconsequential in others. Anatomical variation does not, therefore, always result in dysfunction or discomfort. A persistent question that challenges many is: when does a simple anatomical deviation evolve from an innocuous finding into a medical concern?

It is broadly recognized that the key factors influencing PFJ stability are the tibial tubercle-trochlear groove distance (TT-TG), trochlear morphology, the MPFL, and the height of the patella. Consequently, most orthopaedic surgeons focus on evaluating these parameters before deciding on the appropriate operative treatment.11

Goutallier et al12 initially referred to the TT-TG distance as the tibial tubercle-patella groove (TT-PG), in 1978. Evaluations of this were based on a cohort of 60 patients, mainly aged > 60 years and with osteoarthritis (OA). This demographic does not, however, represent the typical patients with PFJ instability and, on this basis alone, the TT-TG measurements must be approached with circumspection.

The TT-TG distance varies considerably, depending upon an individual’s stature and body dimensions.13 A 20 mm distance can exert a more pronounced effect on the kinematics of the PFJ in shorter individuals. This discrepancy arises because the TT-TG distance is gauged as an absolute metric, rather than as a proportion of the individual’s height and the dimensions of the knee.14 The reproducibility of this measurement among observers is poor, with discrepancies of between 3 and 5 mm having been documented.13 The accuracy of the measurement is also significantly influenced by the degree of knee flexion and the patient’s weightbearing status.14-16 A high TT-TG value of > 20 mm may provoke PFJ instability in some individuals, but not in others.13 Interestingly, this measurement may also have a different effect on PFJ stability between the two legs of the same individual.8,17 Determining the diagnosis of instability on measurements made in millimetres, especially when using static imaging, is too rigid for such a nuanced issue.

Approximately 10% of the general population have high-grade trochlear dysplasia, defined by a sulcus angle of ≥ 154°.18 The incidence of primary patellar dislocation is between 5.8 and 42 per 100,000 individuals annually depending on the age group.17,19 This highlights the discrepancy between structural abnormalities and symptomatic presentations, and raises questions about the appropriateness of surgery in patients with low- to medium-grade dysplasia, particularly in view of the uncertainty surrounding its long-term impact on the development of OA.20

Dejour’s categorization of trochlear dysplasia remains useful,21 albeit with some adjustments. Traditionally, our primary research focus and algorithms for initial treatment have leaned heavily on bone morphology using radiological classifications. There is evidence that this emphasizes distinctions between bony and cartilage morphology, indicating that cartilage might have a different grade of dysplasia when compared with its bony counterpart.22 The patella also tends to mirror the morphology and topography of the trochlea. This adaptability and interplay implies that a flat trochlea may articulate with a flat patella, or a shallow trochlea might still align with a matching type of patella, as they typically develop together from childhood.23 Based on this mutual adaptability, one might wonder: if the PFJ is congruently dysplastic, why would surgeons opt to deepen the trochlea?24,25

Radiological methods for determining the patellar height remain controversial due to the lack of a universally endorsed approach. Discrepancies may arise due to inconsistencies in the landmarks as well as intra- and interobserver disagreements.26 Measurements of the indices of patellar height from radiographs can vary considerably from those derived from MRI scans.27 The techniques used for measurement, which we often depend on, can also be influenced by variables such as the length of the patella and of the patellar tendon, the angle of flexion of the knee, and the tibial slope.

Given the intricacies of quantifying patellar malalignment and malrotation, and the uncertainty about their correlation with dysfunction, it is essential to use a variety of clinical tests and radiological evaluations, and avoid over-reliance on specific numerical values and abstract algorithms.

The management of PFJ instability has evolved since the Lyonnaise school described its initial principles.21 The primary forms of surgical treatment for instability include MPFL reconstruction, tibial tubercle osteotomy, and trochleoplasty. Patients may rarely require a derotation osteotomy. Most patients with instability without significant bony malalignment or severe (grade C and D) Dejour trochlear dysplasia can be treated with isolated MPFL reconstruction. The MPFL is injured in approximately 95% of patients who sustain a dislocation of the PFJ.9 Yet, a torn MPFL on MRI should not automatically be an indication for MPFL reconstruction.28 Many factors contribute to the decision about whether to operate and which operation to perform.

