Overview
The patella, colloquially known as the
kneecap, is the largest sesamoid bone in the body. It is formed within the
quadriceps femoris muscle. It has a role in protecting the knee joint,
protecting the quadriceps tendon from friction forces and has a significant
role in maximizing the biomechanics of knee extension. It articulates with the
femur.
Gross Anatomy
Macro-anatomy
The patella is
triangular in shape with its apex pointed inferiorly to which the patella
ligament is attached. The base (superior border of triangle) is broad and
serves as an attachment point for quadriceps femoris tendon. The lateral border has a tendinous insertion
from vastus intermedius. The medial border has a tendinous insertion from
vastus medialis. Note all four of the quadriceps muscles contribute to the
quadriceps femoris tendon thus it is often referred to as the quadriceps
tendon.
The anterior
surface is convex with rough longitudinal striae and is covered by a thin layer
of the quadriceps tendon which is then continuous with the superficial fibers
of the patella ligament. Between this and the skin is the prepatellar bursa.
The posterior
surface articulates with the femoral trochlea of the distal femur and has a
medial and lateral facet that meet in the midline to form a raised vertical
ridge. The lateral facet is the larger of the two. Below the articular surface
area is a rough convex area not covered by hyaline cartilage to which the
patellar ligament attaches.
Articulations
The patella
articulates with the femoral trochlea and is considered to be part of the knee
joint.
Blood Supply
The patella has
both an extra and intraosseous blood supply. A plexus of blood vessels,
sometimes known as the anastomotic patella ring, makes up the extraosseous
blood supply. This is contributed to by the following; the supreme genicular,
medial superior genicular, lateral supe-rior genicular, medial inferior
genicular, lateral inferior gen-icular, and anterior tibial recurrent arteries(1, 4). The
superior contributions and anastomosis run anterior to the quadriceps tendon, while
the inferior contributions and anastomosis runs posterior to the patella
ligament through the fat pad.
The intraosseus
blood supply is made up of two major systems. One is the midpatellar vessels
which enter the patella on the middle third of the anterior surface. The second
polar vessels enter the distal pole of the patella between the attachement of
the ligamentum patella and the articular surface(1). Due to the distribution of intraosseous blood flow there is a risk
of avascular necrosis (usually of the upper half) following fracture(4).
Nerve Supply
No nerve roots
have been found within the knee joint itself or the patella(1, 5). L2
through to L5 supply the anterior cutaneous innervation of the region.
Anteriomedial innervation is made up of the femoral, obturator and saphenous
nerves. Anterolateral innervation is made up of the later femoral and lateral
sural cutaneous nerves.
Ligamentous attachments
Patella
ligament: The patella ligament is also at times referred to as the patella
tendon, however as it connect bone to bone ligament is perhaps the better
terminology. It originates from the distal apex and margins of the patella and
inserts into the tibial tuberosity. Deep to this ligament is the infrapatellar
fat pad and deep infrapatellar bursa.
Medial
patellofemoral ligament (MPFL): Is a band of retinacular tissue connecting the
femoral medial epicondyle to the medial edge of the patella(6). It has been stated that the MPFL provides 50-80% of the restraining
force to lateral patella dislocation(7).
Lateral
patellofemoral ligament (LPFL): Is a thickening of the knee joint capsule and
the anatomical findings of this ligament vary(8). The ligament is anchored at the lateral femoral epicondyle and
attaches to the lateral patella(8, 9).
Development
At 9 weeks gestation the region of the future patella becomes chondrified forming a hyaline cartilaginous mass that segments the formerly continuous tendinous insertion of the quadriceps and distally fibrocartilagenous transition into the patellar ligament occurs(1). By 23 weeks gestation the patella has lateral facet predominance and increases in relative size until 6 months gestation, after which its growth is relative(2). Ossification begins from a primary ossification center in a centrifugal manner; typically this commences around the age of five years and is completed around puberty(1, 3).
Clinical Anatomy
Patellar fracture
Accounts
for 1% of all fractures usually occurring between the age of 20 and 50 years
being more common in men than women(10). The mechanism of injury is
direct trauma, sudden forceful muscle contraction and iatrogenic (i.e. during
knee arthroplasty)(11).
Plain
film radiography can be useful in diagnosis, an anteroposterior, lateral and
tangential (Merchant) views should be obtained. Many opt for computer
tomography for added detail, including greater articular step assessment, and
pre-operative planning. MRI is at times used for further assessment of soft
tissues and in cases of dislocation(12).
There
are multiple fracture types including transverse, vertical, stellate
(comminuted), apical, marginal and osteochondral. The fracture type and patient
factors such as function will help guide management.
