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THE BONE & JOINT JOURNAL 1235
D. F. Kader,
S. Jones,
F. S. Haddad
From South West
London Elective
Orthopaedic Centre,
Epsom, UK
Correspondence should be
sent to S. Jones; email:
samantha.jones140@nhs.net
© 2023 Kader et al.
doi:10.1302/0301-620X.105B12.
BJJ-2023-1026 $2.00
Bone Joint J
2023;105-B(12):1235–1238.
EDITORIAL
Towards a better understanding of
patellofemoral instability
A TOWER OF BABEL CHALLENGE
Cite this article: Bone Joint J 2023;105-B(12):1235–1238.
A great deal of recent research and debate has
advanced our understanding of instability of
the patellofemoral joint (PFJ),1- 3 but a den-
itive language, understanding, and approach
to management remains elusive. Attempts at a
consensus and at ushing out the unknowns and
key research questions are therefore timely. In this
issue of The Bone & Joint Journal, the authors of
a modied Delphi study have integrated the opin-
ions 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 cartilagi-
nous morphology, and the ligaments which oer
static support.7 Distant static factors, of secondary
signicance, 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 inuences 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 patellofem-
oral ligament (MPFL) acts as the main passive
check rein of the patella during the initial stages of
knee exion, from 20° to 30°.9 The stability of the
PFJ in deeper exion relies on the bony geometry
and cartilaginous cover of the patella and trochlea.
The diagnostic criteria for patellar instability are
opaque. Eorts to quantify instability and adopt an
à la carte approach have been riddled with di-
culty 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 varia-
tions might lead to PFJ instability in some patients
and remain inconsequential in others. Anatom-
ical variation does not, therefore, always result
in dysfunction or discomfort. A persistent ques-
tion that challenges many is: when does a simple
anatomical deviation evolve from an innocuous
nding into a medical concern?
It is broadly recognized that the key factors
inuencing 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 eect on the kinematics of the PFJ
in shorter individuals. This discrepancy arises
because the TT- TG distance is gauged as an abso-
lute metric, rather than as a proportion of the indi-
vidual’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 signicantly
inuenced by the degree of knee exion 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 dierent eect
on PFJ stability between the two legs of the same
individual.8,17 Determining the diagnosis of insta-
bility on measurements made in millimetres, espe-
cially when using static imaging, is too rigid for
such a nuanced issue.
Approximately 10% of the general population
have high- grade trochlear dysplasia, dened 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
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D. F. KADER, S. JONES, F. S. HADDAD
1236
THE BONE & JOINT JOURNAL
age group.17,19 This highlights the discrepancy between struc-
tural 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 develop-
ment 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 classi-
cations. There is evidence that this emphasizes distinctions
between bony and cartilage morphology, indicating that carti-
lage might have a dierent 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 at trochlea may articulate with a at
patella, or a shallow trochlea might still align with a matching
type of patella, as they typically develop together from child-
hood.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 inuenced by variables such as the length of the
patella and of the patellar tendon, the angle of exion 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 specic
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 recon-
struction, tibial tubercle osteotomy, and trochleoplasty. Patients
may rarely require a derotation osteotomy. Most patients with
instability without signicant bony malalignment or severe
(grade C and D) Dejour trochlear dysplasia can be treated with
isolated MPFL reconstruction. The MPFL is injured in approxi-
mately 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 deci-
sion about whether to operate and which operation to perform.
When an isolated MPFL reconstruction will reliably lead to a
successful outcome remains dicult 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 xation and tensioning, and the position
of the tunnel. In view of these potential variations, we might be
comparing fundamentally dierent 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 exemplies the possible discrepancies
in the literature. Although all the following research workers
have dened their reference points as “anatomical”, their nd-
ings are signicantly dierent. 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 identied 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 identied radiologically, it
corresponded to a point just posterior to the posterior femoral
cortex and just anterior to the intersecting point of Blumen-
saat’s line42 and the curved line from the posterior cortex: hence
the name “conuence point”.38,40 This point corresponds to the
centre of rotation of the knee and is best identied radiologi-
cally 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 dicult to accurately drill a 4 mm to 5 mm diag-
onal tunnel in the femur to a landmark that is determined in
millimetres. The precision that this requires makes it dicult 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 deli-
cate 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 medi-
alization 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
VOL. 105-B, No. 12, DECEMBER 2023
TOWARDS A BETTER UNDERSTANDING OF PATELLOFEMORAL INSTABILITY 1237
lateral gutter in exion for whom several procedures may be
required to ensure patellar stability.
In order to progress we must agree on the basic termi-
nology, the stratication of risk factors, and our descriptions
of anatomical landmarks to ensure consistent communication.
Standardizing assessment protocols will minimize subjec-
tivity. Advancing dynamic imaging techniques may also aid in
detecting subtle forms of instability and oer a deeper insight
into the complex interplay of risk factors leading to dislocation.48
In light of these considerations, we should also rene 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
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Follow F. S. Haddad @bjjeditor
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Author information:
D. F. Kader, FRCS (Tr&Orth), MFS EM (UK), Consultant Orthopaedic
Surgeon, President of the British Orthopaedic Sports Trauma &
Arthroscopy Association, South West London Elective Orthopaedic Centre,
Epsom, UK; University of Kurdistan Hewlêr, Erbil, Iraq.
S. Jones, BSc, MSC, PhD, Associate Director for Research, South West
London Elective Orthopaedic Centre, Epsom, UK; Institute of Medical and
Biomedical Education, St George’s, University of London, London, UK.
F. S. Haddad, BSc, MD (Res), MCh (Orth), FRCS (Orth), FFSEM, Professor
of Orthopaedic Surgery, Editor- in- Chief, University College London
Hospitals, London, UK; The Princess Grace Hospital, London, UK; The
NIHR Biomedical Research Centre, London, UK; The Bone & Joint Journal,
London, UK.
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 nancial 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://creative-
commons.org/licenses/by-nc-nd/4.0/
This article was primary edited by J. Scott.
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