When an isolated MPFL reconstruction will reliably lead to a successful outcome remains difficult to determine. It is unclear whether the techniques of reconstruction which are described in the literature consistently adhere to similar standards in relation to the type of graft, its fixation and tensioning, and the position of the tunnel. In view of these potential variations, we might be comparing fundamentally different procedures under the label of MPFL reconstruction.29-35

The management of PFJ instability with MPFL reconstruction is fraught with inconsistencies. The debate around the femoral attachment of the MPFL exemplifies the possible discrepancies in the literature. Although all the following research workers have defined their reference points as “anatomical”, their findings are significantly different. Amis et al36 determined that the MPFL has its origins at the medial epicondyle of the femur. However, Desio et al9 and Schöttle et al37 identified its femoral origin to be 8.8 mm and 1 mm anterior to the posterior femoral cortical extension line, respectively.

Adding to the debate, it is suggested from cadaveric studies that the MPFL connects to an expansive area between the medial femoral epicondyle and the adductor tubercle.38-41 When the midpoint of this attachment was identified radiologically, it corresponded to a point just posterior to the posterior femoral cortex and just anterior to the intersecting point of Blumensaat’s line42 and the curved line from the posterior cortex: hence the name “confluence point”.38,40 This point corresponds to the centre of rotation of the knee and is best identified radiologically intraoperatively.

Consequently, methods of reconstructing the MPFL vary. In some instances, the procedure deviates by between 5 mm and 10 mm from its anatomical attachment. Such variations can result in elevated medial facet contact pressures and medial translation of the patella.43 The long-term complications, such as the possible development of OA, remain uncertain. It is also extremely difficult to accurately drill a 4 mm to 5 mm diagonal tunnel in the femur to a landmark that is determined in millimetres. The precision that this requires makes it difficult to replicate the recommended positions of the tunnel exactly, even if they were in the correct anatomical location.

These problems also beset tibial tubercle osteotomy and trochleoplasty. The decision to perform a trochleoplasty remains highly subjective, setting it apart as the only area yet to see substantial progress. It is clear that the indications for this procedure are determined more by individual preference than by established evidence, making it a weak contributor to the surgical treatment of PFJ instability.44

In conclusion, the management of PFJ instability is a delicate balance between art and science. It involves synthesizing information from a spectrum of clinical tests and radiological evaluations, combined with the expectations of the patient and surgical experience, while avoiding an over-reliance on strict numerical values determined in millimetres.

While nearly 60% of patients with PFJ instability have several anatomical abnormalities,45 priority should be given to rectifying the main anatomical anomaly that would lead to redislocation without creating further local pathology. In essence, MPFL reconstruction may compensate for mild patella alta (Caton-Deschamps < 1.4, the normal being between 0.6 and 1.3)46,47 and minor maltracking. Yet, in patients with severe patella alta, reconstruction might inadvertently introduce further local pathology. In such situations, distalization of the TT should also be considered.

In our practice, surgery – when indicated – involves addressing the soft-tissue disturbance caused by dislocation using MPFL reconstruction with additional distalization of the TT and medialization in selected cases. Trochleoplasty is reserved for severe cases of grade C and D Dejour dysplasia. There is a subset of patients with permanently dislocated patellae that track in the lateral gutter in flexion for whom several procedures may be required to ensure patellar stability.