As
with all fractures one can contemplate a conservative or surgical management
pathway. Factors favoring conservative management are a lack of significant
displacement or articular incongruity, closed fracture, intact extensor
mechanism, when the extensor mechanism forces are perpendicular to the
direction of fracture displacement (vertical fractures) and when patients have
a high co-morbid burden or low functional need (i.e. non ambulatory)(10,
11). Conservative
management involves immobilization of the leg in near full extension for around
six weeks using a cast or leg brace until evidence of healing on radiographs,
the patient can often partially weight bear with crutches(10). Note that controlled and
timely physiotherapy is key to a satisfactory outcome.
There
are multiple surgical options for patellar fixation with very limited
randomized controlled trials comparing the techniques available(10). Aims of operative treatment
are to preserve extensor function, vascularization, and restore articular
congruency. Options include modified tension-band wiring, lag screw fixation,
cerclage, cannulated lag screw with tension band, partial patellectomy, and
total patellectomy(12). Tension band wiring is the
most widely used technique, total patellectomy should be a last resort due to
resulting poor function(11,
12).
Patella dislocation
Patellar
dislocation accounts for 2-3% of all knee injuries and is the second most
common cause of traumatic haemarthrosis (first in children), it usually occurs
between the age of 10 and 20 years(12,
13). The mechanism of
injury can be traumatic or relatively atraumatic on a background of ligamentous
laxity. Classically the patella is displaced laterally out of the trochlear
groove. In the nearly all lateral displacements the MPFL is disrupted(12,
14).
Many
dislocations spontaneously reduce themselves (often on knee extension) and thus
may reduced on attendance to the emergency department. Patient history alongside
examination is therefore important, it is worth remembering soft tissue
swelling and patient apprehension will likely make examination challenging.
Imaging can consist of plain film radiographs and/or computer tomography (CT)
to investigate bony injury and assess for predisposing bony factors. Magnetic
resonance imaging (MRI) can also be used to visualize the soft tissues
including the MPFL.
Management
of non-complicated primary patella dislocation is a topic of debate. Initially
the aim in all patella dislocations (regardless if first time or recurrent) is
to provide symptomatic relief that will consist of; relative rest, light
compression, elevation, ice and analgesia(13). It then becomes unclear on the
advantages between conservative and surgical management. Many will advocate
non-surgical management with rehabilitation following first dislocation,
however some advocate repair of the MPFL(13). However there is minimal
quality evidence available on the topic and a recent Cochrane review was unable
to confirm a significant difference between either approach due to the lack of
quality evidence(15).
For
recurrent dislocations surgery becomes an increasingly attractive option. The
procedures available consist of MPFL reconstruction, trochleoplasty and trochlear
osteotomy. However there is very poor evidence comparing surgical intervention
to rehabilitation in these cases(13,
15).
Acute patella tendon rupture
Acute
patella tendon rupture tends to occur in those under the age of 40 years when
the extensor mechanism is overloaded(16,
17). Rupture is
relatively rare and in the healthy adult a force of 17.5 times the patients
body weight is required(18). However in those with previous
patella tendon pathology, steroid injections, hyperparathyroidism, rheumatoid
arthritis, immunosuppression, systemic lupus erythematosus the risk increases(17).
The
patient will often present with severe sudden onset pain, swelling, inability
to weight bear and weakness of the extensor mechanism(16). On examination one will find
swelling and may find a high riding patella (Patella Alta) and/or weakened
extensor ability. However even in complete ruptures some retinacular fibers may
be intact facilitating extension(16). Initial imaging should be plain
film radiographs demonstrating a high riding patella, some may use
ultrasonography which can in dynamic studies be useful in differentiating between
complete and partial rupture(16). MRI is also a useful
investigation.
In
complete rupture swift surgical repair is necessary to maximize outcome,
however if partial rupture non-surgical management with immobilization in
extension can be undertaken(16). Physiotherapy input is
required in all cases to aid rehabilitation.
Patellofemoral osteoarthritis
Patellofemoral
osteoarthritis (PFOA) is not uncommon and is radiographically evident in
isolation in 24% of those over 50 years and is more common in females(19,
20). The causes are
similar to osteoarthritis in other areas of the body including trauma, obesity
and inflammatory processes. However in the case of PFOA patellar misalignment
and tilt contributes(21).
The
patient will most often complain of anterior knee pain aggravated my going up
or down stairs, squatting and raising from sitting. Stiffening and
pseudolocking may also be a presenting complaint(21). Physical examination may
reveal antalgic gait, mild effusion, crepitus and tenderness. There may also be
muscle wasting.
Plain
film radiography is the initial imaging modality of choice; AP, lateral and
Merchant view should be obtained. Computer tomography can add further detail and
is especially useful if maltracking is suspected, it also gives further
information on whether osteoarthritis is present in other areas of the knee
joint(21).
Conservative
treatment consists of muscle strengthening, exercise, weight loss, activity
modification, oral analgesia and intra-articular corticosteroid injection.
However in some cases this will not result in adequate symptom relief and
surgery may be considered.
There
are many techniques with potential use in PFOA. The procedure can be joint
preserving (lateral release, tibial tuberosity osteotomy) or joint replacing
(patellofemoral replacement, total knee arthroplasty)(21,
22). The principle of
correct patient selection for the correct procedure is key in the case of PFOA
intervention.