In order to progress we must agree on the basic terminology, the stratification of risk factors, and our descriptions of anatomical landmarks to ensure consistent communication. Standardizing assessment protocols will minimize subjectivity. Advancing dynamic imaging techniques may also aid in detecting subtle forms of instability and offer a deeper insight into the complex interplay of risk factors leading to dislocation.48

In light of these considerations, we should also refine our approach to reconstruction. Clear indications for various surgical procedures, particularly the more invasive ones like trochleoplasty, become imperative. While the topic remains contentious, placing emphasis on blinded, independent clinical reviews marks a step forward in ensuring impartiality and the unbiased reporting of outcome

Correspondence should be sent to Samantha Jones. E-mail:


1. Clark D , Metcalfe A , Wogan C , Mandalia V , Eldridge J . Adolescent patellar instability: current concepts review . Bone Joint J . 2017 ; 99-B ( 2 ): 159 170 . Crossref PubMed Google Scholar

2. Amis AA , Oguz C , Bull AMJ , Senavongse W , Dejour D , Biomechanics M . The effect of trochleoplasty on patellar stability and kinematics: a biomechanical study in vitro . J Bone Joint Surg Br . 2008 ; 90-B ( 7 ): 864 869 . Crossref PubMed Google Scholar

3. Utting MR , Mulford JS , Eldridge JDJ . A prospective evaluation of trochleoplasty for the treatment of patellofemoral dislocation and instability . J Bone Joint Surg Br . 2008 ; 90-B ( 2 ): 180 185 . Crossref PubMed Google Scholar

4. Hurley ET , Hughes AJ , Savage-Elliott I , et al. A modified Delphi consensus statement on patellar instability: part I: diagnosis, nonoperative management, and medial patellofemoral complex repair . Bone Joint J . 2023 ; 105-B ( 12 ): 1259 1264 . Google Scholar

5. Hurley ET , Sherman SL , Chahla J , et al. A modified Delphi consensus statement on patellar instability: part II: medial patellofemoral ligament reconstruction, tibial tubercle osteotomy, trochleoplasty, rehabilitation, and return to sport . Bone Joint J . 2023 ; 105-B ( 12 ): 1265 1270 . Google Scholar

6. Kader DF , Matar HE , Caplan N . Patellofemoral joint instability: a review of current concepts . Journal of Orthopaedics and Trauma . 2016 ; 6 : 235979 . Crossref Google Scholar

7. Conlan T , Garth WP , Lemons JE . Evaluation of the medial soft-tissue restraints of the extensor mechanism of the knee . J Bone Joint Surg Am . 1993 ; 75-A ( 5 ): 682 693 . Crossref PubMed Google Scholar

8. Kerkhoffs GMMJ , Haddad F , Hirschmann MT , Karlsson J , Seil R . Patellofemoral Joint Instability: Where Are We in 2018? ESSKA Instructional Course Lecture Book . ESSKA Instructional Course Lecture Book . Berlin, Heidelberg , 2018 . Crossref Google Scholar

9. Desio SM , Burks RT , Bachus KN . Soft tissue restraints to lateral patellar translation in the human knee . Am J Sports Med . 1998 ; 26 ( 1 ): 59 65 . Crossref PubMed Google Scholar

10. Dejour DH , Mesnard G , Giovannetti de Sanctis E . Updated treatment guidelines for patellar instability: “un menu à la carte.” J Exp Orthop . 2021 ; 8 ( 1 ): 109 . Crossref PubMed Google Scholar

11. Danielsen O , Poulsen TA , Eysturoy NH , Mortensen ES , Hölmich P , Barfod KW . Trochlea dysplasia, increased TT-TG distance and patella alta are risk factors for developing first-time and recurrent patella dislocation: a systematic review . Knee Surg Sports Traumatol Arthrosc . 2023 ; 31 ( 9 ): 3806 3846 . Crossref PubMed Google Scholar

12. Goutallier D , Bernageau J , Lecudonnec B . The measurement of the tibial tuberosity. Patella groove distanced technique and results (author’s transl) . Rev Chir Orthop Reparatrice Appar Mot . 1978 ; 64 ( 5 ): 423 428 . PubMed Google Scholar

13. Caplan N , Lees D , Newby M , et al. Is tibial tuberosity-trochlear groove distance an appropriate measure for the identification of knees with patellar instability? Knee Surg Sports Traumatol Arthrosc . 2014 ; 22 ( 10 ): 2377 2381 . Crossref PubMed Google Scholar

14. Pennock AT , Alam M , Bastrom T . Variation in tibial tubercle–trochlear groove measurement as a function of age, sex, size, and patellar instability . Am J Sports Med . 2014 ; 42 ( 2 ): 389 393 . Crossref Google Scholar

15. Smith TO , Davies L , Toms AP , Hing CB , Donell ST . The reliability and validity of radiological assessment for patellar instability. A systematic review and meta-analysis . Skeletal Radiol . 2011 ; 40 ( 4 ): 399 414 . Crossref PubMed Google Scholar

16. Cooney AD , Kazi Z , Caplan N , Newby M , St Clair Gibson A , Kader DF . The relationship between quadriceps angle and tibial tuberosity-trochlear groove distance in patients with patellar instability . Knee Surg Sports Traumatol Arthrosc . 2012 ; 20 ( 12 ): 2399 2404 . Crossref PubMed Google Scholar

17. Gravesen KS , Kallemose T , Blønd L , Troelsen A , Barfod KW . High incidence of acute and recurrent patellar dislocations: a retrospective nationwide epidemiological study involving 24.154 primary dislocations . Knee Surg Sports Traumatol Arthrosc . 2018 ; 26 ( 4 ): 1204 1209 . Crossref PubMed Google Scholar

18. DeVries CA , Bomar JD , Pennock AT . Prevalence of trochlear dysplasia and associations with patellofemoral pain and instability in a skeletally mature population . J Bone Joint Surg Am . 2021 ; 103-A ( 22 ): 2126 2132 . Crossref PubMed Google Scholar

19. Wolfe S , Varacallo M , Thomas JD , Carroll JJ , Kahwaji CI . Patellar instability . 2023 . https://www.ncbi.nlm.nih.gov/books/NBK482427 ( date last accessed 29 August 2023 ). Crossref PubMed Google Scholar

20. von Knoch F , Böhm T , Bürgi ML , von Knoch M , Bereiter H . Trochleaplasty for recurrent patellar dislocation in association with trochlear dysplasia. A 4- to 14-year follow-up study . J Bone Joint Surg Br . 2006 ; 88-B ( 10 ): 1331 1335 . Crossref PubMed Google Scholar

21. Dejour H , Walch G , Nove-Josserand L , Guier C . Factors of patellar instability: an anatomic radiographic study . Knee Surg Sports Traumatol Arthrosc . 1994 ; 2 ( 1 ): 19 26 . Crossref PubMed Google Scholar

22. van Huyssteen AL , Hendrix MRG , Barnett AJ , Wakeley CJ , Eldridge JDJ . Cartilage-bone mismatch in the dysplastic trochlea. An MRI study . J Bone Joint Surg Br . 2006 ; 88-B ( 5 ): 688 691 . Crossref PubMed Google Scholar

23. Fucentese SF , von Roll A , Koch PP , Epari DR , Fuchs B , Schottle PB . The patella morphology in trochlear dysplasia--a comparative MRI study . Knee . 2006 ; 13 ( 2 ): 145 150 . Crossref PubMed Google Scholar

24. Otto A , Tscholl PM , Pääsuke R , et al. Neither lateral patellar facet nor patellar size are altered in patellofemoral unstable patients: a comparative magnetic resonance imaging analysis . Knee Surg Sports Traumatol Arthrosc . 2020 ; 28 ( 4 ): 1064 1071 . Crossref PubMed Google Scholar

25. Qiu L , Li J , Sheng B , et al. Patellar shape is associated with femoral trochlear morphology in individuals with mature skeletal development . BMC Musculoskelet Disord . 2022 ; 23 ( 1 ): 56 . Crossref PubMed Google Scholar