Quick Anatomy
Key Facts
Development: Cartilaginous patella in
utero, ossification begins around 5 years of age and completed around
puberty.
Blood supply: Extraosseous patellar plexus.
Intraosseous supply from polar vessels and mid patella vessels.
Nerve supply: No nerve root within the
patella itself. L2-5 anterior cutaneous supply.
Aide-Memoire
Summary
The patella is the largest sesamoid bone in
the human body and has a vital role in maximizing the biomechanics of the knee
joint. Clinically pathology of the patella can be a cause of considerable pain
and morbidity in both the young and the older patient. A good working
anatomical knowledge can aid the clinician in their diagnosis and management of
patella pathology.
References
1 Fox AJ, Wanivenhaus F, Rodeo SA. The basic science
of the patella: structure, composition, and function. J Knee Surg. 2012 May;25(2):127-41.
2 Fulkerson JP, Buuck DA. Disorders of the
patellofemoral joint. 4th ed. ed. Philadelphia, Pa. ; London: Lippincott
Williams & Wilkins; 2004.
3 Gray HAotHBPLF, 1918; Bartleby.com, 2000.
http://www.bartleby.com/107/.
[24/02/2016].
4 Scapinelli R. Blood supply of the human
patella. Its relation to ischaemic necrosis after fracture. J Bone Joint Surg
Br. 1967 Aug;49(3):563-70.
5 Scuderi GR. The Patella. New York:
Springer-Verlag; 1995.
6 Amis AA, Firer P, Mountney J, Senavongse
W, Thomas NP. Anatomy and biomechanics of the medial patellofemoral ligament.
Knee. 2003 Sep;10(3):215-20.
7 Wheeless C. Medial Patellofemoral
Ligament. Wheeless' Textbook of Orthopaedics
2015 20/08/2015 [cited 2016 23/02/2016]; Available from: http://www.wheelessonline.com/ortho/medial_patellofemoral_ligament
8 Merican AM, Amis AA. Anatomy of the
lateral retinaculum of the knee. J Bone Joint Surg Br. 2008 Apr;90(4):527-34.
9 Saper MG, Shneider DA. Lateral
Patellofemoral Ligament Reconstruction Using a Quadriceps Tendon Graft.
Arthrosc Tech. 2014 Aug;3(4):e445-8.
10 Sayum Filho J, Lenza M, Teixeira de
Carvalho R, Pires OG, Cohen M, Belloti JC. Interventions for treating fractures
of the patella in adults. The Cochrane database of systematic reviews. 2015;2:CD009651.
11 Frank A. Liporace JRL, George J.
Haidukewych. Fractures of the Patella.
Insall & Scott Surgery of the Knee. Fifth ed: Livingstone; 2012. p.
786-98.
12 Petri M, Jagodzinski M. Patella
Fractures and Extensor Mechanism Injuries.
Skeletal Trauma: Basic Science, Management, and Reconstruction:
Saunders; 2015. p. 1895-905.
13 Tan W, Cosgarea J. Patellar Instability.
In: Miller M, Thompson S, editors. DeLee & Drez's Orthopaedic Sports
Medicine. Fourth ed: Saunders; 2015. p. 1243-57.
14 Brukner P, Khan K, Brukner P. Brukner
& Khan's clinical sports medicine. 4th ed. Sydney ; New York: McGraw-Hill;
2012.
15 Smith TO, Donell S, Song F, Hing CB.
Surgical versus non-surgical interventions for treating patellar dislocation.
The Cochrane database of systematic reviews. 2015;2:CD008106.
16 Phillips K, Costantino TG. Diagnosis of
patellar tendon rupture by emergency ultrasound. J Emerg Med. 2014 Aug;47(2):204-6.
17 Seidenstein A, Farrell C, Scuderi G,
Easley M. Quadriceps and Patellar Tendon Disruption. Insall & Scott Surgery of the Knee. 5 ed:
Livingstone; 2012. p. 696-710.
18 Zernicke R.F. GJ, Jobe F.W. Human
patellar-tendon rupture. J Bone Joint Surg Am. 1977;59:179–83.
19 Duncan RC, Hay EM, Saklatvala J, Croft
PR. Prevalence of radiographic osteoarthritis--it all depends on your point of
view. Rheumatology (Oxford). 2006 Jun;45(6):757-60.
20 Tarassoli P, Punwar S, Khan W, Johnstone
D. Patellofemoral arthroplasty: a systematic review of the literature. Open
Orthop J. 2012;6:340-7.
21 Kim YM, Joo YB. Patellofemoral
osteoarthritis. Knee Surg Relat Res. 2012 Dec;24(4):193-200.
22
Grelsamer RP, Stein DA. Patellofemoral arthritis. J Bone Joint Surg Am. 2006
Aug;88(8):1849-60.