26. Verhulst FV , van Sambeeck JDP , Olthuis GS , van der Ree J , Koëter S . Patellar height measurements: Insall-Salvati ratio is most reliable method . Internet . Knee Surg Sports Traumatol Arthrosc . 2020 ; 28 ( 3 ): 869 875 . Crossref PubMed Google Scholar

27. Picken S , Summers H , Al-Dadah O . Inter- and intra-observer reliability of patellar height measurements in patients with and without patellar instability on plain radiographs and magnetic resonance imaging . Skeletal Radiol . 2022 ; 51 ( 6 ): 1201 1214 . Crossref PubMed Google Scholar

28. Arendt EA , Dahm DL , Dejour D , Fithian DC . Patellofemoral joint: from instability to arthritis . Instr Course Lect . 2014 ; 63 : 355 368 . PubMed Google Scholar

29. Weinberger JM , Fabricant PD , Taylor SA , Mei JY , Jones KJ . Influence of graft source and configuration on revision rate and patient-reported outcomes after MPFL reconstruction: a systematic review and meta-analysis . Knee Surg Sports Traumatol Arthrosc . 2017 ; 25 ( 8 ): 2511 2519 . Crossref PubMed Google Scholar

30. Christiansen SE , Jacobsen BW , Lund B , Lind M . Reconstruction of the medial patellofemoral ligament with gracilis tendon autograft in transverse patellar drill holes . Arthroscopy . 2008 ; 24 ( 1 ): 82 87 . Crossref PubMed Google Scholar

31. Deie M , Ochi M , Sumen Y , Adachi N , Kobayashi K , Yasumoto M . A long-term follow-up study after medial patellofemoral ligament reconstruction using the transferred semitendinosus tendon for patellar dislocation . Knee Surg Sports Traumatol Arthrosc . 2005 ; 13 ( 7 ): 522 528 . Crossref PubMed Google Scholar

32. Drez D , Edwards TB , Williams CS . Results of medial patellofemoral ligament reconstruction in the treatment of patellar dislocation . Arthroscopy . 2001 ; 17 ( 3 ): 298 306 . Crossref PubMed Google Scholar

33. Kim TS , Kim HJ , Ra IH , Kyung HS . Medial patellofemoral ligament reconstruction for recurrent patellar instability using a gracilis autograft without bone tunnel . Clin Orthop Surg . 2015 ; 7 ( 4 ): 457 464 . Crossref PubMed Google Scholar

34. Steiner TM , Torga-Spak R , Teitge RA . Medial patellofemoral ligament reconstruction in patients with lateral patellar instability and trochlear dysplasia . Am J Sports Med . 2006 ; 34 ( 8 ): 1254 1261 . Crossref PubMed Google Scholar

35. Panagopoulos A , van Niekerk L , Triantafillopoulos IK . MPFL reconstruction for recurrent patella dislocation: a new surgical technique and review of the literature . Int J Sports Med . 2008 ; 29 ( 5 ): 359 365 . Crossref PubMed Google Scholar

36. Amis AA , Firer P , Mountney J , Senavongse W , Thomas NP . Anatomy and biomechanics of the medial patellofemoral ligament . Knee . 2003 ; 10 ( 3 ): 215 220 . Crossref PubMed Google Scholar

37. Schöttle PB , Schmeling A , Rosenstiel N , Weiler A . Radiographic landmarks for femoral tunnel placement in medial patellofemoral ligament reconstruction . Am J Sports Med . 2007 ; 35 ( 5 ): 801 804 . Crossref PubMed Google Scholar

38. Stephen JM , Lumpaopong P , Deehan DJ , Kader D , Amis AA . The medial patellofemoral ligament: location of femoral attachment and length change patterns resulting from anatomic and nonanatomic attachments . Am J Sports Med . 2012 ; 40 ( 8 ): 1871 1879 . Crossref PubMed Google Scholar

39. Stephen JM , Kader D , Lumpaopong P , Deehan DJ , Amis AA . Sectioning the medial patellofemoral ligament alters patellofemoral joint kinematics and contact mechanics . J Orthop Res . 2013 ; 31 ( 9 ): 1423 1429 . Crossref PubMed Google Scholar

40. Stephen JM , Kaider D , Lumpaopong P , Deehan DJ , Amis AA . The effect of femoral tunnel position and graft tension on patellar contact mechanics and kinematics after medial patellofemoral ligament reconstruction . Am J Sports Med . 2014 ; 42 ( 2 ): 364 372 . Crossref PubMed Google Scholar

41. Valkering KP , Rajeev A , Caplan N , Tuinebreijer WE , Kader DF . An evaluation of the effectiveness of medial patellofemoral ligament reconstruction using an anatomical tunnel site . Knee Surg Sports Traumatol Arthrosc . 2017 ; 25 ( 10 ): 3206 3212 . Crossref PubMed Google Scholar

42. Blumensaat C . Die Lageabweiehungen und Verrenkungen der Knieseheibe . Ergebn Chir Orthop . 1938 ; 31 : 149 223 . [Article in German].Crossref Google Scholar

43. Gobbi RG , Pereira CAM , Sadigursky D , et al. Evaluation of the isometry of different points of the patella and femur for medial patellofemoral ligament reconstruction . Clin Biomech (Bristol, Avon) . 2016 ; 38 : 8 12 . Crossref PubMed Google Scholar

44. Liu JN , Steinhaus ME , Kalbian IL , et al. Patellar instability management: a survey of the International Patellofemoral Study Group . Am J Sports Med . 2018 ; 46 ( 13 ): 3299 3306 . Crossref PubMed Google Scholar

45. Steensen RN , Bentley JC , Trinh TQ , Backes JR , Wiltfong RE . The prevalence and combined prevalences of anatomic factors associated with recurrent patellar dislocation: a magnetic resonance imaging study . Am J Sports Med . 2015 ; 43 ( 4 ): 921 927 . Crossref PubMed Google Scholar

46. Caton J , Deschamps G , Chambat P , Lerat JL , Dejour H . Patella infera. Apropos of 128 cases . Rev Chir Orthop Reparatrice Appar Mot . 1982 ; 68 ( 5 ): 317 325 . PubMed Google Scholar

47. Phillips CL , Silver DAT , Schranz PJ , Mandalia V . The measurement of patellar height: a review of the methods of imaging . J Bone Joint Surg Br . 2010 ; 92-B ( 8 ): 1045 1053 . Crossref PubMed Google Scholar

48. Tompkins MA , Arendt EA . Patellar instability factors in isolated medial patellofemoral ligament reconstructions--what does the literature tell us? A systematic review . Am J Sports Med . 2015 ; 43 ( 9 ): 2318 2327 . Crossref PubMed Google Scholar

Author contributions

D. F. Kader: Conceptualization, Formal analysis, Writing – original draft.

S. Jones: Writing – review & editing.

F. S. Haddad: Writing – review & editing.

Funding statement

The authors received no financial or material support for the research, authorship, and/or publication of this article.

ICMJE COI statement

F. S. Haddad reports multiple research grants from Stryker, Smith & Nephew, Corin, International Olympic Committee, and NIHR, royalties from Stryker, Smith & Nephew, Corin, and MatOrtho, consulting fees from Stryker, speaker payments from Stryker, Smith & Nephew, Zimmer, and AO Recon, and support for attending meetings from Stryker, Smith & Nephew, AO Recon, and The Bone & Joint Journal, all of which are unrelated to this article. F. S. Haddad is also Editor-in-Chief of The Bone & Joint Journal, a member of the BOSTAA executive committee, and a trustee of the British Orthopaedic Association.

Open access statement

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (CC BY-NC-ND 4.0) licence, which permits the copying and redistribution of the work only, and provided the original author and source are credited. See https://creativecommons.org/licenses/by-nc-nd/4.0/


Follow D. F. Kader @DeiaryKader

Follow F. S. Haddad @bjjeditor

This article was primary edited by J. Scott.